Synthesis And Modification Of Degradable PLA-PBT Copolyesters

During the 20th 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 the recent years, people have payed more and more attention to investigating and developing novel biodegradable polymers as their consciousness of environmental protection improved. Due to their good biodegradability, aliphatic polyesters have been regarded as one of the most important biodegradable polymers and became a hot research topic. However, the poor thermal stability and mechanical properties as well as relative high cost have drastically restricted their application. On the contrary, aromatic polyesters have excellent thermal stability, good mechanical properties and relative low cost, although they could not be degraded. Therefore, incorporation of biodegradable aliphatic units into the molecular chain of aromatic polyesters has been regarded as an effective strategy to obtain novel biodegradable copolyesters. In this dissertation, two main research have been carried out. First, synthesis, characterization and properties of degradable aliphatic aromatic copolyesters; second, modification of degradable aliphatic aromatic copolyesters by nanohybrids or nanocomposites.In the first part, degradable aliphatic-aromatic copolyesters, poly(butylene terephthalate-co-lactate) (PBTL) were synthesized via environment-friendly and economical direct melt polycondensation of terephthalic acid (TPA),1,4-butanediol (BDO) and poly(L-lactic acid) oligomer (OLLA). NMR, FT-IR, DSC, XRD and DMA analysis clearly indicated that the glass-transition temperature, thermal stability and tensile strength were gradually increased with the decrease of aliphatic lactate moieties in the final copolyesters. Hydrolytic and soil degradation results demonstrated that the incorporation of lactate moieties into aromatic polyesters endowed the copolyesters good degradability and their degradation behaviours could be easily tailored through adjusting the lactate molar content in the copolyesters. In order to enhance the thermal stability and mechanical properties, one copolymerized component, rigid diols cyclohexanedimethanol (CHDM) was added in the TPA, BDO and OLLA reactions to synthesize a series of degradable aliphatic-aromatic copolyester (PBCTL). The results demonstrated that CHDM had higher reactivity than BDO. When the content of CHDM was 5 mol%, the weight-average molecular weight of the copolyester was up to 89400 g/mol. The incorporation of CHDM rigid ring structure dramastically improved the thermal stability and mechanical properties of the copolyesters. When the CHDM content increased from 0 mol% to 5 mol%, the glass-transition temperature, initial decomposition temperature and tensile strength were significantly increased from 26.9 0 C,282.5℃and 6.4 MPa to 36.2℃,309.7℃and 19 MPa.To further improve the degradability, one hydrophilic diol, polyethylene glycol (PEG) was selected as copolymerized componet to react with TPA, BDO and OLLA via direct melt polycondensation, and synthesized a series of degradable aliphatic-aromatic copolyester (PBTLG) with different PEG molecular weight and content. The results indicated that adding PEG decreased the viscosity of reaction system, promoted the condensation reaction and finally obtained copolyester with the weight-average molecular weight up to 177000 g/mol. Water absorption and water contact angle measurements showed that the introduction of hydrophilic PEG remarkably improved the hydrophilicity and significantly increased the degradation rate of the polyesters. For example, the weight loss of PBTLG1000-1.0 was up to 39% after 40 days at 60℃in PBS.In the second part, nano-SiO2 was first selected as one nanoparticle to copolymerize with TPA, BDO and OLLA via in situ melt polycondensation and prepare a novel degradable aliphatic-aromatic copolyester nanocomposites. The results revealed that during the polycondensation, the abundant hydroxyl groups on the surface of nano-SiO2 provided potential sites for in situ grafting with the simultaneous resulted copolyester, so that the SiO2 nanoparticles were chemically wrapped with copolyester main-chains. The copolymer chains grafted onto SiO2 particle surface not only effectively impeded nanoparticles aggregated and made good disperse, but also enhanced the interfacial adhesion between SiO2 inorganic phase with the organic phase copolyester matrix. So the thermal stability and mechanical properties of the PBTL/SiO2 nanocomposites substantially increased. Hydrolytic degradation indicated that the degradability of the composites did not be obviously affected due to the introduction of SiO2.Secondly, a new POSS nanocompound containing two functional groups, BH-POSS, was synthesized via Michael addition reaction of amino POSS compounds (AI-POSS) with hydroxyethyl acrylate (HEA). Based on the polycondensation of TPA, BDO and OLLA, BH-POSS and two other POSS compounds (AI-POSS and PEG-POSS) were introduced into the resulting copolyesters and prepared a novel kind of degradable aliphatic-aromatic copolyester nanohybrids containing POSS. The results suggested that AI-POSS just dispersed physically and aggregated seriously in the composites, but BH-POSS could take part in the copolymerization to make their nanocage structure covalently linked to the main-chains of copolyesters, which effectively prevented the agglomeration of POSS nanoparticles and made POSS dispersed uniformly in the matrix, thus the thermal stability and mechanical properties of the nanohybrids were significantly enhanced. Hydrolytic degradation showed that the incorporation of PEG-POSS containing hydrophilic groups improved the degradability of the nanocomposits to some extent.