Ferenc Horkay, National Institutes of Health
Noshir Langrana, Rutgers University
Mitsihuro Shibayama, University of Tokyo
Sandip Basu, Agilent Technologies
Symposium Support Aldrich Materials Science
E2: Structure and Mechanical Properties
Monday PM, December 02, 2013
Sheraton, 3rd Floor, Commonwealth
2:30 AM - *E2.01
Swelling in Crosslinked Networks: From Unexplained Peaks to Strain Energy Functions
Gregory McKenna 1 Ben Xu 2 Xiaojun Di 1 Jinrong Wu 3
1Texas Tech University Lubbock USA2Avery Dennison Corporation Mentor USA3Sichuang University Sichuang ChinaShow Abstract
The swelling and mechanical properties of networks are intimately tied together through the free energy function as it is affected either by chain stretching (elastic free energy) or through the mixing of the polymer with the solvent (usually through a Flory-Huggins estimate). Frenkel, then Flory and Rehner (FFR) developed a simple way of looking at this problem by simply balancing the elastic and mixing contributions to the chemical potential at swelling equilibrium. Much work has been done to examine this model, often in a way that suggests that something is missing or that the model itself is incorrect. In this presentation we examine the FFR model and show that it is correct for polymer networks swollen in organic liquids. We demonstrate that anomalous peaks in the swelling activity parameter S (or dilational modulus) are artifacts of experimental methods and we further show that, once the peak in S is removed, quantitative agreement is obtained between mixing and elastic contributions to the free energy with the simple addition of a cross-link dependent interaction parameter.
3:00 AM - *E2.02
Fluid Permeation and Nonlinear Elasticity of Slide-Ring Gels with Movable Cross-Links
Kenji Urayama 1
1Kyoto Institute of Technology Kyoto JapanShow Abstract
Slide-ring (SR) gels with movable cross-links along network strands have received much attention as a novel class of polymer gels. The network topology of SR gels is variable in response to imposed stress or deformation whereas that of classical gels with fixed cross-links is invariable. The polyrotaxane-based SR gels are synthesized by intermolecular cross-linking of alpha-cyclodextrin contained in poly(ethylene glycol). The SR gels are expected to exhibit novel properties and functions resulting from the movable cross-links. In this talk, we focus on the fluid-permeation behavior of the membranes of SR gels under imposed pressure (p), and the nonlinear elasticity revealed by biaxial stretching. It has been well known that the fluid permeation of classical gels obeys the Darcy's law: The steady-state fluid velocity (v) is linearly proportional to p. The proportionality constant (f) corresponding to the friction coefficient between gel network and fluid is independent of p for the classical gels. Here, we demonstrate that the membranes of SR gels exhibit a peculiar p dependence of f, where f sharply varies between two different values within a narrow p range. It indicates that SR gels are promising polymer membrane materials that enable the on-off control of fluid permeation by imposed pressure, which can be developed to separation membranes and drug delivery systems with novel functions. Further, we show a unique feature in nonlinear elasticity for SR gels revealed by the experiments using unequal biaxial strains. The effect of the strain in one direction on the stress in the other direction in SR gels is much smaller than that in classical gels. This feature results in no explicit strain-coupling term in the strain energy density function of SR gels.
4:00 AM - *E2.03
Correlation between Mechanical Properties and Structure in Polymer Gels with Controlled Network Structure
Takamasa Sakai 1
1University of Tokyo Tokyo JapanShow Abstract
Recently, we have developed a novel gel system (Tetra-PEG gel) by new network formation method, “AB-type crosslink-coupling”; the network is formed by the combination of two mutually reactive tetra-arm prepolymers with same shape. Our previous study revealed that the Tetra-PEG gel has a homogeneous polymer network with small amount of structural defects. Although the connectivity and spatial heterogeneity was observed, the degree of heterogeneity is extremely smaller than that of conventional gels. In this study, we focus on the correlation between mechanical properties and structural parameters of polymer gels. We tuned the structural parameters including the polymer volume fraction (f_0), polymerization degree of network strands (M = 5k-40k g/mol), and reaction conversion (p). We investigated the values of p by infrared (IR) measurement, elastic modulus (G) and ultimate elongation ratio (lambda;max) by stretching measurement, and fracture energy (T0) by tearing measurement.
