A study of the orientations of fluorescent molecules incorporated in uniaxially oriented poly(ethylene terephthalate) tapes

Abstract

The theory of the polarized fluorescence from fluorescent molecules incorporated in an oriented solid polymer is developed for a very general model of the optical behaviour of the molecules and it is concluded that any observed intensity can be expressed in terms of the components of a fourth rank tensor. Only twelve of these components are independent for samples of orthotropic symmetry and only six for samples of uniaxial symmetry. The effects of the birefringence of the polymer and the significant polarization-dependent absorption of the exciting light on the observed intensities are considered. The results of the analysis are presented for the uniaxial case in a form which allows values of cos 2θ and cos 4θ to be calculated from the experimental measurements of intensity. θ is the angle between the unique axis in the polymer and a unique axis in a fluorescent molecule and the bars denote the averages over the sample. The theory has been applied to molecules of a stilbene derivative (VPBO) incorporated at concentrations of 50–200 ppm in poly(ethylene terephthalate) (PET) of low crystallinity. It is concluded that the unique axes of the VPBO molecules are more highly oriented than the PET chains, but the two distributions of orientations appear to be uniquely related over the range of drawing temperatures (65–90°C) and draw ratios (1–5·85) used.