Synthesis of Dispersed and Self-Assembled Metal Particles in Epoxy via Aqueous to Organic Phase Transfer Technique

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A convenient and effective method of dispersing gold (Au), silver (Ag) and copper (Cu) particles in epoxy is described. Particles were synthesized in aqueous phase and subsequently dispersed in toluene-epoxy with or without the presence of curing agent. Sodium borohydride and 2-propanol were used as reducing and phase transfer agents respectively. The surface plasmon resonance (SPR) at ~ 540-620 nm, ~ 525 nm and ~450 nm confirms the presence of Cu, Au and Ag nanoparticles in the composites. No chemical interaction occurred between the metal particles and the epoxy or cured epoxy. Increase in epoxy content caused the size of the particles to decrease for all metals. The average particle size for Au, Ag and Cu in 1% and 10% epoxy are 3.9 nm and 2.5 nm, 10.1 nm and 3.2 nm as well as 8.1 nm and 5.9 nm respectively. Similar decreasing trend was also observed upon addition of curing agent. In all cases, metal nanoparticles exhibit self-assembly with inter-particle spacing of <10 nm. The absence of any curing peak in the DSC thermograms suggests that the composites are well cured before any of the DSC analyses were performed. The Tg of the metal-cured epoxy is higher than neat epoxy, that is 71.3 C, 69.8 C and 62.0 C for Cu, Au and Ag respectively.

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_Avestia Publishing_
_International Journal of Theoretical and Applied Nanotechnology_
_Volume 3, Year 2015_
_ISSN: 1929-1248_
_DOI: 10.11159/ijtan.2015.002_

# Synthesis of Dispersed and Self-Assembled Metal Particles in Epoxy via Aqueous to Organic Phase Transfer Technique

Noorul Ain Mohd Akib 1 , Nor Hashila Mohd. Hirmizi 1 , Wei Leng Tan 1 ,
Noor Hana Hanif Abu Bakar 1 , Mohamad Abu Bakar 1 *, Jamil Ismail 1, 2 ,

Cheng Hoon Teoh 1, 3 , Chun Hwa See 3, 4

1Nanoscience Research Laboratory, School of Chemical Sciences,
Universiti Sains Malaysia, 11800 Penang, Malaysia
naakib06@yahoo.com; nhashila@gmail.com; weileng728@gmail.com; hana_hanif@usm.my; bmohamad@usm.my

2Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang,
Lebuhraya Tun Razak, 26300, Gambang, Kuantan, Pahang, Malaysia

jamishoot@yahoo.com;

3Intel Technology (M) Sdn. Bhd., Bayan Lepas FTZ, Phase III, 11900 Penang, Malaysia
cheng.hoon.teoh@intel.com;

4BCI Technology Sdn Bhd., Lot 7, Jalan BS 7/22, Taman Perindustrian Bukit Serdang,
Seksyen 7, 43300 Sri Kembangan, Malaysia

chun.hwa.see@bcichemical.com.my

_Abstract-_ _A convenient and effective method of dispersing gold_
_(Au), silver (Ag) and copper (Cu) particles in epoxy is described._
_Particles were synthesized in aqueous phase and subsequently_
_dispersed in toluene-epoxy with or without the presence of_
_curing agent. Sodium borohydride and 2-propanol were used as_
_reducing and phase transfer agents respectively. The surface_
_plasmon resonance (SPR) at ~ 540-620 nm, ~ 525 nm and ~450_
_nm confirms the presence of Cu, Au and Ag nanoparticles in the_
_composites. No chemical interaction occurred between the_
_metal particles and the epoxy or cured epoxy. Increase_ _in epoxy_
_content caused the size of the particles to decrease for all metals._
_The average particle size for Au, Ag and Cu in 1% and 10% epoxy_
_are 3.9 nm and 2.5 nm, 10.1 nm and 3.2 nm as well as 8.1 nm and_
_5.9 nm respectively. Similar decreasing trend was also observed_
_upon addition of curing agent._ _In all cases, metal nanoparticles_
_exhibit self-assembly with inter-particle spacing of <10 nm. The_
_absence of any curing peak in the DSC thermograms suggests_
_that the composites are well cured before any of the DSC_
_analyses were performed. The Tg of the metal-cured epoxy is_
_higher than neat epoxy, that is 71.3 °C, 69.8 °C and 62.0 °C for_
_Cu, Au and Ag respectively._

_Keywords_ : Copper, Gold, Silver, Particles, Epoxy,
Composite.

