Flexible methylene chain length in crosslinked networks as a measure of the brittleness of γ-irradiated low density polyethylene sheet

Abstract

Low density polyethylene (LDPE) sheets, untreated and pre-crosslinked by thermal reaction with dicumyl peroxide (DCP) in the molten state, have been γ-irradiated with doses up to 200 Mrad at room temperature in air. The elongation at break ( γ B ) of both materials was decreased by the irradiation. The radiation resistance of pre-crosslinked LDPE, as measured by the characteristic dose which leads to a 50% decrease in γ B , ( D 1 2 = 110 Mrad ) was 2·6 times greater than that of untreated LDPE ( D 1 2 = 42 Mrad ). The gel fraction (f g ) of untreated LDPE was increased by the irradiation, while that of pre-crosslinked LDPE with an initial f g value of 0·803 showed negligible change. The density (d p ) of both LDPEs increased and the weight swelling ratio (S w ) of the polymer network decreased with increasing radiation dose. The changes in these quantities for untreated LDPE were more rapid than for pre-crosslinked LDPE. The γ B value was universally correlated with both the d p and S w values, whether the samples were of untreated or pre-crosslinked LDPE. Characteristic values of d p and S w associated with a 50% decrease of residual γ B value for both LDPEs were obtained as 0·924 g/cm 3 and 4·3, respectively. The molar concentration of crosslinks (Φ) in the irradiated LDPE sheets was evaluated from the S w value by the use of Flory's theory. The flexible methylene chain length between two crosslink points, derived from the Φ value, was shown to be a universal molecular measure of the brittleness of irradiated LDPE sheets.