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 Ϸ 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).

Soft Elasticity in Main Chain Liquid Crystal Elastomers

“…In the second and third regions, the system is only partially reversible, as shown in Figure 6, and can only be brought back to the original pre-stretched state by raising the temperature above T I or putting the sample in a solvent. The second and third elastic regions are termed, respectively, as anelastic and plastic in earlier reports [20,50,65,67] but to avoid any ambiguity we shall use the terms "non-elastic reversible" and "non-elastic irreversible" following Ricco and Pegoretti [69]. Here, "non-elastic" means that stress and strain are not linearly proportional and "non-reversible" implies that the specimen does not instantaneously return to strain-free state upon removal of stress.…”

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

“…This phenomenon has been widely observed and studied on highly deformed glassy polymers [1,9 -23], where the exothermal peak, interpreted as an evidence of the relaxation process connected to the deformation recovery, usually extends from the deformation temperature up to glass transition. The wider extent of the energy release process for semicrystalline polymers, already reported for PET [25] and PBT [27], has been attributed to the mobility gradient due to the interphase between the crystalline domains and the amorphous regions [25 -27,49]: as a consequence, the relaxation process for semicrystalline polymers spreads up to temperatures well above T g even if it can be still attributed to a release of internal energy stored in amorphous regions [1].…”

“…The amount of energy internally stored for a given deformation level seems to be affected by the loading conditions, such as strain rate [1,14] and deformation temperature [1,12,15,25,27], but the available literature data are quite scarce. Since most of mechanical properties of polymers are strongly dependent on time and temperature, it is therefore of general interest to study how parameters like strain rate and deformation temperature may affect their energy storage ability.…”

“…For glassy polymers deformed below glass transition temperature, several studies [9,10,15,22,28 -31,34,36] have shown that the strain recovery reaches completion in a short time by bringing the polymer to or above its glass transition temperature ðT g Þ: This total recovery occurs even at large deformations (up to more than 50%) and it concerns also the microstructural state since the polymer recovers its initial properties [5,38,39]. On the other hand, for highly deformed semicrystalline polymers the strain recovery has been proven to be not complete even at temperatures well above T g [26,27].…”

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

“…Some peculiar aspects of semicrystalline polymers are the extension of the strain recovery processes at temperatures much higher than T g [19,20,22,23,26,31,34], and the appearance of a certain irreversible deformation, even when heated at temperatures much higher than T g and within a few degrees of the melting point [20,22,23,34]. This research group recently studied the strain recovery behaviour of highly deformed semicrystalline polymers such as nylon-6 (PA6), poly(ethylene terephthalate) (PET), and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN) tested in tension [19,34], and of poly(butylene terephthalate) (PBT) tested in compression [20]. Similar to amorphous polymers [27,29], the strain recovery data have been treated according to a timeetemperature superposition approach, thus obtaining strain recovery master curves.…”