Epoxy resins belong to the group of thermosets and are widely used
as surface coatings, matrix for composites, and adhesives [1], Normally
the cured epoxy material exhibits a three dimensional network structure
and the resulting structure leads to brittleness, due to the high
cross-link density. The common approach to deal with epoxy brittleness
involves the incorporation of fibers, rubbers, thermoplastics,
carbon-based materials, various micro- or nano-sized fillers, leading to
the formation of multi component materials like blends or
micro/nanocomposites [2]. The most common types of nano-sized fdlers
used in the preparation of epoxy nanocomposites ranges from metallic
powders [3-5], inorganic oxides [6-8], (semi) conductive particles [9],
to carbonaceous fillers including carbon black [10], graphite [11],
graphene [12], micro/nano carbon fibers [13-15], and carbon nanotubes
[16]. The "passivation" (for protection) and the
"adhesion" (for maintaining the structural integrity) achieved
as a result of the curing (crosslinking) process make the epoxy based
materials inevitable in the modern industry.

Previous research works clearly indicate the role of micro/
nano-sized particles of metallic copper as well as copper oxides as
fillers in determining the multifunctional capability of epoxy-based
composite materials. Bagwell et al. [17] reported that short shaped
copper fibers improved the fracture and impact toughness of an epoxy
matrix, along with significant improvement in its electromagnetic
shielding effectiveness and electrical conductivity. Wu et al. [18]
compared the smoke suppression effect of different transitional metal
oxides on the epoxy resin treated with a halogen free flame retardant
and found that the copper oxide was most effective in substantially
decreasing the maximum smoke density as well as the smoke density rate
of ternary composite (2 wt% metal oxide filler-epoxy composite-flame
retardant) system when compared to other transition metal oxides.

Cuprous ([Cu.sup.+1]) oxide and Cupric ([Cu.sup.+2]) oxide are the
two commonly existing oxides of copper. Basically both are
semiconducting oxide materials but they differ in their color,
stability, electronic characteristics, and hence of course in their
applications. They have currently attracted considerable interest in the
fields of both condensed matter physics and materials chemistry
especially when the cost, easiness while handling, stability, and yield
during production along with eco-friendliness become a great concern.
Copper oxides have been reported to improve the adhesion strength of
epoxy materials [19], Larsen et al. [20] investigated the changes in the
tribological behavior of an epoxy resin-PTFE composite system by
incorporating Cupric Oxide (CuO) nanoparticles and the best results are
seen at a CuO content in the range of 0.1%-0.4%. Recently Zabihi et al.
[21] reported that for epoxy-nano CuO composite material, the optimum
crosslink density and better thermal stability achieved for 5% loading
level of nano CuO. According to Nazari et al. [22], CuO nanoparticles
(up to 4 wt%) were able to improve the mechanical and physical
properties of self compacting concrete and recover the negative effects
of polycarboxylate superplasticizer especially on the split tensile
strength. Their report also claims that the CuO nanoparticles could
improve the pore structure of concrete and shift the distributed pores
to harmless pores.

Though there were few reports on the use of cupric ([Cu.sup.+2])
oxide nanoparticle based epoxy composites [19-22], up to the best of our
knowledge nobody reported the use of cuprous([Cu.sup.+1]) oxide
nanoparticles (nCOP) as a reinforcing filler in epoxy resins. Cuprous
oxide filler particles in nano regime with octahedral morphology were
synthesized through hydrazine reduction method. These nCOP were
homogenously dispersed in epoxy network in different proportions along
with the conservation of its octahedral morphology to build up a novel
nanocomposite material. The major aim of present investigation is to
study the dependence of cure reaction, microstructure, glass transition
temperature ([T.sub.g]), mechanical properties, and thermal stability of
the matrix resin as a function of nCOP content. Finally, a relationship
has been established between the filler domain distribution in the
nanocomposite and their ultimate performance.

EXPERIMENTAL

Preparation of nCOP

All chemicals of reagent grade quality were used without further
purification. Nanosized cuprous oxide particles (nCOP) were prepared
through a typical chemical reduction method using hydrazine hydrate as
the reducing agent described by Wang et al. [23]. One millimole of
copper sulfate pentahydrate (Sigma Aldrich) was added to 1% solution of
poly ethylene glycol (PEG, Mw ~ 4000, Merck) in deionized water, stirred
well, and kept for 2 h. For carrying out the reduction sufficiently, the
pH of the cupric salt-PEG solution was kept around 9 by adding NaOH
solution. Then required amount of hydrazine hydrate (24% solution, Sigma
Aldrich) was added to the reaction mixture, a reddish brown precipitate
was obtained. Then, the solution was centrifuged; the precipitate was
collected and washed with distilled water and absolute ethanol for
several times each, and dried in vacuum at 50[degrees]C for 2 h.

Preparation of Epoxy-nCOP Nanocomposites

Commercially available diglycidyl ether of bisphenol-A (DGEBA)
epoxy resin (R 180), and its aliphatic amine curing agent, H180 (a
mixture of triethylenetetramine and isophoronediamine) from Nuplex
epoxies, NZ were used as received in the mixing ratio 5:1.
Nanocomposites with 0, 1, 3, 5, and 10 phr nCOP in epoxy matrix were
prepared. The amine to epoxide ratio was kept same in all the systems
under investigation. In the composite preparation step, first the
required amount of nCOP was mixed with epoxy resin, stirred overnight
using a magnetic stirrer at 60[degrees]C followed by ultrasonication for
30 min. The above mixture was then degassed in a vacuum oven at
60[degrees]C for 10 min. The mixture was poured into a preheated open
metal mould and cured at 60[degrees]C for 5 h and post cured the samples
at 80[degrees]C for 2 h. Freshly prepared epoxy-nCOP mixtures with
curing agent were used for differential scanning calorimetric (DSC)
analysis.

