polymer nanocomposites as dielectrics and electrical...

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 11, No. 5; October 2004 763

Polymer Nanocomposites as Dielectrics and ElectricalInsulation-perspectives for Processing Technologies,Material Characterization and Future Applications

T. TanakaGraduate School of Information, Production and System LSI

Waseda University, 2-7 Hibikino, Wakamatsu-kuKitakyushu-shi, 808-0135, Japan

G. C. MontanariDepartment of Electrical Engineering

High Voltage and Material Engineering LaboratoryUniversity of Bologna, 40136 Bologna, Italy

and R. Mulhaupt¨Institute of Macromolecular ChemistryAlbert-Ludwig University of Freiburg

Stefan-Meier-Str. 31, D-7904 Freiburg i. Br., Germany

ABSTRACTPolymer nanocomposites are defined as polymers in which small amounts ofnanometer size fillers are homogeneously dispersed by only several weight percent-ages. Addition of just a few weight percent of the nano-fillers has profound impacton the physical, chemical, mechanical and electrical properties of polymers. Suchchange is often favorable for engineering purpose. This nanocomposite technologyhas emerged from the field of engineering plastics, and potentially expanded itsapplication to structural materials, coatings, and packaging to medicalrrrrrbiomedi-cal products, and electronic and photonic devices. Recently these ‘hi-tech’ materi-als with excellent properties have begun to attract research people in the field ofdielectrics and electrical insulation. Since new properties are brought about fromthe interactions of nanofillers with polymer matrices, mesoscopic properties areexpected to come out, which would be interesting to both scientists and engineers.Improved characteristics are expected as dielectrics and electrical insulation. Sev-eral interesting results to indicate foreseeable future have been revealed, some ofwhich are described on materials and processing in the paper together with basicconcepts and future direction.

Index Terms — Nanocomposite, polymer nanocomposite, nanofillers, advancedmaterials, dielectrics, electrical insulation.

1 INTRODUCTIONHE development of nanocomposites represents aTvery attractive route to upgrade and diversify proper-

ties of ‘‘old’’ polymers without changing polymer composi-tions and processing. In contrast to conventional filledpolymers, nanocomposites are composed of nanometer-

Ž .sized fillers ‘‘nanofillers’’ which are homogeneously dis-tributed within the polymer matrix. Due to their very highspecific surface areas, a few percent nanofillers can self-

Manuscript recei®ed on 25 February 2004, in final form 7 May 2004.

assemble to produce skeleton-like superstructures espe-cially when anisotropic fillers with high lengthrdiameter

Ž .ratio aspect ratio are used. In comparison with the con-ventional micrometer-sized fillers, the same volume frac-tion of nanofillers contains billion-fold number ofnanoparticles. As a result, most of the polymer ofnanocomposites is located at the nanofiller rpolymer in-terface. The conversion of bulk polymer into interfacialpolymer represents the key to diversified polymer proper-ties. As a function of the nanofiller aspect ratio it is possi-ble to reinforce the polymer matrix and to improve thebarrier resistance against gas and liquid permeation. An

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Tanaka et al.: Polymer Nanocomposites as Dielectrics and Electrical Insulation-Perspecti©es764

important aspect of nanocomposite formation relates toprocessing technology. While many prefabricatednanoparticles are difficult to disperse and require specialsafety precaution for their handling, modern nanocompos-ites are formed in situ via shear-induced intercalation andexfoliation as illustrated by the effective diffusion of poly-mer in between organophilic nanoparticles.

