Engineering polydimethylsiloxane with two-dimensional graphene oxide for an extremely durable superhydrophobic fabric coating

Abstract

A type of nanostructured material comprising reduced graphene oxide (RGO) modified polydimethylsiloxane (PDMS) for fabric coating is described. RGO modified PDMS was prepared through an aryl radical assisted addition reaction, followed by hydrosilylation with Si–H terminated PDMS or poly(dimethylsiloxane-co-methylhydrosiloxane) (PDMS-co-PHMS) in the presence of a catalytic amount of Pt catalyst. The introduction of a trace amount of RGO to cross-link with PDMS or PDMS-co-PHMS led to a significant improvement in the hydrophobicity (water contact angle > 150°) of textile fabrics. The superhydrophobicity of the fabric coating, which was readily achieved by a facile dipping coating process, is long-term resistant to strong acids, corrosive alkalis, hot water and 200 cycles of laundry. This type of RGO modified PDMS or PDMS-co-PHMS coating materials makes use of the inherent morphological anisotropy of fabrics which latter provides a hierarchical roughness on the micro scale to further enhance the surface hydrophobicity, showing potential use in various textile applications.

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Introduction

Superhydrophobic coatings, which deliver bio-mimicking water-repellent,¹ anti-sticking,² contamination prevention,³ reducing fluid drag⁴ and self-cleaning functions,⁵ have gained immense commercial and academic interest due to their wide applicability in various fields, including energy conversion,⁶ waterproof textiles,⁷ smart structural coating materials,^8–10 and microfluidics.¹¹ Numerous artificial superhydrophobic surfaces have been developed by means of a low surface energy material in conjunction with a particular surface roughness.^12–15 On the one hand, the morphologies of a superhydrophobic surface can be facilely controlled by various nanostructures, such as nanoparticles,¹⁶ nanowires,^17,18 nanotubes,¹⁹ graphene nanosheets,²⁰ and diamond-like carbon.²¹ On the other hand, perfluoropolymers or fluorine-containing materials were widely used for superhydrophobic coating^12–25 because of their low surface energy, low friction coefficient and reduced adhesion to surfaces. Nevertheless, practical applications of superhydrophobic coating are far below the expectations due to the high cost and potential risk of fluorochemicals for human health as well as the environment.

In recent years, scientific communities are moving towards the development of sustainable and fluorine-free materials for superhydrophobic coating.²⁶ For example, Erbil and Demirel et al. reported a simple and inexpensive method to prepare gel-like porous superhydrophobic coating using isotactic polypropylene.²⁷ The surface roughness of isotactic polypropylene could be well controlled by selecting suitable solvents and temperature, and thus this type of materials could be applied to a variety of surfaces. Lim et al. reported a superhydrophobic coating strategy by electrospinning organically modified silicates into a fibrous web.²⁸ The electrospun hybrid fabrics maintained superhydrophobicity even after heat treatment at 500 °C, indicating strong thermal wetting stability. Xue et al. reported that hydrophobic polydimethylsiloxane-based coatings were self-roughened on textiles via a nonsolvent-induced phase-separation method. The obtained superhydrophobic and superoleophilic materials were demonstrated as excellent filters for continuous oil–water separation.²⁹ Sun et al. described a method of simple physical absorption of RGO and PDMS on fiber. The superhydrophobic fabric coating involved in a three-step dipping in ammonia solution, RGO and PDMS–toluene solution, respectively. This PDMS-treated cotton exhibits selective absorption of organics and oils from water with an absorption capacity up to 11 to 25 times its weight.^29b Nine et al. also reported a method to prepare graphene-based superhydrophobic composite coatings with robust mechanical strength, self-cleaning, and barrier properties.^29c

