Response of linear high polymers to hydrostatic pressure

The bulk compressibility of linear high polymers has been found to be a function of temperature, pressure, and time. Linear high polymer melts are highly compressible. Addition of pressure causes a structural change in some linear high polymer melts. In the case of amorphous polymers, such as polystyrene, a glassy state response to hydrostatic compression was found at a temperature well above the second‐order transition temperature. In the case of polycrystalline materials, such as polyethylene, pressureinduced crystallization can take place at temperatures above the melting point. In either case, the process requires a higher pressure as temperature is increased. This process is found to be time dependent. There are indications that such structural changes could not take place above a specific temperature limit for any given material, regardless of the amount of pressure. By subjecting a melt of polyethylene to pressure and then cooling, a material that is more dense, more rigid, and more crystalline than the original injection‐molded specimen can be obtained. The effects of the bulk compressibility on the fabrication condition of plastics as well as product properties are discussed.