Journal of Applied Science and Engineering, Vol. 19, No. 4, pp. 401-408 (2016)
DOI: 10.6180/jase.2016.19.4.03
Preparation of Solvent-dispersible Nano-silica
Powder by Sol-gel Method
Chao-Ching Chang1,2, Jo-Hui Lin1 and Liao-Ping Cheng1,2*
1
Department of Chemical and Materials Engineering, Tamkang University,
Tamsui, Taiwan 251, R.O.C.
2
Energy and Opto-Electronic Materials Research Center, Tamkang University,
Tamsui, Taiwan 251, R.O.C.
Abstract
Solvent dispersible nano-silica powder was prepared by a dual-step sol-gel process: first, SiO2
nanoparticles were synthesized through acid-catalyzed hydrolysis and condensation of tetraethyl
orthosilicate in 2-propanol aqueous solution. Then, the particles were surface-modified by means of
the capping agent trimethylethoxysilane (TMES). The formed product, termed TSiO2 nanopowder,
was dispersible in many organic solvents, and the dispersibility was found to depend on the amounts of
TMES bounded to the SiO2 nanoparticles. FTIR spectra of TSiO2 samples confirm Si-O-Si linkage
being formed between TMES and SiO2 through the capping reaction. The sizes of TSiO2 dispersed in
various solvents, as determined by dynamic light scattering (DLS), fell largely over the range 2-20
nm for solvents with solubility parameters of 16-29.6 MPa1/2. TEM imaging of the nanoparticles
indicated that they were well separated with the largest identifiable size of ~10 nm, agreeing with the
results obtained from DLS.
Key Words: Nanoparticles, Dispersible, Sol-gel, Silica
1. Introduction
Inorganic nanoparticles are widely used to fabricate
organic-inorganic composites with enhanced mechanical, thermal, optical, etc., properties suited to various applications [1-16]. The performances of the composites
are, however, dependent upon the size, size distribution,
and how uniform the particles disperse in the organic matrix. For example, the inorganic domain for a hard coating, such as that applied on lenses or glasses, generally
has to be less than ~100 nm to avoid deterioration of optical clarity [16].
For nano-silica derived from the sol-gel process, particle aggregation occurs naturally due to the presence of
*Corresponding author. E-mail: [email protected]
active -OH groups on the particle surface. These -OH
groups tend to form hydrogen bonds or undergo condensation reactions mutually to yield Si-O-Si linkages between neighboring particles. Hence, as the solvent of the
sol is removed, such as to form powdery products, large
irreversible aggregates (secondary particles) will form,
which are no longer dispersible in the original solvent.
To prevent aggregation of nanoparticles, it is generally
necessary to deactivate the -OH groups on the particle
surface. Physical means such as incorporation of chelating agents and surfactants, and various chemical modification approaches are commonly adopted to achieve this
purpose. For example, surfactants can serve as a nanoreactor or template for syntheses of independent nanoparticles that are encapsulated in the micelles of surfactant molecules [17,18]. On the other hand, the amount
402
Chao-Ching Chang et al.
of surface -OH can be reduced by reaction with a modifier, such as those bearing RSi-X, R-OH, or R-NCO
species on the molecule [19-23]. For example, by bonding with both 3-(trimethoxysilyl)propyl methacrylate
(MSMA) and trimethylethoxysilane (TMES) on nanosilica, Huang et al. were able to prepare a paste-like material consisting of ~98% nano-silica and 2% solvent,
which remained stable and dispersible over a prolonged
storage period (> 6 months) [23].
Dried silica powders have been utilized in a number
of industrial applications, such as fillers in filter films,
matrix of a catalyst, reinforcing component for powder
coatings, etc. However, it is often noted that the sizes of
the silica clusters in the sample can be rather large (> 500
nm) in these cases, due to serious particle-particle aggregation, which may downgrade the quality of the products. It is, therefore, of great interest to prepare nanosilica particles that do not aggregate during drying, and
can easily be dispersed in organic solvents. In this research, TMES was employed as a capping agent to treat
silica nanoparticles that were synthesized from an acidcatalyzed sol-gel process. As TMES is mono-functional,
it reduces effectively the amount of -OH groups on the
particle surface. Therefore, even after vacuum-dried, the
obtained nano-silica powder (termed TSiO2) can still be
dispersed in various organic solvents without changing
significantly the average particle size (< 10 nm). The
preparation and characterization of TSiO2 are detailed in
the sections given below.
