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

In this paper, low-frequency mechanical spectroscopy has been used to study poly(methyl methacrylate) (PMMA) melt around the glass transition temperature Tg 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 Tg. 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.