Phase Behavior of Polymer-Grafted Nanoparticles

A common strategy to control the nanoparticle (NP) distribution in composites is to graft polymer chains onto the NP surfaces. Grafted polymer NPs can exhibit rich phase behavior due to complex interplays among chain conformational entropy, depletion effects, and enthalpic interactions. I will describe the use of a relatively new method, theoretically-informed Langevin dynamics (TILD) simulations, to determine the structure of polymer brushes on single grafted NPs and to calculate equilibrium phase diagrams for grafted NPs in solution and in polymer melts. The TILD method is sufficiently fast to allow simulations of tens of densely-grafted NPs, enabling simulation of NP-dense phases and self-assembled structures. In this talk I will focus on two systems. The first consists of PMMA-grafted NPs dispersed in a SAN homopolymer melt. The phase diagram is calculated from direct simulations of the NP-dense and NP-dilute phases. Adding small volume fractions of PMMA homopolymer to the composite shifts the phase boundary so that the PMMA-grafted NPs are more miscible in the polymer matrix, in qualitative agreement with experimental results from Composto and coarse-grained MD simulations from Jayaraman. The second system consists of NPs grafted with two immiscible polymers and dispersed in a selective solvent. In this case the polymers microphase separate on the surface of the particle. A Janus structure is obtained for sufficiently small NPs, while the polymers grafted to larger NPs tend to form disordered patchy structures on the NP surface. Janus-patterned NPs self-assemble in solution into double-walled vesicles when the volume fraction of solvophilic chains is between 0.2 and 0.3, similar to the range required for vesicle formation in diblock copolymers in selective solvent. These simulations establish the TILD method as an efficient tool for exploring complex NP-polymer phase behavior.<br/><br/>This work was performed at the Center for Integrated Nanotechnologies. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525.