_© Copyright 2015 Authors - This is an Open Access article_
_published_ _under_ _the_ _Creative_ _Commons_ _Attribution_
_License terms (http://creativecommons.org/licenses/by/3.0)._

_Date Received: 2014-09-29_
_Date Accepted: 2015-03-24_
_Date Published: 2015_ _06_ _08_

_Unrestricted use, distribution, and reproduction in any medium_
_are permitted, provided the original work is properly cited._

1. Introduction
Nanocomposites are comprised of two or more
components of which at least one is in the nanometer
dimension [1]. The nanosize component in the
nanocomposite exposes enormous surface area and
interfaces when compared to the component in the
conventional composite. Hence, nanocomposites may
exhibit properties that promise a wider or better
performance and the ensued applications as compared
to conventional composites. For example,
nanocomposites comprising of polymer and nanosize
fillers such as metal oxide or carbon has been shown to
exhibit unique electrical [2 - 4], thermal [5, 6] and optical

[7, 8] properties.

The conductive characteristic of metal-polymer
nanocomposites has created intense interest among
researchers. It is known that the distribution or topology
of nanosized metal particles in a polymer exert effect on
these properties. The metal entities can form an interparticle network that creates a path where electron(s)
can migrate from one particle to another hence
facilitating electrical or thermal conduction [9]. On the
other hand, particle aggregation creates voids between

particles which disturb the continuous path or linkage

[10]. Thus, well dispersed particles that form interparticle network/assembly along with narrow interparticle spacing are much desired for conduction.

Epoxy resin is a material that is widely used in
electronic packaging. It offers properties such as high
impact strength and adhesion [11]. Incorporation of
metal or carbon based fillers into the resin may affect the
electrical property of the resultant composite e.g. from
an insulator to a conductor [9, 12]. Silver is a frequently
used metal in such composites [13, 14] due to its high
thermal and electrical conductivity [15, 16]. Copper and
gold are also conductors that are comparable to silver.
Gold is the most stable against oxidation however, is
expensive. Copper, on the other hand, is cheaper and
therefore a good substitute to silver and gold.

The available physical techniques such as milling
process [17] employed in the preparation of metalpolymer nanocomposites usually lead to wider
distribution and poor dispersion of the particles. There
are also some chemical routes that have been reported
which instantly provide a better particle distribution and
dispersion. These chemical routes include
microelmusion [13], chemical reduction [18, 19] and UV
reduction [20]. Even so, when particles are successfully
incorporated and dispersed into the polymer matrix,
these particles tend to agglomerate upon heat treatment
(such as curing) forming secondary clusters via the
interruption of inter-particle arrangements and
consequently this leads to poor conduction [13].

Rong et al. [13] have shown the effect of
temperature on silver-epoxy nanocomposite systems.
They found that heating the system at temperatures
which exceeds the melting point of the metal causes the
metal phase to grow and aggregate. This indirectly leads
to a lower conductivity, which is a handicap in electronic
packaging industries. Recently, Chen and co-workers

[21] have successfully prepared highly conductive epoxy
composited with silver coated copper flakes via _in-situ_
generation and sintering silver nanoparticles during
curing process. The sintering process not only prevent
the oxidation of Cu, it also increase the contact area
among the metal flakes, thus improve its conductivity.
Nam et al. [22] and Chen et al. [23] also reported that the
thermomechanical and dielectric properties as well as
electrical and thermal conductivity of the silver-epoxy
nanocomposites can be altered via the addition of silica
particles. Chen and co-workers [24, 25] demonstrated
the role of ethanol as diluents to control the dispersion
of silver nanoparticles and nanowires to fill in the gap in

between the silver flakes in a composite. They found that
the existence of silver particles/wires provides extra
paths for electron conduction, thus higher conductivity
is achieved.

