Transparent nanocomposites of high refractive
index based on epoxy resin and TiO2 nanoparticle
Dinh Huong Nguyen, Hana Kim and Dai Soo Lee*
School of Semiconductor and Chemical Engineering, Chonbuk National University,
Jeonju, 561-756, S. Korea,
E-mail: dslee@jbnu.ac.kr
A stepwise sol-gel method to synthesize stable colloidal TiO2 by the hydrolysis and condensation
reactions of titanium tetra isopropoxide (TTP) was investigated. The colloidal TiO2 was
characterized by transmittance electron microscopy (TEM). The surface modification was carried
out using 3-glycidoxypropyltrimethoxysilane (GPTMS). The particles sizes of the modified TiO2
observed by TEM were in the range of 3-8 nm maintaining the origin size of the TiO2 particles.
The nanocomposites based epoxy resin and modified TiO2 showed strong UV absorption but
maintain high transmittance in visible region. High resolution TEM images of nanocomposite
confirmed homogenous and fine dispersion of TiO2 nanoparticles in epoxy resin. The refractive
index of the nanocomposites increased linearly with TiO2 content. At 60% TiO2 content, the
transmittance of nanocomposite was 98.4% and refractive index of the nanocomposite was 1.657.
Keywords: Epoxy resin, nanocomposite, transparent, refractive index, TiO2.
1. INTRODUCTION
Recently, epoxy resins became popular as encapsulating
compound for LED, electronic parts, and optical devices
because of their excellent optical, mechanical, chemical,
and thermal properties.1 However, due to low refractive
index of the resin, metal oxide particles of high refractive
index such as TiO2 or ZrO2 is incorporated.2-5
Furthermore, the incorporation of the nanoparticles also
improves UV absorption6,7 physical, mechanical8 as well
as photo-stable9 properties of the nanocomposites. In
general, to obtain highly transparent nanocomposites,
fine dispersion of small particles (less than 25 nm to
minimize light scattering) in polymer matrix are
required.10 There are several ways to prepare
nanocomposites. (i) Direct mixing of nanoparticles with
polymer matrix using high energy mixing machine: this
process usually result in poor dispersion of
nanoparticles.11 (ii) Modification-dispersion method: the
nanoparticles are modified by surfactant or coupling
agents to improve the dispersion of nanoparticle in
solvent before dispersing in polymer matrix.6,7,10 The
dispersibility of this method is usually better than that of
direct mixing method but it is difficult to obtain fine
dispersion of nanoparticles because of aggregated
structures of high surface energy of nanoparticles. (iii)
In-situ sol-gel method: the nanoparticles are formed from
their precursors during polymerization. In this process,
very fine dispersion of nanoparticles was obtained in
polymer matrix.9 However, the precursors or monomer
must be miscible and stable in the mixture. Moreover, it
is difficult to prepare high concentration of nanoparticle
in polymer because of lager amount of precursor is used
and generated large amount of by products. (iv) Direct
sol-gel method: the homogeneous fine dispersion of
colloidal nanoparticles prepared by sol-gel process are
directly dispersed in polymer matrix without
precipitation-redispersion.12 This process allows
incorporations of large amount of fillers, maintaining
fine dispersion of nanoparticles. Therefore, fine
dispersion of nanoparticles in polymer matrix can be
obtained.
In case of TiO2, because of very fast hydrolysis of
titanium precursors such as titanium tetra isopropoxide
TTP, unstable colloidal solutions are precipitated
immediately when water is added, unless large amount
of HCl of HNO3 was used to stabilize colloidal TiO2
solution.12,13 Therefore, controlling the hydrolysis and
condensation reactions are necessary to prepare
homogenous, stable colloidal TiO2 solution. Up to
now, no reports on controlling the hydrolysis and
condensation reactions were reported. Moreover,
polymer nanocomposites based on unmodified TiO2
result in strong yellow color.7,10 Such problems are
still obstacles to be overcome to prepare homogeneous
transparent nanocomposites. In this study, we report
stepwise sol-gel method to synthesize colloidal TiO2 by
the controlled hydrolysis and condensation reactions of
TTP in the presence of small amount of HCl catalyst.
