Infrared studies of chain folding in polymers. II. Polyethylene terephthalate

Abstract

An infrared method has been developed for analyzing the spectra of a semicrystalline polymer for bands arising from the folded chain. The basis of the assignment is the contrasting behavior of crystalline and fold bands during annealing and degradation. The method has been applied to polyethylene terephthalate (PET), and as a result the 988-cm-¹ band has been assigned to a fold conformation. This assignment was utilized in studying the effects of crystallization, annealing, and deformation upon folding in PET. Melt- and solution-crystallized PET exhibit a high degree of folding, as measured by the 988-cm-¹ band. Studies of crystallization by annealing the glass indicate multiple modes of folding. Strain-induced crystallization produces no regular folding, as measured by the 988-cm-¹ band, but heat setting does. Deformation of crystalline samples containing folds disrupts the regular fold, which re-perfects itself upon subsequent annealing. Folding in polymers has been the subject of much discussion and speculation, particularly in conjunction with the intensive work being done in the areas of polymer crystallization and morphology [1]. Much of the controversy rests on the inability to directly detect the presence or absence of the folded chain or to determine its structure.