Laser correlation spectroscopy of amorphous polymethylmethacrylate

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Abstract
We have studied the light scattered from amorphous polymethylmethacrylate (PMMA) using the correlation technique for a range of temperatures from 6 to 165 °C encompassing the glass transition temperature (Tg∼120 °C). The data were analyzed in terms of two exponential decays, and the angular dependence of each of the corresponding relaxation frequencies was examined. The results for the high frequency relaxation mode are angular independent and fall reasonably well on two straight lines of different activation energy (∼8 kcal/mole at high temperatures and ∼1 kcal/mole at low temperatures), indicating the presence of two coupled relaxation mechanisms. The low frequency relaxation results are quite sensitive to the inhomogeneities of unannealed samples and have in this case irregular angular dependences. The angular dependence disappears for samples close to Tg and for annealed samples. Below Tg, this relaxation process has a fairly constant frequency of about 3 Hz independent of temperature. Above Tg, its frequency increases very rapidly with temperature to reach 130 Hz at 165 °C and follows the backbone main‐chain relaxation frequency measured by other techniques.