Let us compare filled polymers with filler-added poly-mers. Nanofiller-added polymers or polymer nanocompos-ites might be differentiated from micro-filler-filled poly-mers in three major aspects that the nanocomposites con-tain small amounts, are in the range of nanometers in sizeand have tremendously large specific surface area. Allthese characteristics would reflect on their material prop-erties. The first thing to attract interest arises from thedifference in content. Conventional filled polymers usu-ally contain a large amount of the fillers, e.g., more than50 wt%. Therefore those materials are really mixtures ofpolymers with mineral fillers, resulting in big change ordifference in material properties from polymers per se. Inthe case of nanofillers, the amount less than 10 wt% isenough, so that some of intrinsic polymeric properties, e.g.,density, must remain almost unchanged even after theybecome nanocomposites. The second characteristics of in-terest are expected from filler size difference. Size is dif-ferent by three orders of magnitude in length between thetwo kinds of materials, which would cause much more dif-ference, i.e., roughly by nine orders in their number den-sity. Therefore the distance between neighboring fillers aremuch smaller in nanocomposites than in conventionalfilled polymers. In many cases, the inter-filler distancemight be in the range of nanometers, if fillers are homo-geneously dispersed. The last, but not least, difference isconcerned with the high specific surface area of fillers.The specific surface is represented by the inverse size, andthen is three orders larger for nonocomposites than thatfor conventional filled polymers. Interaction of polymersmatrices with fillers is expected to be much more in theformer than in the latter. In contrast to many conven-tional fillers, some nanofillers are composed of polyelec-trolyte nanoplatelets which disassemble and get dispersedduring processing. This formation of nano-scaled poly-electrolyte complexes can have a major impact on the di-electric behavior.

These nanocomposite features offer new opportunitiesfor designing a totally different world of dielectrics. In fact,the size of fillers and the inter-filler distance are in therange of nanometers, and the fillers would interact chemi-cally and physically with polymer matrices, resulting in theemerging of intermediate or mesoscopic properties thatbelong neither to atomic nor macroscopic frame. Polymernanocomposites have attracted scientists and engineers inthat these materials are potentially endowed with unex-pectedly excellent properties. Recent scientific discoveriesand technical breakthroughs in the materials indicate their

upgrade from simple commodity plastics to ‘hi-tech’ mate-rials with exceptional properties. The nanocomposites aremade in the form of polymers containing a small amountof nanometer size fillers, or nanofillers. Polymer matricesinteract with such nanofillers and are chemically bondedtogether in many cases. These can be called filler-addedpolymers rather than conventional filled polymers or filledresins. Debye shielding length is in the range of nanome-ters in metal, and so might be better watched in case thatthe effect of electrode in contact with nanocomposites islarge.

There are several preceding areas of investigation andapplication. In the field of engineering plastics, materialselection has been made on the basis of property dia-

w x w 3xgrams such as Young’s modulus GPa vs. density Mgrmw x w 3xor the yield strength GPa vs. density Mgrm from me-

chanical properties’ point of view. Materials with lighterŽ 3.weight density: less than 1.5 Mgrm and more mechani-

Ž .cal strength E s 5�20 MPa, � s0.4 to 1.1 MPa areypreferred. Much more expectation is naturally directed tothermal endurance or thermal stability, stability againstaggressive chemicals, impermeability against gas, waterand hydrocarbons, recyclability through re-processing andless leakage of small molecules such as stabilizers. In com-parison to traditional composites, nanocomposites are

Ž . Ž .certainly advantageous in 1 homogeneous structure, 2Ž . Ž .no fiber rupture, 3 optical transparency, and 4 im-

proved or unchanged processability.

In the field of food packaging industry, new materialswith low permeability against oxygen, carbon dioxide, ni-trogen and water vapor are expected. Often they shouldbe biodegradable. They should be processed byinjectionrcompression molding, film blowing andror cast-ing.

What can one expect from nanocomposite applicationsin dielectrics and electrical insulation? Top priority wouldbe dielectric breakdown strength. Generally polymerswould have intrinsic breakdown strength as high as 10

Ž .MVrcm s1GVrm at room temperature, but in practicetheir breakdown strength is much lower and is deter-mined by internal defects. Therefore, nanofillers shouldhave the effect to increase practical dielectric breakdownstrength, irrespective of defects contained. In this regard,the resistance against partial discharge and treeing isgreatly concerned. Mechanical strength and thermal con-ductivity would be also involved if compact insulation de-sign is required. Permittivity and dissipation factors as di-electric properties should be as low as possible for electri-cal insulation, while the former is required to be as highas possible for capacitors. Flame retardancy is preferredfor cable insulation used in the radiation field, whiletracking resistance is of interest for outdoor insulators. Fordc application, formation of space charge in insulationshall be contrasted. Recyclability is generally required toprotect environments. All the performances described

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above, as well as other characteristics, might be improvedby means of nonocomposite technology.

Processing methods suitable for nanocomposites as wellas their applications have been developed by in situ reac-

Ž .tive blending, melt-mixing, thermo-kinetic sheared mix-ing, extrusion, blowing and injection. Materials tried fornanocomposites are polyolefin such as polyethylene,polypropylene and polyethyleneterephthalate, epoxy, elas-tomers such as silicone rubber and EPRrEPDM,polyamide, and polyimide for example.

