The motion of molecular segments in poly(methylmethacrylate) (PMMA) is investigated in dependence on temperature and hydrostatic pressure with dielectric and dynamic-mechanical experiments. Molecular motion can indirectly be seen in the specific volume and in the specific heat capacity. Relaxation times, usually plotted in activation diagrams, can be calculated by a detailed model for the molecular structure of the polymer melt, where the free-volume theory is replaced by the dislocation concept. The model allows us to correlate the thermal properties with the dynamic behavior, and is able to calculate specific heat capacity curves.