First, we compared G measured by a stretching measurement, and that predicted from the reaction efficiency (p) using affine (G_af ) and phantom (G_ph) network models. As for the 10k and 20k Tetra-PEG gels, G_ph and G corresponded well with each other in a wider range than the other gels, suggesting that their elasticities is roughly predicted by the phantom network model. As for the 5k Tetra-PEG gel, the downward deviation of G from G_ph was increasingly pronounced with decreasing f_0. On the other hand, the 40k Tetra-PEG gel shows distinct behavior; G was above G_ph and near G_af. In order to discuss the whole tendency, G/G_af was plotted against f_0/ f*, f* is the overlapping polymer volume fraction of the prepolymer. In this figure, all of the data fall onto a single curve. In the range from f* to 3.0f*, the elastic moduli were well predicted by the phantom network model. The downward deviation below f* is due to the formation of elastically ineffective loops. In the range above 3.0f*, G = G_af increased with an increase in f_0 and approached to 1.0. This data strongly suggests that trapped entanglements are introduced to the network or that the model shifts to the affne network model, or both in the region above 3.0f*. Only from these results, we cannot distinguish whether the deviation from the phantom network model prediction is originated from trapped entanglements or from the change in models. Thus, we investigate the trapped entanglements by the tearing measurement. The results in tearing measurement indicated that there are few trapped entanglements. Taking into account these data, it is strongly suggested that the model predicting the elastic modulus shifts model with increasing _0 and N. In the presentation, the results of ultimate elongation ratio will be also discussed.
4:30 AM - E2.04
Control of Double Network Structure and Mechanical Behavior of Thermoresponsively Reinforced, Shear Thinning Protein Hydrogels
Matthew J. Glassman 1 Bradley D. Olsen 1
1Massachusetts Institute of Technology Cambridge USAShow Abstract
Responsively tough, injectable biomaterials are potentially useful for the minimally-invasive surgical implantation of durable matrices, either for delivering cellular or molecular cargo, or to act as robust fillers for soft tissue reinforcement. Associative protein hydrogels are well-suited for use as injectable materials, exhibiting cyto-protective shear-banding behavior and immediate recovery post-injection. However, the low yield stress required for the injectability of these materials is typically incompatible with the high resistance to deformation and fracture that is needed to maintain implant integrity under physiological stresses. One approach to accomplish both injectability and toughness is to engineer a shear thinning hydrogel with a low yield stress at low temperatures that is reinforced at body temperature due to thermoresponsive block copolymer self-assembly. Toward this end, triblock copolymer hydrogels containing artificially engineered associative protein midblocks and poly(N-isopropylacrylamide) (PNIPAM) endblocks have been developed, exhibiting responsive and reversible toughening over clinically-relevant temperature ranges. The PNIPAM endblocks exhibit lower critical solution behavior in this hybrid protein-polymer gel, and heating above the transition temperature to 37°C leads to endblock desolvation and aggregation into nanoscale domains. The formation of these reinforcing domains leads to large increases in the gel&’s elastic modulus and yield stress in shear, as well as improved resistance to compressive failure, erosion, and creep. The relationship between macromolecular design, nanostructure formation, and gel mechanics has been investigated in detail using small-angle neutron scattering (SANS) and oscillatory shear rheology, revealing important principles for controlling network self-assembly and achieving improved reinforcement. In particular, large micellar cores, high PNIPAM volume fractions, and high densities of midblock associations led to the stiffest hydrogels, with elastic moduli reinforced by a factor of 14 to approximately 130 kPa at 37°C. Stress relaxation times were seen to increase by up to 50-fold for gels with the largest micelles and packing fractions. For large enough endblocks, midblock associations were seen to promote endblock segregation even below the PNIPAM transition temperature, and these swollen micellar structures introduced long relaxation times into the gels. These studies demonstrate that control of the double network structure is critical for tuning the gel&’s mechanical behavior over a broad range for use in various biomedical applications.
4:45 AM - E2.05
High-Strength Elastomers Inspired from Double Networks Hydrogels
Costantino Creton 1 2 3 Etienne Ducrot 1 2 3 Markus Bulters 4
1ESPCI ParisTech Paris France2Universitamp;#233; Pierre et Marie Curie Paris France3CNRS Paris France4DSM Ahead Geleen NetherlandsShow Abstract
We propose here a generic method to reinforce weak elastomers without using fillers. This method could be particularly interesting in the biomedical or high tech applications where pure polymers with specific physical properties (transparency, resistance to UV or temperature) are used but have poor mechanical properties.
