Solid-state deformation of polyethylene and nylon and its effects on their structure and morphology

Abstract

Solid-state deformation processes have been used to achieve polymers of high tensile moduli. The processes are based on achieving continuity for the oriented high-strength covalent bonds of the polymer chain. This is accomplished by the pulling out of chain folds and the subsequent extension of the long polymer chains which run both through and between crystal lamellae. Three major processes are used in solid-state deformation of semicrystalline polymers. In common cold drawing, the polymer is stretched at or below the crystalline melting point. In cold extrusion, a plug of solid polymer is forced by a ram through an orifice of smaller cross-sectional area to achieve draw. The third process, hydrostatic extrusion, is similar to cold extrusion except that the solid plug is surrounded by a pressure transmitting fluid which exerts a hydrostatic pressure on the plug that forces it through an orifice. Each method has distinct advantages and disadvantages that are described. Proposed molecular models for the three solid-state deformation processes generally consider the breakup of crystalline lamellae, their orientation in the deformation direction, and the pulling out of folded chains. These unfolded chains form tie-molecules between and among the disrupted lamellae. At highest deformation, fibril formation is observed which involves the partially extended tiemolecules. It is these chain-extended tie-molecules which are responsible for the unusually high tensile properties for drawn semicrystalline thermoplastics in the orientation direction.