The surface modification with a silane compound was
carried out to prepare homogenous colloidal TiO2
solution, and characteristics of epoxy resin/TiO2
nanocomposites of transparent high refractive index are
discussed in this paper.
2. EXPERIMENTAL
2.1 Materials and samples preparation
Synthesis colloidal TiO2
Step 1
Step 3
Step 2
IPA = 20 ml
HCl = 0.01 mol
H2O = 0.05 mol
Dropwise
Dropwise
85oC
IPA = 120 ml
TPP = 0.1 mol
IPA = 20 ml
H2O = 0.05 mol
IPA = 20 ml
H2O = 0.05 mol
IPA = 20 ml
H2O = 0.05 mol
Dropwise
Step 4
Stirring
Stirring
Stirring
3 hours
3 hours
3 hours
Dropwise
Stirring
3 hours
3 hours
Evaporation
Curing
Room
temperature
Stirring
Stirring
Stirring
Dropwise
Dropwise
Hybrid films
Epoxy resin
Catalysts
IPA = 20 ml
H2O = 0.05 mol
Step 6
TiO2 epoxy resin hybrids
2 hours
IPA = 20 ml
GPTMS = 0.009 mol
Step 5
Silane modification of TiO2
Scheme 1. Schematic procedures for preparing colloidal TiO2, modification of TiO2, and epoxy resin TiO2
nanocomposites.
O
O Ti OH
O
OCH3
+
H3CO Si
OCH3
O
O
O
O
O Ti O
Si
O
O
O
O
+
CH3OH
Scheme 2. Surface modification reaction of GPTMS with OH groups on the surface of TiO 2
Titanium tetraisopropoxide (TTP), isopropylalchol
(IPA), 3-glycidoxypropyltrimethoxysilane (GPTMS), and
aluminum triacetylacetonate (Al(acac)3) were purchased
from Aldrich. A cycloaliphatic epoxy resin, 3’-4’epoxy
cyclohexane methyl 3’-4’ epoxy cyclohexyl carbonate,
was obtained from Daicel Chemical Industry Ltd. The
typical synthetic procedure for the preparation of
nanocomposites is given in Scheme 1. The process was
divided into three stages. The first stage is a stepwise
syntheses of colloidal TiO2. Typically, 0.1 mol of TTP
and 120 ml of IPA were mixed in glass reactor and heated
up to 85 oC. The step 1 was begun by adding a mixture of
0.01 mol of HCl catalyst and 0.05 mol of H2O in 20 ml
IPA dropwise (4 min.) into the reactor under stirring
while maintaining temperature at 85 oC. The step 2, 3,
and 4 were proceeded after every 3 hours. In each step,
mixture of 0.05 mol of H2O (enough for hydrolysis of
one group of titanium isopropoxide and condensation) in
20 ml IPA were added dropwise at the same speed. After
step 4, transparent solution was obtained. In the second
stage, surface modification was carried out by adding
dropwise a mixture of 0.009 mol of GPTMS (30 Phr of
TiO2) in 20 ml of IPA into the reactor and the reactions
were continued for 2 hours. Then excess amount of water,
0.05 mol in 20 ml IPA, was added dropwise to the reactor
and maintained for 3 hours for complete hydrolysis of
alkoxy group of TTP and GPTMS. It was cool down to
room temperature and transparent solution of TiO2
modified with GPTMS was obtained. In the conventional
sol-gel method, all the reactants are incorporated in one
step. The amount of HCl and H2O for the conventional
method were equal to that added in stepwise method.
