Preparation And Characterization Of Tercopolymers And Their Electrospun Nanofibers Derived From ε-CL, TMC And LLA

Biodegradable synthetic aliphatic polyesters are important biomaterial, which were widely studied and applied recent years. Applications of these homo- and copolymers are found in such areas as biodegradable sutures, drug delivery, artificial skin, nerve guides, tissue engineering, and so forth. To suit for different biomedical requirement, copolymerization of different monomers is one of interesting studies for exploring novel biomaterials of aliphatic polyesters. And tri-components copolymerizatin should be an effectual approach for exploring the applications of aliphatic polyesters in biomedical field. In this paper, random and block tercopolymers with different molar ratio were synthesized by bulk ring-opening polymerization .ofε-CL, TMC and LLA, using Sn (Oct)₂ as catalyst, and the electrospinning nanofibers of the tercopolymers were also prepared. The factors of effecting copolymerization, morlecular microstructure, the thermal properties and the mechanical properties of the tercoplymers were investigated. The nonisothermal crystal behavior and nonisothemal kinetics of the tercopolymers were studied. The effect of the electrospinning parameters on the morphology and structure of the nanofibers were discussed. The degradability of the tercopolymers and the nanofibers were also researched.Firstly, the effect of reaction temperature and reaction time on the molecular weight and intrinsic viscosity of the tercoplymers were investigated. The number average weight, intrinsic viscosity and the yield of the tercopolymer increased with the increase of the reaction temperature and the reaction time. The synthesis of the tercopolymers was carried out at 160℃for 25 hours, and the catalyst concentration of 0.05wt% of total monomer composition was selected. In order to obtain the block tercopolymer, copolymerization were carried out at two steps: firstly, PCL pre-polymer were synthesized, then the block tercopolymer were prepared by copolymerization of pre-PCL, TMC and LLA at 160℃for 24 hours.The sequence structure of the random and block tercopolymer was characterized by using 2D NMR which can provide the structural information of molecular framework. The results indicated that apart from the sequences of PLLA, PTMC and PCL, several other new sequences were also presented in the tercopolymer. So the random copolymer was obtained via direct bulk ring-opening copolymerization of TMC, LLA andε-CL by using Sn (Oct)₂ as catalyst at 160℃for 25 hours. The ABA type block copolymer with central block (B block) of PCL and A block of Poly(TMC-co-LLA) was attained by ring-opening copolymerization of pre-PCL with terminal hydro group, TMC and LLA, using Sn (Oct)₂ as catalyst at 160℃for 24 hours. FTIR analysis sported the conclusion obtained from 2D NMR. Qualitative analysis of ¹H and ¹³C NMR on sequence long of the random and block tercopolymers indicated that the changes of molar ratio and reaction temperature effected the both of molar sequence and composition of the tercopolymers.DSC, XRD and mechanical properties tester were employed to study the thermal properties and the mechanical properties of the tercopolymers. The results showed that with the increase of LLA component, and the decrease of TMC component in the random tercopolymers, the T_g decreased gradually. Due to blocky tendency of the PLLA in the tercopolymer, different crystilization existed in the samples of TP1 and TP2, which led to the properties of TP1 and TP2 were different from the others. It was obvious that the crystility and the thermal properties of the block copolymers were different from the random copolymers. The block copolymers with different molar ratio possessed different thermal properties and mechanical properties. The T_g, T_m and W_c decreased gradually with the increase of LLA component, and the decrease of TMC component in the block tercopolymers. Sample Q1 had maximal tensile stress and minimal tensile strain, and Q5 possessed maximal tensile strain and minimal tensile stress.DSC method was used to discuss the nonisothermal crystal behavior and nonisothemal kinetics of the block tercopolymers. The results indicated that the crystal rate of the block copolymer increased gradually with increasing the LLA component, and decreasing the TMC component in the block copolymer. The crystal rate of block copolymers with different component was higher than that of crystal rate of PCL homo-polymer. The crystal temperatures of the block copolymers were higher than that of crystal temperature of PCL at the same relative crystalinity. The crystal temperatures of the block copolymers with different molar ratio at the same relative crystalinity gradually decreased. Ozawa equation adapted to describe the nonisothemal kinetics of the block tercopolymers except for sample Q1. According to Kissinger method, the crystal active energy of the block copolymers was obviously lower than that of crystal active energy of PCL. The crystal active energy of the block copolymers with different components decreased with decreasing LLA component, and with increasing TMC component in the block copolymers. When weight fraction of TMC component in the block copolymer was of 50, the crystal active energy of the block copolymer was minimal.According to the changes of weight and intrinsic viscosity of the tercopolymers after degradation, the biodegradability of the random and the block tercopolymers were investigated by using GPC, DSC, XRD methods. The results confirmed