When subjected to external loading, polymeric materials behave in a manner intermediate between elastic solids and viscous fluids. Their mechanical properties depend on a material’s viscous flow, which, in turn, is influenced by (i) temperature, with its different magnitudes determining a ductile or brittle behaviour and (ii) time, through the effect of a deformation rate and long-term relaxation. Short-term viscoelastic properties (loss and storage moduli) of a studied semi-crystalline thermoplastic polymer were obtained using Dynamic Mechanical Analysis, while its long-term viscoelastic properties (compliances) were determined using creep tests.