| Hot stage atomic force microscopy (AFM) has been used to study in real time the influence of the film thickness on the crystallization of ultrathin films of poly(epsilon-caprolactone) (PCL) and poly(epsilon-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends (60/40 and 80/20 compositions), and the influence of the residual solvent on the crystallization of 6 nm PCL films.; In PCL ultrathin films (200 to 1 nm) crystallized at low supercooling, hexagonal lamellar crystals are oriented flat-on relative to the substrate. For the thinner films (15 nm and below), the morphology is distorted, the growth rate decreases with the film thickness, the growth becomes non-linear, and the melting process is perturbed. Overgrowths are observed above and below the basal lamellae, and their number increases with the film thickness. At higher supercoolings, flat-on dendritic morphologies are observed for the thinner films, whereas thicker films exhibit flat-on axialites, covered by few branched edge-on lamellae for the larger thicknesses, indicating a transition from the axialitic to the spherulitic morphology. Electron diffraction confirmed the flat-on orientation observed by AFM.; In PCL/PVC blend ultrathin films (120 to 10 nm), lamellae have also been observed in a flat-on orientation, covered by an increasing number of polygonal overgrowths when the film thickness, the supercooling, and the PCL content of the blend increase. These three factors influence the morphology of the crystal aggregates. Unusual S-shaped lamellae, and concentric overgrowths have been observed, related to the presence of PVC. The growth is non-linear, and the growth rate decreases with the film thickness.; Finally, the growth rate and the width of the dendrites grown at 30°C after melting at 64°C for different periods of time have been observed to decrease with the amount of residual solvent trapped in 6 nm PCL films. For the first time, it has been observed that edge-on lamellae grow faster than flat-on lamellae due to the very small film thickness. A spinodal dewetting is also observed.; These observations are discussed in terms of mobility of the polymer chains in the melt, restricted by the limited thickness of the films, the presence of PVC chains or the evaporation of the residual solvent. |
