Influence of phase miscibility and morphology on crack resistance behaviour and kinetics of crack propagation of nanostructured binary styrene–(styrene/butadiene)–styrene triblock copolymer blends

Abstract

The crack toughness behaviour of nanostructured polymer blends based on two SB triblock copolymers with different molecular architectures were studied, where one component is a thermoplastic (LN3) and the other a thermoplastic elastomer (LN4), using the essential work of fracture (EWF) concept. The crack resistance behaviour was correlated to the morphological features from transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) and to the phase miscibility characteristics as revealed from the nature of the tan δ peak in dynamic mechanical analysis curves. Swelling of the polystyrene-layer thickness with increasing LN3 content could be observed from the fast Fourier transform analysis of the TEM images. An increase in the crack toughness behaviour between 60 and 80 wt.% LN3 was observed and is attributed to a change from cylindrical to lamellar morphology, as revealed from the magnitude of non-EWF ( βw p) and the crack resistance ( R-curves) curves. The kinetics of stable crack propagation is discussed with respect to deformation mechanisms, post-yield crack-tip blunting and stable crack propagation behaviour. It was observed that the crack-tip opening displacement rate was more sensitive to phase miscibility. R-curves analysis, in this study, fundamentally establishes that, while crack initiation (increases linearly with the increase in LN3 content) and crack propagation (increases as the morphology changes from cylindrical to lamellar) are dependent on composition and morphology, respectively, the crack propagation stability (d δ/d a) is highly sensitive to phase miscibility. The time-resolved crack propagation studies offer new dimensions to understand the kinetic aspects of fracture behaviour, while the strain field analysis explains the time-dependent deformation behaviour to characterize the time dependence of the strain energy dissipation modes.