Crazing in Thin Films of “Monodisperse” Polystyrene
A technique to study the relationship between mechanical behavior and craze microstructure of thin films of narrow disperse polystyrene is described. Films about 2-4 μm thick were tested in a conventional tensile machine (Instron). Craze kinetics were observed through an optical microscope equipped with a motor-driven camera. In this way, a direct correlation was established between the mechanical behavior and craze growth kinetics. Effects of molecular weight, thermal history and strain rate on both mechanical behavior and craze kinetics were studied, and their relationship was elucidated. Statistical analysis revealed that the phenomenon of crazing is quite complex and depends on several material and experimental variables. The average distance between crazes was found to follow Poisson distribution, thus confirming the random nature of this phenomenon. The macroscopic strain (elongation) of the material during the entire deformation history was explained in terms of the number of crazes per unit area, craze extension and the distribution of crazes. On the basis of experimental results, a model is suggested from which the elastic modulus of crazed material can be derived.
KeywordsHigh Strain Rate Thermal History Lower Strain Rate Molecular Weight Polymer Molecular Weight Sample
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