Some of the favorable results have been obtained fordielectrics such as polyethylene, silicone, epoxy,polyamide, and polyimide. Fillers such as Al O , TiO ,2 3 2SiO and layered silicate have been selected. Material2combinations are epoxy- TiO , PE- TiO , EPDM- Al O ,2 2 2 3PE- SiO , polyimide- SiO , epoxy-layered silicate, and2 2polyamide-layered silicate. Outstanding results based onpioneering works that several research groups have beentackling are described in the paper.

On the basis of the above description, the followingitems are reviewed in the paper: Concept of Nanocompos-ites, Materials of Interest, Processing Methods, Experi-mental Work and Results and Discussion on ElectricalProperties.

1.1 CONCEPT OF NANOCOMPOSITESIt seems difficult to define a term of nanocomposites

clearly. The paper deals with what are called polymernanocomposites. They are usually considered as three-di-mensional composites of polymers with inorganicnanofillers dispersed. In addition to those, two-dimen-sional nano-layered structures are investigated as nano-metric dielectrics, too. Nano-structured organic-inorganicmaterials and nano-structured polymer-polymer sub-stances that are both chemically synthesized from the be-

ginning are also an important field of investigation. Theformer is out of scope in the paper and the latter is brieflyreviewed.

1.2 NANOMETRIC DIELECTRICSA term of ‘‘Nanometric Dielectrics’’ was used in 1994

w x1 as future research area of dielectrics. It was empha-sized that our interest is shifted from ‘‘Debye relaxationalculture’’ to ‘‘a nanotechnical culture’’. The former is gov-erned by determining statistically averaged properties,while the latter is largely affected by a molecular order ofsmallness of systems and system elements. Therefore, asshown in Table 1, it was suggested that phenomena tooccur at nanometric scale should be explored to clarifyfundamental properties and might open up a new field ofapplications. This term might have been defined asnanometer size dielectrics to investigate dielectric phe-nomena in nanometer scale. A term of ‘‘NanoDielectrics’’

w xwas proposed in 2001 2 to explore nanometric dielectricsand dielectrics associated with nanotechnology and toproduce molecularly tailored materials. This conceptseems to be associated with nanostructured ceramics andtailored nanocomposites.

Structures of nanometric dielectrics and their charac-teristics are listed in Table 1 based on the description

w xmade in the reference 1 . This deals with bulks and lay-ered structures, and especially focuses on the interfacialaspect of nanometric phenomena, which are treated notonly by the classical macroscoptic theories but also by thequantum mechanical theories. This might represent thatthe phenomena we have to face in this field are meso-scopic in between macroscopic and atomic scales. It cov-ers not only electrical insulation but also electronics, liv-

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ing systems, MEMS, batteries and the like. It is interest-ing to note that, in connection to STM and AFM embed-

Ž .ded-polymers, a field effect display FED has been devel-Ž .oped using carbon nanotubes CNT , which is considered

a next generation display device. Here STM and AFMstand for scanning tunneling and atomic force micro-scopes, respectively.

1.3 COMPOSITE TO NANOCOMPOSITE[ ]MATERIALS 3

In general, composites consist of two phases, i.e., a ma-trix and a disperse phase. Nanocomposites are named,when the disperse phases are in the range of 1 to several

Ž .hundreds in nanometer nm . What is called a ‘‘single nanoparticle’’ of 1 to 10 nm exhibits characteristics differentfrom either atoms or low molecular bulk materials.Fullerens, carbon nanotubes, dendrimers that have everattracted people are in the range of 1 nm order, and thenmaterials can be called nanocomposites, when they arecombined.

Composites were developed as structural materials,where carbon fiber reinforced plastics developed in 1960’swere a typical example. Emphasis was then placed on theimprovement of mechanical strength and thermal en-durance. Therefore other functional properties were setaside. However, the advent of nanotechnology has changedsuch a situation and has opened a completely differentworld of nanocomposites as functional materials, not sim-ply as structural materials, but materials with optical,electrical, electronic, magnetic, chemical and biologicalfunctions

Generally nanocomposites are, thus, a material systemof the form that nanoparticles are dispersed in continuousmatrices. But functional nanocomposites might includelayered structure devices such as photovoltaic cells, junc-tion devices such as diodes and transistors, and surfacemodified materials such as DNA chips the surface of whichare fixed with biomolecules, if they are all in nanometerscale. Layered devices consisting of organic and inorganiccompounds and even mono-molecule transistors might besome of the ultimate goals as nanostructured devices.

