Molecular Dynamics Study Of Starlike Polymer Brushes In ?-Solvent And Mixed Binary Brushes

Densely grafted polymer layers,commonly known as polymer brushes,have been s-tudied and received ample attention in recent years because of their important applications in technical coatings,biomaterials or colloid stabilization.As the properties of brushes made of linear chains have been thoroughly studied in the past several years,recently more and more attention is being paid on more complex systems,e.g.branched polymer brushes or mixed polymer brushes.These systems are interesting for a couple of reasons:Since branched polymers are offering an enhanced number of end groups which could be func-tionalized,they are regarded good candidates for stimuli-responsive surface layers.They are further expected to offer improved lubricant and anti-fouling properties.Most importantly,branched polymer brushes are known to play vital roles in biological materials.Compared with pure polymer brushes,mixed brushes have unique properties that make they to be per-fect candidates of switchable layers.Molecular dynamics simulations were performed to study the properties of starlike polymer brushes in ?-solvent and mixed polymer brushes made by linear and starlike poly-mers.The background and theories regarding polymer and polymer brushes are introduced in Chapter 1,2,3.In Chapter 4,Starlike polymer could be considered as the simplest dendrimer,which is of great importance to understand the properties of dendrimer.In our study,the height and average concentration of starlike polymer brushes in ?-solvent are calculated,indicating that Alexander-de Gennes scaling theory is invalid for starlike polymer brushes in ?-solvent neither.Further on,for star polymer brushes,we implemented a simple Flory-Huggins type mean-field model that was proposed by Merlitz et.al for linear brushes in ?-solvent.It ex-plains the deviation from molecular dynamics simulations,resulting from ignoring the high-er order terms of Taylor expansion of ln(1-c).In order to figure out how the solvent quality affects properties of polymer brushes,simulation results of starlike polymer brushes in good solvent and ?-solvent are compared.First of all,both of them show a dual-population struc-ture(completely stretched and collapsed)of polymer inside brushes when the grafting den-sity is sufficiently large.The difference lies in that in ?-solvent it requires a higher grafting density(0.1 for ?-solvent,0.02 for good solvent)until two-population structure appears.It is shown that molecules inside a starlike brush can switch between completely stretched and collapsed states.The conformation of starlike polymer is not static,but rather in dynamics equilibrium.The fliprates of brushes in different solvent qualities are calculated.It shows a higher frequency in good solvent.The reason is that the macromolecules easily stretch in good solvent compared with ?-solvent.The interaction between two beads in ?-solvent contains not only repulsive part,but also attractive part,indicating that polymer brushes in ?-solvent collapsed easily.As a consequence,brushes in ?-solvent have higher average concentration,which induces the entanglement of polymers and stronger friction of flipping between stretched and collapsed states.In Chapter 5,we also investigate the structural,surface and dynamic properties of bi-nary polymer brushes in good solvent and poor solvent,composed of functional 4-arm star polymers and chemically identical linear polymers of different molecular weights.The de-pendency of linear chain length,grafting density and temperature on the structure of brush are investigated and compared with recent self-consistent field studies[1,2].Both monomer density profiles,end-monomer distributions show that increasing linear chain length has a dramatice effect on the structure of brush,i.e.the relative position of star and linear chains intriguingly exchanges.Particularly,just slightly increasing linear chain induces a transi-tion of linear conformation from collapsed to stretched.Similar effect is observed when increasing grafting density in the intermediate linear chain regime.At low grafting density,linear chains retract to the bottom of brush,meanwhile,stars domain in the upper layer of brush.With increasing grafting density,a certain fraction of linear chains squeeze out.At high grafting density,Most linear chains are stretched to the surface of brush.Our findings are summarized in the phase diagram.Further on,we studied these mixed binary brush that are immersed in changing quality solvent(from good solvent to poor solvent)by decreasing temperature.We can see.that the composition in the periphery part of binary brush with intermediate linear chain length and grafting density transits from the mixture of free ends of linear chains and stars to free ends of stars.The quantity of these latter effects are in-sufficient to build switchable systems.However,it gives a hint that modifying the chemical properties of different spices to obtain solvent selectivity will be helpful to build a smart environment-responsive coating.Results and perspective of the future work are summarized in the last chapter.