densation of TEOS in the presence of water/IPA solutions, as shown previously [14,23]. Briefly, TEOS was
mixed with IPA to form a homogeneous solution. Then,
HCl(aq) (pH 1.2) was added to this solution under continuous agitation. The molar ratio of TEOS:H2O:IPA was
set to be 1:4:1.16. The reaction was allowed to proceed
for 3 h, cf. Scheme 1(a). Using dynamic light scattering
method, it was found that with an extended period of
storage (typically one week), aggregation of the SiO2
particles occurred in the sols [23]. For this reason, the
-OH groups on the SiO2 particles were end-capped by
reaction with the capping agent TMES, which is a monofunctional ethoxylsilane, cf. Scheme 1(b). Appropriate
amounts of TMES, IPA, and HCl(aq) (pH 0.6) were slowly
added into the as-prepared SiO2 sol under vigorous agitation. After reaction for 3 h at room temperature, TMEScapped silica (TSiO2) was obtained. The compositions
of various chemical species for this reaction are listed in
2. Experimental
2.1 Materials
Tetraethoxysilane (TEOS, > 98%) was purchased
from Fluka. Trimethylethoxysilane (TMES, 97%), 2-propanol (IPA, 99.8%), and hydrochloric acid (37 % in water) were purchased from Aldrich. All materials were
used as received.
2.2 Preparation of Surface Modified Nano-silica
Powder
The silica sol was prepared by hydrolysis and con-
Scheme 1. Schematic representation of the paths for synthesis
of TMES modified SiO2.
Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method
Table 1. The “R” values in the table stand for the mole
ratio of TMES/(TMES + TEOS). Subsequently, vacuum
distillation was applied at 50 °C to remove the volatile
species such as various alcohols and water in the TSiO2
sol. After 1 h of vacuum operation, weight of the sample
approached constant (c.f., Figure 1), and the product appeared as a white powdery solid.
2.3 Characterization
Infrared absorption spectra of the TSiO2 were obtained using a Fourier Transform Infrared Spectrophotometer (Nicolet MAGNA-IR spectrometer 550, USA).
An appropriate amount of the TSiO2 sol was dropped
onto a KBr disc, and then the solvent was evaporated at
25 °C in a vacuum oven. For all scans, the spectra were
collected over the wavenumber range of 400-4000 cm-1
with a resolution of 4 cm-1. TEM micrographs of the silica particles were taken using Hitachi H-7100, Japan.
The samples were prepared by dropping IPA-dispersed
TSiO2 on a standard copper grid, and then IPA was reTable 1. Molar compositions of various species for the
capping reaction
Sample coad
R4
R5
R6
R7
a
TMES
IPA
H 2O
Ra
0.67
1
1.5
2.33
1.34
2
3
4.66
0.67
1
1.5
2.33
0.4
0.5
0.6
0.7
403
moved by vacuum at room temperature. The size and size
distribution of silica particles in various sols were determined by the dynamic light scattering (DLS) method, using Malvern Zetasizer Nano ZS, at 25 °C.
3. Results and Discussion
3.1 Chemical Structure Analyses by FTIR
Scheme 1(a) depicts the synthesis of SiO2 by hydrolysis and condensation of alkoxysilanes under acidic condition. FTIR analyses for this reaction have been performed previously by many authors and the results were
well documented [24-26]. Figure 2 shows the FTIR
spectra of the TSiO2 (R5) formed at various times during
the course of its synthetic reaction, Scheme 1(b). The
absorption band at 946 cm-1 corresponds to the stretching vibration of Si-OH groups on the particle, whose intensity decreases significantly during the initial 30 min
and then gradually reaches a constant level for the remaining 2.5 h. The broad band around 3320 cm-1 is assigned to various -OH groups, e.g., those on SiO2 or water [25]. This band follows a trend similar to that observed for Si-OH. The Si-CH3 signal of TMES is located at 851 cm-1 [24], which grows as the reaction proceeds. Based on the above observations, it is confident
to put that reaction between TMES and the hydroxyl
groups of SiO2 has occurred to form ºSi-O-Si(CH3)3
species on the particle surface. Figure 3 compares the
R = TMES/(TMES + TEOS).
Figure 1. Weight of sample during solvent removal by vacuum distillation.
Figure 2. FTIR spectra of a modified SiO2 (R5) at various
times during its synthesis.
