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

Abstract

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.

Isoprene/Styrene Tapered Multiblock Copolymers with up to Ten Blocks: Synthesis, Phase Behavior, Order, and Mechanical Properties

“…In recent years several reports described the synthesis of multiblock copolymers via chain growth polymerization. ,,− While the full potential of these materials is not limited to thermoplastic elastomers, the following considerations need to be made. ,,, First, phase separation represents a basic requirement for applications. Based on the self-consistent mean-field theory (SCFT) by Leibler and the random-phase approximation by de Gennes, symmetrical AB diblock copolymers with equal volume fractions for each block ( f A = f B ) exhibit phase segregation when χ N ≥ 10.5, where χ is the Flory–Huggins parameter and N = N A + N B is the overall degree of polymerization. , While in usual block copolymers the interaction parameter depends on the choice of monomers, N can be simply adjusted by variation of the molecular weight. , Theoretical and experimental studies reveal a reduction in the order–disorder transition temperature for single AB diblock subunits in symmetrical, linear (AB) n diblock copolymers when increasing the number of blocks. − However, the overall molecular weight required for phase-separated structures increases due to the larger number of blocks.…”

Comprehensive Structural Assessment of Linear Block Polymers by NMR and SEC

“…This high sensitivity to sequencing makes it difficult to pinpoint the exact structure. The complexity may thus be explained by a structure bridging the S and S - alt - MA blocks that is either a tapered or a random array of small blocks of S - alt - MA and S. There is virtually no way for NMR to distinguish the two blocks . A pure linear diblock S - b - (S - alt - MA) would show both little signal in the “semi-alternating” region of the 13 C NMR spectrum and similar DECRA-resolved components.…”

Multiblock Copolymers of Norbornene and Cyclododecene: Chain Structure and Properties

“…Chain structure in copolymers is usually characterized by the average lengths of blocks consisting of chemically or structurally different monomer units. Even a moderate tendency toward blockiness in the monomer unit distribution can result in considerably different properties relative to completely random polymers of the same composition [ 1 , 2 , 3 , 4 , 5 ]. Recent reviews [ 6 , 7 , 8 , 9 , 10 ] discuss a number of well-established techniques nowadays available for the synthesis of multiblock copolymers.…”