Synthesis Of Biodegradable Copolyesters And Antibacterial Polyesters Via Melting Transesterification And Their Nanocomposites

During the20th century the advent and rapid development of synthetic polymer materials greatly improved the level of human's life, but a huge number of plastic waste also caused serious environmental pollution. In recent years, people have paid more and more attention to investigating and developing novel biodegradable and antibacterial polymers materials as their consciousness of environmental protection and contamination of polymer material surfaces by micro-organisms improved. Due to their good biodegradability and biocompatibility, aliphatic polyesters have been regarded as one of the most important biodegradable polymers and become a hot research topic. However, the poor thermal stability and mechanical property as well as relative high cost have drastically restricted their application. Poly(butylene succinate)(PBS) has also been known as an important biodegradable aliphatic polyester and has attracted much interest because of its good degradation property, excellent processability and physical property. However, the product of PBS has poor biomedical application owing to its high hydrophobicity and crystallinity as well as low degradation rate. On the contrary, aromatic polyesters have excellent thermal stability and good mechanical property, although they could not be degraded. Therefore, the design and development of novel biodegradable aliphatic/aromatic copolyesters by incorporation of biodegradable aliphatic units into the molecular chain of aromatic polyesters have become increasingly important. Carbon nanotubes (CNTs) are unique nanostructured materials with remarkable physical and mechanical properties, such as high elastic modulus, as well as remarkable thermal and electrical conductivity, and provide a new approach for improving the biodegradable polyesters' properties and multi-function. Currently,N-halamine antibacterial polymers have shown to be disinfectants which provide almost instant and total kill of a wide range of micro-organisms. In this dissertation, four main researches have been carried out. Firstly and secondly, synthesis and characterization of aliphatic copolyesters based on PBS and biodegradable aliphatic/aromatic copolyesters via melting transesterification, thirdly, synthesis of antibacterial and biodegradable aliphatic polyesters, fourthly, modification of biodegradable aliphatic polyesters by carbon nanotubes.In the first part, a series of aliphatic biodegradable poly(butylene succinate-co-D, L-lactide) copolyesters were synthesized with the aim of decreasing the crystallinity of poly(butylene succinate)(PBS) and improving its degradation rate by incorporation of D, L-oligo(lactic acid)(OLA) into the PBS molecular chains. Due to poly(ethylene glycol)(PEG) is a non-toxic and good hydrophilic polymer, the introduction of OLA and PEG into the PBS molecular chains might also be useful to modify the hydrophilicity of poly(butylene succinate-co-ethyleneoxide-co-D, L-lactide) copolyesters while maintaining its biodegradation rate. Moreover, in combination with biobased diol isosorbide (ISO), monomers such as dimethyl succinate (DMS),1,4-butanediol (BDO) and PEG were used in the synthesis of copolyesters with relatively higher glass transition temperature. The results demonstrated that the incorpation of OLA, PEG and ISO, reduced the degree of crystallinity of PBS effectively. The thermal stability, crystallinity and tensile strength were gradually decreased with the increase of aliphatic lactide units in the resulted copolyesters. Enzymatic degradation and hydrophilicity test results demonstrated that the the incorpation of OLA and PEG into PBS chains endowed the copolyesters good degradability and hydrophilicity, and their degradation behaviors could be easily tailored through adjusting the OLA molar content in the copolyesters.Secondly, poly(butylene terephthalate-co-D, L-lactide) aliphatic/aromatic copolyesters with good biodegradable and mechanical property, were successfully prepared by the melting transesterification between OLA and aromatic polyester poly(butylene terephthalate)(PBT) in the presence of BDO without any catalysts. The results demonstrated that the averaged sequence length of butylene terephthalate segment played a leading role in the solubility, thermal and biodegradable properties of copolyesters. The PB10LA40copolyester with a PBT/BDO/OLA starting molar ratio of50/10/40and reacted at260℃for4h exhibited the lowest values of Tm, Tm, ΔHc, ΔHm, good degradation behavior and biocompatibility. In order to improve the hydrophilicity of copolyesters, OLA, poly(butylene terephthalate)(PET) and PEG were mixed together and underwent the transesterification to synthesize poly(ethylene terephthalate-co-ethyleneoxide-co-D, L-lactide) copolyesters in the absence of any additional catalysts. The results demonstrated that the decrease of crystallization and melting temperatures was mainly caused by the substantial transesterification due to the lagre amount of OLA, resulting in the copolyesters with short sequence length of ethylene terephthalate segment. The increasing of the molar content of soft PEG segment led to the increase of tensile strength and tensile modulus of copolyesters. All the copolyesterrs showed sharp weight loss during the period of2and3months in the soil.Thirdly,3-(N, N-di-β-hydroxyethylaminoethyl)-5,5-dimethylhydantoin was adopted as diol comonomer and was combined with both DMS and BDO to synthesize antibacterial copolyesters via melting transesterification. Furthermore, novel aliphatic poly(s-caprolactone)(PCL) with pendent propargyl groups were successfully prepared by ring-opening polymerization and subsequently used to graft antibacterial hydantoin moieties via click chemistry by a copper(Ⅰ)-catalyzed azide-alkyne cycloaddition reaction. The results demonstrated that the incorporation of hydantoin moieties into PBS main chains by copolymerization and grafting hydantoin onto PCL polymer chains via click chemistry, could lead to the copolymers exhibiting better thermal stabilities than PBS and PCL at a lower degree of crystallinity. After chlorination, hydantoin structure with N-H groups could successfully be transformed into N-halamine wih N-Cl, which could release oxidative Cl+and comprised an antimicrobial activity regarding E. coli.PBS/SWCNT-APTES and PBSG/MWCNT-APTES nanocomposites with excellent dispersion of CNTs and mechanical property were successfully prepared through silication and physical blending between PBS, poly(butylene succinate-co-ethylene glycol)(PBSG) aliphatic polyesters and acyl aminopropyltriethoxysilane functionalized single-and multi-walled carbon nanotubes (SWCNT-APTES and MWCNT-APTES). PBS and PBSG chains were covalently attached to the SWCNT-APTES and MWCNT-APTES by hydrolysis, respectively. The incorporation of SWCNT-APTES and MWCNT-APTES enhanced the thermal stabalities and nonisothermal crystallization rate of the PBS and PBSG in the nanocomposites, while remaining the crystal structure. Compared to nanocomposites prepared by blending, nanocomposites prepared by hydrolysis showed significant improvement in the dispersion of SWCNT-APTES and MWCNT-APTES in the polymer matrices, as well as interfacial adhersion, resulting in the improving of mechanical properties of nanocomposites.