Phase-mixing and molecular dynamics in poly(urethane urea) elastomers by solid-state NMR

Dynamic Monte Carlo simulations of strain-induced crystallization in multiblock copolymers: effects of dilution

Multiblock copolymers containing alternating semicrystalline and molten blocks are good thermoplastic elastomers. Their crystallization in the stretching process is however complicated by the dilution effects, prior microphase separation and contrast chain rigidity of the molten blocks. We designed our systematic investigation with three integrated steps, and herein, as the first step, we considered only the dilution effects without prior microphase separation and contrast chain rigidity. We compared two extreme situations of local dilution separately corresponding to parallel-posited and antiparallel-posited block copolymers upon strain-induced crystallization. Our dynamic Monte Carlo simulations of diblock and tetrablock copolymers demonstrated that the stretching introduces a constraint on the diffusion of locally posited crystallizable blocks along the stretching direction for crystallization and thus enhances the dilution effects to result in a higher diversity in crystal stabilities. We observed that the strain-induced crystallization of parallel-posited copolymers behaved like the melt crystallization of homopolymers; in contrast, the strain-induced crystallization of antiparallel-posited copolymers yielded crystallites near the block junction, which are relatively small and less stable due to their local dilution suppressing their melting points. Similar to the case of spider dragline silks, two contrasting stabilities of crystallites in semicrystalline multiblock copolymers explain their good toughness. Our modeling approach paves the way toward a better understanding of the structure-property relationship in the semicrystalline thermoplastic elastomers.

Interplay of Structural Factors in Formation of Microphase-Separated or Microphase-Mixed Structures of Polyurethanes Revealed by Solid-State NMR and Dielectric Spectroscopy

A set of aromatic-oxyaliphatic polyurethanes (PUs) with different mass fractions of components also containing fluorinated fragments was synthesized and studied using various solid-state NMR techniques and dielectric spectroscopy. In contrast to the common model suggested by Cooper and Tobolsky in 1966, the rigid domains of microphase separated PUs are formed, not only by units containing urethane bonds, but also by oxyethylene fragments that form a common rigid phase. The urethane bonds and oxyethylene fragments are incorporated into both rigid and soft phases. Good agreement with the Cooper and Tobolsky model is observed only when solubility parameters are significantly different for the hard and soft segments, such as hydrocarbon aromatics and perfluoroaliphatic blocks.

Dynamics of supersonic microparticle impact on elastomers revealed by real-time multi-frame imaging

Understanding high-velocity microparticle impact is essential for many fields, from space exploration to medicine and biology. Investigations of microscale impact have hitherto been limited to post-mortem analysis of impacted specimens, which does not provide direct information on the impact dynamics. Here we report real-time multi-frame imaging studies of the impact of 7 μm diameter glass spheres traveling at 700-900 m/s on elastomer polymers. With a poly(urethane urea) (PUU) sample, we observe a hyperelastic impact phenomenon not seen on the macroscale: a microsphere undergoes a full conformal penetration into the specimen followed by a rebound which leaves the specimen unscathed. The results challenge the established interpretation of the behaviour of elastomers under high-velocity impact.