CHARACTERIZATION TECHNIQUES

X-ray Diffraction

The XRD analysis of nCOP was performed by a Bruker AXS D8-Advanced
X-ray diffractometer with a scanning step of 0.02 in the glancing angle
range from 20[degrees] to 80[degrees] with an operation voltage and
current maintained at 40 kV and 40 mA. The average particle size nCOP
was calculated from the diffractogram using the Debye-Scherrer method
[24] using Eq. 1

D = 0.9 [lambda]/[beta] Cos [theta], (1)

where [lambda] is wave length of X-Ray (0.1541 nm), [beta] is FWHM
(full width at half maximum), [theta] is the diffraction angle, and D is
particle size.

Transmission Electron Microscopy

The morphology of the as prepared nCOP and epoxy-nCOP
nanocomposites was examined using JEOL transmission electron microscope,
model JEM 2100, with an accelerating voltage of 200 kV. The
nanoparticles of copper oxide were first ultrasonicated in ethanol
before taking the image while ultrathin sections of nanocomposites'
specimens (100 nm thickness) were obtained at room temperature using an
ultra microtome fitted with a diamond knife transferred to
carbon-film-coated Cu grids.

Scanning Electron Microscopy

The morphology of the as prepared nCOP as well as impact fracture
surfaces of epoxy-nCOP nanocomposites was examined using Hitachi S4000
FESEM scanning electron microscope. The sample surfaces were sputter
coated with platinum before taking the micrographs.

Evaluation of Kinetic Parameters Using DSC Analysis

The cure reaction in neat epoxy and the nanocomposite samples were
followed using TA 2920 differential scanning calorimeter. The instrument
was calibrated using indium standard. Nitrogen was used as the purge
gas. Five to ten milligrams of sample was placed in Aluminum Hermetic
DSC pan for the measurements. Both dynamic and isothermal scans were
performed to follow the cure kinetics.

Dynamic DSC Analysis. Dynamic DSC measurements were performed at
the heating rates 20, 10, 5, and 2.5[degrees]C/min for neat epoxy resin
and its composite mixtures from 25[degrees]C to 180[degrees]C. The
integrated area of the exothermic curves was used to determine the total
heat of reaction [DELTA][H.sub.tot]. The heat flow curves were used to
approach the dynamic kinetic modelling of the cure process. Kissinger
[25] and Ozawa [26] suggested methods to find the activation energy
during the cure reaction.

According to Kissinger method, the relationship between activation
energy E, the heating rate q, and the temperature [T.sub.p] at which the
exothermic peak has its maximum were described as in Eq. 2:

E = - R d(ln [q/[T.sup.2.sub.p]])/d(1/[T.sub.p]), (2)

where R is the gas constant (8.3144 J/(K mol)). The plot between
ln(q/[T.sup.2.sub.p]) and [T.sup.-1.sub.p] can be obtained. In Ozawa
method, ln(q) is plotted against [T.sup.-1.sub.p] and the activation
energy E, heating rate q, and peak exotherm temperature Tp are related
according to Eq. 3

E = -[R/1052] [d(ln q)/d(1 / [T.sub.p])]. (3)

In both approaches, the activation energy can be calculated from
the slope of the linear fit of experimental data.

Isothermal DSC Analysis. Isothermal measurements were done at 100,
90, 80, 70, and 60[degrees]C, respectively. The cure reaction was
assumed to be complete when the isothermal curve leveled off to a
straight line. The area of the peak under the isothermal curve at
different intervals was used to determine the corresponding degree of
conversion (a) at that time. The degree of conversion a at time t can be
defined using Eq. 4:

[alpha] = [DELTA][H.sub.t]/[DELTA][H.sub.T], (4)

where [DELTA][H.sub.t] is the heat of cure at time t and
[DELTA][H.sub.T] is the total heat of cure of the system under
investigation (determined from dynamic cure schedules). Autocatalytic
model [27-29] has been widely used to describe complex kinetics of cure
reactions and is widely used in modelling software for industry.
According to this model, the rate of reaction and the conversion (a) are
related to one another as follows (Eq. 5)

where [k.sub.1] and [k.sub.2] are the apparent rate constants, m
and n are the kinetic exponents of the reaction, and m + n gives the
overall reaction order. The kinetic constants [k.sub.1] and [k.sub.2]
depend on temperature according to Arrhenius equation (Eq. 6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (6)

where [A.sub.i] is the pre exponential constant, [E.sub.i] is the
activation energy, R is the gas constant, and T is the absolute
temperature.