Table 2 shows many examples of nanocomposites andmaterial combinations in such a wide range of applicationfields that include structural and materials engineering,electronics and electrical engineering, optics and opto-electronics, catalysts, filtration membranes, bio-nano-technology and yarn. This table includes not only polymernanocomposites but also inorganic-inorganic nanocompos-ites for reference, and even somewhat different types ofnanocomposites such as meso porous materials filled withpolymer chains inside. Nanocomposites consist of a matrixpolymer and a disperse filler for disperse type, and mate-rial 1 and material 2 for layered structure and copolymertypes.

1.4 DISPERSE PHASE AND[ ]NANO-PARTICLES 4

Nano-particles dispersed in nanocomposites are de-fined to lie in either size between 1 to several 100 nm.Table 3 shows examples of disperse phase ranging fromsub-nm to 1000 nm. Substances in size of a few nanome-

Ž .ters 1 to 10 nm exhibit quite different characteristics fromeither atoms and molecules smaller than 0.3 nm, or bulkmaterials, as described above. Small-particle nanocompos-ites are more important as functional materials thanstructural materials. For example, melting point of wateris markedly lowered due to the surface effect, if it is 10nm in size inside nano-porous materials, and ice crystalsformed in carbon nanotubes smaller than 1 nm in innerdiameter exhibit behaviors different from bulk ice. Fur-thermore, quantum effects emerge in the few-nanometerregion as electrical and optical properties. Quantum dotsare one of the examples. Polymer membrane exhibits re-verse osmosis, if its fine holes are smaller in inner diame-ter than 1 nm, while it shows ultra filtration, when theholes are in the range of 10 to several tens nm. Fine holesin artificial dialysis membranes are close to 1 nanometerin inner diameter, and then prevent proteins from passingthrough. Metal nanoparticles, fullerenes, carbon nan-otubes, giant organic compounds, and mono or accumu-lated molecular layers are some of the examples ofnanocomposites with particles having very small nanome-ter size.

In the range of submicrometers, the size of dispersephase become more apparent in structural materials thanin functional materials, and then impact-resistant polymernanocomposites and high-toughness ceramic nanocompos-ites are often optimized in filler size in this range. Whenthe disperse phase is in the range of 10 to 100 nm, struc-tural and optical properties are markedly affected. Rub-bers and metals are reinforced by carbon black and byparticles, respectively, where the smaller size would makebetter effect. Optical glass prefers the transparency andthen limits the upper size of fillers. Titanium dioxide isoptimized in its size for cosmetics so that it may pass visi-ble light and scatter ultraviolet light. Chemical tips underdevelopment, where a series of chemical reactions such asmixing, reaction and analysis is held on a tiny tip, wouldrequire technologies in size of this range. Semiconductortips are finely processed in the range of 50 nm by lithogra-phy.

Carbon black, which has been long used to reinforcerubbers, is generally several tens of nm. On the other hand,commercial polymer alloys include components hundred

Žnm in size. Among them, ABS acrylonitrile-butadiene-.styrene terpolymer resin is endowed with optimum im-

pact resistance by controlling the size of disperse phase inthe range of several hundred nm. Polymerrlayered claynanocomposites are reinforced by introducing very thindisperse phase clay layers. For example, in the case of

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nylonrmontmorillonite nanocomposite, montmorillonitelayers become separated by 1 to 2 nm being exfoliatedinto nylon matrix. Disperse phase of several tens of nm isobtained for aliphatic polyamiderall aromatic polyamidealloy by the solution blend method, resulting in significantmechanical reinforcement. However, this method is veryexpensive, and then the melt compound method is beingexplored for production on a commercial base. The meltcompound method utilized for PETrall aromatic polyesteralloys gives inadequate reinforcement, since dispersephase is limited to around 1 �m in size due to processingdifficulty. Methods for dispersion should be improved inthis case, including development of compatibilizers.

