Strain-induced crystallization in polyethylene terephthalate

Although crystallization under stress occurs in nearly all synthetic fiber making, giving fine fibrillar fiber structures, it has been relatively little studied; and the literature, centered mainly round coarser spherulitic crystallization, does not take us very far in understanding it. However, a simple experiment shows that it is very important. If a filament of amorphous polyethylene terephthalate is stretched over a hot plate between two roller moving at different speeds, stretching can take place either without crystalization at low tension, or with considerable crystallization at high tension, at the same temperature and speed, simply according to the method of starting the machine. A model visco-elastic theory based on Maxwell's equation is, therefore, developed to describe this behaviour using the simplest possible assumptions. Further experiments then show that this simple and partly empirical theory fits the observed behaviour of polyethylene terephthalate reasonably well, and the simple assumptions made, therefore, seem to give a first approximation to the course of strain induced crystallization. They correspond with the idea of exremely rapid crystallization from a nucleus to a small structural unit, the density of nucleation being dependent on the stress. Parallel work on the streching of spherulitic structures is then mentioned, and it is finally suggested that stress induction and rapid growth to a small structural unit may be common to both spherulitic crystallization under internal stresses, and fibrillar crystallization under an externally applied stress.