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Surface hydrophilic-hydrophobic reversal coatings of polydimethylsiloxane-palygorskite nanosponges
Applied Clay Science 189 (2020) 105546
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Surface hydrophilic-hydrophobic reversal coatings of polydimethylsiloxanepalygorskite nanosponges Mingliang Pei, Changou Pan, Dan Wu, Peng Liu
T
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State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanosponges Palygorskite Hydrophilic-hydrophobic reversal Polydimethylsiloxane Coatings
Fibrous clay mineral could be used to fabricate hydrophobic and superhydrophobic coatings, owing to the easily designed surface micro-nanoscale roughness and porous surfaces. Here, polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponge-based coatings were fabricated via the UV-induced thiol−ene interfacial click reaction of the thiol modified palygorskite nanorods (Pal-SH) with polydimethylsiloxane diacrylate. The surface hydrophilic-hydrophobic reversal function of the PDMS-Pal nanosponges-based coatings was reported for the first time. The surface hydrophilic-hydrophobic reversal mechanism was proposed as the conformational transformation of the PDMS chains during treating with different solvents, revealed by SEM and XPS analysis. Such understanding is expected to design functional fibrous clay-based nanosponges for various applications.
1. Introduction As a kind of plentiful natural environment-friendly nanomaterials, clays have been widely used in various fields, such as polymer nanocomposites (Bitinis et al., 2011; Kotal and Bhowmick, 2015; Zhu et al., 2019), adsorbents for heavy metal ions and organic pollutants (Awad et al., 2019; Liu and Zhang, 2007; Zhu et al., 2016), biomaterials (Gaharwar et al., 2019; Mousa et al., 2018; Rodrigues et al., 2013), catalysts (Glotov et al., 2019; Massaro et al., 2017), Pickering emulsifiers (Machado et al., 2019), packaging materials (Bumbudsanpharoke and Ko, 2019), phase change materials (Lv et al., 2017), energy based materials (Ummartyotin et al., 2016), etc. The surface modification could improve efficiently their surface physical and chemical properties for potential applications (Liu, 2007). The surface hydrophilic-hydrophobic property of the nanoclays could be modulated by intercalation with hydrophobic or hydrophilic polymers (Sangian et al., 2018). Owing to the hydrophobic property, the clay-based hybrids have been designed as insulation material in the construction industry (Madyan et al., 2017), adsorbent for organic contaminants (Wang et al., 2015) and oil recovery (Hsu et al., 2010), Pickering emulsifier for inverse emulsion polymerization (Voorn et al., 2006), and clay-based nanocomposites with fine dispersion (Chiu et al., 2007). Almost all these works were focused on the layered clays, such as montmorillonite and kaolinite. Recently, the fibrous clay mineral, namely palygorskite or attapulgite with ideal formula of Si8Mg5O20(OH)2(H2O)4·4H2O), has also
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been used to fabricate hydrophobic and superhydrophobic coatings, by introducing low surface energy materials such as silico- and fluorinecontaining compounds (Li et al., 2013; Zhang et al., 2018). Compared with the planar layered clays, the fibrous clay easily produce surface micro-nanoscaled roughness (Yu et al., 2018) and porous surfaces (Liang et al., 2014), enhancing the hydrophobic and superhydrophobic nature. In our previous work, it was found that the crosslinked gels could be produced by the thiol−ene interfacial click reaction between the thiol modified palygorskite nanorods (Pal-SH) with nitrile butadiene rubber (Pan and Liu, 2018). The porous palygorskite-based hybrid is expected to provide surface micro-nanoscale roughness. Such feature could be used to fabricate the hydrophobic and superhydrophobic coatings. Based on this hypothesis, the polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponges were designed via the UV-induced thiol−ene interfacial click reaction of Pal-SH nanorods with polydimethylsiloxane diacrylate (Scheme 1), as hydrophobic coatings. Owing to the hydrophilic Pal nanorods and the hydrophobic PDMS linkages, the PDMS-Pal nanosponges-based coatings showed a surface hydrophilic-hydrophobic reversal property by treating with solvents with different polarities. 2. Experimental section 2.1. Materials and reagents Palygorskite (Pal) nanorods (Jiangsu Goldstone Attapulgite R&D
Corresponding author. E-mail address: [email protected] (P. Liu).
https://doi.org/10.1016/j.clay.2020.105546 Received 15 November 2019; Received in revised form 26 February 2020; Accepted 3 March 2020 Available online 13 March 2020 0169-1317/ © 2020 Elsevier B.V. All rights reserved.
Applied Clay Science 189 (2020) 105546
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Scheme 1. Schematic illustration of the fabrication of the PDMS-Pal nanosponges.
2.4. Analysis and characterization
Center Co. Ltd., Xuyi, China) were dried at 120 °C for 24 h before use. 3Mercaptopropyltrimethoxysilane (MPTMS, 97%) was purchased from J &K Scientific Co. Ltd., Beijing, China. China. Silicone modifier (TECH2210, Mn = 1000 Da) was obtained Tiger Polymer Technology (Shanghai) Co., Ltd. All other reagents were analytical grade and used as received.
