Influence of the crystallinity on the transport properties of isotactic polypropylene

Samples of polypropylene of varying crystallinity were obtained by blending isotactic with atactic polypropylene, and the crystallinity determined by X-ray diffraction and differential scanning calorimetry. Crystallinity ranged between 20 and 75%. The transport properties of dichloromethane were analysed varying the activity of the vapour. We observed that the sorption decreases, as the crystallinity increases, proportionally to the decrease of the amorphous fraction. As matter of the fact, the specific sorption, normalized by the amorphous fraction, does not depend on the crystallinity. At variance, a simple correlation between the thermodynamic diffusion coefficient and the crystallinity was not obtained; at low values of this parameter, up to 40%, the zero concentration diffusion coefficient is independent of it. A sharp transition separates a range of crystallinities, in which the diffusion parameter decreases, increasing the crystallinity, due to the tortuosity of the path, and shows that the presence of the impermeable crystals is important only for values higher than 50%. (C) 1999 Elsevier Science Ltd. All rights reserved.