The effect of molecular weight (MW) on the isothermal crystallization of polypropylene (PP) has been studied using a differential scanning calorimeter (DSC). PP samples with Mw ranging from 7×104 to 30×104 were prepared with controlled oxidative degradation. Based on the experimental data of crystallization peak time

it is obvious that the isothermal crystallization rate of PP decreases with decreasing MW. This result is consistent with the phenomena of smec-tic structure formation in fiber spinning process and injection moulding.