Specific Heat of Amorphous Polymethyl Methacrylate and Polystyrene Below 4°K

Measurements of the specific heat of polymethyl methacrylate and polystyrene have been performed using the heat pulse method between 0.5 and 4.3°K. Above 2°K the specific heat is about double that expected on the basis of the Debye theory and acoustic measurements. Below 2°K the experimental data approach the Debye value. The excess specific heat can be fit by two Einstein modes. For polymethyl methacrylate the Einstein temperatures are 4.9 and 17.5°K containing 0.014% and 1.0% of all modes, respectively. For polystyrene the corresponding modes are 5.5 and 16°K with strengths of 0.038% and 1.8% respectively. The data support a model of pendant side groups near voids capable of independent low frequency torsional oscillations. The barrier to internal rotation of the pendant group is estimated.