- Open Access
Preparation of superhydrophobic and transparent micro-nano hybrid coatings from polymethylhydroxysiloxane and silica ormosil aerogels
© Nagappan et al.; licensee Springer. 2014
Received: 3 September 2014
Accepted: 15 September 2014
Published: 2 December 2014
Superhydrophobic and transparent polymethylhydroxysiloxane (PMHOS)/silica ormosil aerogel hybrids were prepared successfully by mixing of PMHOS with various weight percentages of silica ormosil aerogels (as synthesized from methyltriethoxysilane (MTES) and methyltrimethoxysilane (MTMS) precursors) in separate seal perfume glass vials. The hybrids were spin coated on glass substrate at 1000 rpm for 60 seconds and used for further analysis. The surface morphology and chemical compositions of the hybrids were analyzed by high resolution scanning electron microscopy, high resolution transmission electron microscopy, atomic force spectroscopy, adsorption and desorption isotherm, and X-ray photoelectron spectroscopy. The transparency, thermal decomposition and static contact angle (SCA) of each sample were measured by UV-Visible spectrophotometer, TGA and drop shape analysis system, respectively. The spin coated substrates showed good superhydrophobic properties, thermal stability as well as transparency on the glass substrates.
1 1 Background
Transparency of a substrate is an important factor for coating applications. Transparent substrates with several surface properties such as superhydrophilic (contact angle, CA, ≤5°), hydrophilic (CA, <90°), hydrophobic (CA ≥90°) and superhydrophobic (CA ≥150°) surfaces were prepared using various organic and inorganic hybrid materials –. These types of substrates were used widely in a range of applications such as anti-stain coating, anti-fogging, solar cell, flexible substrate fabrication, self-cleaning, and anti-icing coatings –. The coating methods such as dip coating, spin coating, spraying, and casting methods were used for the preparation of transparent substrates –. Nakajima et al. prepared transparent superhydrophobic surface by spin coating method under the sublimation of aluminum acetylacetonate (AACA) followed by fluorosilane treatment . In another method, the authors also fabricated similar type of transparent superhydrophobic surface using AACA and titanium acetylacetonate (TACA) and fluorosilane . The obtained superhydrophobic surface showed almost 100% transparency when the concentration of TiO2 was lower than 20%. Meanwhile, increasing the concentration of TiO2 would lead to reduce the transparency of the coated substrate. Xu et al. recently prepared highly transparent superhydrophobic substrates by spin coating of fluorinated silica nanoparticles on silica wafer or other substrates . The spin coated substrates can produce over 95% of transparency with superhydrophobicity. Fluorine based silane precursors of low surface energy were used widely for changing the hydrophobic surface to superhydrophobic surface ,. This is due to the formation of thin layers of hydrophobic low surface energy materials on the pretreated hierarchical substrates. On the other hand, this might depend on the concentration of low surface energy material used for surface modification. Increasing the concentrations of fluoro silane to other precursor would enhance the surface properties. In another case, long chain alkyl or aromatic compounds were also used for surface treating of hydrophobic surfaces to produce superhydrophobic surface ,.
Recently we developed novel bio-inspired superhydrophobic hybrid micro-nanocomposites with mesoporous, highly stable to various drying temperatures, non-stick and self-cleaning properties on any substrate –. We used lotus leaf (LL) powder, polymethylhydroxysiloxane (PMHOS), and phenyl substituted silica ormosils (PSiOr) for the preparation of superhydrophobic hybrid micro-nanocomposites in ethanol/methanol/H2O. Meanwhile, the hybrid micro-nanocomposites suspension showed transparency lower than 70% on a glass substrate by drop or spin coating method . This is due to the presence of LL powder. Superhydrophobic PMHOS powder was synthesized from polymethylhydrosiloxane (PMHS) based on the reported procedure elsewhere . PMHS is a low molecular weight siloxane based material, which is used widely in many applications such as fabrication of superhydrophobic surface and materials, catalysis, transparent surface fabrications, actuators, and etc. ,,–. This is due to ease of availability and non-toxic properties of PMHS. The introduction of functional silica ormosil aerogel to the superhydrophobic powder would enhance the surface property of the material by the formation hierarchical particles in the suspension .
In this work, we discussed the fabrication of transparent, and almost complete superhydrophobic surfaces by the introduction of methyltriethoxysilane (MTES) or methyltrimethoxysilane (MTMS) to the PMHOS using methanol/H2O. The suspension that was prepared by mixing PMHOS and silica ormosil aerogel obtained from MTES or MTMS in the absence of LL powder in methanol/H2O showed enhanced transparency on the coated substrate than the substrate prepared in the presence of LL powder as well as in methanol/ethanol/H2O solvents .