After 6 hours reaction at 85 oC the translucent solution
was obtained with precipitation of white particles. In
the third stage, the nanocomposites were prepared by
adding epoxy resin, Cell2010, and 0.04Phr of Al(acac)3,
an initiator, into the modified TiO2 solution. It was
coated on PET film, solvent evaporated and cured by
cationic polymerization at 110 oC for 3 hours. Different
samples were obtained by varying TiO2 content from
0%, 10%, 20%, 30%, 40% and 60% and denoted 1, 2, 3,
4, 5, 6 respectively. Nananocomposite based on
unmodified TiO2 was prepared by the same procedure
without doing step 5 as a comparison.
2.2 Characterization
All measurements were performed at room temperature.
XRD diffractometer (Rigaku D/MAX-2500, Cu Kα
radiation target) was employed to determine the
structure of TiO2 powder. FTIR spectra were recorded
(a)
(b)
Figure 1. TEM image of (a) TiO2 (b) TiO2 modified
with GPTMS (scale bar is 100 nm).
Transmittance (%)
(a)
(b)
(c)
1081 951
4000
3500
3000
2500
2000
1500
1000
500
a
-1
Wave number (cm )
Figure 2. FTIR spectra of GPTMS (a), TiO2 (b), and
TiO2 modified with GPTMS (c).
(b)
(a)
Figure 3. Optical images of epoxy resin containing
different TiO2 nanoparticles of 30wt%: (a) Unmodified
TiO2 (0.25 mm thickness); (b) Modified TiO2 (0.26 mm
thickness).
Table 1. Thickness
nanocomposites
Sample.
1
2
3
4
5
6
and
refractive
TiO2 GPTMS Thickness
%
%
µm
0
0
21
10
3
30
20
6
33
30
9
39
40
12
19
60
18
11
index
of
Refractive
index
1.505
1.530
1.550
1.575
1.603
1.657
using FTIR (JASCO 4100). TiO2 powders for XRD and
FTIR were obtained by adding n-heptanes into the TiO2
solution for precipitation. They were washed several
time with methanol and dried at 60 oC for 5 hours before
the characterization. UV spectra were obtained with a
UV-670 Jasco spectrophotometer. High resolution
transmittance electron microscopy (HR-TEM, JEOL
JEM-2010) was employed to observe nanoparticles in the
nanocomposites. X-Ray mapping was done with a
scanning electron microscopy (SEM JSM-6400). The
refractive indexes of nanocomposites were determined by
an Abbe refractometer using a Nippon Optical Works co.
Ltd.
(b)
Figure 4. TEM images of epoxy resin containing 10%
TiO2 nanoparticles: (a) Low magnification (Scale bar
is 50nm); (b) High magnification (Scale bar is 10 nm.)
3.1 Sol-gel synthesis of colloidal TiO2
The colloidal TiO2 prepared from conventional sol-gel
method was translucent and showed precipitation after
6 hours of reaction, indicating unstable colloidal
properties. It is well known in literatures that TTP
reacts very fast with water and results in unstable
colloids and precipitation.7,13 On the other hand, in
stepwise sol-gel method, the transparent and stable
colloidal TiO2 were obtained. XRD results (not shown
here because of limited space) confirm amorphous
structure of TiO2. TEM image of TiO2 is shown in
Figure 1(a). The particle sizes of TiO2 are less than 10
nm. Some big particles were observed due to
agglomeration of small amorphous TiO2 particles
during drying of the sample for TEM observation. It is
postulated that the amount of water added stepwise
following the Scheme 1 resulted in the controlled
hydrolysis and condensation reaction. The stepwise
reactions were given as follow:
Step 1:
2Ti(OIP)4 + H2O => 2Ti(OIP)3OH + 2IPA
Ti(OIP)3OH + Ti(OIP)3OH
H2O
=> Ti(OIP)3OTi(OIP)3 +
Ti(OIP)4 + Ti(OIP)3OH => Ti(OIP)3OTi(OIP)3 + IPA
Over all reaction of step 1
3. RESULTS AND DISCUSSION
2 Ti(OIP)4 + H2O => 2 Ti(OIP)3OTi(OIP)3 + 2 IPA
Over all reaction of step 2
1.70
2 -Ti(OIP)3 + H2O => 2 -Ti(OIP)2OTi(OIP)2-+ 2 IPA
Over all reaction of step 3
1.65
Refractive index
2 -Ti(OIP)2 + H2O => 2 -Ti(OIP)OTi(OIP)-+ 2 IPA
Over all reaction of step 4
2 -Ti(OIP) + H2O => 2 -TiOTi-+ 2 IPA
1.60
1.55
(b)
(a)
1.50
0
10
20
30
40
50
60
70
TiO2 content (wt%)
Figure 7. Refractive indexes of epoxy resin/TiO2
nanocomposite.
