**3. Results and discussion**

#### **3.1. FTIR spectrum of pure epoxy**

FTIR spectrum of pure epoxy is shown in **Figure 5**. In the below figure, peaks corresponding to the presence of functional groups in the epoxy system and are listed in **Table 1**.

The epoxy resin contains several polar groups which can interact with the surface –OH groups on the nanoparticles through hydrogen bonding rather easily, similar to the bonding between the –OH groups and H2 O molecules in the atmosphere. An uncured DGEBA epoxy resin is polar in nature and contains two epoxide groups at both ends.

#### **3.2. Hydrogen bond formation in polymer and nanoparticle**

In the fabrication process of the epoxy nanocomposites, the filler (oMMT) is first mixed with DGEBA based epoxy subsequently the addition of hardner (DETDA) into the epoxy and oMMT nanoparticle mix to initiate the curing process. When DETDA hardner is added to the epoxy and oMMT particle mix, the epoxide group open up and form hydrogen bond with the free –OH groups on the nanoparticle surface in addition to reacting with amine groups of the hardner as shown in **Figure 6**.

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

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

Electrical breakdown voltage of nanocomposite insulator was documented. The dielectric

FTIR spectrum of pure epoxy is shown in **Figure 5**. In the below figure, peaks corresponding

The epoxy resin contains several polar groups which can interact with the surface –OH groups on the nanoparticles through hydrogen bonding rather easily, similar to the bonding between

In the fabrication process of the epoxy nanocomposites, the filler (oMMT) is first mixed with DGEBA based epoxy subsequently the addition of hardner (DETDA) into the epoxy and oMMT nanoparticle mix to initiate the curing process. When DETDA hardner is added to the epoxy and oMMT particle mix, the epoxide group open up and form hydrogen bond with the free –OH groups on the nanoparticle surface in addition to reacting with amine groups

O molecules in the atmosphere. An uncured DGEBA epoxy resin is

to the presence of functional groups in the epoxy system and are listed in **Table 1**.

polar in nature and contains two epoxide groups at both ends.

**3.2. Hydrogen bond formation in polymer and nanoparticle**

*<sup>t</sup>* KV/mm (2)

*V*

**Figure 4**.

146 Optimum Composite Structures

breakdown strength (DES) was computed using,

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

here, V = Electrical breakdown voltage in kilo volts,

*E* = \_\_

t = thickness of nanocomposite sample in m.

**3. Results and discussion**

**3.1. FTIR spectrum of pure epoxy**

of the hardner as shown in **Figure 6**.

the –OH groups and H2


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

**Figure 6.** Hydrogen bonding between –OH groups on the nanoparticle surface and epoxide groups in epoxy during curing.

Since, there is abundantly free –OH groups are available on the nanoparticle surface, the hydrogen bonded epoxy segments will be more in number nearer to the nanoparticle surface. Development of the hydrogen bonds at interface region of epoxy and nanofiller is due to strongly bonded first nanolayer and tightly bounded second nanolayer of nanofiller and epoxy segments. This is in accordance with multi core model [1].

H2

O molecule is highly prone to hydrogen bonding with free hydroxyl groups present on the OMMT particle surfaces as shown in **Figure 7**. Hence, there will be a tendency for all the –OH

erties of the epoxy based nanocomposites [12]. Although, it is very difficult to fully remove the

the nanoparticles. One of the important processes which has been adopted in the present work is to adequately dry the OMMT particles before they are adding into the polymer matrix.

With reference to the **Figure 7**, the width of the peak, intensity of the peak and –OH infrared peak positions are influenced by the existence of hydrogen bonding in the oMMT nanofiller and epoxy matrix. However, it is well known that for –OH bands i)width of peak, and ii) intensity of the peak increases as the incorporation of oMMT particles in epoxy resin increases which is considered as sign of adding up of hydrogen bonds in the nanofilled filled composites.

**)**

of the epoxy nanocomposites are shown in the **Figure 8**. It is noted from

value of pure epoxy resin as 442.15 K. It is also noted from the plot

O molecules will tend to influence the thermal and electrical prop-

O molecules

O molecules are

by 278.15 K as

O molecules, the experi-

O layer on the surface of

O layer on the surface of the OMMT

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149

groups present in the oMMT nanoparticle surface to form hydrogen bonds with H2

particles. When such oMMT particles having high concentration of surface H2

that inclusion of 2 wt.% oMMT, into epoxy results in increased value of Tg

formation of hydrogen bonds between the surface –OH groups and H2

ments carried out have to be controlled to reduce the formation of a H2

in the atmosphere which may result in the formation of a H2

**Figure 7.** FTIR spectra of epoxy-oMMT nanocomposites.

**3.4. Analysis of the glass transition temperature (Tg**

introduced into epoxy, the H2

The variations in Tg

the figure, about the T<sup>g</sup>

compared to pure epoxy.

#### **3.3. FTIR spectra of epoxy-oMMT nanocomposites**

The FTIR spectrum of epoxy-oMMT nanocomposites is shown in **Figure 7**. The FTIR spectra clearly show that the incorporation of the filler particles leads to formation of hydrogen bonds. The interaction mechanism between nanofiller particles and base epoxy is the formation of hydrogen bond with free surface hydroxyl (–OH) groups present on the nanofiller particle outer surface and the base polymer epoxy. One of the major understand with the hydrogen bonding is that they will not exhibit any such new peaks in FTIR spectra.

The band at 3444 cm−1 correspond to the stretching vibration of hydroxyl (–OH) groups attached to the nanoparticle surface. The –OH groups may be attached on the surface of nanofiller particles as free –OH groups, or as –OH groups attached to absorbed H<sup>2</sup> O molecules. When the band appears at 1638 cm−1, this corresponds to the bending vibrations of absorbed H2 O molecules. The presence of the FTIR peaks at the above mentioned wavelengths reports the presence of hydroxyl (-OH) groups on the nanofiller surfaces. This result obtained through FTIR measurements in the present work have been well supported by several researchers [9–11].

The reason for the presence of H2 O molecules on the surfaces of oMMT particles are affinity of H2 O molecules present in atmosphere to get bonded to the surface OH groups on the oMMT particles through hydrogen bonding. H2 O is a polar particle or molecule and O2 attached to the

**Figure 7.** FTIR spectra of epoxy-oMMT nanocomposites.

Since, there is abundantly free –OH groups are available on the nanoparticle surface, the hydrogen bonded epoxy segments will be more in number nearer to the nanoparticle surface. Development of the hydrogen bonds at interface region of epoxy and nanofiller is due to strongly bonded first nanolayer and tightly bounded second nanolayer of nanofiller and

**Figure 6.** Hydrogen bonding between –OH groups on the nanoparticle surface and epoxide groups in epoxy during curing.

The FTIR spectrum of epoxy-oMMT nanocomposites is shown in **Figure 7**. The FTIR spectra clearly show that the incorporation of the filler particles leads to formation of hydrogen bonds. The interaction mechanism between nanofiller particles and base epoxy is the formation of hydrogen bond with free surface hydroxyl (–OH) groups present on the nanofiller particle outer surface and the base polymer epoxy. One of the major understand with the hydrogen

The band at 3444 cm−1 correspond to the stretching vibration of hydroxyl (–OH) groups attached to the nanoparticle surface. The –OH groups may be attached on the surface of nanofiller par-

ecules. The presence of the FTIR peaks at the above mentioned wavelengths reports the presence of hydroxyl (-OH) groups on the nanofiller surfaces. This result obtained through FTIR measurements in the present work have been well supported by several researchers [9–11].

