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RISM theories for polymers and multi-site molecules: applications to polymer blends near surfaces and hybrid theory/simulations
Tormey, Caleb A.
Tormey, Caleb A.
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2016
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Abstract Polymers are one of the most important materials in modern industry. An area of increased interest is understanding polymer blend behavior at surfaces. Understanding of these processes can help to improve material properties such as adhesion, viscosity and surface chemical composition. Current research into blends of polymers with the same monomer makeup but different architectures have been an area of increasing research. Previous work has examined blends of linear and branched systems. However, blends of polymers with cyclic chains are of increasing interest as new synthetic techniques have allowed cyclic blends to be more easily studied. Cyclic polymers have interesting properties because of their lack of end groups and more compact size when compared with linear polymers of the same molecular weight (MW). Wall Polymer Reference Interaction Site Model (Wall-PRISM) studies were conducted to compare with Neutron Reflectivity (NR) studies of cyclic/linear blends near surfaces. To accomplish this a new cyclic model was developed based on the linear Non-overlapping Freely Jointed chain model. The Wall-PRISM calculations were also performed over a range of densities and with different surface stiffnesses to study effects on chain packing at the surface. Abstract Even with increased computational power, large polymer systems can often still prove costly to simulate. Self-consistent PRISM (SC-PRISM) is a state-of-the-art hybrid approach that simulates only a single chain with effective interactions calculated by theory to mimic the influence of the surrounding chains. The recent introduction of Two-Chain SC-PRISM, simulating two chains with effective interactions, showed improvement of results over SC-PRSIM for polyethylene melts when compared with MD studies. However, numerical problems with using polymer chain simulation together with PRISM theory have kept Two-Chain SC-PRISM from being applied to a wide range of polymer systems. I have developed new methods for incorporating the simulation data to work with PRISM theory. This new method was tested on several small molecule systems with both hard sphere and attractive Lennard-Jones potentials and showed overall improvement over the single molecule version of RISM/PRISM. This new method was then extended to allow for attractive potentials, overcoming another limitation of conventional RISM/PRISM. With the new method tested and verified on smaller molecular systems, a foundation is laid for applying the Two-Chain SC-PRISM method to more complex polymer systems such as polyolefin blends and those with attractive interactions such as polymer electrolyte systems.
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