Simulations Of The Response Of Star-shaped Polyelectrolyte Brushes Immersed In High-valent Salt Solution To External Electric Fields

Polyelectrolyte brushes play a very important role in modifying and adjusting the surface properties of materials.Through the continuous efforts of researchers,breakthrough in the fabrication of polyelectrolyte brushes have been made and these polyelectrolyte brushes have found applications in the fields of drug delivery and controlled release and bionics.But as far as the current research is concerned,most of the research on polymer brushes is still limited to charged linear brushes,and the less studied star brushes are almost exclusively neutral ones.Star brushes have advantages over the linear brushes previously studied.It has better grafting stability,can further improve anti-inflammatory and anti-fouling performance,and is also widely used in biological materials such as airway epithelial cells.Moreover,the star brush has multiple end monomers that can carry different functional groups,which is helpful for adjusting the performance of the surface of the substance.Last but not least,the unique internal stratification phenomenon of the star brush can be used as a molecular switch to achieve the desired smart sensing effect.In this thesis,Langevin's molecular dynamics simulations were performed to quantitatively investigate the height of the brush,the distribution of particles in the system and the competitive adsorption of charged monomers and trivalent salt co-ions on the grafting electrode with opposite surface charges,as well as the collapse of the star polymer brush onto the grafting electrode by changing the charge fraction of the monomers on the grafted four-arm star chains,the concentration of trivalent salt under an external electric field.Three different scenarios of external electric fields were examined in the simulations: stretching electric field,compressing electric field,and no external field.The compressing electric field is the main focus of this study.The simulation results show that the average charged fraction of grafted star chains and salt concentration play a critical role in the competitive adsorption of charged monomers and trivalent salt co-ions under a compressing electric field.When the charge fraction is very low and the trivalent salt ion concentration is also very low such that the total amount of charges on a grafted star chain molecule is roughly equivalent to the total charges of a trivalent salt ion,the trivalent salt coins are more competitive than the charged monomers on the star chains in adsorbing onto the grafting electrode.It was also found that when the charge fraction of star chains is relatively low,the addition of trivalent salt co-ions does not cause charge overcompensation on the grafted electrode.In this thesis,the collapse of charged star brushes in the presence of added trivalent salt counter ions was studie.In sharp contrast to the dominant effect of inter-chain bridging between a trivalent salt counter ions and multiple charged monomers from different chains in the collapse of fully charged linear polymer brushes,the effect of intra-chain bridging was found to be dominant over that of inter-chain bridging in the collapse of fully charged star brushes.Simulation results showed that there is a wide distribution of the number of charged monomers bound to a trivalent salt ion for fully charged star brushes.The weight fractions of intra-chain and inter-chain bridging modes were found to decrease drastically with decreasing charge fraction of the grafted star chains.