Low temperature specific heat of heterocyclic polymer networks: Effect of network density

Abstract

Measurements of low frequency Raman scattering and low temperature specific heat between 2 and 20 K have been performed in two glassy, heterocyclic polymer networks. The effective network densities of heterocyclic polymer networks are varied by changing the ratio of bi- and mono-functional isocyanate monomers, leaving the overall chemical structure essentially unchanged. A boson peak at ∼ 20 cm⁻¹ characterizes the Raman spectra at room temperature of both samples. Below 10 K, the specific heats deviate from a cubic temperature dependence, as predicted by the Debye theory, and reveal an excess specific heat, having the shape of a well-defined peak in a C _p/T ³ plot with a maximum at ∼ 5 K. The increase in effective network density is accompanied by a slight decrease in excess specific heat. These observations have been explained in terms of additional low-energy vibrations associated with the monomers building the polymer networks.

Keywords:

• amorphous polymers
• thermal properties

Acknowledgements

We dedicate this paper to the memory of Professor Valery P. Privalko, who inspired this work and provided the samples for the present study. It was an honour and privilege to have known a man of such significant achievement and personality, who was innovative and enthusiastic, open to new ideas and always willing to share his thoughts with colleagues. We also thank Professor Alexei Sokolov for helpful discussions and assistance with Brillouin measurements.