Symposium S.CT04 : Tailored Interphases for High Strength and Functional Composites—Advances in Experiments, Simulations and AI-Based Design

We summarize recent results from our research group on assembling of robust functional synthetic and biocomposite materials from orthogonal nanoscale components for advanced nanophotonic applications. For flexible biophotonic materials, uniformly aligned chiral structures is critically important to be stabilized to improve mechanical performance without compromising controlled helical morphologies and propagating phase separation. In this aspect, we discuss mixed amorphous polysaccharide polymers and nanocrystals in terms of their hierarchical organization, intercalated structures, ability to form periodic patterned surfaces, and resulting chiral optical properties. Furthermore, we consider how highly emissive semiconducting and carbon quantum dots can be integrated into synthetic polymer matrices, chiral biomolecular structures, and survive microfabrication/printing processing for arresting quenching trends, stimulating bright patterned emission, and prompting directed lasing phenomena.

The distribution of nano-inclusions at the interface or within soft films is determined by multiple key factors. These factors include the molecular composition, interfacial profile, morphology of the films and the characteristics of the inclusions along with predominant interactions in the systems. We will discuss the role of these factors on the interfacial behavior of nano-inclusions on the surface of spherical soft films, or vesicles. Furthermore, we will discuss the organization of the inclusions in vesicles as a function of nano-inclusion effective chemistry, dimension and concentration. These investigations have been contingent upon the ability to resolve large spatiotemporal scales. Hence, the studies have required the adoption of coarse-grained models used in conjunction with mesoscale simulation methods such as Dissipative Particle Dynamics and Molecular Dynamics. Our observations have the potential to guide the design of composite materials which require precise control on their characteristics including their shape or the dispersion of the inclusions.

Exfoliated Transition Metal Dichalcogenides (TMDs, MX₂) have attracted considerable attention for infrared optical elements due to their high refractive index and extreme nonlinearities (e.g. MoS₂, TiS₂). Recent high-yield, sonication-free, surfactant-free, exfoliation methods (i.e. Redox Exfoliation) now provide access to oxidatively-resistant and colloidally-stable dispersions at high TMD content (10% v/v), and in an expanded range of polar solvents (e.g. acetonitrile, acetone, alcohols). In addition to transforming approaches to nanocomposite fabrication, ink formulation, and film processing, these characteristics enable direct organic hybridization of Group IV-VI TMDs. For example, Grignards (R-MgBr) are not accessible via traditional Liquid Phase or Li-Intercalation methods due to their extreme reactivity with labile hydrogens (water), amides (NMP), and oxygen. However, the availability of Group VI TMDs dispersions in anhydrous THF enables synthesis of alkyl grafted MoX₂, which has zero surface charge, is dispersible in chloroform, and affords subsequent surface-initiated polymerization, all while retaining optoelectronic monolayer properties. In addition, classic redox intercalation chemistries can be applied to Group V monolayer dispersions. For example, a primary alkyl amine reacts at 70^oC in anhydrous acetonitrile with semi-metallic, single layer NbS₂ via charge transfer from the amine to the half-filled Nb d-orbitals. The resultant organic-grafted NbS₂ is a semiconductor, with enhanced oxidative stability (> 7 days ambient) and dispersiblity in non-polar dichloromethane that allows for ambient handling and processing. Such access to numerous hybridization approaches will greatly expand the electrical, optical, and chemical property suite of TMDs and enhance the ability to fabricate device quality films and hetero-structures for optical filters, GRIN optics, and non-linear absorbers.

Polymer-ligated nanoparticles (NPs) exhibit an exciting variety of physical properties that depend sensitively on the dimension and composition of NPs as well as the surface chemistry of tethered polymer hairs. Herein, we report on a robust nanoreactor strategy for in-situ crafting of monodisperse stable polymer-ligated NPs with well-controlled size and shape as well as tunable plasmonic (e.g., Au) and luminescence (e.g., perovskite MAPbBr₃ and CsPbBr₃) properties. Central to this strategy is to judiciously design unimolecular amphiphilic star-like block copolymer as nanoreactor for yielding NPs intimately and permanently ligated by polymer hairs. The diameter of polymer-ligated NPs can be precisely tuned by modulating the length of inner hydrophilic block of star-like block copolymers. In the case of Au NPs, the perpetual anchoring of photoresponsive or thermoresponsive polymers on the surface renders the attractive feature of self-assembly and disassembly of NPs on demand by capitalizing on light of different wavelengths or heating/cooling, respectively. Such self-assembly/disassembly process is revealed by tunable surface-plasmon resonance absorption of Au NPs and the reversible transformation of Au NPs between their dispersed and aggregated states. For perovskite NPs, the resulting polymer-ligated MAPbBr₃/SiO₂ core/shell NPs and CsPbBr₃ plain NPs display concurrently a stellar set of significantly improved stabilities (i.e., colloidal stability, chemical composition stability, photostability, water stability), while possessing appealing solution processability, which are unattainable by conventional methods. We envision that the amphiphilic star-like block copolymer nanoreactor strategy may provide a versatile platform for crafting diverse organic-inorganic nanohybrids stably-ligated with polymer of different functionalities (e.g., semiconducting, ferroelectric, photo-responsive, thermal-responsive, or pH-sensitive) for a spectrum of applications in bioimaging, biosensors, photonic materials and devices, perovskite-based LEDs, lasers, high-energy ionization radiation detections, multiphoton emission, and scintillators.

Nanotubes used as nanochannels for fluid transport have been of increasing interest. It has been observed that carbon nanotubes (CNT) have uniformly higher permeability compared to other nanochannels, such as polycarbonate membranes, albeit CNTs can achieve smaller diameters [1]. Previous theories attribute the high permeability to nanotube atomic smoothness [2]. However, recent molecular dynamics simulations show that the structure of water at higher densities can align to that of CNTs of small diameter [3]. Furthermore, there are conflicting analyses as to whether the fluid flux is higher for CNT or boron nitride nanotubes (BNNT) [4]–[6]. Therefore, we want to better understand the structural ordering at the atomic scale of nanotube-water interfaces using empirical methods. Here, we use cryogenic electron microscopy (Cryo-EM) techniques to hyperquench water with BNNT into a vitrified state and observe the BNNT/water interface using a transmission electron microscope (TEM) at 80keV. A cryo-transfer holder capable of maintaining the sample at 90-125K facilitates the sample transfer into the TEM. Raman spectroscopy performed in-situ during TEM imaging will enable direct observation of beam-induced phase transformations. We will present results relevant to the preferential ordering of water at the interface with BNNT.

References
[1] J. K. Holt et al., “Fast Mass Transport Through Sub – 2-Nanometer Carbon Nanotubes,” Science (80-. )., vol. 312, pp. 1034–1038, 2006.
[2] A. I. Skoulidas, D. M. Ackerman, J. K. Johnson, and D. S. Sholl, “Rapid Transport of Gases in Carbon Nanotubes,” Phys. Rev. Lett., vol. 89, no. 18, pp. 13–16, 2002.
[3] A. Barati Farimani and N. R. Aluru, “Existence of Multiple Phases of Water at Nanotube Interfaces,” J. Phys. Chem. C, vol. 120, no. 41, pp. 23763–23771, 2016.
[4] E. Secchi, S. Marbach, A. Niguès, D. Stein, A. Siria, and L. Bocquet, “Massive radius-dependent flow slippage in carbon nanotubes,” Nature, vol. 537, no. 7619, pp. 210–213, 2016.
[5] M. E. Suk, A. V. Raghunathan, and N. R. Aluru, “Fast reverse osmosis using boron nitride and carbon nanotubes,” Appl. Phys. Lett., vol. 92, no. 13, pp. 3–5, 2008.
[6] S. Joseph and N. R. Aluru, “Why are carbon nanotubes fast transporters of water?,” Nano Lett., vol. 8, no. 2, pp. 452–458, 2008.

Stress induced mechanical failure of polymeric materials is a result of the breakage of covalent bonds. The occurrence of bond breakage and formation is only observed during reactions. This study focuses on the novel bond-breaking capabilities of the Interface Forcefield (IFF-R). Traditional IFF models simulated systems using harmonic potentials. The IFF-R model incorporates Morse potentials; thereby, eliminating the restoring force experienced by bonded atoms stretched at large distances. This enables accurate predictions of mechanical responses in a variety of periodic systems. This study shows moduli and strength predictions of a single-walled carbon nanotube, poly(acrylonitrile) crystal, cellulose β crystal, and steel FCC lattice to be comparable to experimental values. Mechanical property predictions using IFF-R models are realized magnitudes faster than reactive forcefield (ReaxFF).

Combinations of different materials into hybrid/composite structures are highly sought after as to enable multifunctionality in many of today’s demanding applications. 2-dimensional graphene can combine with porous nanostructures, such as metalorganic frameworks (MOFs), zeolites, and porous carbons, to yield hybrids with improved surface, interface, and activity characteristics. In this work, we will present examples of such material hybridizations via in-situ microstructural tuning of the interface between the involved counterparts. The resulting composites were tested for their gas separation performance, and specifically optimized to exhibit enhanced capture activity for carbon dioxide as adsorbents, as well as nanofillers in mixed matrix polymer-based membranes.

To reveal the full potential of high surface area materials, strategies to tailor their surface characteristics and tune their active sites with high spatial precision and order are needed. In this work, we employ porous materials having surface areas of several hundreds of m² per g and we apply customized chemistry based surface tailoring as to spatially create active sites and subsequently graft functional molecules on them. Such examples of surface features can have a multitude of applications, yet we focus on tuning the affinity of the materials to bound with selective species from mixtures as to enable separation. In particular, innovative CO₂-phylic amines are immobilized on the treated surfaces to yield multifunctional adsorbents for carbon dioxide capture. Examples of silica- and graphene-based materials with controlled covalent positioning of various aminosilanes on the modified surface sites will be presented. Controlled grafting ensured stability and sustainable adsorption, tuned effects of locality, spacing, orientation, and interconnectivity of the functionalities on the surface, and minimized diffusion limitations while maximizing capture and release performance for the adsorbate species.

MXenes are an emerging class of two-dimensional layered transition metal carbides, carbonitrides, and nitrides. Their intriguing properties have attracted significant attention as promising materials for next-generation coatings and devices. One of the significant challenges of this material (e.g. Ti₃C₂T_x) is the rapid oxidation into TiO₂ anatase through hydrolysis in water, though water has proved to be the most effective solvent for solubilizing exfoliated Ti₃C₂T_x. Organic solvents routes for processing of MXenes and functionalization of its surface with specific ligands provide avenues to tailor the surface properties, which could eventually influence the colloidal stability as well as oxidative stability. Herein, we discuss the influence of various organic solvents such as ethanol, isopropanol, toluene, and N, N-Dimethylformamide on Ti₃C₂T_x colloidal stability, oxidative stability at room temperature, and thermo-oxidative properties. We also demonstrate the use of silane and catechol ligands for tailoring the physical properties of Ti₃C₂T_x.

