Study On The Thermal Stability Of Copolyesters Based On Poly(Trimethylene Terephthalate)

In this thesis, the thermal stability and degradation products of a series of poly(ethylene-co-butylene terephthalte) (PET-PTT) and poly(trimethylene-co-butylene terephthalate) (PBT-PTT) copolyesters were investigated.Firstly, the thermal stabilities of PET-PTTs and PBT-PTTs under nitrogen and air atmosphere were tested via thermogravimetry analysis, and were compared with those of PET, PTT and PBT homopolymers. It was found that the thermal stabilities of PET-PTTs under nitrogen lay between PTT and PET, moreover, as the content of PET segments increased the thermal stability of PET-PTT copolymers would be enhanced. On the other hand, the stabilities of PBT-PTTs, PBT and PTT were found to be very close. When studied under air, all the samples would show two weight-loss stages (the second was weak), and the degradation behavior during the first stage was similar with that under air although the onset degradation temperature decreased nearly 50℃. It was deduced that the oxygen in air acted as a catalyzer in the first weight-loss stage and the second stage should be owing to the further oxidative of the first stage. The Friedman and Chang methods were used to calculate the decomposition activation energy and reaction order. The decomposition kinetic parameters of PET-PTTs and PBT-PTTs derived by Friedman method under air atmosphere were about 50% lower than that undernitrogen, so did the parameters obtained from Chang method.Secondly, thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) was used to study the composition of degradation products of PET-PTTs and PBT-PTTs under nitrogen. Although the onset degradation temperature of all samples decreased about 15℃, the thermal stability behavior of those samples were accord with that obtained from single TG study. There were some difference in peak intensity, peak profile and absorption frequency between the FTIR spectra of degradation products of each samples, however, the absorption bands were almost same. It was proved that the degradation products of PET-PTT were CO₂, aromatic compound containning -C(=O)-O-C-, -O-CH₂-CH₂-O and -O-CH₂-CH₂-CH₂-O- group, as well as a little of diphenyl and terphenyl. Due to the -O-CH₂-CH₂-CH₂-CH₂-O- unit in PBT chain, which could react via the six-member cyclic transition state, the absorption of the wide band around 2966 cm^-1 should be owing to the 1,3-butadiene and tetrahydrofuran produced during the degradation process. The 2966 cm^-1 absorption band could be found for PBT-PTT-15 and PBT-PTT-30, but not for PBT-PTT-85. The degradation products of PBT-PTTs were found to be CO₂, aromatic compound containning -C(=O)-O-C-, -O-CH₂-CH₂-O and -O-CH₂-CH₂-CH₂-O- group, as well as a little of diphenyl and terphenyl.The pyrolysis behaviors of PET-PTTs and PBT-PTTs were investigated by pyrolysis gas chromatography-mass spectroscopy (Py-GC/MS). It was found something different from both PET and PTT homopolymers yielded in the degradation process would present in the pyrolysates of PET-PTT copolyesters. However, the composition of the deviation was minor potentially because of the short molecular sequence length of PET or PTT. The PET-PTT copolyesters yielded more volatile products than PET and PTT homopolymers. For PBT-PTT copolyesters, 1,3-butadiene and tetrahydrofuran were found again in the pyrolysis products. The PBT-PTT copolyesters yielded more volatile products as the PET-PTT copolyesters compared with the pyrolysates of their homopolymer. However, a new phenomenon was found for PBT-PTTs, i.e., as the interlink probability of PBT and PTT chain unit became higher, the difference in number and kind of pyrolysates of copolyester from homopolymer were increased. Moreover, for both PET-PTTs and PBT-PTTs the higher content of a component (PET, PTT or PBT) in the copolyesters was, the higher amount of relative pyrolysates of that component would appear.