The excitation spectrum of epoxy resins; neutron diffraction, specific heat and thermal conductivity at low temperatures

Abstract

The thermal conductivity and specific heat from 0.1 to 100K and the inelastic neutron scattering spectrum has been measured for an epoxy resin Araldite MY 750 which has been hardened with primary diamines. These have the form H₂N(CH₂)_nNH₂ and specimens were made with n=2, 3, 4 and 12. The specific heat results indicate that the density of the low-lying two-level state (TLS) increases for the specimens with a longer length of cross-link (i.e. for n=12) and if phonons are scattered by the TLS this should lead to a lower thermal conductivity for these samples than for those with smaller values of n. In fact the thermal conductivity is higher. The problem can be resolved if one assumes, as other workers have suggested, that the phonons are scattered primarily by those excitations which are responsible for the excess T³ term in the specific heat. This term is much smaller for the specimen with n=12 than for those with n=2, 3, 4 and it would appear that these excitations (whatever they are) do have a much stronger influence on determining the thermal conductivity than do the ordinary TLS. The inelastic neutron scattering measurements do not show any significant differences between the various specimens.