Currently Gas sensors have a great impact in direct monitoring of environmental hazardous gas as well widely used in healthcare such as exhaled breath analysis. In this work we report a ZnO/Silicon nanowires (ZnO/p-Si NWs) based p-n heterojunction array based Nitric Oxide (NO) gas sensor operating at room temperature with extremely high response at least down to 0.5 ppm with noise limited response ~ 10 ppb. Utilization of cost effective chemical technique for fabrication of sensor on silicon is compatible with wafer level processing and easily connecting with silicon IC technology. The vertically aligned Si NWs array has been made by electroless etching method and the ZnO nanostructure was made by chemical solution deposition and spin-coating. We observe that the heterostructure leads to a synergetic effect where the sensing response is more than the sum total of the individual components, namely the ZnO and the Si NWs. The response is much enhanced in the p-n junction when the n-ZnO nanostructure interfaces with p-Si NW compared to that in the n-n junction formed by ZnO on n-Si NW. Extensive cross-sectional electron microscopy and composition analysis by line EDS allowed us to make a physical model. The comparison of the simulation results with the experiment point out the device parameters that enhance the device response. The characteristics values of the parameters of ZnO/Si NWs heterojunction for the best fits obtained from the simulation and it can be seen that all the parameters undergo change in the electrical model and this leads to enhancement of current in the device on gas exposure. The top layer of ZnO takes part in electrical current conduction. The Si NWs also has an all-round layer of ZnO that also acts as chemical sensing gate to modulate the depletion layer on the surface of the NW. The main inference from the simulation is that the observed high performances of the sensor device depends on change in resistances of the constituents as well change in the reverse saturation current at the ZnO/p-Si NW p-n junction

A novel route to fabricate a hybrid ceramic matrix composite by utilizing preceramic polymers, chopped carbon nanofiber (CNF) precursors and subsequent additive manufacturing was introduced in this study. An allyl hydrido polycarbosilane (SMP-10) and 1,6-dexanediol diacrylate (HDDA) were mixed with a photo initiator to form a photo sensitive resin. The resulting resin was loaded with distinct weight percentages of polyacrylonitrile (PAN) nanofiber. These mixtures were 3D printed followed by pyrolysis. The end objective of the pyrolysis cycle is that the plycarbosilane resin is converted into a silicon carbide matrix, with the PAN converted into reinforcing carbon nanofibers. The impact of the CNF percentages on structural and mechanical properties was investigated using scanning electron microscopy, transmission electron microscopy, and nano-indentation characterization techniques, respectively. The prepared precursor resin proved to have outstanding photo-curing properties and the ability to transform to the silicon carbide phase at temperatures as low as 850 ^oC. The result of this work showed that ceramic matrix composite components can be successfully fabricated using 3D printing and a specific pyrolysis cycle. The obtained ceramic hybrid composite was fully dense with nearly linear shrinkage and a shiny, smooth surface after pyrolysis. Furthermore, around 60% retained weight after pyrolysis to 1350 ^oC was confirmed by thermogravimetric analysis. In terms of crystallography, the ceramic matrix composite appeared to have three coexisting phases including silicon carbide, silicon oxycarbide, and turbostratic carbon. The results are very promising to fabricate hybrid composites working at high temperatures with improved mechanical properties and complex geometries.

We successfully synthesized Ge_xSe_1-x (x = 0.225) glass samples and doped the samples with commercially produced (Protein Mods) carbon nanotubes (CNTs). We investigated the glass transition temperature (T_g) using Modulated Differential Scanning Calorimetry (MDSC). The glass samples without the CNTs have a a T_g of ~220°C and the T_g was found to be independent of starting materials from different suppliers as well as water-bath temperature. CNTs, being a very hygroscopic material as well as oxygen absorbing material, needed to be cleaned under vacuum with the hot water-bath. We found that the T_g decreases when 5% and 10 % CNTs by mass is added to the glass samples as compared to the base Ge_xSe_1-x glass. The decrease in T_g indicates the occurrence of an intermediate phase (reduced-stress glass phase) at lower temperature, which could be potentially useful in material science applications.

Abstract:
Glancing angle deposition (GLAD) radio frequency (RF) magnetron sputtering enables production of polycrystalline CdTe with different microstructural properties including grain size, grain orientation, and crystal phase. In GLAD sputtering, the normals extending from the centers of the sample and sputtering target are at controllable angles with respect to each other. At lower source flux angles, films grow as the expected more energetically stable cubic zinc blende CdTe phase whereas at higher source flux angles the metastable wurtzite phase forms. During the GLAD process different microstructure and crystal phases are produced in the film due to increased atomic scale self-shadowing effects at higher source flux angles which result in limited diffusion of ad-atom precursors on the substrate. This wurtzite phase CdTe produced at high source flux angles is used to tailor the interface between the n-type hexagonal wurtzite CdS window layer and p-type cubic zinc blende CdTe absorber in the standard CdS/CdTe solar cell. Application of this wurtzite phase CdTe interlayer results in better lattice matching to both hexagonal CdS and cubic CdTe leading to higher photovoltaic device performance. Using spectroscopic ellipsometry (SE) and x-ray diffraction (XRD), the optical and microstructural properties of the GLAD CdTe interlayers before and after CdCl₂ treatment are obtained. CdS/CdTe heterojunction solar cells are fabricated with GLAD interlayers prepared using source flux angles from 0⁰ (normal incidence) to 80⁰, at substrate temperatures from room temperature to 250 ⁰C, and with CdCl₂ post-deposition treatment times up to 30 minutes. Photovoltaic device performances are compared between devices with and without the introduction of these GLAD CdTe interlayers. Improvements in photovoltaic device performance parameters including open circuit voltage, short circuit current, fill factor, and power conversion efficiency are related to GLAD CdTe interlayer characteristics.

Continuing on our work in the surface modification of carbon fibres, we investigated the validity of tethering polymers to a carbon fibre surface.¹ Evaluation of both ‘Graft To’ and ‘Graft From’ approaches were undertaken and how these modifications affected performance/the-innate physical properties of the underlying material was determined.
During our ‘Graft From’ study, we found that the use of an in situ polymerisation protocol^2-3 resulted in the generation of carbon fibres with an electric blue colour. The source of colour is proposed to be thin-film interference, similar to that observed in the Morpho butterfly. Swelling the polymer with a solvent resulted in a cascade of colours through the visible spectrum during evaporation and polymer shrinkage.
In addition to this, the tensile strength of the fibre increased (13-30%), the interfacial shear strength increases in epoxy polymers (180-320%), and an unusual ability to reversibly form and reshape the fibre was also found. ⁴

(1) Randall, J. D.; Eyckens, D. J.; Servinis, L.; Stojcevski, F.; O'Dell, L. A.; Gengenbach, T. R.; Demir, B.; Walsh, T. R.; Henderson, L. C. Carbon 2019, 146, 88-96.
(2) Tessier, L.; Deniau, G.; Charleux, B.; Palacin, S. Chem. Mater. 2009, 21, 4261-4274.
(3) Deniau, G.; Azoulay, L.; Bougerolles, L.; Palacin, S. Chem. Mater. 2006, 18, 5421-5428.
(4) Eyckens, D. J.; Arnold, C. L.; Randall, J. D.; Stojceveski, F.; Hendlmeier, A.; Stanfield, M. K.; Pinson, J.; Gengenbach, T. R.; Alexander, R.; Soulsby, L. C.; Francis, P. S.; Henderson, L. C. ACS Appl. Mater. Interfaces 2019, Accepted 10-10-2019.

This study elucidates the effects of interface tailoring in carbon fiber reinforced polymer (CFRPs) composites with surface grown carbon nanotubes (CNTs) on the quasistatic, rheological, and viscoplastic performance of CFRPs. Carbon nanotubes were grown on the surface of PAN-based carbon fibers using a non-destructive technique to enhance the adhesion between the fibers and the polymeric matrices. The topology of the growth was controlled by patterning the catalyst deposition via physical vapor deposition. Quasistatic tensile tests were utilized to probe the effect of the CNTs growth topology on the strength and stiffness of the hybrid composites. The rheological behavior of the composites was elucidated using dynamic mechanical analysis (DMA). The viscoplastic behavior was investigated using creep and load relaxation tests at different thermomechanical environments. The patterned growth of the CNTs improves both the mechanical and rheological performance of the composite. This study demonstrates “interlocking” mechanism improves adhesion due to the enhanced surface area of the interface between the hybrid fibers and the epoxy matrix. Furthermore, results show that tailoring the interface with patterned grown CNTs reduces the stress relaxation and hinders the steady-state creep strain rate.

The properties of fluids, when confined at nanometer scales, differ greatly from their bulk properties and are generally dominated by interfacial effects. In this interfacial region, the nature of the wall–fluid intermolecular interactions can have a significant effect on the fluids, introducing symmetry breaking, structural frustration and confinement-induced entropy loss into nanoscale interactions under confinement. In this presentation, several examples regarding how different degrees of nanoconfiment alter the structural organization and interfacial properties of soft matter ranging from non-biological (e.g. ionic liquids and silica colloidal suspensions) to biological ones (e.g. biopolymer solutions) will be introduced. This talk will particularly focus on describing the mechanisms of how biomacromolecules such as silk fibroin (SF) proteins self-assemble into hierarchical structures at the multiple-length levels in greater details. The presentation will conclude with some perspectives on new fundamental insights for rational design of SF-based materials with desired interfacial features in use of their fabricating superior functional materials and devices.

Thermosensitive bottlebrush polymers (BBPs) are a type of graft copolymers in which thermosensitive polymer side-chains are grafted on a linear polymer backbone. These side-chains can undergo a coil-to-globule conformational change in response to a change in the surrounding temperature. This also results in a change in the overall shape of the BBPs because of which they can be used for biomedical applications. Here, we will present the results of our recent coarse-grained (CG) molecular dynamics (MD) simulations study of poly(N-isopropylacrylamide) (PNIPAM; transition temperature= 305 K) BBPs of Worm-like, Cone-like, Dumbbell-like shapes. The CG MD simulations were performed at 300 K (below transition temperature) and 320 K (above transition temperature). The analysis of simulation trajectories showed that the shape of BBPs has significant impact on the conformations of individual side-chains.

Molecular self-assembly involves the use of hydrogen bonds and other noncovalent interactions between molecules to create supramolecular structures. A goal of theory is to be able to predict and understand what structures will arise for any given set of conditions, and how the self-assembly can be directed so as to produce useful functional structures. In this talk I will describe recent studies in my group in collaboration with the Stupp and Mirkin groups at Northwestern, and with others, with the goal of making nanostructured materials with broad applications including actuation with various stimuli.

One area of interest concerns studies of peptide amphiphile self-assembly to produce cylindrical micelles and ultimately fiber materials that can be used for applications in biomedicine and for making materials that can be photoactivated for robotic functions. Here we have developed all-atom and coarse-grained models, and specialized molecular dynamics methods, that are capable of describing assembly into fibers, leading to an understanding of biological functions and photoactuation.

In a second direction we are interested in the coupling of self-assembly chemistry involving DNA attached silver and gold nanoparticles to create a new generation of bottom-up nanomaterials of interest for sensing and optical devices. Here we show both coarse-grained and all-atom approaches to the assembly, and how these can be used to understand the formation of plasmonic array structures, including recent studies aimed at understanding programmability of dynamic optical properties.

Debundling and dispersion of carbon nanotubes (CNTs) in polymer solutions play a major role in the preparation of carbon nanofibers due to early effects on interfacial ordering and mechanical properties. First, we analyzed the propensity to achieve homogeneous dispersions of CNTs in polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA) precursor solutions in the solvents DMSO, DMAc, and DMF. Molecular dynamics simulations at 25 and 75 °C with accurate interatomic potentials for graphitic materials that include virtual π electrons show tendencies for PMMA wrapping while PAN exhibits significant interactions only as the concentration is increased or solvent evaporated. Computed conformations, solubility, and temperature dependences correlate well with experimental data from spectroscopy, light scattering, and viscosity measurements.