that degradable behaviors between the random copolymers and the block copolymers were different. After degradation time of 225 days, owing to extremely low degradation rate of PTMC, the weight loss and the intrinsic viscosity drop of the random copolymers with different molar ratio decreased with increasing TMC component in the copolymers. However, for the block tercopolymers, the weight loss and the intrinsic viscosity drop increased with the increase of TMC component in the copolymers due to amorphous areas of in the copolymers. After degradation time of 225 days, although the macromolecular chains seriously broken, the molecular weight of degraded products was still high, which could not well dissolved in degradation solution. So, the drops of intrinsic viscosity of the random the block copolymers were high, and the weight losses of the random the block copolymers were low.The results obtained from GPC presented that degradation of the tercopolymers firstly occurred in the amorphous areas and the edge of the crystal areas. With proceed of degradation, the GPC curves obviously broadened and shifted to low molecule weight area. With the further proceed of degradation, crystal oligmer increased, thus, molecule weight distribution presented more peaks. DSC and XRD analysis indicated that the degradation of the random copolymers mainly occurred in PLLA unite during degradation. With the increase of TMC component in the tercopolymers and the increase of degradation time, the T_g of the copolymers gradually decreased. The primary reason for decreasing the T_g was formed oligemer due to macromolecular chain broken during degradation. After degradation time of 225 days, the degradation of the tercopolymers occurred not only in amorphous area but also in partial crystal area. For the block tercopolymers, serious degradation did not happened in the crystal area of PCL, and the PLLA in the block tercopolymers degraded not only in amorphous area but also in crystal area. The electrospinning nano-structured fibers of synthesized random and block tercopolymers were prepared via electrospinning method. The effects of the blended solvent molar ratio, concentration, voltage, and TCD on the morphology and diameter of the electrospun fibers were investigated by SEM. The mechanical properties, crystilisitity and thermal properties of the nano-structured fibers were characterized by mechanical property tester, DSC and XRD. The results showed that the average diameter of the electrospun fibers decreased with decreasing molar ratio of MC to DMF and solution concentration. However, as the molar ratio of MC to DMF and the solution concentration were lower enough, the fibers with beads also appeared. The average diameter of nanofibers slightly decreased with the increase of applied voltage and TCD. The nano fibers with average diameter range of 300～400 ran were prepared by selecting the process conditions (DMF/MC=40/60, concentration=30 wt%, voltage = 12 KV and TCD= 12 cm). Due to the formation of crystal defects of PLLA block in the random tercopolymer during electrospinning, the melting peak located at 48.7℃in DSCcurve of sample TPE-1 appeared. Due to the molecular orientation during electrospinning, crystal exothermal peak of sample TPE-1 located at 17.64V appeared. The T_g, T_m and the W_c of the nanofibers were all higher than that of corresponding copolymers.Studies on the degradation behaviors of electrospinning nanofibers indicated that the degradation behaviors between nanofibers of random and block copolymers were different. Firstly, the intrinsic viscosity of the block copolymer nanofibers gradually decreased with degradation time, and the intrinsic viscosity of the random copolymer nanofibers obviously decreased during degradation time of 45 days, then, he intrinsic viscosity gradually decreased with degradation time. Secondly, due to the effect of crystal behaviors, after degradation time of 225 days, the drop extent of intrinsic viscosity of the block copolymer nanofibers was lower than that of corresponding copolymers. However, the drop extent of intrinsic viscosity of the random copolymer nanofibers was higher than that of corresponding copolymers due to higher surface area.SEM observation of morphology change of the nanofibers after degradation presented that clear fiber fracture was seen with increasing degradation time of TPE-1. After degradation time of 225 days, the fiber membrane became powder. The degradation of sample TPE-2 carried out under polymer membrane with small holes. After degradation time of 225 days, the copolymer membrane morphology with more holes and blocks evolved into crisp membrane with lot of holes distributed on the surface of the membrane. For the block copolymers, after degradation time of 225 days, different extents of the fibers fracture were observed. The SEM observation also showed that degradation extent of random copolymer fibers was higher than that of the block copolymer fibers.The tri-component copolymers synthesized in this work possess positive significance for extending applications of aliphatic polyesters based onε-CL, TMC and LLA in biomedical field. Effective analysis of 2D NMR on sequence structure of the tercopolymer provided theoretical reference for analyzing sequence structure of complex tri-component copolyester. The result tercopolymers and corresponding electrospun nanofibers are expected to have potential uses as biodegradable suture, never regeneration, anti-adhesion membrane, longer time drug release, tissue engineering scaffold and other biomedical materials.