Effect of boiling and stretching on the optical and mechanical properties of polyester fibres
PET fibres, having an irregular cross-sectional shape, were boiled for different intervals of time and stretched using a stress-strain device, then examined by a two-beam interferometric method. Refractive indices and birefringence were measured. The results were used to calculate virtual and isotropic refractive indices, strain and stress optical coefficients and other structural parameters for various strain values. Also, the number of network chains per unit volume, N c , the work per unit volume, W, the average work per chain, W', the storable elastic energy of the network, W, the reduction in entropy, AS, and the number of crystals per unit volume, Y, were determined. These parameters changed on boiling and stretching, and reorientation occurred. An empirical formula is suggested to correlate the change in certain parameters with the draw ratio, and its constants are determined.