A functional material that has been used for a long timeis silver salt for photo films. Size of silver bromide as aphotosensitizer is usually larger than 1 �m, and is re-quired to be several tens to hundreds in nm for more finepictures. Nano-particles dispersed by the sol-gel methodfor functional optical glass are several tens of nm in size.Blended polymer alloys are often used as functional mate-rials. In this case, the size of disperse phase is similar tothat for structural nanocomposites. Block copolymers usedfor bulk materials and transparent materials are providedwith micro phase separation structures of several nanome-ters. In case of surface modification by graft, a surfacelayer is thinner than that. Surface layers of enzymes sur-

face-immobilized to biomaterials and physiologically ac-tive substances, DNA probes, DNA chips, protein chips,and columns immobilized to polymixins for septicemiatreatment are as thin as below 1 nm. Nano thick films areoften formed by the vacuum evaporation method and thespin coating method, and they are in the range of severaltens to hundreds of nm. Monomolecular and accumulatedmolecular layers typical for LB films are often thinner than1 nm. Functional particles such as noble metal nano-par-ticles, carbon nanotubes, fullerens and dendrimers are inthe order of 1 nm.

2 MATERIALS OF INTEREST[ ]2.1 POLYMER NANOCOMPOSITES 3, 4

Polymer nanocomposites are understood as polymers inwhich nanofillers are homogeneously dispersed, but alsolayered structures in nanometric scale shall be included inthis category. In disperse type, nanofillers are usually or-ganically modified to be miscible with companion poly-mers. Polymerrlayered silicate or clay nanocomposites aremost popular now as polymer nanocomposites, in whichsilicate or clay is micaceous in shape. They could be datedback to 1990, when they were industrially manufacturedas structural materials or engineering plastics for the firsttime. This success initiated further significant research ac-

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tivities on a variety of nanocomposites for other purposes,too, e.g, nanoparticles such as TiO , SiO and Al O can2 2 2 3be mixed into polymers to form nanocomposites.

2.1.1 POLYMER/LAYERED SILICATE[ ]NANOCOMPOSITES 5

Various polymers are feasible as layered silicatenanocomposites, which include vinyl polymers, condensa-

Ž .tion step polymers, polyolefins, specialty polymers, andbiodegradable polymers, as shown in Table 4. Layered sil-icates belong to the family of 2:1 phyllosilicates, as shown

w xin Figure 1 4 . Their crystal structure consists of layersmade up of two tetrahedrally coordinated silicon atomsfused to an edge-shared octahedral sheet of either alu-minum or magnesium hydroxide. The layer thickness isaround 1 nm, and the lateral dimensions of these layersmay vary from 3 nm to severalymicrometers or larger,which depend on the particular layered silicate. Stackingof the layers leads to a regular van der Waals gap betweenthe layers called the interlayer or gallery. Isomorphic sub-

Ž 3qstitution within the layers for example, Al replaced byMg2q or Fe2q, or Mg2q replaced by Li1q generates neg-ative charges that are counterbalanced by alkali and alka-

w xFigure 1. Structure of 2:1 phyllosilicates. Drawn based on 5 .

line earth cations situated inside the galleries. This type oflayered silicate is characterized by a moderate surface

Ž .charge known as the cation exchange capacity CEC . Thischarge in not locally constant, but varies from layer tolayer, and should be considered as an average value over

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the whole crystal. Montmorillonite, hectorite and saponiteare the most commonly used layered silicates.

There are two types of structure: tetrahedral-sub-stituted and octahedral substituted. In case of tetrahe-drally substituted layered silicates, the negative charge islocated on the surface of silicate layers, and hence, thepolymer matrices can react more readily with these thanwith octatedrally- substituted material. Two distinct butinterrelated features characterizes layered silicates. Thefirst is the ability of the silicate particles to disperse intoindividual layers. The second is the ability to finely tunetheir surface chemistry through ion exchange reactionswith organic and inorganic cation. Pristine layered sili-cates usually contain hydrated Naq or Kq ions. These are

Žonly miscible with hydrophilic polymers such as poly eth-. Ž . Ž . Ž .ylene oxide PEO , or poly vinyl alcohol PVA . To make

these miscible with other polymers, their surface shouldbe converted to an organophilic one, which can be real-ized by ion-exchange reactions with cationic surfactantssuch as alkylammonium and alkylphosphonium cations.

