| In recent years,as people pay more and more attention to their health and surrounding environment,bio-polyester materials have been favored by materialists and medics because of their excellent bio-compatibility and bio-degradability.With simple molecular structure and good mechanical properties,these polyester materials have been widely used for medical devices and implants.Now most studies of these polyesters are focusing on the modification of the macroscopic mechanical properties,but the inherent mechanisms on the nanoscale are still unclear.In this thesis,by using atomic force microscope(AFM)-based single molecule force spectroscopy(SMFS)and quantum mechanical(QM)calculations,the inherent single-chain elasticities and the inter-chain interactions(in aggregation)of several typical medical bio-polyester materials(PGA,PCL and P3HB)have been studied,and then the mechanism of the influence of the molecular structure and chain conformation to the macro-mechanical properties of these materials can be revealed at single molecule level.From the SMFS and QM-based theoretical model fitting results,it has been found that the inherent single-chain elasticities of PGA,PCL and P3 HB can be obtained from the single chain force curves in nonane,which cannot be affected by the solvent quality.The inherent stretching elasticity of these polymers can be respectively described well by QM-FRC model,QM-WLC model and QM-FJC model,and the fitting parameters are all related to the length of one repeating unit.According to the obtained single-chain elasticity of PLLA previously,one can find that the inherent single-chain elasticity of PGA is same to that of PCL,and the inherent single-chain stretching elasticity of P3 HB is identical to that of PLLA.However,because of the difference of side chain groups,there is a clear deviation between the force curves of PGA and PLLA.This is due to the steric hindrance effect of methyl groups on the side chains of PLLA and P3 HB,which limits the rotation of ester bonds on the main chain,and finally leads to the difference of the inherent single-chain elasticities during these polyesters.When these polyester chains in water,because there is no special interaction between the polyester chain and water molecules,the obtained single chain elasticity is same to the inherent elasticity,which can be reflected by that the force curve can be superposed very well in the middle and high force regions.Only in the low force region,there is a long platform about 80 pN in the force curve caused by the stretching of collapsed chain because of the hydrophobic interaction.In aggregation state,because there is no special supramolecular structure between their molecular chains,the mechanical elasticities of the chain of PGA and P3 HB are both identical to the inherent elasticities obtained at the single molecule level.However,the mechanical elasticity of the chain of PCL is different from that obtained at the single molecule level,which may be caused by the stretching of folded PCL chain formed by the intra-chain interactions.Based on the above results,it can be concluded that the weak elasticity difference during these polyesters at single molecule level may be the origin of the different macroscopic properties. |
