Crazing in thin films of styrene–butadiene–styrene block copolymers

Abstract

SynopsisThe mechanism of craze initiation and growth and its relationship to mechanical properties has been studied in thin films of styrene-butadiene-styrene (SBS) block copolymers. Optical microscopy and transmission electron microscopy were used to examine three copolymers which had a spherical rubber domain morphology but varied in rubber content from 20 to 50%. With increasing rubber content, the crazes became longer and less numerous. Widening of the crazes was at least partially responsible for the higher strains achieved in the copolymers, especially for the composition with the highest rubber content where the crazes widened to form micronecks. Transmission electron microscopy revealed that craze initiation and growth at the craze tip occurred by cavitation in the polystyrene phase. Cavitation of the continuous phase rather than the rubber domains was attributed to the concentration of chainend flaws in the polystyrene. Crazes in the block copolymers followed a meandering pathway and the boundaries between c r d and uncrazed material were indistinct. Incorporation of fibrillated rubber particles into the craze fibrils strengthened the craze. At higher rubber content, the craze widened in the stress direction by voiding and fibrillation, which produced a cellular morphology.

Cavitation-Induced Stiffness Reductions in Quantum Dot–Polymer Nanocomposites

“…Cavities formed during tensile drawing processes of polystyrene or its block copolymers tend to nucleate early in the elastic regime. 3,50,57 While the aggregates in the 5% QD-SEBS nanocomposites are on average the same size as the TP and NR composites, the standard deviation is much greater, meaning that there are some This is in line with with previous analytical theories of the mechanical properties of cavitated composites. 3,59 In these studies, the underlying assumption was that cavities were present immediately after the start of tensile testing but not before.…”

“…visible voids or crazes (>~200 nm) have been observed in the early elastic region (~1.5% strain, before the yield point) in SEBS polymer 50. This indicates that voids, too small to be optically resolved, can be readily nucleated due to the brittle nature of polystyrene 50.…”

“…This indicates that voids, too small to be optically resolved, can be readily nucleated due to the brittle nature of polystyrene 50. …”

“…,33,50 Previous work in the literature on similar CdSe and CdS QD-polymer nanocomposite systems with identical QD surface chemistry involves nanocomposites which were processed via film casting. In these studies, only increases in the Young's modulus at all concentrations were observed at all concentrations,22,[51][52][53] indicating that the processing technique in this work is critical for the stiffness reductions observed.As noted above, only rule of mixtures theories which accounted for cavitation3,8 predict decreases in the Young's modulus of polymer composites, albeit smaller than observed in this work.…”

Correlation between Molecular Architecture, Morphology, and Deformation Behaviour of Styrene/Butadiene Block Copolymers

“…[29] For SBS triblock copolymers, rupture of styrene domains (cavitation in the PS phase) was reported by Baer and coworkers. [37] Using oriented samples of SBS thermoplastic elastomers (TPEs), Keller and Odell [36] were able to show the fragmentation of styrene domains at high strains. These results indicate that the cavitation mechanism proposed for SB diblock copolymers is not valid for SBS triblock copolymers irrespective of the type of microphaseseparated structures present in them.…”