Morphology of uniaxially oriented poly(ethylene terephthalate)

Abstract

AbstractThe morphological character of uniaxially oriented poly(ethylene terephthalate) (PET) films was investigated as a function of draw ratio. Dynamic mechanical, infrared, and crystallite‐size measurements were made on the samples. In addition, selective degradation experiments and molecular weight determinations were employed. The dynamic mechanical measurements indicated a sharp decrease in irregular folds for draw ratios of 3.0 and higher, which also coincided with the essentially complete disappearence of regular folds (from the 988 cm−1band in the infrared spectra) in unannealed samples. Infrared studies of drawn samples annealed under different conditions gave evidence in support of a structure in which the chains are stretched out. Apparent crystallite‐size measurements showed a sudden increase in length of the crystals in the direction of the draw beyond a draw ratio of 3.0. Molecular weight measurements showed a large increase in average chain length in the residue after selective degradation of amorphous material and folds; undrawn and slightly drawn samples gave a much lowerMn. Based on these observations, it is postulated that for higher draw ratios and present drawing conditions, the crystals are of the straight chain type, somewhat similar to the fringed‐micelle crystal concept.