45525-AC7
Simulating the Tensile Properties of Highly Regular Polymer Networks
The goal of this research is to study via molecular simulation alternative strategies to form regular templates that could be used to generate idealized polymer network topologies with minimal defects. Progress has been made on several fronts: 1) We have implemented Molecular dynamics (MD) codes to simulate iso-strain deformations to complement the Monte Carlo (MC) codes used so far for iso-stress deformations. This has allowed us to obtain the saw-tooth elastic behavior (characteristic of modular supertough microstructures) which appeared as a staircase curve in the iso-stress simulation of semiflexible diamond networks. We are now applying these methods to simulate networks of several types of bicontinuous connectivity. Several regular 3D networks have been realized experimentally in systems of neat block copolymers and surfactant systems such as the gyroid (with three-fold junctions), the double-diamond (with four-fold junctions), and the plumber’s nightmare (with six-fold junction). These structures are “bicontinuous” in that two separate networks can bee observed occupied by the minority component; these networks interweave but never intersect each other even though each has full 3D periodicity. These network phases are of practical interest because their 3D interconnectivity confers materials with good mechanical and electrical properties (several experimental realizations of this concept have already been reported). Simulations of polymer networks with such bicontinuous topologies are expected to show a step-wise elastic behavior and have higher elastic modulus as junction functionality increases. 2) Optimization of methods for the simulation of complex ordered structures. Expanded ensemble methods, designed to sample a range of an order parameter “lambda” of interest, can be optimized to overcome the difficulties associated with traversing large free-energy barriers or rugged landscapes. We extended the optimization strategy of Trebst et al. (that attempts to finding biasing weights for inter-lambda transitions that maximize the number of round trips that the system performs between the lower and upper bounds) by targeting the minimization of the mean round-trip for a Markovian walk over the lambda space. It is expected that this method will help us speed up the equilibration of complex mesostructures (e.g., from copolymer systems forming bicontinuous phases and colloidal systems forming ordered patterns).
3) Simulation of colloidal-based two-dimensional (2D) ordered morphologies as potential templates for regular 2D networks. Colloidal self-assembly has long been cited as a means to create periodic materials for photonic, optoelectronic, solar cell, and polymer nanocomposites. It holds promise for the simple fabrication of low-cost, large-scale devices. Nevertheless, structures that are accessible with spherical particles have generally been restricted due to the isotropic interactions in spherical particles, which fail to capture the selective functionalities and geometries required to encode the ordering of more elaborate assemblies. In collaboration with experimental researchers at Cornell, we used simulations to study the assembly in 2D of hard peanut-shaped particles and found them to organize into a little known “degenerate crystal” phase. In this structure, particle lobes tile a triangular lattice while their orientations uniformly populate the three underlying crystalline directions.
4) Simulation of colloidal-based 3D ordered morphologies as potential templates for regular 3D networks. The phase behavior of suspensions of tetragonal parallelepipeds “TPs” was studied by using Monte Carlo simulations to gain a detailed understanding of the effect of flat-faceted particles on inducing regular local packing and long-range structural order. A TP particle has orthogonal sides with lengths a, b, and c, such that a=b and its aspect ratio is r=c/a (r=1 for a perfect cube). The phase diagram for such perfect TPs was mapped for 0.125