Subsequent heating and drawing of the precursor gels results in PAN/CNT composites. MD simulations reveal the structure at the atomic scale and relationships to mechanical and thermal properties for different degrees of polymer crystallinity and CNT diameter. Glass transition temperatures correlate with the amount of CNT/polymer interfacial area per unit volume and were predictable with +/-5 K accuracy relative to experiments. Tg increases for higher CNT volume fraction and inversely with CNT diameter. An important aspect is the knowledge of CNT bundle size from experiment, which affects the effective surface area. Computed interfacial shear strengths increase when PAN has at least some degree of crystallinity (at least 50% crystalline).

Third, testing of mechanical properties up to failure has been performed for a wide range of over 500 CNT and defective CNT morphologies to explore the role of structural defects and new information from machine learning models. Hereby, we utilize a new force field, IFF-R, that enables bond breaking in high accuracy and computational speed. Suitable feature representations and processing of the data through reinforcement learning and Bayesian uncertainty quantification will be discussed, and the resulting correlations between structure, Young’s modulus, and tensile strength.

Ongoing questions include the application of supervised learning approaches to polymer composites that have more degrees of freedom and to experimental data from tomography and X-ray diffraction that will become available in large quantities. Progress and opportunities will be discussed.

Assemblies of colloidal nanocrystals and magic-sized-clusters (MSCs) are widely used in a variety of applications due to their unique electronic, optical and magnetic properties¹. However, these materials show a surprisingly limited range of elastic moduli (~0.5-5 GPa) and hardnesses (50-500 MPa),² which presents a challenge for applications in which mechanical robustness is critical. These poor mechanical properties originate from the organic ligands during nanocrystal synthesis that result in weak interparticle interactions in particle assemblies. In this work, we conduct ligand strategies to significantly improve the mechanical properties of colloidal nanocrystal solids. We also report on our ongoing efforts to extrapolate these strategies to assemblies of MSCs.

Our preliminary result shows that inorganically linked nanocrystal solids possess significantly increased elastic moduli and hardness. More specifically, in the case of 3.5 nm CdSe nanocrystal material system, we demonstrate an elastic moduli and hardness increase by a factor of 50 once the native long oleic acid ligand is substituted with a few atomic layers of SnS₂ phase. Ligand exchange is carried out through a solution phase ligand exchange³ and is confirmed through a combination of FTIR, TEM&EDS, and nanoindentation experiments. The nanoindentation experiments demonstrates that the elastic moduli/hardness of 3.5 nm CdSe nanocrystal solid increase from 0.43GPa/34MPa to 30GPa/1.6GPa once native long oleic acid ligand is replaced with a few atomic layers of SnS₂ phase.

This observed mechanical reinforcement is due to greatly enhanced mechanical coupling between the neighboring nanocrystals. First of all, the covalent Sn-S bond is much stronger than the van der waals interaction between organic molecules. Second, inorganic ligands occupy much smaller spacing as compared long oleic acid molecules, which suggests enhanced van der waals interaction and electrostatic interaction between the neighboring nanocrystals.⁴ In order to quantitatively understand the underlying mechanism, we performed molecular dynamics (MD) simulations. These simulations also allow us to predict other mechanical properties (bending moduli, fracture toughness etc) that are hard to measure directly on inorganically linked nanocrystal solids.

The similarities between colloidal nanocrystals and MSCs suggest it may be possible to extrapolate these ligand exchange strategies to MSCs. MSCs have the advantage of being perfectly monodisperse as well as small enough for full-fidelity density functional theory simulations. This opens possibilities for an in-depth understanding of the mechanical properties enhancements afforded by ligand exchange strategies.

Reference:
1. J.-H. Choi, H. Wang, S. J. Oh, T. Paik, P. Sung, J. Sung, X. Ye, T. Zhao, B. T. Diroll and C. B. Murray, Science, 2016, 352, 205-208.
2. M. R. Begley, D. S. Gianola and T. R. Ray, Science, 2019, 364, 1250-+.
3. M. V. Kovalenko, M. Scheele and D. V. Talapin, Science, 2009, 324, 1417-1420.
4. M. B. Zanjani and J. R. Lukes, J Chem Phys, 2013, 139.
5. T. T. Zhu, B. W. Zhang, J. Zhang, J. Lu, H. S. Fan, N. Rowell, J. A. Ripmeester, S. Han and K. Yu, Chem Mater, 2017, 29, 5727-5735.

Composite materials are increasingly replacing metals as the top choice for airframe structures because the advantages they offer, including light weight, high strength, high fatigue resistance, flexibility in manufacturing, and fuel efficiency. For Air Force platforms, composites offer the opportunities in enhanced thermal management, electromagnetic absorption, integrated sensing, and other capabilities not achievable in conventional monolithic components. Among the routes to achieving game-changing improvements in composite materials, the Low Density Materials program currently emphasizes: 1) materials discovery to increase the temperature capabilities of matrix resin, fiber sizing, reinforcing fibers; 2) mechanisms and thermodynamics of polymer-to-ceramic conversion for tailored nanoscale heterostructures; 3) understanding of interfacial phenomenon at the molecular level in heterogeneous systems; 4) concepts and processing methods for integrated multifunctionality; and 5) multiscale modeling and experimental validation of composite behavior. The details will be reviewed at the panel discussion.

The deployment of Magnesium (Mg) alloys have increased significantly in the automotive, aerospace and defense industries due to their lightweight structural integrity. In structural applications, these materials components are exposed to a wide range of strain rates such as explosive forming, high speed impact and shock loadings. These alloy components are placed in extreme loading environments where high strain rates are commonly achieved. Therefore, the evolution of failure mechanisms from strain localization and formation of adiabatic Shear Bands (ASBs) and the mechanical behavior at high strain rates is of desirable interest by the aerospace and automotive industries. However, studies on how failure emanate from ASBs formation to crack initiation and propagation in these materials with regard to their mechanical responses under high strain rates such as shock loadings and impact are limited. Deformation and damage accumulation in Mg alloys at high strain rate loading conditions such as impact are not fully understood. Thus, the correlation between the accuracy of the reported mechanical data and microstructural features that leads to the credence of failure mechanisms within Mg is not well understood. It is therefore critical to extend the understanding of the mechanism of failure of Mg at high strain rates of loadings by exploiting other techniques. In this work, the dynamic mechanical impact response and ASBs nucleation of heat treated AZ31B Mg alloys at different strain rates and impact momentums are characterized in-situ during impact at room temperature using the Direct Impact Hopkinson Pressure Bar (DIHPB) coupled with an in-situ 3D Digital Image correlation (DIC) and high speed and high resolution thermal imaging techniques. DIC provides an independent measurement of surface strains directly on test specimen during deformation to monitor stain evolution and strain localizations. Both the microstructure of the initial un-deformed and post deformed specimen were characterized. It is found that the flow stress increased gradually to (~50 %) with increasing strain rates and impact momentums and the AZ31B Mg exhibits strain rate sensitivity at dynamic loadings. This is due to an increasing selective and intensive grain refinement evolving at grain boundaries (GBs) which increases strain hardening under dynamic compression. Strain to failure decreases at higher impact momentum. Evaluation of DIC results indicated that strain occurrence is linear at the onset of deformation but becomes heterogenous at later stage of deformation with multiple nucleation sites of strain localization leading to ASB at ~80% of the specimen length after ~340 us of deformation. Region of peak nonlinearlity of maximum strain concentration leads to ASB formation with characteristic fine grains, voids and crack initiation at grain boundaries observed at the extreme end of specimen using DIC with ~22 °C temperature rise observed with insitu high speed and high resolution thermal imaging camera. The temperature rise is however, lower with respect to the homologous temperature of Mg and hence higher degree of strain hardening behaviour is induced after deformation. Sub grains ~0.3um sizes is found with disintegrated precipitate in voids at ASBs regions and mostly at GBs. In addition, deformation twinning frequency increases(~30 %) with broader thickness at higher impact momentum with disintegrated precipitates on micro-twins boundaries and GBs during plastic deformation at ASB surroundings. Meanwhile at lower impact momentum, freckled pattern selective grain refinement is observed. These results suggest that ASB forms in AZ31B Mg alloy despite its brittle nature that leads to crack initiation and ultimate failure under high strain rates and large strains of deformation and can be used to extend the understanding of the mechanism of failure in Mg alloys.

In mussels, the adhesive proteins that are instrumental for attachment to wet surfaces are known to contain high levels of 3,4-dihydroxy-L-alanine (DOPA), often located adjacent to amino residues such as lysine (Lys). The special synergistic relationship between catechols and amines is a subject of high interest, not only for understanding native proteins but also for informing the design of bioinspired polymer systems. Other research groups have shown that catechol and amine functional groups act synergistically to enhance adhesion at wet surfaces, however reports of catechol-amine interfacial phenomena on a single molecule level have been limited. In this talk we will describe single molecule force spectroscopy (SMFS) measurements that are providing new insights into interactions between Lys-DOPA peptides and various surfaces. Molecular mechanics investigations of catechol and catecholamine polymers are providing new insights into design of novel polymer adhesives, for example pressure sensitive adhesives and high-strength thermoset adhesives.

Elucidating the role of organic ligands that are attached to or incorporated within inorganic nanostructures, for example, at the interface of colloidal nanocrystals or as included in the inorganic framework of two-dimensional hybrid organic-inorganic perovskites, is essential in realizing desired functional properties for applications. However, although general guidelines are provided by experimental explorations, an understanding of the underlying mechanisms for efficient materials development is often lacking. Herein, by employing first principles calculations for the design of materials that exhibit optical tunability characteristics, we first discuss atomically precise quantum dots. We highlight challenges in modeling the structure of semiconductor nanoclusters with organic ligands. In this context, we report on the derivation of a potential energy surface using machine learning that overcomes, in part, some of these limitations, and on the importance of the level of theory applied for accurate prediction of the optical response. In addition, we discuss effects of incorporating large chromophores as the ammonium cation in 2D hybrid organic-inorganic perovskites, which have proven promising for optoelectronic applications. Dependent on the organic moiety type, band alignments indicate variability in quantum-well types for a range of materials we examined, as well as structural distortion. Absorption spectra demonstrate tunability in the optical properties, including enhanced absorption and red-shifts, providing recommendation for the choice of the organic cation and motivation for further synthesis and experimental characterization.

In this talk I will present our recent work studying polymer nanocomposites (PNCs)using molecular dynamics (MD) simulations and Polymer Reference Interaction Site Model (PRISM) theory calculations with coarse-grained (CG) models. Specifically, we are interested in linking molecular design of PNCs comprised of grafted nanoparticles in a polymer matrix to their morphology. In this talk I will focus our work on the impact of strength of graft-matrix attraction on interpenetration of matrix and graft chains (known as grafted layer wetting) and dispersion/aggregation of grafted particles in matrix. Previously for PNCs with attractive graft-matrix interactions, we had shown that wetting/dewetting and dispersion/aggregation are two distinct phase transitions, the former being a continuous transition and the latter being a first-order transition as a function of graft-matrix interaction or temperature. Recently, we found that as the strength of graft-matrix attraction increases, we can also tune the effective size and the hardness of the polymer grafted particle in the PNC. As the attraction between the graft and matrix chains increases, the graft polymer chains extend to make favorable graft-matrix contacts and increase the grafted layer wetting by matrix chains. This leads to larger and ‘harder’ grafted particles compared to analogous fillers with purely entropic (athermal) graft-matrix interactions. Due to the increasing size and hardness of grafted particles with increasing graft-matrix attraction, the PNC structure changes from an aggregated/dispersed morphology governed by entropic driving forces at athermal graft-matrix interaction to a dispersed morphology due to favorable weak graft-matrix attraction, and ultimately, to a correlated fluid of hard grafted particles at stronger graft-matrix attraction. We see these trends in PNCs with high (densely grafted) and low grafting density, and with equal graft and matrix chain lengths as well as cases where the matrix chain lengths are greater than the graft chain lengths.

