Revealing The Influences Of Hydrogen Bonds On The Properties Of Two Common Synthetic Polymers By Single-molecule Force Spectroscopy

Hydrogen bond(H-bond)is a kind of weak interaction,which can be formed within or between molecules.It plays an important role in the fields of biology,chemistry,materials,etc.Due to the great influence on the properties of substances,the bonding energy of H-bond has received a lot of attention.However,most of the research is still on the qualitative level.In order to further understand the properties of H-bond,it becomes more and more necessary to study the type,strength,kinetics and effects at the single-molecule level.In this paper,the influences of H-bonds on the properties of two common synthetic polymers have been investigated by single-molecule force spectroscopy(SMFS).First,the strength of intrachain H-bonds of single poly(vinyl alcohol)(PVA)chain with different degrees of hydrolysis has been quantitatively investigated in vacuum and nonane.The nonlinear correlation between the H-bond strength and the degrees of hydrolysis implies that there are H-bonds synergistically in the PVA chain.Then,the single-chain behavior of PVA in water has been studied by SMFS and steered molecular dynamics(SMD)simulation.The results indicate that a complex structure consisting of water molecules and PVA remains stable upon stretching,which may be the reason for the good water-retaining property of PVA.The energy cost of bound water rearrangement(E_w)of PEG is close to those of biological macromolecule,which may be an important factor for the excellent biocompatibility of PEG.(1)This thesis has measured the intrachain H-bond strength of PVA with different degrees of hydrolysis,and the synergy of intrachain hydrogen bonds was found at the single molecule level..First,the inherent elasticity of the single PVA chain with a degree of hydrolysis of?99%was obtained in dimethyl sulfoxide(DMSO)and 8M urea solution(H-bond breaker).In a vacuum environment,the force-extension(F-E)curve of PVA that is only affected by intrachain bonding-bond is obtained.The single-molecule force spectrum experiment under the same environment was carried out on PVA molecules with hydrolysis degree of?80%and?89%.The intrachain H-bond energy of three PVAs with different hydrolysis degree in vacuum was obtained,which are 2.88±0.05 kJ/mol,3.45±0.04 kJ/mol and 4.75±0.04 kJ/mol(the degree of hydrolysis is from low to high),respectively.We found that there is a nonlinear proportional relationship between the H-bond energy in the chain and the degree of hydrolysis.To verify the accuracy of the intrachain H-bond energy obtained by PVA in vacuum,this thesis conducted a SMFS experiment under non-polar organic solvent nonane in PVA with different degrees of hydrolysis.The three PVA molecules with different degrees of hydrolysis still show the same H-bond energy trend as in the vacuum environment,that is,the change in hydrolysis degree and the H-bond energy are in a nonlinear ratio.This indicates that the strength of H-bonds in the PVA chain mainly depends on the synergistic effect of adjacent H-bonds.(2)This paper has investigated the single-chain behavior of PVA in water by SMFS and SMD.The reasonable structure of PVA in water is proposed,which may be the reason why PVA has good moisture retention.First,this paper compares the F-E of PVA obtained in deionized water with the quantum mechanical calculations-based freely rotating chain model(QM-FRC model),and the results obtained are completely different from the energy consumption of other single molecules due to H-bond during the force stretching process.In response to this anomaly,this paper conducted a SMFS experiment in different concentrations of salt solution.The F-E curves of PVA obtained in different salt solution can be superposed well,indicating that PVA is not ionized in water.We conducted SMD simulations on PVA in water to study the specific interaction modes and types of H-bonds of PVA in the water environment.Based on the SMFS and SMD results,we proposes a H-bonded outer membrane structure of the"wire plastic outer layer"formed by PVA polymer in water.This H-bonded outer membrane structure makes the outer layer of the PVA molecular chain always wrapped with water molecules,which may be the reason for the good water-retaining property of PVA.(3)In this work,we have investigated the relationship between the biocompatibility and the bound water of Poly(ethylene glycol)(PEG)by SMFS.To discern the effect of bound water on PEG in an aqueous solution,the single-chain inherent elasticity of PEG should be determined first as a reference.The inherent elasticity of PEG is obtained in a small-sized nonpolar organic solvent,nonane,which is confirmed by QM-FRC model.Then,SMFS experiments have been performed in phosphate-buffered saline(PBS)to study the effect of bound water on PEG.The shoulder plateau in the F-E curve of PEG obtained in PBS should be caused by the rearrangement of the bound water(water bridge)during the PEG elongation.This assumption has been confirmed by the two-state QM-FRC model(TSQM-FRC model),which takes the effect of the water bridge into account.This bound water rearrangement will consume additional energy(E_w)besides that for the inherent elasticity of the chain.This E_w is calculated to be?1.59 k_BT/unit(3.93 kJ/mol)by integrating the area between the F-E curves of PEG obtained in PBS and nonane,which is closed to those of biomacromolecules.Inspired by the relationship between the low E_w and the behaviors of biomacromolecules,we have speculated the relationship between the biocompatibility and the bound water of PEG.