The Effect Of The Internal Rotation Barrier Of The C-C Single Bond On The Flexibility Of Single-Chain Macromolecule

The conformational interconversion of polymer chain has attracted the attention of scientists for decades.With the development of characterization methods,a series of theoretical results that describe the conformational interconversion of the polymer chain,have been reported.To date,many theoretical models have been proposed to describe the conformational interconversion of polymer chain.However,investigations on the conformational interconversion of the polymer chain under different environments?non-polar organic solvents and high vacuum?at the single chain level are rarely reported.In this thesis,atomic force microscope?AFM?-based single molecule force spectroscopy?SMFS?has been used to study the conformational interconversion of the polymer chain in nonpolar organic solvents and high vacuum.Firstly,the single-chain elasticity of polymer chain in CCl₄ solution is obtained,which can be well fitted by the QM-FRC model.Secondly,the internal rotation barrier and the conformational interconversions frequency of C-C single bond have been studied.Finally,the single-chain elasticity of polymer chain under high vacuum has been studied.On this basis,the single-chain elasticities of polymer chains under different environments were compared.Based on these investigations,the main conclusions in this thesis can be summarized as follows:?1?The internal rotation barrier of C-C single bond in the n-alkane molecule?ethane,propane and n-butane?have been studied.The barrier contribution factor???is introduced to calculate the effect of the side group on the internal rotation barrier of C-C single bond.By combining previous research results,it is found that the barrier contribution factor is in the range of 0.36???1.08.On this basis,a new method is proposed for calculating the internal rotation barrier of small molecule around C-C single bond and the internal rotation barrier of polymer with C-C backbone.The internal rotation barrier of polymer with a C-C backbone is in the range of 16.46-25.17 kJ/mol?3.9-6.0 kcal/mol?.According to the results,we can roughly estimate that the conformational interconversions frequency of the C-C single bond is in the range of 1.89×10⁸ Hz?k?4.3×10⁹ Hz.?2?The results from SMFS showed that the single-chain elasticity of the polymer with C-C backbone can be fitted well with the QM-FRC model of PE,in which the length of the rotating unit?l_b?equals to the length of a C-C single bond?0.154 nm?.This result indicates that the C-C bonds in a single polymer chain are freely rotated in dilute CCl₄ solution,although there are a few conformations that cannot be achieved.The Collision theory was used to explain the phenomenon.It is found that the effective collision frequency between solvent molecules and the polymer chain is equivalent to the conformational interconversion frequency.It is found that the time scale of conformational interconversion is much less than the time scale of the SMFS experiments.This result indicates that the single-molecule experiments are carried out under an equilibrium condition,where the effect of the internal rotation barrier on the conformational interconversion cannot be observed.?3?Polymers in the?solution behave like an ideal chain,which have the largest conformation entropy.Therefore,the single-molecule experiments of PS are carried out in the?solution?cyclopentane,T=296 K?.It is found that the force-extension curve obtained in cyclopentane and octane can be superposed well in the whole force region.Besides,the two curves can be well fitted by the QM-FRC model of PE with l_b=0.154 nm.These results indicate that a single polymer chain in organic solvents has similar single-chain elasticity with that in the?solution at the single-chain level.?4?High vacuum is a relatively isolated system,where the interference from the environment on the inherent elasticity of the polymer chain can be thoroughly eliminated.Therefore,single-molecule experiments are carried out in high vacuum.It is found that the experimental curves of PEMAM can be superposed well with the QM-FRC fitting curve?l_b=0.154 nm?nearly in the whole force region?F>100 pN?.These results indicate that the single PEMAM chain shows a pure elastic behavior in high vacuum?or an unperturbed state?when F>100 pN.In addition,the experimental curves of PEMAM in nonpolar organic solvents and high vacuum are compared.It is found that the two kinds of curves can also be superposed well when F>100 pN.These results indicate that the effect of nonpolar organic solvents on the single-chain elasticity of polymer can be ignored at the single-chain level.Then,the reason why a polymer chain has similar single-chain elasticity in high vacuum to that of an ideal chain is explained.