You-Yi Xu

Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer Films

This study aims to explore the fundamental surface characteristics of polydopamine (pDA)-coated hydrophobic polymer films. 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 lyophilicity (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 behavior of pDA is not strongly dependent on the nature of the substrates. 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 novel functional polymer materials.

Antifouling and Antimicrobial Polymer Membranes Based on Bioinspired Polydopamine and Strong Hydrogen-Bonded Poly(N-vinyl pyrrolidone)

A facile and versatile approach for the preparation of antifouling and antimicrobial polymer membranes has been developed on the basis of bioinspired polydopamine (PDA) in this work. 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. Furthermore, the modified membranes showed remarkable antimicrobial activity after iodine complexation with the PVP layer. 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. The strategy of material surface modification reported here is substrate-independent, and applicable to a broad range of materials and geometries, which allows effective development of materials with novel functional coatings based on the mussel-inspired surface chemistry.

Surface modification of PVDF porous membranes via poly(DOPA) coating and heparin immobilization.

Based on the strong adhesive behavior of poly(3,4-dihydroxy-l-phenylalanine) (or poly(DOPA)) on solid surface, poly(vinylidene fluoride) (PVDF) microporous membranes were surface-modified by the self-polymerization of DOPA in aqueous solution. Subsequently, heparin was immobilized covalently onto the obtained PVDF/poly(DOPA) composite membranes by the coupling between heparin and poly(DOPA) coating. The modified membranes were subjected to a long-term washing, and the firm immobilization of poly(DOPA) and heparin was confirmed by X-ray photoelectron spectroscopy (XPS). The results of water contact angle measurements showed that the hydrophilicity of PVDF membranes was significantly improved by the incorporation of poly(DOPA) and heparin. The effects of poly(DOPA) and heparin on membrane surface morphologies were also investigated by scanning electron microscopy (SEM).

Immobilization of bovine serum albumin onto porous polyethylene membranes using strongly attached polydopamine as a spacer

Based on the self-polymerization and strong adhesion characteristics of dopamine in aqueous solution, a novel and convenient approach was developed to immobilize protein onto porous polyethylene (PE) membranes. A thin polydopamine (pDA) layer was formed and tightly coated onto PE membrane by dipping simply the membrane into dopamine aqueous solution for a period of time. Subsequently, bovine serum albumin (BSA) was bound onto the obtained PE/pDA composite membranes via the coupling between BSA and the reactive polydopamine layer. The firm immobilization of polydopamine layer and BSA was verified by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The results of water contact angle measurement showed that the hydrophilicity of PE membrane was significantly improved after coating polydopamine and binding BSA. The experiments of blood platelet adhesion indicated that BSA-immobilized PE membrane had better blood compatibility than the unmodified PE and the PE/pDA composite membranes. The investigations on hepatocyte cultures and cell viability revealed that the polydopamine coating endowed PE membrane with significantly improved cell compatibility. Compared to BSA surface, polydopamine surface is more favorable for cell adhesion, growth, and proliferation.

Microporous polypropylene hollow fiber membrane: Part I. Surface modification by the graft polymerization of acrylic acid

Surface modification of microporous polypropylene hollow fiber membranes was performed by graft polymerization of acrylic acid. The effects of temperature, polymerization medium, monomer concentration and multifunctional cross-linker on the graft polymerization were studied. The grafting degree on the polypropylene membrane can be greatly increased upon the addition of divinylbenzene (DVB). Scanning electron microscopy (SEM) pictures demonstrated that PAA grafted mainly on the surface of the membrane. Mercury porosimetry analysis revealed that the grafting also took place within the pores. Contact angle and absorbed water measurements showed excellent wetting properties with water for the grafted membranes. The grafted membrane did not show high flux for ultrapure water because the grafting of AA plugs the pores of the membrane.

Microporous polypropylene and polyethylene hollow fiber membranes. Part 3. Experimental studies on membrane distillation for desalination

Three polypropylene (PP) and one polyethylene (PE) microporous hollow-fiber membranes were used in direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD) for the desalination of simulated seawater. The influence of feed temperature and feed flow on distillate pure water flux was investigated. The comparison of the PP and PE membranes in DCMD and VMD was carried out. It was found that the water flux increased with the feed temperature and feed flow in both DCMD and VMD. The data also showed that, compared with the PP membranes, higher water flux could be obtained by using PE membranes in both the DCMD and VMD processes.