Fourier transform infrared spectroscopy (FT-IR) was performed on a Tensor 27 spectrometer (Germany) at a frequency range of 4000–400 cm−1 with an increment of 2 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed using a VG Scientific ESCALAB 250Xi-XPS photoelectron spectrometer (ThermoFisher Scientific, USA) with an Al Kα X-ray resource. The binding energies were calibrated by the C1s binding energy at 284.7 eV. The morphological analysis of the PDMS-Pal nanosponges was carried out on JEM-1200 EX transmission electron microscope (TEM, JEOL Ltd., Japan) and field emission scanning electron microscope (FESEM, SU8200 Hitachi). Static contact angle analysis was carried out on a Tantec CAM-Micro Contact Angle Meter equipped with microinjectors and flat needles, with 2.0 μL of water and diiodomethane (DIM), (n = 5, RSD < 3%).
2.2. Preparation of PDMS-Pal nanosponges The thiol modified palygorskite (Pal-SH) nanorods with a surface thiol content of 1.31 mmol/g were prepared via the self-assembly of MPTMS onto the surfaces of the Pal nanorods (Liu et al., 2018), as following: 2.00 g Pal nanorods and 2.00 mL of MPTMS were dispersed into 40 mL absolute ethanol with ultrasonication for 2 h, and then the mixture was refluxed for 8 h. The product was separated by centrifugation (10,000 rpm for 8 min), washed thoroughly with ethanol, and dried at 40 °C. 100 mg of Pal-SH nanorods and TECH-2210 were dispersed into 3.0 mL methanol with ultrasonication (Table 1). Then 0.50 mL of the dispersion was coated onto a glass wafer (26 × 76 mm) by using a QTS bar applicator (Shanghai Longtuo Instrument Equipment co., LTD, Shanghai, China). The coated glass wafer was irradiated immediately under a high-pressure mercury lamps (GY1224) (365 nm, 500 W, at a distance of 15 cm) with an energy density of 6.0 mW/cm2 at room temperature for 10 min. The PDMS-Pal nanosponges were scraped from the as-prepared coatings for chemical structure and morphological analysis.
3. Results and discussion 3.1. Preparation and characterization of PDMS-Pal nanosponges The PDMS-Pal nanosponges were designed via the UV-induced thiol−ene interfacial click reaction of Pal-SH nanorods with polydimethylsiloxane diacrylate (Scheme 1) with different feeding amounts of PDMS (0.05, 0.10, or 0.15 mL) (Table 1). After the self-assembly of MPTMS, weak CeH stretching vibrations below 3000 cm−1 and inplane bending vibration at 1370 cm−1 appeared in the FT-IR spectrum of the product (Fig. 1), indicating the successful silylation of Pal nanorods. In the FT-IR spectrum of the PDMS-Pal nanosponges (Fig. 1), besides the absorbance peaks of Pal nanorods, the strong characteristic absorbances of polydimethylsiloxane diacrylate appeared, CeH stretching from CH3 groups at 2962 cm−1, C]O stretching vibration from ester group at 1730 cm−1, CH3 symmetric deformation of SiCH3 bond at 1262 cm−1, SieC stretching at 801 cm−1, asymmetric stretching vibrations of –CH3 group at 1411 cm−1, indicating the successful thiol−ene interfacial click reaction (Liao et al., 2018). Furthermore, the C]C bond in vinyl groups were remained at 1534 m−1, demonstrating that some polydimethylsiloxane diacrylate molecules had been introduced with only one terminated vinyl group. The morphology of the PDMS-Pal nanosponges prepared with different feeding amounts of PDMS (0.05, 0.10, or 0.15 mL) was compared
2.3. Surface hydrophilic-hydrophobic reversal tests The as-prepared PDMS-Pal nanosponge-based coatings were immersed into water and CH2Cl2 for 10 min and dried at 60 °C for 20 min, respectively. Table 1 Preparation conditions for the PDMS-Pal nanosponges. Samples
Pal-SH (mg)
CECH-2210 (mL)
Methanol (mL)
PDMS-Pal-1 PDMS-Pal-2 PDMS-Pal-3
100 100 100
0.05 0.10 0.15
3.0 3.0 3.0
2
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M. Pei, et al.
Fig. 3. XPS survey spectra of the pristine Pal, Pal-SH and PDMS-Pal nanosponges. Fig. 1. FT-IR spectra of the pristine Pal, Pal-SH and PDMS-Pal-2 nanosponges. Table 2 Surface atomic concentrations of the pristine Pal, Pal-SH and PDMS-Pal-2 nanosponges after treating with water and CH2Cl2.
with TEM technique. With less polydimethylsiloxane diacrylate, 0.05 mL/100 mg Pal-SH, the product, PDMS-Pal-1 nanosponges, was composed of few Pal nanorods (Fig. 2a). While loose PDMS-Pal-2 nanosponges were obtained with 0.10 mL polydimethylsiloxane diacrylate per 100 mg Pal-SH (Fig. 2b). Further increasing the feeding amount of polydimethylsiloxane diacrylate to 0.15 mL per 100 mg Pal-SH, the compact PDMS-Pal-3 nanosponges were formed (Fig. 2c), due to a higher crosslinking degree. So the loose PDMS-Pal-2 nanosponges are expected for hydrophobic coating application, because of their excellent porous structure. XPS technique was also used to reveal the successful fabrication of the PDMS-Pal-2 nanosponges (Fig. 3). After the surface modification of the Pal nanorods with MPTMS, the surface atomic concentrations of the main elements (O, Fe, Mg and Al) in Pal mineral decreased, except that Si and C elements increased (Table 2). Furthermore, S element, which could not be detected in the pristine Pal nanorods, appeared with a surface atomic concentration of 1.43%. The results demonstrated the successful surface modification of Pal nanorods with MPTMS. After the UV-induced thiol−ene interfacial click reaction, the surface atomic concentration of the main elements (Fe, Mg, Al and S) of the Pal-SH nanorods decreased, while those of Si, O and C increased distinctly, indicating the successful thiol−ene interfacial click reaction of the PalSH nanorods with polydimethylsiloxane diacrylate, as shown in Scheme 1.
