Investigation On Polyethyleneimine And Its Graft Copolymer By Single-molecule Force Spectroscopy

Polyethyleneimine(PEI)is a cationic weak polyelectrolyte.Its graft copolymer polyethyleneimine-graft-polyethylene glycol(PEI-g-PEG)has great potential for applications in gene delivery,drug release and resistance to protein adsorption.However,the attention of related studies has been mainly focused on their applications,and there is a little research on their structural properties,so there is a lack of sufficient understanding.In order to deepen the understanding of them,their mechanical properties are investigated at the single molecule level.In this thesis,the single-chain elasticity of branched PEI and PEI-g-PEG in a nearunperturbed state is investigated using atomic force microscopy-based single-molecule force spectroscopy(AFM-SMFS)techniques and quantum mechanics(QM)theoretical models.The effect of solution environment on their single-chain mechanical properties and their conformational changes is also investigated.Based on the above studies,the main conclusions can be summarized as follows:(1)The single-chain elasticity of branched PEI in nonpolar organic solvent is obtained by using AFM-SMFS technique.Combined with QM theoretical calculation,it is found that the single-chain elasticity can represent the inherent elasticity of PEI.In addition,by studying the single-chain elasticity of PEI in aqueous solutions with different p H,it is found that the proportion of amine group protonation of PEI in aqueous solution is affected by the p H of the solution.And the proportion of amine group protonation in PEI chain segments and the free monovalent counter ions in solution could affect the interaction between PEI and water molecules.(2)The single-chain elasticity of PEI and PEG in nonpolar organic solvent and aqueous environment was obtained using the AFM-SMFS technique.By comparing the forceextension curves of PEI-g-PEG,PEI and PEG,it is found that the SMFS experiments of PEIg-PEG in nonane mainly reflected the mechanical behavior of the main-chain PEI.It is speculated that PEG curls up into a spherical shape in nonane to expose the PEI with larger molecular weight.The SMFS experiments of PEI-g-PEG in aqueous environment mainly reflects the mechanical behavior of the PEG.It is speculated that the PEG disperses in the aqueous solution in an extended conformation and wraps the PEI.Characterizing the conformation of PEI-g-PEG in different environments at the single-molecule level can provide a reference for studying the conformational changes of other graft copolymers in different environments.(3)The force-extension curves of PEI-g-PEG in different polar solution environments are compared to find the main factors affecting the elasticity of PEI-g-PEG single chains.Subsequently,comparing the force-extension curves of PEI-g-PEG obtained in solutions with different water contents,combined with the TSQM model,it is found that the higher the water content in the solution,the stronger the interaction between the solution and PEI-g-PEG polymer chains.Therefore,the water content is the main factor affecting the mechanical properties of PEI-g-PEG.This study can deepen the understanding of the mechanical properties of single-chain of PEI and its graft copolymer PEI-g-PEG,which can contribute to the in-depth research and application of PEI and PEI-g-PEG.