On the mechanism of craze fibril breakdown in glassy polymers

• Published in: Macromolecules 1990-05, Vol.23 (11), p.2926-2934
• Main Author: Berger, Larry L
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Mechanical properties; Physical properties; Polymer industry, paints, wood; Properties and testing; Technology of polymers
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma00213a019

Optical and transmission electron microscopy were used to investigate the mechanism of craze fibril growth and breakdown in a series of neat glassy polymers of poly(methyl methacrylate) (PMMA), poly( alpha -methylstyrene) (P alpha MS), and polystyrene (PS), as well as in fully compatible blends of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and PS, all at a homologous T sub g --T = 75 deg C. The neat polymers were examined over a broad range of molecular weight M, and the PPO--PS system was studied as a function of the weight fraction of PPO ( phi ) in the blend. During a slow strain rate test, in which approx 40 independent film squares were simultaneously monitored, the median tensile strains at which crazing, epsilon sub c , and craze fibril breakdown, epsilon sub b , occurred in each polymer were determined. For the neat polymers, with M < M sub c (where M sub c is the critical molecular weight for entanglement effects on the zero-shear rate viscosity) no stable craze formation was observed (i.e. epsilon sub b -- epsilon sub c approx 0); for M = approx (2-20)M sub c , epsilon sub b -- epsilon sub c increased strongly with M and at M > 20M sub c , epsilon sub b -- epsilon sub c increased only weakly with increasing M. For the PPO--PS blends, epsilon sub c was found to be roughly constant for 0.5 < phi < 0.68 whereas epsilon sub b -- epsilon sub c increased markedly at values of phi = > 0.64. In all cases, craze fibril breakdown was traced to the formation of a small pear-shaped void at the craze /bulk interface. The morphology and statistics of the craze breakdowns were combined with a detailed description of the craze microstructural parameters (i.e. the craze fibril diameter and craze fibril spacing) to advance a molecular model of craze breakdown. At these strain rates, craze breakdown is believed to occur by two events; random chain scission to form the surfaces of the craze fibrils, and stress-mediated chain disentanglement of a group of surviving strands at the craze/bulk interface. The predictions of this model are in satisfactory agreement with the empirically determined molecular weight (or phi ) dependence of ( epsilon sub b -- epsilon sub c ). Graphs, Photomicrographs. 42 ref.--AA