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(95c) Designing of High-? Block Oligomers for Assessing 1-Nm Domains and Understanding the Effects of Molecular Weight on ? and the Effect of Dispersity on Blend Phase Diagrams

Authors: 
Chen, Q., University of Minnesota
Lodge, T. P., University of Minnesota
Hillmyer, M. A., University of Minnesota
Siepmann, J. I., University of Minnesota
Molecular dynamics simulations are used to design a series of high-χ block oligomers(HCBOs) that can self-assemble into a variety of mesophases with domain sizes as small as 1 nm. The exploration of these oligomers with various chain lengths, volume fractions, and chain architectures at multiple temperatures reveals the presence of ordered lamellae, perforated lamellae, and hexagonally-packed cylinders. Interestingly, the phase behavior of these oligomers is distinct from that of either solvent-free surfactants or block polymers in detail. The simulations reveal that the behavior of these HCBOs is a product of interplay between both “surfactant factors” (head group interactions, chain flexibility, and interfacial curvature) and “block polymer factors” (χ, chain length N, and volume fraction f). Gibbs ensemble Monte Carlo simulations are performed for oligomeric blends consisting of poly(ethylene-alt-propylene) (PEP) and poly(ethylene oxide) dimethyl ether (PEO). It is found that an empirical model with a single adjustable parameter kij is used to quantify this molecular weight dependence, and it allows for the accurate prediction of χ of PEP/PEO mixtures with arbitrary molecular weights. When PEO is relatively monodisperse (Ð < 1:2), the phase diagram is insensitive to molecular weight distribution (MWD) and Ð, despite differentiation in molecular partitioning observed from simulations. However, the coexistence curve for mixtures containing PEO with a bimodal distributionand a large dispersity (Ð = 1:76) differs dramatically from that for mixtures containing low-dispersity PEO, which suggests that the former mixture can no longer be treated as a binary system.