Plasticity in most metallic materials depends essentially on dislocation glide. Metals, which display a wealth of gliding systems, are those having a face centered cubic lattice. On the other hand, metals with hexagonal close packed (hcp) structure, like Mg, or Ti, have a complex deformation behavior [1-2]. In these materials, both dislocation glide and twinning are fundamental mechanisms of plastic deformation. Another hcp metal, cobalt, with distinctly lower stacking fault energy, has been so far studied much less.
The straining mechanisms of cobalt, as shown by acoustic emission experiments and electron microscopy observations, is characterized by two principal stages: the first stage of work hardening is driven by planar glide of dislocations, while twinning is the most active deformation mechanism in the second stage [3].
Important insight into twinning has been recently provided by advanced electron microscopy techniques, such as scanning electron microscopy coupled with the electron backscatter diffraction technique, which is a powerful tool to capture the twinning process appearing in different stages of plastic straining [4].
In the present work, polycrystalline cobalt was monotonously strained in tensile test up to a plastic strain corresponding to the second stage of work hardening. The electron backscattered diffraction technique was used to acquire a large quantity of data, which allows for statistical analyses. The relationships between grain orientation, grain size, twin volume fraction, twinning modes, and twin variants have been established.
The results show that the principal twin mode is the {10-12} tension type and that multiple twin variants of this mode can be present inside the same grain. The occurrence of a specific twin variant in a grain is related to the local strain accommodation within this grain; consequently, the higher frequency of the occurrence corresponds to the variants with larger misorientations between them. The statistical analysis shows that the grain orientation has a more pronounced impact on twinning characteristics than the grain size. The Schmid Factor analysis adapted to twinning is used to give a general view of the cobalt behavior, taking into account the significance of grains oriented favorably for twin formation. This approach may also explain cases, in which grains are not favorably oriented, twinning is not expected, but it occurs in reality. The results will be compared with the behavior of other hcp metals.

[1] A. Ghaderi and M. R. Barnett, Sensitivity of deformation twinning to grain size in titanium and magnesium, Acta. Mater. 59 (2011) 7824–7839.
[2] N. Dixit, K. Y. Xie, K. J. Hemker and K.T. Ramesh, Microstructural evolution of pure magnesium under high strain rate loading, Acta. Mater. 87 (2015) 56–67.
[3] M. Martinez, G. Fleurier, F. Chmelík, M. Knapek, B. Viguier, E. Hug, TEM analysis of the deformation microstructure of polycrystalline cobalt plastically strained in tension, Mater. Charact. 134 (2017) 76–83.
[4] L. Capolungo, P.E. Marshall, R.J. McCabe, I.J. Beyerlein and C.N. Tomé, Nucleation and growth of twins in Zr: A statistical study, Acta. Mater. 57 (2009) 6047–6056.

Tensile tests on Cu/Zr-based metallic-glass (MG) micro-wires show observable first yield point, followed by shear fracture on further straining. Microscopy examination reveals discrete shear bands decorating the free surfaces of yielded and fractured MG micro-wires. Both the first-yield and the fracture stress scatter statistically as expected, but surprisingly, they do not exhibit any significant dependence on the wire length.
Fundamentally, while it has been widely accepted that glass plasticity takes place via shear transformation zones (STZs), the knowledge gap between such atomic-sized STZs and the above-mentioned micro/macroscopic plasticity phenomena remains huge. In this work, molecular dynamics (MD) simulations were carried out to delineate the detailed process by which shear bands form from discrete STZs. The results show that the STZs have an increasing tendency to emerge and operate close to one another in a correlated manner along the strain path. This process leads to shear localization in the form of shear bands.
An analytical model is then proposed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, as prior shear events triggers the emergence of new STZs, successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization.
Finally, the MD simulations also indicate that the first observable yield point cannot be due to the emergence and operation of one single STZ. Instead, yield or fracture is controlled by the average or extreme behavior of many STZs or shear bands operated in different locations in the wire, which explains length independence of the MG wire strength – a fact also observed in other glass wires.

Materials with difficult-to-attain combination of multiple properties - mechanical, electrical, chemical, optical, thermal, and transport, – represent the quintessential bottleneck for nearly all modern technologies. Nanocomposites with molecular, nano-, meso-, and microscale levels of structural engineering can provide such property combinations, while intrinsic ability of nanoscale components to self-assemble make them suitable for simplicity of synthesis.
Biomimetic nanocomposites exemplified by a variety of nanostructured nacre-like materials provide a generalized approach to engineer materials with multiple difficult-to-attain properties. As the continuation of this research, we learn that the unique mechanics of tooth enamel can be replicated combining out-of-plane nanoscale columns with molecular precision of layer-by-layer (LBL) assembly between them. As a result of that, these composites reveal remarkably high vibrational damping unusual for stiff materials that imparts them resilience to aging.
The novel type of biomimetic nanocomposites are those based on aramid nanofibers (ANFs). They spontaneously assemble into three-dimensional percolating networks reminiscent of cartilage. The nanoscale structure of ANF composites reveal nanoscale porosity that can be controlled by nanofiber branching. The latest results from multiple groups demonstrate that ANF composites resolve some of the essential property bottlenecks for ion-selective membranes, dendrite-resistant electrolytes, and structural batteries.
One of the emerging fields for biomimetic nanocomposites are optical devices. The high strain and strong polarization rotation make possible metaoptical devices with wide-angle diffraction gratings for LIDARs and highly efficient quarter wave plates for THz scanners. Both devices can be used for machine vision and biomaterials imaging.

Composites offer a wide variety of uses in aerospace, automotive industry, additive manufacturing, and energy storage applications. Discovered in 2011, MXenes have emerged as a new class of two dimensional (2D) materials consisting of transition metal carbides and nitrides. Synthesized MXenes have rich chemistry and 2D morphology offering a combination of high metallic conductivity and hydrophilicity for easy solution processing. Additionally, MXenes have been shown to have excellent mechanical properties with Young’s modulus of 330 GPa, surpassing both GO and rGO making it one of the strongest solution-processable 2D materials. Thus, they can improve mechanical and thermal stability as well as electrical conductivity of polymers. They can also reinforce ceramics and metals.

The most commonly studied MXene, titanium carbide (Ti3C2), has been combined with various polymer systems producing new multifunctional nanocomposites. When introduced into thermoplastics like PVA, improved mechanical properties were observed at 10 wt% polymer loading with a 34% increase in tensile strength compared to pure Ti3C2 films. Additional studies have shown electrical properties can be successfully transferred to the polymer composite extending its use to electrodes in energy storage systems as well as electromagnetic interference shielding for future electronics. In addition to simple direct mixing with polymers, Ti₃C₂ MXene has also been incorporated into more complex polymer fibers for integration into textile applications. A recent study on electrospinning of MXene-PAN nanofibers showed MXene content could be increased up to 35 wt% and retain high areal capacitance three times that of pure PAN nanofibers. This presentation will provide an overview of the current MXene-reinforced composites field, including ceramic- and metal-matrix, and the various applications.

Surface Initiated Atom Transfer Radical Polymerization (SI- ATRP) in its various modifications has emerged as a versatile toolbox to control and tailor the properties and interactions of interfaces and to enable the synthesis of hybrid materials with unprecedented property combinations. The resulting materials have attracted interest not only because the high-level structural control of the architecture of polymer-tethered surfaces enables tailoring of the interactions, microstructure and properties of particulate-based materials but also because the confinement of chains on surfaces alters the mechanism of termination reactions that limit conventional polymerization processes.
This presentation will review recent examples of the application of SI-ATRP for the synthesis of functional polymer materials. Examples to be covered include the synthesis of ultra-high molecular weight polymers, materials for optical applications as well as thermal interface materials. Recent examples of the interplay between phononic and photonic properties in brush particle-based materials will be shown to illustrate how the subtle control of polymer chains at interfaces can instigate novel physical properties in hybrid materials that cannot be realized in ‘classical’ composite materials that are fabricated by mixing of particle and polymer constituents. This will provide the basis for a discussion of ‘guidelines’ for the fabrication of novel functional materials that harness ‘chemical confinement’ of polymer chains.

Nanocomposite property enhancements originate substantially from interfacial effects. We focus on two examples of nanocomposite interfacial effects. In Example 1, modification of filler surfaces can improve dispersion and compatibilize the polymer-filler interface, leading to property enhancements relative to nanocomposites containing unmodified filler prepared by conventional melt mixing or solvent blending. For the first time, we isolate the effect of interactions between matrix polymer and polymer-grafted nanoparticles from dispersion quality, revealing how the interactions affect the properties of polypropylene/halloysite nanotube (PP-HNT) hybrids. PP-HNT nanocomposites with pristine filler or PP-grafted filler were prepared by solid-state shear pulverization, leading to excellent dispersion in both cases. Optimal interactions with PP-grafted nanofiller leads to faster PP crystallization and higher yield strength; in contrast, Young’s modulus is determined by the filler dispersion and not filler modification. In Example 2, for the first time, we have decoupled chain dynamics in nanocomposite interfacial layers from chain dynamics of the matrix in poly(2-vinylprydine) (P2VP)-silica nanocomposites. The interfacial T_g (glass transition temperature) and the matrix T_g (both associated with α-relaxation dynamics) were determined by fluorescence. The interfacial layer T_g is higher than the matrix T_g due to H-bonding between P2VP and hydroxyl groups on the silica surface. At the same filler dispersion, 2.1 kg/mol P2VP matrix polymer leads to much greater T_g enhancements in both the interfacial regions and the matrix relative to the use of 110 kg/mol P2VP. We hypothesize that low molecular weight P2VP preferentially aligns at the filler interface, leading to higher H-bonding density and increased T_g.

The functionalization of electrospun polymer fibers has created potentially radical materials for sensing, tissue scaffolds, flexible electronics and filtration. Fundamentally, the composite material design is enabled by electroless deposition, wherein conformal coatings (either as complete films or individual nanoparticles of copper) can be achieved on the default random configurations of electrospun fibers through solution-based precipitation reactions. Advantageously, fiber alignment can also be achieved through design modifications in the electrospinning process
We have electrolessly deposited copper on random and aligned PAN fibers utilizing silver nanoparticles as catalytic seeds. Fiber sizes range from diameters of hundreds of nanometers to a few microns. Prior work has established that coating conformity is strongly modulated by density of catalytic seeds: metallic films are obtained with a high density of silver seeds (48 particles/μm²), and discrete particles are observed for low density seeding (15 particles/μm²). X-ray crystallography has been used for phase identification and crystallite size measurements of both the seeds and subsequent copper coatings.
We investigate the changes in the chemistry of the fibers due to the exposure to the electroless plating solutions using Raman spectroscopy, focusing on effects of fiber dimensions on surface chemistry and amenability to nucleation events. In addition, tensile tests are carried out on the metallized aligned fibers at distinct strain levels to investigate possible delamination events. Previous strain-to-failure tests on conformally coated, randomly aligned fibers mats showed good adhesion of the copper particles on the fibers. In parallel, we examine the mechanical behavior of the metallized fibers under an equi-biaxial stress state utilizing a novel “leaky” bulge testing system. We demonstrate the links between the mechanics of coated fiber mats in aligned and random configurations to identify the impact of plating on the stiffness of the resulting structure, a critical parameter for use in filtration applications where fluid pressure can alter the achievable minimum pore size.