Therefore, understanding the nature and
exploring methods to improve the composite material
performance is desired. This is especially true for the
preparation and performance of metal-filled composites
for application as conductive materials which has
remained a challenge to scientists. As an alternative
route, this study focuses on the dispersion of copper,
gold and silver nanoparticles in epoxy by employing the
phase transfer method. The effect of epoxy, with or
without the (ambient temperature) amine curing agent,
on the dispersion of metal particles is investigated.
Several techniques to determine particle dispersion are
reported. These techniques include suspension [26],
turbidity [27], CO adsorption [28] and the approximate
methods via SEM and optical images. In this work, the
fractal technique was employed to determine particle
dispersion [29]. Other characterization techniques were
also used to evaluate the nature in which particles are
dispersed. This includes UV-Vis, TEM and FTIR.

2. Experimental
All the works described were performed at
Universiti Sains Malaysia, Penang. The percentage
volume per volume (% v/v) refers to the volume
percentage of epoxy or epoxy-curing agent with respect
to toluene. For calculations involving the epoxy-curing
agent, the ratio of epoxy resin to curing agent is 1:1 by
volume. All experiments were carried out at atmospheric
conditions.

2. 1. Materials and Equipments
Unless otherwise stated, all chemicals were
obtained commercially and were used as received. Silver
nitrate (Johnson Matthey), hydrogen tetrachloroaurate
(III) trihydrate (Aldrich), copper acetate (BDH), sodium
borohydride (Riedel-de Haen), toluene (Fisher
Scientific), 2-propanol (R&M Chemicals) and N-cetylN,N,N-trimethylammonium bromide, CTAB (Merck)
were employed. The epoxy resin, diglycedil ether
bisphenol A, DGEBA (Clear 331) and the room
temperature amine curing agent (Clear 2963) were
obtained from Dow Plastics (M) Sdn Bhd.

The optical properties of metal organosols were
measured using a HITACHI U2000 UV-vis
spectrophotometer within the range of 200 to 700 nm.
The FTIR spectrum was recorded in the region of 4000

to 650 cm -1 using a Thermo Nicolet IR200 spectrometer.
The samples were cast in between ZnSe windows to form
a thin film. The morphology of the metal particles was
investigated using a Philip CM 12 transmission electron
microscope (TEM) operating at 80 kV. The TEM samples
were prepared by dropping the organosol onto a carbon
coated copper grid. The solvent was allowed to
evaporate off before imaging. Subsequently, the
“Analysis Docu” Version 3.2 (Soft Imaging System GmbH,
Munster, Germany) computer program was used to
analyze the particle diameters. The average particle size
was calculated from more than 300 particles and the
standard deviation (SD) using equation (1),

(SD) 2 = [Σ (xi – x) 2 ]/N (1)

Where xi is the particle size, x is the average
particle size and N is the total number of particles.
A Perkin Elmer Analyst 100 atomic absorption
spectrometer (AAS) was used to determine the metal
content in the organosols. For the AAS determinations,
as much as 1 ml of the respective organosol was pipette,
digested with aqua-regia and then made up to 25 ml with
distilled water. The thermal behaviors of the composites
were studied using a Perkin-Elmer Pyris differential
scanning calorimeter (DSC) at a heating rate of 20 o C min -

1 under nitrogen atmosphere. In the first heating cycle,
the sample was heated from 30 o C to 200 o C at a rate of
20 o C min -1 . Samples were then held at this temperature
for a minute. The sample was then quenched to -50 o C
and held for another 3 minutes. This was followed by the
second heating run from -50 o C to 200 o C at the same
heating rate.

