Polymer property enhancement by solid‐state forming

The alteration of the physical properties of a polymer by the use of forming techniques which are applied below the melt or glass transition temperature of the polymer has been discussed. The influence of cold rolling (rolling accomplished at room temperature) on the mechanical properties of amorphous polymers was first briefly reviewed (1). The specific effect of rolling on the toughness of polycarbonates, as measured by notched Izod impact strength, was discussed in detail and is summarized in (2). The role of crystallinity in further enhancing the mechanical properties of polycarbonate and the combined effect of rolling and crystallinity was also described and is discussed in more detail in (3). Other amorphous polymers were also discussed, particularly the effect of rolling on the notched Izod impact strength (4). For most polymers studied, a large increase in impact strength occurs for small reductions in thickness and a maximum occurs at a given thickness reduction beyond which the ductility of the material is exhausted. The increase in impact strength is not only a result of molecular orientation but also a function of residual stress placed in the sheet by the rolling process. In addition to the studies on amorphous polymers, recent studies (5, 6) were described on the high‐pressure nonisothermal compression molding of linear polyethylenes. Mold pressures up to 80,000 psi were used and several different polyethylenes having various molecular weights (up to 10 6 ) were investigated. It was shown that the modulus of elasticity of polyethylene can be increased by a factor of two when the mold pressure is increased to 80,000 psi. A detailed discussion of the changes in morphology responsible for this increased stiffness is presented in (6).