2008 Annual Report Conference on Electrical Insulation Dielectric Phenomena

Reduction of Permittivity in Epoxy Nanocompositesat Low Nano-filler Loadings

Santanu Singha and M. Joy ThomasHigh Voltage Laboratory, Department of Electrical Engineering

Indian Institute of ScienceBangalore, India 560 012

Abstract- Experimental studies reveal a reduction in the valuesof permittivity for epoxy nanocomposites at low filler loadings ascompared to neat epoxy over a wide frequency range. Thispermittivity reduction is attributed to the interaction dynamicsbetween nanoparticles and epoxy chains at the interface regionand interestingly, this interaction has also been found to influencethe glass transition temperatures (Tg) of the examinednanocomposite systems. Accordingly, a dual nanolayer interfacemodel for an epoxy based nanocomposite system is analyzed toexplain the obtained permittivity characteristics.

I. INTRODUCTION

Since the concept of dielectric systems based on polymernanocomposite technology came to be known, the permittivitycharacteristics of such systems have been continuouslyevaluated for a better understanding of the behaviors.Literature reports several experimental studies on thepermittivities of polymer nanocomposites with nano-fillers anddepending on the filler type, the permittivities were observed toeither increase [1] or decrease [2, 3] as compared to the neatepoxy value. An increase in permittivity is usually expected,but the lowering is rather surprising and available reports cite areduction in polymer chain mobilities as the prime reason forthis observation [4]. Today, a very clear understanding of thepermittivity behaviors in nanodielectric systems is still lackingand hence this study tries to throw more insights into thisaspect. An epoxy nanocomposite system is considered for theinvestigation and the obtained permittivity characteristics areanalyzed together with the glass transition behaviors of thematerials. The interaction mechanisms at the nanocompositeinterface are discussed using a dual nanolayer interface model.

II. EXPERIMENTAL DETAILS AND SAMPLE PREPARATION

The epoxy used in this study is a bisphenol-A epoxy resin(CY1300) along with hardener (HY956), both supplied byHuntsman. As for the filler materials, commercially availableand highly pure grades of TiO2 [APS-50 nm] and A1203[APS-45 nm] nanoparticles procured from Sigma Aldrich wereused. The nano-filler loadings considered for the present studyare 0.1%, 0.5% and 1% by weight.The process for nanocomposite sample preparation with a

good dispersion of nanoparticles and the details of sampleconditioning prior to the dielectric measurements are availablein an earlier article by the same authors [5]. For experiments inthe present study, round samples of 75 mm diameter and 1 mm

thickness are molded. The permittivities are measured at roomtemperature using two different dielectric spectrometersdepending on the frequencies of measurement - (a) 4291BAgilent impedance analyzer with 16453A dielectric test fixture(106_109 Hz) and (b) 4284A Agilent LCR meter with 16451Bdielectric test fixture (400-106 Hz). The values of permittivityreported in this work are average values of 5 samples with anerror within 2%. The measurements of glass transitiontemperature (Tg) are performed using a Mettler DifferentialScanning Calorimetry (DSC) equipment with the rate of rise oftemperature set at 5 °C/minute and the values reported here areaverage values of 3 measurements with error within 1%. FTIRstudies are also performed for the materials used in this studyusing a Perkin-Elmer make equipment. The samples areprepared as KBr pressed pellets and they are scanned in thetransmission mode between wavelengths of 400-4500 cm-'.

III. RESULTS AND DISCUSSIONS

A. Permittivity characteristicsThe variations of the real permittivity with respect to

frequency for the epoxy nanocomposites with TiO2 and A1203nano-fillers are shown in figures 1 and 2 respectively. From theabove figures, it can be seen that the nanocompositepermittivities are lower than unfilled epoxy permittivity atcertain filler loadings at all the frequencies. This characteristicis intriguing since we know that the addition of fillers to apolymer usually enhances the permittivity of the compositesystem. A reduction in the effective permittivity of the epoxynanocomposite is possible only if there is a reduction in theelectrical polarization mechanisms in the material. Amongst allthe active polarization processes in the nanocomposite, thefillers cannot reduce the permittivity and in the case ofinterfacial polarizations, they are unlikely to occur over theexamined range of frequencies. Therefore, the polarizationsassociated with the epoxy component of the epoxynanocomposite can only cause a lowering of the permittivity.The polarization behavior in epoxy is due to the orientation

