Dynamic Behavior Of Block Copolymers

Author : Youngmi Seo
Publisher :
ISBN 13 :
Total Pages : 135 pages
Book Rating : 4.:/5 (1 download)

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Download or read book Structure and Dynamic Properties of Interfacially Modified Block Copolymers from Molecular Dynamics Simulations written by Youngmi Seo and published by . This book was released on 2017 with total page 135 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nanostructured diblock copolymers, in which one can take advantage of the different properties of two types of monomers, are of interest in transport applications such as battery electrolytes. One can modify structural properties of diblock copolymers by inserting a midblock with a different composition between the two pure blocks. We study linearly tapered block polymers (TBPs) which are AB diblock copolymers modified by adding a linear gradient region (taper) between the blocks in which composition varies from one species to the other. This gives an additional tuning parameter, the length of the taper, to control the microphase separation behavior, e.g. increased accessibility of the bicontinuous double gyroid phase, or dynamic properties potentially of interest in transport applications. Here, we use molecular dynamics (MD) simulations to investigate both the structural and dynamic properties of TBPs and related block polymer systems. The MD results are consistent with the theoretical results in which the double gyroid network structure is more accessible with a taper and both the effective segregation strength and domain spacing decrease. Interestingly, the inverse TBPs, which have the B-side of the taper bonded to the pure A block, have a significantly shorter domain spacing compared to the normal TBPs. This is related to the polymer conformations; in contrast to typical diblocks or normal TBPs, inverse tapered chains can exist in several distinct conformations. For instance, inverse tapered chains may fold back and forth across the interface, bridge between multiple domains, or stretch over two layers of lamellae depending on the taper length. This conformational behavior was quantitatively analyzed as a function of the taper length. Theoretical work had predicted that the 50% inverse TBPs have a unique phase diagram; the larger of the two pure end blocks can phase separate into double gyroid phases. The MD results also show such a double gyroid structure for the 50% inverse TBPs. To understand the potential impact of tapers on ion dynamics, we investigated the diffusion of a small amount of penetrants in both normal and inverse TBPs. We considered uncharged monomer-sized penetrants, which have favorable interactions with A monomers of a strength that is chosen to make an analogy with the solvation of ions in the higher dielectric microphase of certain experimentally relevant copolymers. With normal tapering, both polymer and penetrant diffusion parallel to the lamellae increases as taper length increases, but inverse tapers show a non-monotonic variation in polymer diffusion. This is explained by analyzing differences in dynamic behavior between inverse tapered polymers with different types of conformations. However, regardless of the type of the midblocks, the penetrants’ diffusion constant values generally collapsed on a single curve when plotted against blockiness (average block length as a fraction of chain length) or number of A-B bonds per chain which can represent the degree of local A-B mixing. This implies that the local A-B mixing of monomers (rather than polymer conformation) is the most important factor in determining penetrant diffusion. These insights may allow improved design of nanostructured membranes for small molecule transport.