In recent studies on the reinforement of hydrogels. Gong et al have shown that hydrogels synthesized with two interpenetrating networks with very different levels of crosslinking and conformations of chains have a fracture toughness significantly enhanced relative to a single homogeneous network. Their mechanical properties are enhanced through breaking the bonds of the more crosslinked and highly stretched minority network while avoiding crack propagation through the less crosslinked and unstreched majority network.
We applied this method to acrylic elastomers and succesfully prepared poly(alkyl acrylate) elastomers containing isotropically prestretched chains at different volume fractions, using sequential swelling/polymerization steps. Samples containing prestretched chains show an impressive enhancement of properties compared to networks polymerized in one step. Both initial modulus and fracture toughness are enhanced while retaining a negligible hysteresis and residual deformation upon unloading, which is impossible in simple networks.. Our best samples show a 50 time increase in true stress at break and in fracture toughness, making those material as tough as some filled elastomers. Our methodology holds great promise to improve the extensibility, toughness and tune the non-linear elasticity of elastomers previously thought to be mechanically too weak to be used in mechanically demanding applications.
 Gong, J. P.; et al, Y. Adv. Mater. 2003, 15, 1155-1158.
 Tanaka, Y.; et al J. of Physical Chemistry B 2005, 109, 11559-11562.
 Gong, J. P. Soft Matter 2010, 6, 2583.
 Brown, H. R. Macromolecules 2007, 40, 3815-3818.
5:00 AM - E2.06
Mechanical Properties of the Strongly Deswollen Polymer Network
Takuya Katashima 1 Ung-il Chung 1 Takamasa Sakai 1
1The University of Tokyo Tokyo JapanShow Abstract
We investigated the effects of swelling and deswelling on the mechanical properties of Tetra-PEG ion gels with variable polymer volume fractions at the measured condition (phi;_m). Tetra-PEG gels were prepared by the AB type crosslink-coupling between the two symmetrical tetra-arm prepolymers with tuning the network strands length and polymer volume fractions. The use of an ionic liquid as diluent enables us to measure the mechanical properties of strongly deswollen Tetra-PEG gels above the melting point of PEG. The drastic increase in the elastic modulus was observed in the high phi;_m region due to the unusually contracted conformation of the network strands, called supercoiling. The Obukhov model, which predicts the elasticity of polymer networks using the concentration at preparation (phi;_0) and measured condition (phi;_m) as variables, can describe the phi;m-dependence of the elastic modulus in all phi;_m regions. Furthermore, we analyzed the stress-elongation relationships for the swollen and deswollen networks. We estimated the fractal dimensions based on the Pincus blob concept. We, for the first time, observed the crossover of the phi;_m-dependence of the fractal dimension from normal scaling to supercoiling, and the dependence of the fractal dimension on the strand length. The extensibility at break increased with an increase in phi;_m and an increase in the network strand length. These results did not obey the familiar Kuhn model, but were better explained by our model. These findings will help to understand the structure and formation mechanism of supercoiling.
5:15 AM - E2.07
Fabrication of Piezoelectric-Rubber of Large Piezoelectric Property
Shogo Mamada 1 Naoyuki Yaguchi 1 Masanori Hansaka 1 Masafumi Yamato 2 Hirohisa Yoshida 2
1Railway Technical Research Institute Kokubunji-shi Japan2Tokyo Metropolitan University Hachoji-shi JapanShow Abstract
The piezoelectric-rubber is expected to be a material that is used at the places where conventional piezoelectric materials cannot be applied, because it has flexibility and can be formed into arbitrary shape. However, so far there have been the following issues that in order to increase the piezoelectric property of piezoelectric-rubber, a large amount of the large size piezoelectric-ceramic particles must be mixed in the rubber, resulting in loss of the elastic property of the piezoelectric-rubber.
In the previous investigation, it was confirmed that the orientation of the piezoelectric-ceramic particles in the direction normal to the surface of the rubber was effective in increasing d33 which indicates the piezoelectric coefficient when the input load is applied and the output electric charge is taken both in the direction of thickness of rubber. On the other hand, it is considered that the piezoelectric property of the “Oriented-Type” in which the piezoelectric-ceramic particles is oriented in the rubber is strongly affected by rubber&’s property as the matrix of the Oriented-type, because the volume of the rubber of Oriented-type is larger than that of the PZT particles and the stress on the oriented particles is affected by the property of matrix.
