Mobility Gradient of Polymer Chains near a Solid Interface

Polymer composites have been widely used in a variety of engineering fields. The performance and functionality of the composites are closely related to the quality of the interface between polymer and filler. Thus, it is important to study polymer behavior at the filler interface. The local conformation of rubbery chains in direct contact with a quartz substrate was here examined by interface-sensitive sum-frequency generation (SFG) spectroscopy. SFG signals, which could be obtained from functional groups only oriented at the interface, were clearly observed for the rubber polymer in a film at room temperature which was much higher than the bulk glass transition temperature (T_g). When the film was thermally annealed, rubbery chains at the quartz interface changed their conformation to one with a lower energy state, accompanied by the randomization of both the main and side chain parts. The characteristic temperature, at which interfacial chains started to lose their orientations, was much higher than the bulk T_g. Also, the extent found to be more remarkable for the spin-coated film than for the solvent-cast one. This implies that the stress accumulated at the interface, which resulted from the centrifugal force during the spin-coating process, accelerates the mobility of chains there. Then, the relaxation dynamics of rubbery chains at the quartz interface were directly probed as a function of distance from the quartz surface using time-resolved evanescent wave-induced fluorescence anisotropy (TRFA), dielectric relaxation spectroscopy (DRS) and SFG spectroscopy. We found the presence of the dynamics gradient of chains in the interfacial region with the quartz surface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders.