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Micromachines 2015, 6, 855-864; doi:10.3390/mi6070855
micromachines
ISSN 2072-666X
www.mdpi.com/journal/micromachines
Article
Effects of Fumed and Mesoporous Silica Nanoparticles on the
Properties of Sylgard 184 Polydimethylsiloxane
Junshan Liu
1,2,
*, Guoge Zong
1
, Licheng He
1
, Yangyang Zhang
1
, Chong Liu
1
and Liding Wang
1
1
Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University
of Technology, Dalian 116024, China; E-Mails: zongguoge@163.com (G.Z.);
helicheng868@163.com (L.H.); 15104504983@163.com (Y.Z); chongl@dlut.edu.cn (C.L.);
wangld@dlut.edu.cn (L.W.)
2
Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education,
Dalian University of Technology, Dalian 116024, China
* Author to whom correspondence should be addressed; E-Mail: liujs@dlut.edu.cn;
Tel.: +86-411-8470-7713 (ext. 2171); Fax: +86-411-8470-7940.
Academic Editor: Hongrui Jiang
Received: 21 April 2015 / Accepted: 2 July 2015 / Published: 8 July 2015
Abstract: The effects of silica nanoparticles on the properties of a commonly used Sylgard
184 polydimethylsiloxane (PDMS) in microfluidics were systemically studied. Two kinds
of silica nanoparticles, A380 fumed silica nanoparticles and MCM-41 mesoporous silica
nanoparticles, were individually doped into PDMS, and the properties of PDMS with these
two different silica nanoparticles were separately tested and compared. The thermal and
mechanical stabilities of PDMS were significantly enhanced, and the swelling
characteristics were also improved by doping these two kinds of nanoparticles. However,
the transparency of PDMS was decreased due to the light scattering by nanoparticles. By
contrast, PDMS/MCM-41 nanocomposites showed a lower coefficient of thermal
expansion (CTE) owing to the mesoporous structure of MCM-41 nanoparticles, while
PDMS/A380 nanocomposites showed a larger elastic modulus and better transparency due
to the smaller size of A380 nanoparticles. In addition, A380 and MCM-41 nanoparticles
had the similar effects on the swelling characteristics of PDMS. The swelling ratio of PDMS
in toluene was decreased to 0.68 when the concentration of nanoparticles was 10 wt %.
Keywords: polydimethylsiloxane; PDMS; Sylgard 184; silica; nanoparticles
OPEN ACCESS
Micromachines 2015, 6 856
1. Introduction
Polydimethylsiloxane (PDMS) has been widely used in microfluidics due to its flexibility,
biocompatibility, transparency, low cost and ease of fabrication [1–7]. However, PDMS has a high
coefficient of thermal expansion (CTE) and a low elastic modulus, which makes its poor thermal and
mechanical stabilities and hinders many practical applications. For example, the metal microelectrode
has become one of the essential compositions of microfluidic chips. However, wrinkles or even cracks
were caused in metal films deposited on PDMS substrates owing to poor stabilities of PDMS [7–9]. In
addition to stabilities, swelling of PDMS is another important issue [10–12]. The swelling ratios of
PDMS are very large in common organic solvents, such as a swelling ratio of 1.15 in toluene [10].
Hence, it is difficult to perform experiments in organic media for PDMS microfluidic chips.
To improve the stabilities of PDMS, many kinds of nano materials, such as carbon nanotubes
(CNTs) [13–15], silica nanoparticles [16–18], and graphene [19] have been doped into PDMS. Wu et al.
reported that the elastic modulus of PDMS was increased to 2.34 MPa by adding carbon nanotubes [13].
Camenzind et al. reported that the elastic modulus of PDMS was increased to 3.18 MPa by admixing
fumed silica nanoparticles [16]. Chen et al. reported a method for embedding CNT sheets in PDMS,
and the CTE of PDMS was reduced to 6 ppm/°C in the direction parallel to CNT alignment [15].