Samples
Pal Pal-SH PDMS-Pal-2 (H2O) PDMS-Pal-2 (CH2Cl2)
Surface atomic concentration (%) O
Si
Fe
Mg
Al
C
N
S
43.6 40.3 42.7 21.5
18.4 25.5 21.9 15.4
11.3 6.3 0.2 0.1
9.3 3.5 – –
7. 8 6.4 3.0 0
6.3 14.1 29.5 61.7
3.4 2.4 1.7 0
0 1.4 1.0 1.3
3.2. Hydrophilic-hydrophobic property of the as-prepared coatings The static water contact angle (SWCA) of the as-prepared PDMS-Pal nanosponge-based coatings were measured and plotted in Fig. 4. For the PDMS-Pal-1 nanosponges with a low PDMS amount, a hydrophilic coating was formed with an initial SWCA of 77.9°. And the value decreased over time. The water droplet infiltrated into the coating within 2.5 min. As for the loose PDMS-Pal-2 nanosponges, the as-prepared coating showed hydrophobic property with an initial SWCA of 92.0° and the water droplet infiltrated into the coating within 4 min. The asprepared PDMS-Pal-3 nanosponge coating showed an initial SWCA of 86.6° and the water droplet infiltrated into the coating within 9 min. The PDMS-Pal-2 nanosponge coating showed the best hydrophobic property due to their loose porous structure, while the PDMS-Pal-3 nanosponge coating possessed the longest water droplet retention period because of their compact structure, as shown in the TEM images
Fig. 2. TEM images of PDMS-Pal-1 (a), PDMS-Pal-2 (b), and PDMS-Pal-3 (c) nanosponges. 3
Applied Clay Science 189 (2020) 105546
M. Pei, et al.
100
160 a
140
As-prepared Treated with H2O
Treated with CH2Cl2
120
Treated with CH2Cl2
100
CA (o)
CA (o)
80
b
As-prepared Treated with H2O
60 40
80 60 40
20
20 0
0 0.0
0.5
1.0 1.5 Time (min)
2.0
140
0
2.5
c
1
As-prepared Treated with H2O
100
Treated with CH2Cl2
CA (o)
120
2 Time (min)
3
4
80 60 40 20 0 0
1
2
3
4 5 6 Time (min)
7
8
9
10
Fig. 4. Change of the static water contact angle (SWCA) over time of the as-prepared coatings, after treated with water and CH2Cl2 of the PDMS-Pal-1 (a), PDMS-Pal2 (b), and PDMS-Pal-3 (c) coatings.
Still), suggesting that the coatings can be applied as self-cleaning surfaces in outdoor environments (Torun et al., 2019). The compact PDMSPal-3 nanosponge coating changed from hydrophilic to hydrophobic surface with an initial SWCA of 120.1°, although the infiltration time of the water droplet was only 1 min. Such results indicated the surface hydrophilic-hydrophobic reversal property of the proposed PDMS-Pal nanosponge coatings. The similar tendency was found in the static diiodomethane contact angle (SDCA) measurements after treating with different solvents. After treating with CH2Cl2, the SDCA values of the coatings with bigger PDMS-Pal nanosponges increased obviously. Especially for the loose PDMS-Pal-2 nanosponge coating, it became oleophobic with an initial SDCA of 101.1° and the SDCA value declined to about 60° in 2.5 min. Even though, the SDCA values were still higher than those of the compact PDMS-Pal-3 nanosponge coating (40–50°). The results also indicated the structural transformation of the bigger PDMS-Pal nanosponges during treating with CH2Cl2. The PDMS-Pal-1 nanosponge coating did not show obvious surface hydrophilic-hydrophobic reversal property, it should be due to the small size and less PDMS content. As for the PDMS-Pal nanosponge coatings with bigger PDMS-Pal nanosponges, obvious surface hydrophilic-hydrophobic reversal occurred after treating with CH2Cl2, but not with water. The structural transformation during treating with CH2Cl2 was more distinct for the loose PDMS-Pal-2 nanosponge coating, because of their loose structure with relatively lower crosslinking degree. For all three PDMS-Pal nanosponge coatings, the surface property did not change obviously after treating with water. The reason should be the similar strong polarity of methanol and water, which was used for the fabrication of the nanosponge coatings and post-treating, respectively. To reveal the structural transformation during treating with CH2Cl2,