`
`
Figure 5. X-Ray mapping images of epoxy resin
containing 40% TiO2 nanoparticles. (a) Titanium (b)
Silicone (scale bars 5000 nm)
Figure 2 show FTIR spectra of GPTMS, unmodified
TiO2 and modified TiO2. In Figure 2b, the peak at ~1620
cm-1 was attributed to adsorbed water on the surface of
TiO2. In the spectrum of TiO2 modified with GPTMS
(Figure 1c), the bands region ~2960 to 2860 cm-1 and
broad peak at 3230 cm-1 were assigned to the alkyl
groups and hydroxyl groups of modified TiO2
respectively. The small peak at 1081 cm-1 corresponds to
Si-O-C or Si-O-Si bonds. The new peak at 951 cm-1 in
the spectrum is attributable to the stretching vibration
band of Ti-O-Si bonds.14,15 Furthermore, the peak
intensity of OH group on modified TiO2 decreases in
comparison with that of TiO2. Those result confirmed the
reaction of silane groups with hydroxyl groups on the
surface of TiO2 particles. The reaction is shown in
Scheme 2. Figure 1(b) shows TEM image of TiO2
modified with GPTMS. It is clearly observed in Figure
2(b) that after modification, the sizes of modified TiO2
are in the range of 3-8 nm and less agglomeration are
observed than unmodified TiO2.
100
Transmittance (%)
80
60
(1)
(2)
(3)
(4)
(5)
(6)
40
20
0
300
400
500
600
700
Wave length (nm)
Figure 6. UV spectra of epoxy resin/TiO2 nanocomposite
3.2 Nanocomposites.
Figure 3 shows optical images of nanocomposites based
on epoxy resin and unmodified TiO 2 or modified
TiO2. Both of them are homogeneous and transparent,
but the nanocomposites of modified TiO2 show less
yellowing than that of unmodified one. It is generally
known that TiO2 nanoparticle is a strong oxidant
showing catalytic activities. Thus, polymers having
ester units in contact with TiO2 particle show yellowing.
7,10
After TiO2 was modified with GPTMS, it covers
the surface of TiO2 and prevents the direct contact
between polymers with the surface of TiO2. Thus,
yellowing of the nanocomposite was decreased.
Figure 4 shows TEM images of the nanocomposite
containing 10% of modified TiO2. At low
magnification, the sample was observed homogeneous.
Furthermore, at high magnification fine dispersion of
modified TiO2 particles in the range 3-8 nm were
observed, remaining in the original size without
aggregation in epoxy matrix. At high TiO2 content, Xray mapping images were done and images shown in
Figure 5 were obtained. It is clearly observed in Figure
5 that both silicone and titanium were detected and
dispersed homogenously in epoxy resin. From TEM
and X-ray mapping images, it can be concluded that
homogenous and fine dispersion of modified TiO2 in
epoxy resin was achieved.