Plant-based polysaccharides such as cellulose and its derivatives are receiving increasing interest in a large variety of applications because they represent an environmentally friendly alternative to plastic. Many of them are commonly used in diverse industrial applications, such as food additives and for biomedical devices due to their non-toxic and water-soluble nature. Moreover, the self-assembly nature and responsiveness of cellulosic bio-polymers makes them also extremely attractive for smart photonic applications including sensing. Among various types of cellulose and its derivatives, hydroxypropyl cellulose (HPC) encompasses all these desirable properties.
Hydroxypropyl cellulose is a liquid crystal polymer, which can form a cholesteric liquid-crystalline phase allowing Bragg-like reflection. The reflected colours can be simply controlled by changing the nature of the solvent, concentration, temperature. Recently, we demonstrated for the first time that a simple aqueous solution of HPC can be used as a photonic strain sensor that displays the applied strain by shifting its colour.
In this seminar, I will introduce how such properties can be enhanced to create a solid-state film with desired optical appearance.

MXenes represent an emerging class of two-dimensional layered transition metal carbides and nitrides with intriguing properties, which have attracted significant attention as a promising material for next-generation functional composites. Control of size (lateral and thickness), layered spacing, and surface chemistry enables tuning of optoelectronic properties and improvement of processibility. To this end, we present fundamental studies aimed at understanding the role of surface terminations and functional groups on the electronic, chemical, and structural properties of MXene flakes. Ultra-thin nanosheets of MXenes are surface modified with catechol [dopamine, pyrocatechol, and tetrachlorocatechol (TCC)] chemistries. UV-Vis spectroscopy shows oxidation of catechol to quinone, confirming the formation of a charge transfer complex with the MXene layer. The reaction kinetics and subsequent colloidal stability are different each catechols, reflecting the different terminal groups (NH₂ vs. Cl). FT-IR, XPS, Raman, and AFM-IR support surface binding; XRD reveals changes in layer spacing, further confirming surface modification. These experimental studies are corroborated by theoretical calculations of the electronic properties of both the native and functionalized MXene surfaces. Finally, these functionalized thin flakes exhibit significantly improved thermo-oxidative stability and dispersibility in non-aqueous solvents. Overall, such approaches to surface termination opens up new avenues for synthesis of hybrid functional architectures based on MXenes.

Light-weight one-dimensional nanomaterials such as carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) possess impressive physical, chemical and mechanical properties and are promising reinforcement agents for the development of enhanced multifunctional composite materials. Although BNNTs and CNTs share similar mechanical properties and thermal conductivity they can impart a different set of functionalities in composite material. CNTs are electrically conductive and are usually employed to improve the electrical conductivity of a matrix. On the other hand, BNNTs show transparency in the visible region and are electrically insulating. Thus, BNNTs are very promising nano-fillers for insulating polymer composites. Additionally, BNNTs have higher resistance to oxidation and the ability to shield neutron radiation.
High nanotube (NT) contents are often necessary in order to achieve thermal or electrical conductivities requirements for a particular application. However, the incorporation of well dispersed nanotubes into polymer composites can be challenging, especially at high loadings due to re-agglomeration of the nano-filler and a significant increase in viscosity. Here we show the fabrication of CNT-polyurethane (TPU) and BNNT-TPU composite sheets of variable composition using a scalable one-step filtration process. Using this fabrication method the composition of the nanocomposites, which defines the characteristics and properties of the material, can be precisely controlled. Consequently, materials with tailorable properties (e.g. porosity, stiffness, strength and toughness), shapes and functionalities can be fabricated. The trend in mechanical and electrical properties was understood in terms of the NT-TPU interfacial interactions and morphological changes occurring in the nanocomposite sheets as a function of increasing the TPU content. At specific NT-TPU weight ratios (e.g. 35:65 weight ratio for CNT-TPU composites) a better NT exfoliation and optimal NT surface coverage was achieved, which in turn led to an optimal packing of NT−TPU fibers. This optimal packing produced the highest improvement in tensile properties and electrical conductivity in CNT-TPU sheets. Due to their straightforward production and handling, high nanotube content, and superior properties in comparison to dispersed nanotube composites, the resulting conductive, lightweight sheets are of interest for various applications including improved damage tolerance, fire retardancy and electrical properties of laminate composites, and electromagnetic shielding. On the other hand, thermally conductive (4 W/mK), electrically insulating sheets were produced with BNNT materials of different quality/purity. The quality of the BNNT materials was evaluated using several characterization techniques and a recently reported methodology for assessment of relative quality by absorption spectroscopy of regiorandom poly(3-hexyl-thiophene) aggregated on BNNTs. Nano-microscale roughness, which is essential for superhydrophobicity, can also be integrated on the NT-TPU fabrics surface by a novel approach that incorporates a nano-micron size array of functionalized nanotubes on the sheet surface producing highly non-wetting surfaces. Sheets can also be applied as coatings on other structures (e.g. glass fiber reinforced plastics) to create a multifunctional self-cleaning surfaces.

Gas separations via selective transport in polymer membranes are dominating the membrane technologies due to their ease of processing and inexpensive production. However, these polymer membranes fall short of selectivity and permeability. Due to their exceptional structural features, metal organic frameworks (MOFs) have emerged as promising fillers to enhance the performance of a polymer membrane. In these MOF-polymer mixed matrix membranes (MMMs), the polymer-MOF interface is known to play an important role in determining the gas separation mechanism. However, understanding the structure of this MOF-polymer interface and the factors that govern their compatibility using existing experimental characterization methods is very challenging. In this research, we have performed all-atom molecular dynamics (MD) simulation of a model polymer-MOF MMMs with IRMOF-1 and 5 different polymers. Simulation trajectories were analyzed to understand the effect of different functional groups and rigidity of polymers on the structure of the MOF-polymer interface. Results of this study can be useful for creating MMMs with IRMOF for separation applications.

In order to develop shear-based solid phase processing methods for achieving bulk nanostructured metallic alloys, we aim to better understand the fundamental atomic scale mechanisms of local and global deformation mechanisms and microstructural evolution in polycrystalline materials under shear deformation. To achieve this aim, we employed synchrotron-based in situ and ex situ high-energy x-ray diffraction capabilities under high pressure and shear deformation, using a newly designed high-speed rotational diamond anvil cell. The obtained synchrotron-based XRD results were also correlated with detailed microstructural characterization before and after shear deformation using transmission electron microscopy and atom probe tomography. Atomic scale computational modeling using density functional theory and molecular dynamics was additionally complemented with experimental results to obtain mechanistic insights. Our results on shear induced structural and chemical modifications of several model metallic alloys such as Al-Si, Cu-Nb and Cu-Ni provide new insights on the unique role of shear deformation in formation of metastable states, as well as in modifying the phase transformation pathways of these alloy systems.

The interface and interphase between a matrix and the fillers will influence the stress transfer, energy transport, stabilization of dispersant, degree of confinement or bonding, and, other new property generations. Both physical (e.g., molecular wrapping on filler surfaces or self-assembly) and chemical (e.g., surface functionalization) can modify the matrix-filler interfacial interactions. This talk will introduce two kinds of interface/interphase manipulations for precisely controlling nanoparticle morphologies. Our unique manufacturing and the resulted advanced composites have potential applications in wearable, robotics, biomedical, and other areas.

The first example was the use of polymer-graphene interphase design to achieve two dimensional (2D) nanoparticle orientation management. As well known that atomically thin 2D materials are challenging to be aligned as compared with their allotropes, such as carbon nanotubes. Free-standing graphene in ambient conditions will wrinkle, crumple or fold due to their instability of thermodynamic states. Our unique design of the different phases consisted of macromolecules and 2D graphene allowed the material system to take advantage of the interphase structural evolutions for confining, exfoliating and aligning the nanoparticles. Different polymers such as semicrystalline polyvinyl alcohol (PVA) and thermoplastic polyurethane were used to study the interphase engineering and their influence on graphene or similar nanocarbons for mechanical enhancement or functionality incorporations. The unique material system of composite fibers were used as piezo- and chemi-resistive sensors.

The second demonstration utilized layer-by-layer-based deposition techniques. Both additive manufacturing (e.g., 3D printing) and dip-coating methods were used on the same processing platform. One dimensional carbon nanofibers (CNF) were used as an example to be selectively deposited on polymer surfaces with pre-printed patterns. The control of the surface patterns and the nanoparticle assembly conditions (e.g., thermodynamic parameters, nanoparticle interactions, solid-liquid-air contact lines, etc.) led to selective deposition and preferential alignment of CNF. As a result, the conductive paths on the substrate were developed to be anisotropic; following this characterization, the multifunctional sensitivity to strain, temperature, chemical liquids and volatile organic compounds (VOCs) were also displayed.


Jambhulkar, S., Xu, W., Ravichandran, & Song, K. (2019). Selective Deposition and Preferential Alignment of Nanoparticles on Surface Patterns. Under review.
Ravichandran, D., Xu, W., Franklin, R., Kanth, N., Jambhulkar, S., Shukla, S., & Song, K. (2019). Fabricating Fibers of a Porous-Polystyrene Shell and Particle-Loaded Core. Molecules, 24(22), 4142.
Xu, W., Jambhulkar, S., Verma, R., Franklin, R., Ravichandran, D., & Song, K. (2019). In situ alignment of graphene nanoplatelets in poly (vinyl alcohol) nanocomposite fibers with controlled stepwise interfacial exfoliation. Nanoscale Advances, 1, 2510-2517.

Polyolefin (polyethylene, and ethylene-propylene-diene (EPDM) elastomers) composites were studied in order to elucidate and quantify the effects of the interfaces with ceramic and carbon nanofillers, and particularly of the interphase contributions on the dielectric properties of the composites. For ceramic nanoparticles, it was shown that the interphase contributions dominate the composites’ dielectric properties, often being antagonistic to the contributions of the ceramic fillers: Ceramic fillers with systematically varied dielectric nature –spanning orders of magnitude in dielectric constants– were studied, and the interphasial contributions effectively overwhelmed the ceramic filler permittivities and dominated the dielectric performance of the composites. For composites containing both ceramic and carbon-black nanofillers, proper design of the interphases is crucial, so as to capitalize on the high effective permittivity provided by the carbon while, at the same time, preventing mobile charges to undertake large lengthscale transport (conductivity) or high dissipation of energy in localized vibrations (dielectric loss); in these systems, a third nanofiller is introduced as a means to control conductivity and minimize losses and E-field breakdown.