O molecules present in atmosphere to get bonded to the surface OH groups on the oMMT

O molecules on the surfaces of oMMT particles are affinity of

O is a polar particle or molecule and O2

band appears at 1638 cm−1, this corresponds to the bending vibrations of absorbed H2

O molecules. When the

O mol-

attached to the

epoxy segments. This is in accordance with multi core model [1].

bonding is that they will not exhibit any such new peaks in FTIR spectra.

ticles as free –OH groups, or as –OH groups attached to absorbed H<sup>2</sup>

**3.3. FTIR spectra of epoxy-oMMT nanocomposites**

148 Optimum Composite Structures

The reason for the presence of H2

particles through hydrogen bonding. H2

H2

H2 O molecule is highly prone to hydrogen bonding with free hydroxyl groups present on the OMMT particle surfaces as shown in **Figure 7**. Hence, there will be a tendency for all the –OH groups present in the oMMT nanoparticle surface to form hydrogen bonds with H2 O molecules in the atmosphere which may result in the formation of a H2 O layer on the surface of the OMMT particles. When such oMMT particles having high concentration of surface H2 O molecules are introduced into epoxy, the H2 O molecules will tend to influence the thermal and electrical properties of the epoxy based nanocomposites [12]. Although, it is very difficult to fully remove the formation of hydrogen bonds between the surface –OH groups and H2 O molecules, the experiments carried out have to be controlled to reduce the formation of a H2 O layer on the surface of the nanoparticles. One of the important processes which has been adopted in the present work is to adequately dry the OMMT particles before they are adding into the polymer matrix.

With reference to the **Figure 7**, the width of the peak, intensity of the peak and –OH infrared peak positions are influenced by the existence of hydrogen bonding in the oMMT nanofiller and epoxy matrix. However, it is well known that for –OH bands i)width of peak, and ii) intensity of the peak increases as the incorporation of oMMT particles in epoxy resin increases which is considered as sign of adding up of hydrogen bonds in the nanofilled filled composites.

#### **3.4. Analysis of the glass transition temperature (Tg )**

The variations in Tg of the epoxy nanocomposites are shown in the **Figure 8**. It is noted from the figure, about the T<sup>g</sup> value of pure epoxy resin as 442.15 K. It is also noted from the plot that inclusion of 2 wt.% oMMT, into epoxy results in increased value of Tg by 278.15 K as compared to pure epoxy.

nanolayer near to the nanoparticle surface and this interfacial polymer nanolayer influences

or remain constant. In the present work, the results obtained for 2 wt.% nanocomposites, may be due to few cross links but for higher loading, the nanoparticle-polymer interaction

with the published literatures [16, 17]. The cross linking density reduces due to etherification mechanism and curing agent preferably tends to attach to the surfaces and very thin layer of hardner or curing agent will surround the nanofiller particles. This thin layer of curing agent will keep the curing agent around the nanofiller particles from reacting with the epoxy resin. This 'curing agent concentration' mechanism will also cause decrease in cross-link density

1.75 ns, that is size or area of the free volume and free volume content increases from 7.17 to

may be because of addition of nanofiller in polymer matrix creates additional free volume. It further suggests that, the filler will not occupy the pre-existing free volume cavities due to

versus filler content (wt.%) of epoxy-oMMT nanocomposites.

and 2.28 to 2.45% respectively. However, at 7 wt.% oMMT loading, both τ<sup>3</sup>

) with 2, 5 and 7 wt.% of oMMT in epoxy nanocomposites is shown in

and 2.14% respectively. The modifications has taken place

tion, repulsive interaction or the interaction is neutral and as such Tg

decreases at 5 and 7 wt.% of filler loading.

The variation of free volume content or fractional free volume (Fv

The addition of oMMT content in epoxy resin (2-5 wt.%), causes τ<sup>3</sup>

found to be repulsive and therefore Tg

**3.5. Characterization of free volume content**

decrease slightly to 1.74 ns, 7.34 nm3

. These nanoparticle and polymer interactions can be developed as attractive interac-

