Low-temperature specific heat of heterocyclic polymer networks: effect of network density

Measurements of low frequency Raman scattering and low temperature specific heat between 2 and 20K 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 at20 cm1 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 Cp/T3 plot with a maximum at 5K. 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.