Superhydrophobic coating for textile and fabrics is one of important applications in our daily life. The general route towards superhydrophobic fabric coating usually involves in the controlled deposition of, for example, the designed nanomaterials and polymer composite onto fibers. For instance, Ramaratnam et al. demonstrated that the deposition of an ultrathin coating layer on fabrics, consisting of non-fluorinated, hydrophobic polymer and reactive silica nanoparticles, led to the formation of an ultrahydrophobic textile surface.³⁰ Recently, Wang et al. coated cotton fabric with ZnO@SiO₂ nanorods to mimic the hierarchical structure and create a superhydrophobic surface.³¹ However, the main drawback of this method is the fragility of coating materials which is not able to resist abrasion and harsh chemical environments. Moreover, if nanoadditive and polymeric materials are only physically blended, the organizational coating structure will not be durable during practical use. Therefore, covalently integrating textiles with low surface energy materials plays a substantial role in enhancing the long-term durability of the superhydrophobicity.³²

Materials and methods

Chemicals

Graphite powder (<45 μm, ≥99.99%), poly(dimethylsiloxane-co-methylhydrosiloxane), trimethylsilyl terminated, (PDMS/PMHS), (M_n ∼ 13 000, methylhydrosiloxane 3–4 mol%) and hydride terminated PDMS (MW 600–800 and 4500–5000) were purchased from Sigma-Aldrich. The preparations of graphene oxide and 4-propargyloxybenzenediazonium tetrafluoroborate were according to reported procedures.³³ Monodisperse hydride terminated polydimethylsiloxane was purchased from Gelest Inc. Other chemicals were purchased from Sigma-Aldrich and used as received. Water was purified with a Millipore Milli-Q water system.

Instrumentation and measurements

Scanning electron microscopy (SEM) images were taken using a JEOL JSM 6700F operated at an acceleration voltage of 5.0 kV. Contact angle (CA) measurements were carried out on a ramé-hart Contact Angle Goniometers using liquid droplets of 5 μL in volume. UV-Vis analyses were performed on a Shimadzu UV-3600 UV-Vis-NIR spectrophotometer (1 mm quartz cell used). Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 instrument in ATR mode at a resolution of 4 cm⁻¹ accumulating 32 scans. The ¹H NMR spectra were recorded on a Bruker DRX 400 MHz NMR spectrometer at room temperature and chemical shifts were recorded in parts per million (ppm). Dynamic light scattering experiments were performed on a Brookhaven 90 plus spectrometer with a temperature controller. An argon ion laser operating at 633 nm was used as a light source.

Preparation of 4-vinyloxybenzenediazonium tetrafluoroborate

4-Vinyloxybenzenediazonium tetrafluoroborate was prepared according to a modified procedure. A portion of nitrosonium tetrafluoroborate (0.19 g, 1.61 mmol) was dissolved in acetonitrile (10 mL), and the solution was cooled to −30 °C. 4-(Vinyloxy)aniline (0.20 g, 1.34 mmol) in acetonitrile (10 mL) was added dropwise while stirring. After addition was complete, stirring was maintained for 30 min at −30 °C and then continue to stir for one more 1 hour after removing cooling bath. 200 mL ether was poured into the resulting solution to give a precipitate. The product was collected by filtration and washed with ether.

Preparation of vinyloxy-containing RGO

RGO (5.0 mg) was dispersed in 30 mL DMF, and then ultrasonication for 2 hours. The as-prepared 4-vinyloxybenzenediazonium tetrafluoroborate (20.0 mg) dissolved in DMF solution (5.0 mL) was added into the RGO suspension. The mixture was heated to 45 °C and stirred vigorously for 8 hours. After the reaction, the resulting suspension was kept for further application.

Preparation of vinyloxy-containing RGO modified PDMS (PDMS@RGO) by approach I

PDMS/PMHS (1.0 g) was dissolved in toluene, and the as-prepared vinyloxy-containing RGO dispersion (1 mL, 2 mL, 5 mL, 10 mL and 20 mL for PDMS@RGO 1–5, respectively) was then added. Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution (20 μL) was dissolved in minimum toluene and added to the above solution slowly under N₂. Once the addition was complete, the suspension was stirred at 80 °C overnight. The suspension was then concentrated to remove solvent and washed by water and acetone, and finally the remaining liquid was vacuum-dried to give oily PDMS@RGO 1–5.