can increase, decrease,

), free volume size (Vf

to increase from 1.72 to

, Vf

) and

151

and Fv

decreases and the present results are in good concord

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the Tg

and thereby Tg

longest life time (τ<sup>3</sup>

**Figure 9**.

7.45 nm3

**Figure 9.** Plot of τ<sup>3</sup>

, Vf and Fv

**Figure 8.** DSC plots of epoxy-oMMT nanocomposite.

With the incorporation of 5 and 7 wt.% of nanofiller T<sup>g</sup> value decreases. The interfacial interaction of polymer nanocomposites and following in the formation of nanolayer in nanocomposites are concluded in the literature followed. The formation of nanolayer and its influence on Tg is also been reported. The higher value of in Tg [13, 14] when nanofiller has been added is reported in some cases whereas, some others report a smaller value of Tg due to addition of nanoparticles into base polymer matrix [15], but the majority of the proposed theories are uncertain till today. The influence of oMMT nanofiller on the curing reaction and glass transition values of the epoxy nanocomposites is investigated by DSC in order to understand the molecular mobility in the nanocomposites. When the nanofiller concentration is increased from 5 to 7 wt.%, the values of Tg in epoxy nanocomposites is observed to decrease. Similar to the results obtained in the case of epoxy filled with oMMT nanofillers in the present work, reduced value of Tg have been reported for nanoalumina filled PMMA composites [16]. The increase in Tg with the addition of 2 wt.% of nanofiller is due to few cross links that developed between the polymer matrix and nanoparticle. However, the reduction in Tg values at 5 and 7 wt.% of nanofiller loading may be due to the many reasons such as changes in molecular weight, tacticity, cross-linking density and the presence of residues from incomplete reactions. The other possible reasons could be the size of the nanofillers which are certainly larger than the free volume hole sizes (discussed in Section 2.4) in the matrix and therefore the possible slide between the chains can result in increased free volume.

Sun and co-authors [17] reported that the Tg depression is associated with the improved polymer chain dynamics because of the additional free volume at the resin-nanofiller interface. Becker and co-authors [18] remarked, interfacial interactions between polymer chains and positively or negatively charged nanofiller surface leads to the development of a polymer nanolayer near to the nanoparticle surface and this interfacial polymer nanolayer influences the Tg . These nanoparticle and polymer interactions can be developed as attractive interaction, repulsive interaction or the interaction is neutral and as such Tg can increase, decrease, or remain constant. In the present work, the results obtained for 2 wt.% nanocomposites, may be due to few cross links but for higher loading, the nanoparticle-polymer interaction found to be repulsive and therefore Tg decreases and the present results are in good concord with the published literatures [16, 17]. The cross linking density reduces due to etherification mechanism and curing agent preferably tends to attach to the surfaces and very thin layer of hardner or curing agent will surround the nanofiller particles. This thin layer of curing agent will keep the curing agent around the nanofiller particles from reacting with the epoxy resin. This 'curing agent concentration' mechanism will also cause decrease in cross-link density and thereby Tg decreases at 5 and 7 wt.% of filler loading.

#### **3.5. Characterization of free volume content**

With the incorporation of 5 and 7 wt.% of nanofiller T<sup>g</sup>

**Figure 8.** DSC plots of epoxy-oMMT nanocomposite.

is also been reported. The higher value of in Tg

from 5 to 7 wt.%, the values of Tg

Sun and co-authors [17] reported that the Tg

reduced value of Tg

150 Optimum Composite Structures

increase in Tg

Tg

tion of polymer nanocomposites and following in the formation of nanolayer in nanocomposites are concluded in the literature followed. The formation of nanolayer and its influence on

of nanoparticles into base polymer matrix [15], but the majority of the proposed theories are uncertain till today. The influence of oMMT nanofiller on the curing reaction and glass transition values of the epoxy nanocomposites is investigated by DSC in order to understand the molecular mobility in the nanocomposites. When the nanofiller concentration is increased

to the results obtained in the case of epoxy filled with oMMT nanofillers in the present work,

7 wt.% of nanofiller loading may be due to the many reasons such as changes in molecular weight, tacticity, cross-linking density and the presence of residues from incomplete reactions. The other possible reasons could be the size of the nanofillers which are certainly larger than the free volume hole sizes (discussed in Section 2.4) in the matrix and therefore the

mer chain dynamics because of the additional free volume at the resin-nanofiller interface. Becker and co-authors [18] remarked, interfacial interactions between polymer chains and positively or negatively charged nanofiller surface leads to the development of a polymer

is reported in some cases whereas, some others report a smaller value of Tg

between the polymer matrix and nanoparticle. However, the reduction in Tg

possible slide between the chains can result in increased free volume.

