The microstructure of a polyethylene terephthalate matrix near to a void under uniaxial draw

Abstract

Under the appropriate conditions, a void can generate within a polymer matrix, when it is subjected to large strains. In this work, the local microstructure of a polyethylene terephthalate (PET) matrix near to such a void was studied through the use of a model system. The approach adopted was to mimic the void growth as a function of uniaxial stretching using a macroscopic sample of PET film, and evidence suggests that the PET matrix around the model void does possess structural features similar to those of a PET matrix which surrounds a microscopic void. Two separate but complementary methods, namely numerical prediction and direct measurement, were employed to characterise the matrix immediately adjacent to the void. The structure of the PET matrix was predicted by combining a previously established relationship between strain and molecular orientation, and strain distribution measured on the model sample, while direct measurements were performed using infrared and Raman spectroscopy. The results obtained demonstrated that a very varied and complicated structure exists in the material immediately surrounding a void, which is formed as a result of stretching. Uneven deformation due to expansion of the void results in a shell of polymer of high molecular orientation and crystallinity at its boundary. On continued drawing, the shell resists further deformation leading to a slower rate of void growth.