Figure 6 shows UV spectra of epoxy resin/TiO2
nanocomposites. It is observed in Figure 6 that pure
epoxy resin shows very high transmittance in visible
region and slight absorption in UV region. When TiO2
were incorporated, the nanocomposites show strong
UV absorption but maintain high transmittance in
visible region. This is due to characteristic properties of
TiO2
nanoparticles.16
The
transmittance
of
nanocomposites at UV region (420 nm) and at visible
region (700 nm) are slightly different. This implies that
the yellow index are relatively very low in comparison
with previous studies.10,13 The transmittance of
nanocomposites slightly decreased with increasing
TiO2 content. At loading of 60% TiO2, the
transmittance was 98.4% at wave length of 700 nm.
Those results are attributed by fine dispersion of the
small size TiO2 particles in epoxy matrix.
The refractive index of the nanocomposites is shown
in Figure 7 and summaried in Table 1. The refractive
index of nanocomposites increased linearly with TiO2
content. It is in agreement with reports in literature.2,3,10
2.
3.
4. CONCLUSIONS
4.
The colloidal TiO2 prepared by conventional sol-gel
method was translucent and showed precipitation. On the
other hand, the colloidal TiO2 prepared by stepwise solgel method was homogenous, transparent, and stable.
XRD data confirmed amorphous nature of TiO2. The
modified TiO2 was confirmed by new peak appearing at
951 cm-1 in FR-IR corresponding to Ti-O-Si bonds.
Particle sizes of modified TiO2 observed in TEM were in
range of 3-8 nm. The nanocomposites bases on modified
TiO2 showed less yellowing than that of unmodified one
due to GPTMS covering the surface of TiO2 particle and
preventing contact between epoxy resins with TiO2. The
nanocomposite of modified TiO2 show strongly UV
absorption but maintain high transmittance in visible
region. It was attributed to fine dispersion of TiO2
nanoparticles in epoxy resin which was confirmed by
TEM and X-ray mapping images. The refractive index of
the nanocomposite increased linearly with increasing the
content of TiO2.
5.
REFERENCE
1.
R. A. Ortiz, M. D. L G. Cisneros, G. A. Garcı´a,
Polymer, 46, 10663 (2005).
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
C. Guan, C. L. Lu, Y. F. Liu, B. Yang, J. Appl.
Polym. Sci., 102, 1631 (2006).
J. G. Liu, Y. Nakamura, T. Ogura, Y. Shibasaki, S.
Ando, M. Ueda, Chem. Mater. 20, 273 (2008).
W. F. Ho, M. A. Uddin, H. P. Chan, J. Appl.
Polym. Sci. 102, 1631 (2006).
M. Sangermano, B. Voit, F. Sordo, K. J. Eichhorn,
G. Rizza, Polymer 49, 2018 (2008).
N. Nakayama, T. Hayashi, Composites Part A, 38,
1996 (2007).
N. Nakayama, T. Hayashi, J. Appl. Polym. Sci.,
105, 3662 (2007).
Y. Chen, S. Zhou, G. Gu, L. Wu, Polymer, 47,
1640 (2006).
M. Sangermano, E. Borlatto, F. D. D. Bytner, A.
Priola, G. Rizza, Prog. Org. Coat., 59, 122 (2007).
Y. Imai, A. Terahara, Y. Hakuta, K. Matsui, H.
Hayashi, N. Ueno, Euro. Polym., 45, 630 (2009).
D. Sun, W. N. Everett, M. Wong, H. J. Sue, N.
Miyatake, Macromolecules, 42, 1665 (2009).
S. Wu, G. Zhou, M. GuO, Optic. Mater., 29, 1793
(2007).
J. L. H. Chau, H. W. Liu, W. F. Su, J. Phys. Chem.
Solids ,70, 1385 (2009).
M. Sabzi, S.M. Mirabedini, J. Zohuriaan-Mehr, M.
Atai, Prog. Org. Coat., 65, 222 (2009).
Z. Li, B. Hou, Y. Xu, D. Wu, Y. Sun, W. Hu, F.
Deng, J. Solid State Chem., 178, 1395 (2005).
Q. Qu, H. Geng, R. Peng, Q. Cui, X. Gu, F. Li, M.
Wang, Langmuir, DOI:10.1021/la100121 n.