Book Description
Polymer glasses are important engineering materials and undergo various mechanical instabilities in service such as crazing, brittle failure, shear yielding and necking. Although our understanding of these mechanical instabilities starts from 70 years ago, most of previous works borrow the treatment from solid mechanics. A molecular level understanding of these nonlinear mechanical behaviors is still missing and challenge. In this dissertation, we utilize the hybrid structure proposed recently to understand the various mechanical instabilities as well as the origin of the stress in the post-yield region. Different from previous treatments of emphasizing the inter-segmental contribution in stress, we are mainly focused ourselves on the role of chain network (intra-segmental contribution). Our experimental data shows intra-segmental contribution plays a deterministic role in the nonlinear region. Specifically speaking, the existence of chain network ensures the success of load transfer during deformation, which helps the glass to tolerate structural heterogeneities. A sufficient strong chain network could completely suppress crazing and necking, and it could also delay the brittle-ductile transition to toughen the material. On the other hand, we further elucidate the role of chain network by studying the nature of the stress in the post-yield region. Thermal shrinkage and elastic yielding phenomena of cold drawn polymer glasses are studied. Based on a systematic study of the phenomena, we conclude there is a portion of elastic stress (intra-segmental stress) in the post-yield stress and this elastic component comes from the distortion of covalent bonds in the chain backbone.
