**2. Experimental studies**

## **2.1. Matrix and fillers**

between epoxy and oMMT nanofiller has been investigated using Fourier transform infrared spectroscopy (FTIR). The cross linking between polymer and nanofiller was measured to determine the glassy state of the nanocomposite called glass transition temperature (Tg

using differential scanning calorimetry (DSC). Further, the positron annihilation spectroscopy (PALS) has been utilized to determine free volume as outlined in the multi-core model [1]. Many researchers have theoretically estimated the free volume of nanocomposites and there is no experimental data on the evaluation of free volume. In the present work, PALS has been used in the accurate evaluation of free volume. A brief explanation of nanocomposite interface dynamics, free volume estimation and the effect of intermolecular interactions and hydrogen bonding are discussed. The effect of these results on electrical property such as

The interaction of nanoparticles with the surrounding polymer matrix by means of three layers is described by Multi-core model [1] as shown in **Figure 1**. It consists of (i) Primary layer also referred as bonded layer, (ii) Secondary layer as referred as called bound layer, (iii) tertiary layer also referred as called loose layer, and the next fourth layer which overlaps all the above three layers called electric double layer. Primary layer represents a type of transition layer which is firmly attached to the carbonless nanofiller or inorganic nanofiller and carbon based organic base matrix polymer by compatabilizer or hardner. Secondary or bound layer addressed as interfacial layer or region consists of a region or area of layer of polymer chains

**Figure 1.** Multi-core model for nano-particle-polymer interfaces (source: Toshikatsu Tanaka and co-authors [1]).

dielectric strength (DES) at room temperature was studied.

**1.1. Multi-core interface model**

142 Optimum Composite Structures

) by

The Bisphenol A diglycidyl ether based Epon 828 epoxy resin (DGEBA) with epoxy equivalent weight (EEW) of 188 g/mol and curing agent such as Epikure W which is chemically called diethyl toluene diamine (DETDA) with an amine hydrogen equivalent weight of 45 g/ mol were used in the present work. These materials were supplied by M/s. Miller-Stephenson Chemical Company, USA. The nanoclay used in the present work is called as Nanomer 1.30E supplied by M/s. Nanocor, USA. This nanoclay was surface treated with surface functionalizer namely octadecylamine mainly used for uniform dispersion of nanoparticle epoxy resin polymer. This surface functionalized nanoclay is called organically modified montmorillonite clay represented as oMMT.

#### **2.2. Fabrication process of nanocomposite**

One of major challenges in the processing of nanocomposites is the non-uniform mixing of curing agent. Non uniform mixing of nanofiller, resin, and the curing agent or hardner may also results in an improper curing. The schematic diagrams of processing method and curing cycle are shown in **Figures 2** and **3** respectively.

Steps for processing of epoxy-oMMT nanocomposites:

**Step1:** The viscosity of Epon 828 resin was high at room temperature; and therefore, it was difficult to mix. In order to reduce the viscosity before mixing, the resin was preheated in an oven at 60°C for about 2 hours. The filler was dried in an oven at 100°C for 24 hours.

**Step2:** After reducing the viscosity, a known weight of epoxy resin was taken.

**Step3:** The required weight of the oMMT and curing agent were added to the epoxy resin.

**Step4:** Epoxy resin, oMMT, and curing agent mixture were mixed using IKA high shear mixer (T-T18 ULTRA TURRAX Basic) at a speed of 24,000 RPM for 45 minutes.

**Step5:** After mixing, degassing was carried out in a vacuum oven for 45 minutes.

**Step6:** The, the mixture was then transferred into aluminum molds and degassed again for 30 minutes. After degassing, the aluminum molds were placed in an oven, and the materials were cured based on the time–temperature curing cycle shown in **Figure 3**. The dimensions of cured sheet of epoxy-oMMT nanocomposites used for the investigation had an area of 200 mm × 200 mm and thickness of 3 mm.

#### **2.3. Measurements**

#### *2.3.1. Interface dynamics*

#### **(i) Fourier transform infrared spectroscopy (FTIR)**

Nanoparticles chemistry and chemical bonding type that existed between polymer and nanoparticles of cured polymer nanocomposites in the present study were characterized through FTIR measurements.

