Characterization of static and dynamic properties of a low-density polyethylene. Evidence for translocation of fluorescent probes upon stretching and determination of relaxation processes at the mesoscopic level

Abstract

Fluorescence from anthracene guest molecules (AN) has been used to study secondary α, β and γ relaxation processes in unstretched and stretched low density polyethylene (LDPE) in the temperature range 15–400 K. The LDPE was characterized by several different techniques. It contains about 4 vinylidene, vinylene and vinyl type CC bonds and about 320 CH3 groups per 10 000 CH2 units. The unstretched polymer is 31% crystalline and has a melting point, Tm, of 109°C. The 5x stretched polymer is 51% crystalline and Tm = 110°C. Although the degree of crystallinity of the films suffers a large change upon macroscopic stretching, the melting temperature (determined by d.s.c. and fluorescence spectroscopy) and the relaxation temperatures do not. However, plots of total fluorescence intensity or fluorescence intensity ratios at different wavelengths vs temperature indicate that dynamic processes involved with the excited singlet states of anthracene are dependent upon polymer stretching. These results, in combination with others obtained previously, suggest that stretching reduces the average amount of free volume at guest sites and induces a translocation of anthracene molecules toward sites at the crystalline-amorphous interfaces.