Crazing Micromechanism in Glassy Atatic Polystyrene and Its Blends with Poly(2,6-dimethyl-1,4-diphenyl oxides) by AFM

摘要

Crazing in glassy polymer thin films was found to follow a micronecking process similar to that of local shear deformation in ductile polymers. The void fraction in the fully necked craze region was determined, and a close-packed fibril structure was concluded. The local stress and strain within the craze were obtained from AFM topographic data by the Bridgman's plasticity analysis. The stress/ strain curve of craze fibrillation was subsequently determined where an apparent strain softening was found in the initiation of fibrillation, which was then followed by strain hardening as fibrils were drawn into the neck region. Strain rate was found to peak at the craze boundaries, consistent with the surface drawing mechanism from TEM results. During craze fibrillation, the local strain rate of the drawn polymer increased with drawing strain until a critical strain Eh was reached; beyond that, the strain rate decreased with the strain. The critical strain Eh, identified as the onset of strain hardening, was found to decrease with entanglement density Ve in the low-ve, craze-forming regime but become a constant in the high-ve regime where crazing was replaced by shear yielding. The transition between crazing and shearing is controlled by the tendency of strain localization in the entanglement network.