(Fig. 2). The oleophilic-oleophobic property of the as-prepared PDMS-Pal nanosponge-based coatings were also evaluated with diiodomethane (DIM). As shown in Fig. 5, the as-prepared PDMS-Pal-1 nanosponge coating gave an initial static diiodomethane contact angle (SDCA) of 23.6°, while the other two around 30°. The as-prepared PDMS-Pal-1 nanosponge coating showed the best oleophilic property, similar as the hydrophilic-hydrophobic tests (Fig. 4). It might be due to the smooth coating surface with low roughness resulted from the small nanosponges with few Pal nanorods. 3.3. Surface hydrophilic-hydrophobic reversal property After the as-prepared PDMS-Pal nanosponge-based coatings were immersed in water for 10 min and dried at 60 °C for 20 min, the SWCA values were near to those before treating with water (Fig. 4). However, the SWCA values changed distinctly after treating with CH2Cl2 in which PDMS could be dissolved, maybe due to the structural transformation. For the small PDMS-Pal-1 nanosponges with less PDMS, the infiltration time of the water droplet into the coating was shortened to 1 min, although the initial SWCA was remained similar as before treating with water. While the initial SWCA values of the bigger nanosponges (PDMSPal-2 and PDMS-Pal-3) increased obviously, demonstrating that the PDMS content and structure showed important effects on the surface hydrophilic-hydrophobic property of the proposed PDMS-Pal nanosponge coatings. After treating with CH2Cl2, the loose PDMS-Pal-2 nanosponge coating could remain hydrophobic surface for 1.5 min, with an initial SWCA of 143.4°, showing superhydrophobicity. When a water droplet was dropped to the surface of the treated PDMS-Pal-2 nanosponge coating, it completely bounced off the coating (Supplementary Video 4
Applied Clay Science 189 (2020) 105546
M. Pei, et al.
50
120
a
45
As-prepared Treated with H2O
40
Treated with CH2Cl2
CA (o)
CA (o)
30
60 45
20
30 0.2
0.4 0.6 Time (min)
0.8
60 55
Treated with CH2Cl2
75
25
0.0
As-prepared Treated with H2O
90
35
15
b
105
15
1.0
c
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Time (min)
As-prepared Treated with H2O
50
Treated with CH2Cl2
o CA ( )
45 40 35 30 25 20 0.0
0.5
1.0 1.5 Time (min)
2.0
2.5
Fig. 5. Change of the static diiodomethane contact angle (SDCA) over time of the as-prepared coatings, after treated with water and CH2Cl2 of the PDMS-Pal-1 (a), PDMS-Pal-2 (b), and PDMS-Pal-3 (c) coatings.
Fig. 6. SEM images of the PDMS-Pal-2 coatings after treating with water (a) and CH2Cl2 (b).
originated from the Pal nanorods. While after treating with CH2Cl2, a significantly higher surface C atomic concentration was found, meaning the surface of Pal nanorods had been covered with a PDMS skin. The XPS results revealed the hydrophilic Pal nanorods-riched surface and hydrophobic PDMS-rich surface after treating with strong polar solvent water and weak polar solvent CH2Cl2, respectively. So the surface hydrophilic-hydrophobic reversal mechanism of the PDMS-Pal-2 nanosponge coating could be explained with the conformational change of the PDMS linkers and brushes, as shown in Scheme 2.
the surface morphology and surface atomic concentrations of the PDMS-Pal-2 nanosponge coating were analyzed with SEM and XPS techniques. The Pal nanorods could be seen clearly from the PDMS-Pal2 nanosponge coating after treating with water (Fig. 6a), however, the nanorods were overshadowed after treating with CH2Cl2 (Fig. 6b). The results indicated the structural transformation of the PDMS-Pal-2 nanosponge coating after treating with solvents with different polarities. That is to say, due to the shrinkage of the PDMS linkers and brushes in the strong polar solvents such as methanol and water, a Pal nanorodsriched surface was resulted, showing a hydrophilic surface. While after treating with low polar solvent CH2Cl2, in which the conformation of the PDMS linkers and brushes could change easily from shrinking to spreading chain (Zhang et al., 2012), so a PDMS-rich surface was resulted, showing a hydrophobic surface. The XPS technique was used to reveal the surface hydrophilic-hydrophobic reversal mechanism shown in Scheme 2. Clearly, the PDMSPal-2 nanosponge coating after treating with water possessed higher surface atomic concentrations of O, Si, Fe, Al elements (Table 2),
4. Conclusions In summary, facile strategy was developed to fabricate of the polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponge-based coatings, via the UV-induced thiol−ene interfacial click reaction of the thiol modified palygorskite nanorods (Pal-SH) with polydimethylsiloxane diacrylate. Because of the hydrophilic/hydrophobic property of the resultant PDMS-Pal nanosponges-based coatings 5
Applied Clay Science 189 (2020) 105546
M. Pei, et al.
Scheme 2. Schematic illustration of the surface hydrophilic-hydrophobic reversal mechanism of the PDMS-Pal nanosponges.