The DSC Model 821 of Thermal Analysis instrument was operated in nitrogen atmosphere to determine the glass transition temperature as per ASTM D-3428-99. The PALS in pure epoxy resin and oMMT filled nanocomposites were recorded using the positron lifetime spectrometer with a time resolution of 220 ps [2]. The PAL spectra obtained were analyzed by employing the computer software PATFIT [3]. This software decomposes a PAL spectrum

the o-Ps from the free volume sites present mainly in the amorphous regions of the polymer matrix. Nakanishi and co-authors [4] proposed equation 1, which is referred to the previous research works of Tao [5] and Eldrup [6] utilized to compute radius(R) of the free volume cell

<sup>R</sup> <sup>+</sup> ΔR <sup>+</sup> \_\_\_1

Here, ΔR is the fitting parameter and has been found to be 1.656 Angstrom (Å) for solid

<sup>2</sup><sup>π</sup> sin(

\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 2 π R <sup>R</sup> <sup>+</sup> ΔR)]

‐1

.

) is calculated as Fv = CVf

(o-Ps lifetime).

molecular media [7]. The free volume size is evaluated as Vf = (4/3) πR<sup>3</sup>

<sup>=</sup> <sup>τ</sup><sup>3</sup> <sup>=</sup> 0.5 [1‐ \_\_\_\_\_ <sup>R</sup>

with intensities I2

with intensity I3

is due to trapping of positrons at

Improved Dielectric Properties of Epoxy Nano Composites

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145

is due to pick-off annihilation of

(1)

I3

, where,

into three discs. The lifetime component τ<sup>2</sup>

**Figure 3.** Curing cycle for epoxy-oMMT nanocomposites.

**Figure 2.** Processing of epoxy-oMMT nanocomposites.

the defects. The longest life components τ<sup>3</sup>

λ3

The fractional free volume or the free volume content (Fv

from the noted values of τ<sup>3</sup>

\_\_\_\_\_\_\_\_\_\_\_\_\_ <sup>1</sup>

[7, 8].

C = 0.00018 nm3

#### **(ii) Differential scanning calorimeter (DSC)**

The nanocomposites melting and glass transition temperature were calculated using DSC model 821 of Thermal Analysis instruments.

#### **(iii) Positron annihilation lifetime spectrometer (PALS)**

The PAL spectra of the nanocomposites have been traced by means of positron lifetime spectrometer with time resolution of 220 picoseconds.

#### **(iv) Dielectric strength (DES)**

The electrical breakdown measurements were carried out using HV AC Test Set of M/s. W.S. Test Systems Pvt. Ltd., Bangalore.

#### **2.4. Experimental techniques**

The bonding of nanoparticles with the base polymer and the chemical nature of cured polymer material were characterized through FTIR measurements using Perkin Elmer make, model spectrum-GX FT-IR as per ASTM D 7214-07a.

**Figure 2.** Processing of epoxy-oMMT nanocomposites.

also results in an improper curing. The schematic diagrams of processing method and curing

**Step1:** The viscosity of Epon 828 resin was high at room temperature; and therefore, it was difficult to mix. In order to reduce the viscosity before mixing, the resin was preheated in an

**Step3:** The required weight of the oMMT and curing agent were added to the epoxy resin.

**Step4:** Epoxy resin, oMMT, and curing agent mixture were mixed using IKA high shear mixer

**Step6:** The, the mixture was then transferred into aluminum molds and degassed again for 30 minutes. After degassing, the aluminum molds were placed in an oven, and the materials were cured based on the time–temperature curing cycle shown in **Figure 3**. The dimensions of cured sheet of epoxy-oMMT nanocomposites used for the investigation had an area of

Nanoparticles chemistry and chemical bonding type that existed between polymer and nanoparticles of cured polymer nanocomposites in the present study were characterized

The nanocomposites melting and glass transition temperature were calculated using DSC

The PAL spectra of the nanocomposites have been traced by means of positron lifetime spec-

The electrical breakdown measurements were carried out using HV AC Test Set of M/s.

The bonding of nanoparticles with the base polymer and the chemical nature of cured polymer material were characterized through FTIR measurements using Perkin Elmer make,

oven at 60°C for about 2 hours. The filler was dried in an oven at 100°C for 24 hours.

