Abnormal brittle-ductile transition for glassy polymers after free and constrained melt stretching: The role of molecular alignment

Abstract

By comparing the brittle-ductile transition (BDT) behavior between uniaxial width-Free Melt pre-Stretched (FMS) and width-Constrained Melt pre-Stretched (CMS) poly(methyl methacrylate) (PMMA), the effect of molecular alignment on the mechanical response of glassy polymers has been investigated. Unexpectedly, during further mechanical characterization, CMS samples exhibit higher ductility than FMS samples along both the machine (stretching) direction (MD) and the transverse direction (TD). Combining the results from molecular orientation and stress relaxation, it is proposed that chain stretching and aligned chain density is responsible for the ductile deformation along the MD and TD, respectively. Along the MD, CMS samples exhibit more significant chain stretching than FMS samples due to width constraint, leading to a lower chain activation energy barrier in CMS samples. Upon further deformation, chain activation is then accelerated and shear yielding can be achieved at lower temperature. Along the TD, the orientation function remains constant for CMS samples but negatively increases for FMS samples, and the lower aligned chain density in FMS samples increases the activation energy barrier for plastic deformation. As a result, crazing prevails during further deformation for FMS samples and causes weaker ductility. The mode of BDT in glassy polymers is considered from multiple scales.