vanishing over time with contact with different solvents, the hydrophilic or weak hydrophobic PDMS-Pal nanosponge-based coatings could transform into hydrophobic coatings after treating with CH2Cl2, and even superhydrophobic surface for potential self-cleaning applications. Based on the SEM and XPS analysis of the PDMS-Pal nanospongebased coatings after treating with strong polar water and weak polar CH2Cl2, the surface hydrophilic-hydrophobic reversal mechanism was proposed as the conformational transformation of the PDMS chains. The unique surface hydrophilic-hydrophobic reversal function is expected to explore the potential applications of the PDMS-Pal nanosponges as functional adsorbents or coatings. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2020.105546.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Surface hydrophilic-hydrophobic reversal coatings of polydimethylsiloxanepalygorskite nanosponges Mingliang Pei, Changou Pan, Dan Wu, Peng Liu
T
⁎
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanosponges Palygorskite Hydrophilic-hydrophobic reversal Polydimethylsiloxane Coatings
Fibrous clay mineral could be used to fabricate hydrophobic and superhydrophobic coatings, owing to the easily designed surface micro-nanoscale roughness and porous surfaces. Here, polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponge-based coatings were fabricated via the UV-induced thiol−ene interfacial click reaction of the thiol modified palygorskite nanorods (Pal-SH) with polydimethylsiloxane diacrylate. The surface hydrophilic-hydrophobic reversal function of the PDMS-Pal nanosponges-based coatings was reported for the first time. The surface hydrophilic-hydrophobic reversal mechanism was proposed as the conformational transformation of the PDMS chains during treating with different solvents, revealed by SEM and XPS analysis. Such understanding is expected to design functional fibrous clay-based nanosponges for various applications.
1. Introduction As a kind of plentiful natural environment-friendly nanomaterials, clays have been widely used in various fields, such as polymer nanocomposites (Bitinis et al., 2011; Kotal and Bhowmick, 2015; Zhu et al., 2019), adsorbents for heavy metal ions and organic pollutants (Awad et al., 2019; Liu and Zhang, 2007; Zhu et al., 2016), biomaterials (Gaharwar et al., 2019; Mousa et al., 2018; Rodrigues et al., 2013), catalysts (Glotov et al., 2019; Massaro et al., 2017), Pickering emulsifiers (Machado et al., 2019), packaging materials (Bumbudsanpharoke and Ko, 2019), phase change materials (Lv et al., 2017), energy based materials (Ummartyotin et al., 2016), etc. The surface modification could improve efficiently their surface physical and chemical properties for potential applications (Liu, 2007). The surface hydrophilic-hydrophobic property of the nanoclays could be modulated by intercalation with hydrophobic or hydrophilic polymers (Sangian et al., 2018). Owing to the hydrophobic property, the clay-based hybrids have been designed as insulation material in the construction industry (Madyan et al., 2017), adsorbent for organic contaminants (Wang et al., 2015) and oil recovery (Hsu et al., 2010), Pickering emulsifier for inverse emulsion polymerization (Voorn et al., 2006), and clay-based nanocomposites with fine dispersion (Chiu et al., 2007). Almost all these works were focused on the layered clays, such as montmorillonite and kaolinite. Recently, the fibrous clay mineral, namely palygorskite or attapulgite with ideal formula of Si8Mg5O20(OH)2(H2O)4·4H2O), has also
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been used to fabricate hydrophobic and superhydrophobic coatings, by introducing low surface energy materials such as silico- and fluorinecontaining compounds (Li et al., 2013; Zhang et al., 2018). Compared with the planar layered clays, the fibrous clay easily produce surface micro-nanoscaled roughness (Yu et al., 2018) and porous surfaces (Liang et al., 2014), enhancing the hydrophobic and superhydrophobic nature. In our previous work, it was found that the crosslinked gels could be produced by the thiol−ene interfacial click reaction between the thiol modified palygorskite nanorods (Pal-SH) with nitrile butadiene rubber (Pan and Liu, 2018). The porous palygorskite-based hybrid is expected to provide surface micro-nanoscale roughness. Such feature could be used to fabricate the hydrophobic and superhydrophobic coatings. Based on this hypothesis, the polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponges were designed via the UV-induced thiol−ene interfacial click reaction of Pal-SH nanorods with polydimethylsiloxane diacrylate (Scheme 1), as hydrophobic coatings. Owing to the hydrophilic Pal nanorods and the hydrophobic PDMS linkages, the PDMS-Pal nanosponges-based coatings showed a surface hydrophilic-hydrophobic reversal property by treating with solvents with different polarities. 2. Experimental section 2.1. Materials and reagents Palygorskite (Pal) nanorods (Jiangsu Goldstone Attapulgite R&D
Corresponding author. E-mail address: [email protected] (P. Liu).
https://doi.org/10.1016/j.clay.2020.105546 Received 15 November 2019; Received in revised form 26 February 2020; Accepted 3 March 2020 Available online 13 March 2020 0169-1317/ © 2020 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration of the fabrication of the PDMS-Pal nanosponges.
2.4. Analysis and characterization
Center Co. Ltd., Xuyi, China) were dried at 120 °C for 24 h before use. 3Mercaptopropyltrimethoxysilane (MPTMS, 97%) was purchased from J &K Scientific Co. Ltd., Beijing, China. China. Silicone modifier (TECH2210, Mn = 1000 Da) was obtained Tiger Polymer Technology (Shanghai) Co., Ltd. All other reagents were analytical grade and used as received.
