Energy storage and strain-recovery processes in highly deformed semicrystalline poly(butylene terephthalate)

Abstract

Nonelastic deformation of semicrystalline poly(butylene terephtalate) (PBT) was investigated by calorimetric measurements and strain-recovery tests. Differential scanning calorimetry on PBT specimens deformed both below and above their glass-transition temperature (T g 50 °C) showed the presence of a broad exothermal peak whose area represents the energy released for the nonelastic strain recovery. This energy became more and more pronounced as the strain level increased, and it decreased as the deformation temperature increased, even if a significant contribution was detected on specimens deformed at temperatures much higher than T g . For two temperature conditions (21 and 100 °C), strain-recovery master curves were built showing the following two distinct deformation components: one recoverable with time and another one irreversible, this latter one arising from relatively low levels of strain. The recoverable component can be erased by heating the material at temperatures much higher than its T g , close to the onset of the melting process. On the other hand, the irreversible strain component does not recover even if the material is brought close to the onset of the crystals melting. The shift factor for the strain-recovery master curves was compared with the shift factor for the construction of the dynamic storage modulus master curve obtained in the linear viscoelastic regime (small strain).