Theory And Computer Simulation Study On Conformation Of Polymer Brushes In Mixed Solvent

polymer brushes are a special polymer system formed by immobilizing one end of a set of polymer chains onto a substrate.With the advantages of simple preparation and easy modification,polymer brushes are widely used in biomedical fields such as biosensors,antibacterial coatings and protein binding and immobilization.Most of the applications of polymer brushes are realized based on their conformational changes.In good solvents polymer brushes will be in the extended swollen state;on the contrary,in poor solvents polymer brushes will be in the collapsed state.The conformational changes of polymer brushes are more rich when they are in binary mixed solvents,where the co-nonsolvent effect has aroused strong interest among researchers.However,there is still no unified understanding of the physical mechanism of the co-nonsolvent effect.This in turn has led to no systematic study of the mechanism of the co-nonsolvent effect of each parameter on polymer brushes.In this thesis,a combination of theoretical and computer simulations is used to investigate the possible conformational changes of polymer brushes in binary mixed solvents,and the effect of each parameter on the co-nonsolvent effect of polymer brushes.In this thesis,we introduce the theory and model of polymer brushes.In the box model theory,we extend the calculation of polymer brushes in implicit solvents to explicit solvents.In the explicit solvent environment,the conformational changes of polymer brushes in single solvents and binary mixed solvents are systematically investigated in this thesis by fully considering the interactions between different components.However,the fine structure of polymer brushes cannot be accurately described in some cases because the box model is relatively crude and ignores the factors that different polymer chains may be in different conformations.Based on this,this thesis presents the derivation process of polymer brushes under the self-consistent field theory.We adopt the lattice point self-consistent field model and firstly write the canonical partition function of the polymer brush system with the aid of Hamiltonian quantities;then use the Hubbard-Stratonovich transformation to transform the multi-body interaction in the system into a single-chain partition function in an effective auxiliary field;then use the mean-field approximation to obtain the system of self-consistent field equations;finally,we solve the system of equations numerically.Based on the box model theory,computer simulations were carried out in this thesis in a single solvent and a binary mixed solvent,respectively.In the single solvent we set the solvent-monomer interaction parameterx_{p B} to vary in the range of-1～1 and investigated the effect of chain length and graft density on polymer brushes.The results showed that the polymer brush thickness increased with the increase of chain length when the solvent quality and graft density were fixed;the polymer brush thickness increased with the increase of graft density when the solvent quality and chain length were fixed;and the polymer brush thickness increased with the increase of solvent quality when the chain length and graft density were fixed.In binary mixed solvents,the parameter space of the polymer brush system is more diverse and the conformational changes are more complex due to the introduction of another group of components.The simulation results show that in binary mixed solvents,fixing a certain solvent quality,the polymer brush thickness increases with the increase of another solvent quality.To some extent,this is qualitatively consistent with the variation pattern in polymer brushes in a single solvent system.In addition,the polymer brush thickness varies with the volume fraction of the co-solvent(φ_Br)in the particle bath.For example,atx_{p A}=0 andx_{p B}=-7,the polymer brush thickness undergoes a non-monotonic change with increasingφ_Br,i.e.,a co-non-solvent effect occurs.This change trend is closely related to the factors such as graft density,chain length,and the quality difference between the two solvents.In this thesis,we found that the co-solvent effect does not occur in polymer brushes in a mixture composed of a good solvent and a bad solvent.In the mixture of good/no-thermal solvent and good solvent,the polymer brush will have co-solvent effect,and the larger the quality difference between the two solvents,the more significant the co-solvent effect of the polymer brush.Increasing the grafting density will weaken the co-solvent effect of polymer brushes,and even when the grafting density increases to a certain value,the co-solvent effect no longer occurs.Increasing the polymer chain length increases the thickness of polymer brushes.The chain length does not affect the co-solvent effect of polymer brushes,nor does it change the internal composition of the brush layer.Similarly,based on the self-consistent field theory,computer simulations were carried out in a single solvent and a binary mixed solvent in this thesis,respectively.In a single solvent,the effects of chain length and solvent quality on the conformational changes of polymer brushes are analyzed in this thesis.The simulation results show that for a given chain length and graft density,both the monomer density distribution curve and the terminal monomer density distribution curve become flatter and more widely distributed as the solvent quality decreases,i.e.,the polymer brush thickness increases with the increase of solvent quality.The above pattern was also shown when increasing the graft density and chain length,indicating that increasing the chain length and graft density would also increase the polymer brush thickness.The above conclusions are qualitatively consistent with the laws obtained in the box model.In the binary mixed solvent,the effect of graft density and solvent quality difference on the polymer brush structure was analyzed in this thesis.With the increase of graft density and solvent quality difference,the polymer monomer and terminal monomer are gradually distributed farther away from the substrate,indicating that increasing the graft density or the quality difference of the two solvents will increase the polymer brush thickness.Moreover,by comparing the simulation results under box model theory and self-consistent field theory,it is found that both are consistent in analyzing the change pattern of the overall structure of polymer brushes,but if we want to analyze the polymer monomer distribution and terminal monomer density distribution,it is more reliable with the help of self-consistent field theory.