A CHAIN-INTRINSIC FLUORESCENCE STUDY OF ORIENTATION-STRAIN BEHAVIOR IN UNIAXIALLY DRAWN POLY(ETHYLENE-TEREPHTHALATE) FILM

The chain-intrinsic fluorescence technique was used to investigate the development of orientation in the noncrystalline phase of PET film du ring drawing at 90 degrees C. It was confirmed that orientation develo ps faster at higher strain rate. In the low draw ratio (precrystalliza tion) regime, the orientation-strain data at high strain rate can be f itted to the affine network model, but at low strain rate, where relax ation effects are substantial, this model is not applicable. We found considerable deviation from the linear stress-optical law: it appears that a stress threshold must be reached before significant orientation takes place and that the threshold value increases with strain rate. Crystallization onset seems to increase the rate of development of ori entation at the lower strain rate, probably by providing additional ju nction points. At both high and low strain rates, however, there event ually occurs an abrupt decrease in the rate of development of orientat ion (and crystallinity) which coincides with a sharp increase in stres s. By comparing our data to some results in the literature on uncrysta llizable PEMT, we infer that the slowing of orientation development ar ises from chain slippage and that the increase in stress largely resul ts from the reinforcing effect of crystallites and an increase in poly mer viscosity arising from the interconnection of crystallites.