The conversion versus reaction time at different temperatures of
the neat epoxy and their composites with different nanoparticles
contents (phr) were plotted. The experimental value of the rate of
reaction (d[alpha]/dt) and conversion ([alpha]) for the complete course
of the reaction was computed and adjusted with the kinetic equation.
Also the experimental curves of rate (d[alpha]/dt) versus conversion
([alpha]) for all the systems at different temperatures were compared
with the theoretical model as predicted by Kamal [28]. The kinetic
parameters [k.sub.1], [k.sub.2], m, and n were estimated without any
constraints on them by fitting the experimental data of (d[alpha]/dt)
versus ([alpha]) at different temperatures using a nonlinear
least-square procedure and the values of the reaction kinetic parameters
determined by the fitting process at various curing temperatures were
tabulated.

Thermal Characterization of Epoxy-nCOP Nanocomposites

The thermal stability of the neat epoxy and nCOP filled epoxy
nanocomposites was studied via thermogravimetric analysis (TGA) (TA
Instruments, Model Q-500). All the samples were heated from 30[degrees]C
to 700[degrees]C under nitrogen flow (60 mL/ min) at a heating rate of
10[degrees]C/min.

Mechanical Analysis of Epoxy-nCOP Nanocomposites

A dynamic mechanical analyzer (TA DMA Q800) was used for measuring
the viscoelastic properties of the neat resin as well as the
nanocomposites. Rectangular specimens cured at room temperature were
used for the analysis. The analysis was done in single cantilever mode
at a frequency of 1 Hz and the samples were heated from room temperature
to 200[degrees]C at a heating rate of 5[degrees]C/min.

Tensile tests were performed using an Instron 4201 at room
temperature and constant cross head speed of 0.1 mm/min. At least five
samples from each composition were tested according to ASTM standard
D638-08.

RESULTS AND DISCUSSION

Morphological Characterization of Synthesized nCOP

The adopted method for the production nCOP was the room temperature
chemical reduction of cupric ([Cu.sup.+2]) salt using hydrazine hydrate
as the reducing agent in a polyol (poly ethylene glycol, PEG- Mw ~ 4000)
medium. Hydrazine hydrate in basic aqueous solution is a suitable
reducing agent for the preparation of nCOP as elucidated by Wang et al.
[23]. The chemical reactions involved in the synthesis method can be
described as:

The reaction mixture, that is, cupric ([Cu.sup.+2]) salt in 1%
aqueous solution of PEG, was a clear solution, light blue in color
before being alkylated or reduced. Upon adding sodium hydroxide, a
bright blue precipitate was produced in the reaction mixture due to
formation of cupric ([Cu.sup.+2]) hydroxide. Then required amount of
hydrazine hydrate was added to the reaction mixture. Initially the color
of the solution changes to forest green and gradually to brown red upon
the completion of reduction. The forest green color is characteristic of
cuprous ([Cu.sup.+1])(oxide nanoparticles generated in the reaction
mixture [30], As the reduction proceeds, the color of the reaction
medium changes to reddish brown as more and more cuprous ([Cu.sup.+1])
oxide (nCOP) nanoparticles were formed. The main advantage of this
method over other techniques is the high purity of the nanoparticles
obtained as the bye products of the reduction reaction (water and
nitrogen gas) easily escape from the reaction mixture.

In the experiment, 1% solution of poly ethylene glycol (Mw ~ 4000)
was used as the solvent for controlling the formation of nCOP. The PEG
added to the reaction mixture itself acts as the stabilizer for the
nanoparticles produced [31]. The stabilizer can reduce the Gibbs'
free energy of the surface of the nanoparticles and hence it can prevent
the grains from merging into larger ones [23], Also the active sites for
reduction were markedly decreased and diluted, which should be
beneficial for a mild and controllable reducing reaction [32], Besides
assisting in their stabilization, the hydroxyl groups from the long PEG
chains adhered to the surface of the nCOP can definitely improve its
compatibility while using as filler for a polymer matrix during the
preparation of various composite materials. The crystal structure of the
thus produced nCOP was confirmed by X-ray diffraction (Fig. la). The XRD
spectrum contains five peaks that are clearly distinguishable. All of
them can be perfectly indexed to crystalline copper (I) oxide not only
in peak position, but also in their relative intensity.

The lattice constant calculated from the XRD pattern is a = 0.42618
nm which is in good agreement with the reported value of 0.4269 nm,
given in the International Center of Diffraction Data card (JCPDS le no.
05-0667) confirming the formation of a single cubic phase Cuprous
([Cu.sub.+1]) oxide ([Cu.sub.2]O) with cuprite structure. The peaks with
20 values of 29.6, 36.5, 42.4, 61.5, and 73.6 correspond to the crystal
planes of 110, 111, 200, 220, and 311 of the crystalline Copper (I)
oxide, respectively. No characteristic peaks of Cu metal or cupric
([Cu.sub.+2]) oxide (CuO) are observed in the XRD patterns indicating
that phase-pure cuprous ([Cu.sub.+1]) oxide is readily obtained in the
solution phase by reduction using hydrazine hydrate. The broadness of
the peaks was used to calculate crystallite size of nCOP particles using
Debye-Scherrer equation [24] and the mean size was found to be about 160
nm.

[FIGURE 1 OMITTED]

Electron microscopy (scanning electron microscopy [SEM] and
transmission electron microscopy [TEM]) was used to further identify the
morphology of the as synthesized nCOP. The SEM image showed that the
prepared nCOP displays a lot of stacked spheres or semi spheres almost
uniform in diameters in the range less than 1 [micro]m. It appears that
the stacked structures have rough surfaces and may be composed of
smaller nanoparticles. The more clear and distinct morphology of the as
synthesized nCOP was demonstrated in the TEM image (Fig. 1b).

