In-situ addition of graphene oxide for improving the thermal stability of superhydrophobic hybrid materials

In-situ addition of graphene oxide for improving the thermal stability of superhydrophobic hybrid materials

Polymer xxx (2016) 1e11 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer In-situ addition of gra...

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Polymer xxx (2016) 1e11

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

In-situ addition of graphene oxide for improving the thermal stability of superhydrophobic hybrid materials Saravanan Nagappan, Chang-Sik Ha* Department of Polymer Science and Engineering, Pusan National University, Busan 46241, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2016 Received in revised form 28 December 2016 Accepted 29 December 2016 Available online xxx

Superhydrophobic mesoporous hybrid materials were synthesised by the in-situ self-hydroxylation and condensation of polymethylhydrosiloxane in ethanol and sodium hydroxide solutions in the presence of a cetyl trimethylammonium bromide (CTAB) as a surfactant and graphene oxide (GO). The samples were analysed by a range of characterisation techniques, such as Fourier-transform infrared and Raman spectroscopy, 29Si cross polarisation magic angle spinning nuclear magnetic resonance spectroscopy, Xray diffraction, X-ray photoelectron spectroscopy, surface area analysis, high resolution scanning electron microscopy, and high resolution transmission electron microscopy. The superhydrophobic hybrid powder was used for the detection and separation of chloroform in water from water/chloroform mixtures. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Superhydrophobic Polymethylhydroxysiloxane Cetyl trimethylammonium bromide Graphene oxide Chloroform detection and separation

1. Introduction In the recent days, superhydrophobic materials have attracted considerable attention in environmental applications, such as sorption and the separation of various oils and organic solvents, selective removal of metal ion adsorption, and gas sensors [1e10]. In general, superhydrophobic surfaces can be prepared easily by enhancing the surface roughness or by reducing the surface energy of the materials using low surface energy materials [11,12]. Fluorine-based low surface energy materials are used widely to prepare superhydrophobic materials [13]. Fluorine-based compounds, however, are more expensive than other compounds, which has limited their use in some practical applications. Several polymers, silane precursors, metal ions, and various methods have been used for the synthesis and fabrication of superhydrophobic materials and surfaces [14e19]. Polymethylhydrosiloxane (PMHS) is a well-known siloxane material used in a range of applications, such as stable hydrophobic coatings, highly transparent substrates, reducing agents for organic synthesis, biocompatible materials, micro-pattern devices, and nano necklace fabrication [20e22]. Yang et al. synthesised a nontemplate superhydrophobic micro-mesoporous hybrid using

* Corresponding author. E-mail address: [email protected] (C.-S. Ha).

PMHS and tetraethoxysilane (TEOS) and reported that more than one week was needed to reproduce the superhydrophobic hybrid micro/mesoporous material [23e25]. PMHS exhibited selfhydroxylation and condensation properties in sodium hydroxide solutions [11,23e25]. Therefore, superhydrophobic mesoporous material were synthesised in a one-pot process (within 2e3 days) using PMHS (without a cross-linking agent and surfactant). The superhydrophobic material was used for the preparation of novel superhydrophobic hybrid materials using natural leaf powder for oil spill capture and metal ions adsorption [11,26]. Graphene oxide (GO) is a well-known carbon material because of its excellent properties, such as easy dispersion in water and other organic solvents, electrical insulator, and readily modified by chemical or physical methods [27,28]. GO is anchored with covalently bonded functional groups, such as carbonyl or carboxyl, epoxy, and hydroxyl groups [29]. The above functional groups present in GO makes it a hydrophilic nature. The functional groups are active to react with various active molecules to form covalent or weak Van der Waals force of attraction. The addition of GO can enhance the thermal, mechanical, electrical, and other properties of the final materials [30,31]. GO is used widely in energy, environmental and biological applications due to the excellent properties [28]. In this study, cetyl trimethyl ammonium bromide (CTAB, 99%) was introduced as a surfactant and the solvent removal temperature was maintained at 150  C for further condensation of the

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hydrolysed siloxane network. The surfactant was used to ensure the porous structure of the synthesised material and was removed from the material using a solvent extraction method. Furthermore, the in-situ addition of graphene oxide (GO) to the pre-hydrolysed

PMHS was also studied. This paper reports the results of a detailed study of the structural, thermal properties of the superhydrophobic hybrid materials made from PMHS and GO or PMHS in the presence of CTAB surfactant by various characterisation tools.

