Tailoring crystalline structures and mechanical properties of injection-molded polyethylene by tuning the relaxation time of molecular chains

Abstract

The relaxation times of commercial polymeric materials are generally very short and most of the oriented molecular chains tend to relax or disorient during practical processing. In the present work, the relaxation time of high-density polyethylene (HDPE) was greatly prolonged by subjecting HDPE pellets to a certain dose of electrical irradiation. Gas-assisted injection molding (GAIM), with a strong shear flow during the secondary melt flow process, was utilized to prepare the molded bars. Macroscopically, the sub-skin layer of virgin HDPE sample was characterized by typical shish-kebabs, whereas random lamellar dominated in the gas channel region. With the enhancing of the relaxation time of polymer materials, the gas channel zone of PE-10 sample exhibited regularly aligned oriented lamellae. A high and homogeneous orientation structure in PE-10 sample led to remarkable mechanical enhancement from 36.2 and 931.2MPa of virgin HDPE sample to 52.5 and 1251.4MPa of PE-10 specimen for tensile strength and modulus, respectively. More importantly, under the same processing condition, all the skin, sub-skin and gas channel zones of PE-25 specimen exhibited shish-kebab crystals owing to the further increasing relaxation time. With this large scale of homogeneous orientation structure, PE-25 sample exhibited a further reinforcement to 66.8 and 1715.7MPa for the tensile strength and modulus, respectively. Our findings demonstrated that the flow-induced crystalline structures of injection-molded bars can be tailored by tuning the relaxation time of polymer materials and it provides a new method to self-reinforce polymer products by controlling the crystalline structures.