Using photothermal techniques and the Open Photoacoustic Cell (OPC) we measure thermophysical properties, such as thermal diffusivity, thermal conductivity, and heat capacity per unit volume of 5 polymer foils, poly(ethylene terephthalate) PET, of different molecular weights. It is shown that the physical properties influence strongly the thermal ones, and that photoacoustics can be used as a monitoring technique in the application of polymers.