Compared to traditional separation technologies such as distillation and absorption, membrane separation offers distinct advantages in terms of separation efficiency, operational ease, and energy conservation, garnering widespread attention from researchers(K Scott, 1997, Tijing, et al., 2015, Wang and Chung, 2015). Hydrophobic microporous membranes play a crucial role in oil-water separation and membrane contactor processes, where the degree of hydrophobicity significantly influences membrane performance(Wang and Chung, 2015, Zhang, et al., 2018). In the process of oil-water separation, In the process of oil-water separation, if the hydrophobicity of the membrane is insufficient, the spreading ability of the oil phase on the membrane surface will decreases, and the oil-water phase will not be effectively divided, and the water will be entrained by the oil through the membrane pores(Tan, et al., 2018, Wang, et al., 2015). Hence, enhancing membrane hydrophobicity is imperative to ensure the successful operation of relevant membrane processes(Chu, et al., 2015, Remanan, et al., 2018).
It is well known that the hydrophobicity of a membrane surface is determined by both the surface energy of the material and its surface microstructure(Roach, et al., 2008). Constructing microstructures or reducing the surface energy of hydrophobic membranes proves effective in enhancing hydrophobicity(Roach, Shirtcliffe and Newton, 2008, Yan, et al., 2011). Polyvinylidene fluoride (PVDF), with its low surface energy and excellent physical, chemical, and mechanical properties, is a widely used material for preparing hydrophobic membranes(Wang, et al., 2017, Zuo and Chung, 2017). However, PVDF hydrophobic membranes without rough surfaces struggle to achieve a water contact angle exceeding 150°. To address this limitation, researchers often adopt the "lotus leaf effect" principle, constructing micro-nano hierarchical structures on the membrane surface to achieve superhydrophobic performance(Himma, et al., 2019).
Various methods for enhancing the hydrophobicity of PVDF membrane surfaces through the construction of rough structures have been documented in several papers, including spraying, electrospinning, coating, immersion-deposition, co-mingled particles, phase separation, template methods, and more (Hamzah and Leo, 2017, Li, et al., 2019, Su, et al., 2016, Tao, et al., 2014, Wei, et al., 2018). Su et al. prepared three-dimensional superhydrophobic membranes with high porosity via simultaneous electrospraying of silica/DMAc colloids and electrospinning of PVDF/DMAc solutions. The membrane exhibits a high WCA of 163° and a low SA of 3°, it also shows superior performance in both membrane distillation and oil-water separation(Su, Li, Dai, Gao, Tang and Cao, 2016). In another approach, hydrophobic PVDF hollow fiber composite membranes were created using the dilute solution coating process to develop a specialized surface structure akin to the dual micro-nano structure found on lotus leaves. These membranes exhibited excellent performance in separating dyes from water through VMD(Li, Feng, Shi, Zhang, Du, Qin and Qin, 2019). SiO2 nanoparticles are strongly fixed to the surface of the “adhesive” PVDF membranes via delayed phase inversion. Oil/water filtration membranes with micro–nano coarse structures and low surface energies were obtained, and it has good stability and recoverability in oil / water separation(Wei, Dai, Lin, An, He, Chen, Chen and Zhao, 2018). Hamzah et al. modified PVDF membranes mixed with TiO2 nanoparticles, and then performed membrane distillation of saline with phenolic compound. The fouled membrane shows self-cleaning property under UV irradiation(Hamzah and Leo, 2017). Mimi et al. used PET non-woven fabric as a mold to prepare micro-nano hierarchical intelligent superwetting PVDF membrane. It is suitable for separating various oil-in-water, water-in-oil emulsion and has excellent separation efficiency, permeability, recyclable and anti-fouling properties(Tao, Xue, Liu and Jiang, 2014).
The mentioned methods have proven effective in enhancing the hydrophobicity of PVDF membranes, yet they come with drawbacks such as poor nanoparticle adhesion to the membrane surface, low mechanical strength of the surface microstructure, polymer adhesion to the template, and challenges in continuous production. These limitations hinder practical applications(Remanan, Sharma, Bose and Das, 2018, Tian, et al., 2011, Wang, Tang and Li, 2017). In this regard, our research group proposed the preparation of superhydrophobic iPP microporous membranes by combined hot rolling embossing with thermally induced phase separation (TIPS) and superhydrophobic PVDF microporous membranes by combining immersion precipitation with rolling embossing(Sun, et al., 2020, Sun, et al., 2019). Notably, these methods are characterized by scalability and continuity, presenting a novel avenue for large-scale and continuous improvement of membrane surface hydrophobicity. During the preparation of superhydrophobic membranes through the combination of hot rolling embossing with TIPS, it is crucial to pre-evaporate the liquid membrane in the air for a specified duration. This step is essential to adjust the gelation state, ensuring successful membrane replication during the rolling procedure.
In this study, we focused on the preparation of superhydrophobic PVDF microporous membranes using hot rolling embossing combined with TIPS. With fixed roller structure, roller temperature, and water bath conditions, we delved into the impact of pre-evaporation on the membrane's structure and properties. Initially, pre-evaporation experiments were conducted to identify the optimal pre-evaporation time range for the liquid film before rolling pressing. Subsequently, SEM, XRD, AFM were employed for characterization, and membrane performance metrics such as water contact angle, porosity, pore size, N2 flux, and mechanical stability were thoroughly examined. Finally, the oil-water separation capabilities of the PVDF microporous membrane were put to the test.