The advantages of bulge testing as a method of mechanical property analysis for polymer films was described. Deformation and rupture mechanisms of glassy polymer films during bulge testing were investigated by in situ molecular aggregation structure analyses. Heat-pressed polycarbonate (PC, weight-average molecular weight (Mw) = 38k) and poly(methyl methacrylate) (PMMA, Mw = 70k and 1.2 M (PMMA-70k and PMMA-1.2 M)) films were employed as transparent glassy polymers. Evaluations of molecular aggregation structures during bulge testing were conducted by polarized high-speed camera observation and wide-angle X-ray diffraction (WAXD) measurements. PC and PMMA-1.2 M films showed a yield point in their stress-strain curves, indicating ductility, while PMMA-70k film showed brittleness. WAXD measurements revealed that the distances between the main chains slightly increased with increasing strain. With increasing film strain above the yield point, a craze-like morphology with thinning of the film was seen for the PC film, likely due to the flexible PC molecules. Although it is difficult for PMMA molecules to orient along applied stress, a transition to the thinner state with the formation of wrinkled structures occurred for the PMMA-1.2 M film above the yield point. The transition to the thinner state, which corresponds to a discontinuous decrement of the cross-section area by deformation, might be categorized as two-dimensional crazing and/or shear deformation. PMMA-70k did not show such behavior because of the small number of entanglements. The use of a small amount of films in terms of thickness and size and the capability of observation of the entire film were given as advantages of the bulge test.
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