Introduction

Oil is the most important energy source and raw material for human production, and it includes different types such as, petroleum, animal fat and vegetable oil (Gryznova et al. 2019; Amina et al. 2022). However, leak cases of during oil transportation have caused serious environmental pollution in recent years (Zhang et al. 2013). Oil floating on water surfaces restricts the flow of oxygen into water columns, which in turn threatens the survival of aquatic organisms. More seriously, fish and fishery products may be exposed to poisoning by the intermediate products of oil degradation, and they become harmful to humans when they enter the food chain (Hassoun and Emir Çoban 2017). Therefore, research on oil–water separation technology has practical importance, and many methodologies like mechanical oil boom (Etkin and Nedwed 2021), in situ burning of oil (Faksness et al. 2022), oil spill remediation dispersants (John et al. 2016), biodegradation technology (Xue et al. 2015) and oil-absorbing materials (Zhang et al. 2019) have been extensively investigated for the removal of oil pollutants from water bodies. Among these methods, porous oil-absorbing materials are considered highly promising for oil–water separation applications due to their low-cost, short treatment period, absence of secondary contamination and the possibility of oil recycling (Hao et al. 2019; Deng et al. 2020; Liu et al. 2021). These commonly used porous materials for oil–water separation including activated carbon, resin sponges, cotton fabrics and aerogels (Wang et al. 2022; Sun et al. 2021; Rana et al. 2016). Herein, melamine formaldehyde resin sponge (MFRS) possesses numerous attractive advantages, such as chemical and mechanical durability, porous and lightweight, environmentally friendly features, etc. (Wang et al. 2023). However, these porous materials are mostly hydrophilic, hence demonstrating very low efficiency in oil–water separation.

Nanostructure modification and chemical modification of surfaces have been widely studied to develop porous materials with hydrophobic and lipophilic properties for efficient oil–water separation. Nanostructure modification is achieved by the construction of geometric folds on the surfaces of materials to hinder the infiltration of water droplets into the materials, thereby improving their hydrophobicity (Wu et al. 2019; Chu et al. 2023). Meanwhile, chemical modification of surfaces involves procedures that alter the surface chemistry of materials to introduce new functionalities, characteristics and performances to the materials (Wang et al. 2020; Pan et al. 2021; Jing et al. 2023). The principle for the chemical modification of surfaces to improve the hydrophobicity of materials is based on the introduction of compounds containing hydrophobic functional groups with low surface energy (Shi et al. 2022). Compounds that commonly contain hydrophobic functional groups include fluoropolymers, silane coupling agent, polydopamine and oil (Lv and Cheng 2021; Zhang et al. 2022). Oils containing hydrophobic functional groups, such as polydimethylsiloxane (PDMS), do not only exhibit hydrophobic properties, but also display lipophilicity. PDMS-modified porous materials have received extensive research interest in oil–water separation applications owing to their inexpensive nature, as well as their chemical and mechanical stability (Liu et al. 2019). Jin et al. (2015) prepared PDMS/cotton via a solution-immersion process and achieved a water contact angle greater than 157°. Wang and coworkers (Wang et al. 2019) fabricated PDMS@starch coatings for effective separation of oil–water mixture and emulsions, and investigations confirmed its strong hydrophobicity. Chen et al. (2016) modified sponges with PDMS via a solution-immersion process and attained efficient separations of hexane, toluene, octadecene, silicone oil and motor oil from an oil–water mixture. Chu et al. (2022) synthesized PDMS/cellulose nanofiber by an electroless deposition method, and the material exhibited an oil absorption capacity greater than 60% of its volume. However, the weak chemical bonds between the porous materials and PDMS resulted in reduced selectivity for oil–water separation in recycling experiments. This is a limitation for the practical application of this method.

