Synthese Of Pheaa Polymer Brushes And Study On Its Surface/Interface Properties And Applications

Polymer brushes are defined as one-end tethered polymer chains with high density on surface or interface. Due to the advantages of simplicities of preparation and structure regulation, polymer brushes have become a versatile platform for the surface modification and functionalization, and therefore, have shown great potential in the the field of surface and interface property modulation, nano-hybrid materials, bio-medicine, etc. Poly(N-hydroxyethyl acrylamide)(PHEAA) brushes, as a newly developed hydrophilic polymer, have shown strong hydration properties and excellent biocompatibility. Due to these characteristics, it has been applied to the fabrication of antifouling surfaces, cotrolled drug release system, and other devices for biological purpose. However, the applications of PHEAA brushes in aqueous lubrication and water treatment membrane have not been reported yet. Herein, poly HEAA brushes on silica wafer and PP membrane were prepared by “graft from” and “graft to” methods, respectively. The morphologies, surface hydration, and aqueous lubrication of the resultant brushes, as well as their effect on the comprehensive performance of PP membrane in water treatment were systemically investigated.PHEAA brushes with well-controlled thickness were prepared via surface-initiated atomic transfer radical polymerization(SI-ATRP). Ellipsometry, atomic force microscope(AFM) were used to characterize the thickness and surface morphology of the polymer brushes respectively, and then the surface friction coefficients of polymer brushes in water was measured by a macroscopic friction test. The results showed that the surface morphology and hydration property of PHEAA brushes highly depended on the thickness of brushes, and both of them showed significant effect on the surface lubrication performance of polymer brushes. For thin film of ~9.5 nm, the surface was covered with isolated clusters, leading to high roughness. As the film thickness increased, the brushes became denser, and the surface roughness was generally decreased. The hydration also decreased as increasing the thickness, likely due to the increased intramolecular hydrogen bonds in the high molecular weight polymer which inhibit the binding of water in the brushes. The combined effect of surface morphology and surface hydration results in the high surface coefficients of the thick film and thin film. Only when the thickness is in an optimal range(20~50) nm, the films show excellent surface lubrication. For example, the brushes with thickness of 21.5 nm showed an extremely low surface friction coefficient of ~0.013, which is comparable to many surfaces of superlubricity.To enhance the load bearing and wear resistance of PHEAA brushes, a new strategy of fabricating cross-linked structrure was proposed. To this end, different content of cross-linker were added during SI-STRP to obtain brushes with different crosslinking density. Surface morphology, lubricating performance, especially the load bearing and wear resistance of the cross-linked PHEAA brushes were studied. It was shown that the presence of a cross-linker in the ATRP resulted in high surface roughness, leading to a significant increment of surface friction to ~2, albeit the water contact angle was lower. However, the strategy of cross-linking showed great success in enhancing the load bearing and wear resistance of polymer brushes, the deformation of cross-linked polymer brushes was small when under load, and the water molecules absorbed in the cross-linked network were squeezed out forming water lubrication film on surface. As a result, the friction coefficient decreased with an increase of load, opposite to cross-linked polymer brushes. Moreover, the wear resistance of the polymer brushes was also improved, as evidenced by the fact that the friction coefficient of cross-linked gel brushes was stable at 2000 s, while the friction coefficient of uncross-linked brushes increased quickly(at ~700 s).Due to the excellent hydration property, PHEAA brushes were also employed in the surface modification of polypropylene membrane with aim of improving the performance of PP membrane in water treatment. In this case, a combine technique of biomimetic adhesion and “graft to” method was used to realize the graft of PHEAA brushes onto polypropylene membrane. Breifly, a series of PHEAA with different chain length were first synthesized by reversible addition-fracture method transfer radical polymerization(RAFT). Then, the thioester group on the ends of PHEAA chains was reduced to thiol by Na BH4. For the surface modification PP membrane, polydopamine was first deposited on surface of PP membrane in an alkaline solution, and PHEAA brushes with different chain length were subsequently grafted onto PP membrane surface through Michael addition reaction. UV-vis spectrophotometer, video-based optical contact angle measuring system, scanning electron microscopy(SEM) to confirm the graft of PHEAA brushes and characterize the evolution of pore morphology of PP membrane. Filtration system was then used to evaluate the performance of modified PP membrane in water treatment, including the water flux, protein pollution resistance, and recovery rate. Specifically, the effect of molecular chain length of PHEAA brushes on grafting amount, hydrophicility of PP membrane and antifouling performance was also investigated. It was found that PHEAA brushes can successfully graft to PP membrane surface by dopamine deposition and addition reaction, and grafting amount depended on the chain length of PHEAA. The grafting of PHEAA brushes significantly improved the hydrophicility of PP membrane, which in turn increase water flux and enhanced fouling resistance capacity of the membrane. The performance of modified PP membrane also showed dependence on chain lenghth of PHEAA. The PP membrane modified with PHEAA brushes of 200 chain length showed the highest water flux recovery rate of ~72%.