Dynamic Crosslinking of LDPE by Nitroxide Radical Coupling of a Dicyclopentadiene Dicarboxylic Acid and Its Dynamic Properties

LDPE was crosslinked with novel dynamic or conventional crosslinking agents during melt processing. Both crosslinkers were synthesized by the esterification of Thiele's acid or adipic acid with 4-hydroxy-TEMPO. ¹H-NMR showed that a temperature of 170 °C and a reaction time of 24 min are required for a successful crosslinking. The concentrations of crosslinking agents were 1.45, 2.9, and 5.8 mol%. Conventionally crosslinked LDPEs show a decrease in soluble content in hot xylene with increased crosslinker concentrations, while dynamically crosslinked LDPEs show no change after thermal treatment, indicating the scission of dynamic crosslinks. The rheology of both crosslinked LDPEs at 130 °C shows that the stress release is slower than that of neat LDPE, confirming crosslinking, while at 170 °C a shift in the stress release and also a shift in the flow properties of dynamically crosslinked LDPE towards those of neat LDPE are observed, both indicating the cleavage of dynamic crosslinks. Compared to neat LDPE, the mechanical properties of both crosslinked LDPEs show an increase in Young's modulus and tensile strength and a decrease in elongation and creep when the concentration of both crosslinkers is increased. By increasing the processing temperature to 170 °C, the crystallinity index decreases, leading to a rather small improvement in the mechanical properties.