Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate in water, hexane, and methanol, which are "nonsolvents" for dPMMA, were examined along the direction normal to the interface by specular neutron reflectivity (NR). The interfaces of dPMMA with the liquids were diffuse in comparison with the pristine interface with air; the interfacial width with water was thicker than that with hexane. Interestingly, in water, the dPMMA film was composed of a swollen layer and the interior region, which also contained water, in addition to the diffused layer. The interface of dPMMA with hexane was sharper than that with water. Although there were slight indications of a swollen layer for the dPMMA in hexane, the solvent molecules did not penetrate significantly into the film. On the other hand, in methanol, the whole region of the dPMMA film was strikingly swollen. To conserve mass, the swelling of the film by the nonsolvents is accompanied by an increase in the film thickness. The change in the film thickness estimated by NR was in excellent accord with the results of direct observations using atomic force microscopy (AFM). The modulus of dPMMA in the vicinity of the interfaces with liquids was also examined on the basis of force-distance curves measured by AFM. The modulus decreased closer to the outermost region of the film. The extent to which the modulus decreased in the interfacial region was consistent with the amount of liquid sorbed into the film.

Dynamic mechanical analysis was successfully used, for the first time, to characterize polymer thin and ultrathin films supported on substrates. This method allowed us to uncover the effects of the free surface and the substrate interface on the segmental dynamics in polystyrene (PS) films with various thicknesses. As the film thickness decreased, the distribution of relaxation times for the segmental motion became broader, a change mainly due to surface and interfacial effects. Interestingly, PS ultrathin films sandwiched with SiO x layers exhibited a relaxation process corresponding to the interfacial segmental motion in addition to that in the internal region of the films. The results obtained in this study imply that two contrasting effects exist: the effect of the free surface accelerates the segmental motion, whereas interfacial interactions produce the opposite effect.

Segmental mobility of a typical amorphous polymer, polystyrene, at the interface with a solid substrate was examined noninvasively by fluorescence lifetime measurement using evanescent wave excitation. Glass transition temperature (Tg) was discernibly higher at the interface than in the bulk. Measurements at different incident angles of excitation pulses revealed that Tg became higher the closer to the interface. This is the observation for a Tg gradient of polymers at the interface.

Monodisperse polystyrene (PS) films with various thicknesses were spun-coated on silicon wafers with native oxide layer. Surface relaxation behavior in the PS films was studied as a function of thickness by lateral force microscopy (LFM). In the case of a thick PS film, a clear lateral force peak corresponding to surface Ra-relaxation process of segmental motion was observed at a temperature much lower than the bulk glass transition temperature, Tg. As the film became thinner than 3-4 times the radius of gyration of an unperturbed consistent chain, the peak on lateral force vs temperature curve started to broaden out with decreasing thickness and eventually split into two peaks. The appearance temperature of the surface R a-relaxation peak was invariant with respect to the film thickness even in such an ultrathin state, meaning that surface Tg was insensitive to the thickness. On the other hand, the ultrathinning-induced relaxation process, called surface β-relaxation, was strongly dependent on the thickness in terms of relaxation temperature and apparent activation energy. Finally, possible origins of the surface β-relaxation process were proposed.

No abstract