Yamauchi’s group has investigated thermal, mechanical, optical and swelling properties of PDMS/silica
nanocomposites [17,18]. The mechanical and thermal stabilities were effectively enhanced, and the
swelling characteristics of PDMS were also improved. However, in their studies, a hard PDMS
(X-32-3095, Shin-Etsu Chemical Co., Ltd, Tokyo, Japan), which does not require a curing agent, was used.
In this paper, we systematically studied the effects of silica nanoparticles on the properties of the
most commonly used PDMS in microfluidics, Sylgard 184 (Dow Corning Corporation, Midland, MI,
USA) [3,5,10,11,20]. The thermal and mechanical stabilities, swelling characteristics and transparency
were all examined. Two kinds of silica nanoparticles, fumed silica nanoparticles and mesoporous silica
nanoparticles were doped and compared.
2. Experimental Section
2.1. Materials
A380 fumed silica nanoparticles were purchased from Evonik Degussa (Essen, Germany), and
MCM-41 mesoporous silica nanoparticles were provided by Nanjing XFNANO Materials (Nanjing,
China). These two types of nanoparticles are abbreviated henceforward as A380 and MCM-41,
respectively. Specific characteristics of these two types of silica nanoparticles are listed in Table 1.
According to the density of silica (2.2 g/cm
3
) and the particle size and pore volume of silica
nanoparticles listed in Table 1, the mass of a single nanoparticle was approximately calculated. A
single A380 nanoparticle was about 4 × 10
−19
g, and a single MCM-41 nanoparticle was about
1 × 10
−14
g.
PDMS (Sylgard 184) consisted of a base and a curing agent was purchased from Dow
Corning Corporation.
Micromachines 2015, 6 857
Table 1. Specific characteristics of silica nanoparticles (SSA, specific surface area).
The Type
SSA (m
2
/g)
Particle size (nm)
Pore diameter (nm)
Pore volume (cm
3
/g)
A380
~380
~7
-
-
MCM-41
~800
~300
3.5–4
0.8–0.9
2.2. Fabrication of PDMS/Silica Nanocomposites
During the process of fabricating PDMS/silica nanocomposites, the agglomeration of nanoparticles
is inevitable. In order to obviate the agglomeration and achieve a homogeneous dispersion, magnetic
stirring and sonication were often used [13,19]. In addition, organic solvents were often used to help
the dispersion of nanoparticles, such as toluene [21,22] and xylene [13,19]. Our group once used
toluene to dilute PDMS for making a thin film [23], hence, here, toluene was also chosen as the dilute
solvent. In this study, PDMS/silica nanocomposites were fabricated by combining magnetic stirring,
sonication and the dilute solvent. The whole fabrication procedures were illustrated in Figure 1:
(a) Silica nanoparticles were mixed with toluene. The mixture was magnetically stirred for 1 h, and
ultrasonically oscillated for 3 h. (b) The nanoparticles in toluene were mixed with the PDMS base.
Similarly, the mixture was stirred for 1 h and oscillated for 3 h. (c) Toluene in the mixture was
thoroughly evaporated in chemical hood. (d) The PDMS base and curing agent were mixed at a weight
ratio of 10:1. The mixture was stirred uniformly, and then put into a vacuum oven to remove bubbles.
(e) The mixture was poured on a glass plate, and cured at 80 °C for 2 h. Then the PDMS/silica
nanocomposites were peeled off from the glass plate.
2.3. Characterization
Silica nanoparticles were observed by a transmission electron microscope (TEM) (Tecnai G2 F30,
FEI Company, Hillsboro, OR, USA), as shown in Figure 2. The thermal expansion property of
PDMS/silica nanocomposites was examined by a thermomechanical analyzer (Q400, TA Instruments,
New Castle, DE, USA). A specimen with a dimension of 8 mm × 5 mm × 5 mm was prepared. During
the test, the temperature was increased from room temperature to 180 °C at a rate of 5 °C/min. The
CTE of PDMS/silica nanocomposites was determined based on the curves from the analyzer. The
elastic modulus of PDMS/silica nanocomposites was measured by using a universal tensile testing
machine (5657, Instron Company, Norwood, MA, USA). The specimen was fabricated according to
American Society for Testing of Materials (ASTM) standards, and the rate of crosshead motion was
kept at 50 mm/min. To measure the swelling property of PDMS/silica nanocomposites, a specimen
with a dimension of 25 mm × 25 mm × 2 mm was put into toluene, and the weight of the specimen
was recorded every half an hour until the swelling of nanocomposites reached equilibrium. In
accordance with the change in weight caused by absorption, the swelling ratio was calculated [10]. The
transmittance of PDMS/silica nanocomposites was measured by a UV-Vis spectrophotometer
(Cary300, Agilent Technologies, Santa Clara, CA, USA), and the thickness of the specimen was
100 μm.
