Polymer stress relaxation in biaxially stretching-induced crystallization

Biaxially stretching-induced polymer crystallization is a common yet fundamental process in the industrial processing for plastic thinfilms, bottles and foams, in which polymer stress relaxation plays an important role. By means of dynamic Monte Carlo simulations, we have previously investigated the role of polymer stress relaxation in uniaxial stretching-induced crystallization under various strain rates and temperatures. Hereby we extended our parallel study to biaxially stretching-induced crystallization and compared the results also to the cases without stress relaxation. At high temperatures, we observed a significant retardation of onset strains for biaxial crystallization, which could be attributed to the retardation of intermolecular crystal nucleation due to an enhanced stress relaxation among cross-stretching polymers. In consequence, fewer crystallites are generated to yield larger crystallites with a higher content of chain-folding. The more chain-folding favors a continuous biaxial stretching towards ultrathin polyethylene films with thickness down to 12 nm.