404
Chao-Ching Chang et al.
Figure 3. FTIR spectra of TSiO2 nanoparticles prepared with
different amounts of TMES (R-values) after 3 h of
reaction.
spectra of TSiO2 nanoparticles prepared with different
added amounts of TMES (R-value). Obviously, as the Rvalue is raised, the intensity of the Si-CH3 band (with respect to Si-OH) increases. That is, more Si-OH has been
converted to Si-O-Si(CH3)3 when more TMES is added.
However, it is noted that the degree of increment becomes less significant for higher R values. For example,
between R = 0.4 and 0.5, the intensity ratio of Si-CH3/
Si-OH changes significantly from 0.13 to 1.37. However, between R = 0.6 and 0.7, the ratio increases only
45% form 2.27 to 3.3. In other words, R = 0.7 is approaching the saturated dosage of TMES for the capping
reaction.
tive TSiO2 sol (R5 in Table 1). The sizes of the particles
in the sol fall over a narrow range of ca. 1.5-9 nm, with
the maximum number fraction located at 2.7 nm, which
is close to that of SiO2 sol before end-capped with TMES
[23]. Such is consistent with the fact that capping of the
-OH groups can halt the growth/joining of SiO2 particles, and thus maintain the particle size. Liquid species,
such as methanol, ethanol, water, etc., in the TSiO2 sols
can be removed by vacuum-distillation to yield solid products. For the cases of R £ 0.4, considerable particle aggregations are found to occur during the late-stage of solvent removal, and eventually flaky monolithic samples
are obtained, which is no longer dispersible in common
solvents. That is, at these levels of TMES dosages, the
particles still have considerable amounts of -OH groups
on their surfaces, which condense with each other upon
contact to form irreversible covalent bonds. In contrast,
for the cases of R = 0.5-0.7, the vacuum-dried products
appear as white fine powders, and can readily be dispersed in many organic solvents. Table 2 lists the solubility parameters of the tested solvents (dispersants) along
with the measured sizes of the re-dispersed TSiO2 nanoparticles (DLS). For R5 dispersed in IPA, the average
size of the particles is ~2.6 nm, essentially the same as it
is in the original synthesized sol, which confirms that
Table 2. Sizea of TSiO2 particles dispersed in various
solvents (d: solubility parameter)
Dispersant
3.2 Particle Size from DLS and TEM
The particle sizes of various freshly synthesized sols
were measured by means of DLS. As an example, Figure
4(a) shows the size distribution profile of a representa-
Figure 4. Particle size distribution of a TSiO2 sol (R = 0.5) as
determined by DLS.
Name
Decane
Hexane
Toluene
Acetone
Tetrahydrofyran
Dimethylsulfoxide
Butanol
Isopropanol
Ethanol
Methanol
H 2O
a
b
R4
d
3 1/2
(cal/cm )
R5
R6
R7
´b
´
´
´
´
´
´
´
´
´
´
7.3
6.4
5.8
6.4
5.3
4.5
4.8
2.6
2.9
2.9
´
18
´
´
´
´
´
6.1
4.1
4.4
´
´
85
´
´
´
´
´
6.9
5.3
5.4
´
´
06.6
007.24
08.9
009.77
09.9
10.8
11.4
11.6
12.7
14.5
23.2
Determined by DLS.
´: Non-dispersible.
Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method
the TMES moiety (-Si(CH3)3) on the particle surface has
effectively prohibited bond formation between neighboring particles. As a result, even in the compacted powder form, the nanoparticles are able to regain their sizes
simply by dispersing in IPA. Table 2 also indicates that
the R5 sample is dispersible in a relatively wide range of
organic solvents (specifically, polar and non-polar ones)
with particle size less than 10 nm. For all the tested solvents, the solubility parameters fall over the range 6.623.2 (cal/cm3)1/2. It is interesting to find that the measured particle size decreases with increasing solubility
parameter of the solvent. For instance, the particle diameter rises to 7.3 nm, corresponding to an aggregation of
~3 particles, when it is dispersed in decane. This may be
associated with the polarity and/or hydrogen bonds formation between residual -OH on the particle and the dispersant. The real causes, however, are sophisticate and
beyond the scope of the present research. For the R = 0.6
and 0.7 cases, the formed nanoparticles are dispersible
only in the three tested alcohols (ethanol, IPA, and 1-butanol) and decane, and the particle sizes are somewhat
larger than R5 in the same alcohol. The fact that higher
R values give rise to particles with smaller amounts of
-OH groups on the particle surface is expected to play a
role; as is evident, the decreased polarity has rendered
the particles somewhat dispersible in non-polar solvent
like decane (d = 6.6 (cal/cm3)1/2).
