Strain recovery of post-yield compressed semicrystalline poly(butylene terephthalate)

Cubic specimens of a semicrystalline poly(butylene terephthalate) (PBT) have been compressed up to post-yield deformation levels with a fast (3.0 102 s1) and a slow (1.5 104 s1) strain rate at three different temperatures (25 C, 45 C, and 100 C, i.e. below, close and above the glass transition temperature of the material, Tg, respectively). Differently from literature results reported for amorphous polymers, semicrystalline PBT shows that, after a post-yield deformation, recovery occurs also at temperatures higher than Tg, and that an irreversible deformation, 3irr, is set in the material. The irreversible strain component has been evaluated as the residual deformation after a thermal treatment of 1h at 180°C.
After unloading, isothermal strain recovery has been monitored for time periods of 1 h at various temperatures. From the obtained data, strain recovery master curves have been constructed by a timeetemperature superposition scheme. The features of the recovery process for the various deformation conditions have been analysed. In particular, it appears that specimens deformed below Tg show a lower irreversible component, whereas, when deformed above Tg, they display a higher irreversible deformation and a slower recovery process. Moreover, the effect of deformation rate appears particularly marked for samples deformed above Tg.