Fourier transform infrared spectroscopy (FT-IR) was performed on a Tensor 27 spectrometer (Germany) at a frequency range of 4000–400 cm−1 with an increment of 2 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed using a VG Scientific ESCALAB 250Xi-XPS photoelectron spectrometer (ThermoFisher Scientific, USA) with an Al Kα X-ray resource. The binding energies were calibrated by the C1s binding energy at 284.7 eV. The morphological analysis of the PDMS-Pal nanosponges was carried out on JEM-1200 EX transmission electron microscope (TEM, JEOL Ltd., Japan) and field emission scanning electron microscope (FESEM, SU8200 Hitachi). Static contact angle analysis was carried out on a Tantec CAM-Micro Contact Angle Meter equipped with microinjectors and flat needles, with 2.0 μL of water and diiodomethane (DIM), (n = 5, RSD < 3%).
2.2. Preparation of PDMS-Pal nanosponges The thiol modified palygorskite (Pal-SH) nanorods with a surface thiol content of 1.31 mmol/g were prepared via the self-assembly of MPTMS onto the surfaces of the Pal nanorods (Liu et al., 2018), as following: 2.00 g Pal nanorods and 2.00 mL of MPTMS were dispersed into 40 mL absolute ethanol with ultrasonication for 2 h, and then the mixture was refluxed for 8 h. The product was separated by centrifugation (10,000 rpm for 8 min), washed thoroughly with ethanol, and dried at 40 °C. 100 mg of Pal-SH nanorods and TECH-2210 were dispersed into 3.0 mL methanol with ultrasonication (Table 1). Then 0.50 mL of the dispersion was coated onto a glass wafer (26 × 76 mm) by using a QTS bar applicator (Shanghai Longtuo Instrument Equipment co., LTD, Shanghai, China). The coated glass wafer was irradiated immediately under a high-pressure mercury lamps (GY1224) (365 nm, 500 W, at a distance of 15 cm) with an energy density of 6.0 mW/cm2 at room temperature for 10 min. The PDMS-Pal nanosponges were scraped from the as-prepared coatings for chemical structure and morphological analysis.
3. Results and discussion 3.1. Preparation and characterization of PDMS-Pal nanosponges The PDMS-Pal nanosponges were designed via the UV-induced thiol−ene interfacial click reaction of Pal-SH nanorods with polydimethylsiloxane diacrylate (Scheme 1) with different feeding amounts of PDMS (0.05, 0.10, or 0.15 mL) (Table 1). After the self-assembly of MPTMS, weak CeH stretching vibrations below 3000 cm−1 and inplane bending vibration at 1370 cm−1 appeared in the FT-IR spectrum of the product (Fig. 1), indicating the successful silylation of Pal nanorods. In the FT-IR spectrum of the PDMS-Pal nanosponges (Fig. 1), besides the absorbance peaks of Pal nanorods, the strong characteristic absorbances of polydimethylsiloxane diacrylate appeared, CeH stretching from CH3 groups at 2962 cm−1, C]O stretching vibration from ester group at 1730 cm−1, CH3 symmetric deformation of SiCH3 bond at 1262 cm−1, SieC stretching at 801 cm−1, asymmetric stretching vibrations of –CH3 group at 1411 cm−1, indicating the successful thiol−ene interfacial click reaction (Liao et al., 2018). Furthermore, the C]C bond in vinyl groups were remained at 1534 m−1, demonstrating that some polydimethylsiloxane diacrylate molecules had been introduced with only one terminated vinyl group. The morphology of the PDMS-Pal nanosponges prepared with different feeding amounts of PDMS (0.05, 0.10, or 0.15 mL) was compared
2.3. Surface hydrophilic-hydrophobic reversal tests The as-prepared PDMS-Pal nanosponge-based coatings were immersed into water and CH2Cl2 for 10 min and dried at 60 °C for 20 min, respectively. Table 1 Preparation conditions for the PDMS-Pal nanosponges. Samples
Pal-SH (mg)
CECH-2210 (mL)
Methanol (mL)
PDMS-Pal-1 PDMS-Pal-2 PDMS-Pal-3
100 100 100
0.05 0.10 0.15
3.0 3.0 3.0
2
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Fig. 3. XPS survey spectra of the pristine Pal, Pal-SH and PDMS-Pal nanosponges. Fig. 1. FT-IR spectra of the pristine Pal, Pal-SH and PDMS-Pal-2 nanosponges. Table 2 Surface atomic concentrations of the pristine Pal, Pal-SH and PDMS-Pal-2 nanosponges after treating with water and CH2Cl2.
with TEM technique. With less polydimethylsiloxane diacrylate, 0.05 mL/100 mg Pal-SH, the product, PDMS-Pal-1 nanosponges, was composed of few Pal nanorods (Fig. 2a). While loose PDMS-Pal-2 nanosponges were obtained with 0.10 mL polydimethylsiloxane diacrylate per 100 mg Pal-SH (Fig. 2b). Further increasing the feeding amount of polydimethylsiloxane diacrylate to 0.15 mL per 100 mg Pal-SH, the compact PDMS-Pal-3 nanosponges were formed (Fig. 2c), due to a higher crosslinking degree. So the loose PDMS-Pal-2 nanosponges are expected for hydrophobic coating application, because of their excellent porous structure. XPS technique was also used to reveal the successful fabrication of the PDMS-Pal-2 nanosponges (Fig. 3). After the surface modification of the Pal nanorods with MPTMS, the surface atomic concentrations of the main elements (O, Fe, Mg and Al) in Pal mineral decreased, except that Si and C elements increased (Table 2). Furthermore, S element, which could not be detected in the pristine Pal nanorods, appeared with a surface atomic concentration of 1.43%. The results demonstrated the successful surface modification of Pal nanorods with MPTMS. After the UV-induced thiol−ene interfacial click reaction, the surface atomic concentration of the main elements (Fe, Mg, Al and S) of the Pal-SH nanorods decreased, while those of Si, O and C increased distinctly, indicating the successful thiol−ene interfacial click reaction of the PalSH nanorods with polydimethylsiloxane diacrylate, as shown in Scheme 1.
