FRACTURE SURFACES OF POLYSTYRENES HAVING DIFFERENT MOLECULAR WEIGHTS

Compression moulded samples of narrow molecular weight distribution polystyrene have been fractured and the fracture surfaces studied by electron micrography. Three different molecular weight polymers were used with molecular weights of 35 000; 82 000 and 220 000. The lowest molecular weight polymer gives a generally smoother fracture surface than the other polymers, a characteristic which is ascribed to the virtual absence of entanglement. When broken at -196°C, all three polymers give nodular areas and the nodule size is observed to depend on the molecular weight. The actual sizes are of the same order as the expected end-to-end molecular distance in a theta solvent. When fractured at room temperature the high molecular weight material, and also a commercial wide distribution polymer of similar molecular weight, gives rise to very small, highly extended filaments. It is suggested that these filaments are formed by the rapid, adiabatic extension of material which has been heated and softened during the propagation and separation of a craze. However, although these features are well developed and indeed striking, the electron micrograph information by itself does not unambiguously prove that melting has occurred, although it certainly supports this concept. Neither can it be unequivocally shown that the fibrils are solid and not hollow. © 1969.