The Study Of Nano-mechanics For Aromatic Polyesters

Over the years,the rapid development of China's polymer materials field,the gradual emergence of aromatic polyesters also appeared in the public eye,as a synthetic polymer materials,due to its better mechanical properties,cheap,easy to process and other properties,widely used in capacitors,fibers,engineering plastics and other fields.However,aromatic polyesters are less degradable in the environment and easily pollute the environment.In recent years,most people have been studying the degradation properties of materials and the modification of macroscopic mechanical properties.However,few people have studied the microscopic mechanical properties of the materials from the nanoscale.In this thesis,the single-molecule force spectroscopy technique,the single-chain elasticity theoretical model based on quantum mechanical(QM)calculations,and NAMD molecular dynamics simulations are used as research tools to investigate the intrinsic elasticity of three aromatic polyesters(PET,PTT,and PBT)in the nonpolar organic solvent nonane,so as to explain the intrinsic connection between the macroscopic mechanical properties of polyester materials and the single-molecule chains at the single-molecule level.The obtained single-molecule force spectrum curves,combined with the QM single-chain elasticity theoretical model fitting analysis,find that all three aromatic polyester molecules,PET,PTT,and PBT,are able to obtain their intrinsic elasticity in nonane at low concentrations,with no relation to the solvent quality.After fitting the theoretical model of single-chain elasticity,PET and PTT can be fitted by the QM-WLC model with the fitting parameters are respectivelyl_p(28)0.4 5nm?l_p(28)0.53nm,which correspond to exactly half of the length of the molecular chain structure units of PET and PTT,respectively.PBT cannot be fitted by the QM-WLC model,but can be fitted by the QM-FRC with the fitting parameter are _bl(28)0.63nm,which is exactly the length of the sub-methyl of the PBT molecular structure This means that the fitting parameter of the single-chain elasticity theory model describes the results related to the structure of the individual molecular chains of the polymer.Subsequently,combined with NAMD molecular dynamics simulations,the single-chain elasticity of trimers of PET,PTT and PBT molecules in aqueous solution was systematically investigated.The force curves of PET,PTT and PBT derived from the single-molecule force spectroscopy experiments and molecular dynamics simulations can overlap well,and all of them can characterize the elasticity of single-molecule chains.This also indicates that the three polyester molecular chains have no special interaction with water molecules in aqueous solution.Secondly,comparing the intrinsic elasticity of the aromatic polyesters in this thesis,PET and PTT can overlap well throughout the force region,and the intrinsic elasticity curves of both are the same,which is due to the benzene ring and ester bond conjugation on the main chain leading to the blocked rotation of their individual molecular chains that play a dominant role,but the number of methylene groups in the molecular structure is small enough to affect the elasticity of the main chain,thus indicating that these two polyester materials show rigidity.PET is more rigid than PBT,and also consumes more energy during the stretching of individual molecular chains.Through mathematical calculations,it is found that the difference in the number of methylene groups results in an energy difference are 0.292k J mol^-1unit^-1,This may be one of the reasons for the large macroscopic mechanical differences between PET and PBT.Finally,in combination with several aliphatic polyesters(PGA,PCL,PLA)previously studied by the group and aromatic polyesters to do a comparison of the intrinsic elasticity,it was found that aromatic polyesters are more rigid than those of aliphatic polyesters,this difference between molecular chains may be due to the main chain molecular structure more determine the mechanical properties of the polymer,the benzene ring and carboxyl groups form a conjugate structure affect the intrinsic elasticity of polyester molecules.This is one of the reasons for the large macroscopic mechanical differences between these two types of polyester materials.