Dynamic‐mechanical and tensile properties of poly(vinyl chloride). Influence of thermal history and crystallinity

The dynamic‐mechanical properties as a function of temperature and the low‐ and high‐speed tensile properties at 23°C have been determined on specimens of conventional suspension‐polymerized PVC and of low‐temperature‐polymerized PVC roll milled and then compression molded at different temperatures. It has been found that the main transition α and the shear modulus above T α depend on the thermal history and are strongly affected by crystallinity, whereas the dynamic‐mechanical spectrum below T α is not influenced by these parameters. Room‐temperature tensile modulus and yield properties are very little affected by processing history and crystallinity. The elongation at break and the fracture energy, on the contrary, increase, at any fixed strain rate, for conventional PVC with milling temperature. The same trend has been found for low‐temperature PVC, but the elongation at break‐versus‐temperature curve is shifted, as a whole, toward higher temperatures by approximately 50°C. Such results are discussed in terms of homogeneity of the specimens, which is controlled by the melting process of the crystallites. Stereoscanning electron micrographs of fracture surfaces appear to substantiate these conclusions.