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Cuprous oxide nanoparticles in epoxy network: cure reaction, morphology, and thermal stability.

INTRODUCTION

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)

d[alpha]/dt = ([k.sub.1] + [k.sub.2][[alpha].sup.m]) [(1 - [alpha]).sup.n], (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:

[Cu.sup.2+] + 20[H.sup.-] [right arrow] Cu[(OH).sub.2]. 4Cu[(OH).sub.2] + [N.sub.2][H.sub.4] [right arrow] 2[Cu.sub.2]O [down arrow] + 6[H.sub.2]O + [N.sub.2] [up arrow].

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

OSE = [integral] [([Wt.sub.(|oss%)]of nanocomposites).sub.T] - [([Wt.sub.(loss%)] of virgin matrix).sub.T], (7)

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.

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Anu Tresa Sunny, (1) Poornima Vijayan P., (1,2) Thresiamma George, (3,4) Kim Pickering, (5) Suresh Mathew, (1,4) Sabu Thomas (1,6,7)

(1) School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686560, Kerala, India

(2) Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar

(3) Department of Science, Holy Kings' College of Engineering Science & Technology, Pampakuda, Ernakulam, Kerala, India

(4) Advanced Molecular Materials Research Centre, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686560, Kerala, India

(5) Department of Materials and Process Engineering, The University of Waikato, Hamilton, New Zealand

(6) Faculty of Applied Sciences, International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686560, Kerala, India

(7) Universiti Teknologi MARA, Faculty of Applied Sciences, 40450 Shah Alam, Selongor, Malaysia

Correspondence to: Sabu Thomas; e-mail: sabut@sancharnet.in

DOI 10.1002/pen.24116
TABLE 1. Activation energy for neat epoxy and their nanocomposites
obtained using Ozawa and Kissinger methods.

                       Activation energy (kJ/mol)

Composition         Ozawa method   Kissinger method

Neat epoxy             66.97            64.35
Epoxy-1 phr nCOP       62.69            59.76
Epoxy-3 phr nCOP       62.20            59.28
Epoxy-5 phr nCOP       61.72            58.84
Epoxy-10 phr nCOP      61.24            58.01

TABLE 2. The reaction kinetic parameters for neat epoxy and their
composites obtained using Kamal's approach.

                                 T                       [k.sub.1]
                                                       ([10.sup.-3]
Composition         ([degrees]C)   (K)    m      n     [min.sup.-1])

Neat epoxy               60        333   0.12   1.84      0.0005
                         70        343   0.02   1.57      0.0012
                         80        353   0.05   1.29      0.0049
                         90        363   0.48   1.68      0.0096
                        100        373   0.33   1.77      0.0276
Epoxy-1 phr nCOP         60        333   0.23   1.41      0.0009
                         70        343   0.14   1.64      0.0018
                         80        353   0.58   1.01      0.0062
                         90        363   0.06   1.55      0.0090
                        100        373   0.20   1.17      0.0390
Epoxy-3 phr nCOP         60        333   0.16   1.51      0.0016
                         70        343   0.15   1.60      0.0043
                         80        353   1.21   1.47      0.0070
                         90        363   0.21   1.63      0.0250
                        100        373   0.53   1.14      0.0314
Epoxy-5 phr nCOP         60        333   0.21   1.82      0.0019
                         70        343   0.07   1.49      0.0046
                         80        353   1.11   1.36      0.0069
                         90        363   0.98   1.78      0.0271
                        100        373   0.75   1.52      0.0421
Epoxy-10 phr nCOP        60        333   0.39   1.92      0.0020
                         70        343   0.82   1.59      0.0046
                         80        353   0.64   1.74      0.0070
                         90        363   0.73   1.07      0.0291
                        100        373   1.09   1.19      0.0465

                      [k.sub.2]
                    ([10.sup.-3]    [E.sub.a1]   [E.sub.a2]
Composition         [min.sup.-1])   (kJ/mol)     (kJ/mol)

Neat epoxy             0.0027
                       0.0121
                       0.0563         101.68       123.46
                       0.0945
                       0.3792
Epoxy-1 phr nCOP       0.0031
                       0.0157
                       0.0830          92.45       109.91
                       0.0998
                       0.2503
Epoxy-3 phr nCOP       0.0055
                       0.0371
                       0.0524          79.31       107.25
                       0.2072
                       0.4165
Epoxy-5 phr nCOP       0.0059
                       0.0395
                       0.0756          82.06       106.5
                       0.2138
                       0.4328
Epoxy-10 phr nCOP      0.0060
                       0.0418
                       0.0856          83.64       109.82
                       0.2764
                       0.4661

TABLE 3. Variation of glass transition temperature and tensile
properties of epoxy-nCOP nanocomposites as a function of filler
content.

                                       Tensile            Tensile
                     [T.sub.g]         modulus           strength
Composition         ([degrees]C)        (MPa)              (MPa)

Neat epoxy              82.3       1870 [+ or -] 50   39 [+ or -] 2.2
Epoxy-1 phr nCOP        84.7       1937 [+ or -] 52   44 [+ or -] 2.3
Epoxy-3 phr nCOP        90.3       2904 [+ or -] 52   56 [+ or -] 2.3
Epoxy-5 phr nCOP        91.7       2633 [+ or -] 54   45 [+ or -] 2.3
Epoxy-10 phr nCOP       92.8       2756 [+ or -] 58   39 [+ or -] 3

TABLE 4. Thermogravimetric data for neat epoxy and epoxy-nCOP
nanocomposites.

                    [T.sub.10%]    [T.sub.40%]    [T.sub.80%]
Composition         ([degrees]C)   ([degrees]C)   ([degrees]C)

Neat epoxy              234            360            514
Epoxy-1 phr nCOP        261            373            519
Epoxy-3 phr nCOP        298            374            521
Epoxy-5 phr nCOP        271            372            513
Epoxy-10 phr nCOP       267            377            500

                    [T.sub.max]          Char at
Composition         ([degrees]C)   650([degrees]C) (%)

Neat epoxy              518               0.32
Epoxy-1 phr nCOP        518               0.81
Epoxy-3 phr nCOP        519               2.01
Epoxy-5 phr nCOP        520               2.1
Epoxy-10 phr nCOP       496               6.23
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Author:Sunny, Anu Tresa; P., Poornima Vijayan; George, Thresiamma; Pickering, Kim; Mathew, Suresh; Thomas,
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Date:Oct 1, 2015
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