KINETICS OF CRAZING IN POLYBUTADIENE POLYSTYRENE BLENDS

The crazing behavior of blends of polystyrene (PS) and a low molecular weight polybutadiene (PB)was examined as part of a continuing study o f toughening mechanisms in thermoplastic polymers. These PS/PB blends attain high levels of toughness from the stress-activated plasticizing action of the polybutadiene (PB), a mechanism that is active only in the region of a growing craze. The plasticization is therefore localiz ed and leads to enhanced toughness without loss in stiffness. The net result of this plasticization is a reduction in craze flow stress acco mpanied by an increase in craze velocity, which, in turn, allows the s pecimen to reach substantial strains-to-fracture in uniaxial deformati on under an imposed strain rate. The ability of the PB to plasticize a growing craze is expected to be a function of the mobility of the PB. To investigate the role of diluent mobility, tensile tests and craze velocity measurements were conducted at -20-degrees-C and compared wit h previously published results collected at 23-degrees-C. Although the blends displayed high levels of toughness at 23-degrees-C, the blends tested at -20-degrees-C showed brittle behavior. Craze velocities mea sured at -20-degrees-C were 2 orders of magnitude lower than the resul ts at 23-degrees-C. Addition of 3 wt % PB at -20-degrees-C led to craz e velocities only as large as those found in pure PS at 23-degrees-C. Comparison of the craze velocities with an asymptotic theoretical mode l describing the dependence of the craze velocity on the PB content sh owed good agreement with the results at -20-degrees-C and only fair ag reement with the room-temperature results. (C) 1994 John Wiley & Sons, Inc.