Heat Capacity of Amorphous Polymers at Low Temperatures

Abstract

RECENTLY, Starkweather¹ has shown that the model for the heat capacity of chain crystalline polymers proposed by Stockmayer and Hecht² can be very, useful in understanding thermal motions of polymers at low temperatures. The model assumes that groups in the chain vibrate as units and that there are strong-primary bonds between the units and weak interactions between adjacent chains. The temperature dependence of the heat capacity, C_v, can be expressed in terms of two ratios, C_v/3Nk and T/T_m, where N is the number of vibrating groups per gm., T_m equals hν_m/k, where h and k are Planck and Boltzmann constants, and ν_m is the maximum frequency of vibration. Starkweather¹, using published C_p data and the Stockmayer–Hecht model, was able to calculate the parameters, T_m, ν_m, 3NK, and the weight of the repeating unit for crystalline polytetrafluoro-ethylene and for conventional 55 per cent crystalline polyethylene. It is the purpose of this communication to extend this previous work^1,2 to amorphous chain polymers and to present the calculated values for the above parameters for polystyrene (PS) and poly-methylmethacrylate (PMMA).