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 (T g), including T g’s approaching that predicted by the Fox equation. Composition-weighted average T 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 M 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 (M 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 T 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 T g behavior.

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A review of recent literature reveals that magnetic resonance experiments can quantify interfacial chain content in tapered and inverse-tapered copolymers in their end-use, solid-state forms. Chemically dissimilar chain segments organize into nanoscale domains according to copolymer chain structure, sizes for which depend upon whether diblock or multiblock versions of discrete, tapered, or inverse-tapered chain designs are used. Broad calorimetric glass-transitions can be further resolved through variable-temperature solid-state MAS NMR methods, revealing that some high-Tg components in copolymers can exhibit dynamics usually associated with low-Tg polymers, while some low-Tg components can exhibit chain dynamics characteristic of high-Tg polymer segments. The amount, distribution, and temperature-dependence of this dynamic and compositional heterogeneity can be systematically varied for copolymers with the same chemical composition by tailoring the arrangement of monomers in the chain. In sequence-controlled copolymers of styrene and butadiene, comparison to microscopy data indicates that solid-state NMR methods can quickly and non-invasively yield reasonable estimates of interphase fractions by quantifying “rigid butadiene” and “mobile styrene” segments in their tapered and inverse-tapered copolymers. These developments in which solid-state NMR has been used to understand relationships between chain structure, overall morphology, and differential ordering and dynamics within and between interfaces resulting from sequence-controlled polymerizations are reviewed and described in a format suitable for non-NMR specialists.

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