Micromachines 2015, 6 858
Figure 1. Fabrication procedures of PDMS/silica nanocomposites. (a) Dispersion of silica
nanoparticles in toluene under magnetic stirring and ultrasonic vibration; (b) mixing silica
nanoparticles with PDMS base under magnetic stirring and ultrasonic vibration;
(c) evaporation of toluene; (d) mixing PDMS base with curing agent and removing
bubbles; and (e) cast molding of PDMS/silica nanocomposites on a glass plate.
Figure 2. Transmission electron microscope (TEM) images of silica nanoparticles. (a) A380
fumed silica nanoparticles; (b) MCM-41 mesoporous silica nanoparticles.
3. Results and Discussion
It is worth noting that the Sylgard 184 PDMS base itself contains silica filler. However, these silica
filler are dimethylvinylated and trimethylated silica [24], and different from the silica nanoparticles
(A380 and MCM-41) used in this study. Moreover, during the curing process, the vinyl of the silica
Micromachines 2015, 6 859
filler and the silicon hydride groups of the curing agent would undergo a hydrosilylation reaction to
form a Si-C bond [24]. Therefore, it is believed that the silica nanoparticles used in this work would
not interact with the silica filler, but with the whole three-dimensional PDMS network.
3.1. The CTE of PDMS/Silica Nanocomposites
Strain-temperature curves of PDMS/silica nanocomposites were shown in Figure 3, and the CTE of
PDMS/silica nanocomposites was calculated from the slope of the curves. The CTE of pure PDMS
was 301 ppm/°C. The CTE of PDMS decreased with the silica concentration. The decrease of the
CTE was mainly attributed to the following two factors. On one hand, the CTE of silica is only
0.54 ppm/°C [25], which is almost four orders of magnitude lower than that of pure PDMS. Hence the
CTE of PDMS can be decreased by adding silica nanoparticles. On the other hand, covalent bonds
between silica nanoparticles and PDMS and hydrogen bonds between silica nanoparticles formed [16,26].
The bonds caused large interaction between PDMS and silica nanoparticles, and the interaction
restricted the heat deformation of PDMS.
Figure 3. Strain-temperature curves of PDMS with different concentrations of (a) A380
fumed silica nanoparticles and (b) MCM-41 mesoporous silica nanoparticles; (c) CTEs of
PDMS/A380 and PDMS/MCM-41 nanocomposites. Data were shown as mean ± standard
deviation (SD). #, p < 0.05, compared with control of PDMS/A380 nanocomposites;
*, p < 0.05, compared with control of PDMS/MCM-41 nanocomposites (one-way analysis
of variance, Tukey’s post hoc test).
Compared with A380, MCM-41 reduced the CTE of PDMS more efficiently. At a concentration of
10 wt %, the CTE of PDMS/MCM-41 nanocomposites was 272 ppm/°C, while that of PDMS/A380
nanocomposites was 280 ppm/°C. The width of the PDMS chain is about 0.7 nm [27], and the pore
diameter of MCM-41 is 3.5–4 nm (Table 1). A portion of PDMS chains could be easily penetrated into the
pores of MCM-41 due to capillary force [28]. According to the research by Yamauchi’s group [17,18], the
thermal expansion of these PDMS chains confined inside the pores of MCM-41 was restricted by the
framework of MCM-41. These confined PDMS chains made few contributions to the CTE of PDMS.
