Preparation And Characterization Of PET-PCT Copolyester And Its Ionomer

A series of poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) copolymers (PET-PCTs) were prepared through direct esterification technique from terephthalic acid, ethylene glycol (EG) and 1,4-cyclohexane dimethanol (CHDM). Furthermore, two sulfonated PET-PCTs (SPET-PCTs) containing ionic groups were prepared via interchange esterification from dimethyl terephalate (DMT), 5-sulfoisophthalate' sodium salt (3～5mol%, with respect to DMT), EG and 10mol% of CHDM.The chemical composition and sequence length distribution of the copolyesters were investigated by means of ¹H-NMR and ¹³C-NMR. The results demonstrated that PET-PCTs were random block copolyesters and the content ratio of PCT/PET was always greater than the loading ratio of CHDM/EG, moreover, the number-average sequence length of each kind of block was in proportion to its content in the copolymer. SPET-PCT samples were analyzed via FT-IR, the appearance of absorption peak located at 628cm^-1, which was characteristic of the stretching vibrations of C-S bond, confirmed that the ionic group -SO₃Na had been chemically linked in the polymer chains.The crystallization behavior of the copolyesters was studied by means of DSC. It was found that the glass transition temperature(Tg) of PET-PCT increased with the increasing content of PCT units, non-crystallization polymer would be obtained when the CHDM content reached to 15mol%. On the other hand, the annealed samples with low PCT content showed two melting temperatures(Tm) within the low and high temperature region respectively, furthermore, the values of Tm and melting enthalpy decreased with the content of PCT units. Therefore, copolyesters with different crystallinities or even the amorphous products could be obtained by controlling the loading ration of EG/CHDM. Concerning the SPET-PCT samples, no melting points have been found in their DSC curves, indicating that those ionomers were amorphous.The WXRD results showed that with the low content of CHDM, the copolyester possessed a similar crystalline diffraction to that of PET, and then gradually became amorphous with the increasing of CHDM content. However, SPET-PCTs never showed crystalline diffraction peaks. These results were all in concordance with those of DSC and POM.The densities of PET-PCTs and SPET-PCTs were measured in a density gradient column. The density of PET-PCT decreased with the increscent content of CHDM while the density of ionomer was lower than that of PET-PCT with a same CHDM content. TGA showed that PET-PCTs possessed an excellent thermal stability, i.e., the onset degradation temperatures were higher than 400℃and the temperature of the maximum weight loss rate were higher than 435℃. The analysis on thermal decomposition kinetics of PET-PCTs has also been performed with Friedman method, the activation energy and the reactive constant of thermal degradation of PET-PCT decreased slightly in contrast with PET, so did the ionomers. However, the thermal stability difference between any copolyester was little.Pyrolysis gas chromatograph-mass spectrogram has been used to investigate of pyrolysates of the copolymers. It has been found the components of pyrolysate were rather different under various temperatures. Especially, the characteristic pyrolysate products were distinct, affording rich information to identify the structure of those copolymers.The micro-phase structures of copolymers have been investigated by using dynamic rheology measurement (ARES-RFS) and dynamic mechanical analysis (DMA). A divergence from the linear relationship between storage modulus(G') and frequency(freq) has been found within the low freq region, and this terminal region effect would shift to higher freq region with the increasing content of PCT in the copolymers. Moreover, two Tg's have been observed from DMA curves for the samples soever annealed or not. Therefore, it could be concluded that the micro-phase separation had taken place in PET-PCT copolyesters.