What controls the lamellar orientation at the surface of polymer films during crystallization?

The crystalline morphologies of poly(bisphenol A hexane ether) (BA-C6) films were investigated using atomic force microscopy (AFM). The glass transition temperatures (T(g)s) of the BA-C6 films on silicon wafers were determined using an ellipsometer, and the results indicated that the T, increased dramatically when the thickness of the films was smaller than about 30 nm. The lamellar orientation of the BA-C6 films with thicknesses of about 33 and 970 nm was studied at temperatures varying between the T, and the melting point of the polymer (T-m). In the 33 run films, we found that the edge-on lamellae predominantly formed at the temperatures close to the bulk T-g (T-g similar to 35 degrees C), while some edge-on and mostly flat-on lamellae developed at the temperatures near T-m (T-m similar to 98 degrees C). In addition to temperature, the film thickness was found to have significant influence on the observed lamellar orientation at the surface. The concentration of the edge-on lamellae increased as the film thickness increased. These observations were explained on the basis that polymer chain mobility is strongly affected by the thickness of the films, temperature, and the interactions between the polymer and substrate. A three-layer T-g model was proposed for supported polymer films.