Therefore, PDMS/MCM-41 nanocomposites showed a lower CTE at the same concentration of
silica nanoparticles.
Moreover, it was observed that the viscosity of the mixture of PDMS and A380 was apparently
larger than that of the mixture of PDMS and MCM-41. At the same concentration of nanoparticles, due
to a smaller size, the number of A380 is about 4 orders of magnitude larger than that of MCM-41, and
Micromachines 2015, 6 860
the distance among A380 is also smaller. Therefore, we believed that the interaction among A380
could be stronger, and the relative motion among A380 could be more difficult, so the viscosity of the
mixture of PDMS and A380 was larger. It became very difficult to mix PDMS base and A380 when
the concentration of A380 was over 10 wt %. While the concentration of MCM-41 was easily
increased up to 20 wt %, and, correspondingly, the CTE of PDMS was decreased to 241 ppm/°C.
3.2. The Elastic Modulus of PDMS/Silica Nanocomposites
The elastic modulus (E) of PDMS/silica nanocomposites was calculated based on the stress-strain
region below 40%. Stress-strain curves of PDMS are shown in Figure 4a,b, and effects of
concentrations of nanoparticles on the modulus are shown in Figure 4c. The elastic modulus of pure
PDMS was 1.38 MPa. It can be seen that silica nanoparticles significantly improved the elastic
modulus of PDMS. The modulus of PDMS with 20 wt % MCM-41 was 9.44 MPa, which is almost
seven times of that of pure PDMS. The increase of the elastic modulus mainly resulted from two
aspects. First, the elastic modulus of silica is at least four orders of magnitude larger than that of
PDMS, and, therefore, the modulus of PDMS increased with the amount of nanoparticles. Second,
hydrogen bonds between silica nanoparticles and covalent bonds between silica and PDMS increased
the resistance to deformation [29].
In addition, the elastic modulus of PDMS/A380 nanocomposites was apparently larger than that of
PDMS/MCM-41 nanocomposites at the same concentration. At a concentration of 10 wt %, the elastic
modulus of PDMS/A380 nanocomposites was 7.83 MPa, while that of PDMS/MCM-41
nanocomposites was only 5.44 MPa. As stated above, the interaction among A380 and between A380
and PDMS could be stronger due to the smaller size of A380, and the stronger interaction made a
larger elastic modulus.
Figure 4. Stress-strain curves of PDMS with different concentrations of (a) A380 fumed
silica nanoparticles and (b) MCM-41 mesoporous silica nanoparticles; (c) Elastic moduli
of PDMS/A380 and PDMS/MCM-41 nanocomposites. Data were shown as mean ± standard
deviation (SD). #, p < 0.05, compared with control of PDMS/A380 nanocomposites;
*, p < 0.05, compared with control of PDMS/MCM-41 nanocomposites (one-way analysis
of variance, Tukey’s post hoc test).
3.3. Swelling of PDMS/Silica Nanocomposites
The swelling property of PDMS/silica nanocomposites in toluene was examined. The swelling ratio
(R) was calculated as (W−W
0
)/W
0
where W
0
and W denoted weights of the PDMS before and after
Micromachines 2015, 6 861
absorbing toluene. As shown in Figure 5, at the beginning, the swelling ratio of PDMS increased
quickly, and reached the maximum after about 2 h. Then the ratio kept nearly constant with time. The
swelling ratio of pure PDMS was 1.142. The ratio remarkably decreased with the silica concentration,
and the ratio was even down to 0.486 when the concentration of MCM-41 was 20 wt %, which is
almost 60% lower than that of pure PDMS. The silica nanoparticles increased the crosslinking density
of PDMS [26]. According to the Flory-Rehner rubber swelling theory [30], the increase of crosslinking
density decreased the swelling ratio of PDMS.