of dipolar groups present in the material and hence, restrictionin the mobility of these dipolar groups can reduce thepolarizations in epoxy. The occurrence of such a process ispossible as published literature report a reduction in themobilities of polymer chains in polymer nanocomposites [6].An inhibition in the chain mobilities are reported to occur dueto interactions between polymer chains and nanoparticles at theinterface region [6, 7] and due to chain entanglements [8].

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Unfilled epoxy value

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Epoxy-TiO2 Nanocomposites

A - 0.1% TiOB - 0.5% TiOC - 1% TiO,D - Unfilled epoxy

10 1lFrequency [Hz]

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Fig. 1. Variations of real permittivity with respect to frequency at differentnano-filler loadings for epoxy-TiO2 nanocomposites.

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Fig. 2. Variations of real permittivity with respect to frequency at differentnano-filler loadings for epoxy-AI203 nanocomposites.

Fig. 3. Glass transition temperature (Tg) variations in epoxynanocomposites.

Based on these viewpoints, there is a need to understand theinterface characteristics of the epoxy nanocomposites in thepresent study and this is done through an analysis of the glasstransition temperatures (Tg) of those materials.

B. Glass transition temperature study (Tg)The variations of Tg with respect to nano-filler loadings for

the examined nanocomposite systems are shown in figure 3.The figure clearly shows that the values of nanocomposite Tgin all the systems are lower than the value measured forunfilled epoxy. The addition of 0.5% by weight of nano-fillersreduced the Tg by around 10 °C, but at 1% loading, Tg began toshow an increase in all the systems. Similar reductions in Tgare also reported for alumina/PMMA nanocomposites [9].Literature suggests that interactions between polymer chainsand nanoparticles result in the formation of an interface regionon the nanoparticle surface and the characteristics of thisregion can cause a change in the values of Tg [6, 7]. Thesenanoparticle-polymer interactive systems are predicted to beattractive, repulsive or neutral and accordingly, Tg can increase,decrease or remain constant. For the current investigation, let'sexamine two different arguments to understand the nature ofinteraction between nanoparticles and epoxy chains based onthe results of Tg and permittivity variations:a) Tg values reduce up to 0.5% filler loading (figure 3)suggesting that the interaction between nanoparticles andepoxy chains is a repulsive one wherein the epoxy chains in theinterface region relaxes faster. But then, if the interface has arepulsive system, with increasing filler loadings beyond 0.5%,Tg in the epoxy nanocomposites should have reduced further,which is not the case.b) In figures 1 and 2, nanocomposite permittivities areobserved to reduce below that of pure epoxy and that wasprobably due to a restriction in the mobility of epoxy chainsdue to strong bonding between the nanoparticles and epoxychains. This means that the interface possesses an attractivesystem and hence, addition of nano-fillers to epoxy has toresult in higher values of Tg as compared to pure epoxy, whichagain is in contrary to the obtained results. On these lines,computer simulations by Starr et al. [10] have shown that evenif the interfacial system is attractive, the nanocomposite system

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can still have a faster relaxation depending on the interactionbetween bound polymer chains and the remaining polymer inthe bulk of the material. Therefore, there is a possibility thatthe addition of nanoparticles into a polymer matrix introducestwo different simultaneously occurring interaction processes atthe interface region.To account for the variations in Tg and permittivity in epoxy

nanocomposite systems, the above understanding suggests thatthe interaction mechanism between nanoparticles and epoxychains at the interface region should have epoxy segmentswhich are both immobilized as well as loose and this isanalyzed and explained using a dual nanolayer interface modelsimilar to the one suggested by Tsagarapoulos et al. [6].

