Preparation And Physical Properties Of Poly(Glycolic Acid) And Its Copolyester

As a new type of biodegradable engineering plastic,poly(glycolic acid)(PGA)is the simplest linear biodegradable polyester.PGA has been widely used in the fields of oil and gas exploration,packaging materials,and biological medicine,such as drug nano-carriers,prostheses,and absorbable surgical sutures,due to its excellent biocompatibility,degradability,mechanical strength,and gas barrier property.Generally,PGA can be prepared through the melt polycondensation of glycolic acid(GA)and ring-opening polymerization of glycolide.Although the high-molecular-weight(HMW)PGA can be prepared through the ring-opening polymerization of glycolide,the synthesis process of glycolide is very complicated,resulting in the high price of PGA.In this thesis,we systematically investigated the polymerization process of PGA and its copolyesters through the melt polycondensation route,as well as their structures and properties.Firstly,we compared the four metal salt catalysts bismuth(?)trifluoromethanesulfonate[Bi(OTf)₃],zinc acetate[Zn(Ac)₂],zinc acetate dihydrate[Zn(Ac)₂·H₂O],and titanium(?)isopropoxide[Ti(Oi Pr)₄]and a metal-free catalyst bis(trifluoromethane sulfonimide)(Nf₂NH)to prepare PGA by the melt polycondensation of GA.It is found that Bi(OTf)₃ showed the best catalytic activity.Under the catalysis of Bi(OTf)₃,the effects of esterification temperature(160-190?)and catalyst content(0.05-0.25 wt%)on the intrinsic viscosity,thermal stability,and melting temperature(T_m)of PGA were studied.The optimal polymerization conditions of PGA was that the catalyst content was 0.2 wt%,the esterification and melt polycondensation temperatures were 180 and 210°C,respectively.Under such optimal polymerization conditions,the prepared PGA was light gray and had a intrinsic viscosity higher than 0.35 d L/g.Moreover,the PGA had 5 wt%thermal decomposition temperature of?250°C and T_m of?220°C,showing good thermal stability.Solid-state polycondensation is a common method to improve the molecular weight of polyesters by removing small molecules at a relatively low temperature.Thus,the solid-state polycondensation was further used to improve the molecular weight of PGA.Under the catalysis of Bi(OTf)₃ with the same solid-phase polycondensation time(12 h),the influences of solid-phase polycondensation temperatures(190-220°C)and catalyst contents(0.05-0.20 wt%)on the intrinsic viscosity of PGA were studied.The optimal solid-phase polycondensation temperature was 210°C and the catalyst content was 0.2 wt%.Under such solid-phase polycondensation conditions,the intrinsic viscosity of PGA can be increased from 0.36 d L/g to0.59 d L/g.Secondly,the incoporation of GA units into polyethylene terephthalate(PET)chains through copolymerization is expected to achieve the degradability of PET,due to the excellent degradation performance of PGA.The four promising polymerization routes by using different monomers and by varying feeding sequences were ultilized to prepare poly(ethylene terephthalate-co-glycolic acid)[P(ET-co-GA)]copolyesters.¹H nuclear magnetic resonance spectroscopy results demonstrate that P(ET-co-GA)copolyesters were successfully prepared by the four polymerization routes.P(ET-co-GA)copolyesters with different GA contents were prepared by one-pot melt polycondensation by varying feeding ratio.The effects of GA content on crystallization behavior,thermal properties,and degradability of P(ET-co-GA)were studied.The thermal decomposition temperatures of PET and P(ET-co-GA)with different GA contents were higher than 300°C;this indicats that the incoporation of GA units into PET has no significant influence on the thermal stability of copolyesters.Wide-angle X-ray scattering results show that the PET-rich P(ET-co-GA)copolyesters formed PET-type crystals.The characteristic diffraction peaks of PET-rich P(ET-co-GA)shifted to lower q side with increasing GA content,suggesting that the inclusion of GA units in the crystal lattice led to the expansion of PET crystal lattice.Moreover,the incorporation of GA obviously affected the melting and crystallization behaviors of P(ET-co-GA).When the GA content increased from 0 to 11.98%,the melting temperature and the crystallization temperature decreased from 247.40°C to 214.20°C and from 201.10°C to 145.50°C,respectively.The introduction of GA units into PET chains obviously improved the degradation rate of copolyesters during degradation in simulated seawater and in phosphate buffer solution containing lipases.The degradation rate of copolyesters accelerated as the GA content increased.Finally,P(ET-co-GA)with the intrinsic viscosity of higher than 0.30 d L/g was prepared after optimizing the one-pot melt polymerization conditions,and its intrinsic viscosity was further improved to 0.70 d L/g by applying solid-phase polycondensation.Meanwhile,the preparation of P(ET-co-GA)was also achieved in a 2.5 L polycondensation reactor by one-pot melt polymerization.The intrinsic viscosity of P(ET-co-GA)was higher than 0.60 d L/g,which can be processed by injection molding and has good processability.