The unmodified SiO2 nanoparticles tend strongly to
gather into large clusters upon dispersant removal during
drying. Such is clearly demonstrated in TEM imaging of
the particles, which involves a vacuum step for sample
preparation [23]. As the solvents are gradually removed
from the sol, particle-particle contacts become frequent
and the interactions between them are enhanced. Eventually aggregates are formed by hydrogen bonding or condensation between hydroxyl groups on their surface. By
contrast, aggregation phenomenon is not evident for the
TSiO2 nanoparticles (R = 0.5–0.7) since their surface
-OH amounts have been greatly reduced. Figure 5 indicated that the particles are of circular shape and well separated with size over the range 3-10 nm, consistent with
that obtained from DLS measurements.
405
As is recognized, SiO2 nanoparticles may either be
negatively or positively charged, depending on the acidity
of the sol. After modified by TMES, the charge density
on the surface changes because the polar Si-OH groups
have been partly replaced by Si-O-Si(CH3)3. Such effect is manifested in Figure 6 in terms of zeta potentials
of the particles (SiO2 and TSiO2) dispersed in sols with
addition of hydrochloric acid of different pH values. It
appears that increasing acidity causes shifting of the
zeta potential from negative to positive values for both
types of particles. As a result, the isoelectric point (pH
where zeta potential is identically zero) can be identi-
Figure 5. TEM image of TSiO2 particles with R = 0.5.
Figure 6. Zeta potential of SiO2 and TSiO2 dispersed in sols
of different pH values.
406
Chao-Ching Chang et al.
fied to be pH 1.2 for TSiO2 and pH1.6 for SiO2. Further,
it is found that the zeta potential of TSiO2 is always
more negative than that of SiO2 over the tested pH range,
which implies that particle-particle repulsive forces
are stronger for TSiO2 than for SiO2. This effect explains
partially why TSiO2 particles are more stable and less
prone to aggregation in the sol.
Processing to Materials and Innovative Applications,”
Applied Organometallic Chemistry, Vol. 15, No. 5, pp.
331-343 (2001). doi: 10.1002/aoc.169
[3] Aegerter, M. A., Almeida, R., Soutar, A., Tadanaga, K.
and Yang, H., “Coatings Made by Sol-Gel and Chemical Nanotechnology,” Journal of Sol-Gel Science and
Technology, Vol. 47, No. 2, pp. 203-236 (2008). doi:
10.1007/s10971-008-1761-9
4. Conclusions
[4] Wang, J. Y., Zhou, G. G., Xu, W. H., Liu, W. L., Cai, X.
X., Liu, Q. Z., Wang, X. Q. and Wu, Y. Z., “Facile Syn-
SiO2 nanoparticles synthesized from acid-catalyzed
sol-gel process were surface-modified by reaction with
the capping agent TMES. The formed products, termed
TSiO2 nanopowder, exhibited different level of solvent
dispersibility, depending on the amounts of TMES attached to the SiO2 surface. Successful capping was verified by FTIR spectra of SiO2 and TSiO2 samples, which
indicated increment of Si-O-Si linkage and decrease of
-OH groups after capping reaction. The sizes of TSiO2
particles dispersed in various solvents, as determined by
DLS, fell largely over the range 2-20 nm for solvents
with solubility parameters of 16-29.6 MPa1/2. TEM
imaging indicated that the particles were well separated
without any evidence of aggregation, and the largest identifiable particle size was ~10 nm, consistent with the
DLS data.
thesis of a Superhydrophobic Surface with Modified
Hollow Silica Nanoparticles,” RSC Advances, Vol. 4,
No. 93, pp. 51603-51608 (2014). doi: 10.1039/C4RA
09218J
[5] Jeevajothi, K., Subasri, R. and Soma Raju, K. R. C.,
“Transparent, Non-fluorinated, Hydrophobic Silica
Coatings with Improved Mechanical Properties,” Ceramics International, Vol. 39, No. 2, pp. 2111-2116
(2013). doi: 10.1016/j.ceramint.2012.07.019
[6] Girdthep, S., Komrapit, N., Molloy, R., Lumyong,
S., Punyodom, W. and Worajittiphon, P., “Effect of
Plate-like Particles on Properties of Poly(Lactic Acid)/
Poly(Butylene Adipate-co-terephthalate) Blend: a
Comparative Study between Modified Montmorillonite
and Graphene Nanoplatelets,” Composites Science and
Technology, Vol. 119, No. 23, pp. 115-123 (2013).
doi: 10.1016/j.compscitech.2015.10.005
Acknowledgement
[7] Chang, C. C., Huang, F. H., Lin, Z. M. and Cheng, L.
