CHAIN SCISSION IN POLYSTYRENE BY IMPACT FRACTURE

The number of chain scissions n(s) per unit fracture area by impact in high-molecular weight polystyrene is determined to be approximately 3.3 X 10(14)/cm2 at room temperature. This is almost 20 times larger than would be expected if chain scissions took place only at, or very close to, fracture surfaces. This result was obtained by measuring the molecular weight decrease and the total fracture area of the impact fragments by using size exclusion chromatography and statistical particle size measurements, respectively. The large n(s) strongly indicates that significant chain breakage occurs during crazing before the propagation of cracks. An average craze thickness before breakdown under impact is estimated from n(s) to be around 2-mu-m. In a diluted polymer, n(s) is found to be significantly lower than the extrapolated value, assuming a linear dilution of entangled chain crossings at the fracture surface. This low chain scission density, however, can be explained by taking into account the reduction of craze breakdown strain in the diluted polymers. Finally, the broken chain ends of polystyrene appear to be stable under ambient conditions.