Synthesis And Properties Of Biodegradable Liquid Crystalline Polymeric Materials

A number of biodegradable aliphatic polyesters have been developed because of their good biodegradability. However, their poor thermal and mechanical properties provide obstacles to their further application. The combination of aromatic and aliphatic units in the same polyester chain has been investigated for a long time as an attractive approach to obtain novel products encompassing biodegradability and high performance properties.It is well known that thermotropic main-chain liquid crystalline (LC) polyesters possess more excellent thermal stability, processability and self-reinforced mechanical strength than the simple aromatic copolyesters due to the presence of highly oriented aromatic moieties. However, poor solubility, high melting point, and inadequate degradability make LC polyesters of limited application in biodegradable materials. Thus it is of considerable interest to develop LC polyesters with enhanced degradability. The basic concept for this is introducing LC segments into the biodegradable aliphatic polyester backbones, which will combine the biodegradability of aliphatic polyesters and the liquid crystallinity of aromatic polyesters.' From this point of view, this thesis is aimed at exploring the possibility of developing a series of liquid crystalline aromatic-aliphatic copolymers with better physical properties as well as still having biodegradability.To increase the thermal and elastic properties of the aliphatic polyester poly(butylene succinate) (PBS), a series of potentially biodegradable liquid crystalline aromatic/aliphatic random copolyesters are prepared by melt polycondensation of a new mesogenic monomer MTB, dimethyl succinate, and 1,4-butanediol. The synthesized copolyesters are characterized by means of ~1H-NMR, DSC, TGA, XRD and PLM. The MTB content is varied so that the effects of the mesogen content on the thermal and mechanical properties, degradable behaviors and mesophase are examined respectively. It is found that introducing rigid rod mesogens can increase the thermal stability and the elastic properties, while it reduces the melting temperature, the crystallization temperature, the degree of relative crystallinity and the hydrolytic degradation rate. The copolyester P100 is able, to show the schlieren texture characteristic of nematics by PLM.Owing to the poor copolycondensability of the aliphatic units with the aromatic ones, its degree of polymerization is usually quite limited, and thus aromatic/aliphatic copolyesters with high molecular weight cannot be easily achieved. Therefore, chain extended biodegradable liquid crystalline aromatic/aliphatic copolyesters are synthesized by melt polymerization and solution polymerization of the prepolymers and chain extender hexamethylene diisocyanate (HDI). The effects of reaction temperature, HDI content, and catalyst on chain extension are examined respectively. Furthermore, the effects of chain extension reaction on polyesters molecular weights, thermal and mechanical properties, and biodegradable behaviors are investigated respectively. The catalysis mechanism and the copolyesters morphological textures are also investigated. It is found that increased reaction temperature and an excess amount of chain extender can not only activate the chain extension reaction, but also accelerate the side reactions such as crosslinking. The inherent viscosities of the copolyesters are remarkably increased under the action of catalyst, leading to an increase in the tensile strength. The degree of relative crystallinity, the melting temperature, and the rate of degradation decrease after chain extension.Biodegradable polymers used in the medical field are ideally composed of metabolites found in the living body. Novel biodegradable-cum-photoactive liquid crystalline homopolymer and copolyester are prepared by melt polycondensation of ferulic acid (FA), 4-hydroxybenzoic (HBA) and D,L-lactic acid (LA) in the presence of acetic anhydride and a transesterification catalyst. FA, HBA, and LA have been confirmed nontoxicy and well biocompatible in the body. It is found that modestly increasing LA content and adjusting feed ratios can enhance the solubility and biodegradability of the copolyesters, still retaine the liquid crystallinity. The obtained copolyester can degrade in phosphate buffer solution (PH 7.2) with enzyme proteinase K, and can crosslink by UV-irradiation at ambient temperature due to the photoactivity of FA moieties.Two series of biodegradable segmented aliphatic-liquid crystalline copolymers are prepared by solution polymerization of poly(L-lactic acid) (PLA), mesogenic diol prepolymer (MD, FBH), and chain linker HDI. The mesogenic diol content is varied so that the effects of the mesogen content on the thermal and physical properties, degradation behaviors and morphological textures are examined respectively. It is found that introducing mesogens units can increase the thermal stability and the elastic properties, while reduces the melting temperature, the degree of crystallinity and the biodegradation rate. 20 mol% MD or 30 mol% FBH content is sufficient to impart a liquid crystalline texture to the copolymers.