2. 2. Preparative Method
A 0.2 ml of 5x10 -6 mol ml -1 aqueous solution of the
respective metal salt and 2 ml of 1x10 -5 mol ml -1 aqueous
solution of CTAB were mixed and made up to a total of 5
ml with distilled water. As much as 0.18 ml of 1x10 -4 mol
ml -1 aqueous solution of chilled NaBH4 was added to the
mixture. The reaction mixture was stirred for 3 minutes.
Subsequently, 5 ml of the previously prepared epoxytoluene or epoxy/curing agent-toluene solution was
then added (see Table 1) and this was stirred vigorously
to form an emulsion. Then 5 ml of 2-propanol was added
while continuous stirring. Finally, the stirring was
stopped and the mixture was then left for phase
separation. A metal-filled polymer composite was
obtained upon casting and drying the respective
organosol.

Table 1. Experiments performed with various matrices.
Experiment Matrix*

1 1 % vol. Epoxy
2 1 % vol. Epoxy/curing agent

3 10 % vol. Epoxy
4 10 % vol. Epoxy/curing agent

- % vol. refers to the percentage volume of epoxy or
epoxy/curing agent relative to toluene

3. Results and Discussion
The various CTAB-stabilized metal nanoparticles
were first synthesized via the reduction of the respective
metal salts with sodium borohydride in an aqueous
phase. The overall reaction for the typical borohydride
reduction is shown in the generalized equation (2) below

[30 - 32]. A brilliant colored hydrosol was obtained
depending on the metal used i.e. brown, wine-red and
yellow for copper, gold and silver respectively.

M n+ + nNaBH4 + 3nH2O → M 0 + nH3BO3 + nNa + +
(7/2)nH2 (2)

Where M (n) = Ag (1), Cu (2) or Au (3)

The nanoparticles in the hydrosol were then
transferred with the aid of 2-propanol into toluene
containing either epoxy or epoxy/curing agent via the
formation of emulsion and under vigorous stirring.
When stirring was stopped, the mixture slowly separates
into two layers where the bottom aqueous layer
becomes colorless while the upper organic layer exhibits
a luminous color that is characteristic of the respective
metal organosol. However, these organosols are darker
in their respective color when the curing agent is
present.

UV-vis absorption spectra of the various metal
organosols confirm the availability of the metal
nanoparticles. This is shown in Figure 1. The spectra
show the surface plasmon resonance (SPR) bands at ~
500 nm, ~ 525 nm and ~450 nm corresponding to the
presence of copper [33], gold [34] and silver [35]
nanoparticles respectively. It is interesting to note that
the absorption spectra for copper and gold in the
epoxy/curing agent is broader when compared to their
respective metal particles in epoxy. This is significantly
obvious for the copper-epoxy/curing agent sample and
seems independent of the Cu content percentage. In the
case of copper-epoxy/curing agent sample, two peaks

are also observed at approximately ~540 nm and ~620
nm as compared to one peak for Cu-epoxy at ~500nm.
This leads to the belief that two various size ranges of
particles may exist in the Cu epoxy/curing agent sample.
Several works have reported that the surface plasmon
resonance for Cu nanoparticles exhibits a maximum in
the range of 560-610 nm [36, 37]. However, this is
strongly influenced by several factors including the
refractive index of the surrounding medium, its nature
whether colloidal or film, the distribution of the particles
as well as their size and shape [36, 38]. This may explain
the variation in the position of the SPR band observed in
this work when compared to other work. For the gold
epoxy/curing agent organosols, the position of the SPR
band seems to be similar to the gold epoxy organosols.