Scheme 1. Synthesis of superhydrophobic polysiloxane/GO hybrid material.

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The superhydrophobic hybrid powder was also applied to the detection and selective removal of chloroform in a waterchloroform mixture. 2. Experimental details 2.1. Materials Polymethylhydrosiloxane (PMHS, Mn ~1700 to 3200), cetyl trimethylammonium bromide (CTAB, 99%), graphite powder (<20 mm, synthetic), and sodium nitrate (NaNO3, 99%) were purchased from Sigma-Aldrich. Anhydrous ethanol (special grade, 99.8%) and chloroform (CHCl3, 99%) were acquired from Carlo Erba Reagents Co. Ltd. Sodium hydroxide (NaOH, 97%) and hydrogen peroxide (H2O2, 30% conc.) were supplied by Junsei Chemical Co. Ltd. Sulfuric acid (H2SO4, 97%) and hydrochloric acid (HCl, 35% conc.) were purchased from Matsuneon Chemical Ltd. Potassium permanganate (KMnO4, 99%) was obtained from Katayama Chemical Industries Co. Ltd. Double deionised water was used in all experiments. All chemicals were used without further purification.

2.3. Synthesis of graphene oxide (GO) GO was synthesised using a slight modification of the Hummers method [32]. Graphite (5 g) was taken into a three neck round bottom flask (250 mL) and cooled to 0  C for 1 h. H2SO4 (115 mL) was added to the graphite and stirred slowly followed by the addition of NaNO3 (2.5 g) and KMnO4 (15 g). Water (200 mL) was added slowly and the suspension was kept for 24 h followed by the addition of H2O2 (25 mL) to the suspension. The precipitate was filtered after 10 min (Advantec filter paper), washed with a 5% HCl solution, followed by dispersion into water, centrifuging three times at 4000 rpm for 30 min, and vacuum drying at 40  C overnight. The obtained black colour precipitate was called GO. 2.4. In-situ addition of GO to the pre-hydrolysed polymethylhydrosiloxane PMHS (4.7 g) was dissolved in anhydrous ethanol (70 mL) in a

2.2. One-pot synthesis of superhydrophobic powder Superhydrophobic powder was obtained by a one-pot approach with slight modifications according to the procedure reported elsewhere [11]. Briefly, PMHS (4.7 g) was dissolved in anhydrous ethanol (70 mL) in a 100 mL glass container and hydrolysed by the addition of sodium hydroxide (0.008 g in 2 mL of water) and stirred for 30 min, followed by the addition of CTAB (1.170 g in 8 mL water) solutions with continuous stirring for 24 h. The gel was condensed further upon drying at 150  C for 2 h. The obtained superhydrophobic product was ground into a powder followed by removing the template by sonicating the superhydrophobic powder in water/ethanol (1:0.5 vol ratios) for 5 min (adjusting the pH of the suspension to pH 2 using 0.5 M HCl) followed by filtering, washing with excess of water and ethanol. The washing cycle was carried for several times (average 5 times), drying at 150  C for 2 h and grinding into a fine product. The obtained sample was called PCT (Scheme 1). Fig. 2. Raman spectra of (a) GO, (b) PCTGO 1%, (c) PCTGO 5%, and (d) PCTGO 10%.

Fig. 1. FTIR spectra of (a) PMHS, (b) PCT before removing the surfactant, (c) PCT, (d) GO, (e) PCTGO 1%, and (f) PCTGO 10%, where the spectrum of PCT before removing the surfactant was also provided for comparison.

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Fig. 3.

