The dependence of polymer properties on structure has been of great practical importance. In this study, the microstructures of two types PET copolymer sheets made from different polymerization process and catalyst systems have been characterized in terms of density, crystallinity, modified structural factor, conformational isomer contents, as well as spherulite sizes. The relation of microcrystallinity to physical aging and environmental stress cracking was systematically evaluated.
It is found that physical aging occurs in both amorphous and semi-crystalline PET stored above room temperature, but below the glass transition. Some physical property changes resulting from aging were recorded. Results indicate that after aging, samples become denser, exhibit higher glass transition temperatures, higher enthalpy of relaxation values (ΔH), as well as less trans and more gauche isomers. Tensile mechanical evaluations show that aging causes elongation to break to decrease dramatically. There is a correlation between the enthalpy of relaxation and the mode of failure. The critical ΔH also decreases linearly as levels of crystallinity increase. Activation energies for onset of embrittlement were calculated and morphological contributions to failure are discussed.
The mechanism of physical aging was investigated. Differential scanning calorimetry (DSC) measurements were conducted to record a series enthalpy of relaxation values of the storage at different temperature for various times. The data were further analyzed with a semi-empirical Cowie-Ferguson (CF) model. Comparisons were made among different samples and storage temperatures. Results indicate that the rate of aging is accelerated by temperature and that the presence of a crystalline phase reduces the amount of amorphous regions available for aging and hinders the relaxation process. Dynamic differential scanning calorimetry (DDSC) experiments illustrated the existence of two amorphous regions in semi-crystalline PET, the contribution of each region to aging was resolved for samples aged at 65°C. Moisture absorption investigations demonstrated that moisture is one of the important factors affecting aging and that its effects cannot be ignored even for samples with high crystallinity.
Environmental stress cracking resistance (ESCR) behavior of semi-crystalline PET was studied as a function of crystalline content and aging effect. With increasing crystallinity, critical stress was found to decrease linearly. Tie-molecules theory was used to explain the behavior. Physical aging reduced the ESCR of PET and lead to shortened service lifetime of materials.