Dependence of relaxation processes in a low-density polyethylene with different crosslink densities investigated by fluorescence spectroscopy

Relaxation processes of one type of low-density polyethylene with three different degrees of crosslinking were studied using the fluorescence of 1-pyrenyl groups attached covalently to polymer chains. The average molecular weights between crosslinks (M_c) of the polyethylenes were 125, 76.5, and 7 kg mol⁻¹. As assessed by changes of the integrated fluorescence intensity and the full width at half maximum of the 0-0 emission band with temperature, chain mobility decreases with increasing crosslinking density. Chain mobility in the polyethylene with the smallest M_c value appears to be strongly inhibited up to 220 K, while motions of small segments of the polymer chains were observed for samples with lower crosslinking degrees at temperatures similar to those of the pristine (non-crosslinked) polymer. These data are discussed in terms of local versus bulk properties of the films.