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Si-NMR spectra of (a) PMHS, (b) PCT, (c) PCTGO 1%, and (d) PCTGO 10%.

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Fig. 4. HRSEM images of (a) PCT, (b) GO, (c) PCTGO 1%, (d) PCTGO 5%, and (e) PCTGO 10%.

100 mL glass container, followed by the addition of sodium hydroxide (0.008 g in 2 mL of water) solution and stirred for 30 min. A binary mixture of CTAB (1.170 g) and GO (0.01 g) dispersion (in a constant amount of water (8 mL)) was added to the solution and stirred for 24 h. The gel was condensed further by drying at 150  C for 2 h and followed similar experimental procedure for the removal of template. The resulting material was named as PCTGO 1% (Scheme

1). Similar experiments were also carried out with the addition of various weight percentages of GO and the samples were called PCTGOX, where X is the contents of GO (2.5%, 5%, 7.5%, and 10%). 2.5. Characterisation The functional groups were characterised by Fourier transform

Fig. 5. HRTEM images of (a) PCT, (b) GO, (c) PCTGO 1%, (d) PCTGO 5%, and (e) PCTGO 10%.

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infrared spectroscopy (FTIR, JASCO (FTIR-4100)), Raman spectroscopy (FRS-100S, Bruker), and 29Si cross polarisation nuclear magnetic resonance (29Si CP NMR) spectroscopy using a 400 MHz Avance II þ Bruker Solid-state NMR at the Korea Basic Science Institute (KBSI) Daegu Centre. FTIR and Raman spectra were recorded at the scanning range from 400 to 4000 cm1 and 3002000 cm1, respectively. 29Si CP NMR spectroscopy was carried out at a radio frequency of 79.5 MHz with a 2 ms contact time and a repetition delay of 3 s and a spinning rate at 7 kHz. The surface morphology of the samples were analysed by loading the samples on carbon tape and coated with osmium tetraoxide (Hatfield, PA19440) prior to analysis by high resolution scanning electron microscopy (HRSEM, Hitachi S-4800). The nano surface morphologies of the samples were tested by high resolution transmission electron microscopy (HRTEM, JEM 2011 at 200 kV). The HRTEM sample was prepared by dispersing the samples in ethanol followed by loading on a copper grid and drying. The X-ray diffraction (XRD) patterns were obtained using a Miniflex goniometer at a scanning range from 1.2 2q to 80 2q, scanning speed of 1 /min and a scanning rate of 0.02/min. The nitrogen adsorption and desorption isotherms were measured using a Micromeritics ASAP 2020 V3.04 G surface area and pore size analyser. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distribution was determined using the Barrett-Joyner-Halenda (BJH) method. The thermal stability of the pristine and hybrid materials was checked in a nitrogen atmosphere and compared by thermogravimetric analysis (TGA, Q50 V6.2, Build 187, TA instruments, U.S., heating rate, 10 min1). X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific (U.K), Multi Lab) was carried out using Al Ka (hn ¼ 1486.6 eV) and Mg Ka (hn ¼ 1253.6 eV) radiation in the binding energy analysis range, 0e600 eV, in constant analyser energy mode. All the optical images were taken in a 13 megapixels (rear side) camera of LG G3 Cat6 mobile with the resolution 4160  3120 pixels.

