Deformation of oriented high density polyethylene shish-kebab films

Abstract

Decrystallization is defined as the mechanically induced reduction and reorganization of the crystalline phase. The decrystallization of oriented high density polyethylene (HDPE) was observed here by transmission electron microscopy (TEM). A special drawing technique was used to form thin films with an idealized, highly oriented, shish-kebab morphology. These oriented films were uniaxially elongated at room temperature along or perpendicular to the chain direction, and then viewed by TEM. The use of such a well defined initial morphology reduced the deformation of polyethylene as a whole to that of three structural elements: long shish crystals, chain folded kebabs, and the non-crystalline phase. The non-uniform deformation characteristic of spherulitic film, in which the elongation direction is parallel to lamellar normals in one region and perpendicular to them in another, was thus avoided. High strain transformed the shish-kebab morphology into filaments which were generally non-crystalline and of uniform diameter, with occasional crystalline remnants within them. Upon decrystalliz ation, internal “defects” were generated within crystallites, facilitating subsequent yielding. Stressed kebabs decrystallized and fed into the shish core. “Defect” generation also resulted in a reduction in crystal thickness along the chain direction, indicating that such a reduction can occur in the absence of thermally induced melting. Crystals underwent <001) crystal shear and chain slip, reducing crystal width, as well as martensitic transformation to the monoclinic crystalline form. No subsequent recrystallization was detected by darkfield studies. The influence of the initial shish-kebab morphology of undeformed films on the growth of crazelike structures was discussed.