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

Assessing suitability of amorphous polymers in durable products requires understanding of long-term effects of physical aging on the material properties. This work shows four polyesters with varying diol composition (poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) (PETG1 and PETG2 with ∼30 and ∼60% 1,4-cyclohexylenedimethylene (CHDM), respectively), poly(ethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PETT) with ∼30% 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) and poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PCTT) with ∼80% CHDM and ∼20% TMCD) exposed to thermal treatment at 20, 30 and 40 °C below their respective glass transition temperatures for up to 504 h to accelerate physical aging. The enthalpy relaxation was investigated by differential scanning calorimetry and compared to mechanical changes manifested as tensile yield strength increase. The physical aging rates were found to depend on both chemical structure and composition of CHDM and TMCD segments, where the introduction of TMCD inhibited physical aging. Arrhenius and Vogel-Fulcher-Tamman models were used to fit horizontal shift factors and evaluate the time and temperature dependencies for each polyester. From this study, the two models showed no significant differences in ability to describe the effects of physical aging. The Arrhenius activation energies, E a , were all in the range 118–244 kJ mol−1, were both PETG1 and PETG2 showed no significant difference between E a for enthalpy relaxation and yield strength increase, whereas PETT and PCTT showed ∼19 and ∼107% difference between the two, respectively, suggesting that the relationship between the two phenomena is not independent of chemical structure. The difference between the activation energies suggests that the time scales for physical aging are different when observed as enthalpy relaxation and yield strength. Four PET copolyesters were exposed to thermal treatment at 20, 30, and 40 °C below their respective Tg for up to 504 h to accelerate physical aging, which are rate dependant. Arrhenius and Vogel-Fulcher-Tamman model showed no significant differences in ability to describe the effects of physical aging, with their Arrhenius activation energies ranging 118–244 kJ mol−1, suggesting different time scales for aging in regards to their enthalpy relaxation and yield strength, which are not independent of chemical structure. [Display omitted] • Four glycol modified polyesters are submitted to accelerated physical aging at three different temperatures. • Arrhenius and Vogel-Fulcher-Tamman models were able to describe the results equally well. • Physical aging rate depended on chemical structure, where copolymer TMCD inhibits enthalpy relaxation with yield strength increased. • A de-coupling between thermodynamic and mechanical effects of physical aging was found. [ABSTRACT FROM AUTHOR]