Accelerated physical aging of four PET copolyesters: Enthalpy relaxation and yield behaviour

The Effect of Physical Aging on the Viscoelastoplastic Response of Glycol Modified Poly(Ethylene Terephthalate)

“…Aging causes a large decrease in activation energy in both copolyesters, but generally the values are higher for PECT than PETT suggesting a larger effect of temperature in PECT, which also fits well with the experimental data. This observation also agrees with previous studies of enthalpy relaxation in PECT and PETT, where activation energies of 218-224 and 134-159 kJ mol − 1 where found, respectively [30]. However, from the data shown in Fig.…”

“…The T g difference implies that more energy is needed to initiate molecular mobility in PETT compared to PECT, and therefore it is notable, that the rate of interaction-rearrangement is found to be highest in PETT at the same temperature (30 • C). Our previous studies of physical aging rates, measured as enthalpy loss at T g -20 • C, also suggested, that the enthalpy relaxation occurs faster in PECT than in PETT at the same temperature relative to T g , suggesting that structural relaxation should occur faster in PECT [30]. However, even when the backbone of PETT is more sterically hindered, the nature of the intermolecular forces in the two polyesters is the same.…”

“…Chemical structures and glycol ratios have previously been confirmed by hydrogen-and carbon nuclear magnetic resonance spectroscopy and infrared spectroscopy [30]. Average molecular weights were estimated from gel permeation chromatography as 14.0 and 14.8 kDa for PETT and PECT [30], respectively. Both injection moulded materials were found to be amorphous by DSC, showing neither melting nor crystalline peaks (Fig.…”

“…Solid lines: results of numerical analysis. [30]. The moduli of PETT and PECT differ only slightly showing that PETT is stiffer than PECT.…”

“…However, the incorporation of CHDM or TMCD in specific levels is known to reduce the crystallization rate significantly compared to PET. Polyesters with CHDM/EG and TMCD/EG composition of 30/70 have previously been found to show no crystalline peaks when scanned with DSC before and after aging at T g -20 for 24 h [30]. Therefore no formation of crystallites are expected at these temperatures.…”