A common strategy to control the nanoparticle (NP) distribution in composites is to graft polymer chains onto the NP surfaces. Polymer-grafted 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. First, I will discuss the behavior of polymer-grafted NPs in polymer melts. Phase diagrams are calculated from direct simulations of the NP-dense and NP-dilute phases. Fluctuations in polymer conformational entropy are important for agreement with experiment. In ternary systems with small amounts of added homopolymer that is chemically identical to the polymer grafts, the added homopolymer shifts the phase boundary and tends to localize near the NP/matrix interface. Second, I will describe simulations of polymer-grafted NPs in solution, and in particular, a system consisting 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.
Biography
Amalie L. Frischknecht is a Principal Member of Technical Staff at Sandia National Laboratories, and a staff scientist at the Center for Integrated Nanotechnologies (CINT), a DOE Nanoscale Science Research Center and user facility at Sandia and Los Alamos National Labs. She received her PhD in Physics from the University of California, Santa Barbara, in 1998, and went on to postdocs at ExxonMobil Research and Engineering Company (1998-2000) and at Sandia National Laboratories (2000-2003), before becoming a member of technical staff at Sandia in 2003. Amalie is a Fellow of the American Physical Society (APS), and recently served in the Chair line of the APS Division of Polymer Physics. She previously chaired the Gordon Research Conference on Polymer Physics (2018). Her research interests are in the statistical mechanics and molecular simulation of complex fluids, including polymer nanocomposites, ion-containing polymers, and charged soft matter systems.