Slow dynamics of the α and α' relaxation processes in poly (methyl methacrylate) through the glass transition studied by mechanical spectroscopy

摘要

In this paper, low-frequency mechanical spectroscopy has been used to study poly (methyl methacrylate) (PMMA) melt around the glass transition temperature T_g for shedding light on its unique relaxation behaviors. The mechanical spectra show an asymmetrical broad structure with a maximum peak on the high-temperature side and a shoulder peak on the low-temperature side. The shoulder peak corresponds to the α relaxation due to the local segmental motion; while the maximum peak is assigned to the α' relaxation due to the slow motion of longer chain segments or chains. The α relaxation mode has a stronger temperature dependence of relaxation time than the α' mode, causing the α' mode to merge with the α mode with decreasing temperature toward T_g. Time-temperature superposition (TTS) breaks down in the entire temperature range due to the different friction coefficients of the α and α' relaxations. For only the α' process, TTS is found to hold for PMMA melt, where the high-frequency decay of the mechanical loss decreases like a power law of the frequency with an exponent of -1/3. Moreover, compared with the dielectric results, the relaxation times of β and α relaxations from mechanical measurements are about one order of magnitude slower than those from dielectric experiments.