Preparation of hydride terminated PDMS modified RGO (PDMS@RGO) by approach II

Hydride terminated PDMS (100 mg, MW 600–800, and 4500–5000) was dissolved in toluene (5 mL), and the as-prepared vinyloxy-containing RGO dispersion (in DMF 10 mL) was then added. Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution (20 μL) was dissolved in minimum toluene and added to the above solution slowly under N₂. Once the addition was complete, the mixture solution was stirred at 80 °C overnight. The resulting material was filtered by polycarbonate film (0.2 micron, 47 mm) or centrifuged, and extensively washed with deionized water and acetone in order to get rid of excess PDMS, salts, and other un-reactive components, yielding the RGO@PDMS 6–7, corresponding to the products of PDMS with MW 600–800 and 4500–5000, respectively.

Coating procedure

PDMS@RGO and RGO@PDMS products were dissolved into chloroform and toluene, respectively, according their properties. The ratio between PDMS@RGO and chloroform is 100 μL PDMS@RGO 1–4 dissolved into 2 mL chloroform, respectively. The same ratio was applied to pure PDMS for comparison. For RGO@PDMS products, 2 mg sample 6 and 7 was dispersed into 2 mL toluene as coating solution. A piece of textile was immersed in above solution for 5 minutes and then it was taken out, followed by air-dry in a fume hood. Finally, the textile was dried in vacuum-oven for half an hour at 40 °C.

Washing durability

The superhydrophobic stability of a fabric coating in the laundry conditions was evaluated according to the procedures described in the American Association of Textile Chemists and Colorists (AATCC) Test Method 61-2006. The test was performed using a standard color-fastness to washing laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China) equipped with 500 mL (75 mm × 125 mm) stainless-steel lever-lock canisters. The fabric was laundered in a rotating closed canister containing 200 mL aqueous solutions of an AATCC standard WOB detergent (0.37%, w/w) and 10 stainless steel balls, the sizes of the fabric samples were 50 mm × 100 mm (2.0 in. × 4.0 in.) for the experimental test. During laundering, the temperature was controlled at 50 °C, and the stirring speed was 40 rpm. After 45 min of laundering, the laundered sample was rinsed with tap water, and then dried at room temperature without spinning. The contact angle was subsequently measured. This standard washing procedure is equal to five cycles of home machine launderings. Thus, we used the equivalent number of home launderings in this work.³⁴

Results and discussion

Polydimethylsiloxane (PDMS) is a very popular polymer for surface coating³⁵ because it is non-toxic, low cost, easy fabrication, flexible, and optical transparent. Herein, we describe preparation of fluorine-free superhydrophobic fabric coating materials, comprising covalently bonding reduced graphene oxide (RGO) to the backbone of Si–H terminated linear polydimethylsiloxane (PDMS) or its copolymer poly(dimethylsiloxane-co-methylhydrosiloxane) (PDMS/PMHS) in one synthetic step. Using this strategy, we demonstrated a trace amount of RGO (0.026–0.26%, wt%) could intrinsically lead to significant improvement in hydrophobicity of the materials, particularly when the RGO modified PDMS was applied onto fabrics. To the best of our knowledge, RGO covalently modified PDMS has yet been explored so far. In comparison with most prevailing blending methods,^19,36,37 the foremost considerable challenges come from how to efficiently and covalently incorporate two dimensional nanofiller RGO into the low-reactive PDMS as well as achieving a good dispersion of RGO in the PDMS polymer matrix. Most importantly, the covalent bonding between RGO and polymer matrix plays a key role in determining the performance of superhydrophobic coating during their practical applications.⁶ In this work, an efficient hydrosilylation was selected to create covalent bonding between RGO and PDMS polymer matrix.

