Relaxation behavior of poly(methyl methacrylate) at a water interface

The relaxation behavior of poly(methyl methacrylate) (PMMA), spin-coated on a silicon wafer, at the water interface was examined by lateral force microscopy as a function of temperature and scanning rate. Even in water, the lateral force peak which was assigned to the segmental motion of PMMA plasticized by water molecules was clearly observed in the temperature domain. The apparent activation energy for the plasticized α_a- relaxation process was much smaller than those for the original α_a-relaxation processes at the intact surface and in the bulk. The depth profile of the glass transition temperature (T_g) of the PMMA film in water was obtained, showing that T_g decreases with proximity to the water phase. The T_g depression observed here was best explained in terms of the water content of the film, rather than a confinement effect.