It can be seen that the particles are having octahedral morphology
with smooth surfaces with an average edge length of about 200 nm.
Octahedral morphology is reported as the thermodynamically stable shape
of cuprous oxide crystals [33, 34], The selected area diffraction
patterns show various diffraction rings of cubic cuprous oxide and the
pattern obtained denotes the polycrystalline nature of the material. The
interplanar distances were consistent with the standard values for cubic
cuprous oxide. Hence, the morphological analysis revealed that the
product has regular shape, small size, narrow size distribution, and
high purity. We believe that more information regarding the growth
mechanism of cubic morphology of nCOP is beyond the scope of this
article and hence not included in this discussion.

Morphological Characterization of Epoxy--nCOP Nanocomposites

TEM analysis shows fine and homogeneous dispersion of nCOP
throughout the epoxy matrix. The variations in contrast and shape of the
octahedral morphology of nCOP in the epoxy matrix were mainly due to
difference in electron scattering from different depth regions of the
section. For epoxy-3 phr nCOP nanocomposites, as shown in Fig. 2, it is
observed that individual nCOPs are randomly dispersed or embedded within
the matrix without any aggregation. The embedded or interwoven nCOPs in
the epoxy matrix indicate the extreme compatibility of poly ethylene
glycol coated nano-sized cubic cuprous oxide particles (nCOP) with the
epoxy matrix. However, as the nCOP content in the epoxy matrix
increases, for example, as in epoxy-5 phr nCOP nanocomposites, the nCOPs
seldom appear as fine particles but are often observed as their stacks
as depicted in Fig. 2. These stacks were uniformly ordered, with an
average domain size of about 1 [micro]m. Such stacking or aggregation of
nCOPs restricts the mobility of the polymeric chains during their curing
process, hence significantly reduces the degree of crosslinking of epoxy
matrix. A more detailed discussion regarding this is given in coming
sections. The well-defined octahedral morphology of nCOP is preserved in
its epoxy based nanocomposites as observed in the TEM results. For the
TEM investigation, ultra thin sections of the samples were cut using an
ultra microtome, but in the image no pull outs of nCOP domains were
seen. This obviously indicates the good compatibility between the nCOP
phase and the epoxy matrix. A plausible explanation for this may be the
presence of PEG (-CH2-CH2 O- - - -CH2-CH2- OH) chains adhered to the
surface of nCOP which can improve the interaction between epoxy chains
and the nCOP domains at their interphases.

To get more insight into the interfacial interaction of epoxy and
nCOP domains in the nanocomposites, the impact fracture surfaces were
investigated in detail using scanning electron microscopy. Figure 3a-e
shows the SEM images of the fracture surfaces of the epoxy
nanocomposites containing different amounts of nCOP. On the fracture
surface of the neat epoxy resin, smooth linear cracks (indicated by
white arrows) propagating with a uniaxial orientation were observed
(Fig. 3a). But when nCOP is added to the epoxy resin, the roughness of
fracture surface was increased as observed by the presence of circular
cracks on the fracture surface. This is a substantial evidence for the
interaction between the epoxy phase and nCOP domains. More over detailed
investigation of Fig. 3a-e reveals that most protruded fracture surface
is obtained for the epoxy-3 phr nCOP nanocomposites. This clearly
indicates that the maximum interfacial interaction is achieved at this
composition. Short ranged linear cracks propagating uniformly in all
directions along the stretched surface were also observed in the image.
This morphology may be due to the presence of more adsorbed matrix
layers on the homogenously oriented nCOP domains, showing the intimate
contact and good adherence of the polymer to the nCOP domains. As the
nCOP content exceeds above 3 phr, the nanocomposites become more brittle
as shown by the SEM images (Fig. 3d and e). The interesting and typical
breakage phenomenon of the epoxy-3 phr nCOP nanocomposites upon impact
fracture tests indicates that the more homogenous is the dispersion of
nCOP in epoxy phase, the stronger will be the interfacial adhesion
between nCOPs and epoxy matrix and the sufficient will be load transfer
from the matrix to nCOP. Thus, a combination of better dispersion and
adhesion of nCOP in epoxy is leading to better characteristics.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Cure Kinetics Using Differential Scanning Calorimetric Analysis

DSC measurements provide the kinetic variables required for the
solution of heat/mass-transfer equations: the heat flow (proportional to
d[alpha]/dt) and the heat generation (proportional to [alpha] or the
conversion). Information on the kinetics of the cure reaction allows the
optimization of processing parameters and thereby controlling the
properties of the products.