Samples
Pal Pal-SH PDMS-Pal-2 (H2O) PDMS-Pal-2 (CH2Cl2)
Surface atomic concentration (%) O
Si
Fe
Mg
Al
C
N
S
43.6 40.3 42.7 21.5
18.4 25.5 21.9 15.4
11.3 6.3 0.2 0.1
9.3 3.5 – –
7. 8 6.4 3.0 0
6.3 14.1 29.5 61.7
3.4 2.4 1.7 0
0 1.4 1.0 1.3
3.2. Hydrophilic-hydrophobic property of the as-prepared coatings The static water contact angle (SWCA) of the as-prepared PDMS-Pal nanosponge-based coatings were measured and plotted in Fig. 4. For the PDMS-Pal-1 nanosponges with a low PDMS amount, a hydrophilic coating was formed with an initial SWCA of 77.9°. And the value decreased over time. The water droplet infiltrated into the coating within 2.5 min. As for the loose PDMS-Pal-2 nanosponges, the as-prepared coating showed hydrophobic property with an initial SWCA of 92.0° and the water droplet infiltrated into the coating within 4 min. The asprepared PDMS-Pal-3 nanosponge coating showed an initial SWCA of 86.6° and the water droplet infiltrated into the coating within 9 min. The PDMS-Pal-2 nanosponge coating showed the best hydrophobic property due to their loose porous structure, while the PDMS-Pal-3 nanosponge coating possessed the longest water droplet retention period because of their compact structure, as shown in the TEM images
Fig. 2. TEM images of PDMS-Pal-1 (a), PDMS-Pal-2 (b), and PDMS-Pal-3 (c) nanosponges. 3
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100
160 a
140
As-prepared Treated with H2O
Treated with CH2Cl2
120
Treated with CH2Cl2
100
CA (o)
CA (o)
80
b
As-prepared Treated with H2O
60 40
80 60 40
20
20 0
0 0.0
0.5
1.0 1.5 Time (min)
2.0
140
0
2.5
c
1
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100
Treated with CH2Cl2
CA (o)
120
2 Time (min)
3
4
80 60 40 20 0 0
1
2
3
4 5 6 Time (min)
7
8
9
10
Fig. 4. Change of the static water contact angle (SWCA) over time of the as-prepared coatings, after treated with water and CH2Cl2 of the PDMS-Pal-1 (a), PDMS-Pal2 (b), and PDMS-Pal-3 (c) coatings.
Still), suggesting that the coatings can be applied as self-cleaning surfaces in outdoor environments (Torun et al., 2019). The compact PDMSPal-3 nanosponge coating changed from hydrophilic to hydrophobic surface with an initial SWCA of 120.1°, although the infiltration time of the water droplet was only 1 min. Such results indicated the surface hydrophilic-hydrophobic reversal property of the proposed PDMS-Pal nanosponge coatings. The similar tendency was found in the static diiodomethane contact angle (SDCA) measurements after treating with different solvents. After treating with CH2Cl2, the SDCA values of the coatings with bigger PDMS-Pal nanosponges increased obviously. Especially for the loose PDMS-Pal-2 nanosponge coating, it became oleophobic with an initial SDCA of 101.1° and the SDCA value declined to about 60° in 2.5 min. Even though, the SDCA values were still higher than those of the compact PDMS-Pal-3 nanosponge coating (40–50°). The results also indicated the structural transformation of the bigger PDMS-Pal nanosponges during treating with CH2Cl2. The PDMS-Pal-1 nanosponge coating did not show obvious surface hydrophilic-hydrophobic reversal property, it should be due to the small size and less PDMS content. As for the PDMS-Pal nanosponge coatings with bigger PDMS-Pal nanosponges, obvious surface hydrophilic-hydrophobic reversal occurred after treating with CH2Cl2, but not with water. The structural transformation during treating with CH2Cl2 was more distinct for the loose PDMS-Pal-2 nanosponge coating, because of their loose structure with relatively lower crosslinking degree. For all three PDMS-Pal nanosponge coatings, the surface property did not change obviously after treating with water. The reason should be the similar strong polarity of methanol and water, which was used for the fabrication of the nanosponge coatings and post-treating, respectively. To reveal the structural transformation during treating with CH2Cl2,
(Fig. 2). The oleophilic-oleophobic property of the as-prepared PDMS-Pal nanosponge-based coatings were also evaluated with diiodomethane (DIM). As shown in Fig. 5, the as-prepared PDMS-Pal-1 nanosponge coating gave an initial static diiodomethane contact angle (SDCA) of 23.6°, while the other two around 30°. The as-prepared PDMS-Pal-1 nanosponge coating showed the best oleophilic property, similar as the hydrophilic-hydrophobic tests (Fig. 4). It might be due to the smooth coating surface with low roughness resulted from the small nanosponges with few Pal nanorods. 