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Applied Surface Science 254 (2008) 2158–2161

Stable superhydrophobic surfaces over a wide pH range

Li Guo a,b, Wenfang Yuan a,b, Junping Li a,b, Zhijie Zhang a,*, Zemin Xie a

a Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of Chinab Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

Received 23 July 2007; received in revised form 31 August 2007; accepted 31 August 2007

Available online 6 September 2007

Abstract

A stable superhydrophobic surface was fabricated by solidifying poly(epoxy-terminated polydimethylsiloxane-co-bisphenol A) [P(ETPDMS-

co-BPA)] copolymer on a rough substrate. The low surface energy of the copolymer and the geometric structure at micrometer scale of the surface

contribute to the superhydrophobic property. The as-prepared surface shows stable superhydrophobicity over a wide pH range (1–14) and the

wettability is excellent stable to heating, water, corrosive solution and organic solvent treatments. The procedure is simple and time-saving as well

as utilizing non-fluorine-containing compounds.

# 2007 Elsevier B.V. All rights reserved.

PACS : 68.08.Bc; 68.35.Ct; 81.05.Lg

Keywords: Superhydrophobic; Stable; Epoxy-terminated polydimethylsiloxane; Bisphenol A

1. Introduction

Wettability is an important property of solid surfaces, which

is affected by two factors, i.e., the chemical composition and

geometrical microstructure [1,2]. Superhydrophobic surfaces

with a water contact angle (CA) greater than 1508 can be

obtained by combining surface roughness with low surface

energy and have attracted much attention for both fundamental

research and practical applications [3–17], such as dust-free

coatings, covering to resist water, frog condensation and snow

sticking [18–20]. Recently, stable superhydrophobic surfaces

that can be used over a wide pH range have aroused great

interest for their resistance to corrosive liquids [21–26].

However, high cost, complex process and harsh chemical

treatment have limited their practical applications.

Herein reported is a stable superhydrophobic surface over

the entire pH range, which was fabricated by a facile method,

coating a poly(epoxy-terminated polydimethylsiloxane-co-

bisphenol A) [P(ETPDMS-co-BPA)] copolymer thin film on

a roughly etched silicon substrate and solidifying at ambient

atmosphere. The as-prepared surface kept its superhydropho-

* Corresponding author. Tel.: +86 10 62554494; fax: +86 10 62554494.

E-mail address: zhangzj@iccas.ac.cn (Z. Zhang).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.08.089

bicity even after heating at 250 8C in air for 12 h. Such surfaces

are also stable to water, corrosive liquids (acidic and basic

solutions) and organic solvents.

2. Experimental

2.1. Materials

1,1,3,3-Tetramethyldisiloxane (TMDS) and octamethylcy-

clotetrasiloxane (D4) were purchased from Chenguang Second

Chemical Factory. Allyl glycidyl ether (AGE) was purchased

from Jintan Eastchina Coupling Agent Factory. TMDS and

AGE were purified by distillation. D4 was purified by

fractionation. All the other reagents were all analytical grade

and used as received. Rough silicon substrate with geometrical

structures of patterned square pillars (20 mm high, 10 mm long

and with a spacing of 15 mm between the silicon pillars) was

made of flat silicon wafer by photolithography and an

inductively coupled plasma deep-etching technique [27].

2.2. Measurements

Nuclear magnetic resonance (NMR) spectra were recorded

in CDCl3 solutions with a Bruker WM 300 spectrometer. The

microstructures of the surfaces were characterized on a Hitachi

L. Guo et al. / Applied Surface Science 254 (2008) 2158–2161 2159

S-4300 scanning electron microscope (SEM). Contact angles

were measured on an OCA20 contact-angle system at ambient

temperature. The transmittance of visible light was evaluated

by Shimadzu UV-1601PC.

2.3. Synthesis and characterization of P(ETPDMS-co-

BPA)

The synthesis route of P(ETPDMS-co-BPA) is shown in

Scheme 1.

