The Structure Change Of Polymer Nanochannel And Its Effects On Electroosmotic Flow

The electroosmotic flow has been widely applied in fluid conveying of microfluidics and nanofluidics system, drug delivery, biochemical analysis and electronic cooling, etc. With the development of microfluidics and nanofluidics technology and the extension of its application fields, the nanochannel has become the main factor that restricts the rapid development of the microfluidic system. The surface modification is one of the effective new technologies. It gives many new featuresto the nanochannel, and greatly broaden the application fields of microfluidics research. The block copolymer, composed of two or more than two kinds of polymer monomer composition with different properties, and amphiphilic polymer with two-phase affinity function can be made by artificial chemical synthesis technology. These two special polymers will show different properties with the nature of simple linear polymers, random copolymer as well as mixed homopolymer, and has potential application value in biosensor, smart valve, colloid stability, etc. This paper presents the research of neutral block polymer self-assembly behavior, valve control research of amphiphilic copolymer, electroosmotic control of polyelectrolyte and ampholyte in the nanochannel to accomplish the research of nanochannel surface modification method.This paper firstly studied the self-assembly phase behavior of symmetric diblock copolymer in multi-walled cylindrical nanochannel. The diblock copolymer can form a spiral layered structure in the non-selective nanotubes and the layer thick will increase with the increase of the adsorption intensity between the wall particles and polymer monomer. The constraint space size has significant effects on the conformational changes of copolymer. The end-to-end distance of the diblock copolymer increases with the increase of the ratio of outer diameter of nanotube. The studies have shown that the constraint space size effect on the phase behavior of copolymer is weak and the end-to-end distance of the copolymer will increase with the increase of the adsorption intensity between the wall particles and polymer monomer in the selective nanotubes. For the amphiphilic triblock copolymer, the copolymer can form ordered structure only when the interaction strength parameter between the wall particles and the fore and aft monomer of diblock copolymer is lower. For the amphiphilic triblock asymmetric copolymer, the copolymer can form stratified columnar structure when the adsorption intensity between the wall particles and the end block monomer is bigger. The analysis and discuss on the reasons for the formation of copolymer and orderly conformation and change law have important research significance to search for a new polymer structure of nano channel through exploring the adsorption potential energy, cohesive energy and reject potential energy with the change of adsorption intensity.Secondly, the research shows that the channel open or close in the nanochannel with surface grafting amphiphilic triblock copolymer can be controlled through temperature adjustment and solvent properties adjustment. There is a threshold temperature for the temperature sensitive copolymer. Under the condition of the threshold temperature, the average height of the copolymer can achieve the minimum or maximum, corresponding to the “open” or “close” state of nanochannel. The response to temperature changes of the temperature sensitive copolymer with high grafting density is not obvious, which should be not used as a nano valve control technology. By contrast, the medium grafting density copolymer is more suitable for the temperature controlled nanovalve. For the triblock uniform amphiphilic copolymer A10B10C10 with the hydrophobic block A and C, hydrophilic block B, the copolymer conformational change will be obvious when B block monomer hydrophilic intensity changes. If the hydrophobic monomer intensity of block A and C increase, the average height of copolymer chain has a tendency to increase, but the increase rate is slow.Thirdly, we studied the microchannel flow driven by electric field in the horizontal direction with a lower surface grafted of negatively charged polyelectrolyte brushes and upper surface grafted of positively charged polyelectrolyte brushes. The studies show that the velocity curve of electroosmotic flow along the direction perpendicular to the flow is an s-shaped curve. That is, there is a wave peak of electroosmotic flow velocity at the bottom of the flow channel area, and a wave valley at upper area. This is different from ordinary electroosmotic flow. The concentrated distribution of anion at the upper and of cation at the bottom area due to the channel surface modified by the polyelectrolyte brush leads to speed difference between the anion and cation ion under the effect of electric field. Adjusting the electric field intensity, the proportion of charged polyelectrolyte brushes and grafting density are important parameters of adjusting electroosmotic flow. The anionic speed at the upper area is reduced with the increase of the variables above, but the cation speed at lower area will increase, at the same time, the velocity difference of anionic and cation is also increased. This will help ion separation of anionic and cation. The research results can provide theoretical reference for the design of microfluidics functional channel.The studies have shown that the microfluidic channel surface grafted polyampholyte brush with zero net charge can effectively inhibit electroosmotic flow. The electroosmotic flow is closely related to the polymer grafting density. The average height of polyampholyte brush will increase with the increases of grafting density, and the inhibition effect of seepage will also increase. The effect on the ion density distribution of anionic and cation will increase in the microchannel along with the increase of polyampholyte brush charged ratio, which make the free anionic and cation ion in layered distribution state along the direction of flow channel section. This special ion distribution leads to concave phenomenon of electroosmotic flow velocity curve, that is, the center velocity is low, but velocity near the wall is higher, which is completely different from traditional plug mode of electroosmotic flow velocity curve.For the polyampholyte brush with different distribution of charged monomers and same charged proportion, the electroosmotic flow inhibitory effect is more apparent when the distribution of different charged monomer is more uniform. We designed a microfluidic chip with the surface of grafted polyelectrolyte brush to inhibit the electroosmotic flow based on the research above. This new microfluidic chip can greatly improve the control precision of the microfluidic flow in the microchannel with advantages of simple structure, high precision, strong controllability and self adaptive to different electric field intensity.Finally, based on the inorganic alumina AAO membrane as working fluid channel of electroosmotic pump, this paper puts forward a kind of electroosmotic pump solutions with polymer nanochannel.The simulational and experimental study on the surface modification of nanochannel has been done in this work. We have analysed the structure change of polymers and the effects on the electroosmotic flow. This work developed a new method to modify the nanochannel with polyampholyte and polyelectrolyte brushes. The study of the interaction between the polymer brushes and fluid has important academic meaning, and the achievments could directly be applied to the design of microfluidic chip and the development of electroosmotic pump.