# Ultrahydrophobicity of Polydimethylsiloxanes-Based Multilayered Thin Films

Authors: Hongyan Gao; Karen Xiaohe Xu; Bin Chen; Li-Zhu Wu; Chen-Ho Tung; Hai-Feng Ji
Journal: Journal of Nanotechnology
 (2009)

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## Abstract

The formation of polydimethylsiloxanes (PDMSs)-based layer-by-layer multilayer ultrathin films on charged surfaces prepared from water and phosphate buffer solutions has been investigated. The multilayer films prepared under these conditions showed different surface roughness. Nanoscale islands and network structures were observed homogeneously on the multilayer film prepared from pure water solutions, which is attributing to the ultrahydrobic property of the multilayer film. The formation of nanoscale islands and network structures was due to the aggregation of PDMS-based polyelectrolytes in water. This work provides a facile approach for generating ultrahydrophobic thin films on any charged surfaces by PDMS polyelectrolytes.


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## Body

## 1. Introduction

Ultrahydrophobic  surfaces are now in
demand of many industrial  processes  and of interests to biological  researches
[1, 2]. The
ultrahydrophobic surface allows the drops of water to bead up on the surface
and freely roll off the surface. Water contact angle measurement is commonly
used to characterize surface hydrophobicity. Water contact angles greater than 150° have been achieved on different ultrahydrophobic surfaces
[3–6].In general, the two requirements [3]
for ultrahydrophobic surfaces are the hydrophobic characteristics of the
materials and the presence of highly porous network textures or nano- to
microscale projection areas on the surfaces. The hydrophobic nano/microscale projections
create a rough surface texture that sufficiently lowers the surface tension to
reach the ultrahydrophobicity. Our results in this work were developed based on
this hypothesis. However, it should be noted that hydrophilic materials could
also form ultraphydrophobic surface
[7, 8] and the causes
of ultrahydrophobic phenomenon are still under extensive investigation.The microscale projections have been delicately
micromachined
[4–6] using
photolithographic methods or deposition
[9–13], and the highly
porous textures
[3, 14–19] could be introduced
by evaporation, acidification, photochemistry process, and so forth. Most of
the hydrophobic materials used for developing ultrahydrophobic surfaces were
perfluorocarbons because of their highly hydrophobic nature. Several other
hydrophobic materials for creating ultrahydrophobic surfaces include
polypropylene, [3] polyethylene, [20], alkylketene dimmer [21], and so forth.Polydimethylsiloxanes (PDMSs) are industrially important
materials that have been used for adhesives, sealants, lubricants, as well as in defoaming agents, damping fluids, heat transfer fluids,
cosmetics, and other applications [22]. Besides its characteristically viscoelastic property, PDMS is also known as a hydrophobic material. Ultrahydrophobicity has been
achieved on a microfabricated PDMS surfaces featured with 5-micron-wide pillars
[23]. In
this paper, we report a facile approach to generate an ultrahydrophobic surface
by layer-by-layer (LbL) self-assembling a PDMS-based multilayer thin film on
virtually any surfaces. LbL method [24] allows formation of ultrathin organized
films on almost any metal or nonmetal surfaces through alternate adsorption of
oppositely charged components, such as linear polycations and polyanions. The
oppositely charged species are held together by strong ionic bonds and form
long-lasting, uniform, and stable films that are often impervious to a
solvent.Recently, LbL self-assembly technique has been applied to develop
ultrahydrophobic films
[25–28]. In these
ultrahydrophobic films, pre- or posttreatments were conducted to introduce
micro- and nanoscale textures. In this paper, the PDMS thin film was prepared by
using a simple LbL process based on PDMS polyelectrolytes without pre- or
posttreatments. The surface characteristics of these PDMS-based multilayer thin
films prepared from pure water and buffer solutions were investigated.