value decreases. The interfacial interac-

[13, 14] when nanofiller has been added

in epoxy nanocomposites is observed to decrease. Similar

depression is associated with the improved poly-

have been reported for nanoalumina filled PMMA composites [16]. The

with the addition of 2 wt.% of nanofiller is due to few cross links that developed

due to addition

values at 5 and

The variation of free volume content or fractional free volume (Fv ), free volume size (Vf ) and longest life time (τ<sup>3</sup> ) with 2, 5 and 7 wt.% of oMMT in epoxy nanocomposites is shown in **Figure 9**.

The addition of oMMT content in epoxy resin (2-5 wt.%), causes τ<sup>3</sup> to increase from 1.72 to 1.75 ns, that is size or area of the free volume and free volume content increases from 7.17 to 7.45 nm3 and 2.28 to 2.45% respectively. However, at 7 wt.% oMMT loading, both τ<sup>3</sup> , Vf and Fv decrease slightly to 1.74 ns, 7.34 nm3 and 2.14% respectively. The modifications has taken place may be because of addition of nanofiller in polymer matrix creates additional free volume. It further suggests that, the filler will not occupy the pre-existing free volume cavities due to

**Figure 9.** Plot of τ<sup>3</sup> , Vf and Fv versus filler content (wt.%) of epoxy-oMMT nanocomposites.

bigger size of the fillers but creates additional free volume probably at the interface. The results also indicate that the layers of oMMT will result in favorable interaction with the epoxy resins and thereby the segmental motion is hindered. Hence, the significant increase in F<sup>v</sup> percentage is not observed. The decrease in Tg is justified by the decrease in free volume content.

Because of this cause, charge carriers are accelerated over shorter distances and have reduced mobility and kinetic energy. This process is considered as a scattering mechanism. The energy of charge carriers is distributed more evenly in the polymer and thus causes less damage in the material and prolongs the lifetime and service of the epoxy. This scattering process decreases the electric field at the electrodes and increase the voltage required for charge injection.

These fillers can also act as barriers for the penetration of the charge carriers throughout the depth of the sample. In case of 5 wt.% oMMT, the number of nanoparticles is much more and the inter particle distance is also less than 100 nm. This inter particle distance was justified by morphological studies. Hence, there is a possibility of overlapping of the loose polymer regions in the nanocomposites leading to the reduction in the loose polymer regions. This polymer layer along with a large number of fillers can also obstruct the discharge path, thereby dielectric strength increases. The increase in dielectric strength values is well supported by reduction in dielectric constant values and increase in free volume content at 2 and

When oMMT filler content is increased to 7 wt.%, there is a reduction in the dielectric strength occurs due to the overlapping of the tightly bound polymer regions over the interface, since the inter particle distances are comparable to the filler diameter. The interphase region close to the nanoparticle is found to be conductive [11]. As the conductive interphase regions tends to overlap, the pure polymer region is reduced, leading to an easier conducting path for the charge transfer and thereby a reduction in the dielectric strength is observed. With the addi-

free charge carriers are available, but reduction in free volume leads to an easier conduction path for the charge carriers and thereby DES decreases. The reduction in DES at 7 wt.% of oMMT filled nanocomposite is well supported by increase in dielectric constant values and free volume content of the nanocomposites. This has been well supported by many authors. Many researchers [19] have reported that layered silicate nanofillers modify the trapping property of both isotactic and syndiotactic PP. Roy and co-authors [20] have reported that

**i.** Existence of hydrogen bond between nanofillers and epoxy polymer chains in the nano-

**ii.** The interaction takes place between epoxy and nanoparticle in small region around the oMMT nanofiller surface called as "interface region" and the nature of interaction is

**iii.** The interaction between the oMMT and epoxy chains has a direct effect on dielectric strength characteristics of the nanocomposite and accordingly, a three core interface

found to be depends on the chemical bond of the oMMT and epoxy.

model has been used to elucidate the characteristics of the interface region.

has taken place, and hence more

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153


, and free volume measurements.