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and 1030-1130 cm1 on its FTIR spectrum were due to the presence of Si-H, SiCH3, and broad Si-O-Si groups (Fig. 1a) [20]. The Si-H peak of PMHS at 2170 cm1 disappeared completely and a broad Si-O-Si peak was transformed to two sharp peaks at 1118 cm1 and 1035 cm1, indicating the hydroxylation and condensation of PMHS under basic conditions in ethanol (Fig. 1b [11,26]. The C-H stretching and bending vibrations of the alkyl substitutions present in CTAB and PMHS appeared at 2965 cm1, 2906 cm1, 2804 cm1, 1260 cm1, 882 cm1, and 768 cm1 (Fig. 1b). On the other hand, strong asymmetric and symmetric C-H stretching peaks appeared at 2965 cm1, 2906 cm1, and 2804 cm1 due to the presence of a surfactant (Fig. 1b) that leached out from PCT by an extraction method using ethanol and aqueous HCl (Fig. 1c). Fig. 1def shows the FTIR spectra of the synthesised GO and PCT modified with various weight percentages of GO (1e10%). The synthesised graphene oxide showed the presence of broad hydroxyl (O-H) groups at 3600-3400 cm1, strong carbonyl (COOH) stretching peaks and carbon-carbon double bonds (C¼C) at 1730 cm1 and 1621 cm1, respectively (Fig. 1d) [33]. The GO sample also showed an O-H deformation vibration peak, and the CO stretching vibrations of -OH groups and C-O-C groups at 1364 cm1 and 1046 cm1, respectively (Fig. 1d). The addition of GO to the prehydrolysed polysiloxane solution showed similar peaks of PCT with slight modifications in the stretching and bending

3. Results and discussion 3.1. Functional groups identification of superhydrophobic hybrid material The main functional peaks in PMHS at 2170 cm1, 1260 cm1,

Fig. 6. XRD patterns of (a) PCT, (b) GO, (c) PCTGO 1%, (d) PCTGO 2%, (e) PCTGO 5%, (f) PCTGO 7.5%, and (g) PCTGO 10%.

Fig. 7. (A and B) N2 adsorption and desorption isotherms and pore size distribution curves of (a) PCT, (b) PCTGO 1%, (c) PCTGO 2%, (d) PCTGO 5%, (e) PCTGO 7.5%, and (f) PCTGO 10%.

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vibration peaks of the materials (Fig. 1e and f), while the characteristic GO peaks are observed more distinctly as the GO contents were increased (see also Fig. S1). Fig. 2 presents the Raman spectra of the synthesised GO and PCT-GO. The GO sample showed strong G and D bands at 1595 cm1 and 1350 cm1, indicating the successful synthesis of GO nanosheets (Fig. 2a) [34]. The ratio of the D band to G band intensity (ID/ IG) for GO is 1.06. The results indicate the defect-free synthesis and maintenance of the structural properties of GO nanosheets, whereas the intensity of the G band was much weaker (not visible) and the D band peak position shifted towards a higher wavenumber (1414 cm1) by adding 1% of GO powder to the PCT (Fig. 2b). This is due to the presence of an excess of siloxane groups in PCT by the hydrolysis and condensation reaction of PMHS than the presence of GO in the material (ID/IG, 1.15). On the other hand, the intensity of the G band appeared and the D band intensity increased with increasing GO content to 5% to the prehydrolysed PMHS (Fig. 2c, ID/IG, 1.18). The G band intensity increased and a broad D band appeared similar to that of the GO synthesised by further increasing the GO content (10%) (Fig. 2d, ID/IG, 0.98). The results indicate a uniform distribution of GO nanosheets in the prehydrolysed PMHS suspension. The other peaks in Fig. 2bed at the range, 400-800 cm1, indicate the presence of Si-O-Si and Si-C groups in polysiloxane. The 29Si CP NMR spectrum of PMHS showed weak and sharp alkyl substituent peaks at d 9.0e10.0 ppm and 35.0 ppm, which indicates the presence of trimethylsiloxy ((CH3)3SiO, M1) end groups on both ends of the silicone network and (CH3SiO2H) groups in PMHS (Fig. 3a) [20], whereas the hydrolysis and condensation of PMHS revealed the disappearance of a Si-H peak and the appearance of weak and strong peaks at 59 ppm and 66 ppm, which is due to the formation of siloxane networks (CH3Si(SiO)2(OH), T2), and (CH3Si(SiO)3, T3) by the crosslinking of PMHS in basic medium (Fig. 3b) [11]. The in-situ addition of GO to the prehydrolysed samples also showed similar peaks, indicating that the addition of GO would not affect the hydrolysis and condensation of PMHS (Fig. 3c and d). 3.2. Surface morphological studies of superhydrophobic hybrid material The surface morphology of the synthesised as well as after the in-situ addition of GO was examined (Fig. 4). PCT showed the surface morphology of a porous three dimensional network structure (Fig. 4a) due to the faster condensation of siloxane networks and the aggregation of particles in the presence of a surfactant at 150  C. The synthesised GO powder showed a layered morphology by the stacking of a few layers of GO sheets on the surface (Fig. 4b). The addition of GO to the pre-hydrolysed polysiloxane suspension showed some changes in the surface morphology of the samples (Fig. 4cee). The porous three dimensional network structures of PCT were reduced by increasing the GO weight percentage. This is due to the good distribution and bonding of GO sheets to the PCT surface. The HRTEM images of PCT and GO revealed the porous