To resolve this problem, porous materials modified with PDMS-based coatings were developed to improve the durability and recyclability of the modified material by introducing more functional blocks. Tong et al. (2022) synthesized a porous rGO/PEI/PDMS sponge in six steps and achieved excellent recyclability due to the enhancement of the interface interaction between the rGO and PDMS by PEI as the adhesive layer. Moreover, Wen et al. (2021) reported a progressive graft polymerization method for the sequential introduction of functional glycidyl methacrylate (GMA), diethylenetriamine (DETA) and PDMS block on the surface of cotton fabrics, and the resulting material displayed satisfactory recyclability and stability in oil–water separation. However, such a step-by-step grafting process is often tedious, signifying low operational feasibility and discouraging large-scale applications. This drawback became the major motivation for the current study. We therefore designed and synthesized a PDMS-based copolymer containing various functional parts, and the porous material (such as MFRS) was modified by copolymers via a one-step method. Interestingly, the constituent acrylate double bond and epoxy group in glycidyl methacrylate (GMA) played significant roles in the fabrication process. While the acrylate double bond was co-polymerized with PDMS, the epoxy group reacted with a variety of functional groups to form firm chemical bonds. Hence, the PDMS-PGMA copolymer exhibited promising potentials for practical applications.

Herein, we synthesized a PDMS-PGMA copolymer whose chemical structure was confirmed by 1H NMR and FT-IR spectra. Then, we modified melamine formaldehyde resin sponges (MFRS) with PMMA, PDMS-PMMA or PDMS-PGMA via a simple solution-immersion. Furthermore, we compared the water contact angle (WCA) and oil–water separation efficiency of the modified MFRS. Moreover, the elastic property and geometry morphology of the PDMS-PGMA/MFRS were investigated. Benefiting from the introduction of functional blocks in the PDMS-PGMA copolymer, PDMS-PGMA/MFRS exhibited efficient capabilities for the separation of oil–water mixture. Finally, we provided explanations for the decrease in the separation efficiency of PDMS-PGMA/MFRS in recycling experiments. We believe that this work will inspire the fabrication of recyclable, durable, and efficient oil–water separation porous materials.

Experimental section

Materials

Melamine formaldehyde resin sponge (MFRS, Sichuan Super Poly New Material Co., Ltd.), N, N-dimethylformamide (DMF, Tianjin Fuyu Fine Chemical Co., Ltd., AR), plant oil (Yihai Kerry Arawana Holdings Co., Ltd.), polydimethylsiloxane (PDMS-OH, Laizhou Xintai Chemical Co., Ltd., CP), triethylamine (TEA, Tianjin Fuyu Fine Chemical Co., Ltd., AR), 4-dimethylaminopyridine (DMAP, Shanghai Fuzhe Chemical Co., Ltd., AR), 2-bromoisobutyryl bromide (BiBB, Shanghai Darui Fine Chemicals Co., Ltd., AR), dichloromethane (CH2Cl2, Tianjin Fuyu Fine Chemical Co., Ltd., AR), sodium bicarbonate (NaHCO3, Shanghai Hongguang Chemical Factory, AR), cuprous chloride (CuCl, Tianjin Dengfeng Chemical Reagent Factory, AR), magnesium sulfate anhydrous (MgSO4, Shanghai Shanpu Chemical Co., Ltd., AR), N,N,N′,N′-tetramethylethylenediamine (TMEDA, Sinopharm Chemical Reagent Co., Ltd., AR), methanol (CH3OH, Tianjin Damao Chemical Reagent Factory, AR) and glycidyl methacrylate (GMA, Aladdin, AR) were straightway used without purifications.

Tetrahydrofuran (THF, Tianjin Hedong District Hongyan Reagent Factory, AR) and cyclohexanone (CYC, Tianjin Chemical Reagent Sixth Factory, AR) were purified by adding calcium hydride to solutions under magnetic stirring for 24 h, respectively, followed by distilling of the reaction solution and collecting the distilled liquid.

Synthesis of copolymers

The copolymers of both PDMS-PGMA and PDMS-PMMA were synthesized by similar procedures, the synthesis steps of PDMS-PMMA are described in detail in previous work (Niu et al. 2014), and the PDMS-PGMA was synthesized in the following steps.

Firstly, we have prepared the PDMS-Br. A Schlenk flask was pumped out of the air and fed with N2 in an ice-bath, and the THF solutions of PDMS-OH, TEA and DMAP were introduced under the protection of N2. Followed by addition of a certain amount of BiBB solution, and then the reaction was carried out at room temperature under magnetic stirring for 24 h to obtain a white suspension. After that, the suspension was removes large particles by centrifugation and filtration, and the THF solution was removed by rotary evaporation to obtain the crude products. Next, the crude products were redissolved in CH2Cl2, washed with saturated NaHCO3 solution and dried with anhydrous MgSO4, before performing filtering operations. Lastly, the PDMS-Br was obtained after conducting rotary evaporation to remove CH2Cl2.