5 wt.% oMMT filled epoxy-oMMT.

tion of 7 wt.% of oMMT into epoxy matrix, reduction in Tg

composites has been established through FTIR, Tg

deep trap sites have been identified in SiO<sup>2</sup>

lated current (TSC) measurements.

The following remarks are drawn:

**4. Conclusions**

#### **3.6. Effect of interface on dielectric strength (DES)**

The dielectric strength (DES) is the vital properties of dielectric insulators. With reference to the discussion in Section 2.4, the properties of epoxy nanocomposites are mainly explained by interfacial interaction of polymer and of nanofillers. This interfacial area is responsible for the interaction of the electric field between the base epoxy and nanofiller. The DES of the nanocomposites depends largely on nanofiller content and even a very less quantity of nanofiller can cause improvement. When nanofiller particles are incorporated into the epoxy matrix, there is a change in morphology of the epoxy due to the interfacial interaction of epoxy with the oMMT nanofiller.

The **Figure 10** shows the variation of Dielectric strength, free volume with respect to nanofiller loading. With the addition of 2 wt.% oMMT into epoxy matrix, it is observed that T<sup>g</sup> increases due to increase in cross linking density and hence less free charge carriers are available leading to slightly higher value of DES than that of pure epoxy. A further increase in the filler loading up to 5 wt.% of oMMT shows an increase in the DES above that of 2 wt.% of nanocomposite. In this case the Tg decreases, and hence more free charge carriers are available.

Here the effect of third interface layer also called loose layer comes into scenario. The loose polymer layers contain more traps or free volumes as discussed in Section 2.4. These charge carriers are easily and more frequently trapped in trap sites rather than in the base epoxy.

**Figure 10.** Plot of DES, Fv with respect to filler content (wt.%) of epoxy-oMMT nanocomposites.

Because of this cause, charge carriers are accelerated over shorter distances and have reduced mobility and kinetic energy. This process is considered as a scattering mechanism. The energy of charge carriers is distributed more evenly in the polymer and thus causes less damage in the material and prolongs the lifetime and service of the epoxy. This scattering process decreases the electric field at the electrodes and increase the voltage required for charge injection.

These fillers can also act as barriers for the penetration of the charge carriers throughout the depth of the sample. In case of 5 wt.% oMMT, the number of nanoparticles is much more and the inter particle distance is also less than 100 nm. This inter particle distance was justified by morphological studies. Hence, there is a possibility of overlapping of the loose polymer regions in the nanocomposites leading to the reduction in the loose polymer regions. This polymer layer along with a large number of fillers can also obstruct the discharge path, thereby dielectric strength increases. The increase in dielectric strength values is well supported by reduction in dielectric constant values and increase in free volume content at 2 and 5 wt.% oMMT filled epoxy-oMMT.

When oMMT filler content is increased to 7 wt.%, there is a reduction in the dielectric strength occurs due to the overlapping of the tightly bound polymer regions over the interface, since the inter particle distances are comparable to the filler diameter. The interphase region close to the nanoparticle is found to be conductive [11]. As the conductive interphase regions tends to overlap, the pure polymer region is reduced, leading to an easier conducting path for the charge transfer and thereby a reduction in the dielectric strength is observed. With the addition of 7 wt.% of oMMT into epoxy matrix, reduction in Tg has taken place, and hence more free charge carriers are available, but reduction in free volume leads to an easier conduction path for the charge carriers and thereby DES decreases. The reduction in DES at 7 wt.% of oMMT filled nanocomposite is well supported by increase in dielectric constant values and free volume content of the nanocomposites. This has been well supported by many authors. Many researchers [19] have reported that layered silicate nanofillers modify the trapping property of both isotactic and syndiotactic PP. Roy and co-authors [20] have reported that deep trap sites have been identified in SiO<sup>2</sup> -XLPE nanocomposites through thermally stimulated current (TSC) measurements.
