No abstract

Summary: A mechanical model was developed to describe qualitatively and quantitatively the stress‐strain‐time behavior of a prepared shape memory crosslinked polyethylene during hot stretching, stress relaxation under 200% strain at high temperature and strain recovery of the heat shrinkable polymer. The stress‐strain, the stress relaxation and the irrecoverable strain behavior of the model were established by driving the constitutive equation, which could qualitatively represent the behavior of the real material. By choosing significant values for the parameters of the proposed model, an excellent fit was obtained between the experimental behavior of the polymer and that predicted by the model. It was also revealed that the main source responsible for the imperfect recovery of the induced strain observed was the stress relaxation occurring during the stretch holding‐cooling time step.

Abstract. Silane method is a preferred method in crosslinking polyethylene to modify its properties. Here, the silane grafting reactions of low density (LDPE), linear low density (LLDPE) and high density polyethylene (HDPE) were compared, all with a fixed amount of silane (vinyltrimethoxysilane) and peroxide (dicumylperoxide). Processing for silane grafting was carried out in an internal mixer, and FTIR spectra were used for comparing the silane grafting efficiencies. The effect of polymer physical form and pre-mixing the components was also determined. Molecular structure parameters were analyzed to investigate their effect on silane grafting efficiency. In case of LDPE and LLDPE, the probable interfering effect of two types of antioxidants on silane grafting reactions was studied. Ethylene propylene diene monomer (EPDM) was added to LDPE to evaluate its effect on silane grafting efficiency. Amongst different grades of polyethylene, LLDPE had a better efficiency in silane grafting. Branchings, PDI, Mw, and MFI are all determining factors and should be considered in comparing the silane grafting efficiency between different grades of polyethylene. In case of incorporating antioxidant, different results were observed when the polymer under study was different. EPDM as an additive would enhance silane grafting efficiency.

No abstract

Shape memory was induced in crosslinked low‐density polyethylene by a heating‐stretching‐cooling cycle. The effect of crosslink content on thermal properties and temperature dependence recovery behavior was studied experimentally. The importance of stretching temperature and crosslink content on recovery behavior could be reasonably explained by the observed changes in the thermal properties which were attributed to the differences in crystalline structures and mechanism of crystal formation during the heating‐stretching‐cooling process. A mechanical model was developed to describe qualitatively and quantitatively the temperature dependence recovery behavior of the prepared shape memory crosslinked polyethylene at nonisothermal state under various conditions by driving constitutive equations using a set of model constants. These model constants were determined with the help of a set of optimization codes using a genetic algorithm method. By choosing a suitable set of model constants one can describe with high accuracy the temperature dependence recovery behavior of any shape memory polymer.