Subsequently, pre-determined amounts of CuCl were transferred to a Schlenk flask, the air in Schlenk flask was pumped out and replaced by N2. The cyclohexanone solutions of PDMS-Br, TMEDA and GMA (quality of GMA/PDMS-Br = 10:1) then introduced to the above-mentioned Schlenk flask; it was reacted under magnetic stirring at 80 °C for 24 h. After natural cooling to 50 °C, the reaction mixture was conducted dilute by THF, filtering operations to remove Cu2+ and rotary evaporation to remove THF, followed by precipitation of solid products via addition of the aforementioned mixture into methanol. Finally, the PDMS-PGMA copolymer powder was obtained after drying the collected solid products.

Modification of MFRS

In this paper, a solution-immersion process (Camalan and İhsan Arol 2022) was used to functionalize MFRS; we used PMMA, PDMS-PMMA and PDMS-PGMA to modify MFRS, respectively. The procedure for sponge modification is basically similar, and the PDMS-PGMA-modified melamine formaldehyde resin sponge (PDMS-PGMA/MFRS) was fabricated as follows. Firstly, the homogeneous polymer solution was prepared by sufficient disperse 2.5 g PDMS-PGMA powder in 50 mL DMF. Then, the tailored MFRS (length × width × height = 6 cm × 2.5 cm × 2 cm) was completely immersed in the aforementioned polymer solution for 1 h. After that, the excess solution in MFRS is squeezed out and the sponge is placed in an oven to performing fully dry treatment.

Elasticity performance test of MFRS

Firstly, the initial height of the modified and unmodified sponge was measured by a ruler. Then, apply a ceramic sheet to compress the sponge to 1/4 of its original height, and 10 s later, the ceramic sheet was removed slowly. The height was recorded after the sponge was restored to its original shape in 5 min. To assess the elastic properties of sponge, the above operation is repeated for 10 times.

Oil/water separation experiment of MFRS

Oil/water mixture were prepared by introducing 15 mL plant oil to 85 mL DI water and processing 30-min ultrasonic treatment. Then, the modified or unmodified sponges were transferred to a conical flask containing 100 mL oil/water mixture. After that, the conical flask was placed in a running thermostatic shaker for 10 min oil/water separation. Following by remove the sponge and calculate oil/water separation efficiency by measure the volume of water and oil in separated solution. ηWater is the calculated ratio of the volume of water after separation to the initial volume of water, and ηOil is the calculated ratio of the volume of oil that absorbed by sponge to the initial volume of oil; the formula is as follows.

$$\begin{aligned} & \eta_{{{\text{Water}}}} = V_{1} /V_{2} \times 100\% \\ & \eta_{{{\text{Oil}}}} = \, \left( {V_{4} - V_{3} } \right)/V_{4} \times 100\% \\ \end{aligned}$$

where V1 and V3 represent the volume of water and oil in the separated solution, while V2 and V4 refer to the volume of water (85 mL) and oil (15 mL) in the initial oil–water mixture.

Meanwhile, to evaluate the influence of pH on the oil–water separation efficiency of PDMS-PGMA/MFRS, the PDMS-PGMA/MFRS was immersed in alcohol solutions of pH = 1, 4, 10, 13 for 24 h, respectively, before conducting oil–water separation experiments.

Furthermore, to assess the recyclability of PDMS-PGMA/MFRS, the modified sponges squeezed out the adsorbed oil and washed for five times with alcohol after each use. Then, the used PDMS-PGMA/MFRS is dried for reuse.

Characterization

An OCA20 contact angle goniometer (Data Physics, Germany) was used to measure water contact angle (WCA) of accurate 300 µL DI water droplets on modified MFRS exposed to air. The chemical structures of unmodified and modified MFRS were obtained using a Tensor 27 of Bruker Optics in a FT-IR spectrometer for spectral range 400–4000 cm−1. A JEOL 7800F Field Emission Scanning Electron Microscope was used to obtain SEM images of MFRS before modified and modified by PDMS-PGMA. A Bruker AV-500 spectrometer using (Methyl sulfoxide)-d6 or CDCl3 as solvent was used to collect proton nuclear magnetic resonance (1H NMR) data for obtained PDMS-PGMA copolymer. The electronic universal mechanical testing (CITEMA, Shanghai) was used to test the stress–strain curves of unmodified and PDMS-PGMA-modified MFRS as 0.01 mm s−1 squeezing speed.

