Crosslink density dependence of polymer degradation kinetics: Photocrosslinked acrylates

A series of crosslinked polyurethane acrylate films with glass transition temperatures ranging from −49 °C to +65 °C was prepared by photopolymerization of solvent-free resins. The kinetics of thermo-oxidative (in air) and thermal (in N2) degradation of these crosslinked acrylate networks at temperatures from 100 °C to 400 °C was monitored thermogravimetricly as a function of crosslink density. Initial degradation rate of polyurethane network decreased with the increase of crosslink density. Apparent activation energies of degradation were found to be temperature and crsosslink density dependent and ranged from 12.6 kJ mol−1 to 25.1 kJ mol−1 in 200–300 °C interval and 33.5 kJ mol−1 to 58.6 kJ mol−1 in 300–400 °C interval. The polyacrylate thermal stability increase with crosslinking was correlated with decreased chain segments mobility leading to the reduced oxygen and volatile products diffusion rate, rate of cyclic decarboxylation, and increased radicalʹs recombination in a cage. Data indicated that the cyclic decarboxylation, rather than oxygen uptake or volatile products escape rate control the thermal degradation kinetics.