The straightforward “one-step” synthesis of two types of PDMS@RGO hybrid materials are schematically depicted in Scheme 1. The RGO was coated with vinyloxy phenyl through an aryl radical assisted addition reaction to the RGO surface.^32,38 Afterwards, the vinyloxy phenyl covered RGO reacted with PDMS/PMHS or Si–H terminated PDMS in the presence of a catalytic amount of Pt catalyst. In order to improve the compatibility between organic and inorganic components, two types of approaches have been developed to integrate RGO with PDMS/PMHS or PDMS. In the 1st approach, manifold covalent carbon–silicon bonds were formed between PDMS/PMHS backbone and RGO surface through a one-step hydrosilylation process in toluene. The multiple reaction sites offer a strong covalent link between nanoscale RGO building blocks and organic matrixes PDMS/PMHS, preventing from phase separation in final hybrid materials. It is noteworthy to point out that during the hydrosilylation, a trace amount of vinyloxy phenyl covered RGO (wt% from 0.026% to 0.258%) not merely serves as a crosslinking agent to assist in the formation of hybrid coating materials, but also offers two-dimensional platforms to significantly improve the hydrophobicity of PDMS/PMHS matrix. Five RGO modified PDMS/PMHS materials were designated as PDMS/PMHS@RGO 1–5 in synthesis approach I (Table S1†). PDMS/PMHS@RGO composites were able to homogeneously disperse in dichloromethane and chloroform to form a stable suspension (Fig. S1†). In the 2nd approach, the as-prepared vinyloxy phenyl coated RGO was employed as a platform in which Si–H terminated PDMS can be uniformly coated on the RGO surface by a similar facile hydrosilylation process. The resulting suspension was filtered through a polycarbonate film (0.2 micron, 47 mm) or centrifugation (>10 000 rpm) and subsequently washed with chloroform and acetone to obtain PDMS modified RGO. PDMS@RGO 6 and 7 showed good dispersibility in toluene in comparison with corresponding PDMS modified graphene oxide (GO) (Fig. S2†).

Scheme 1 A graphical representation of preparation of PDMS/PMHS and Si–H terminated PDMS decorated RGO through hydrosilylation.

The resultant PDMS@RGO hybrid materials were first analyzed by ¹H NMR (Fig. S3†), FTIR (Fig. S4 and S5†) and UV-Vis (Fig. S6†) spectroscopies, confirming the formation of covalent binding between vinyloxy phenyl coated RGO and Si–H moieties of PDMS/PMHS. For example, the integration of proton at δ 4.72, corresponding to Si–H in the ¹H NMR spectrum progressively decreases with the increase of RGO as evidenced by the reduction in integration ratios of Si–H and Si–CH₃ (δ: 0.1) from 1 : 549 to 1 : 860, 1 : 1046 and finally 1 : 2185 for unreacted PDMS/PMHS, PDMS/PMHS@RGO 1–3. In addition, by increasing the content of vinyloxy phenyl modified RGO to react with PDMS/PMHS matrix, gradual gelation and eventually solidification are observed for polymers prepared by approach I. This trend can be explained by the dense crosslinking due to the presence much more anchorage groups in the vinyloxy phenyl covered RGO in hydrosilylation reaction. As indicated in Fig. 1, gradually increasing the amount of RGO during the hydrosilylation process, as-prepared PDMS/PMHS@RGO changes from liquid (PDMS/PMHS@RGO 1–3, wt% of RGO 0.026%, 0.052% and 0.129%, respectively) to gel (PDMS/PMHS@RGO 4, wt% of RGO 0.258%) and finally to solid state (PDMS/PMHS@RGO 5, wt% of RGO 0.515%). This is also reinforced by the increase of viscosity and decrease of hydrodynamic diameter with the increase of RGO. In addition, PDMS/PMHS@RGO 1–4 can be homogeneously dispersed in chloroform, but, PDMS/PMHS@RGO 5 cannot be fully dispersed in chloroform because of more densely cross-linking.