C. Dual nanolayer interface modelAs per the dual nanolayer interface model (figure 4), an

interface region is formed on the nanoparticle surface whichconsists of two epoxy nanolayers - (a) the ISt nanolayer isformed closest to the nanoparticle surface and is assumed to betightly bound and highly immobile and (b) the 2nd nanolayer isan extended epoxy layer over the Ist nanolayer where thechains are loosely bound and they tend to relax faster. Theeffective properties of the nanocomposite will be governed bythe behaviors of this dual nanolayer interface region on eachnanoparticle in the epoxy matrix and hence, the filler loadingwill play a significant role on the characteristics.

At 0.1% and 0.5% nano-filler loadings, the presence of ahigher volume fraction of the 2nd nanolayer of loose epoxysegments as compared to the ISt nanolayer of tightly boundepoxy results in a reduction in the values of Tg for both thenanocomposite systems as compared to neat epoxy. Furtherdetails are available in ref. [5]. At 1% filler loading, the plot infigure 3 shows that there is an increase in the values of Tg inboth the nanocomposite systems. This observation suggeststhat there is an increase in the concentration of immobile epoxyregions at 1% nano-filler loading. Earlier experiments byTsagarapoulos et al. [6] on different nanocomposite systemswith 7 nm SiO2 fillers had indicated the occurrence of anoverlap of the loose polymer layers at the interface around 10%filler loadings by weight. Compared to this observation, thepresent study utilizes nanoparticles with sizes of approximately50 nm and this larger particle size can have two effects - (1)the number of nanoparticles in epoxy at very low filler loadingscannot induce an overlap of the loose epoxy segments at theinterface region and (2) at a fixed nano-filler loading, thefraction of immobilized epoxy segments in the first interfacialnanolayer may be higher as compared to the case of a smallersized nanoparticle. As a consequence of the latter phenomenon,a small change in the nano-filler loading will lead to a largerenhancement in the fraction of immobilized polymer segments

present in the first nanolayer of the interface region. It isprobably due to this reason that an increase in the value of Tg isobserved in the present study when the filler loading isincreased from 0.5% to 1%.

D. Interaction at the first interfacial nanolayerThe interaction process between polymer and nanoparticles

depends on the properties of the respective materials. If there isan interaction between nanoparticles and epoxy in the form of achemical reaction, they will introduce chemical changes in thebulk of the nanocomposite which will result in the formation ofnew chemical bonds. Hence, there is a need to examine thesurface chemistry of the nano-fillers and also the chemicalnature of the epoxy nanocomposite systems.The FTIR spectra of both neat epoxy and TiO2 filled

nanocomposite samples are shown in figures 5a and 5brespectively and a comparison of the spectra do not show anydistinct changes in the formation of peaks suggesting that nonew bonds are formed in the nanocomposite. The FTIR spectraof TiO2 and A1203 nanoparticles are shown in figures 6a and 6brespectively and two bands are common in both the figures -one around 3500 cm-' and another around 1630 cm-'. The bandat 3500 cm-' corresponds to the stretching vibration ofhydroxyl (OH) groups attached to the nanoparticle surface andthey can be present as free OH groups attached to the surfaceor as OH groups attached to absorbed water (H20) moleculesor both. The band at 1630 cm-' corresponds to the bendingvibrations of absorbed H20 molecules. The presence of theFTIR peaks at both the above wavelengths suggests that OHgroups are certainly present on the nanoparticle surfaces andthat a portion of these OH groups are due to absorbed H2Omolecules. The presence of H2O molecules on the nanoparticlesurfaces are due to the affinity of water present in atmosphereto get bonded to the surface OH groups on the nanoparticlesthrough hydrogen bonding. H20 is a polar molecule and theoxygen attached to it is highly prone to hydrogen bonding withfree OH groups present on the nanoparticle surface. In a similarprocess, when nanoparticles are introduced into epoxy, theepoxide group (dipole with a pair of electrons on the oxygen)attached at both ends of an uncured epoxy resin can alsoinvolve in hydrogen bonding with the free OH groups presenton the nanoparticle surface as shown in figure 7. As per thenanocomposite preparation technique, the fillers are first mixedto the base epoxy resin followed by the addition of hardener toinitiate the curing process. When hardener is added, theepoxide group opens up and forms strong hydrogen bonds withthe free OH groups on the nanoparticle surface in addition toreacting with the amine groups of the hardener. Because of theabundance of free OH groups on the nanoparticle surface, thedensity of hydrogen bonded epoxy segments will be high at theregion just close to the nanoparticle surface. These hydrogenbonds result in the formation of the strongly bonded and tightlybound first nanolayer of epoxy segments at the interface regionas per the dual nanolayer model. Therefore, the inhibition inthe mobility of epoxy chains leading to the reduction in thenanocomposite permittivity at certain filler loadings isprobably due to this strong hydrogen bonding.