To stirred 0.25 mol TMDS and 0.25 mol D4 was added

2.5 mL sulfuric acid and stirred at room temperature. After 6 h,

10 mL water was added and stirred for about 12 h. Raw product

was rinsed by water until it became neutral and dried with

anhydrous magnesium sulfate. After filtration, the filtrate was

vacuumed at 80 8C and gave pure a,v-dihydrogen poly-

dimethylsiloxane. 1H NMR (d, ppm): 0.08 [m, OSi(CH3)2O],

0.19 [d, OSi(CH3)2H], 4.71 (m, SiH).

0.2 mol a,v-Dihydrogen polydimethylsiloxane was slowly

added at 90 8C to 0.5 mol AGE containing a catalytic amount of

H2PtCl6. After addition completed, the mixture was then stirred

for 10 h at 100 8C. The mixture was treated with activated

charcoal to remove colored impurities and the catalyst, and then

filtered to get epoxy-terminated polydimethylsiloxane (ETPD-

MS). 1H NMR (d, ppm): 0.02–0.09 (m, SiCH3), 0.54 (m,

SiCH2), 1.61 (m, SiCH2CH2), 2.61 and 2.79 (m, –CHCH2O–),

3.14 (m, –CHCH2O–), 3.37–3.48 (m, CH2OCH2CH) 3.68 (d,

CH2OCH2CH).

Into a 50 mL round bottom flask equipped with a magnetic

stirrer was charged 5 mmol ETPDMS, 5 mmol bisphenol A

(BPA) and triphenylphosphine (1 wt.% relative to the total

monomers). The mixture was stirred at 110 8C for 7 h to yield

P(ETPDMS-co-BPA) copolymer. 1H NMR (d, ppm): 0.05–0.11

(SiCH3), 0.52 (m, SiCH2), 1.62 [m, SiCH2CH2, Ar2C(CH3)2],

2.62 (s, OH), 3.45 (m, CH2OCH2CH), 3.59 (m, CH2OCH2CH),

4.00 (d, ArOCH2), 4.14 (m, CHOH), 6.80 and 7.11 (d, ArH).

Scheme 1. Synthesis route of P(E

2.4. Fabrication of coatings

The prepared P(ETPDMS-co-BPA) copolymer was dis-

solved in tetrahydrofuran (THF) at a concentration of 10 mg/

mL and mixed with curing agent (1,6-diisocyanatohexane,

200 mol% relative to copolymer). This solution was cast and

coated onto clean glass slides or roughly etched silicon

substrates, and then dried and solidified naturally in an ambient

atmosphere.

3. Results and discussion

The SEM images of the P(ETPDMS-co-BPA) films on a flat

glass slide and on a rough silicon substrate are shown in Fig. 1a

and b, respectively. Compared with Fig. 1a, which shows the

film on flat glass slide, Fig. 1b shows a typical SEM image of

the film on a rough silicon substrate. The images show that the

films on glass slide and silicon pillars are both flat without

obvious complex geometrical nanostructures. Fig. 1c and d

show the shapes of water droplets on flat and rough surfaces,

respectively. The as-prepared P(ETPDMS-co-BPA) film on flat

substrate has a water CA of 105.1 � 0.98. On rough substrate,

the film shows superhydrophobic property with high water CA

of 152.2 � 1.18 and low CA hysteresis of approximately 108(with advancing and receding CAs of 153.2 � 0.38 and

142.9 � 0.68, respectively).

A theoretical consideration is necessary to explain the

superhydrophobicity of the resulting P(ETPDMS-co-BPA) film

coated rough surface. The relationship between the CA of water

on rough surface (ur) and that on corresponding flat surface (u)

is described by Cassie equation [2]

cos ur ¼ f 1 cos u � f 2

where f1 and f2 are the fractions of the liquid–solid and liquid–

air contact area, respectively (i.e., f1 + f2 = 1). It is easy to

deduce that ur, the CA on rough surface, enhances with the

TPDMS-co-BPA) copolymer.

Fig. 1. (a and b) Typical SEM images of P(ETPDMS-co-BPA) film on flat and rough substrates. (c and d) The shapes of water droplets on flat and rough surfaces.

L. Guo et al. / Applied Surface Science 254 (2008) 2158–21612160

increase of f2, the fraction of air. From the equation, the value of

f2 in our experiment is calculated to be 0.844, which means that

the trapped air within the microstructures plays a very impor-

tant role in the superhydrophobicity.

