PHOTOCHEMICAL SENSING OF SEMICRYSTALLINE MORPHOLOGY IN POLYMERS - PYRENE IN POLYETHYLENE

Pyrene was inserted into a low-density polyethylene matrix. Fluorescen ce spectra as a function of temperature and the differential scanning calorimetry (DSC) trace were recorded simultaneously. Along with the u sual vibronic bands, a low-intensity band at 365 nm appears at higher energies with respect to the 0-0 transition in the pyrene fluorescence spectra. The fluorescence intensity of this small band increased with temperature, and an isoemissive point was observed to occur at 368 nm . This emission was interpreted as arising from pyrene molecules locat ed in the outer rigid interfacial region of polymer crystallites. Its temperature-dependent fluorescence was interpreted in terms of electro n-phonon coupling; two phonons which coincide with fundamental vibrati ons of polyethylene were necessary to fit experimental data. Coupling with a high-energy phonon was possible at low temperature, whereas, ab ove the beta relaxation temperature, phonon coupling occurs with a low er-energy phonon. The a relaxation was detected as a maximum in fluore scence intensity since above its characteristic temperature, nonradiat ive processes begin to operate.