AbstractA study was made of the structural and property changes that occur when poly(ethylene terephthalate) films are oriented, crystallized, and thermally relaxed. X‐ray diffraction, infrared, density, and stress‐strain measurements were used to develop information on the amount and direction of orientation of both the crystalline and amorphous regions of these films. Three structural factors influence resultant film properties: (1) the kind and perfection of crystallite orientation, (2) the amount and direction of thetrans‐gaucheisomerization in the amorphous regions, and (3) the amount of crystallinity. Interactions of these factors result in substantially different film properties and relaxation behavior. Of special significance is the influence of “taut” extended amorphous regions produced by orientation which generate useful structure‐property
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