Brillouin scattering in polymethyl methacrylate from 4 to 300 K: Temperature dependence of the Grüneisen constant and thermal properties

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Abstract
Measurements of Brillouin scattering and thermal expansion of amorphous polymethyl methacrylate (PMMA) have been carried out between about 4 and 300 K. Temperature dependence of the ratio of the anti‐Stokes to Stokes intensities is measured between about 2 and 15 K. This agrees with the result calculated from the quantum theory of Brillouin scattering based on the Debye theory. The lattice Grüneisen constant γL is calculated from the above two measurements, Brillouin scattering and thermal expansion. Below about 100 K, this seems to agree with the results calculated from cell theory by Curro and independently by Simha et al. Above this temperature, there is a small deviation from the theoretical result. The thermodynamic Grüneisen constant γT is also calculated by using the bulk modulus calculated from the sound velocity obtained from Brillouin scattering. At low temperature, γT seems to coincide with γL. Using the ratio γT to γL, the intermolecular specific heat Cv (inter)is obtained. The temperature dependence of Cv(inter) agrees with the calculated one from the Debye specific heat theory. Using the results of the sound velocity and Cv(inter), the temperature dependence of the thermal conductivity is obtained, and compared with the experimental result. These nearly agree with each other.