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45525-AC7
Simulating the Tensile Properties of Highly Regular Polymer Networks
The goal of this research is to study via molecular simulation idealized topologies of polymer networks wherein chains are regularly interconnected (via end-linking to multifunctional crosslink sites) and present minimal defects. While no single aspect of the work has been developed to the point to become a finished paper, progress has been made on several fronts:
1) We are implementing Molecular dynamics simulation (MD) codes to complement the Monte Carlo (MC) codes that have been using thus far. Our previous MC simulations generate force-strain curves by imposing a stress and measuring the resulting strain. To do the opposite, e.g., set the strain and measure the resulting stress, MD is more convenient than MC because the former readily allows access to the full stress tensor from which the deforming stress can be obtained. 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. MD will also allow us to simulation of network breakage. By setting an upper bound to the stretching force on individual bonds, MD should allow for bond breakage events which will eventually lead to an estimation of the ultimate strength and strain. Such information is also needed to calculate the ‘toughness' of the system from the area under the stress curve. We have been testing and developing this latter calculation with the help of an undergraduate researcher.
2) Simulating networks of bicontinuous phases. Numerous regular three-dimensional networks have been realized experimentally in systems of neat block copolymers and surfactant systems. Examples o such network phases include 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 (the minority-block in copolymers or the minority solvent in surfactants); 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 in such applications as porous membranes, catalytic supports, and photonic materials. We have already realized the formation of such bicontinuous phases in copolymer system with a polymeric additive. The next step consists of either (a) To edge away the majority component (as has been demonstrated experimentally to create porous membranes with several systems) and allow the minority component in these networks to crosslink, the result will be regular network architectures. These double networks should possess higher elastic modulus than single networks, and networks with high junction functionality should be stronger than those with lower crosslink functionality. (b) To make one of the block components (or the additive) “glassy” keeping the other “rubbery”; this combination of features could give the material both rigidity and toughness.
3) Simulating colloidal-based two-dimensional (2D) ordered morphologies as potential templates for regular 2D networks. “Triblock” systems consisting of nanoparticle(A)-polymer(B)-nanoparticle(C) where the chemistries A, B, and C have enthalpic disparity, has been simulated to try to create network morphologies in thin films (i.e., quasi 2D membranes); our results thus far have demonstrated that the nanoparticles tend to give a scaffold for the polymer to form distinct morphologies, some of which appear to have a well defined order.
4) Developing mesoscopic models to predict the mechanical properties of networks containing structural features spanning over several length scales (up to microns). Here the elementary “units” of the model are not polymer segments (as in the MC and MD methods discussed above) but either entire chains or chain section equivalent to a full entanglement length and are represented by ideal density clouds of segments. The models have been so far tested to represent entangled melts and is now being extended to represent networks; it has also the potential to be used to represent nanocomposites.