3.3. Surface hydrophilic-hydrophobic reversal property After the as-prepared PDMS-Pal nanosponge-based coatings were immersed in water for 10 min and dried at 60 °C for 20 min, the SWCA values were near to those before treating with water (Fig. 4). However, the SWCA values changed distinctly after treating with CH2Cl2 in which PDMS could be dissolved, maybe due to the structural transformation. For the small PDMS-Pal-1 nanosponges with less PDMS, the infiltration time of the water droplet into the coating was shortened to 1 min, although the initial SWCA was remained similar as before treating with water. While the initial SWCA values of the bigger nanosponges (PDMSPal-2 and PDMS-Pal-3) increased obviously, demonstrating that the PDMS content and structure showed important effects on the surface hydrophilic-hydrophobic property of the proposed PDMS-Pal nanosponge coatings. After treating with CH2Cl2, the loose PDMS-Pal-2 nanosponge coating could remain hydrophobic surface for 1.5 min, with an initial SWCA of 143.4°, showing superhydrophobicity. When a water droplet was dropped to the surface of the treated PDMS-Pal-2 nanosponge coating, it completely bounced off the coating (Supplementary Video 4
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50
120
a
45
As-prepared Treated with H2O
40
Treated with CH2Cl2
CA (o)
CA (o)
30
60 45
20
30 0.2
0.4 0.6 Time (min)
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60 55
Treated with CH2Cl2
75
25
0.0
As-prepared Treated with H2O
90
35
15
b
105
15
1.0
c
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Time (min)
As-prepared Treated with H2O
50
Treated with CH2Cl2
o CA ( )
45 40 35 30 25 20 0.0
0.5
1.0 1.5 Time (min)
2.0
2.5
Fig. 5. Change of the static diiodomethane contact angle (SDCA) over time of the as-prepared coatings, after treated with water and CH2Cl2 of the PDMS-Pal-1 (a), PDMS-Pal-2 (b), and PDMS-Pal-3 (c) coatings.
Fig. 6. SEM images of the PDMS-Pal-2 coatings after treating with water (a) and CH2Cl2 (b).
originated from the Pal nanorods. While after treating with CH2Cl2, a significantly higher surface C atomic concentration was found, meaning the surface of Pal nanorods had been covered with a PDMS skin. The XPS results revealed the hydrophilic Pal nanorods-riched surface and hydrophobic PDMS-rich surface after treating with strong polar solvent water and weak polar solvent CH2Cl2, respectively. So the surface hydrophilic-hydrophobic reversal mechanism of the PDMS-Pal-2 nanosponge coating could be explained with the conformational change of the PDMS linkers and brushes, as shown in Scheme 2.
the surface morphology and surface atomic concentrations of the PDMS-Pal-2 nanosponge coating were analyzed with SEM and XPS techniques. The Pal nanorods could be seen clearly from the PDMS-Pal2 nanosponge coating after treating with water (Fig. 6a), however, the nanorods were overshadowed after treating with CH2Cl2 (Fig. 6b). The results indicated the structural transformation of the PDMS-Pal-2 nanosponge coating after treating with solvents with different polarities. That is to say, due to the shrinkage of the PDMS linkers and brushes in the strong polar solvents such as methanol and water, a Pal nanorodsriched surface was resulted, showing a hydrophilic surface. While after treating with low polar solvent CH2Cl2, in which the conformation of the PDMS linkers and brushes could change easily from shrinking to spreading chain (Zhang et al., 2012), so a PDMS-rich surface was resulted, showing a hydrophobic surface. The XPS technique was used to reveal the surface hydrophilic-hydrophobic reversal mechanism shown in Scheme 2. Clearly, the PDMSPal-2 nanosponge coating after treating with water possessed higher surface atomic concentrations of O, Si, Fe, Al elements (Table 2),
4. Conclusions In summary, facile strategy was developed to fabricate of the polydimethylsiloxane-palygorskite (PDMS-Pal) nanosponge-based coatings, via the UV-induced thiol−ene interfacial click reaction of the thiol modified palygorskite nanorods (Pal-SH) with polydimethylsiloxane diacrylate. Because of the hydrophilic/hydrophobic property of the resultant PDMS-Pal nanosponges-based coatings 5
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Scheme 2. Schematic illustration of the surface hydrophilic-hydrophobic reversal mechanism of the PDMS-Pal nanosponges.
vanishing over time with contact with different solvents, the hydrophilic or weak hydrophobic PDMS-Pal nanosponge-based coatings could transform into hydrophobic coatings after treating with CH2Cl2, and even superhydrophobic surface for potential self-cleaning applications. Based on the SEM and XPS analysis of the PDMS-Pal nanospongebased coatings after treating with strong polar water and weak polar CH2Cl2, the surface hydrophilic-hydrophobic reversal mechanism was proposed as the conformational transformation of the PDMS chains. The unique surface hydrophilic-hydrophobic reversal function is expected to explore the potential applications of the PDMS-Pal nanosponges as functional adsorbents or coatings. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2020.105546.
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