Fig. 1 Viscosity and particle size changes of PDMS/PMHS@RGO hybrid materials 1–4 containing different amount (wt%) of RGO at room temperature. The dashed line was simulated by Origin Software. Inset: the photographs show the appearance of suspensions.

A certain amount of PDMS/PMHS@RGO 1–4 and PDMS@RGO 6–7 was dispersed into chloroform and toluene, respectively, to form a coating solution, which was directly applied onto the fabrics by dip-coating. After the fabric coating treatment, a significant change in hydrophobicity was observed. The contact angle and detailed morphology of various hybrid materials coated fiber tissue were indicated in Fig. 2 and 3. The water contact angle measurement revealed that fabrics coated with PDMS@RGO hybrid materials showed a nearly sphere-like water droplet with a water contact angle of 150–160°. Such spherical droplets (10 μL) were stable and are able to maintain their spherical morphology on the fabrics for extended periods of time. The sliding angles of PDMS/PMHS@RGO 1–4 are below 10° which are summarised in Table S1.† The above observation is in contrast to the case of PDMS@RGO 6 where no high contact angle was found upon fabric coating, which may be due to its short PDMS side chains (MW 400–600). Note however that, no contact angle could be observed upon pure water dropping onto the un-coated fabrics, where the water completely spreads into the fabric as shown in Fig. 2a. In contrast, a low contact angle of <50° can be observed by pure PDMS coated polyester and cotton textile at the same conditions. Importantly, the coating solution can be applied to various natural or man-made fibers, e.g. polyester, cotton, asbestos cloth, etc., using the same dip-coating method to achieve similar superhydrophobicity and stain resistance (Fig. S7†). These results are comparable to the reported silica-assisted fluoro-polymer coating on fabrics.

Fig. 2 Wetting behavior of water on the different treatment fabrics. Top panel: photograph and optical image of blue-colored water droplets on the polyester textiles. Bottom panel: on the cotton textiles (10 μL for each drop). (a) Un-coated; (b) PDMS/PMHS coated; (c) PDMS/PMHS@RGO 1; (d) PDMS/PMHS@RGO 2; (e) PDMS/PMHS@RGO 3; (f) PDMS/PMHS@RGO 4; (g) PDMS@RGO 6; (h) PDMS@RGO 7.

Fig. 3 The SEM images of (A) un-coated, (B) PDMS/PMHS@RGO 3 and (C) PDMS@RGO 7 dip-coated fiber tissues at different magnification.

In a different control experiment, the coated fabrics were also stained with pigment-containing water for stain resistant test. After immerged into a blue pigment aqueous solution, the PDMS@RGO hybrid materials coated fabric was easily cleaned by rinsing with water (Fig. S8†). The excellent stain resistance suggests that it has significant potential uses for fabrics in anti-fouling of organic contamination applications. The comparison tests demonstrated that a trace amount of RGO plays a key role in determining the superhydrophobic properties.

Moreover, in a different control experiment, the coating-dependent fiber morphology changes could be clearly observed by SEM images. Original un-coated fabrics possess a rather rough surface as shown in Fig. 3A. Consistent with contact angle analysis, a set of distinct smooth shells inherit the original shape of the fabric weave observed (Fig. 3B and C), corresponding to coating layer of the PDMS@RGO hybrids. Although the boundary between the PDMS and RGO could not be seen due to the unconducive coating materials and a trace amount of RGO applied, the existence of RGO particles are evenly distributed throughout the shells were observed in Fig. 3C. In comparison with coating on the glass slide, the coating materials take advantage of the inherent morphological anisotropy of fabrics which latter provides a hierarchical roughness in the micro scales to further enhance the surface hydrophobicity. Based on such superhydrophobicity features, a mechanism is proposed as depicted in Fig. S9,† which combines the surface roughness and surface chemistry to mimic lotus leaves.