**Step2:** After reducing the viscosity, a known weight of epoxy resin was taken.

**Step5:** After mixing, degassing was carried out in a vacuum oven for 45 minutes.

(T-T18 ULTRA TURRAX Basic) at a speed of 24,000 RPM for 45 minutes.

cycle are shown in **Figures 2** and **3** respectively.

200 mm × 200 mm and thickness of 3 mm.

**(i) Fourier transform infrared spectroscopy (FTIR)**

**(ii) Differential scanning calorimeter (DSC)**

model 821 of Thermal Analysis instruments.

trometer with time resolution of 220 picoseconds.

model spectrum-GX FT-IR as per ASTM D 7214-07a.

**(iii) Positron annihilation lifetime spectrometer (PALS)**

**2.3. Measurements**

144 Optimum Composite Structures

*2.3.1. Interface dynamics*

through FTIR measurements.

**(iv) Dielectric strength (DES)**

**2.4. Experimental techniques**

W.S. Test Systems Pvt. Ltd., Bangalore.

Steps for processing of epoxy-oMMT nanocomposites:

**Figure 3.** Curing cycle for epoxy-oMMT nanocomposites.

The DSC Model 821 of Thermal Analysis instrument was operated in nitrogen atmosphere to determine the glass transition temperature as per ASTM D-3428-99. The PALS in pure epoxy resin and oMMT filled nanocomposites were recorded using the positron lifetime spectrometer with a time resolution of 220 ps [2]. The PAL spectra obtained were analyzed by employing the computer software PATFIT [3]. This software decomposes a PAL spectrum into three discs. The lifetime component τ<sup>2</sup> with intensities I2 is due to trapping of positrons at the defects. The longest life components τ<sup>3</sup> with intensity I3 is due to pick-off annihilation of the o-Ps from the free volume sites present mainly in the amorphous regions of the polymer matrix. Nakanishi and co-authors [4] proposed equation 1, which is referred to the previous research works of Tao [5] and Eldrup [6] utilized to compute radius(R) of the free volume cell from the noted values of τ<sup>3</sup> (o-Ps lifetime).

$$\frac{1}{\lambda\_{\odot}} = \tau\_{\odot} = 0.5 \left[ 1 \cdot \frac{\text{R}}{\text{R} + \Delta \text{R}} + \frac{1}{2\pi} \sin \left( \frac{2 \,\pi \,\text{R}}{\text{R} + \Delta \text{R}} \right) \right]^{\text{-1}} \tag{1}$$

Here, ΔR is the fitting parameter and has been found to be 1.656 Angstrom (Å) for solid molecular media [7]. The free volume size is evaluated as Vf = (4/3) πR<sup>3</sup> .

The fractional free volume or the free volume content (Fv ) is calculated as Fv = CVf I3 , where, C = 0.00018 nm3 [7, 8].

**Figure 4.** Insulation breakdown test method by needle-plate electrode geometry.

The electrical breakdown voltage was measured at 300.15 K as per ASTM D-149 as shown in **Figure 4**.

Electrical breakdown voltage of nanocomposite insulator was documented. The dielectric breakdown strength (DES) was computed using,

$$E = \frac{V}{l} \,\mathrm{KV/mm} \tag{2}$$

**Figure 5.** FTIR spectra of pure epoxy.

~1582 ~1508 ~1458

~560

**Wave number (cm−1) Functional groups** ~3399 OH groups

~3036 Corresponds to the C–H stretch in aromatics

~2965 Corresponds to asymmetrical C–H stretch of –CH3

~2932 Corresponds to asymmetrical C–H stretch of –CH2

~1298 Corresponds to asymmetrical –CH2 deformation ~1247 Corresponds to asymmetrical aromatic C–O stretch ~1181 Corresponds to asymmetrical aliphatic C–O stretch ~1085 Corresponds to symmetrical aromatic C–O stretch

~916 Corresponds to epoxide ring vibrations

**Table 1.** FTIR peaks corresponding to functional groups in pure epoxy.

~1608 }Corresponds to C–C stretching vibration in aromatic

~874 Corresponds to –CH out of plane deformation in aromatic

group

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group

here, V = Electrical breakdown voltage in kilo volts,

t = thickness of nanocomposite sample in m.
