Physical aging and nanostructure of poly(methyl methacrylate): Effect of methanol

The effect of physical aging on the macroscopic thermophysical properties of amorphous polymers is well documented. The problem of physical aging of amorphous polymers at a nanoscopic scale is addressed in this work. To achieve this goal, we take advantage of low-frequency Raman scattering spectroscopy (LFRS) that makes it possible to investigate the structure at such a scale and its evolution by observing the LFRS intensity in the spectral range of the boson peak. The physical aging is usually observed according a given thermal history (i.e., quenching and isothermal aging below the liquid glass transition temperature). A particular procedure is considered out in this paper. Namely, the specimen of poly(methyl methacrylate) experiences a physical aging at room temperature (i.e., far below its conventional Tg) during a drying process following immersion in methanol until saturation. It is shown that this procedure increases the physical aging rate but in an inhomogeneous way. This observation together with low-frequency mechanical spectroscopy measurements is in agreement with a nonhomogeneous structure model of amorphous matter. (C) 2001 American Institute of Physics.