It has been demonstrated that commercial cotton (Fig. 4a) and polyester textile (Fig. 4b) coated with various concentrations of PDMS@RGO hybrid materials by dip coating exhibited the superhydrophobic properties even at a very low concentration of PDMS@RGO hybrid materials. Furthermore, the long-term stability of the superhydrophobic coating in harsh environments and laundry conditions is exceptionally important for many aspects of practical applications. As can be observed, water droplets are spherical with an average contact angle of 155° after immersing the coated fabrics in an aqueous H₂SO₄ solution (pH = 1) and an aqueous KOH solution (pH = 14) for the various time periods. For example, the long-term etching performances of PDMS/PMHS@RGO 3 at acidic and alkaline conditions are displayed in Fig. 4c and d, suggesting the superhydrophobic fabric coating layer exhibited excellent resistance to both a strong acid and strong corrosive alkaline solutions. Furthermore, this type of superhydrophobic coating also showed excellent thermal stability to boiling water (Fig. S10†). Experimental results showed that the coated fabrics had no change in superhydrophobicity after boiling the coated fabrics in water for 2 hours. Washing is also an important factor cause of degradation to superhydrophobic coatings during their practical uses. The superhydrophobic stability of fabric coating in the laundry conditions by using the AATCC Test Method 61-2006 was examined. As shown in Fig. 4e, the contact angles of textile fibers coated with PDMS/PMHS@RGO 3 did not show significant degradation after washing under a standard machine laundry process, which was equivalent to wash more than 200 times by a washing machine. In addition, the superhydrophobic stability of fabric coating in the abrasion with sandpaper was examined. The methodology of scratch test is illustrated in Fig. 4f. Sandpaper (1200 mesh) served as an abrasion surface, with the superhydrophobic fabric to be tested facing this abrasion material. While a pressure (10 kPa) was applied to the textile, the sandpaper was moved back and forth with a speed of 0.5 cm s⁻¹. Clearly, experimental results showed that the coated fabrics had no change in superhydrophobicity after being abraded repeatedly, where water droplets displayed contact angle above 160°. Therefore, the coating not only granted the fabrics a super water-repellent feature, but also showed excellent durability against both washing and abrasion.

Fig. 4 The contact angle changes of (a) polyester textiles and (b) cotton textiles after coated with different concentration of PDMS/PMHS@RGO 3 and 5% PDMS/PMHS; and the water contact angles of PDMS/PMHS@RGO 3 coated cotton textile changes versus various etching time periods in (c) an aqueous H₂SO₄ solution (pH = 1), in (d) an aqueous KOH solution (pH = 14), and (e) washing cycles. (f) Scratch test: 5 kPa and 10 kPa; (g) schematic illustration of the scratch test employed to evaluate the mechanical durability of the PDMS/PMHS@RGO 3 coated textile. Inset: wetting behavior of water drop on PDMS/PMHS@RGO 3 coated cotton textile after scratching.

Conclusions

In summary, we have designed two types of PDMS@RGO hybrid materials for extremely durable superhydrophobic fabric coating, and a facile dip-coating has been utilized for deposition of PDMS@RGO hybrid materials onto different types of commercial textiles. These coatings exhibit superhydrophobicity with exceptional corrosion resistance to boiling water, strong acid and strong alkali. Such PDMS@RGO hybrid materials also manifest excellent long-term performance after repeated washing for a long time. Given the simplicity of the protocol, the approach demonstrated in this work would provide a promising route for PDMS@RGO hybrid materials to offer a variety of applications in the areas of textile processing, packaging materials, paper industry, antifouling clothing, sportswear and even biomedical devices.

Acknowledgements

The authors would like to thank A*STAR of Singapore (Grant no. 1321760011) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Additional information on the synthesis, UV-Vis, FTIR, and ¹H NMR spectra. See DOI: 10.1039/c6ra14362h

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