Results and discussion

The chemical structure of the PDMS-PGMA copolymer was deduced by 1H NMR spectrum, as shown in Fig. 1a. The chemical shift recorded at 0.1 ppm (a) and 1.8 ppm (b) was attributed to the Si–(CH3) and –CH2– of the PDMS block (Liang et al. 2011). Similarly, the peaks observed at 3.2 (d), 2.7 and 2.8 (f), 3.8 and 4.3 (e) ppm indicate the presence of epoxy group, and 0.9 ppm (c) belonging to –CH3 from GMA block, further confirming the existence of GMA repeat units in the copolymer (Hameed et al. 2010). Therefore, the 1H NMR spectrum provided evidence for the successful synthesis of the PDMS-PGMA copolymer. Meanwhile, the 1H NMR spectra of PDMS and PDMS-PMMA were investigated (Fig. S1). Compared to the chemical shift at 3.65 ppm (OH–CH2–CH2–), 0.07 ppm (–Si(CH3)2–O–) and 0.81 ppm (α-CH2 to the Si atom) from PDMS (Fig. S1a) (Subramanyam and Kenedy 2009; Li et al. 2013), some new peaks at 1.74, 1.85 and 3.55 ppm belonging to the protons of methyl, methylene and methoxy in PMMA chain (Li et al. 2013) can be clearly observed (Fig. S1b), which provided direct evidence of successful synthesis of PDMS-PMMA copolymer.

Fig. 1
figure 1

a 1H NMR spectrum of PDMS-PGMA copolymer (Inset—the chemical structure of PDMS-PGMA). b FT-IR spectra of MFRS, PDMS-PGMA copolymer and PDMS-PGMA/MFRS

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Moreover, Fourier transform infrared spectrometer (FT-IR) spectra were used to study functional groups of MFRS, PDMS-PGMA and PDMS-PGMA/MFRS (Fig. 1b). The signals of melamine formaldehyde resin sponge (MFRS) observed at 3424 cm−1 (N–H and O–H), 2959 cm−1 (C–H), 1688 cm−1 (C=N), 809 cm−1 (triazine ring) and 999 cm−1 (–CH2OH) confirmed that MFRS is condensed from hydroxymethyl melamine (Xiong et al. 2020). Similarly, in PDMS-PGMA copolymer, the absorption bands recorded at 800 cm−1 (C–Si–C) and 1091 cm−1 (Si–O–Si) verified the existence of PDMS (Hameed et al. 2010), while the signals at 1727 cm−1 (C=O) and 906 cm−1 (epoxy group) validated the introduction of GMA block in the copolymer (Zhou et al. 2019). Besides, the absorption bands of PDMS-PGMA/MFRS were similar to those of PDMS-PGMA, which was ascribed to the sufficient coating of PDMS-PGMA on MFRS.

Furthermore, the chemical structure of PDMS-PMMA/MFRS was further investigated by FT-IR spectrum in Fig. S2. Apart from the absorption bands at 3429 cm−1 (N–H and O–H), 2964 cm−1 (C–H), 1454 cm−1 (C–N) and 815 cm−1 (triazine ring) from MFRS (Chen et al. 2008), the signals observed at 1737 cm−1 (C=O) and 1016/1099 cm−1 (Si–O–CH3) were derived to the PMMA and PDMS block of PDMS-PMMA, respectively, further confirming the chemical structure of PDMS-PMMA/MFRS.

The major composition of the sponge is melamine formaldehyde resin, which was synthesized by an additive reaction between formaldehyde and melamine, followed by a condensation reaction. Hydroxymethyl melamine was obtained in the additive reaction (Scheme 1a), and the subsequent condensation reaction partially removed water, while adequate amounts of hydroxyl groups were still retained [as confirmed by FT-IR spectra: 3424 cm−1 (O–H), 999 cm−1 (–CH2OH)]. These hydroxyl groups were available for reaction with the epoxy groups, thereby leading to the grafting of the PDMS-PGMA copolymer on MFRS (Scheme 1b), which was further confirmed by the FT-IR spectra of MFRS and PDMS-PGMA/MFRS (Fig. 1). Interestingly, the PGMA block of PDMS-PGMA bonded with MFRS to form a stable coating, and the PDMS block afforded superhydrophobic properties on MFRS.

