Surface Tailoring Of Polymeric Separation Membranes Based On The Self-polymerization And Assembly Of Dopamine

Polymer membranes play important roles in current separation and purification technologies. However, membrane fouling, blood coagulation and immunologic rejection resulting from contact with membranes limit their further developments and applications in water treatment, biomedical fields, and so on. It is clear that all these adverse effects are closely related to the poor hydrophilicity, biocompatibility and antimicrobial properties of the polymer membranes. A promising strategy for solving these problems may be construction of novel functional layers that could endow the membranes with improved performances. Recently, the bionic researchers have found that mussel-inspired polydopamine (PDA) could be incorporated into/onto materials on the basis of the self-polymerization and adhesion behavior of dopamine. PDA has good compatibility with various materials, and could serve as a versatile platform for surface functionalization via simple secondary treatments. In the present dissertation, the self-polymerization and deposition of dopamine on different substrates were systematically studied. Then PDA particles/films were incorporated into/onto polymer membranes by blending or surface modification process, and different functional layers based on PDA were designed to enhance the antifouling, antimicrobial properties and biocompatibility of the membranes. It is expected to establish a versatile approach of surface modification for hydrophobic polymer membranes.This study first explored the self-polymerization and deposition behavior of dopamine under different conditions and the fundamental surface characteristics of polydopamine (PDA) coatings. A poly(vinylidene fluoride)(PVDF) film was surface modified by dip coating in an aqueous solution of dopamine on the basis of its self-polymerization and strong adhesion feature. The self-polymerization and deposition rates of dopamine on film surfaces increased with increasing temperature as evaluated by both spectroscopic ellipsometry and scanning electronic microscopy (SEM). Changes in the surface morphologies of PDA-coated films as well as the size and shape of PDA particles in the solution were also investigated by SEM, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The surface roughness and surface free energy of PDA-modified films were mainly affected by the reaction temperature and showed only a slight dependence on the reaction time and concentration of the dopamine solution. Additionally, three other typical hydrophobic polymer films of polytetrafluoroethylene (PTFE), poly(ethylene terephthalate)(PET), and polyimide (PI) were also modified by the same procedure. The liquid affinity and surface free energy of these polymer films were enhanced significantly after being coated with PDA, as were those of PVDF films. It is indicated that the deposition of PDA is not strongly dependent on the nature of the polymers. This information provides us with not only a better understanding of biologically inspired surface chemistry for PDA coatings but also effective strategies for exploiting the properties of dopamine to create advanced functional materials.Based on the strong adhesion effect between polydopamine nanoparticles (PDA NPs) and PVDF, a PVDF/PDA blend membrane was prepared via non-solvent induced phase separation (NIPS) process. The size and shape of PDA NPs as well as the surface morphologies of PVDF/PDA membranes were observed by TEM and SEM. Data of water contact angle measurements showed that the hydrophilicity of the blend membranes was improved obviously compared with that of the unmodified PVDF membrane. The water permeability and antifouling properties of the PVDF membranes were both enhanced after blending modification as evaluated by protein filtration tests. Results from tensile test indicated that an appropriate amount of PDA NPs was able to improve the toughness of the as-prepared blend membranes. Moreover, the PVDF/PDA blend membranes had satisfying long-term stability in aqueous environment. The PVDF/PDA blend membranes were able to undergo secondary treatments based on the potential reactivity of PDA NPs. Initiators containing Br groups were immobilized on the blend membranes via covalent conjugation with PDA. Then zwitterionic polymer brushes were incorporated onto the membranes through atom transfer radical polymerization (ATRP) of sulfobetaine methacrylate (SBMA). The surface morphologies of membranes before and after grafting were observed by SEM and AFM. The surface hydrophilicity of the modified membranes was remarkably improved by compared with the pure PVDF membrane. Experiments of protein filtration showed that membrane fouling obviously reduced after surface grafting. Results of platelet adhesion test indicated that the incorporation of poly SBMA suppressed the adhesion and activation of platelets, and improved the in vitro hemocompatibility of PVDF membranes significantly. The stability and reactivity of PDA NPs offer opportunities to realize surface functionalization of the blend membranes, which enhances their surface designability and allows effective development of novel functional polymer membranes.It is well-known that a tightly adherent PDA layer can be generated over a wide range of material surfaces through a simple dip-coating process in dopamine aqueous solution. The resulting PDA coating is prone to be further surface-tailored and functionalized via secondary treatments because of its robust reactivity. Herein, a typical hydrophobic polypropylene (PP) porous membrane was first coated with a PDA layer and then further modified by poly(N-vinyl pyrrolidone)(PVP) via multiple hydrogen-bonding interactions between PVP and PDA. Data of water contact angle measurements showed that hydrophilicity and wettability of the membranes were significantly improved after introducing PDA and PVP layers. Both permeation fluxes and antifouling properties of the modified membranes were enhanced as evaluated in oil/water emulsion filtration, protein filtration, and adsorption tests. The PVP layer immobilized on the membrane had satisfying long-term stability and durability because of the strong noncovalent forces between PVP and PDA coating. Furthermore, the modified membranes showed remarkable antimicrobial activity after iodine complexation with the PVP layer. In the other work, a functional layer was immobilized onto PDA-coated membranes via covalent bonding with better long-term stability. A thin PDA layer was first formed and tightly coated onto PE membranes by dipping simply the membranes into dopamine aqueous solution for a period of time. Subsequently, heparin or bovine serum albumin (BSA) was bound onto the PE/PDA composite membranes via Michael-type addition or Schiff base reaction. The results of water contact angle measurement showed that the hydrophilicity of PE membranes was significantly improved after modification. And the water flux of the modified membranes increased under proper modification conditions. Moreover, the heparin or BSA-immobilized PE membranes had better blood compatibility than the unmodified PE and the PE/PDA composite membranes. The PDA and BSA layers endowed PE membranes with significantly improved cell compatibility. Compared to BSA surface, PDA surface is more favorable for cell adhesion, growth, and proliferation. The strategy of material surface modification is substrate-independent, and applicable to a broad range of materials and geometries, which allows effective development of materials with novel functional layers based on the mussel-inspired surface chemistry.