Three different types of polymerrlayered silicatenanocomposites are thermodynamically achievable,namely intercalated, flocculated and exfoliated nancom-

w xposites, as illustrated in nanometric scale in Figure 2 5 .In intercalated nanocomposites, the insertion of a poly-mer matrix into the layered silicate structure occurs in acrystallographically regular fashion, regardless of the clayto polymer ratio. Intercalated nanocomposites are nor-mally interlayer by a few molecular layers of polymer.Properties of the composites typically resemble those ofceramic materials. Flocculated nanocomposites are con-ceptually the same as intercalated ones. However, silicatelayers are sometimes flocculated due to hydroxylatededge-edge interaction of the silicate layers. In exfoliatednanocomposites, the individual clay layers are separatedin a continuous polymer matrix by an average distance thatdepends on clay loading. Usually, the clay content of exfo-liated nanocomposites is much lower than that of interca-lated nanocomposites.

2.1.2 POLYMER/METAL OXIDENANOCOMPOSITES

In a composite, the polymer in the vicinity of the filleris strongly affected by the presence of the filler and thearea surrounding the filler particle is called the interphase

w xor the interaction zone, as shown in Figure 3 6 . Fornanocomposites, the volume fraction of the polymer ad-joining the filler is large due to the large surface area. It iswell known that the polymer chains interacting with thesurface of the filler have altered properties such as crys-tallinity, crosslink density, mobility or conformation.Therefore, the interface chemistry and interfacial strengthis a much more critical parameter in nanocomposites thanin traditional composites. Further, since these interactionzones overlap at relatively low volume fractions, it has been

Figure 2. Schematic illustration of three different types ofw xpolymerrlayered silicate nanocomposites. Drawn based on 5 .

proposed that a small amount of nanofillers can have aprofound affect on material properties. But unfortunately,such interaction zones are not necessarily well character-ized. It was also suggested that free volume in such inter-

w xaction zones would be affected by nano-fillers 7, 8 .

2.2 DIELECTRICS AND ELECTRICALINSULATING MATERIALS

Polymer matrices with dispersed inorganic nanoparti-cles are now a target for investigation in the dielectric field.Performance improvement as dielectrics and electrical in-sulation is expected. Both thermoplastic resins and ther-moset resins are being considered and their companion

Ž .nanoparticles are selected from clay layered silicates , sil-Ž . Ž . Ž .ica SiO , rutile TiO , and alumina Al O . Disperse2 2 2 3

phases, nanoparticles or nanolayers of interest vary in sizeas indicated in Table 3. Layered silicates or clays are inthe range of a few nm in thickness, and in the range of100 nm in other two dimensions. Size of nanoparticles ofinorganic substances such as SiO , TiO and Al O are2 2 2 3chosen to be 30 to 40 nm. Polymer materials such as PE,PP, EVA, EPDM, PA, PI and epoxy are under investiga-tion.

Figure 3. Illustrated morphology of polymers around metal oxidesw x10 .

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w xFigure 4. Intercalation and exfoliation process for polymer nanocomposites 19 .

3 PROCESSING METHODS UNDERDEVELOPMENT

Composite materials consist of at least two differentkinds of material phases, and therefore inherently havethe interfaces with which they both contact each other. Inorder to form a stable composite system, it is requisite toincrease the compatibility between them, and decrease theinterfacial tension between them to the utmost. Especiallynanocomposite materials include nanoparticles that aredispersed in nanometer scale separation even with theirsmall addition as shown in Table 3, and are enormouslylarge in their total surface areas. Thus it is technically re-quired to form such a material system stably without ag-glomeration and phase separation for production ofnanocomposites There are several methods for nanocom-posites such as intercalation, sol-gel, molecular compositeand direct dispersion, as shown in Table 5. Interfacialstates between both of the two phases are modified incertain ways so as to form their stable systems. In general,the interfacial tension is required to be below 10y4 Nrmfor polymer alloys of micrometer phase separation type,and below 5 x 10y4 Nrm for polymer composites with mi-crometer-sized fillers. In case of nanocomposites, thesevalues should be far less than those.

