Craze formation in long chain branched poly(styrene) as revealed by in situ transmission electron microscopy: Influence of deformation temperature

摘要

The craze structure of linear and long chain branched poly(styrene) (PS) was investigated as a function of temperature by means of in situ transmission electron microscopy (TEM). At room temperature, high entanglement density and a degree of molecular mobility result in homogeneous crazes for branched PS and fine fibrillated crazes for linear PS. With increasing temperature up to near (but still below) the glass transition temperature, the mobility of polymer chains increases, longer units of macromolecules are mobile, and disentanglement becomes more probable. This behavior results in a transition from the formation of homogeneous crazes to fibrillated crazes in long chain branched PS, and in the formation of coarser fibrils and greater distances between the fibrils in both linear and long chain branched PS. The increase in the deformation rate has the opposite effect on the craze structure compared to the temperature increase. After breaking the craze fibrils, a high stress concentration is suddenly produced in the active zone, at the tip of the craze, and the entanglements act as chemical cross-links in the short time available which restricts the mobility of polymer chains, reduces disentanglement, and hence increases the probability of homogeneous crazes formation. [References: 27]