Construction Of Modified Sponge With Liquid-like Molecular Brush Coating And Its Application In Oil Absorption And Oil-water Separation

Environmental pollution caused by crude oil spills and oily wastewater has become a global problem.Porous sponges are considered to be an ideal absorption material for the removal of oil pollution because of their low cost and ease of operation.Many researchers have prepared superhydrophobic-superoleophilic sponges with wettability selectivity for oil-water separation by various surface modifications,showing excellent absorption performance.Simultaneous optimization of the absorption and desorption speed of absorbents towards oil is highly desirable for their recyclable usage.Therefore,how to achieve the simultaneous optimization of absorption speed and desorption ratio of absorbents towards oil is still a challenge,because these two properties are generally conflicting.Herein,an ingenious strategy is proposed to tackle the above challenge via surface modification of sponges with flexible linear polydimethylsiloxane(LPDMS)brushes.The LPDMS brushes feature liquid-like properties at room temperature owing to its extremely low glass transition temperature,and act as a covalently-grafted lubrication layer throughout the three-dimensional network channels of the sponge,which can reduce friction between oil and sponge channel.Compared with the prevalent cross-linked polydimethylsiloxane(CPDMS),the sponge modified by linear LPDMS molecular brush can not only improve the absorption rate of viscous crude oil,but also improve the desorption ratio.The main contents of this paper are as follows:(1)A surface modification method was developed to modify the highly flexible liquid-like molecular brush(LPDMS)on the melamine sponge which can be produced in a large scale.The surface of the sponge was modified with polydopamine,Si O₂,and LPDMS molecular brushes in a step-by-step method,and a superhydrophobic-superoleophilic sponge(LPDMS@MF)with liquid-like surface characteristics was prepared.The changes of the sponge surface structure before and after modification were observed by thermal field emission scanning electron microscopy(SEM).The successful modification of liquid-like molecular brushes on sponges was demonstrated by XPS and FT-IR analysis.The selective wetting of LPDMS@MF was demonstrated by testing the contact angle of water droplets(160°)and oil droplets(0°).Furthermore,the advancing contact angle(45.7±0.5°)and receding contact angle(28.6±0.4°)of the viscous crude oil were measured on a flat glass surface(LPDMS@Glass),which was modified with the same modification method of LPDMS@MF.(2)The absorption capacity of LPDMS@MF was tested for a variety of oils and organic solvents,and it was found that the absorption capacities of hexane,diesel oil,mineral oil,peanut oil,and cetane could reach 78 g/g,88 g/g,89 g/g,98 g/g,and 84 g/g,respectively,showing extremely high absorption capacities.Further,we tested the cyclic performance of LPDMS@MF.The cyclic absorption-desorption of n-hexadecane was tested,and it was found that the absorption capacity of n-hexadecane hardly decreased after 20 times of cyclic test.By testing the separation of oil-water mixture and water-in-oil emulsion,it was found that the separation efficiency could reach more than 99%.By testing the acid(p H=3),alkali(p H=13)and high temperature(200?)resistance of LPDMS@MF,it was found that LPDMS@MF has excellent acid and alkali resistance and high temperature resistance,indicating that LPDMS@MF can perform oil-water separation under harsh environment.(3)The absorption performance of LPDMS@MF for viscous crude oil was tested.Compared with the cross-linked PDMS modified sponge(CPDMS@MF),the absorption speed of LPDMS@MF for viscous crude oil increased by 2.5 times(the average absorption speed in the first 50s).By testing the absorption of crude oil on water,it was found that LPDMS@took less time than CPDMS@MF to finish absorbing crude oil(2 ml)on water.Compared to the prevalent CPDMS modification strategy,the sponge modified with LPDMS brushes exhibits significantly enhanced absorption speed(2.5 times as large as the CPDMS-modified one),and superior desorption ratio increased by 6.4%towards viscous crude oils.(4)The inherent mechanism of LPDMS@MF with excellent absorption and desorption performance was revealed.The highly flexible PDMS molecular chains reduce the advancing contact angle of high-viscosity crude oil,thus enhancing the absorption drive to the crude oil,while the liquid-like properties increase the receding contact angle to the viscous crude oil,thus reducing the desorption resistance.In addition,the interface slip on the liquid-like surface further enhances the absorption speed and desorption ratio of LPDMS@MF to crude oil.