The layer exfoliation and intercalation method is one ofthe present major methods for polymer nanocomposites.There are two processes available at present to conduct it,i.e., the polymerization process, and the melt compoundprocess. The former was first available in public in 1987and has been favored since then. The latter, however,seems to have been a main stream for this type ofnanocomposites, since it costs less in equipment and ren-ders the flexibility in application. The sol-gel method nowattracts much more attention, because it might be com-paratively easily modified to suit to industrial manufac-ture. Molecular composites are to be manufactured by thesolution re-precipitation method, and by the melt com-pound method. The latter is a new method in this manu-facturing process that is expected to emerge. It is neces-

Ž .sary to develop nanometric liquid crystal polyester LCPfibrils to realize an industrial manufacture line for themolecular composites.

3.1 INTERCALATION METHODThe intercalation method is the most popular for poly-

mer nanocomposite formation. This is the method to in-tercalate monomers or polymers between layers of inor-ganic layered substances to cause to disperse them intopolymers during a process of polymerization or melt com-pounding by exfoliating the layered substances each byeach layer. Layered silicates are often used in this method,as shown in Figure 4.

w xThere are three methods in this category 5 . The firstone, i.e., intercalation of polymer or pre-polymer from so-lution is based on a solvent system in which the polymeror pre-polymer is soluble and silicate layers are swellable.The layered silicate is first swollen in a solvent, such aswater, chloroform, or toluene. When the polymer and lay-ered silicate solutions are mixed, the polymer chains in-

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tercalate and displace the solvent within the interlayer ofthe silicate. Upon solvent removal, the intercalated struc-ture remains, resulting in polymerrlayered silicatenanocomposite. In the second one, i.e., in-situ intercala-tive polymerization method, the layered silicate is swollenwithin the liquid monomer or a monomer solution so thatthe polymer formation may occur between the interca-lated sheets. Polymerization is initiated either by heat orby an organic initiator or catalyst fixed through cation ex-change inside the interlayer before the swelling step. Thelast one, i.e., melt intercalation method involves anneal-ing, statically or under shear, a mixture of the polymerand organically modified layered silicate above the soften-ing point of the polymer. This method has great advan-tages over either in-situ intercalative polymerization orpolymer solution intercalation. Firstly, it is environmen-tally benign due to the absence of organic solvents. Sec-ondly, it is compatible with current industrial process, suchas extrusion and injection molding. The melt intercalationmethod allows the use of polymers that were previouslynot suitable for the other two methods.

3.2 SOL GEL METHODThe sol gel method is characterized by the fact that in-

organic or composite organic-inorganic materials are madeat relatively low temperatures, and in principle, consists ofhydrolysis of the constituent molecular precursors andsubsequent polycondensation to glass-like form. It allowsincorporation of organic and inorganic additives during theprocess of formation of the glassy network at room tem-perature. This method has been traditionally utilized tofabricate glasses and ceramics. Recently, at the same time,it has been used for polycrystals, porous composites, andorganic-inorganic composites. Sol-gel reaction is started

Ž .from metal alkoxide, M OR n. It should be melted inwater, alcohol, acid, ammonia, and the like in order to behomogeneously dispersed. Metal alkoxide is hydrolyzedthrough reaction with water and turns out to be metalhydroxide and alcohol. There are many kinds of metalsutilized such as Na, Ba, Cu, Al, Si, Ti, Zr, Ge, V, W and

Ž .Y. Silicon alkoxides such as tetraethoxysilane TEOS andŽ .tetramethoxysilane MTEOS are often used. In case of

TEOS, for example, an amorphous polymer with three di-mensional network structures of silica is formed by poly-

w xmerization reaction followed by hydrolysis 3 .

Si OC H q H O™ OC H Si-OHqC H OHŽ . Ž .2 5 2 2 5 2 54 3�Si-OHqHO-Si�™�Si-O-Si �qH O2

�Si-OHq OC H Si-™�Si-O-Si �qC H OHŽ .2 5 2 53

The sol gel method was not considered suitable for massproduction, since it used water as media in general. How-ever, it is now expected to become a key technology in thenear future, as new modified methods such as a continu-ous sol-gel have been developed recently. Various compa-

nies have introduced highly functional organosols of silicacid, produced either by solrgel condensation of te-traethoxsilane, or acidification of sodium silicates, fol-lowed by functionalization with various trisalkoxysilanes.Another very versatile solrgel route led to the industrialpreparation of dispersable boehmite nanofillers. In theSasol process aluminum or magnesium metal is activatedby etching off the surface oxide layer. Reaction with alco-hol produces alkoxides and hydrogen. Upon hydrolysis thealuminumalcoxides from boehmite mineral that is ob-tained as nanoparticle dispersion. The boehmite mineralscan be rendered organophilic by reaction with carboxylicor benzenesulfonic acids. The by-product alcohol is recy-cled in this process. In contrast to natural organophilicboehmite, the solrgel reaction product is much easier toredisperse and does not possess other metal ions as impu-rities.