As indicated, the copolymer used to fabricate the surfaces

consists of both a thermally stable and hydrophobic component

(polydimethylsiloxane) and a corrosion- and organics-repellent

component (bisphenol A), which make the surfaces have many

excellent properties.

Fig. 2 shows the relationship between pH value and CA on

the superhydrophobic surface. There is no obvious fluctuation

of the CA values within the experimental errors over a pH range

from 1 to 14. All CA values are in the range from about 150.28to 152.48, indicating that pH values of the aqueous solution

have little or no effect on CAs for as-prepared surface. This

phenomenon indicates that such surface is superhydrophobic

for not only pure water but also corrosive liquids, such as acidic

Fig. 2. The relationship between pH value and CA on the superhydrophobic

surface of P(ETPDMS-co-BPA) film on rough substrate.

and basic aqueous solutions, thus can be used in all pH

environments (in the pH range from 1 to 14). To the best of our

knowledge, the superhydrophobic surfaces that could be used

over a wide pH range have been reported only in a few studies.

Jiang and co-workers reported that the nanostructured carbon

films prepared from aligned polyacrylonitrile nanofibers show

superhydrophobic in the pH range from 1.07 to 13.76 [21], and

then got the polyaniline/polystyrene composite film, through

electrospinning method, with superhydrophobicity and con-

ductivity over a wide pH range conditions [22]. Guo et al.

reported the fabrication of superhydrophobic surfaces by

covering NaOH solution treated copper, aluminum and its alloy

with perfluorononane or vinyl-terminated polydimethylsilox-

ane [23,24]. Wang et al. fabricated the superhydrophobic

surfaces suitable for all pH environment by curing the mixture

of 2,2-bis(3,4-dihydro-3-methyl-2H-1,3-benzoxazine) propane

(BA-m benzoxazine) and nanoparticles (22 nm precipitated

hydrated silica) on glass slide, followed by modification with

BA-m benzoxazine [25]. Qu et al. reported a superhydrophobic

surface to the whole pH solution droplets prepared by etching

steel, Cu alloy and Ti alloy with HNO3-H2O2 or HF-H2O2

solutions and then covered with fluoroalkylsilane [26].

Compared with above reported methods to fabricate super-

hydrophobic surfaces, the method herein employed is simple

and time-saving, without using fluorine-containing compounds,

and can be used in all pH environments for corrosive liquids.

Further studies indicated that the as-prepared P(ETPDMS-

co-BPA) film coated surfaces had other significant properties.

When the superhydrophobic surfaces were heated in air for 12 h

at 200 and 250 8C, respectively, the surfaces remained

superhydrophobic and the CAs of water droplets stayed

essentially intact when compared with those for unheated. The

P(ETPDMS-co-BPA) films were immersed in water, ethanol,

hexane and ethyl acetate for 12 h, as well as in 10 wt.% H2SO4

solution and 10 wt.% NaOH solution for several hours, and

Fig. 3. Effect of heating on the optical transmittance of P(ETPDMS-co-BPA)

film on glass slides: (A) without heating; (B) heating at 200 8C in air for 12 h;

(C) heating at 250 8C in air for 12 h.

L. Guo et al. / Applied Surface Science 254 (2008) 2158–2161 2161

found essentially no change in water contact angle. As shown in

Fig. 3, the films also had excellent transparence and remained

transparent even after heating at 250 8C in air for 12 h.

4. Conclusions

In conclusion, we have fabricated a stable superhydrophobic

surface over a wide pH range through a simple, time-saving

method without using fluorine-containing compounds. The

superhydrophobicity can be attributed to both the low surface

energy of the copolymer and the microstructure of the surface.

The as-prepared surface shows superhydrophobic characteristic

over the entire pH range, that is, not only for pure water but also

for corrosive liquids such as acidic and basic solutions. The

superhydrophobic surface also possesses marvelous environ-

mental stability to thermal, water, corrosive solution and

organic solvent treatments in terms of water CA. The

P(ETPDMS-co-BPA) film shows excellent transparent even

after high temperature heating treatment in air. Therefore, our

results are very important for interface science research and

applications.

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