Therefore the influence of the matrix property on piezoelectric property of Oriented-Type was investigated. In the investigation, the Young&’s modulus of matrix was focused as the matrix property. The lower the Young&’s modulus of matrix is, the higher d33 of Oriented-Type is, because the stress applied to oriented PZT particles in Oriented-type increases. Four kinds of matrix materials were investigated: Silicone gel, Silicone rubber, Urethane rubber and Poly-methyl-methacrylate. The Young&’s modulus of matrix materials are as follows: Silicone gel is 5MPa; Silicone rubber, 70 MPa; Urethane rubber, 180MPa; and Poly-methyl-methacrylate, 600MPa. The piezoelectric-ceramic particles to be mixed with the matrix materials are the same particles of Lead Zirconate Titanate (PZT).
As the result of investigation, the sample made of the silicone gel which had the lowest Yang&’s modulus in the selected matrix materials had the largest d33. The large increase as compared with the previous investigation was observed. The d33 of the fabricated sample was more than 60 pC/N when the concentration of PZT particle was about 10 Vol%. The Oriented-type using silicone gel as matrix is expected to be the vibration-isolating materials having the property of sensor, because the volume of silicone gel is 90 Vol% and Young&’s modulus is 12MPa even if the sample is the piezoelectric-rubber of Oriented-Type.
5:30 AM - E2.08
Effect of Methanol/Water Mixtures on the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)
Derrick C Mancini 1 Subramanian K.R.S. Sankaranarayanan 2 Ganesh Kamath 3 Sanket A. Deshmukh 2
1Argonne National Laboratory Argonne USA2Argonne National Laboratory Argonne USA3University of Missouri Columbia Columbia USAShow Abstract
Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its lower critical solution temperature (LCST) around 305K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, experiments suggest that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fractions) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 moles fraction of methanol, LCST gets elevated to temperatures above 32°C and cannot be detected up to 100°C. In the present study we have used MD simulations to investigate the effect of solvent mixed with methanol on conformational transitions and the LCST of PNIPAM. We employ polymer consistent force-field (PCFF) and CHARMM force-field to study the PNIPAM and water-methanol mixtures of different mole fractions of methanol namely, 0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions, at fully atomistic level were carried out at 260, 278, 310, and 340 K. Simulated trajectories were analyzed for different structural properties such as radius of gyration of PNIPAM, extent of hydration of PNIPAM etc. Different dynamical properties such as diffusion coefficient, hydrogen bonding life-times, residence time of water and methanol near PNIPAM were also studied at 260, 278, 310, and 340 K.
5:45 AM - E2.09
Using Separation Techniques to Produce Natural Rubber Latex with Small-Sized Particles to Be Applied in the Fabrication of Homogenous LbL (Layer by Layer) Films
Mariselma Ferreira 1 Gero Decher 2 Christiane Pinto Davi 1
1Universidade Federal do ABC Santo Andramp;#233; Brazil2Institut Charles Sandron Strasbourg FranceShow Abstract
Natural Rubber latex (NRL) is a colloidal system constituted of rubber particles (20-45%) ranging from 3 nm to 5 µm dispersed in a liquid serum . The NRL has bioactive properties that have recently been explored to produce films using polyelectrolyte multilayers technique . However, the production of homogenous films may be influenced by the distribution of rubber particles size. In this sense, we separated the NRL to keep structures bellow 200 nm, aiming the production of homogeneous thin films using layer-by-layer technique. The separation of NRL occurred by a sequence of centrifugations. First centrifugation at 12,225 G (9,000rpm) for 2h removed the bigger particles; followed by a sequence of 3 centrifugations at 42,206 G (22,000 rpm) for 1h. On each step the liquid on the bottom was collected and submitted to the next centrifugation step. By this mean, the NRL was reduced in 75% of the initial weight. This serum was finally filtrated in 0.2 µm membrane and the solution obtained was analyzed. The presence of poly-cis-isoprene in the sample was confirmed by FTIR peaks attributed to CH2 (2858, 2926 cm-1), CH3 (2964 cm-1), C=C (1664 cm-1) and C=CH (840 cm-1). The presence of proteins was confirmed by the FTIR peaks of C=O (1737 cm-1), C-O (1099 cm-1), O-O (1013 cm-1) and NH (1580 cm-1). Furthermore, there is a broad peak at 3330 cm-1 that indicates the presence of both NRL (