Dynamic DSC Studies

The effect of nCOP (0, 1, 3, 5, and 10 phr) on the epoxyamine cure
at different heating rates (20, 10, and 5[degrees]C/min) was illustrated
by the dynamic DSC data (Fig. 4a). At a particular heating rate, the
area of the DSC peak increases for nanocomposite samples with the
increase in its filler content. The larger area of the exotherm refers
to a faster conversion, which in turn denotes that the presence of nCOP
which facilitates the cross linking process of epoxy at a faster rate.
The increase of the heat of reaction relative to neat polymer also
emphasizes the higher crosslinking density in the nanocomposites as
reported in literature [35, 36). For concentrations > 5 phr, the
reaction heat decreases as nCOP distribution becomes more heterogeneous
at these stages. As nCOP content increases, the inter particle distance
between the filler particles in a matrix decreases. As a result, the
particle-particle interaction predominates over the particle-matrix
interactions due to the high characteristic surface energy of
nanoparticles and hence acts as a hindrance for effective cross-linking
in different zones of epoxy matrix. This is a substantial evidence for
the mutual dependence catalytic efficiency of nanofillers and its number
of parts inside the composite material. Zabihi et al. [35] reported
similar results based on investigations using ZnO nanofiller on the
curing reaction of epoxy and concluded that the reaction heat of the
sample having 5 phr ZnO nanoparticles was higher than that of the
others. More recently Karasinski et al. [36] reported similar results
where they have discussed the catalytic effect of different (zinc oxide
and aluminum oxide) nanoparticles in the epoxy-amine reaction and found
that even 3% of [Al.sub.2][O.sub.3] or ZnO nanoparticles could induce
meaningful changes on the matrix cure reaction. But the acid-base
interaction between zinc oxide surfaces and epoxy/hardner moieties leads
to more modified interfaces and hence catalyze the curing reaction more
favorably leading to extended conversion and hence a higher crosslinking
density compared to [Al.sub.2][O.sub.3].

[FIGURE 4 OMITTED]

The peak temperature ([T.sub.p]) at the maximum heat flow shifted
to a higher temperature region with increasing heating rate (Fig. 4a).
This peak-shifting phenomenon caused by the increasing scanning rate
depends on the activation energy associated with each reaction [37],
Based on this peak-shifting phenomenon during the dynamic DSC analyses,
Kissinger [25] and Ozawa [26] suggested methods to find the activation
energy during the dynamic DSC analysis of the cure reaction. According
to the Kissinger method (Eq. 2 and Fig. 4b) ln(q/[T.sup.2.sub.p]) was
plotted against (1/[T.sub.p]) while in Ozawa approach (Eq. 3 and Fig.
4c), the reciprocal of the peak temperature [T.sub.p] values
(1/[T.sub.p]) were plotted against the logarithm of the heating rate q
(ln q). There is an excellent linear fit in all cases, indicating that
both models fit the experimental data quite well. The activation energy
was calculated from the slope of the fitted straight lines. The obtained
activation energy values were shown in Table 1. The values obtained from
both the approaches show similar trend, that is, the activation energy
for the curing of the filled system is lower than that of neat epoxy.
But decrease in [E.sub.a] was not significant after 3 phr nCOP loading.
For neat epoxy, the E.d is 64 kJ/mol and for 3 phr loading, the
[E.sub.a] decreases to 59.7 kJ/mol. But for 5 phr-10 phr nCOP loadings,
there is only a marginal decrease in the [E.sub.a] values, that is, only
58.8 and 58.1 kJ/mol, respectively. Also the activation energy obtained
from Ozawa method is slightly higher than that from the Kissinger
method.

[FIGURE 5 OMITTED]

In a similar way Vijayan et al. [37] applied the Kissinger and
Ozawa model to a DGEBA system with liquid as well as solid additives
cured with an anhydride hardener. Their results show that the activation
energy for the nanoclay filled epoxy system is lower than the unfilled
system while that of epoxy/ nanoclay/liquid rubber ternary system is
found to be lower than unfilled system but slightly higher than
epoxy/nanoclay system, indicating the catalytic activity of clay and the
retarding effect of liquid rubber (carboxyl terminated butadiene
acrylonitrile) on the cure reaction. Harsch et al. [38] investigated the
influence of different additives and aggregates on the curing kinetics
of the epoxy resin system by conventional DSC both under dynamic and
isothermal conditions. According to their report the fillers with
surface modification showed an accelerating effect on the reaction
kinetics of the original epoxy resin as the activation energy was
reduced and on adding any kind of aggregate (liquid and solid additives;
modifiers and fillers) to the epoxy resin a reduction of total heat of
reaction was observed. Catalytic action of various other nanomaterials
like barium carbonate [8], calcium carbonate [39], and so forth, on the
cure reactions of DGEBA systems was also available in the literature.
All these studies confirm the fact that the morphology, inherent
characteristics, filler content as well as surfactant chemistry of
nanofillers altogether play important role in the cure reaction of
epoxy.

Isothermal DSC Studies

The generally accepted scheme of an amino-epoxy cure involves three
main reactions of the glycidyl ether: a primary amine group addition to
the epoxy ring, a secondary amine group addition, and the etherification
[40], This is an autocatalytic process because the hydroxyl molecules
formed as one of the reaction products partly protonate the oxygen atom
of the epoxy group, facilitating the ring-opening reaction. The
concentration of the hydroxyl groups increases as the reaction proceeds,
so the cure rate steeply increases. The process will be synergistically
favored when there were excess -OH groups. The reactions of primary and
secondary amines are described by two rate constants, [k.sub.1] and
[k.sub.2] and their ratio depends on the electron-donating properties of
the amines. Normally, the secondary amine addition is not important in
the beginning of the cure because the primary amine addition controls
the overall rate [41], A schematic representation of the plausible
mechanism of the cure reaction is given in Fig. 5. The dynamic DSC
studies were applicable only during the initial stage of the cure
reaction: as these methods were entirely based on the maximum rate of
cure, which occurs approximately at the beginning of the curing
reaction. To complete the investigation of cure kinetics, isothermal DSC
measurements were performed at different cure temperatures viz., 60, 70,
80, 90, and 100[degrees]C. The corresponding conversion versus time
plots for neat epoxy and the epoxyn-COP nanocomposites were shown in
Fig. 6, respectively. A closer view to the experimental data showed that
nCOP introduced to the system seems to act as accelerators for the cure
reaction. This acceleration effect could be accounted on behalf of the
inherent nature of nCOP, dispersion state of nCOP in epoxy, and poly
hydroxyl surface coating of nCOP.

