## 1 Introduction  
The petroleum industry discharges large quantities of oily wastewater, most of which comes from oilfields [1, 2]. This wastewater significantly threatens ecological systems and human health [3, 4]. According to statistics, the volume of oilfield-produced water will increase to 243,000 million barrels per year by 2030, due to increased demand for crude oil and increased global production activity in the absence of economically competent renewable energy sources and their limitations, all of which will be reflected in an increasing volume of oily water associated with oil extraction [5, 6]. The oilfield wastewater stream must be managed to reduce its harmful environmental impact and meet the agenda of the Sustainable Development Goals by 2030 [7]. The difficulty of separating oily water mainly arises from the size of the oil droplets in the water and the stability of the system. According to the size of the oil droplets, oil-water systems are usually divided into floating oil (>150 \(\upmu\)m), dispersed oil (20-150 \(\upmu\)m), emulsified oil (<20 \(\upmu\)m), and dissolved oil [8]. Emulsified oil, particularly heavy crude oil (<20\({}^{\circ}\) API), contains more natural compounds as surfactants, such as asphaltenes and resins, which promote the stability of the emulsion, making the emulsified oil separation more difficult to handle [9; 10]. Membrane technology presents a promising alternative for treating oilfield wastewater, offering higher-quality effluent, lower energy consumption, compact design, low chemical consumption, and cost efficiency compared to conventional methods [11; 12]. However, a major technical obstacle faced by successfully implementing membranes for produced water treatment in oilfields is serious fouling by crude oil, since fouling causes flux decline due to its high adhesion to most membrane surfaces [13; 14; 15; 16; 17; 18]. To date, there is a general lack of crude oil-repellent membranes in the oil and gas sector. Polyvinylidene fluoride (PVDF) is an important polymer used in microfiltration and ultrafiltration membranes due to its excellent flexibility, chemical resistance, thermal stability, and mechanical strength [19; 20; 21]. These characteristics make PVDF membranes particularly suitable for oily water separation. However, PVDF membranes have strong hydrophobicity, so membrane fouling inevitably happens during separation processes [22; 23; 24]. In practical applications, the wettability of the membrane materials, whether hydrophilic or hydrophobic, can be tailored by constructing specially engineered structures on the surface of the substrate material or by introducing hydrophilic or hydrophobic components. Such development strategies improve the membrane separation performance to meet different research needs and industrial requirements [25; 26]. Consequently, the modification of PVDF membranes, likewise other polymeric membranes, has emerged as a significant research area focused on altering their hydrophilic properties [27; 28; 29]. Various modification methods have gained significant attention in the development of super-wettable materials, such as surface grafting [30; 31; 32], surface coating [33; 34; 35], and grafting co-blending [27; 36; 37]. Among these strategies, surface coating with hydrophilic materials stands out for its simplicity and scalability. Hydrogels, in particular, have gained significant interest as promising materials for the hydrophilic modification of separation membranes, owing to their high hydrophilicity and exceptional design flexibility [38; 39]. The high hydrophilicity of polymer hydrogels is attributed to the abundance of hydrophilic functional groups, including the hydroxyl (-OH), amino (-NH\({}_{2}\)), carboxyl (-COOH), and amide (-CONH\({}_{2}\)) groups. These functional groups facilitate the formation of a hydration layer on the membrane surface during the filtration process that repels crude oil droplets, thereby exhibiting a superoleophobic effect crucial for oil-water separation applications [40; 41].