β- and γ-relaxations of low density polyethylene: Study by fluorescent probes

The temperatures of γ- and β-relaxation processes in low density polyethylene are studied by means of fluorescence spectroscopy. In this work, the fluorescent sensors employed, p-vinyliden derivatives of N,N-dialkylaminoaryl compounds, were adsorbed in polymer films, and their fluorescence analysed over the temperature range from −200 to 0 °C. In general, the fluorescence of the probes adsorbed in polyethylene decreases as temperature increases, as a consequence of the enhancement of the free volume fraction that favours the radiationless process of the lowest excited singlet state. Slope changes are observed in the temperature ranges −160 to −130 and −70 to −30 °C, attributed to the γ- and β-transitions, respectively. All fluorescent probes exhibit higher sensitivity at β- than at γ-relaxation. Since the former involves longer range segmental motion, and a greater decrease in rigidity of the probe surrounding allows the amino group rotation around the NCAr bond to happen in probes. The results correlated well with those obtained by means of dynamic mechanical analyses, which indicated the ability of these molecular rotors to sense the weak differences in chain mobility due to β- and γ-transitions of LDPE.