Molecular orientation in hot-drawn poly(ethylene terephthalate) films as determined from the intrinsic polarized fluorescence

The groundstate-stable dimers in the non-crystalline regions of uniaxially drawn poly(ethylene terephthalate) (PET) films were used as chain-intrinsic fluorescent labels for studying the orientation distribution in the non-crystalline regions. As far as indicated by the spectra and the fluorescence decay law, the fluorescent group remains unchanged when the sample is uniaxially oriented by drawing above the glass transition temperature. In contrary to the behaviour of physically incorporated probe molecules, the orientation coefficient f ₂ ^F of the dimers is proportional to the amorphous orientation coefficientf ₂ ^A ; concerning the fluorescence signal, amorphous includes all the material outside the perfect crystal.During deformation, the orientation coefficientf ₂ ^F follows approximately a superposition curve of crystallite-like orientation, separable in the initial range of stretching ratio <2.5, and of true-amorphous orientation of the affine network type that becomes noticably at >3.At temperatures closely aboveT _g, and within the selected range of stretching parameters, the fluorescence intensity of PET remains nearly constant with increasing stretching ratio; at >2.5, where the maximum crystallite orientation is achieved and the increase of amorphous orientation becomes noticably, a slight augmentation of the dimer concentration is observed.