Crystal twinning in simultaneous biaxial stretching of gelation-crystallized ultra-high molecular weight polyethylene

Abstract

Structure and orientation development in simultaneous biaxial stretching of gelation-crystallized ultra-high molecular weight polyethylene (UHMWPE) was examined by means of X-ray pole figures. The dry gel film exhibits a stacked lamellar structure similar to single-crystal mats with preferential crystalc-axis orientation normal to the film surface. The biaxially stretched gelationcrystallized films reveal a complex crystal orientation of (1 1 0), (2 0 0) and (0 2 0) planes. The detailed analysis of these pole figures manifests the occurrence of crystal twinning in addition to the orientation of crystallites. Crystal transformation of orthorhombic to monoclinic structure was not observed during biaxial stretching. The orientation distribution functions of crystallites were calculated assuming affine deformation. The orthogonality of crystal 〈1 1 0〉-c axes and crystala-c axes was further assumed to be conserved during the crystal twinning and biaxial orientation, respectively. Model simulation with contributions of 30% crystal twinning and 70% crystal orientation yields the best fit with the X-ray pole figures of biaxially stretched UHMWPE films. The affine model appears to be valid up to a biaxial draw ratio of 1.8×1.8, after which the results deviate from ideality with increasing draw ratio.