728

Wavenumber (cm-')

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150

145

140

135

130

125

I00 3600 3200 2800 2400 2000 1600 1200 800

Wavenumber (cm-')

Fig. 7. Dual nanolayer interface model.

IV. CONCLUSIONS

400

(b) Epoxy-TiO2 nanocomposite

Fig. 5. FTIR spectra of neat epoxy and epoxy-TiO2 nanocomposite.

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0( 3600 3200 2800 2400 2000 1600 1200 800 400Wavenumber (cm-')

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4000 3600 3200 2800 2400 2000 1600 1200 800Wavenumber (cm- )

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(v) A1203 nanoparticles

Fig. 6. FTIR plot of nanoparticles used in the present investigations.

Epoxy nanocomposites are observed to show lower values ofpermittivity as compared to neat epoxy at low nano-fillerloadings. This reduction in the permittivity is found to bestrongly influenced by the interface characteristics rather thanthe properties of the nanoparticle itself. The properties of theinterface in turn are observed to be dependent on the propertiesof the nanoparticle surface and the nature of interactionbetween the epoxy chains and these nanoparticles. One of theinteresting aspects of this study is that the change in theproperties of the interface is reflected in the nanocompositeglass transition temperatures and based on these results, a dualnanolayer interface model has been used to explain theinteraction dynamics in the nanocomposite.

REFERENCES

[1] P. Gonon and A. Boudefel, "Electrical properties of epoxy/silvernanocomposites," J. App. Phys., Vol. 99, pp. 024308(1)-024308(8), 2006.

[2] J. C. Fothergill, J. K. Nelson and M. Fu, "Dielectric properties of epoxynanocomposites containing TiO2, A1203 and ZnO fillers," IEEE Conf.Electr. Insul. Dielectr. Phenomena (CEIDP), pp. 406-409, 2004.

[3] E. Tuncer, I. Sauers, D. R. James, A. R. Ellis, M. P. Paranthaman, T.Aytug, S. Sathyamurthy, K. L. More, J. Li and A. Goyal, "Electricalproperties of epoxy resin based nano-composites," Nanotechnology, Vol.18, pp. 025703(1)-025703(6), 2007.

[4] T. Tanaka, "Dielectric nanocomposites with insulating properties", IEEETrans. Dielectr. Electr. Insul., Vol. 12, No. 5, pp. 914-928, 2005.

[5] S. Singha and M. Joy Thomas, "Dielectric properties of epoxynanocomposites," IEEE Trans. Dielectr. Electr. Insul., Vol..15, No. 1, pp.12-23, 2008.

[6] G. Tsagarapoulos and A. Eisenberg, "Dynamic mechanical study of thefactors affecting the two glass transition behavior of filled polymers.Similarities and differences with random ionomers," Macromolecules,Vol. 28, pp. 6067-6077, 1995.

[7] A. M. Mayes, "Softer at the boundary," Nature Materials, Vol. 4, pp.651-652, 2005.

[8] R. C. Picu and M. S. Ozmusul, "Structure of linear polymeric chainsconfined between impenetrable spherical walls," J. Chem. Phys., Vol.118, No. 24, pp. 11239-11248, 2003.

[9] B. J. Ash, L. S. Schadler and R. W. Siegel, "Glass transition behavior ofalumina/polymethylmethacrylate nanocomposites," Materials Letters,Vol. 55, pp. 83-87, 2002.

[10] F. W. Starr, T. B. Schroder and S. C. Glotzer, "Effects of a nanoscopicfiller on the structure and dynamics of a simulated polymer melt and therelationship to ultrathin films," Physical Review E, Vol. 64, pp.021802(1)-021802(5), 2001.

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