| Due to excellent biodegradability,biocompatibility,physical and mechanical properties and melt processing properties,poly(L-lactide)(PLLA)is widely used in sheet materials,packaging,agricultural mulch,medical equipment and other applications.However,for a long time,the application of PLLA has faced many problems such as uncontrollable degradation cycle,unclear relationship between the internal structure of the material and degradation performance,and Small selection of comonomers used to control the degradation cycle.For this reason,this article selects the hydrophobic long-segment monomer?-valerolactone(DVL)and L-lactide(LLA)to copolymerize to prepare a series of random and block poly(lactide-valerolactone)copolyester,focusing on the differences in the chemical structure,molecular weight,water absorption,thermal properties,crystallization behavior and other properties of random copolymers and block copolymers under different monomer ratios.The degradation law of the copolymer in phosphate buffer was analyzed,and the influence of key parameters such as structural hydrophilicity and hydrophobicity,crystallization performance,spherulitic defects on the degradation performance of modified polylactide was revealed.The use of the same monomer realizes the controllable preparation of modified polylactide with different degradation cycles to accelerate degradation and delay degradation.The specific research content has the following three parts.The ring-opening copolymerization method was used to synthesize poly(lactide-valerolactone)random copolymers(PVLA-R)in different proportions using LLA and DVL as reactive monomers.The chemical structure,water absorption,thermal properties,crystallization properties and spherulitic defects of PVLA-R were tested.The results showed that PVLA-R were successfully prepared under the four feed ratios,and the hydrophilicity of PVLA-R is better than that of PLLA.The molecular weight,thermal properties,and crystallization properties of PVLA-R with different compositions are different.As the molar ratio of DVL increased from 2.5%to 10%,the reaction yield decreased from 86.2%to 43.8%,the molecular weight of the product decreased from 135000 g·mol-1to 36000 g·mol-1.At the same time,the melting point,cold crystallization temperature,glass transition temperature of the copolymer decreased,crystallinity reduced from 20%to 1.5%,and the crystallization ability is reduced.The crystal structure of PVLA-R is consistent with that of PLLA,but the spherulitic defects of PVLA-R are dominated by holes,which are different from the ring-shaped spherulitic defects of PLLA.The ring-opening copolymerization method was used to synthesize poly(lactide-valerolactone)block copolymers(PVLA-B)in different proportions using LLA and PDVL as reactive monomers.The chemical structure,water absorption,thermal properties,crystallization properties and spherulitic defects of PVLA-B were tested.The results showed that the block copolymers were successfully prepared under the four feed ratios,and the reaction yield was stable around 92%,and the number average molecular weight of the product was100000 to 120000 g·mol-1.The hydrophilicity,crystallinity and crystallization ability of PVLA-B are better than PLLA,but the performance of PVLA-B with different compositions is different.With the increase of DVL content,the hydrophilicity of PVLA-B decreases,the melting point,crystallization temperature,and glass transition temperature decrease slightly,but the crystallinity and crystallization ability increase.In the WAXD spectrum,the diffraction peaks of PVLA-B are dominated by the characteristic diffraction peaks of PLLA,and a faint characteristic peak of PDVL crystal appears at the same time.The spherulitic defects of PVLA-B are mainly ring-shaped,which is consistent with the spherulitic defects of PLLA.In vitro degradation tests of PVLA-R,PVLA-B and PLLA were carried out in a constant temperature and humidity chamber at 37?and a humidity of 50%.The differences in degradation behavior of different modified copolymers were compared through intrinsic viscosity,weight loss,and p H value of the degradation solution.The results show that when the initial DVL feed molar content is 7.5%and 10%,the intrinsic viscosity,residual mass,and p H value of the degradation solution of PVLA-R and PVLA-B all decrease as the degradation progresses,and the degradation rate of PVLA-R is higher than that of PVLA-B.In order to explore the reasons for the difference in degradation,further tests were carried out on the crystalline properties,chemical structure,water absorption and other properties of the copolymer degradation residues.The results show that PVLA-R has already degraded the crystal area after 8weeks of degradation,and its crystal area has weaker hydrolysis resistance than PLLA;the mass loss of PVLA-B after 8 weeks of degradation mainly comes from the amorphous area,and the hydrolysis resistance of PVLA-B crystal area is better than that of PLLA.The lower DVL content in PVLA-R residues(except for products with 10%DVL feed content)indicates that the degradation rate of the DVL segment in PVLA-R is faster than that in the LLA segment;the higher DVL content in PVLA-B residues after degradation indicates that the degradation rate of DVL segment in PVLA-B was slower than that in LLA segment.The water absorption of PVLA-R and PVLA-B degradation residues both increase with the progress of degradation,showing higher hydrophilicity than before degradation.In this paper,the PVLA-R random copolymer and PVLA-B block copolymer were successfully synthesized by the ring-opening polymerization method.Through the study of the in vitro degradation performance of the product,the possibility of delta-valerolactone modified PLLA to regulate the degradation of PLLA was explored,which provided a reference for the future degradation research and application of PLLA. |
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