Theoretical Investigation On Switching Of Stimuli-responsive Biomimetic Macromolecule/Polymer Brushes

Stimuli-responsive behaviors of single-stranded DNA(ssDNA)brushes,poly(N-isopropylacrylamide)(PNIPAM)brushes,mixed brushes and mixed polyelectrolyte brushes have been extensively investigated because of their scientific interest and potential technological applications.In recent years,advances in experiment of these brushes,and their new stimuli-responsive behaviors have been reported experimentally.For example,when ssDNA brushes are exposed to water vapor,the ssDNA brushes show anomalous swelling or collapse;when ssDNA is anchored to the positive charged nanoparticle surface,the states of ssDNA are directly controlled by changing body temperature;PNIPAM brushes can adsorb large amounts of HCl gas,and the morphology of a PNIPAM brush can be affected by HCl gas.In addition,experimental investigations demonstrate that pH can induce switching of mixed polyelectrolyte brushes,and the switching properties are dependent on the pH values.In this thesis,we employ a molecular theory to study the switching and structure of ssDNA brushes,PNIPAM brushes,mixed brushes and mixed polyelectrolyte brushes.The new experimental results can be explained by the molecular theoretical models.We also predicted some new switchings and structures.The research contents of this thesis are organized as follows:In chapter 1,we briefly reviewed the polymer brushes,synthesis of polymer brushes and structural determination of polymer brush,theoretical studies of polymer brushes,focused on the molecular theory,and described the research content of this thesis.In chapter 2,we used a molecular theory to study the anomalous switching of ssDNA brushes induced by water vapor.Here,both ssDNA-water and water-water hydrogen bonds and their explicit coupling to the ssDNA conformations are considered.We find that hydrogen bonding becomes a key element in inducing the anomalous switching of ssDNA brushes.Our results indicate that the combination of ssDNA-water and water-water hydrogen bonds can result in a nonmonotonic dependence of the monolayer height on water vapor concentration:while initially the monolayer shrinks with increasing humidity until it reaches a minimum,and then it increases with increasing humidity.Experiment also showed a hysteretic behavior:when humidity subsequently decreased,the height of grafted layer decreased monotonically with the decreasing water-vapor concentration.We expect that the hysteresis will be attributed to the effects of ssDNA-water and water-water hydrogen bonds.Based on our theoretical model,we predict that ssDNA-water hydrogen bonds and water-water hydrogen bonds hybridization will lead to the hydrogen-bond network formation of 3D ssDNA monolayers.In chapter 3,using a molecular theory,we investigated the temperature-dependent switching and structure of ssDNA brush grafted to charged nanoparticle surface.Here the size,conformations,charge properties of ssDNA,and electrostatic interaction between the charged ssDNA segments and nanoparticle surface are taken into account.The main results are as following:1)For the long ssDNA chains,when the temperature is lower than the critical switching temperature,the ssDNA brush will collapse due to the existence of the electrostatic interaction between ssDNA and the charged nanoparticle surface;2)For the short ssDNA chains,the switching of ssDNA brush cannot happen,and the critical temperature does not exist;3)Since the electrostatic attractive interaction between ssDNA and charged nanoparticle surface weaks dramatically with the increase in temperature,and ssDNA chains will stretch when the electrostatic attractive interaction is insufficient to overcome the elastic energy of ssDNA and the electrostatic repulsion energy.We also predicted that micellar structure will present in ssDNA tethered to charged nanoparticle surface.In chapter 4,we employed a molecular theory to study HCl gas-adsorption/desorption properties of PNIPAM brushes.Here,PNIPAM-HCl hydrogen bonds and theirs explicit coupling to the PNIPAM conformations are considered.We found that hydrogen bonding becomes a key element in determining HCl gas-adsorption/desorption behaviors of PNIPAM brushes.Our results indicate that,when at moderate grafting densities,the association of PNIPAM-HCl hydrogen bonds can result in a dependence of PNIPAM brush-height on HCl concentration,and the morphology of PNIPAM brushes may have a significant effect on the HCl gas-adsorption/desorption properties.We also found the irreversibility of HCl gas-adsorption/desorption can be attributed to the effects of PNIPAM-HC1 hydrogen bonds.Apart from providing a theoretical framework that can interpret the experimental results,our theoretical model predicts that too high grafting densities meight bring unfavourable adsorpting HCl gas for PNIPAM brushes.In chapter 5,we performed a molecular theory to study the switching mechanism of the mixed brushes and pH-induced switching of the mixed polyelectrolyte brushes.For mixed brushes,the possibility of perpendicular segregation in the mixed brushes is explained.The thermodynamic properties of the mixed brushes,such as the lateral pressure-area isotherms,are studied as a function of surface coverage.The structural properties of the mixed brushes were analyzed in detail for various solvent responds.For the mixed polyelectrolyte brushes,we investigated the pH-induced switching of mixed polyelectrolyte brushes.The results indicate that the switching properties of mixed polyelectrolyte brushes are dependent on the pH values.At low pH,negatively charged chains adopt a compact conformation on the bottom of the brush while positively charged chains are highly stretched away from the surface.At high pH values,the inverse transformation takes place.The role of pH determining the polymer chains conformation and charge behavior of mixed polyelectrolyte brushes was analyzed.It is found that there exists a mechanism for increasing strong electrostatic repulsions:stretching of the chains.The H+ and OH-units play a more important role as counterions of the charged polymers do.The collapse of the polyelectrolyte chains for different pH values could be attributed to the screening of the electrostatic interactions and the counterion-mediated attractive interaction along the chains.In the last chapter,the thesis is summarized,and an outlook for the future works on the ssDNA brushes,PNIPAM brushes,mixed brushes and mixed polyelectrolyte brushes is described.The results of this thesis will enable us to modify the designs of ssDNA brush,PNIPAM brush,mixed brush and mixed polyelectrolyte brush nanomaterials for a large variety of applications,and can provide reference for experimental designs of these brushes.