Kinetics and dynamics of thermally-induced shape-memory behavior of crosslinked short-chain branched polyethylenes

Systematic investigation of the relation between shape-memory (SM) behavior and characteristics of the covalent network and the crystalline domains of a crosslinked polymer, i.e., crosslink density and crystallinity, respectively, was performed using homogeneous ethylene-1-octenecopolymers (EOC) as model polymers. The EOCs have been crosslinked by 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (DHBP) decomposition. Two EOCs with a degree of branching of 30 and 60 hexyl side chains per 1000C atoms with each four different crosslink densities were employed. The investigated EOCs differ significantly in crystallinity, melting temperature (T_m) and crosslink density. The crosslinked EOC undergone the programming at a strain of 100% showed high strain fixity ratio (R_f) and strain recovery ratio (R_r). The R_f and R_r values increase with increasing crystallinity and crosslink density, respectively, and decline only slightly in a subsequent SM cycle. The switching temperature (T_sw) is strictly related to T_m and decreases with increasing degree of branching as well as crosslink density in the temperature range of 101–63 °C. T_sw remains nearly unchanged when the programming temperature (T_pr) or the load during SM recovery is varied. The kinetics of SM recovery as characterized by the temperature dependence of recovery rate is controlled by the melting behavior. The specific work generated by the programmed specimen during thermally-induced recovery under constant load, gains with increasing crosslink density, and is proposed as dynamical characteristic of practical relevance. The opportunity of tailoring T_sw by variation of the degree of branching and crosslink density makes such polymers attractive candidates for applications requiring T_sw temperatures in the range from 60 to 100 °C.