As we have mentioned in earlier section, octahedral morphology is
reported as the thermodynamically stable shape of cuprous oxide
crystals. In this morphology, the crystal will be bounded by eight (1 1
1) surfaces. Every "Cu"-containing plane is sandwiched between
two "O"-containing planes which means that every (1 1 1) plane
is terminated by an outer layer of oxygen anions, with a second atomic
layer of Cu-I- cations, and then a third atomic layer of oxygen anions,
and so on. The special (O-Cu-O) 180[degrees] linear co-ordination made
its crystalline surfaces of (1 1 1) possess distinctive chemical
activities [42]. Thus, the (1 1 1) plane was expected to possess a
higher energy status and it is also reported that morphology of the Cu20
is having a significant influence on its catalytic activity [43], Coming
to epoxy-nCOP composites, the 3 phr loading exhibits the most exfoliated
microstructure among all the compositions under investigation. A number
of reports are available in literature that the exfoliated orientation
of fillers in epoxy enhances the curing process. In the case of
exfoliated nanocomposites, there is considerable penetration of polymer
chains in between the nCOP and such diffused orientation can cause
significant changes in the physical characteristics of matrices.

[FIGURE 6 OMITTED]

More over hydroxyl groups are known to have a catalytic effect on
the cure reactions, acting as hydrogen-bond donor molecule. With the
strong acid sites in the poly hydroxyl layer of nCOP, electrophiles
could be effectively activated at the curing temperature (Fig. 5). As
nCOP content increases, the PEG fraction on the reaction site also
increases which provides extra regimes for the crosslinking reaction;
and thus further catalyzing effect occurs during the cure reaction.
However, the nCOP content exceeds 5 phr, due to the agglomeration of the
nanofiller as interparticle aggregation hinders the availability of -OH
groups on the surface of nCOP from acting as a catalyst in the epoxy
amine cure process [38]. Alzina et al. [44, 45] studied the effect of
various organically modified montmorillonites, highlighting the
catalytic effect of MMT water content and alkyl ammonium cations on the
oxirane ring opening and also in the whole epoxy/amine cure reaction
mechanism. Kalaee et al. [7] report that the addition nanoparticles
surface coated with poly hydroxy compounds can modify the kinetic
pathways of the epoxy amine additions, facilitating the ring-opening
reactions. According to Wang et al. [46], acidic treated inorganic
fillers can effectively absorb the hardener and act as a support to
disperse the hardener during the curing procedure of the resin system.
Therefore, it can be inferred

that when there is relatively high concentration of curing agents on
the surface of nCOP at the beginning, they can easily advance into the
unreacted zone in epoxy-nCOP nanocomposites during the initial stage of
the process thereby enhancing the rate. Also it is evident that, at a
given nCOP loading, an increase in the isothermal curing temperature
drives the curing reaction forward besides decreasing the curing time.
The reason for such observation could be correlated to the availability
of more thermal energy in addition to lower viscosity of the compound
which facilitates the formation of the cross linking networks [7],

For further understanding the cure kinetics of epoxy resin in the
presence of nCOP, the experimental value of the rate of reaction
(d[alpha]/dl) and conversion (a) for the complete course of the reaction
were computed and adjusted with the kinetic equation (Kamal-Sourour
method). The experimental curves of (d[alpha]/ dt) versus ([alpha]) for
different epoxy systems at three different temperatures along with the
fitted results are shown in Fig. 7. The kinetic parameters [k.sub.1],
[k.sub.2], m, and n were estimated without any constraints on them by
fitting the experimental data of (d[alpha]/dt) versus ([alpha]) at
different temperatures using a nonlinear least-square procedure and are
listed in Table 2. It was found that the Kamal-Sourour equation fits
very well the experimental data in the whole conversion range for the
selected isothermal temperatures.

[FIGURE 7 OMITTED]

It was observed that [k.sub.1] values are lower than [k.sub.2]
values and the overall reaction order, m + n ranges around 2. Generally,
the (m + n) value increases with temperature. This is attributed to the
a trimolecular mechanism [47] in which, few of the hydroxyl groups in
the molecular chain of the epoxy resin can become proton donor and
participate in the reaction with the increasing curing temperature. But
in our results there is no trend for the systematic variation of either
m or n with temperature for different levels of nCOP loading. As already
reported by many authors, this conclusion was expected to reach
theoretically, as m and n are not dependent on curing temperature and
nanofiller content [7], In addition, the cure kinetic characterizations
show a direct proportionality between both the nth-order and
autocatalytic reaction rate constants and also the curing temperature.
The trend in k, values was found to be poor compared with that for
[k.sub.2]. This is because [k.sub.1] is computed only with the two or
three of the first experimental data that may lead to imprecise
calculated values. It was observed that generally the [k.sub.1] values
of neat and modified epoxies are low compared with those obtained for
[k.sub.2] values. Harsch et al. [38] also observed that when virgin
epoxy resin and its version containing additives and surface-modified
Si[O.sub.2] fillers were subjected to isothermal DSC scans, the
conversion of both materials increased with increasing time and
temperature. For the filled material, the total conversion was less than
the virgin resin and the difference in the conversion was larger at
lower cure temperatures.