Hetero- or homo-interfaces formed by carbon nanostructures of different C hybridizations are ubiquitous in electronic, structural, and energy-functional materials. Understanding how weak interactions and/or chemical bonding at the interface lead to a meso- and macroscopic behavior warrants a multiscale modeling framework. This will be illustrated with the example of carbon nanotube (CNT) fibers. Starting from the energetics and kinetics of various crosslinks (~nm, ~fs) accessible from accurate DFT calculations, we derive scaling relations of their tensile mechanical response from a μm-long coarse-grained model of a CNT bundle. These inter-tube crosslinks can be expected to behave as “hook-and-loop” fasteners, operational in Velcro, which can have the ability to break and reform and thus resist relative motion in a continuous manner. Such a model can incorporate the effect of interface friction due to inter-tube crosslinks or tube-polymer matrix connects, quantitatively evaluate toughness and fatigue processes and look for ways of performance improvement. Same coarse-grained model allows us to also explore the fundamental problem of basic packing defects in the aligned CNT bundles: the ‘twists’ and ‘inclusions’. We show how a finite characteristic size/length L of such defects emerge as a result of interplay between their intrinsic bending stiffness and the elastic response of the surrounding CNT matrix-crystal, and quantify their energetics scaling. In conjunction with synthesis efforts and novel experimentation, such models are crucial in developing novel carbon nanomaterials.

Thermoset vitrimer polymers have recently shown tremendous promise for structural applications such as reshaping and remolding. The trans-esterification reaction plays a vital role in the vitrimer mechanism, in which the efficiency of the reaction is controlled by organic or organometallic catalysts. Understanding the mechanistic insights of the trans-esterification reaction in the bulk phase is difficult due to the highly cross linked complex structure of thermosetting vitrimers. In this work, we employ density functional theory (DFT) to investigate catalytic efficiency and the mechanism of the trans-esterification reaction using transition state theory with model systems that include alkoxy and carboxylic groups as reacting sites for mimicking the trans-esterification reaction. The catalytic efficiency of a number of catalysts, including triazabicyclodecene (TBD), zinc acetate Zn(OAc)₂ Dibutyl tin oxide (DBTO) and Methylimidazole (1-MI) is explored. The mechanism of trans-esterification reaction is explored based on results from fukui indices (i.e measure of electrophilicity and nucleophilicity of atomic sites), partial charges, and transition state theory.

Hierarchical assembly of plasmonic nanoparticles such as gold nanoparticles (AuNPs) into two-dimensional materials (graphene) is of significant interest for nearfield imaging, sensing, and inducing a photo-thermal response in a material. However, uniform decoration of AuNPs onto graphene nano-platelets (GNP) is a significant challenge. This is because GNPs tend to aggregate easily via van der Waals force to form graphite, which perturbs the reaction at the interface. Here we demonstrate a simple solution-phase mixing technique to prepare AuNPs decorated GNPs hybrid materials (AuNPs-GNPs). By carefully optimizing the reaction condition, we were successful in maintaining the structural integrity of GNPs and simultaneously decorating it uniformly with AuNPs. High-resolution transmission electron microscopy (TEM) and high-angle annular dark-field imaging (HAADF-STEM) showed few-layered GNPs (< 10 nm thickness, ~ 200 nm lateral dimension) decorated with AuNPs. Here the diameter of AuNP ranges from 10 to 20 nm, which is controlled by the stoichiometry ratio of gold and carbon. Similarly, the average spacing between AuNP can be controlled by the density of defects on the graphene surface and the reaction conditions. Raman spectroscopy confirms the strain sensitivity of G and 2D modes as a result of the covalent interaction at the interface. Finally, these hybrid structures are embedded in a polymer matrix for studying thermal conductivity and plasmon-phonon interactions. Overall, this kind of hierarchical structure opens up new avenues for the next generation of smart coatings and composites.

We review the state-of-the-art in the molecular design and processing of low density organic-inorganic nanocomposite hybrids at the extreme limits of molecular-scale confinement with tailored internal interface chemistry. We probe the mechanical and thermal properties of nanocomposite hybrids where a stiff inorganic matrix phase confines polymer chains to dimensions far smaller than their bulk radius of gyration. We describe a synthesis strategy which involves tailoring the internal interface chemistry between the matrix and the polymer phase. Tuning the interface allows access to extreme levers of molecular “hyper-confinement” where the confined polymer dynamics and resulting thermal behavior are markedly altered. We demonstrate the strategy with polystyrene and polyimide phases. The infiltration of individual polyimide precursors into a nanoscale porous network where imidization reactions under such confinement increase the molecular backbone stiffness. We find that polyimide oligomers can then undergo crosslinking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that a confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (> 350 °C).

Current state-of-the-art carbon fiber reinforced composite materials have become a standard structural material used in the aircraft industry. Their relatively high strength-to-density and stiffness-to-density ratios make them useful for reducing vehicle mass and thus improving fuel efficiency. However, for manned space exploration beyond the moon (deep space), further decreases in mass are needed for fuel efficiency. Thus, increases in the strength-to-density and stiffness-to-density ratios are necessary.

Carbon nanotube (CNT)-reinforced composites can potentially provide the needed reductions in composite laminate mass. Relative to carbon fiber, CNTs can have higher strength and modulus, and can potentially provide a larger surface area for polymer interaction and load transfer. Further, flattened CNTs (flCNTs) can form self-assembled arrays with a higher packing than round CNTs [1]. Although countless studies have examined CNT/polymer interaction on the molecular level [2], little effort has been devoted to understanding flCNT/polymer interaction.

The objective of this research is to use molecular dynamics (MD) simulation to explore the interaction and load transfer characteristics for a range of different polymer resins with flCNTs. Specifically, the interfacial interaction energy, pull-out frictional force, pull-apart force, and resin wetting contact angles have been predicted. These predictions have been performed for two polyimide systems, two cyanate ester systems, polyurea, and PEEK. The results of these studies will be presented, and the best polymer candidates for these flCNT/polymer composites will be discussed.

References

1. Jolowsky, C., R. Sweat, J.G. Park, A. Hao, and R. Liang, Microstructure evolution and self-assembling of CNT networks during mechanical stretching and mechanical properties of highly aligned CNT composites. Composites Science and Technology, 2018. 166: p. 125-130.

2. Radue, M.S. and G.M. Odegard, Multiscale modeling of carbon fiber/carbon nanotube/epoxy hybrid composites: Comparison of epoxy matrices. Composites Science and Technology, 2018. 166: p. 20-26.

Due to superior strength and stiffness properties, the use of textile composites for aerospace, automotive and marine applications has increased dramatically. The next generation of reinforcements, namely non-crimp fabric (NCF) is being explored for various structural applications. NCF can exploit low angle plies and can also be stacked as non-symmetric plies about the mid-plane. NCF provides excellent laminate strength, and the cost of fabrication is usually substantially lower than traditional composite manufacturing. Composite laminates are bonded together by a thin layer of resin between them, and the interface layer transfers the displacement and force from one layer to another layer. When these layers damage or weaken, adjacent layers separate, which forms the crack between adjacent plies. This also reduces the strength and stiffness of the laminate and can have a significant impact on the useful life of composites. Eventually, this separation of layers causes stress concentration in the plies which leads to the growth of delamination and results in failure of the laminate. Carbon nanofillers, such as nanoparticles, CNTs or CNFs, within the matrix material of composites has offered new avenues for improving the multifunctional properties of polymer matrix composites due to their high aspect ratio. In the present work, the laminates were manufactured using non-crimp carbon fabric in conjunction with and without nanoengineered enhanced epoxy resin. Mode I fracture toughness was evaluated for the composite panels with and without nanoengineered enhanced laminates. A detailed fractographic examination of the failed interfaces was performed by using state of the art imaging equipment such as Helium Ion, Axio Image upright microscope and scanning electron microscope. The study indicated that nanoengineered composites have significantly higher interlaminar properties as compared to the conventional composite laminates.

Vitrimer materials have gained increasing attention since their introduction in 2011, due to their recyclable and dynamic nature. In addition to a traditional glass transition temperature (T_g), vitrimers have a second vitrimer transesterification temperature (T_v) above which dynamic covalent bonds allow for rapid stress relaxation, self-healing, and shape reprogramming. However, in order to take advantage of these unique properties, it is crucial to correctly identify the T_v and understand the impact of various experimental parameters (e.g., heating and applied force) upon its identification. Herein, we present a unique method to identify the T_v and discuss the impact of catalyst concentration upon the Tv. In addition, we present vitrimer nanocomposites with a variety of nanofillers (e.g., graphene and gold-coated graphene nanoplatelets) and identify the impact of nanofiller addition upon the composite T_v. By embedding graphene nanofillers into the vitrimer matrix, the resulting composite demonstrates increased mechanical properties as well as a photothermal response when exposed to near-infrared (NIR) light. These photothermally activated composites exhibit shape memory and shape reconfigurability for actuators and self-healing behaviors.

For optimal materials usage in numerous defense applications, as well as in aerospace systems, materials are to operate in episodes requiring simulteneous multifunctionality. For example, electronics in munitions and high temperature sensor modules are expected to retain its expected electrical, thermal, and mechanical properties or attributes even in extremely high strain gradient operation. Similarly, in hypersonics coatings, high temperature material oxidation stability plus tailored thermal conductivity are needed in extreme high temperature operation. In this presentation, materials design approaches for simulteneous exhibit of selected multifunctionality will be discussed. Further, such desired multifunctionality is expected to be more optimally feasible by taking the materials design to small scale (say, at atomic level) and then linking that to bulk materials performance. Also, materials hybridization at small (atomic or molecular) scale and optimizing the associated heterostructure phases significantly opens up materials performance domain and its multifunctionality. Examples of several computational tools (atomistic, Mesa, continuum scale - ab initiation, NEGF, MD, MD Wave Packets, Tight Binding MD, Molecular Mecjhanics) will be illustrated for tailoring materials functionalities (thermal, electrical, structural) in the material compositional design, along with a few design cases (thermal interface, nano porous carbon, strain-resilient electronics).

Nanocomposite property enhancements originate substantially from interfacial effects. We focus on two examples of nanocomposite interfacial effects. In Example 1, modification of filler surfaces can improve dispersion and compatibilize the polymer-filler interface, leading to property enhancements relative to nanocomposites containing unmodified filler prepared by conventional melt mixing or solvent blending. For the first time, we isolate the effect of interactions between matrix polymer and polymer-grafted nanoparticles from dispersion quality, revealing how the interactions affect the properties of polypropylene/halloysite nanotube (PP-HNT) hybrids. PP-HNT nanocomposites with pristine filler or PP-grafted filler were prepared by solid-state shear pulverization, leading to excellent dispersion in both cases. Optimal interactions with PP-grafted nanofiller leads to faster PP crystallization and higher yield strength; in contrast, Young’s modulus is determined by the filler dispersion and not filler modification. In Example 2, for the first time, we have decoupled chain dynamics in nanocomposite interfacial layers from chain dynamics of the matrix in poly(2-vinylprydine) (P2VP)-silica nanocomposites. The interfacial T_g (glass transition temperature) and the matrix T_g (both associated with α-relaxation dynamics) were determined by fluorescence. The interfacial layer T_g is higher than the matrix T_g due to H-bonding between P2VP and hydroxyl groups on the silica surface. At the same filler dispersion, 2.1 kg/mol P2VP matrix polymer leads to much greater T_g enhancements in both the interfacial regions and the matrix relative to the use of 110 kg/mol P2VP. We hypothesize that low molecular weight P2VP preferentially aligns at the filler interface, leading to higher H-bonding density and increased T_g.

