Kinetics Study On The Assembly Behavior Of Polyelectrolytes And Nanoparticles Based On Electric Field

The essence of the bottom-up nanofabrication technology is molecular assembly.This technology is on a wide variety of technical methods,such as micro-contact printing,polymer grafting,deposition and adsorption,and layer-by-layer assembly,etc.,among which,the deposition and adsorption techniques of multi-materials has been widely used and investigated for the bottom-up nanofabrication.Among the massive popular nanofabrication materials,polyelectrolytes(PEs)and nanoparticles(NPs),which act as important parts of polymer science and nanoscience,play a critical role in the fields of scientific research and technical application,especially electronics,micro-electro-mechanical systems,optics science,and biomedical science,etc.Typical PEs,such as protein,DNA,RNA,chitosan,and various synthetic macromolecules have been widely used in the design and manufacture of functional films,nano-scaffolds,and nano-patterns.Moreover,nanomaterials,such as metallic nanoparticles,carbon nanotubes and graphene are also used to design and fabricate ultramicroelectrodes,microfluidic devices,microelectronic devices,and microreactors by bottom-up method.In this work,kinetics features and assembly behavior of physical deposition and adsorption of several PE and NP systems were studied based on molecular simulation methods,as well as the formation conditions of brush-,nanosphere-,nanogrid-,and nanostripe-patterns and the responsive behavior of PE brushes in external electric fields.This work reveals the physical mechanisms of the deposition and assembly of PEs and nanoparticles in micro/nano scale.Some new phenomena were found.Computer simulation technology is of great importance in the developing of nanofabrication,which can catch the kinetics and conformational behavior in the process of nanofabrication in smaller scale and examine the variables which are difficult to analyze in experimental and theoretical research.Therefore,research on the micro-mechanism of deposition and assembly of PEs and nanoparticles is of important scientific significance and application prospects to thedevelopment of bottom-up nanofabrication technology.The main contents are listed as follows:(1)Effects of solvent selectivity,charge distribution of PE,and external electric field strength on the deposition behavior of flexible amphiphilic diblock polyelectrolytes(ADPEs)in electric field were considered systematically.Flexible ADPEs formed into bilayers which consist of different affinities on hydrophilic surface.In nonselective solvent,hydrophilic blocks formed into a thin and uniform adsorption layer,hydrophobic blocks distribute on the adsorption layer like brush.In selective solvent which is selective to hydrophilic blocks,hydrophilic blocks form into disc-like adsorption structure,and hydrophobic blocks self-assemble into nanospheres distributing on the hydrophilic layer.The electrostatic repulsion between similar charges in PEs loose the deposition structure.Moreover,external electric field compresses the deposition structure of PEs to some extent.In selective solvent,strong electric field reduces the size of hydrophobic nanospheres and hydrophilic disc.(2)Micro-mechanisms of physical deposition behavior of semi-flexible ADPE in selective solvent,which is selective to hydrophilic blocks,depending on the charge distribution and stiffness of PE,and the external electric field strength were revealed.The formation conditions of stripe,network,and local ordered adsorption structures were summarized systematically.In deposition multilayers,semi-flexible ADPEs form into “three-segment” micelles which consist of one hydrophobic center and two hydrophilic ends.Variations of charge distribution and stiffness of PEs result in two kinds of micelles,i.e.short ones and streamlined ones.The increases of PE stiffness and positive charges on hydrophilic blocks reduce the lateral size of hydrophobic centers of PE micelles.Thin adsorption layer of hydrophilic blocks is local ordered.When PE stiffness is small,hydrophilic blocks which belong to two adjacent micelles are perpendicular to each other.Whereas,at large PE stiffness hydrophilic blocks tend to distribute as streamlines.The increase of charges in hydrophilic blocks results in the reduction of the thickness of deposited multilayer structure and the lateral size of hydrophobic micelles.Moreover,external electric field may result in the formation of tilt micelles.(3)The responsive behavior of polyampholyte brushes(PABs)and polyelectrolyte brushes(PEBs)in external electric field were investigated from micro-scale level.The influences of charge sequence,chain rigidity,and the orientation and strength of electric field on the conformational transition were analyzed systematically.Results revealed that the responsive behavior and local structure of PA brushes depend on the charge sequence of polyampholytes(PAs).When positively charged blocks in semi-flexible PAs are grafted on the substrate,the critical electric field strength which induces the collapse of the PABs increases with the increase of block length.However,if the grafted blocks in PAs carry negative charges,the orientation of critical electric field is contrary to the above situation.Without external electric field,PABs maintain in stretching status.Although the external electric field induces the collapse of PABs,local stretching phenomena still exist in PA chains.(4)Electrostatic adsorption behavior of Janus nanoparticles(JNPs),which are oppositely charged in the two semi-spheres,onto charged surfaces were studied from molecular scale.Physical mechanisms of the formation of mono-layer adsorption structure were revealed and analyzed.When the proportion of positive charges on the surface of JNPs is small or the charge density of substrate surface is large,JNPs tend to form into a monolayer and small nanoparticle clusters.When the net charge on JNP surfaces is small,JNPs tend to form into large nanoparticle clusters and thick adsorption structures,and delays occur during the initial adsorption stage.Moreover,when the charge density of adsorption substrate is large,the charge density of adsorption substrate acts as the dominant factor during the adsorption process.Instead,when the charge density of adsorption substrate is small,the influence of the net charge of JNPs on the final adsorption structures becomes significant.(5)Silver NPs were synthesized by hydrothermal synthesis method and loaded onto the surface of chitosan grafted Activated Carbon Fibers(ACF).Antibacterial properties of ACF before and after loading silver NPs were tested.Coarse grained molecular dynamics model of surface-polyelectrolyte-nanoparticle adsorption system was established and used to validate the adsorption mechanism between chitosan and silver NPs.Moreover,effectsof PE rigidity on the chain conformation and kinetics characteristics of PE chains were investigated systematically.Results showed that electrostatic attractions between positive charges carried by PEs and negative charged on NPs are strong enough to make NPs adsorb onto PE surface.When the stiffness of PE chains is small,PE chains wrap around NPs and this behavior promotes the stability of adsorption structure to some extent.Moreover,when PE chains are less stiff,the aspect ratio of NP clusters is smaller.However,when PE chains are stiff,the adsorbed NPs distribute like a string of pearls,which possesses a larger aspect ratio.