However, this curing reaction is not a standard nth order reaction,
as for all the samples, [k.sub.2] value was higher than [k.sub.1] which
suggested that autocatalytic mechanism was more favorable than the nth
order mechanism. A variety of reasons might be given for the large
[k.sub.2] value. There is a greater tendency for the curing agent to
react with the monomer adsorbed on the surface of nCOP and upon
completing the initial cure reaction, the reactants cannot move away but
they would rather sequester together. As a result, they are more
prepared for subsequent localized cure reactions. It is interesting to
note that the increment of the nCOP content increases the rate constant
obtained for the nanocomposites which seems to be connected with the
higher extent of absorbed epoxy chains on the PEG treated nCOP surface
rather than catalytic effect of nCOP. According to Eq. 6, the activation
energy values, [E.sub.1] and [E.sub.2], were calculated for all the
systems under investigation. Typical Arrhenius plots for neat epoxy and
the nanocomposites were given in Fig. 8. [E1.sub.1] and [E2.sub.2] can
be determined from the slope of linear relationship between In
([k.sub.1]) and In ([k.sub.2]) versus 1/T. The numerical values
calculated for the above-mentioned parameters are represented in Table 2
for all the samples. Interestingly, the activation energy required for
the cure reaction reduced by 22% and 13% at the initial ([E.sub.1]) and
final stage ([E.sub.2]) of the cure, respectively, by the incorporation
of 3 phr nCOP.

This observation is consistent with the discussion previously
mentioned that the cure rate of epoxy systems increases upon raising the
nCOP content. Consequently, lower amount of energy for curing together
with shorter cure time of nCOP containing epoxy samples over the neat
resin could offers an excellent approach to meeting the requirements of
the present world.

Glass Transition Temperature (Tg) of Epoxy-nCOP Nanocomposites

[T.sub.g] of epoxy-nCOP nanocomposites as function of filler
content was determined using dynamic mechanical analysis. The variation
of tan [delta] with temperature is given in Fig. 9. The temperature
corresponds to the highest tan [delta] value is taken as Tg. The
[T.sub.g] of all the nanocomposites is found to shift toward higher
temperatures compared with that of neat epoxy (Table 3). A maximum
increase in [T.sub.g] is achieved for 3 phr nCOP loading. This increase
in [T.sub.g] of epoxy-nCOP nanocomposites is mainly due to the good
matrix-filler interaction between epoxy and nCOP filler so that nCOP can
restrict the segmental motion of epoxy chains and the cross-links near
the epoxy-nCOP interface.

Tensile Properties of Epoxy-nCOP Nanocomposites

The dependence of mechanical characteristics of epoxy resin as a
function of nCOP content was studied. Generally, mechanical properties
of cured epoxies depend on their network structure and crosslink
density. The measured tensile properties are tabulated in Table 3.
Tensile property analysis revealed that the tensile modulus increased
with increase in nCOP content. A maximum modulus value is reached for 3
phr nCOP loading, after this the modulus does not vary significantly
with nCOP content. While the tensile strength increased with nCOP
content up to 3 phr of filler and then decreased for higher filler
content. The increase in crosslink density of epoxy-nCOP nanocomposite
up to 3 phr nCOP loading as predicted from cure studies, reflects in the
modulus and tensile strength values of epoxynCOP nanocomposites.