Thermosensitive bottlebrush polymers (BBPs) are a type of graft copolymers in which thermosensitive polymer side-chains are grafted on a linear polymer backbone. These side-chains can undergo a coil-to-globule conformational change in response to a change in the surrounding temperature. This also results in a change in the overall shape of the BBPs because of which they can be used for biomedical applications. Here, we will present the results of our recent coarse-grained (CG) molecular dynamics (MD) simulations study of poly(N-isopropylacrylamide) (PNIPAM; transition temperature= 305 K) BBPs of Worm-like, Cone-like, Dumbbell-like shapes. The CG MD simulations were performed at 300 K (below transition temperature) and 320 K (above transition temperature). The analysis of simulation trajectories showed that the shape of BBPs has significant impact on the conformations of individual side-chains.

Polyolefin (polyethylene, and ethylene-propylene-diene (EPDM) elastomers) composites were studied in order to elucidate and quantify the effects of the interfaces with ceramic and carbon nanofillers, and particularly of the interphase contributions on the dielectric properties of the composites. For ceramic nanoparticles, it was shown that the interphase contributions dominate the composites’ dielectric properties, often being antagonistic to the contributions of the ceramic fillers: Ceramic fillers with systematically varied dielectric nature –spanning orders of magnitude in dielectric constants– were studied, and the interphasial contributions effectively overwhelmed the ceramic filler permittivities and dominated the dielectric performance of the composites. For composites containing both ceramic and carbon-black nanofillers, proper design of the interphases is crucial, so as to capitalize on the high effective permittivity provided by the carbon while, at the same time, preventing mobile charges to undertake large lengthscale transport (conductivity) or high dissipation of energy in localized vibrations (dielectric loss); in these systems, a third nanofiller is introduced as a means to control conductivity and minimize losses and E-field breakdown.

We review the state-of-the-art in the molecular design and processing of low density organic-inorganic nanocomposite hybrids at the extreme limits of molecular-scale confinement with tailored internal interface chemistry. We probe the mechanical and thermal properties of nanocomposite hybrids where a stiff inorganic matrix phase confines polymer chains to dimensions far smaller than their bulk radius of gyration. We describe a synthesis strategy which involves tailoring the internal interface chemistry between the matrix and the polymer phase. Tuning the interface allows access to extreme levers of molecular “hyper-confinement” where the confined polymer dynamics and resulting thermal behavior are markedly altered. We demonstrate the strategy with polystyrene and polyimide phases. The infiltration of individual polyimide precursors into a nanoscale porous network where imidization reactions under such confinement increase the molecular backbone stiffness. We find that polyimide oligomers can then undergo crosslinking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that a confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (> 350 °C).

The distribution of nano-inclusions at the interface or within soft films is determined by multiple key factors. These factors include the molecular composition, interfacial profile, morphology of the films and the characteristics of the inclusions along with predominant interactions in the systems. We will discuss the role of these factors on the interfacial behavior of nano-inclusions on the surface of spherical soft films, or vesicles. Furthermore, we will discuss the organization of the inclusions in vesicles as a function of nano-inclusion effective chemistry, dimension and concentration. These investigations have been contingent upon the ability to resolve large spatiotemporal scales. Hence, the studies have required the adoption of coarse-grained models used in conjunction with mesoscale simulation methods such as Dissipative Particle Dynamics and Molecular Dynamics. Our observations have the potential to guide the design of composite materials which require precise control on their characteristics including their shape or the dispersion of the inclusions.

In mussels, the adhesive proteins that are instrumental for attachment to wet surfaces are known to contain high levels of 3,4-dihydroxy-L-alanine (DOPA), often located adjacent to amino residues such as lysine (Lys). The special synergistic relationship between catechols and amines is a subject of high interest, not only for understanding native proteins but also for informing the design of bioinspired polymer systems. Other research groups have shown that catechol and amine functional groups act synergistically to enhance adhesion at wet surfaces, however reports of catechol-amine interfacial phenomena on a single molecule level have been limited. In this talk we will describe single molecule force spectroscopy (SMFS) measurements that are providing new insights into interactions between Lys-DOPA peptides and various surfaces. Molecular mechanics investigations of catechol and catecholamine polymers are providing new insights into design of novel polymer adhesives, for example pressure sensitive adhesives and high-strength thermoset adhesives.

Exfoliated Transition Metal Dichalcogenides (TMDs, MX₂) have attracted considerable attention for infrared optical elements due to their high refractive index and extreme nonlinearities (e.g. MoS₂, TiS₂). Recent high-yield, sonication-free, surfactant-free, exfoliation methods (i.e. Redox Exfoliation) now provide access to oxidatively-resistant and colloidally-stable dispersions at high TMD content (10% v/v), and in an expanded range of polar solvents (e.g. acetonitrile, acetone, alcohols). In addition to transforming approaches to nanocomposite fabrication, ink formulation, and film processing, these characteristics enable direct organic hybridization of Group IV-VI TMDs. For example, Grignards (R-MgBr) are not accessible via traditional Liquid Phase or Li-Intercalation methods due to their extreme reactivity with labile hydrogens (water), amides (NMP), and oxygen. However, the availability of Group VI TMDs dispersions in anhydrous THF enables synthesis of alkyl grafted MoX₂, which has zero surface charge, is dispersible in chloroform, and affords subsequent surface-initiated polymerization, all while retaining optoelectronic monolayer properties. In addition, classic redox intercalation chemistries can be applied to Group V monolayer dispersions. For example, a primary alkyl amine reacts at 70^oC in anhydrous acetonitrile with semi-metallic, single layer NbS₂ via charge transfer from the amine to the half-filled Nb d-orbitals. The resultant organic-grafted NbS₂ is a semiconductor, with enhanced oxidative stability (> 7 days ambient) and dispersiblity in non-polar dichloromethane that allows for ambient handling and processing. Such access to numerous hybridization approaches will greatly expand the electrical, optical, and chemical property suite of TMDs and enhance the ability to fabricate device quality films and hetero-structures for optical filters, GRIN optics, and non-linear absorbers.

Elucidating the role of organic ligands that are attached to or incorporated within inorganic nanostructures, for example, at the interface of colloidal nanocrystals or as included in the inorganic framework of two-dimensional hybrid organic-inorganic perovskites, is essential in realizing desired functional properties for applications. However, although general guidelines are provided by experimental explorations, an understanding of the underlying mechanisms for efficient materials development is often lacking. Herein, by employing first principles calculations for the design of materials that exhibit optical tunability characteristics, we first discuss atomically precise quantum dots. We highlight challenges in modeling the structure of semiconductor nanoclusters with organic ligands. In this context, we report on the derivation of a potential energy surface using machine learning that overcomes, in part, some of these limitations, and on the importance of the level of theory applied for accurate prediction of the optical response. In addition, we discuss effects of incorporating large chromophores as the ammonium cation in 2D hybrid organic-inorganic perovskites, which have proven promising for optoelectronic applications. Dependent on the organic moiety type, band alignments indicate variability in quantum-well types for a range of materials we examined, as well as structural distortion. Absorption spectra demonstrate tunability in the optical properties, including enhanced absorption and red-shifts, providing recommendation for the choice of the organic cation and motivation for further synthesis and experimental characterization.

Hetero- or homo-interfaces formed by carbon nanostructures of different C hybridizations are ubiquitous in electronic, structural, and energy-functional materials. Understanding how weak interactions and/or chemical bonding at the interface lead to a meso- and macroscopic behavior warrants a multiscale modeling framework. This will be illustrated with the example of carbon nanotube (CNT) fibers. Starting from the energetics and kinetics of various crosslinks (~nm, ~fs) accessible from accurate DFT calculations, we derive scaling relations of their tensile mechanical response from a μm-long coarse-grained model of a CNT bundle. These inter-tube crosslinks can be expected to behave as “hook-and-loop” fasteners, operational in Velcro, which can have the ability to break and reform and thus resist relative motion in a continuous manner. Such a model can incorporate the effect of interface friction due to inter-tube crosslinks or tube-polymer matrix connects, quantitatively evaluate toughness and fatigue processes and look for ways of performance improvement. Same coarse-grained model allows us to also explore the fundamental problem of basic packing defects in the aligned CNT bundles: the ‘twists’ and ‘inclusions’. We show how a finite characteristic size/length L of such defects emerge as a result of interplay between their intrinsic bending stiffness and the elastic response of the surrounding CNT matrix-crystal, and quantify their energetics scaling. In conjunction with synthesis efforts and novel experimentation, such models are crucial in developing novel carbon nanomaterials.

Current state-of-the-art carbon fiber reinforced composite materials have become a standard structural material used in the aircraft industry. Their relatively high strength-to-density and stiffness-to-density ratios make them useful for reducing vehicle mass and thus improving fuel efficiency. However, for manned space exploration beyond the moon (deep space), further decreases in mass are needed for fuel efficiency. Thus, increases in the strength-to-density and stiffness-to-density ratios are necessary.

Carbon nanotube (CNT)-reinforced composites can potentially provide the needed reductions in composite laminate mass. Relative to carbon fiber, CNTs can have higher strength and modulus, and can potentially provide a larger surface area for polymer interaction and load transfer. Further, flattened CNTs (flCNTs) can form self-assembled arrays with a higher packing than round CNTs [1]. Although countless studies have examined CNT/polymer interaction on the molecular level [2], little effort has been devoted to understanding flCNT/polymer interaction.

The objective of this research is to use molecular dynamics (MD) simulation to explore the interaction and load transfer characteristics for a range of different polymer resins with flCNTs. Specifically, the interfacial interaction energy, pull-out frictional force, pull-apart force, and resin wetting contact angles have been predicted. These predictions have been performed for two polyimide systems, two cyanate ester systems, polyurea, and PEEK. The results of these studies will be presented, and the best polymer candidates for these flCNT/polymer composites will be discussed.

References

1. Jolowsky, C., R. Sweat, J.G. Park, A. Hao, and R. Liang, Microstructure evolution and self-assembling of CNT networks during mechanical stretching and mechanical properties of highly aligned CNT composites. Composites Science and Technology, 2018. 166: p. 125-130.

2. Radue, M.S. and G.M. Odegard, Multiscale modeling of carbon fiber/carbon nanotube/epoxy hybrid composites: Comparison of epoxy matrices. Composites Science and Technology, 2018. 166: p. 20-26.

Polymer-ligated nanoparticles (NPs) exhibit an exciting variety of physical properties that depend sensitively on the dimension and composition of NPs as well as the surface chemistry of tethered polymer hairs. Herein, we report on a robust nanoreactor strategy for in-situ crafting of monodisperse stable polymer-ligated NPs with well-controlled size and shape as well as tunable plasmonic (e.g., Au) and luminescence (e.g., perovskite MAPbBr₃ and CsPbBr₃) properties. Central to this strategy is to judiciously design unimolecular amphiphilic star-like block copolymer as nanoreactor for yielding NPs intimately and permanently ligated by polymer hairs. The diameter of polymer-ligated NPs can be precisely tuned by modulating the length of inner hydrophilic block of star-like block copolymers. In the case of Au NPs, the perpetual anchoring of photoresponsive or thermoresponsive polymers on the surface renders the attractive feature of self-assembly and disassembly of NPs on demand by capitalizing on light of different wavelengths or heating/cooling, respectively. Such self-assembly/disassembly process is revealed by tunable surface-plasmon resonance absorption of Au NPs and the reversible transformation of Au NPs between their dispersed and aggregated states. For perovskite NPs, the resulting polymer-ligated MAPbBr₃/SiO₂ core/shell NPs and CsPbBr₃ plain NPs display concurrently a stellar set of significantly improved stabilities (i.e., colloidal stability, chemical composition stability, photostability, water stability), while possessing appealing solution processability, which are unattainable by conventional methods. We envision that the amphiphilic star-like block copolymer nanoreactor strategy may provide a versatile platform for crafting diverse organic-inorganic nanohybrids stably-ligated with polymer of different functionalities (e.g., semiconducting, ferroelectric, photo-responsive, thermal-responsive, or pH-sensitive) for a spectrum of applications in bioimaging, biosensors, photonic materials and devices, perovskite-based LEDs, lasers, high-energy ionization radiation detections, multiphoton emission, and scintillators.