(a)

(b)

However, the bands are slightly broader when compared
to gold epoxy organosols, indicating a larger size
distribution of particles. In contrast, no distinct
difference is observed in the spectra of the Ag
organosols. This indicates that curing of the epoxy resin
affects the particle distribution of Ag in the samples
minimally, if any. X-ray diffraction analyses (not shown)
were also conducted to confirm the state in which the
metal particles existed. However, no peaks attributed to
the metal particles or oxides (for copper) were observed.
This may be due to the low metal content when
compared to the epoxy/curing agent content as well as
the masking effect of the polymer.

(c)

Figure 1. UV-vis absorption spectra of various metal viz. (a) copper, (b) gold and (c) silver in various organosols containing of

1% (v/v) - epoxy (-x-);  epoxy/curing agent (-+-); and 10% (v/v) - epoxy (-■-); epoxy/curing agent (-▲-).

The respective metal content (wrt the metal provided the most metal particle accommodation. The
precusors) in the organosols are summarized in Table 2 presence of the epoxy in the toluene phase assists in the
(column 3). Generally more than 80 % of the metal trapping of the metal particles. However, in the presence
particles are present in the organosols. These results of curing agent, the formation of crosslink networks
show that 2-propanol is an effective transferring agent. within the matrix, further provide effective trapping for
It is soluble in both the aqueous and toluene phase the metal particles. It should be noted that in order to
therefore enables transportation of metal particles from obtain an effective particle transfer, the ratio of 2the former to the latter phase [32]. propanol to toluene as well as the composition of the

Among the various organosols that were employed
in this work, 10% (v/v) of epoxy/curing agent-toluene

provided the most metal particle accommodation. The
presence of the epoxy in the toluene phase assists in the
trapping of the metal particles. However, in the presence
of curing agent, the formation of crosslink networks
within the matrix, further provide effective trapping for
the metal particles. It should be noted that in order to
obtain an effective particle transfer, the ratio of 2propanol to toluene as well as the composition of the
organosol are crucial.

Table 2. The metal content, average size, fractal dimension and correlation coefficient of the various samples.
Organosol Metal % Metal Av. Size Fractal Correlation
content in the and SD Dimension coefficient (R)
organic phase* (nm) (D value)

Av. Size
and SD
(nm)

Fractal
Dimension
(D value)

Correlation
coefficient (R)

1% vol. epoxy Copper
Gold
Silver

1% vol. epoxy Copper 89.4 3.9 ± 1.0 1.79 0.9976
Gold 93.6 10.1 ± 1.5 1.88 0.9957
Silver 93.5 8.1 ± 2.3 1.85 0.9995

1 % vol. epoxy/curing Copper 85.9 3.6 ± 1.0 1.92 0.9743
agent Gold 93.4 3.5 ± 0.6 1.95 0.9979

3.9 ± 1.0
10.1 ± 1.5
8.1 ± 2.3

3.6 ± 1.0
3.5 ± 0.6
6.8 ± 2.6

3.0 ± 1.1
8.5 ± 2.6
6.9 ± 2.1

2.5 ± 0.5
3.2 ± 0.7
5.9 ± 3.2

1.79
1.88
1.85

1.92
1.95
1.96

1.83
2.02
1.95

1.87
1.99
2.10

1 % vol. epoxy/curing Copper 85.9 3.6 ± 1.0 1.92 0.9743
agent Gold 93.4 3.5 ± 0.6 1.95 0.9979

Silver 88.8 6.8 ± 2.6 1.96 0.9988

10 % vol. epoxy Copper 81.3 3.0 ± 1.1 1.83 0.9773
Gold 90.3 8.5 ± 2.6 2.02 0.9989
Silver 92.3 6.9 ± 2.1 1.95 0.9811

Copper
Gold
Silver

10 % vol. epoxy Copper 81.3 3.0 ± 1.1 1.83 0.9773
Gold 90.3 8.5 ± 2.6 2.02 0.9989
Silver 92.3 6.9 ± 2.1 1.95 0.9811

10 % vol. epoxy/curing Copper 90.4 2.5 ± 0.5 1.87 0.9917
agent Gold 95.4 3.2 ± 0.7 1.99 0.9834

