Controlling Macroscopic Properties by Tailoring Nanoscopic Interfaces in Tapered Copolymers

Systematic variation of comonomer concentration in individual polymer chains is responsible for unique phase behaviors in different types of copolymers. Controlled self-assembly of gradient copolymers into desired morphologies is theoretically understood but practically challenging, and identifying heterogeneous phase partitioning of individual comonomers in those resulting morphological regions can be difficult. Building on previous work where improved methods were used to elucidate heterogeneous comonomer partitioning in styrene–butadiene gradient copolymers [Clough Macromolecules 2014, 47, 2625], the arrangement of the styrene and butadiene monomers in only a fraction of the total chain length is used here to significantly perturb the overall morphology and physical properties of copolymers. Importantly, the chemical composition of all copolymers was held nearly fixed in this study. The resulting tapered and inverse tapered block copolymers contain nanometer length scale interfaces that differ from one another and differ dramatically from that observed in a control block copolymer composed of chains with a discrete interface. Evidence is presented that butadiene can reside in rigid environments, styrene can reside in mobile environments, and their relative amounts can be varied based on the gradient design. The connection between the molecular design of the gradient, the resulting nanometer-length scale interfacial structures, and mechanical properties is demonstrated using a combination of variable temperature solid-state NMR, modulated DSC (differential scanning calorimetry), AFM (atomic force microscopy), and rheology experiments

Multiblock Inverse-Tapered Copolymers: Glass Transition Temperatures and Dynamic Heterogeneity as a Function of Chain Architecture

Systematic variation of the size and number of inverse-tapered blocks in styrene–butadiene copolymers results in a wide range of accessible glass-transition temperatures (<i>T</i>_g), including <i>T</i>_g’s approaching that predicted by the Fox equation. Composition-weighted average <i>T</i>_g’s are expected for miscible blends or random copolymers, but such behavior has not previously been reported for block copolymers made from immiscible styrene and butadiene segments. In this work, 50:50 wt % multiblock copolymers with <i>M</i>_n = 120 000 kg/mol were synthesized using an inverse-tapered block design for all blocks except the end blocks. The total composition and molecular weight were held constant, but the type and number of blocks were systematically varied in order to compare contributions from the inverse-tapered chain interfaces to the overall glass transition behavior. Discrete copolymers of similar block number and length were investigated as controls to help separate contributions from the inverse-tapered design and the molecular weight of individual blocks. Some copolymers were intentionally designed such that individual block molecular weights were between the entanglement molecular weight (<i>M</i>_e) of polystyrene (PS) and polybutadiene (PB). A range of intermediate glass transitions was observed, but the inverse-tapered copolymers that satisfied this latter condition were the only copolymers that exhibited a <i>T</i>_g near a composition-weighted average. Solid state NMR reveals dynamic heterogeneity among monomeric components through chain-level identification of relatively large amounts of rigid PB segments and mobile PS chain segments versus that observed in discrete block analogues where essentially all PB segments are mobile and all PS segments are rigid. NMR revealed subtle differences in the temperature-dependent segmental chain dynamics of different inverse-tapered blocks, which were not obvious from the calorimetric studies but which presumably contribute to the longer length scale <i>T</i>_g behavior