Structure and thermal molecular motion at surface of semi-crystalline isotactic polypropylene films

Surface crystalline structure in isotactic polypropylene (iPP) films was explored by in-plane grazing incidence X-ray diffraction measurement. Apparent crystallinity in the surface region was lower than the bulk one. After an etching treatment with a droplet of potassium permanganate solution, a clear crater was formed at the surface, and the step height between etched and intact regions was approximately 3 nm. This means that the iPP surface was covered with 3 nm thick amorphous layer. Then, surface molecular motion in the iPP films was examined by lateral force microscopy. Surface α_a-relaxation process arisen from the segmental motion was observed at about 250 K, and its apparent activation energy was 230±10 kJ mol⁻¹. The both were lower than the corresponding bulk values, indicating that surface molecular motion is more active than the bulk one even in the semi-crystalline iPP films. An iPP film with 1.5 nm thick surface amorphous layer was prepared. In this case, the enhanced mobility was still observed at the surface, but the extent of the enhancement was not remarkable as that for the iPP film with 3 nm thick surface amorphous layer. These results imply that surface mobility is affected by the presence of underneath crystalline phase, if the surface amorphous layer is thin enough.