Characterization of the non-crystalline phase of oriented poly(ethylene terephthalate) by chain-intrinsic fluorescence

Abstract

We have investigated the suitability of the chain-intrinsic fluorescent properties of poly(ethylene terephthalate) for characterizing ‘amorphous orientation’ in semicrystalline films and fibres of this polymer. By developing an apparatus which permits the polarized fluorescence to be measured in the optical axis, we have eliminated possible sources of measurement error. Our results provide further evidence that the increase in fluorescence intensity at high draw ratios arises from light scattering between crystallites and not from an increase in the population of fluorescent dimers. This cause of intensity increase would have no influence on the orientation function, and one can correct for any significant depolarization due to scattered light. For hot-drawn film, the amorphous orientation data obtained from intrinsic fluorescence provide a consistent explanation of the influence of strain rate on the induction and development of crystallinity. For fibres, our intrinsic fluorescence data confirm that high speed spinning results in substantially lower amorphous orientation than spin-drawing, and that amorphous orientation goes through a maximum as a function of spinning speed. These fluorescence results are in very good qualitative agreement with data we obtained from a birefringence/X-ray method; quantitative differences seem to arise from the choice of values for intrinsic birefringence ( Δ o c and Δ o a) required in the latter method. Determination of Δ o c and Δ o a using amorphous orientation data from the fluorescence method provides support for the argument that these values are not truly intrinsic, but are dependent on the structural state of the crystalline and amorphous regions.