## 2. Experimental

### 2.1. Materials

Poly[dimethylsiloxane-co-methyl(3-hydroxy-propyl)siloxane]graft-poly(ethyleneglycol)
[(3-trimethylammonia)propylchloride]ether (PDMS-1, MW~4000), and poly (dimethylsiloxane)-graft-polyacrylates
(PDMS-2, MW 26,000), poly(ethylenimine) (PEI, MW 70,000), sodium
poly(styrenesulfonate) (PSS, MW 70,000), 11-mercapto- undecanoic acid, and aminopropyl triethoxysilane  (ATS) were used as
obtained from (Sigma-Aldrich Co. Milwaukee, USA). The structures of the
PDMS-containing polycationic PDMS-1 and polyanionic PDMS-2 are shown in Schemes1 and 2.Scheme 1
PDMS-1.Scheme 2
PDMS-2.


### 2.2. Instrument

The LbL assembly process was measured by quartz crystal
microbalance technique (QCM, Agilent 53131A 225 MHz) and water contact angle
instrument. In a QCM measurement, the resonator was immersed in a
polyelectrolyte  solution for 20 minutes and dried, and the frequency change was
measured in air. The long-term stability (several hours) of the quartz
resonator frequency was within ±2 Hz. All experiments were carried out in an air-conditioned room at a
temperature of ca.20°C. The
resonators were covered by evaporated silver electrodes (0.16 cm2)
on both surfaces, and their resonance frequency was 9 MHz (AT-cut). The contact angle measurements were
taken by an OCA15 contact angle
meter (Future Digital Scientific, Long
Island, USA).
The OCA software provided a preset
ellipse fitting for accurate contact angle calculation after the image was
recorded. Atomic force microscope measurements were conducted using a tapping mode
of a Quesant AFM (Model-Q-Scope250). Scanning
electron micrograph (SEM) and energy dispersive spectroscopy (EDS) measurements
were conducted on a Hitachi S4800 SEM. The dynamic light scattering (DLS)
results were measured with an automated light scattering system (Precision
Detectors ALS4000, Bellingham, Mass, USA). Attenuated infrared (ATR) spectra
were recorded by using a Varian Nexus 470 ATR spectrometer.


### 2.3. LbL Process

The LbL procedure used for surface
modification in these experiments was as follows. (A) The concentrations of the
polyelectrolytes  used in these  experiments were 3 mg/mL PEI, 3 mg/mL PSS, 1 mg/mL PDMS-1, and 1 mg/mL PDMS-2. Polyelectrolytes, except for PDMS-2 solutions were directly prepared
in pure water or various phosphates buffer solutions (pH 7.0). The PDMS-2 was firstly
dissolved in acetone at 10 mg/mL as a stock solution and then diluted 10 folds
in water or buffer solutions. (B) A substrate carrying a surface charge was
alternately immersed in the oppositely charged  polyelectrolyte solutions. Each
immersion took 20 minutes with 1 minute intermediate water washing. (C) This
cycle was repeated several times until the desired number
of multilayer films was reached. The assembled multilayer films prepared both from water and
buffer solutions were investigated to compare the uniformity and surface characteristics.


## 2.1. Materials

Poly[dimethylsiloxane-co-methyl(3-hydroxy-propyl)siloxane]graft-poly(ethyleneglycol)
[(3-trimethylammonia)propylchloride]ether (PDMS-1, MW~4000), and poly (dimethylsiloxane)-graft-polyacrylates
(PDMS-2, MW 26,000), poly(ethylenimine) (PEI, MW 70,000), sodium
poly(styrenesulfonate) (PSS, MW 70,000), 11-mercapto- undecanoic acid, and aminopropyl triethoxysilane  (ATS) were used as
obtained from (Sigma-Aldrich Co. Milwaukee, USA). The structures of the
PDMS-containing polycationic PDMS-1 and polyanionic PDMS-2 are shown in Schemes1 and 2.Scheme 1
PDMS-1.Scheme 2
PDMS-2.