In this talk I will present our recent work studying polymer nanocomposites (PNCs)using molecular dynamics (MD) simulations and Polymer Reference Interaction Site Model (PRISM) theory calculations with coarse-grained (CG) models. Specifically, we are interested in linking molecular design of PNCs comprised of grafted nanoparticles in a polymer matrix to their morphology. In this talk I will focus our work on the impact of strength of graft-matrix attraction on interpenetration of matrix and graft chains (known as grafted layer wetting) and dispersion/aggregation of grafted particles in matrix. Previously for PNCs with attractive graft-matrix interactions, we had shown that wetting/dewetting and dispersion/aggregation are two distinct phase transitions, the former being a continuous transition and the latter being a first-order transition as a function of graft-matrix interaction or temperature. Recently, we found that as the strength of graft-matrix attraction increases, we can also tune the effective size and the hardness of the polymer grafted particle in the PNC. As the attraction between the graft and matrix chains increases, the graft polymer chains extend to make favorable graft-matrix contacts and increase the grafted layer wetting by matrix chains. This leads to larger and ‘harder’ grafted particles compared to analogous fillers with purely entropic (athermal) graft-matrix interactions. Due to the increasing size and hardness of grafted particles with increasing graft-matrix attraction, the PNC structure changes from an aggregated/dispersed morphology governed by entropic driving forces at athermal graft-matrix interaction to a dispersed morphology due to favorable weak graft-matrix attraction, and ultimately, to a correlated fluid of hard grafted particles at stronger graft-matrix attraction. We see these trends in PNCs with high (densely grafted) and low grafting density, and with equal graft and matrix chain lengths as well as cases where the matrix chain lengths are greater than the graft chain lengths.

Surface Initiated Atom Transfer Radical Polymerization (SI- ATRP) in its various modifications has emerged as a versatile toolbox to control and tailor the properties and interactions of interfaces and to enable the synthesis of hybrid materials with unprecedented property combinations. The resulting materials have attracted interest not only because the high-level structural control of the architecture of polymer-tethered surfaces enables tailoring of the interactions, microstructure and properties of particulate-based materials but also because the confinement of chains on surfaces alters the mechanism of termination reactions that limit conventional polymerization processes.
This presentation will review recent examples of the application of SI-ATRP for the synthesis of functional polymer materials. Examples to be covered include the synthesis of ultra-high molecular weight polymers, materials for optical applications as well as thermal interface materials. Recent examples of the interplay between phononic and photonic properties in brush particle-based materials will be shown to illustrate how the subtle control of polymer chains at interfaces can instigate novel physical properties in hybrid materials that cannot be realized in ‘classical’ composite materials that are fabricated by mixing of particle and polymer constituents. This will provide the basis for a discussion of ‘guidelines’ for the fabrication of novel functional materials that harness ‘chemical confinement’ of polymer chains.

For optimal materials usage in numerous defense applications, as well as in aerospace systems, materials are to operate in episodes requiring simulteneous multifunctionality. For example, electronics in munitions and high temperature sensor modules are expected to retain its expected electrical, thermal, and mechanical properties or attributes even in extremely high strain gradient operation. Similarly, in hypersonics coatings, high temperature material oxidation stability plus tailored thermal conductivity are needed in extreme high temperature operation. In this presentation, materials design approaches for simulteneous exhibit of selected multifunctionality will be discussed. Further, such desired multifunctionality is expected to be more optimally feasible by taking the materials design to small scale (say, at atomic level) and then linking that to bulk materials performance. Also, materials hybridization at small (atomic or molecular) scale and optimizing the associated heterostructure phases significantly opens up materials performance domain and its multifunctionality. Examples of several computational tools (atomistic, Mesa, continuum scale - ab initiation, NEGF, MD, MD Wave Packets, Tight Binding MD, Molecular Mecjhanics) will be illustrated for tailoring materials functionalities (thermal, electrical, structural) in the material compositional design, along with a few design cases (thermal interface, nano porous carbon, strain-resilient electronics).

Materials with difficult-to-attain combination of multiple properties - mechanical, electrical, chemical, optical, thermal, and transport, – represent the quintessential bottleneck for nearly all modern technologies. Nanocomposites with molecular, nano-, meso-, and microscale levels of structural engineering can provide such property combinations, while intrinsic ability of nanoscale components to self-assemble make them suitable for simplicity of synthesis.
Biomimetic nanocomposites exemplified by a variety of nanostructured nacre-like materials provide a generalized approach to engineer materials with multiple difficult-to-attain properties. As the continuation of this research, we learn that the unique mechanics of tooth enamel can be replicated combining out-of-plane nanoscale columns with molecular precision of layer-by-layer (LBL) assembly between them. As a result of that, these composites reveal remarkably high vibrational damping unusual for stiff materials that imparts them resilience to aging.
The novel type of biomimetic nanocomposites are those based on aramid nanofibers (ANFs). They spontaneously assemble into three-dimensional percolating networks reminiscent of cartilage. The nanoscale structure of ANF composites reveal nanoscale porosity that can be controlled by nanofiber branching. The latest results from multiple groups demonstrate that ANF composites resolve some of the essential property bottlenecks for ion-selective membranes, dendrite-resistant electrolytes, and structural batteries.
One of the emerging fields for biomimetic nanocomposites are optical devices. The high strain and strong polarization rotation make possible metaoptical devices with wide-angle diffraction gratings for LIDARs and highly efficient quarter wave plates for THz scanners. Both devices can be used for machine vision and biomaterials imaging.

Plant-based polysaccharides such as cellulose and its derivatives are receiving increasing interest in a large variety of applications because they represent an environmentally friendly alternative to plastic. Many of them are commonly used in diverse industrial applications, such as food additives and for biomedical devices due to their non-toxic and water-soluble nature. Moreover, the self-assembly nature and responsiveness of cellulosic bio-polymers makes them also extremely attractive for smart photonic applications including sensing. Among various types of cellulose and its derivatives, hydroxypropyl cellulose (HPC) encompasses all these desirable properties.
Hydroxypropyl cellulose is a liquid crystal polymer, which can form a cholesteric liquid-crystalline phase allowing Bragg-like reflection. The reflected colours can be simply controlled by changing the nature of the solvent, concentration, temperature. Recently, we demonstrated for the first time that a simple aqueous solution of HPC can be used as a photonic strain sensor that displays the applied strain by shifting its colour.
In this seminar, I will introduce how such properties can be enhanced to create a solid-state film with desired optical appearance.

Composites offer a wide variety of uses in aerospace, automotive industry, additive manufacturing, and energy storage applications. Discovered in 2011, MXenes have emerged as a new class of two dimensional (2D) materials consisting of transition metal carbides and nitrides. Synthesized MXenes have rich chemistry and 2D morphology offering a combination of high metallic conductivity and hydrophilicity for easy solution processing. Additionally, MXenes have been shown to have excellent mechanical properties with Young’s modulus of 330 GPa, surpassing both GO and rGO making it one of the strongest solution-processable 2D materials. Thus, they can improve mechanical and thermal stability as well as electrical conductivity of polymers. They can also reinforce ceramics and metals.

The most commonly studied MXene, titanium carbide (Ti3C2), has been combined with various polymer systems producing new multifunctional nanocomposites. When introduced into thermoplastics like PVA, improved mechanical properties were observed at 10 wt% polymer loading with a 34% increase in tensile strength compared to pure Ti3C2 films. Additional studies have shown electrical properties can be successfully transferred to the polymer composite extending its use to electrodes in energy storage systems as well as electromagnetic interference shielding for future electronics. In addition to simple direct mixing with polymers, Ti₃C₂ MXene has also been incorporated into more complex polymer fibers for integration into textile applications. A recent study on electrospinning of MXene-PAN nanofibers showed MXene content could be increased up to 35 wt% and retain high areal capacitance three times that of pure PAN nanofibers. This presentation will provide an overview of the current MXene-reinforced composites field, including ceramic- and metal-matrix, and the various applications.

The properties of fluids, when confined at nanometer scales, differ greatly from their bulk properties and are generally dominated by interfacial effects. In this interfacial region, the nature of the wall–fluid intermolecular interactions can have a significant effect on the fluids, introducing symmetry breaking, structural frustration and confinement-induced entropy loss into nanoscale interactions under confinement. In this presentation, several examples regarding how different degrees of nanoconfiment alter the structural organization and interfacial properties of soft matter ranging from non-biological (e.g. ionic liquids and silica colloidal suspensions) to biological ones (e.g. biopolymer solutions) will be introduced. This talk will particularly focus on describing the mechanisms of how biomacromolecules such as silk fibroin (SF) proteins self-assemble into hierarchical structures at the multiple-length levels in greater details. The presentation will conclude with some perspectives on new fundamental insights for rational design of SF-based materials with desired interfacial features in use of their fabricating superior functional materials and devices.

Molecular self-assembly involves the use of hydrogen bonds and other noncovalent interactions between molecules to create supramolecular structures. A goal of theory is to be able to predict and understand what structures will arise for any given set of conditions, and how the self-assembly can be directed so as to produce useful functional structures. In this talk I will describe recent studies in my group in collaboration with the Stupp and Mirkin groups at Northwestern, and with others, with the goal of making nanostructured materials with broad applications including actuation with various stimuli.

One area of interest concerns studies of peptide amphiphile self-assembly to produce cylindrical micelles and ultimately fiber materials that can be used for applications in biomedicine and for making materials that can be photoactivated for robotic functions. Here we have developed all-atom and coarse-grained models, and specialized molecular dynamics methods, that are capable of describing assembly into fibers, leading to an understanding of biological functions and photoactuation.

In a second direction we are interested in the coupling of self-assembly chemistry involving DNA attached silver and gold nanoparticles to create a new generation of bottom-up nanomaterials of interest for sensing and optical devices. Here we show both coarse-grained and all-atom approaches to the assembly, and how these can be used to understand the formation of plasmonic array structures, including recent studies aimed at understanding programmability of dynamic optical properties.

We summarize recent results from our research group on assembling of robust functional synthetic and biocomposite materials from orthogonal nanoscale components for advanced nanophotonic applications. For flexible biophotonic materials, uniformly aligned chiral structures is critically important to be stabilized to improve mechanical performance without compromising controlled helical morphologies and propagating phase separation. In this aspect, we discuss mixed amorphous polysaccharide polymers and nanocrystals in terms of their hierarchical organization, intercalated structures, ability to form periodic patterned surfaces, and resulting chiral optical properties. Furthermore, we consider how highly emissive semiconducting and carbon quantum dots can be integrated into synthetic polymer matrices, chiral biomolecular structures, and survive microfabrication/printing processing for arresting quenching trends, stimulating bright patterned emission, and prompting directed lasing phenomena.