Silver 93.3 5.9 ± 3.2 2.10 0.9549

*wrt the metal precursor

Copper
Gold
Silver

89.4
93.6
93.5

85.9
93.4
88.8

81.3
90.3
92.3

90.4
95.4
93.3

The FTIR spectra of neat epoxy and the typical
composites of silver-epoxy and silver-cured epoxy are
shown in Figure 2. The spectrum of the epoxy, shown in
Figure 2(a), exhibits peaks at ~2966 cm -1 and ~2918 cm -

1 that are due to the assymmetric -CH2- stretching while
the peak at ~2872 cm -1 is due to the symmetric
stretching of the aliphatic carbon chains [39, 40]. The
stretching frequencies of the aromatic group are situated
at ~3038 and 3057 cm -1 . The peak at ~1509 cm -1 is due
to the C=C of the 1,4-disubstituted benzene ring [36] of
the epoxy while the characteristic peak of oxirane

[42,43] is located at ~915 cm -1 .

There are no significant change between the peaks
position in the FTIR spectrum of the neat epoxy in Figure
2(a) above, and that of the metal-epoxy composite
shown in Figure 2(b). For example, the oxirane peak is at
915 cm -1 . This verifies that no chemical interactions have
occurred between the metal particles and the epoxy. It
should be noted that traces of water, which is
represented by a broad peak at ~ 3498 cm -1 [40], are
observed in both spectra of Figure 2 (a) and (b). This is
due to the moisture present in the neat epoxy resin
precursor. However, upon the addition of curing agent
into the metal-epoxy, this peak intensity is increased,
shifted and broadened within the range of 3074-3444
cm -1 as shown in Figure 2(c). This is attributed to the
formation of -OH functional groups as a result of
reactions of the amine curing agent and the oxirane [44].
The reaction between epoxy and amine curing agent also
results in the formation of the new carbon-nitrogen
linkages. Figure 3 shows an example of the curing
mechanism between DGEBA epoxy resins and a typical

amine based curing agent [45]. The presence of the
curing agent in the composite also generates an extra
peak at 1625 cm -1 that is attributed to -NH2 [40] of the
unreacted amine of the curing agent itself.

In the silver-epoxy composite sample previously
reported by Rong et al. [13], without prior curing, one
exothermic peak is observed in the thermogram. This is
due to the migration of melted silver nanoparticles and
the formation of larger secondary particles due to the
polymer’s fluidity upon heat treatment. This means the
silver particles tend to rearrange and grow upon heating
resulting in aggregation of the particles. Eventually this
leads to the destruction of existing conducting networks
and thus a decrease in the conductivity.

The DSC thermograms obtained from the first and
second heating of the respective metal-cured epoxy are
shown in Figure 4. The thermograms obtained from the
first heating of the metal-cured epoxy composites
revealed only melting peaks, as shown in Figure 4(a).
These peaks are positioned at 72.3 o C, 55.5 o C and 84.8 o C
for copper, gold and silver nanoparticles respectively.
These melting peaks are absent in the thermograms of
second heating as shown in Figure 4(b). This indicates
that the metal nanoparticles are transformed to
conventional metal nanocrystals during the first heating

[13].

The absence of any curing peak in the DSC
thermograms above suggests that the composites are
well cured before any of the DSC analyses were
performed. This has an important implication to the final
composites because the cured epoxy matrix will restrict
the movement of metal particles and thus limit the

aggregations of the melted particles during the DSC
heating cycles. Hence, our results have shown that
though particle aggregation can occur during heat
treatment of the metal-epoxy composites, the extent of

aggregation can be overcome via dispersing the particles
_in-situ_ and using a RT curing agent. This enables the
fixing of the dispersed particles within the matrix.

Figure 2. Typical FTIR spectra of (a) neat epoxy, (b) silver-epoxy and (c) silver-cured epoxy.

Figure 3. Typical cross-linking (curing) reaction between DGEBA molecule (epoxy resin) and an amine based curing agent [35].