Scheme 1
scheme 1

a Scheme diagram of additive reaction in synthesis melamine formaldehyde resin. b Schematic diagram of modification of MFRS by PDMS-PGMA copolymer

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In order to investigate the elastic properties of the sponges modified by the polymers, we compared the elastic property data of the unmodified MFRS with those of the modified MFRS (PDMS-PGMA/MFRS), as shown in Fig. 2. The original heights of both the MFRS and PDMS-PGMA/MFRS were 2.00 cm (Fig. 2a, b), and the sponges were squeezed to 1/4 of their original heights (Fig. 2a1 and b1). PDMS-PGMA/MFRS was restored to 97.5% of its original height after fivefold squeezing and 96.5% after tenfold squeezing (Fig. 2c). Meanwhile, stress–strain curves were performed to investigate the elastic property of MFRS and PDMS-PGMA/MFRS (Fig. S3). Compared with MFRS, PDMS-PGMA-modified MFRS demonstrated easier deformation under pressure (less than 50% of strain), suggesting a slightly reduced mechanical property after coating. However, the final stress–strain result of PDMS-PGMA/MFRS displayed same stress with pure MFRS, possible due to the unchanged structure of sponge, which was consistent with the conclusions from elasticity performance test. Therefore, we believed that PDMS-PGMA/MFRS did not suffer a significant decrease in elastic performance in comparison with the pure MFRS, indicating that the modified sponge exhibited an excellent elastic property, hence demonstrating its reusability potential.

Fig. 2
figure 2

Images of a unmodified (MFRS) and b PDMS-PGMA-modified MFRS in elastic properties testing procedure. a1 and b1 Sponge compression process, a2 and b2 images of unmodified (MFRS) and PDMS-PGMA-modified MFRS after 10 times of elastic properties testing. c Elastic properties of unmodified and PDMS-PGMA-modified MFRS

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Scanning electron microscope (SEM) was used to investigate the geometry morphology of MFRS and PDMS-PGMA/MFRS, and the micrographs are displayed in Fig. 3. Both the surface and internal section of MFRS presented a porous structure, with an average pore size of nearly 100 µm (Fig. 3a, b), and the appearance presented a smooth and regular feature (Fig. 3a2, b2). Furthermore, the three-dimensional structure of PDMS-PGMA/MFRS is similar to that of MFRS (Fig. 3c, d), suggesting that the polymer modification did not change the porous structure of MFRS. However, the appearance of the PDMS-PGMA-modified MFRS was rough and wrinkled (Fig. 3c2, d2), which is attributed to the polymer coating on MFRS. Wrinkles were observed on both the surface and internal section of PDMS-PGMA/MFRS, indicating the successful modification of MFRS by PDMS-PGMA. Therefore, the geometric wrinkles on the PDMS-PGMA/MFRS surface and the introduced functional groups may have enhanced the hydrophobic properties of MFRS (Zou et al. 2017; Kieu et al. 2019).

Fig. 3
figure 3

Different resolution SEM images of MFRS: a–a2 surface and b–b2 internal section. Different resolution SEM images of PDMS-PGMA/MFRS: c–c2 surface and d–d2 internal section

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We evaluated the corresponding hydrophobicity and water contact angle (WCA) images to compare the hydrophobic properties of the different modified MFRS (Fig. 4). The water droplets were rapidly absorbed on MFRS (Fig. 4e, e1), indicating that MFRS had a strong hydrophilicity, which was probably related to the presence of the hydroxyl groups on the surface of MFRS. Meanwhile, PMMA-modified MFRS exhibited non-uniform hydrophobic properties (Fig. 4e, e2), implying that while hydrophobicity dominated some areas, hydrophilicity characterized the other areas. The WCA values were 126° on the surface of PMMA/MFRS and 61° in the internal section (Fig. 4a) due to the poor adhesion and uneven dispersion of the PMMA coating. However, PDMS-PMMA-modified MFRS exhibited improved WCA with 139° for the surface and 134° for the internal section (Fig. 4b). The enhanced hydrophobic performance may have been influenced by the introduction of the PDMS block (Moquin et al. 2018). Besides, the PDMS-PGMA-modified MFRS demonstrated superhydrophobic properties with WCA of 155° on the surface (Fig. 4c). Such a result was probably influenced by the presence of the hydrophobic groups of PDMS together with the geometric folds formed by the copolymer on MFRS.