## 2.2. Instrument

The LbL assembly process was measured by quartz crystal
microbalance technique (QCM, Agilent 53131A 225 MHz) and water contact angle
instrument. In a QCM measurement, the resonator was immersed in a
polyelectrolyte  solution for 20 minutes and dried, and the frequency change was
measured in air. The long-term stability (several hours) of the quartz
resonator frequency was within ±2 Hz. All experiments were carried out in an air-conditioned room at a
temperature of ca.20°C. The
resonators were covered by evaporated silver electrodes (0.16 cm2)
on both surfaces, and their resonance frequency was 9 MHz (AT-cut). The contact angle measurements were
taken by an OCA15 contact angle
meter (Future Digital Scientific, Long
Island, USA).
The OCA software provided a preset
ellipse fitting for accurate contact angle calculation after the image was
recorded. Atomic force microscope measurements were conducted using a tapping mode
of a Quesant AFM (Model-Q-Scope250). Scanning
electron micrograph (SEM) and energy dispersive spectroscopy (EDS) measurements
were conducted on a Hitachi S4800 SEM. The dynamic light scattering (DLS)
results were measured with an automated light scattering system (Precision
Detectors ALS4000, Bellingham, Mass, USA). Attenuated infrared (ATR) spectra
were recorded by using a Varian Nexus 470 ATR spectrometer.


## 2.3. LbL Process

The LbL procedure used for surface
modification in these experiments was as follows. (A) The concentrations of the
polyelectrolytes  used in these  experiments were 3 mg/mL PEI, 3 mg/mL PSS, 1 mg/mL PDMS-1, and 1 mg/mL PDMS-2. Polyelectrolytes, except for PDMS-2 solutions were directly prepared
in pure water or various phosphates buffer solutions (pH 7.0). The PDMS-2 was firstly
dissolved in acetone at 10 mg/mL as a stock solution and then diluted 10 folds
in water or buffer solutions. (B) A substrate carrying a surface charge was
alternately immersed in the oppositely charged  polyelectrolyte solutions. Each
immersion took 20 minutes with 1 minute intermediate water washing. (C) This
cycle was repeated several times until the desired number
of multilayer films was reached. The assembled multilayer films prepared both from water and
buffer solutions were investigated to compare the uniformity and surface characteristics.


