Revealing The Interfacial Structures And Dynamic Behavior Of Several Typical Soft Matters With Hydrogen-bond Using Sum Frequency Generation Vibrational Spectroscopy And Molecular Dynamics

Soft matter is the materials between solids and ideal fluids,also known as soft condensed matter.The meaning of"soft"refers to that these kinds of materials are very sensitive to the external environments.Even small external environmental stimuli can cause large responses,especially for the surfaces or interfaces,which are in direct contact with the external environments.Therefore,revealing the molecular conformation and dynamics change process of the soft materials on the surfaces or interface under the dynamic environmental perturbations from the molecular scale will contribute to the wider applications of soft matter.Based on that,we investigated the characteristics of molecular structures for several typical soft matter systems with hydrogen-bonding under dynamic environmental perturbations by the nonlinear sum frequency generation（SFG）spectroscopy,molecular dynamics simulation,and theoretical analysis,revealing the intrinsic mechanism behind the environmental stress responses,emphasizing the key role of hydrogen-bonded network structures,and exploring the effective strategies for controlling the molecular structures of soft materials on the surfaces and interfaces.The main contents are as follows:1.The influence of ion exchange on polyelectrolyte brushes which can form strong hydrogen bonding with water molecules.Although the strong polyelectrolyte brushes collapse under the high ion concentration conditions,swelling of the brushes can be observed under the low ion concentration conditions.The effects of salt ions on the hydration characteristics of strong polyelectrolyte sodium polystyrene sulfonate（PSSNa）brush interfaces were studied.The dynamics processes of ion exchange induced molecular reconstruction at PSS brush/aqueous solution interfaces were detected by SFG spectroscopy.A two-step process for the penetration of salt ions into PSS brush layer is summarized and proposed,which reveals the mechanism of the initial ion diffusion and subsequent dissociation of the chain segments near the substrate wall,causing the enhancement of interfacial hydrogen-bonded network structures.Finally,the reliability of the model is verified by coarse-grained molecular dynamics simulations.The manipulation of the interfacial molecular structure can be achieved by using the ionic response characteristics of the strong polyelectrolyte brush at low concentration.2.The influence of shear flow and thermal perturbation on neutral polymers which can formhydrogen-bonded structures with themselves and water molecules.The evolution of molecular chain structures of temperature sensitive polymer brush under shear flow and different temperatures were systematically studied.The relationship of polymer chain deformation on fluid shear stress and temperature dependence is pointed out,namely（a）below the phase transition temperature,the flow shear only affects the outer edge of PNIPAM brush;（b）above the phase transition temperature,the flow shear can affect the whole polymer brush layer and increase the ordering of water molecules.（c）The higher the temperature,the easier the polymer chain is to be stretched.It is revealed that the hydrogen-bonded interaction between polymer chain and water molecule plays a key role in resisting the flow shear force.The theoretical model is confirmed by direct SFG experiments and coarse-grain MD simulations.Such findings are attributed to the balance between the shear flow and the molecular interaction with respect to the polymer chains and adjacent water molecules,thus demonstrating the significant effect of the shear flow on the structural and dynamic behaviors of the polymer chains at the boundaries from the molecular level.3.The influence of shear flow and thermal perturbation on neutral polymers which can onlyform weak hydrogen bonding with water molecules.In this chapter,the conformations of PEG molecular chain adsorbed at the interface under flow shear were studied.It was found that the deformation ability of PEG molecular chain adsorbed at the interface under dynamic flow condition was also dependent on the ambient temperature.However,PEG molecular chain is more easily influenced by external environmental stimuli,which mainly includes that the polymer chains are stretched under the flow shear at room temperature of 20°C.This is due to the weak hydrogen-bonded interactions between the OCH₂ groups and water molecules,which makes the ability of interfacial molecules to resist deformation relatively weak.4.The influence of thermal,humidity change and competitive adsorption of small molecules on water-insoluble polysaccharides which can form strong hydrogen bonding by themselves.The interfacial molecular structures regarding the bound water molecules on the cellulose film surface were studied.It was found that the hardening phenomenon for cellulose materials after a transformation from wetting to drying under the normal environmental condition mainly came from the crosslinked structure mediated by the bound water.Based on SFG and MD simulation,it was found that a small number of surfactant molecules can have a strong interaction with the hydroxyl groups on the cellulose surface,thus weakening the hydrogen-bonding interaction between the cellulose and the water and consequently inhibiting the formation of the cellulose intermolecular network structures containing the bound water.However,too many surfactants covered on the cellulose surface with the monolayer or multilayer micellar structure after the natural drying can limit the evaporation of water and therefore negatively affect the hardening of the cellulose materials.This results help to control the drying property of the cellulose materials and provide a fundamental understanding on the effect of the interfacial bound water with respect to the hardening phenomenon of the cellulose or cotton based materials at the molecular level.