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 (DeltaU(ST)/W) is decreasing as the final deformation level increases. For a given final strain level, the amount of energy stored in specimens deformed under T, increases as either loading or unloading rates increase: in particular, both DeltaU(ST) and DeltaU(ST)/W values are linearly increasing with the logarithm of loading rate. On the other hand, energy storage for specimens deformed at T, results to be practically independent from the loading rate. Moreover, as the deformation temperature increases from 25 to 100 degreesC DeltaU(ST) values markedly decrease, while the ratio DeltaU(ST)/W is almost constant around an average value of about 51%.