Fig. 8. TGA curves of (a) PMHS, (b) PCT (c) PCTGO 1%, (d) PCTGO 2%, (e) PCTGO 5%, (f) PCTGO 7.5%, and (g) PCTGO 10%.

network structure of PCT and the layered morphology of GO (Fig. 5a and b). The addition of GO to the prehydrolysed polysiloxane showed an aggregated surface morphology due to the agglomeration of polysiloxane and GO in the in-situ hydrolysis and condensation reaction (Fig. 5cee). XRD of the synthesised PCT showed a sharp intensity peak at 10.5 2q (003 plane), which indicated the crystalline domain of the alkyl chains. PCT also showed a broad peak at 16.0e30.0 2q with some sharp and weak-peaks, suggesting the crystalline nature of the alkyl chains present in the amorphous siloxane backbone (Fig. 6a) [35]. A sharp intensity peak at 11.4 2q illustrated the successful synthesis of the GO sheets (Fig. 6b). The addition of GO to the pre-hydrolysed polysiloxane showed similar XRD patterns with slight increases in intensities with increasing weight percentage of GO (Fig. 6ceg). The surface area (464 m2 g-1), pore volume (1.02 cm3 g-1) and pore diameter (20.1 nm) of the synthesised PCT showed a nonuniform mesoporous structure with a larger pore diameter (Fig. 7a, Table 1). The addition of smaller amount of GO (1%) to the pre-hydrolysed polysiloxane suspension altered the surface area (400 m2 g-1), pore volume (0.80 cm3 g-1) and pore diameter (92.5 nm) (Fig. 7b and Table 1). The increased pore diameter may be due to the less binding of a GO sheet on the polysiloxane matrix, which partially affects the surface properties. Similarly, the surface areas, pore volumes and pore diameters also showed different behaviours by further increasing the GO contents to a prehydrolysed polysiloxane suspension (Fig. 7cef and Table 1). These were attributed to the better distribution and deposition of GO sheets by physical or chemical bonding with polysiloxanes with increasing GO content.

Table 1 Surface areas, pore volumes, and pore size distributions, and thermal properties of superhydrophobic polysiloxane/GO hybrid materials. Sample

BET surface area (m2g1)

Total pore volume (cm3g1)

BJH pore diameter (nm)

T5( C)

T10( C)

Remaining residual mass at 800  C

PMHS PCT PCTGO1% PCTGO2.5% PCTGO5% PCTGO7.5% PCTGO10%

e 464 400 473 241 205 315

e 1.02 0.80 1.28 0.34 0.20 1.02

e 20.1 92.5 25.5 6.40 3.60 3.60

185 213 419 454 431 427 542

238 310 534 558 575 621 649

18.3 3.60 66.7 67.2 73.0 83.4 85.6

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3.3. Thermal stability and chemical compositions of superhydrophobic hybrid material Fig. 8 shows the thermal stability of the synthesised

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polysiloxane and polysiloxane hybrid materials. PMHS showed good thermal stability due to the presence of strong alkyl chains at the surface and at the terminals of silicon. The thermal stability of PMHS was maintained at more than 100  C and the organic

Fig. 9. XP spectra of (A) PCT, (B) GO, (C) PCTGO 1%, (D) PCTGO 5%, and (E) PCTGO 10%.