## 3. Results and Discussions

Silver electrodes have a naturally
negative charge  on their surfaces. 11-Mercapto-undecanoic acid and ATS were used to develop a charged monolayer  film on the gold
and silicon (or glass) surfaces, respectively, using typical surface
modification procedures [29, 30]. These
monolayer films introduced a layer of charges on the gold or silicon surfaces
that could strengthen the stability of the consequent LbL assembly.In the LbL assembly process, three
bilayers of PEI/PSS were firstly formed on the surfaces that
provided a solid base for further immobilization. Multiple bilayers of PDMS-1/PDMS-2 were formed on the top of these layers.The results showed that the
PDMS-1/PDMS-2 multilayer films on Ag, charged Au and SiO2 surfaces
had
virtually the same properties when they were prepared under the same
conditions. However, the films prepared from water solutions showed
significantly different characteristics from those prepared from buffer
solutions, especially those from higher concentration of buffer solutions.The QCM experiments showed the formation of the
PDMS-1/PDMS-2 multilayer film on the silver-covered quartz crystal resonators
(Figure1). The frequency decreases of the resonators as a function of
multilayer numbers were nearly linear for the multilayer films prepared from both
water solution and a 0.025 M phosphate buffer solution. The frequency change,
however, varied under these  two conditions. The deposition of each polyion bilayer
in water and in the 0.025 M buffer solutions resulted in approximately 1.6 kHz and
7 kHz frequency decrease (ΔF), respectively. These were
corresponding to  1.44×10−6 g
and  6.30×10−6 g of mass
increase on the resonator surfaces for the films from water and from buffer solutions,
respectively, after each polyion bilayer deposition according to the following equation
[31]: (1)ΔF=−1.832×108M/A, where A is the surface area of
the resonator (in the unit of cm2), M is the adsorbed mass, and ΔF is the frequency shift of the quartz
resonator.Figure 1
QCM resonator frequency shifts upon deposition of each
layer of PDMS-1 (odd numbers) and PDMS-2 (even numbers) prepared
from water and buffer solutions.The EDS result (Figure2) of a 12 PDMS-1/PDMS-2 bilayer
film on a gold surface verified the formation of the PDMS film. The film
contains 13.2% of Si, 61.3% of C, and 13.2% of O atoms.Figure 2
Electron disperse spectrum (EDS) of a 12 PDMS-1/PDMS-2
bilayered thin film on a gold surface.Figure3 shows ATR spectra for PDMS-1 deposition, PDMS-2 deposition, and the 12 PDMS-1/PDMS-2
bilayer film on gold surfaces. All these figures showed four characteristic PDMS peaks: the bands at 1260 cm−1 (Si−CH3 symmetric bend), 1100 cm−1 (Si−O−Si symmetric stretch), 1060 cm−1 (Si−O−Si asymmetric stretch), and 800 cm−1 (CH3 rocking)
[32]. PDMS-2 also showed a characteristic sharp peak
for carbonyl group at 1740 cm−1. The ATR spectrum of the multilayer
film was an addition of spectra of PDMS-1 and PDMS-2, confirming the formation
of PDMS-1/PDMS-2 bilayer composites.Figure 3
ATR spectra for PDMS-1 deposition, PDMS-2 deposition, and the 12
PDMS-1/PDMS-2 bilayer film on gold surfaces.The static water contact angle experiments
not only confirmed the formation of the PDMS multilayer films, but also
demonstrated the ultrahydrophobicity of multilayer surfaces prepared under
controlled conditions. For the multilayer films prepared from 0.025 M buffer
solutions, the contact angle change showed a regular pattern (Figure4(a)). The
contact angles, obtained when the PDMS-2 was at the outmost layer (105±5°),
were approximately 20° larger than those obtained when PDMS-1 was at
the outmost layer (85±5°). The 20° contact angle
difference may attribute to the polarity difference between PDMS-1 and PDMS-2.
There were approximately 80 wt.% of the polar polyethylene glycol component in
PDMS-1, while only 20 wt.% of polar polyacrylates  component in PDMS-2. The contact angles of water droplets on a PDMS multilayer film prepared from
0.005 M buffer  solutions (Figure 4(b)) were similar to those obtained from the
0.025 M buffer solutions. The contact angles of water droplets on a PDMS
multilayer film  prepared from 0.001 M buffer solutions showed an increase after
10 layers (Figure 4(c)). For the multilayer films prepared from the water solutions,
the contact angles of water droplets on the first three-bilayered films (Figure
4(d)) showed similar pattern as those in Figure 4(a). After three bilayers, the
contact angle gradually increased. The static contact angles reached a plateau
at approximately 160° after 8 bilayers as shown in Figure 4(d), and the
water droplets bead up on the surface and could freely roll off the surface without
becoming pinned (Figure 5). The contact angles remained at 160° after 8
bilayers when either the PDMS-1 or the PDMS-2 was at the outmost layer. The