P., “Thermal Analyses of Dye Disperse Red 1 Grafted
The authors wish to thank the Ministry of Science
and Technology for financial support (104-2632-E-032001).
onto Silica Nanoparticles,” Journal of Coatings Technology and Research, Vol. 12, No. 4, pp. 731-738
(2015). doi: 10.1007/s11998-015-9670-7
[8] Gao, L. P., Ding, G. J., Li, C. L. and Wang, Y. C.,
References
“Photochromic and Electrochromic Performances of
New Types of Donor/Acceptor Systems Based on Cros-
[1] Sanchez, C., Soler-Illia, G. J. de A. A., Ribot, F., Lalot,
slinked Polyviologen Film and Electron Donors,” Ap-
T., Mayer, C. R. and Cabuil, V., “Designed Hybrid Or-
plied Surface Science, Vol. 257, No. 7, pp. 3039-3046
ganic-Inorganic Nanocomposites from Functional Na-
(2011). doi: 10.1016/j.apsusc.2010.10.113
nobuilding Blocks,” Chemistry of Materials, Vol. 13,
[9] Yeh, J. M., Weng, C. J., Liao, W. J. and Mau, Y. W.,
No. 10, pp. 3061-3083 (2001). doi: 10.1021/cm011
“Anticorrosively Enhanced PMMA-SiO2 Hybrid Coat-
061e
ings Prepared from the Sol-Gel Approach with MSMA
[2] Schmidt, H., “Nanoparticles by Chemical Synthesis,
as the Coupling Agent,” Surface & Coatings Tech-
Preparation of Solvent-dispersible Nano-silica Powder by Sol-gel Method
nology, Vol. 201, No. 3-4, pp. 1788-1795 (2006). doi:
10.1016/j.surfcoat.2006.03.011
407
[18] Yao, L., Xu, G., Dou, W. and Bai, Y., “The Control of
Size and Morphology of Nanosized Silica in Triton
[10] Wu, L. Y. L., Boon, L., Chen, Z. and Zeng, X. T., “Ad-
X-100 Based Reverse Micelle,” Colloids and Surfaces
hesion Enhancement of Sol-Gel Coating on Polycar-
A: Physicochemical and Engineering Aspects, Vol. 316,
bonate by Heated Impregnation Treatment,” Thin Solid
No. 1-3, pp. 8-14 (2008). doi: 10.1016/j.colsurfa.2007.
Films, Vol. 517, No. 10, pp. 4850-4856 (2009). doi:
08.016
10.1016/j.tsf.2008.11.101
[19] Posthumus, W., Magusin, P. C. M. M., Brokken-Zijp,
[11] Klajn, R., “Spiropyran-based Dynamic Materials,”
J. C. M., Tinnemans, A. H. A. and van der Linde, R.,
Chemical Society Reviews., Vol. 43, No. 1, pp. 148-
“Surface Modification of Oxidic Nanoparticles Using
184 (2014). doi: 10.1039/c3cs60181a
3-Methacryloxypropyltrimethoxysilane,” Journal of
[12] Souza, F. L., Bueno, P. R., Longo, E. and Leite, E. R.,
Colloid and Interface Science, Vol. 269, No. 1, pp.
“Sol-gel Nonhydrolytic Synthesis of a Hybrid Organic-
109-116 (2004). doi: 10.1016/j.jcis.2003.07.008
Inorganic Electrolyte for Application in Lithium-ion
[20] Eras, J., Méndez, J. J., Balcells, M. and Canela, R.,
Devices,” Solid State Ionics, Vol. 166, No. 1-2, pp.