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[MeHSiO] substituents attached on the surface decomposed slowly with increasing decomposition temperature [20]. The thermal stability of PMHS was increased by the hydroxylation and condensation of PMHS. This is due to the formation of crosslinked siloxane structures, which required additional energy to break the bonds. The surfactants began to decompose from the materials at over 150  C. Similarly, the alkyl groups lost their strength by volatilisation and depolymerisation at 350e550  C. The 5% and 10% of weight loss results of PMHS and PCT highlight the enhanced thermal stability of the synthesised PCT (Table 1). The addition of GO to the prehydrolysed polysiloxane showed significant enhancement in the thermal stability of the hybrid materials. Increasing the weight percentage of GO sheets would increase the thermal stability (Table 1) because of the excellent thermal stability of GO and the presence of a carbon source in the hybrid materials by a phase transformation of GO to graphene at higher temperatures. Fig. 9 shows the chemical compositions of PCT, GO and PCT-GO hybrid. The spectrum of PCT showed a good distribution of carbon (32%), oxygen (39%) and silicon (29%) in atomic percentage (Fig. 9A) [26], whereas the GO sheets showed high contents of carbon (66%) and oxygen (34%) in the structure (Fig. 9B). The addition of GO sheets to the pre-hydrolysed polysiloxane resulted in a slow increase in carbon content (34e39%) with a concomitant decrease in oxygen (38-36%) and silicon (28-25%) (Fig. 9C-E). The increase in carbon content in the hybrid materials is due to the presence of an excess of carbon by GO with addition to alkyl groups in polysiloxanes. The increase in carbon content in the hybrid materials would increase the thermal stability of the materials. The results proved the higher thermal stability of the synthesised hybrid materials, as shown in Fig. 8. 3.4. Surface properties and chloroform detection in waterchloroform mixture of superhydrophobic hybrid material Pristine PMHS has hydrophobic property (SCA, 100.0 ± 2.0 ) to water [19]. On the other hand, PCT in the absence of surfactant showed superhydrophobic (SCA, 174.2 ± 1 ) property due to the shape of the hierarchical micro-nano particles with a mesoporous

structure and the presence of a hydrophobic methyl group anchored to the surface [11]. Similarly, PCT synthesised in the presence of a surfactant also showed very stable superhydrophobicity (Fig. 10). The addition of hydrophilic graphene oxide to the pre-hydrolysed polysiloxane suspension also imparted superhydrophobicity to the hybrid materials. Moreover, the hybrid materials can show highly stable superhydrophobic property to water droplet. Owing to the presence of hydrophilic graphene oxide, the hybrid material showed strong interactions with water droplet. On the other hand, the presence of highly stable hydrophobic functional group in the hybrid material made the material stable enough to repel water droplets on the surface. Furthermore, we checked the superhydrophobic stability of the hybrid material to water droplet. A water droplet can easily form a layer of highly stable hydrophobic coating on the surface by placing or moving the water droplet on the hybrid material bed due to the presence of hydrophilic and hydrophobic nature and by electrostatic attraction between the hybrid material and water droplet (Fig. 10). Moreover, the hybrid material maintained the extreme hydrophobicity by further addition of water droplets. Several water droplets were combined to form a single droplet (Fig. 10). The droplet can be broken into several smaller droplets by a mild or stronger mechanical force (Fig. 10). Furthermore, the smaller droplets were recombined again into a single droplet by mild agitation. The results highlight the very stable superhydrophobic properties of the hybrid materials. Moreover, this technical advantage may be useful for various applications. PCT and hybrid materials were floated at the water surface, even by strong mechanical shaking (Fig. 11A and B). Furthermore, the PCT-GO hybrid materials were used for the detection of chloroform-in-water [11,36e38]. The experiment was performed with a fixed volume of water (25 mL) and various volumes of chloroform (0.1, 0.2, 0.3, 0.4, and 0.5 mL) in water-chloroform mixtures. A fixed amount of PCT-GO10% hybrid material (0.05 g) was added to the water-chloroform mixture solution and shaken violently (Fig. 11C, before shaking). The hybrid materials adopted a gel-like state by adding a small amount of chloroform that also shrink to the bottom and was dispersed in chloroform by increasing

Fig. 10. Water droplet on the PCT-GO hybrid material surface. (AeF) The water droplets formed by covering a thin layer of hybrid material can split into various sizes and shapes, and can be recombined into a single droplet by mild agitation (Scale bar: 1 cm to the naked eye. See also movie file in the Supporting Data.).

