Experimental analysis and mechanical modeling of effect of stress-relaxation on shape memory and recovery behavior of e-beam irradiated HDPE

摘要

The thermo-sensitive shape memory behavior of electron beam crosslinked high-density polyethylene (EB-XL-HDPE) was analyzed in terms of effects of processing variables (dose, stretch ratio and programming temperature) on the recovery behavior of the EB-XL-HDPE. Shape memory induced above the melting temperature revealed an instant recovery with higher initial recovery temperature as compared to the samples programmed at crystallization temperature, well correlated with the observed differences in their crystallite size, crystal orientation and degree of crystallinity analyzed by wide-angle X-ray scattering (WAXS). The WAXS on orientation of crystalline domains in the stretched specimens confirmed breakdown of crystallites and formation of an uneven crystallite size as a result of stretching of specimens in the semi-crystalline state below melting temperature. With the increase in programming temperature, the relative orientation of stretched EB-XL-HDPE is increased due to greater flow of the plastic phase, thus enhancing the recoverability. A four-element mechanical model was developed to simulate and predict recoverability and stress relaxation behavior of the shape memory EB-XL-HDPE. By adjusting model parameters, the model described precisely the experimentally measured shape memory behavior of EB-XL-HDPE and predicted its recovery behavior. It is concluded that the stress relaxation phenomenon is accountable for partial recovery of the EB-XL-HDPE remained under sustained stress.