[where ● = alkyl or aryl ].

Moreover, from Figure 4(b), the respective glass
transition temperature (Tg) of metal-cured epoxy
composites also support the above findings. Tg of neat
epoxy, cured epoxy and the metal-cured epoxy
composites are tabulated in Table 3. Neat epoxy shows

low Tg at -12.6 o C. Upon curing this epoxy, the Tg is shifted
to a higher temperature _ca._ 42.6 o C. This is due to the
cross-linking reactions that resulted in a decrease in free
volume of the epoxy as well as the restriction in the
movement of the polymer chains. Upon incorporation of

the metal nanoparticles, the Tg of the metal-cured epoxy
is further increased to 71.3 °C, 69.8 °C and 62.0 °C for
copper, gold and silver respectively. This is common for
the inorganic-epoxy composites and is attributed to the
particle-matrix interactions as well as further decrease
in free volume of the matrix [46].

(a)

Table 3. Tg values of neat epoxy, cured epoxy and the metal

cured epoxy composites.
Epoxy/Composite Tg ( ° C)
Neat epoxy -12.6
Cured epoxy 42.6
Cu-cured epoxy 71.3
Au-cured epoxy 69.8
Ag-cured epoxy 62.0

-12.6
42.6
71.3
69.8
62.0

(b)
Figure 4. The DSC thermograms for various metal nanoparticles, (i) Cu, (ii) Au and (iii) Ag, in 10% (v/v) epoxy/curing agent (a)

first heating and (b) second heating scan.

TEM images of the various samples prepared are
shown in Figure 5. Figure 5(a) and (b) demonstrate that
some degree of self-assembly of the particles occurs for
all metals, regardless of the amount of epoxy used.
However, this phenomenon is more obvious for the gold
nanoparticles. Furthermore, it is also found that at
higher contents of epoxy ( _i.e._ 10% v/v) the selfassemblies are less compact and seem to deteriorate
especially for copper and silver. The particles are
dispersed with a narrow space between them. The
average inter-particle spacing of gold, silver and copper
nanoparticles is 1.73 nm, 3.61 nm and 3.15 nm
respectively. It is well known that electrical conduction
and tunneling effect can occur if the spacing between the
particles is small (< 10 nm) [9]. Hence, the inter-particle
spacing in these samples is suitable for electrical
conduction.

The average metal particle sizes and SD values of
the metal-epoxy composites are tabulated in Table 2
(column 4). Generally, as the epoxy content is increased,
the average size of the particles is decreased _i.e_ . from 3.9
± 1.0 nm to 3.0 ± 1.1 nm for copper, from 10.1 ± 1.5 nm
to 8.5 ± 2.6 nm for gold and 8.1 ± 2.3 nm to 6.9 ± 2.1 nm
for silver. Upon curing the epoxy, the particle dispersion
and assemblies remain intact. This is shown in Figure

5(c) and 5(d). Even so, a decrease in the average size as
well as the size distribution of the particles is observed
when the metal particles are formed in these samples
when compared to those prepared in epoxy only.

The particle formation in epoxy can be explained
as due to polymer chains which accommodate the metal
particles and thus prevent aggregation or retard the
Oswald ripening process. The increase in amount of
epoxy from 1 % to 10 % enhances this effect. More
polymer chains are available restricting the metal
particles formed from coming in close contact with each
other and growing. This is reflected by the decrease in
particle size with increase in epoxy content as
mentioned previously. Upon curing the epoxy, crosslinking reactions via the oxirane and amine occurs. This
forms networks that have the ability to lock and trap the
growing particles in the inter-chain voids within the
networks. Thus this creates an environment that
constrains further growth of the metal particles in the
cured matrix when compared to epoxy solely. A similar
phenomenon as for the effect of epoxy content on the
formation of the respective particles, is observed when
the epoxy/curing agent content is increased from 1 % to
10 %.