ultrahydrophobic surface is very stable and the water droplet can sit on the
surface for hours (before all evaporated) with low contact angle hysteresis
[33], indicating ultrahydrophobic characteristics of this PDMS thin
film. The ultrahydrophobic surface is stable and resistant to strong acids, bases,
and organic solvents.
The contact angles were the same at approximate 160° after one-hour
immersing either in a 1 N HCl or a 1 N NaOH. Similar results were observed
after exposing the film to organic solvents, including ether, THF, acetone, CH2Cl2,
benzene, and hexane. However, the film can be readily damaged by mechanical
forces, such as scratches.Static contact angles of water droplets on the alternately
deposited PDMS-1/PDMS-2 (old numbers for PDMS-2 and even numbers for PDMS-1)
multilayer film after each layer of deposition prepared from 0.025 M phosphate buffer (a),
0.005 M phosphate buffer (b), 0.001 M phosphate buffer (c),
and water solution (d).
(a)(b)(c)(d)Figure 5
Image of a water droplet on a 12 PDMS-1/PDMS-2 bilayered thin film prepared
from water solutions. The contact angle
is 159.4°.These contact angle differences
and the ultrahydrophobic phenomenon could be explained by the surface roughness
as shown from   AFM and SEM images in Figures6,  7, and 8. For the
multilayer film prepared from water solutions, SEM images showed the appearance
of nanoparticles on the surfaces after two PDMS-1/PDMS-2 bilayers (Figure 6(a));
these particles accumulated and connected after six PDMS-1/PDMS-2 bilayers
(Figure 6(b)). Islands and network structures at nanometer-scale were formed
after 12 PDMS-1/PDMS-2 bilayers (Figures 6(c)-6(d)). The depth of the cavities
on the 16 PDMS-1/PDMS-2 bilayered films ranged from 200 to 500 nm (Figure 7). The
nanometer-scale islands, with diameters at approximately 100 nm, were widely distributed
on the surfaces. In comparison, a 12 PDMS-1/PDMS-2 bilayered film prepared
from the 0.025 M buffer solutions was rather smooth with a 20 nm roughness
(Figure 8). The root-mean-square average surface roughness data from each AFM
measurement of 16 PDMS-1/PDMS-2 films prepared from 0.025, 0.005, 0.001 M PBS,
and water were 20±2, 24±2, 106±8, and 180±25 nm, respectively.Scanning Electron Micrographs of
(a) 2, (b) 6, (c) 12, and (d) 16 PDMS-1/PDMS-2 bilayered film prepared
from water solutions.
(a)(b)(c)(d)Figure 7
An AFM image of a 16 PDMS-1/PDMS-2 bilayered film prepared
from water solutions.Z scale was in nm.(a) AFM and (b) SEM images of a 12 PDMS-1/PDMS-2 bilayered film prepared
from a 0.025 M buffer solutions.Z scale was in nm.
(a)(b)After
rinsing the multilayer  film samples from water, no salts were observed on the
surfaces using EDS measurements. The formation of
nanometer-scale islands and network structures on the
surfaces could be explained by the aggregation of PDMS-1 and PDMS-2 in pure
water measured from dynamic light scattering experiments. The average particle
sizes of PDMS-1 and PDMS-2 in pure water were156±50 nm and 60±40 nm in
diameter, respectively, which could also be supported from the SEM images as
shown in Figure 6(d). By comparison, in the
0.025 M buffer solutions, the average particle sizes of PDMS-1 and PDMS-2 were
40±20 nm and 15±10 nm, in diameter, respectively, suggesting much less
aggregation than those in buffer solutions, which may be due to less salvation
energy of polyelectrolytes in high ionic strength solutions [34].The ultrahydrophobic film is resistant to acids and bases and the
ultrahydrophobicity remains after half a year in ambient conditions. However, heat
will convert the film to a uniform and smooth surface (Figure9), which loses
its ultrahydrophobic characteristics. The contact angle of this film after heat
was 102±4°.Figure 9
An AFM image of a 16 PDMS-1/PDMS-2 bilayered film prepared from water
solutions after a 2-hour heating at180°C.


## 4. Conclusions

In summary, this work demonstrates  the
formation of polydimethysiloxanes (PDMSs) based layer-by-layer multilayer thin
films on charged surfaces. The multilayer films prepared from water and phosphate
buffer solutions showed different surface characteristics. Ultrahydrobic phenomenon
was observed on the multilayered thin films prepared from water solutions due
to the formation of nanostructured network surface. Dynamic light scattering experiments
showed that both PDMS-1 and PDMS-2 aggregated in pure water, which could be
attributed to the formation of nanometer-scale
islands and network structures on
the surfaces. This work provides a
facile approach to develop an ultrahydrophobic film on possibly any charged
surfaces without pre- or posttreatments. The ultrahydrophobic surface is stable
and resistant to acids and bases, but will be readily damaged by heat and
mechanical forces.



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*Source: 709748-2009-02-03.xml*