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Fig. 11. Superhydrophobic property of PCT and PCTGO10% hybrid materials on the water surface before (A) and after shaking (B). PCT-GO hybrid materials for the detection of chloroform-in-water before (C) and after shaking (D). Dispersibility of PCTGO10% hybrid material in water (W), water-methanol (Me), water-ethanol (Et), and water-chloroform (Chl) mixtures (left to right) (E). Dispersibility of PCTGO10% hybrid materials by increasing the volume of methanol and ethanol in water (F). Separation of chloroform from a chloroform-in-water mixture (G) (See also movie file in the Supporting Data.).

the volume of chloroform in water (Fig. 11D, after shaking). This was attributed to the superoleophilic nature of the superhydrophobic powder, which strongly interacts with oils and organic solvents [11,39,40]. The hybrid materials were dispersed well at the bottom when a higher volume of chloroform was used in

water (Fig. 11D, after shaking). Moreover, there was no dispersion of hybrid materials in water at this stage. Similarly, the hybrid samples showed no dispersibility at lower volumes (0.5 mL) of alcoholic solvent (ethanol, methanol) added to water (Fig. 11E, after shaking). The hybrid materials were dispersed partially by increasing the

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volume of alcohol in water (25 mL) to more than 5 mL (Fig. 11F, after shaking). The results indicate the excellent superhydrophobic property of the hybrid materials and the detection ability of a small quantity of chloroform in water. The hybrid materials were used further for the separation of chloroform in water by a simple gravitation force (Fig. 11G). The superhydrophobic and superoleophilic nature of the hybrid materials separates the chloroform from the chloroform-in-water mixture (Fig. 11G). The continuous separation of chloroform in water was achieved by pouring the chloroform-in-water solution to a glass tube filter. The results highlight the excellent properties and applicability of superhydrophobic hybrid materials. 4. Conclusions Three dimensional porous structures of polysiloxanes with a foam-like morphology were synthesised. The synthesised material showed superhydrophobicity and partial variations in the surface properties, such as morphology, surface area, pore volume, and pore diameter, by introducing CTAB. The in-situ addition of GO to the pre-hydrolysed polysiloxane resulted in the superhydrophobic property of the hybrid material. Increasing the GO amounts to constant amounts of pre-hydrolysed polysiloxane also imparted superhydrophobic properties and enhanced the thermal stability of the synthesised hybrid materials. FTIR spectroscopy, Raman spectroscopy and 29Si-NMR spectroscopy, XRD, BET surface areas, HRSEM, HRTEM, and XPS confirmed the structural changes to the materials. The superhydrophobic hybrid powder showed excellent detection and separation ability of chloroform in a waterchloroform mixture. These results suggest that superhydrophobic hybrid materials can be used in a range of applications, such as photo-catalysts, organic reaction conversion, sorption, and the separation of oils and organic solvents in water.

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Acknowledgements The work was financially supported by the “2016 Post-Doc Development Program” of Pusan National University, Korea and the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT & Future Planning, Korea, Acceleration Research Program (NRFe2014R1A2A1A11054584) and Brain Korea 21 Plus Program (21A2013800002).

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2016.12.072.

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Please cite this article in press as: S. Nagappan, C.-S. Ha, In-situ addition of graphene oxide for improving the thermal stability of superhydrophobic hybrid materials, Polymer (2016), http://dx.doi.org/10.1016/j.polymer.2016.12.072