Smart Oil/Water Separation Materials Based On Stimuli Responsive Block Copolymers

Oil spills and industrial organic pollutants have induced severe water pollution and caused a long-term damaging effect on the ecological environment.Novel,economical,and efficient technologies and materials for oil/water separation are urgently desired.Because of the advantages in recuperability,separating feed,and efficiency,the special wettability-controlled separation has a promising application in the field of wastewater treatment.Smart materials with switchable wettability can realize controllable separation processes by adjusting the external stimuli such as pH,light,temperature,electric fields,and gas.As the most potential candidate for the development of a smart and convenient separation method,stimuli-responsive special wettable materials attract increasing interest whether in the fundamental or practical application studies.Block copolymers,containing two or more chemically different blocks that can exhibit their own properties independently and also affect each other,are good candidates for constructing smart interfaces.In this work,a series of stimuli-(thermal-and pH-)responsive block copolymers were designed and prepared.Based on the obtained copolymers,various special wettable materials were fabricated through using solution-casting method,electrospinning technique,and mussel-inspired chemistry,respectively.Ascribe to the synergy between the surface chemical composition and surface topography,the as-prepared smart materials exhibit specific stimuli-switchable oil/water wettability and can realized layered oil/water separation with high efficiency.The contents of each part are listed as follows:(1)A series of thermal-responsive block copolymers,i.e.,poly(methyl methacrylate)-block-poly(N-isopropyl-acrylamide)(PMMA-b-PNIPAAm),were successfully synthesized via successive copper(0)-mediated reversible-deactivation radical polymerization(Cu(0)-mediated RDRP)technology.Thermal properties of block copolymers with different PNIPAAm chain lengths were investigated by differential scanning calorimetery(DSC)and thermogravimetric analysis.The well-separated glass transition temperature values shown in DSC results indicate that two chemically different blocks,PNIPAAm and PMMA,are incompatible and phase-segregated.The thermal degradation results show that the thermal stability of these copolymers was improved through incorporating the PNIPAAm segments.Because of the synergistic effect of copolymer composition and copolymer film roughness,the obtained reversible thermal-responsive wettability for the resulting copolymer-modified surfaces is different,which is enhanced by incorporating more functional NIPAAm units.Specifically,the variations of the static water contact angle for the surfaces fabricated by PMMA₁₂₀-b-PNIPAAm_n(n=40,54,86,130)are 18.7,20.4,26.8,and 34.3°,respectively.The as-prepared copolymer,especially PMMA₁₂₀-b-PNIPAAm₁₃₀,is a good candicate coating material for constructing smart surface.(2)Two smart membranes were prepared through solution-casting method and electrospinning technology,respectively,based on temperature-responsive copolymer PMMA-b-PNIPAAm.According to the thermo-responsive component PNIPAAm,both membranes exhibited temperature-modulable oil/water wettability.Electrospun fibrous membrane owned an extended transition range of oil/water wettability compared to polymer solution-casting membrane because of its 3D network porous structure of the random entangled fibers.The as-prepared membranes realized gravity-driven oil/water separation with efficiency higher than 98%through regulating temperature.Solution-casting membrane exhibited a water flux of about 6200 L h^-1 m^-2 and an oil flux of about 1550 L h^-1 m^-2.By contrast,characteristics of the high porosity and the large surface-to-volume ration made the electrospun fibrous membrane achieve higher fluxes of about 9400 L h^-1 m^-2 for water and about 4200 L h^-1 m^-2 for oil.As a whole,electrospnining is a powerful and cost-effective method to construct smart membrane with excellent wetting property and separation performance.(3)A smart fibrous membrane was fabricated by depositing pH-responsive copolymer fibers on a stainless steel mesh through electrospinning.The cost-effective precursor material poly(methyl methacrylate)-block-poly(4-vinylpyridine)(PMMA-b-P4VP)was precisely synthesized using Cu(0)-mediated RDRP.pH-responsive P4VP and the underwater oleophilic PMMA confer the as-prepared membrane with switchable surface wettability toward water and oil.Simultaneously,the three-dimensional(3D)network structure of fibers greatly strengthens the oil/water wetting property of the membrane,which is desired by the separation of oil and water mixtures.The as-prepared fiber membrane exhibits gravity-driven pH controllable oil/water separations.Oil selectively passes through the membrane,whereas water is remained in the initial state;after wetting the membrane with acidic water(pH=3),the reverse separation is realized.Both separations are highly efficient and the membrane also exhibits switchable wettability after many cycles of the separation process.This cost-effective and easily mass-produced smart fiber membrane with excellent oil fouling repellency has promising in practical applications for water purification and oil recovery.(4)pH-responsive polymer poly(dimethylsiloxane)-block-poly(4-vinyl-pyridine)(PDMS-b-P4VP)mat was first prepared through electrospinning technology.Differing from PMMA block,PDMS block is flexible and thus it is benefit for underwater oil wettability switching.The PDMS-b-P4VP fibrous mat with a thickness of around 250?m exhibits good pH-switchable oil/water wettability,and is able to effectively separate oil or water from layered oil/water mixtures by gravity-driven through adjusting pH value.Stemming from its porous structure and pH-switchable superwettability,the electrospun PDMS-b-P4VP fibrous mat achieved controllable separations with high fluxes of about 9000 L h^-1 m^-2 for oil(hexane)and 27000 L h^-1 m^-2for water.In addition,extended studies on the polymer/silica nanoparticulates(silica NPs)composite fibrous mats show that the addition of inorganic component improves the thermal stability,pH switchable wettability and separation performance of the fibrous mats(about 9000 L h^-1m^-2 for hexane and 32000 L h^-1 m^-2 for water).It can be concluded from the results that both polymer fibrous mat and sillica-filled composite fibrous mats are good candidates for on-demand layered oil/water mixtures separation.(5)Mussel-inspired V-shaped copolymer based materials for pH controllable oil/water separation was successfully fabricated.Triblock copolymer,polydimethylsiloxane-block-poly(2-hydroxyethyl methacrylate)-block-poly(2-(dimethylamino)ethyl methacrylate)(PDMS-b-PHEMA-b-PDMAEMA)was designed and synthesized through Cu(0)-mediated RDRP.Hydroxyl-containing PHEMA block enabled the covalent reaction to occur between polymer and polydopamine(PDA)functionalized substrates.The mixed polymer brushes of oleophilic/hydrophobic PDMS and pH-responsive PDMAEMA endowed the substrates with good pH responsive oil/water wettability and stability.As a proof of concept,the functionalized mesh can separate a range of different immiscible oil/water mixtures with high separation efficiency over 98.5%and high oil flux of8800?9500 L h^-1 m^-2 and water flux of 7400?7800 L h^-1 m^-2.Additionally,the functionalized sponge realized reversible oil capture and release in aqueous media through adjusting pH value.Because dopamine through self-polymerization can form strong adhesive films on inorganic and organic substrates in the presence of Tris,the method used in current contribution would be also suitable for producing polymer brush functionalized intelligent materials.