Controlling Block Copolymer Self-Assembly for Tailored Performance

The macroscale properties of block copolymers (BCP), for given block chemistries and polymer architecture, are governed by their self-assembled morphologies. Depending on the processing method employed, the BCP may have an equilibrium morphology or a kinetically trapped, non-equilibrium structure. Thus it is important to understand the factors controlling the equilibrium morphology and the process parameters influencing the kinetics of the self-assembly process. This dissertation consists of two research thrusts, wherein the structures of two block copolymer systems are tailored for applications as membranes and elastomers, respectively.

The first research thrust addresses the structure-processing-property relationships for solution processed films of a sulfonated pentablock copolymer, poly(tert- butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene- b-tert-butylstyrene). The solutions contain trace amounts of water due to the hygroscopic nature of sulfonated polymer. Structural characterization studies showed accelerated self-assembly kinetics in model polymer/organic solvent systems with controlled amounts of trace water. This is attributed to the disruption of hydrogen bonds between acid groups, which likely increases the chain mobility. It is also shown that solvent-cast films show a preferential orientation of domains at the surface. Both the bulk morphology and domain orientation is influenced by solvent polarity as well as processing parameters such as drying time. Finally, water vapor transport and proton conductivity measurements show that both bulk structure and surface orientation of domains impact the through-film membrane transport properties.

In the second research thrust, the design of interaction-tuned additives to tailor the morphology and mechanical properties of a commodity thermoplastic, poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) is studied. The polymeric additives used are either polystyrene (PS) or poly(methyl methacrylate-co-cyclohexyl methacrylate) (PrC) which are athermal and enthalpically-compatible additives, respectively, for the polystyrene end-blocks in SEBS. The SEBS/PS formed miscible blends only at low molecular weight and loading of PS. The SEBS/PrC blends, in contrast, formed miscible blends systems with lamellar morphology for a broad range of PrC molecular weight and additive loadings. Consequently, the PrC additives can increase the modulus and yield stress of styrenic BCPs without impacting toughness. Additionally, the PrC additives can elevate the glass transition temperature of the PS block and maintain a high modulus at elevated temperatures.