High-density polyethylene (HDPE) and poly(vinylidene fluoride) (PVF2) were submitted to plane-strain compression at temperatures between the glass transition temperature (T-g) and the melting point (T-m) by using a deep channel die; this device allowed compression along the Z-axis, elongation along the X-axis and no dimensional change along the Y-axis. The thermomechanical and morphological changes occurring during the plastic deformation process and relaxation of the deformed materials were investigated by thermomechanical analysis, differential scanning calorimetry and density measurements. The results show that the deformation process affects the thermodynamic properties of polymers, introducing different characteristics into the material. An increase of amorphous phase was registered in both materials. The plastically deformed semicrystalline polymers showed non-regular linear thermal expansion behaviour in the deformation directions (X and Z). The relaxation process, which is obtained by heating the deformed materials, is characterized by 'shape and temperature memory'.