2 2 Methods
2.1 2.1 Materials
Poly(methylhydrosiloxane) (PMHS, Mn ~1700 to 3200), methyltriethoxysilane (MTES, 99%), methyltrimethoxysilane (MTMS, 98%), oxalic acid (≥99%) and ammonium hydroxide solution (ACS reagent, 28 to 30% NH3 basis) were acquired from Sigma-Aldrich. Sodium hydroxide (NaOH) was supplied from Junsei Chemical Co. Ltd. Anhydrous methanol (special grade) was purchased by Carlo Erba reagents Co. Ltd. Double deionized water (surface tension, 72.8 mN/m) was used in all experiments.
2.2 2.2 Synthesis of methyl substituted silica ormosil (MeSiOr) aerogel
2.3 2.3 Preparation of superhydrophobic hybrid suspension
Preparation of PMHOS/silica ormosil aerogel hybrids of various ratios
Silica ormosil aerogel suspension (g)
2.4 2.4 Characterization
High-resolution scanning electron microscopy (HRSEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEM 2011 at 200 kV) were used to analyze the micro-nano surface morphology. Prior to the HRSEM measurement, the samples were coated with osmium tetraoxide (Hatfield, PA-19440), whereas, prior to the HRTEM measurement, the sample was dispersed well in ethanol and collected on a copper grid. The surface roughness of the spin coated substrates were predicted by atomic force microscopy (AFM) in tapping mode (Digital instruments nanoscope IIIa (USA) - Veeco metrology group) at a scanning rate of 1.49 Hz and on 5 μm data scales. The chemical compositions present in the samples were measured by X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific (U.K), Multi Lab) using the Al Kα (hν = 1486.6 eV) and Mg Kα (hν = (hν = 1253.6 eV) line in the binding energy analysis range of 0–600 eV in constant analyzer energy mode. The surface areas, pore volumes and pore diameters of the samples were obtained from Micromeritics ASAP 2020 V3.04 G, and calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analysis. Thermogravimetric analysis (TGA, Q50 V6.2, Build 187, TA instruments, U.S) was performed in a nitrogen atmosphere at a heating rate at 10°C/min. The transparency of the coated substrates were measured by UV-Visible spectrophotometer (Hitachi U-2010) in the spectral wavelength range of 400–800 nm, under the conditions of 200 nm/min scanning speed, 1 nm sampling interval, and 2 nm slit width. The surface wettability was measured by drop shape analysis system (DSA 100, Krüss GmbH, Germany) at room temperature (500- μL syringe, needle diameter and length (0.5 mm and 38 mm)).
3 3 Results and discussion
BET surface areas, pore volumes, and pore diameters of silica ormosil aerogels, PMHOS and PMHOS/silica ormosil aerogel hybrid
Surface area (m2g−1)(a)
Pore volume (cm3g−1)
Pore diameter (nm)(b)
4 4 Conclusions
In this work, we successfully fabricated transparent substrate with superhydrophobicity on a glass substrate using various ratios and types of silica ormosil aerogels and their hybrid materials. The surface of the materials showed the formation of hierarchical surface morphology with high roughness created on the surface by the introduction of silica ormosil aerogel to the polymethylhydroxysiloxane. The synthesized silica ormosil aerogel and hybrid materials showed disordered meso structures. However, the surface areas, pore volumes and pore diameters of the silica ormosil aerogel were decreased by increasing the volume of methanol. Similarly, the transparency of the spin coated substrates also decreased partially by aggregation but with maintaining reasonable transparency. On the other hand, the thermal stability was increased by increasing the volume percentages of methanol to the silane precursors. The addition of silica ormosil aerogel to the PMHOS showed the enhanced the surface properties as well as the thermal stability of the hybrid material. The obtained results exhibit the important contribution of silica ormosil aerogels to the PMHOS for the enhancement of various properties of the hybrid material in a co-solvent by comparing with the materials obtained from ethanol/methanol/water or ethanol/DMAc/water solvents. On the other hand, the stability of the hybrid suspensions is not so good over a period of time under rest. The main reason is due to the deposition of the hybrid particles in the solvents in a few hours. Meanwhile, superhydrophobic surface can be fabricated easily at any time by simple shaking of the suspension for a few seconds followed by coating on a glass substrate.
This study was supported by the National Research Foundation of Korea (NRF) Grant funded by The Ministry of Science, ICT & Future Planning, Korea (Pioneer Research Center Program (2010-0019308/2010-0019482), NRF-RFBR Joint Research Program (No. 2013K2A1A7076267), and Brain Korea 21 Plus Program (21A2013800002)).
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