Understanding the brittle-ductile transition of glass polymer on mesoscopic scale by in-situ small angle X-ray scattering

The structural evolution of poly (methyl methacrylate) (PMMA) under uniaxial stretching has been studied with in-situ synchrotron radiation small angle X-ray scattering (SAXS) to reveal the brittle-ductile transition (BDT) mechanism. The scattering invariant analysis method was applied to semi-quantitatively analyze the evolution of crazing. For the sample without melt pre-stretching, which exhibits almost the same BDT behavior in different directions. The formed crazes are enabled to grow and propagate stably via shear deformation of the matrix. While for samples with melt pre-stretching, crazing is further promoted when stretching along the transverse direction. With a higher craze growth rate and lower craze fibril fraction, the destructive crazing cannot be fully stabilized by shear banding and premature craze fracture occurs. In contrast, shear banding dominates the deformation when stretching along the pre-stretching direction. Due to the low craze concentration and slow craze growth rate, crazes with higher fibril fraction gradually slip with matrix shearing. The premature craze damage is consequently avoided and an enhanced toughness is achieved. The structural evolution of these samples indicates that the kinetic competition between crazing and shear banding is the key to the macroscopic toughness of glass polymers.