Compared with MCM-41, the reduction of the swelling ratio caused by A380 was expected to be
more effective because the crosslinking density of PDMS/A380 nanocomposites was denser due to its
smaller size [16]. However, as shown in Figure 5, the swelling ratios of PDMS/A380 and
PDMS/MCM-41 nanocomposites were very close at the same concentration. For example, at the
concentration of 10 wt %, the swelling ratio of PDMS/A380 nanocomposites was 0.681, and that of
PDMS/MCM-41 nanocomposites as 0.686. As mentioned above, a portion of PDMS was confined
inside the nanopores in MCM-41. We consider that the absorbing ability of this portion of PDMS was
suppressed by the frameworks of MCM-41, which led to an average smaller swelling ratio of the
whole PDMS.
Figure 5. Time-dependent swelling ratios of PDMS with different concentrations of (a) A380
fumed silica nanoparticles and (b) MCM-41 mesoporous silica nanoparticles in toluene.
3.4. Transparency of PDMS/Silica Nanocomposites
The effect of silica nanoparticles on the transparency of PDMS was studied. The ultraviolet–visible
spectroscopy (UV–Vis) spectra of PDMS/silica nanocomposites are shown in Figure 6. The pure
PDMS had excellent transparency, and the transmittance was above 95% in the range from 350 to
800 nm. The silica nanoparticles can cause significant light scattering, and this will decrease the
transmittance of PDMS. As shown in Figure 6, the transmittance of PDMS decreased with the
concentration of nanoparticles. Moreover, the smaller the wavelength was, the lower the transmittance
of PDMS/silica nanocomposites was.
Furthermore, PDMS/A380 nanocomposites showed much better transparency than PDMS/MCM-41
nanocomposites at the same concentration. At the concentration of 10 wt %, the transmittance of
PDMS/A380 nanocomposites was 81% at 450 nm, while that of PDMS/MCM-41 nanocomposites was
67% at 450 nm. The transmittance of PDMS/silica nanocomposites is mainly related to the size and
Micromachines 2015, 6 862
amount of nanoparticles [31]. Hence, when the concentration of nanoparticles is the same, the
transmittance is dependent on the size of nanoparticles. As shown in Table 1, the size of A380 is much
smaller than MCM-41. Therefore, the scattering loss caused by A380 is smaller, and a better
transparency of PDMS/A380 nanocomposites can be obtained.
Figure 6. Ultraviolet–visible spectroscopy (UV–Vis) spectra of PDMS with different
concentrations of (a) A380 fumed silica nanoparticles and (b) MCM-41 mesoporous
silica nanoparticles.
4. Conclusions
The effects of two kinds of silica nanoparticles on the CTE, elastic modulus, swelling property and
transparency of Sylgard 184 PDMS were investigated and compared. By doping silica nanoparticles,
the stabilities and swelling characteristics of PDMS were remarkably improved, while the
transmittance was decreased. Compared with A380, the reduction of CTE of PDMS caused by MCM-41
was more effective due to its mesoporous structure. Moreover, a higher concentration of MCM-41
could be doped owing to its larger size, and a CTE of 241 ppm/°C was obtained at a concentration of
20 wt %. Owing to the smaller size of A380, the PDMS/A380 nanocomposites showed larger elastic
moduli and better transparency than PDMS/MCM-41 nanocomposites. A380 and MCM-41
nanoparticles had the similar effects on the swelling property of PDMS. In sum, by doping different
types of silica nanoparticles, Sylgard 184 PDMS with different properties can be achieved and applied
for a variety of applications.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51475080, 51321004),
the National High-tech R&D Program of China (2012AA040406), the Fundamental Research Funds for
the Central Universities (DUT14QY21), Dalian Foundation of Science and Technology (2012J21DW002),
and Funds of Key Laboratory of Liaoning Education Department (LZ2014005).
Author Contributions
Junshan Liu, Chong Liu and Liding Wang conceived and designed the experiments; Guoge Zong,
Licheng He and Yangyang Zhang performed the experiments; Junshan Liu and Guoge Zong analyzed
the data; Junshan Liu and Guoge Zong wrote the paper.
Micromachines 2015, 6 863
Conflicts of Interest
The authors declare no conflict of interest.
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