Date of Award

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Committee Member

Dr. Dvora Perahia, Committee Chair

Committee Member

Dr. Gary S. Grest

Committee Member

Dr. Brian Dominy

Committee Member

Dr. Steven J. Stuart

Abstract

Impact of ionizable blocks on structure and dynamics of structured ionic co-polymers in solutions, melts, and thin films has been studied using atomistic molecular dynamics simulations. Much of the interest in ionic block co-polymers derives from their inherent tendency to phase segregate into hydrophobic and hydrophilic domains. Ionic domains in ionic block co-polymers serve as physical cross-linkers and form a long-range percolated cluster assembly, which is a crucial component to transport ions or solvents for varieties of applications from clean energy to biotechnology. One of such ionic co-polymers, where ionic blocks facilitate the transport and other non-ionic blocks provide chemical and mechanical stability, is pentablock (ABCBA) co-polymer. It consists of randomly sulfonated polystyrene (C) in the center, tethered to poly-ethylene-r-propylene (B), terminated on both sides by poly-t-butyl styrene (A). This dissertation focuses on the studies of these co-polymers in their different forms, including the single chains to micelles in solutions, membrane, and followed by water penetration into their thin films. Single chains were studied as a function of nature of solvents including the 1:1 mixture of cyclohexane/heptane and water. This specific hydrophobic solvent is used in industrial casting processing. Water is a highly prevalent substance in the environment and is a by-product of many eletrochemical reactions. We find that a single molecule of the ionic co-block polymer even undergo internal segregation into ionic and non-nonic blocks in both solvents. We then probed the assembly of the pentablock copolymer in solutions to understand the structural diffrences between the micelles formed by the ionic copolymer and van der Waals polymers. Micelles are the building blocks of a membrane, and a key step to engineer controlled polymeric ion transport systems. We find that the ionic network serves as a long lived skeleton of the assembled co-polymers where the hydrophobic blocks are able to migrate in and out of this structure depending on the nature of the solvents. Following the understanding of micellizaion of polymers we moved to melts. We find that the melts form intertwined networks of t-b-PS and center PS blocks. These networks are independent of the degree of sulfonation and have no long-range ordering. The sulfonated groups form different size of clusters where their cohesiveness and morphology affect both collective and segmental dynamics of all the blocks. Studies have been further exteneded to focus on the interfacial behaviour of complex copolymer thin films in solvent environments. We find that inerfacial response of hydropbobic and hydrophilic blocks including their dynamcis differs from the bulk ones. We aslo observe a multi-steps water penetration process. Onset of slow penetration is observed at the early stage where water molecules first transverse the hydrophobic rich surface before reaching to the hydrophilic regime. Water molecules then diffuse along the percolating pathway formed by ionic center block. Interpenetration occurs due to the change in bulk morphology where individual blocks rearrange. Results from these studies provide an engineering tool to design functional polymeric membranes for promising device applications. Overall the works have resolved the interplay between ionic and non-ionic blocks in co-polymers.

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