THE EFFECT OF BIAXIAL ORIENTATION AND CRYSTALLINITY ON THE LONG-TERM CREEP-BEHAVIOR OF POLY(ETHYLENE-TEREPHTHALATE) FILMS BELOW GLASS-TRANSITION TEMPERATURE

The use of thin polymeric films in applications such as flexible circuit boards and dish membrane solar collectors has been gaining popularity. In these and many other applications the films are used under constant loading conditions which subjects them to long‐term creep. In this paper, we present detailed experimental tensile creep results on unoriented films of varying crystallinities and unequal and equal biaxially oriented poly(ethylene terephthalate) (PET) films. The results indicate that the increasc of crystallinity, stretch ratios, and annealing causes reduction in longterm creep strains. Unequal biaxially stretched films exhibited in‐plane anisotropy in their tensile creep behavior. In these films the lowest creep strains are observed in the direction along which the film stretched to the highest stretch ratio. Copyright © 1990 John Wiley & Sons, Inc.