Slow mechanical relaxations of polymethacrylates below 20 K

Abstract We have measured the temperature dependence of sound velocity and internal friction of three polymethacrylates (PMMA, PEMA, PBMA) at audio frequencies using a vibrating reed technique. Below helium temperature, all the samples show the typical behaviour predicted by the tunneling model: a temperature independent damping plateau and a logarithmic variation of sound velocity with temperature. The absorption data obtained at higher temperatures suggest that a cut-off exists in the ratio of tunnel splitting to total energy splitting. For a given temperature the relaxation times are distributed over 10 decades.