Fig. 4
figure 4

Water contact angle (WCA) images of PMMA/MFRS (a–a1 surface and internal section), PDMS-PMMA/MFRS (b–b1 surface and internal section) and PDMS-PGMA/MFRS c–c1 surface and internal section). d and e Hydrophobic images of MFRS modified by different polymers, and high-resolution hydrophobic images of e1 MFRS, e2 PMMA/MFRS, e3 PDMS-PMMA/MFRS, and e4 PDMS-PGMA/MFRS

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To assess the oil–water separation efficiency of the different polymer-modified MFRS, uniform-sized blocks (length × width × height = 6 cm × 2.5 cm × 2 cm) were loaded in a conical flask with oil–water mixture (Fig. 5a, b). The pure MFRS sank to the bottom of the conical flask upon transfer to the mixture, suggesting excellent hydrophilicity. This was followed by the transfer of the aforementioned conical flask to a water bath shaker which was operated for 10 min. After the shaking process, MFRS, PMMA/MFRS and PDMS-PMMA/MFRS sank to the bottom of the conical bottle, and the oil–water mixture presented a cloudy appearance (Fig. 5c, d). However, the MFRS modified by PDMS-PGMA floated on the liquid surface, indicating that its excellent hydrophobic property was maintained, and the separated oil–water mixture was clear in appearance (Fig. 5d). This may be ascribed to the preoccupation of the pores of the modified MFRS with oil due to the interaction of the capillary and Van Der Waals forces, while water was repelled (Li et al. 2018).

Fig. 5
figure 5

a Images of oil–water mixture before separation and b the loading of different polymer-modified MFRS into oil–water mixtures. c Images of different polymer-modified MFRS in oil–water mixture after separation. d Images of oil–water mixture and sponges after separation

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The separation efficiency of the MFRS modified with different polymers was denoted by ηOil and ηWater, where the level of ηOil and ηWater reflect the lipophilicity and hydrophobicity of MFRS, respectively. The separation efficiency represented by ηWater (70.6%) and ηOil (40.0%) exhibited a significant gap for pure MFRS (Fig. 6a), which is attributed to its non-selective absorption of oil–water mixtures. In contrast, the values of ηWater and ηOil were quite similar for PMMA/MFRS, PDMS-PMMA/MFRS and PDMS-PGMA/MFRS, indicating that the lipophilicity and hydrophobicity of the materials are related (Liao et al. 2021). Compared to PMMA/MFRS, PDMS-PMMA/MFRS demonstrated a higher oil–water separation efficiency, which is ascribed to the enhanced hydrophobic performance by the introduction of PDMS block. Moreover, PDMS-PGMA-modified MFRS demonstrated desirable oil–water separation efficiency, with ηWater of 98.8% and ηOil of 99.3%. These reflect favorable hydrophobic properties, which may have been influenced by both the introduction of hydrophobic PDMS block and the tight bonding of PGMA block with MFRS.

Fig. 6
figure 6

a Oil–water mixture separation efficiency of MFRS modified with different polymers. b Recyclability of PDMS-PGMA/MFRS. c Actual and d hydrophobic images of PDMS-PGMA/MFRS d1 before use, d2 after 1 run and d3 8 runs. e Acid/alkaline resistance capability of PDMS-PGMA/MFRS. f Separation efficiency of PDMS-PGMA/MFRS for different oil–water mixtures

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Reusability is an important parameter for the performance evaluation of oil–water separation. The separation efficiency (ηWater) of PDMS-PGMA/MFRS after 4 runs was 96.5%, and 91.5% after 8 runs (Fig. 6b). The volume of PDMS-PGMA/MFRS was significantly reduced after 8 runs (Fig. 6c), whereas its hydrophobicity was still retained (Fig. 6d3). Furthermore, acid and alkali resistance are an important factor in evaluating hydrophobic materials. The PDMS-PGMA/MFRS material was immersed for 24 h in alcohol solutions with different pH, followed by drying prior to oil–water separation experiments. The oil–water separation efficiency (ηWater) displayed a decreasing trend with increasing or decreasing pH (Fig. 6e). The PDMS-PGMA/MFRS exhibited an oil–water separation efficiency of 98.7% after immersion in an unadjusted alcohol solution, and the value was 94.7% for the solution at pH = 1. PDMS-PGMA/MFRS was more unstable in strong bases compared with strong acids, which is reflected by the 72.9% separation efficiency after immersion in an alcohol solution of pH = 13. Therefore, PDMS-PGMA/MFRS exhibited strong acid resistance and weak alkali resistance, which is attributed to the destruction of the hydrophobic functional groups and chemical bonds between the copolymers and MFRS by strong base solutions (Cui and Liu 2021). Besides, PDMS-PGMA/MFRS achieved efficient separation of different types of oil–water mixtures. The separation efficiencies (ηWater) were 97.1%, 99.4%, 98.9% and 98.2% for diesel-water, silicone oil–water, octadecene–water and plant oil–water mixture, respectively (Fig. 6f). Therefore, based on the above-mentioned discussion, the recyclability, acid resistance and effective separation for different oil–water mixtures of PDMS-PGMA/MFRS demonstrate the potential of the material for practical applications.

