Post-yield compressed semicrystalline poly(butylene terephthalate): energy storage and release

Post yield deformation of semicrystalline poly(butylene terephthalate) (PBT) is studied by differential scanning calorimetry (DSC) measurements on compressed specimens after unloading. In particular, the effects of strain level, loading-unloading rate, and deformation temperature are analyzed. DSC traces indicate that a remarkable fraction of the mechanical work of deformation (in the range from 25 up to 62%) is stored in the material after unloading. Final strain dependence of stored energy values for specimens deformed up to 40% follows the general S-shaped trend observed for many amorphous and semicrystalline polymers. The ratio of the stored energy to the mechanical work of deformation (ΔUST/W) is decreasing as the final deformation level increases. For a given final strain level, the amount of energy stored in specimens deformed under Tg increases as either loading or unloading rates increase: in particular, both ΔUST and ΔUST/W values are linearly increasing with the logarithm of loading rate. On the other hand, energy storage for specimens deformed at Tg results to be practically independent from the loading rate. Moreover, as the deformation temperature increases from 25 to 100 °C, ΔUST values markedly decrease, while the ratio ΔUST/W is almost constant around an average value of about 51%.