Translocation of fluorescent probes upon stretching low-density polyethylene films. Comparison between ‘free’ and covalently-attached anthryl groups

Abstract

Fluorescence from (9-anthryl)methyl groups (AN-C) attached covalently to polymer chains has been used to study secondary α-, β- and γ-relaxation processes in two types of stretched and unstretched low-density polyethylene (LDPE) with different degrees of crystallinity and frequencies of branches in the temperature range 55–400 K. Plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths versus temperature indicate that the efficiency of radiationless deactivation processes is very sensitive to film stretching (i.e. micromorphological changes induced by macroscopically applied stress) and to secondary relaxation processes of LDPE (i.e. micromorphological changes induced by temperature). Also, since the efficiencies of intermolecular radiative energy transfer processes in LDPE suffered by AN-C and ‘free’ anthracene (ANH), doped into native films, are much different, the mode of ‘reporter’ incorporation into a film and its treatment thereafter can establish differing local concentration gradients. The results suggest that, although stretching reduces, on average, the amount of free volume available at guest sites and partially orients both AN-C and ANH probes, the AN-C (due to their being constrained to their original positions of incorporation) are more influenced by polymer relaxation processes, and the excited singlet states of both probes are more sensitive to temperature in stretched films than in unstretched ones. Overall, comparisons between the behaviour of the covalently-attached and free lumophores provide insights into how external perturbations, such as stress and temperature changes, affect specific aspects of polymer microstructure and mobility and what is the influence exerted by subtly differing environments on the photophysical properties of the probe.