The dynamics of a side chain liquid crystal copolymer at two different compositions has been investigated in its smectic A and C phases. J(1) (omega (0)) and J(2)(2 omega (0)) spectral densities have been determined for aromatic deuterium nuclei in the side chains and their interpretation has been carried out by means of several available motional models. The dynamic process mainly contributing to relaxation has been found to be the rotation of the phenyl ring about its para-axis, that is best described by a small step rotational diffusion model, where the diffusion constant has an Arrhenius temperature dependence and a Gaussian distribution of the activation energies is present, indicating a remarkable degree of dynamic heterogeneity of this system.