Design,synthesis And Properties Of High Performance Biobased Liquid Crystal Copolyesters

The development and application of bio-based polymers is an important way to achieve the goal of carbon peaking and carbon neutrality,and it is also the main path to achieving green and sustainable development.However,although conventional bio-based polymers such as polylactic acid(PLA)and polycaprolactone(PCL)have excellent biocompatibility and degradability,their mechanical properties and thermal stability are insufficient due to the limitation of their flexible molecular chain structure,which limits application in the field of weight-bearing bone graft repair.On the other hand,aromatic liquid crystal copolyester(TLCP)has excellent mechanical properties(high compatibility with the mechanical properties of weight-bearing bone),thermal stability and processability,but its biocompatibility,hydrophilicity and degradability are poor,which also limits its application in the field of Medical tissue engineering.The introduction of flexible bio-based units into the molecular chain of TLCP is an effective way to improve its hydrophilicity and degradability.Therefore,it is necessary to obtain bio-based liquid crystal polymers with excellent comprehensive performance through molecular structure design and suitable polymerization process by combining liquid crystal properties with the advantages of the bio-based monomer composition.In this paper,to obtain bio-based liquid crystal polymers with excellent comprehensive properties,firstly,a series of high-performance bio-based liquid crystal copolyesters derived from HBA(p-hydroxybenzoic acid),biobased HPPA(p-hydroxyphenylpropionic acid),VA(vanillic acid)and LA(lactic acid)units were designed and prepared by one-step continuous melt polymerization process consisted of acetylation-polycondensation-viscosity enhancement,and their structures and properties were investigated by experimental characterization and molecular simulation techniques.The copolyesters showed relatively low melting points(176-229 °C)and controlled crystallinity(15.3-36.5%)due to increased molecular chain flexibility caused by LA units and multicomponent copolymerization effects.The bio-based liquid crystal copolyesters had a wide nematic liquid crystal phase temperature range,good thermal stability,and shear thinning and viscous melt flow behavior,indicating they had excellent melt processing capabilities.Compared with polyetheretherketone(PEEK)materials,bio-based liquid crystal copolyesters exhibited better mechanical properties,hydrophilicity and biocompatibility due to the combined advantages of liquid crystal properties and molecular structure,of which tensile strength and contact angle were between 95-175 MPa and 72.5-84.8°,respectively,as well as cellular activity was higher than 78.4% after 72 h incubation.Secondly,to further improve the bioactivity of bio-based liquid crystal copolyester,a series of BTLCP(bio-based liquid crystal copolyester)/n-HA composites were prepared by compounding n-HA(nano-hydroxyapatite)through one-pot melt in situ polymerization method,and the structure and properties of the composites were investigated.The addition of n-HA did not reduce the effect of multicomponent copolymerization on molecular chain fluidity,and the composites were also observed relatively low melting points(228-255 °C)and a wide melting range(36-62 °C).The addition of n-HA improved the thermal stability of the composites without disrupting the wide temperature range of the nematic liquid crystal phase,shear thinning and the viscous melt flow behavior,still maintaining the excellent melt processing ability of the liquid crystal copolyester.As compared with HA/PEEK composites,the BTLCP/n-HA composites exhibited better mechanical properties and hydrophilicity due to the combined advantages of liquid crystal properties and chemical composition,with tensile strength and contact angle values ranging from 61-135 MPa and 76.3-85.6°,respectively,which have potential applications in the field of weight-bearing bone graft repair.Finally,proteinase K solution,PBS solution(phosphate buffer)and dilute Na OH solution was used to degrade three bio-based liquid crystal copolyester P-HA35LA10,P-HA25LA20 and P-HA35LA25 with different LA contents,respectively,and the degradation behaviors were investigated by means of mass loss rate before and after degradation,FTIR,SEM,DSC,XPS and other experimental measurements.Copolyester degradation first occurred by erosion on the sample surface,mainly involving the breakage of aliphatic LA ester bonds,and the degradation mass loss rate was positively correlated with the content of flexible LA units.The mass loss rate of copolyester P-HA35LA25 with more LA content was 39.7% and 23.3% after 13 weeks of degradation in an enzyme solution and PBS solution,respectively,and 35.3% after 36 days of degradation in Na OH solution,while the mass loss rate of P-HA35LA10 with less LA content was only 0.1-1.8% in the three kinds of degradation media.The degraded molecular fragments dissolved in the degradation medium and the mass loss rate exhibited an increasing trend with time;the breakage of macromolecular chains affected the thermal properties of the degraded copolyester and could reflect the degradation behavior;Unlike PBS and Na OH solution,proteases also had decomposition effect on aromatic ester bonds,indicating that the degradation mechanism correlated with degradation medium.Other structural parameters such as the crystallinity and composition of the copolyesters also had an influence on the degradation behavior.The copolyesters’ molecular weight gradually decreased and the hydrophilicity increased as degradation proceeded,but still had nematic liquid crystal morphology and certain thermal stability after degradation.Therefore,the regulation of degradation behavior can be achieved by modulating the composition of bio-based liquid crystal copolyester,which is beneficial for its subsequent application.