The effect of annealing on the structure and properties of isotactic polypropylene films

The effect of annealing on the structure and properties of two polypropylene (PP) films, one displaying the monoclinic α-structure and the other the quenched structure, has been investigated using a combination of differential scanning calorimetry (DSC), small-and wide-angle x-ray scattering (SAXS and WAXS), and uniaxial tensile testing. It has been established that upon heat treatment at temperatures >60°C there is an increase in the yield stress of both films, and that this increase can be attributed to an increase in the size and perfection of the crystalline regions. An attempt has been made to quantify the yield mechanisms occurring within these films. It has been found that a good correlation could be obtained between the experimental results and a crystal plasticity mechanism involving the thermal activation of screw dislocations with their Burger vectors parallel to the chain direction.