(a)

(b)

Cu Au Ag

(c)

(d)

Figure 5. TEM micrographs of copper, gold and silver nanoparticles in (a) 1% (v/v) epoxy, (b) 10% (v/v) epoxy, (c) 1% (v/v)

epoxy/curing agent and (d) 10% (v/v) epoxy/curing agent.

In order to have a better understanding on the
homogeneity of the dispersion of metal particles within
the matrix, a fractal technique was applied [29] to the
TEM images of Figure 5. This technique involves drawing variables (i.e. ln M and ln r).
of a few circles with a fix radius, _r_ randomly onto the
fractal image followed by a series of circles of increasing 4. Conclusion
_r_ value. Next, the micrograph is manually digitized using
‘ones’ and ‘blanks’ which corresponds to the presence and the subsequent _in-situ_
and the absence of particles. Then, the number of
particles, _M_, within each circle of different radii, _r_ is
counted. For circles with the same radius, an average
value of _M_ is calculated. The fractal dimension, _D_ is
obtained from the slope of the plot of ln _M_ against ln _r_ as
shown in Figure 6. A smaller fractal dimension reflects a
higher homogeneity of the particles in the matrix [29].
The obtained _D_ values and their correlation coefficient
(R) are tabulated in Table 2 (column 5 and 6). Generally,
the fractal dimension of the metal in epoxy (Figure 5(ab)) is smaller than those in the respective cured epoxy
(Figure 5(c-d)). This means that the metal particles are
better dispersed in epoxy alone. This is obvious as no
compartmental constrain is imposed on the particles in
this matrix. Even so, it is interesting to note that
comparison between the metal nanoparticles
demonstrate that the dispersion of gold particles is least
affected upon addition of the curing agent when
compared to silver and copper. This phenomenon is yet

to be understood. It should be noted that the correlation
coefficient (R) obtained for all the plots are > 0.9. This
suggested a strong and positive association between the
variables (i.e. ln M and ln r).

4. Conclusion
The synthesis of metal particles in aqueous phase
and the subsequent _in-situ_ transfer of these particles to
an organic phase containing epoxy or epoxy-curing agent
were successfully performed. UV-vis spectra confirmed
the formation of metal nanoparticles in the organic
phase. More than 80% of the metal nanoparticles were
transferred into the organic phase according to AAS
analyses. No chemical interactions occurred between the
metal nanoparticles and the epoxy. The DSC analyses
showed no curing peaks in all of the samples, suggesting
that the composites were well cured before DSC analyses
were performed. This eliminates the secondary growth
of metal particles which can be attributed to the RT
curing agent. TEM results revealed that metal particles
show some degree of self-assembly in epoxy. A closepacked particle arrangement was observed especially
for gold nanoparticles. The inter-particle spacing
between the respective metal particles are small ( _ca._ <
10 nm) and thus are suitable for conduction. Overall, all
metal particles showed a trend where the average
particles size decreased as the epoxy and epoxy/curing

agent content is increased. This is due to the particle
growth restriction by the epoxy and curing agent. In
addition it is found that the metal particle dispersion in
epoxy is more homogeneous when compared to those
prepared in the cured epoxy. Based on these results we

have demonstrated a convenient and effective method to
disperse metal nanoparticles in epoxy resin. This
preparation technique also promotes the formation of
small sized nanoparticles which have some degree of
self-assembly.

(a) (b)

Figure 6. Plot ln _M_ versus ln _r_ of various metal particles in (a) 1% (v/v) and (b) 10% (v/v) of epoxy [Au(□);Ag(○);Cu(∆)] and

epoxy/curing agent [Au(■);Ag(●);Cu(▲)].

Acknowledgements
The authors would like to thank INTEL Technology
(M) Sdn Bhd (Grant No. 304/PKIMIA/650341/I104) as
well as Universiti Sains Malaysia (Grant no.
1001/PKIMIA/811091) for the financial assistance
granted to this project.

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