Studies On The Synthesis, Structure And Properties Of Biodegradable Poly(Butylene Succinate-co-butylene Terephthalate)s (PBST) For Fiber Application

As a response to the problem of increasing domestic waste, biodegradable polymers have attracted more and more attention. To date, aliphatic polyesters have been found to be the most attractive biodegradable synthetic polymers and have been applied to textiles because of good biodegradability. Poly(butylene succinate) (PBS), one of the most important aliphatic polyesters, have already been available on the market because of good biodegradability. However, low melting point (114℃) restricts its large-scale application. On the contrary, in spit of non-biodegradability, aromatic polyesters have been known to possess excellent thermal and mechanical performances. Therefore, introducing aromatic unit into the main chains of aliphatic polyesters seems an interesting strategy to produce new aliphatic-aromatic copolyesters, sharing the favorable biodegradability of aliphatic polyester component and the good physical performance of more rigid aromatic counterpart.In the above cases, firstly, a series of aliphatic-aromatic poly(butylene succinate-co-butylene terephthalate) (PBST) copolyesters was efficiently synthesized from the starting materials of dimethyl succinate (DMS), dimethyl terephthalate (DMT) and 1,4-butanediol (1,4-BD) in the presence of tetraisopropoxide titanium (TTiPO) as the bulk poly condensation catalyst. ¹H NMR proved the prepared products of the PBST copolyesters, and the molar ratios of BS to BT comonomers were almost equal to the feeding ratios of DMS to DMT. According to the sequential distribution, it was indicated that the synthesized polymers were randomness. GPC results showed that the weight average molecular weights (M_w) of the PBST copoyesters were in range of 14.35×10⁴-29.45×10⁴, and degree of distribution ranged from 2.1 to 3.2.From DSC investigation, it was found that melting point, fusion enthalpy and degree of crystallinity all increased with the increasing of BT comonomers, then shifted to decrease. When BT molar fraction was 30%, the lowest thermal parameters were obtained. Furthermore, the relations between thermal properties and copolyester components were discussed by Flory equation, which indicated the synthesized polymers were also randomness. WAXD patterns clearly revealed that the transition of crystal lattice structure from monoclinic lattice of PBS crystal to triclinic lattice of PBT crystal with an increase of BT comonomers, and when the BT comonomers content approached 30 mol%, the shift of crystal diffraction patterns from the PBS to PBT began to occur. According to isothermal crystallization kinetics, it could be known that the crystallization behaviors of PBST copolyesters were attributed to the homogeneous nucleation and spherulitic growth. In addition, the equilibrium melting points of PBS and PBST-10, -50, -70 calculated by Hoffman-Week equation were calculated for 117.9℃, 105.9℃, 173.0℃and 192.5℃, respectively.Thermal degradation behavior for the PBST copolyesters were investigated by the thermogravimetry analyzer (TGA) at a scanning rate of 10℃/min both in nitrogen and air. TGA traces showed that all the samples obviously exhibited only one weight-loss stage when they were heated up to high temperature in nitrogen, while two weight-loss stages were found under the atmosphere of air. According to the Friedman model, the kinetic parameters exhibited the dependence on the BT comonomers composition. Thermal stability was enhanced generally with increasing the content of BT comonomers in both nitrogen and air, and was much better in nitrogen than in air. Moreover, the results obtained by the three models indicated most of the kinetic parameters tended to increase significantly with the thermal scanning rate, implying a strong thermal scanning rate dependence of degradation kinetic parameters. Among the three models, the Friedman model presented the lowest activation energy value but the highest thermal degradation reaction order value.The enzymatic degradation was carried out in the presence of a lipase originated from Pseudomonas cepacia (Lipase PS^?). The weight-loss of PBST was found to decrease with increasing BT content, indicative of decreasing enzymatic degradation rate. DSC curves showed the melting points and fusion enthalpies increased with prolonging the degradation time, which implied that amorphous regions were hydrolyzed prior to crystalline regions or macromolecular chains resulted into recrystallization. The results were confirmed by SAXS analysis. From ATR-FTIR, the degradation depth from the surface of PBST-30 was calculated quantitatively for 2.76μm after 50 days. ¹H NMR analysis showed the BT content kept unchanged after degradation, suggesting the Lipase decomposed the PBST copolyesters unselectively. Besides, it could be clearly observed that the surface of the PBST copolyesters became coarser after degradation than original samples, and many holes appeared in SEM photos.The hydrolysis behavior of PBST was conducted at 60℃under predetermined relative humidity of 60, 70 and 90%RH in an IG-400 humidic chamber. According to the results, it could be observed that the reducing extent of M_w of PBST first increased, then slowly decreased when BT molar fraction was more than 30%. The studies on hydrolysis kinetics revealed the logarithmic plots of M_w as a function of hydrolysis time t exhibited good linearity, which suggested the domination hydrolysis mechanism for the PBST copolyesters was autocatalytic random hydrolysis process. By investigating the effects of test conditions on hydrolysis behavior, it could be found that the hydrolysis rate increased with the increase of relative humidity, but was independent of hydrolysis temperature. Furthermore, ¹H NMR analysis for the PBST copolyesters after hydrolysis revealed little change in composition within experimental limits, indicating the hydrolysis behavior had no choice between BS and BT comonomers.In order to study the spinnability of the PBST copolyesters, PBST-70 with different molecular weights prepared by 5 litres synthesis kettle were selected to investigate their rheological behavior by capillary rheometer. The results showed that the apparent viscosity of PBST-70 decreased with the increase of shear rate and temperature. The shear-thinning results indicated the melt solution belonged to non-Newtonian pseudoplastic fluid. The flow activation energy of PBST-70 decreased with increasing shear rate, so the sensitivity to temperature decreased accordingly. When shear rate kept unchanged, the good linearity could be obtained by plotting non-Newtonian index n of PBST-70 fluid to temperature. Higher the temperature was, larger n became. When temperature stayed steady, n decreased with the increase of shear rate. With the increase of temperature, structural viscosity index of PBST-70 tended to decrease, however, the possibility of thermal decomposition increased. Therefore, it was important to select suitable spinning temperature for the spinnability and stability. In the same shear rate, higher the molecular weight of PBST-70 was, larger the shear stress, apparent viscosity and structural viscosity index were, but smaller n became.