Anti-fouling and anti-bacterial polyethersulfone membranes quaternized from the additive of poly(2-dimethylamino ethyl methacrylate) grafted SiO2 nanoparticles

Anti-fouling and anti-bacterial polyethersulfone (PES) membranes were developed by the addition of poly(2-dimethylaminoethyl methacrylate) grafted silica nanoparticles (SiO2-g-PDMAEMA NPs) and further post-quaternization. The SiO2-g-PDMAEMA NPs were first synthesized by grafting PDMAEMA brushes from SiO2 NPs via surface-initiated, reversible addition fragmentation chain transfer (RAFT) polymerization. PES/SiO2-g-PDMAEMA hybrid ultrafiltration (UF) membranes were then prepared from the blending solutions of PES and SiO2-g-PDMAEMA NPs via non-solvent induced phase separation (NIPS) process. The PDMAEMA chains incorporated into the PES membranes were further quaternized by reacting with 1,3-propane sultone (1,3-PS) and methyl iodide (CH3I), respectively. After treatment with 1,3-PS, the resulting zwitterionic PES membranes exhibited excellent hydrophilicity, water permeability, solute rejection and protein anti-fouling properties. The cationic membranes obtained from CH3I treatment showed strong anti-bacterial activity against Escherichia coli (E. coli) and Staphyloccocus aureus Rosenbach (S. aureus). This work presents a convenient strategy for anti-biofouling modification of polymer membranes via surface quaternization of the reactive SiO2-g-PDMAEMA NPs additive.

HYDROPHILIC NANOFILTRATION MEMBRANES WITH SELF-POLYMERIZED AND STRONGLY-ADHERED POLYDOPAMINE AS SEPARATING LAYER *

Inspired by the self-polymerization and strong adhesion characteristics of dopamine in aqueous conditions, a novel hydrophilic nanofiltration (NF) membrane was fabricated by simply dipping polysulfone (PSf) ultrafiltration (UF) substrate in dopamine solution. The changes in surface chemical composition and morphology of membranes were determined by Fourier transform infrared spectroscopy (FTIR-ATR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The experimental results indicated that the self-polymerized dopamine formed an ultrathin and defect-free barrier layer on the PSf UF membrane. The surface hydrophilicity of membranes was evaluated through water contact angle measurements. It was found that membrane hydrophilicity was significantly improved after coating a polydopamine (pDA) layer, especially after double coating. The dyes filtration experiments showed that the double-coated membranes were able to reject completely the dyes of brilliant blue, congo red and methyl orange with a pure water flux of 83.7 L/(m2·h) under 0.6 MPa. The zeta potential determination revealed the positively-charged characteristics of PSf/pDA composite membrane in NF process. The salt rejection of the membranes was characterized by 0.01 mmol/L of salts filtration experiment. It was demonstrated that the salts rejections followed the sequence: NaCl < Na2SO4 < MgSO4 < MgCl2 < CaCl2, and the rejection to CaCl2 reached 68.7%. Moreover, the composite NF membranes showed a good stability in water-phase filtration process.

Flow distribution in a randomly packed hollow fiber membrane module

Abstract Shell side flow distribution in a randomly packed hollow fiber module was analyzed using the random cell model. The module was divided into subchannels. The hydrodynamics in each channel was described according to Happel’s free surface model and then it was just the function of the local cell packing fraction. Based on the theoretical probability density distribution function of local packing fraction, the shell side flow distribution was presented. The results showed that a serious flow mal-distribution occurred in the shell side. More than the 40% fluid flowed through the largest 20% void area of the total module void area (cells with high void fraction), and the smallest 20% void area (cell with low void fraction) carried less than 6% of the flow. This mal-distribution became more serious as the mean packing density increased.

Microporous polypropylene hollow fiber membranes: Part II. Pervaporation separation of water/ethanol mixtures by the poly(acrylic acid) grafted membranes

Polypropylene hollow fiber membranes grafted with poly(acrylic acid) were used for water–ethanol separation by pervaporation. The effects of grafting degree of poly(acrylic acid) (PAA), the cross-linking degree of the grafted-PAA, the sodium counter-ions as well as the operation temperature and the ethanol concentration in feed mixtures on pervaporation properties were investigated. It was found that water-permselective pervaporation membranes could be prepared by the grafting polymerization of acrylic acid on microporous polypropylene hollow fiber membrane surface. The separation factor increased with the increase of grafting degree of PAA in the range of 20–70 wt.%. Incorporating counter-ions as well as multifunctional comonomer into the grafted chains could improve the selectivity but sacrificed the permeation flux. The separation factor of the counter-ion containing membrane decreased according to the sequence Al 3+ >K + >C a 2+ >N a + >L i + and the permeation flux increased following the sequence Al 3+ < Ca 2+ < K + < Na + < Li + . Remarkable increase in permeation flux without decrease in separation factor was observed for Al 3+ as counter-ion in this membrane. These results might be ascribed to the increase of packing density and the decrease of swelling degree of grafted layer on the membrane surface.