FT-IR was used to investigate the functional group variations of PDMS-PGMA/MFRS before and after oil–water separation (Fig. 7a). Peak signal enhancement occurred around 2800–3000 cm−1 (C–H) after oil–water separation measurements. Also, a new peak appeared at 720 cm−1 (–(CH2)n–, n > 4) after 1 run and 8 runs (Guo and Bustin 1998; Huang and Wang 2009). The enhanced signal and new peak appearance are confirmations of the adsorption of long-chain alkanes on the surface of PDMS-PGMA/MFRS. Moreover, we observed that some absorption bands of PDMS-PGMA/MFRS disappeared or were diminished after 1 run and 8 runs of separation, such as 1260 cm−1 (C–O), 1080 cm−1 (Si–O–Si) and 802 cm−1 (C–Si–C) all belonging to the signals of the PDMS block (Zhou et al. 2019). These disappeared or diminished signals should have been masked by the adsorbed alkanes, further suggesting that the PDMS block plays hydrophobic and lipophilic functions in oil–water separation. The WCA of PDMS-PGMA/MFRS slightly decreased on both the surface (151°) and internal section (136°) after 1 run of oil–water separation (Fig. 7b, b1). Similarly, the WCA values were recorded as 135° on the surface and 130° on the internal section after 8 runs of separation (Fig. 7c, c1). Meanwhile, the SEM images of PDMS-PGMA/MFRS after 8 runs of separation exhibited some breakages in sponge connectivity (Fig. 7d, d1), possibly due to the physical overuse and damage after squeezing during oil absorption and discharge processes, which may be the direct reason for the decrease in sponge volume. However, traces of the copolymer were still clearly observed on the surface of MFRS (Fig. 7d2) and are the main contributor to the hydrophobicity of MFRS. Therefore, the decreased oil–water separation efficiency in the recycling experiment should attributed to the combined effect of the declined volume of MFRS and the diminished hydrophobicity of the modified copolymer.

Fig. 7
figure 7

a FT-IR spectra of PDMS-PGMA/MFRS sample before and after oil–water separation. WCA images of PDMS-PGMA/MFRS after b 1 run and c 8 runs of oil–water separation. d SEM images of PDMS-PGMA/MFRS after 8 runs of oil–water separation. e Schematic diagram for reduced sponge volume in recycling experiments

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Conclusion

In summary, we report the synthesis of PDMS-PGMA copolymer, followed by the fabrication of PMMA, PDMS-PMMA and PDMS-PGMA-modified MFRS via a facile solution-immersion process. PDMS-PGMA/MFRS demonstrated superhydrophobicity with WCA values of 155° and 147° on its surface and internal section, respectively. The superhydrophobic property of PDMS-PGMA/MFRS was attributed to both the introduction of hydrophobic PDMS fragments and the geometric fold structures induced by the copolymer coating. Benefiting from the hydrophobicity, PDMS-PGMA/MFRS exhibited efficient oil-absorbing ability in oil–water mixture, which was reflected by the ηOil value of 99.3%. Moreover, PDMS-PGMA-modified MFRS demonstrated favorable recyclability, with a separation efficiency of 91.5% after 8 runs, and this is attributed to the strong chemical bonds between MFRS and the epoxy group of GMA in the copolymer. Besides, PDMS-PGMA/MFRS displayed good acid resistance and was efficient in the separation of various oil–water mixtures. Thus, these promising results reflect the highly efficient, recyclable, acid-resistant and widely adaptable features of the fabricated PDMS-PGMA/MFRS, which shows possibilities for large-scale oil spill removal.