Thermogravimetric Analysis of Epoxy-nCOP Nanocomposites

Simultaneous thermogravimetric and differential thermal analysis
(TGA-DTA) were performed to investigate the effect of nCOP content on
thermal stability of the epoxy matrix. The TGA curves of neat epoxy and
epoxy-nCOP nanocomposites at a heating rate of 10[degrees]C/min under
nitrogen atmosphere are shown in Fig. 10a. The neat epoxy as well as all
the nanocomposites shows similar decomposition profiles and undergo the
degradation mainly as a two-stage process. All the samples exhibit an
initial short stage of weight loss (approximately <5%) around
100-150[degrees]C which may be due to the dehydration as well as the
loss of intercalated compounds, surfactant on the filler surface, or
other traces of impurities apart from the polymer phase. This is
followed by a first major degradation stage around 320-450[degrees]C
which is attributed to the breakdown of uncross linked epoxy leading to
the formation of primarily carbonaceous char.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Then the second stage of degradation process takes place around
450-570[degrees]C in which primary char is further oxidized along with
the thermal degradation of the cross-linked epoxy network. Table 4
tabulates the thermogravimetric data, including [T.sub.10%],
[T.sub.40%], and [T.sub.80%] of degradation which refers to the
respective onset temperature at which 10%, 40%, and 80% loss of the
initial mass occurs, the residual non-volatile material left at
650[degrees]C and maximum weight loss rate ([T.sub.max]). The
[T.sub.10%] and T40% values of all the nanocomposites are higher than
that of neat epoxy, irrespective of the amount of nCOP present. However,
in the final stage of degradation, that is, above 400[degrees]C, the
increase in onset temperature of degradation ([T.sub.80%]) is observed
only up to 1-5 phr of nCOP loading. Further addition of nCOP to epoxy
produces a thermal destabilization effect. The effective dispersion of
nCOP controls the stability of nanocomposite at 1 and 3 phr nCOP
loading. This effect was found to reduce as increasing the nCOP content.
At higher nCOP loading, for example, in the case of 10 phr nCOP loading,
[T.sub.80%] is considerably reduced even from neat epoxy by about
15[degrees]C. At higher nCOP loading, the inter nCOP-nCOP interaction
will predominate over the nCOP--epoxy matrix interactions, resulting in
the formation of nCOP stacks or agglomerates, thus leading to the poor
dispersion of nCOP in the epoxy matrix as evident from TEM results.
Theses stacks of nanoparticles can cause the spatial obstruction on the
formation of high cross-linked molecular structure of epoxy or increased
free volume fractions in the polymer nanocomposites [48]. This
observation is in close agreement with our DSC results, which shows
lower crosslinking ability of the 10 phr nCOP loaded epoxy
nanocomposite. The [T.sub.max] values (obtained from DTA profile) were
found to shift to higher temperature side for the epoxy-nCOP
nanocomposite having lower nCOP content. However, the [T.sub.max] for
epoxy-10 phr nCOP nanocomposite was lower than the neat epoxy. The
amount of char residue increases with respect to the filler loading
irrespective of its dispersion in epoxy phase. All the nCOP filled
system eventually gave significant residual char of about 2-8% at
700[degrees]C. The amount of char was significantly enhanced with the
addition of nanofillers which indicates that the presence of the
nanofiller can affect the degradation pathway. Moreover, it is well
known that inorganic materials such as metal oxides have good thermal
stability and hence the introduction of these metal oxides into organic
materials can improve their thermal stability where they can act as heat
barrier and physical barrier for volatile degradation products.

Calculation of Overall Thermal Stabilization Effect in Epoxyn-COP
Nanocomposites. To have a better comparison of thermal stability of the
neat epoxy with its nCOP filled nanocomposites, it is important to find
out the overall stabilization effect (OSE) for nanocomposites. The
overall stabilization parameter can be defined as

where T is the degradation temperature and Wt(|OSS%, is
corresponding the percentage of weight lost during the degradation [49],
This was calculated via integration of the area under the [DELTA] mass%
versus temperature curves. The OSE values for obtained for different
epoxy-nCOP nanocomposites are presented in Fig. 10b. A high positive OSE
value indicates an improvement in the overall thermal stability of the
polymer nanocomposite in the temperature range 30-700[degrees]C while a
negative value suggests that the overall thermal stability of the
nanocomposite is inferior to that of the unmodified resin. Composite
systems with uniform distribution of nano-sized additives throughout the
matrix phase increase the probability of both chemical and physical
interactions of filler with the matrix resin [48, 49]. The higher
thermal stabilization of nanocomposites is mainly attributed to the
uniform dispersion as well as the high aspect ratio of the nCOP in the
polymer matrix. The epoxy-3 phr nCOP nanocomposite has the highest OSE
value, while epoxy-10 phr nCOP sample has lower OSE which depicts
thermal destabilization of epoxy matrices at higher filler loadings.

CONCLUSION

Ultra fine, phase pure, octahedral cuprous oxide (nCOP)
nanoparticles with cuprite structure were synthesized in the lab. nCOP
filled epoxy nanocomposites achieved a fine and homogeneous dispersion
of nCOP nanoparticle throughout the matrix along with the conservation
of its octahedral morphology. The fracture surfaces revealed
considerable increase in surface roughness of nanocomposites as compared
to neat epoxy which could be concluded as a substantial evidence for the
strong interfacial adhesion of nCOP with the epoxy matrix. In the curing
process, autocatalytic mechanism was observed both in the neat system as
well as in epoxy-nCOP nanocomposites. The overall reaction order, m + n
ranges between 1 and 2. The reaction rate was found to increase with the
addition of the nCOP. The activation energy required for the cure
reaction was reduced by 22% and 13% at the initial ([E.sub.1]) and final
stage ([E.sub.2]) of the cure, respectively, by the incorporation of 3
phr nCOP. We proposed a plausible mechanism showing the involvement of
nCOP on accelerating the cure reaction. The kinetically controlled parts
of reaction could be expressed well by Kamal's phenomenological
model while end of curing process could not be completely expressed by
this model which showed that there it was diffusion controlled. The
epoxy-nCOP nanocomposites showed higher [T.sub.g] compared to neat epoxy
and a maximum [T.sub.g] was achieved for 3 phr nCOP loading. Also the
incorporation of nCOP filler increased the tensile properties like
modulus and strength. The increase in crosslink density of epoxy-nCOP
nanocomposite up to 3 phr nCOP loading reflected in the tensile modulus
and strength values of epoxy-nCOP nanocomposites. The incoiporation of
nCOP delayed the char oxidation thereby improving the thermal stability
of epoxy matrix. In the initial stage of thermal decomposition of epoxy
matrix, all the nCOP-epoxy nanocomposites exhibited a thermal
stabilization effect irrespective of the filler content. Finally, it can
be concluded that nCOP could act as potential low cost nano scale
reinforcement for epoxy polymers. Studies are in progress in exploring
the effect in other polymer systems.