“Chlorotrimethylsilane: a Suitable Reagent for the
83-88 (2000). doi: 10.1016/j.ssi.2003.10.009
Synthesis of Chlorohydrin Esters,” Journal of Organic
[13] Otsuka, T. and Chujo, Y., “Synthesis of Transparent
Poly(Vinylidene Fluoride) (PVDF)/Zirconium Oxide
Chemistry, Vol. 67, No. 24, pp. 8631-8634 (2002).
doi: 10.1021/jo026020w
Hybrids without Crystallization of PVDF Chains,”
[21] Delak, K. M. and Sahai, N., “Amine-Catalyzed Bio-
Polymer, Vol. 50, No. 14, pp. 3174-3181 (2009). doi:
mimetic Hydrolysis and Condensation of Organosili-
10.1016/j.polymer.2009.05.018
cate,” Chemistry of Materials, Vol. 17, No. 16, pp.
[14] Chang, C. C., Lin, Z. M., Huang, S. H. and Cheng, L.
3221-3227 (2005). doi: 10.1021/cm048355v
P., “Preparation of Highly Transparent 13F-Modified
[22] Rao, A. V. and Kulkarni, M. M., “Hydrophobic Pro-
Nano-Silica/Polymer Hydrophobic Hard Coatings on
perties of TMOS/TMES-Based Silica Aerogels,” Ma-
Plastic Substrates,” Journal of Applied Science and
terials Research Bulletin, Vol. 37, No. 9, pp. 1667-
Engineering, Vol. 18, No. 4, pp. 387-394 (2015). doi:
10.6180/jase.2015.18.4.10
1677 (2002). doi: 10.1016/S0025-5408(02)00795-X
[23] Huang, F. H., Chang, C. C., Oyang, T. Y., Chen, C. C.
[15] Chang, C. C., Hsieh, C. Y., Huang, F. H. and Cheng, L.
and Cheng, L. P., “Preparation of Almost Dispersant-
P., “Preparation of Zirconia Loaded Poly(Acrylate)
Free Colloidal Silica with Superb Dispersablility in
Anti-static Hard Coatings on PMMA Substrates,” Jour-
Organic Solvents and Monomers,” Journal of Nano-
nal of Applied Polymer Science, Vol. 132, No. 33, pp.
particle Research, Vol. 13, No. 9, pp. 3885-3897
42411-42416 (2015). doi: 10.1002/app.42411
(2011). doi: 10.1007/s11051-011-0342-y
[16] Chang, C. C., Oyang, T. Y., Chen, Y. C., Hwang, F. H.
[24] Hajji, P., David, L., Gerard, J. F., Pascault, J. P. and
and Cheng, L. P., “Preparation of Hydrophobic Nano-
Vigier, G., “Synthesis, Structure and Morphology of
silica Filled Polyacrylate Hard Coatings on Plastic
Polymer-Silica Hybrid Nanocomposites Based on
Substrates,” Journal of Coatings Technology and Re-
Hydroxyethyl Methacrylate,” Journal of Polymer Sci-
search, Vol. 11, No. 3, pp. 381-386 (2014). doi: 10.
ence, Part B: Polymer Physics, Vol. 37, No. 22, pp.
1007/s11998-013-9540-0
3172-3187 (1999). doi: 10.1002/(SICI)1099-0488
[17] Ju, X., Huang, P., Xu, N. and Shi, J., “Studies on the
(19991115)37:22<3172::AID-POLB2>3.0.CO;2-R
Preparation of Mesoporous Titania Membrane by the
[25] Yu, Y. Y., Chen, C. Y. and Chen, W. C., “Synthesis and
Reversed Micelle Method,” Journal of Membrane Sci-
Characterization of Organic-Inorganic Hybrid Thin
ence, Vol. 202, No. 1-2, pp. 63–71 (2002). doi: 10.
Films from Poly(Acrylic) and Monodispersed Col-
1016/S0376-7388(01)00722-0
loidal Silica,” Polymer, Vol. 44, No. 3, pp. 593-601
408
Chao-Ching Chang et al.
(2003). doi: 10.1016/S0032-3861(02)00824-8
[26] Hegde, N. D., Hirashima, H. and Rao, A. V., “Two
No. 2, pp. 165-171 (2007). doi: 10.1007/s10934006-9021-2
Step Sol-Gel Processing of TEOS Based Hydrophobic Silica Aerogels Using Trimethylethoxysilane as a
Co-precursor,” Journal of Porous Materials, Vol. 14,
Manuscript Received: Apr. 20, 2016
Accepted: May 5, 2016