3.3 MOLECULAR COMPOSITEFORMATION METHOD: POSSIBILITY

FOR NANOCOMPOSITESMolecular composite is a material system wherein dis-

crete reinforcement is achieved with molecular rods. It wascharacterized originally by the fact that a rigid polymer

Ž .such as liquid crystal polyester LCP was dispersed in aflexible polymer matrix in molecular or microfibril dimen-sion. This past method was conducted by such a way thattwo kinds of materials were melted in a co-solvent to beprecipitated afterwards. It was not built up to mass pro-duction. A new method was developed in 1990’s that engi-neering plastics were melt-compounded with small

Ž .amounts of liquid crystal polyester LCP . This was foundto produce composites with excellent properties, and have

w xattracted people’s attention since then 9 . Typical exam-ples of this type are polyamide, poly phenylene ether al-loy, polyethylenetelephthalate, and polycarbonate with all

Ž .aromatic polyester LCP . It should be noted that theyneed the third substance that would function as compati-bilizers. The third substance would work to promote theformation of fibrils and to disperse it in the composites. Itis recognized that the molecular composites have moreadvantage over pristine materials in mechanical proper-ties. In this case, microfibrils to be used are 500 nm intheir size. In order to put nanocomposites of this type inreality, it is necessary to put disperse phase down to oneorder smaller than that we have now

3.4 NANO-PARTICLE DIRECTDISPERSION METHOD

In this method, nano-particles are chemically modifiedon their surfaces to increase compatibility with polymers,and are mixed with a polymer and dispersed homoge-neously without agglomeration. There are several exam-ples such as photo-hardening coating agents with modi-fied silica nano-particles, nano-particle paste of gold orsilver protected by comb-shaped block copolymers, and

1070-9878rrrrr04rrrrr$20.00 � 2004 IEEE772


polyamide 6 nanocomposite with silica nano-particles sur-face-treated by amino butyric acid.

4. EXPERIMENTAL WORK ANDRESULTS

4.1 SURFACE CHANGE OF POLYAMIDENANOCOMPOSITE CAUSED BY PARTIAL

[ ]DISCHARGES 10Polyamide 6 used for experiments was manufactured by

Unitica Co. in such a way that layered silicates, i.e., syn-thetic mica, were exfoliated to about 1 mm thick layers,and they were uniformly dispersed in a polyamide 6 resin

Ž .by in-situ polymerization. Partial discharge PD degrada-tion was investigated for polyamide 6 without nanoscale

Ž .fillers nanofillers and polyamide 6 nanocomposites with2, 4 and 5 wt% addition of organically, grafted in situ withpolyamide. Such materials were subjected to partial dis-

Ž .charge under the IEC b electrode configuration for eval-uation. Comparisons were made as to the surface rough-ness observed by scanning electron microscopy and atomicforce microscopy. It was found that the change in the sur-face roughness was far smaller in specimens withnanofillers than those without fillers, and that the 2wt%addition was sufficient for improvement, as shown in Fig-ure 5. This result indicates that polyamide nanocompositeis more resistive to PDs than polyamide 6 withoutnanofillers.

From SEM image observation, it seems that partial dis-charges would attack the surface of nanocomposite, butdegrade the part of polymer regions between nanofillersselectively, and even step aside from the fillers to intrudeinto their backside. It is to be noted that the nanofillersare more resistant against PDs, and larger in permittivitythan the polymer per se.

A mechanism for PD degradation has been investi-gated. Figure 6 illustrates how PD possibly acts on thesurface of PArlayered silicate nanocomposite for clarifi-cation of the mechanism. The nanocomposite consists of agroup of nanometric spherulites formed aroundnanofillers. Regions between such spherulites are filledwith amorphous PA. PD resistance is stronger for layeredsilicate than for polyamide. PA spherulite regions seem tohave stronger PD resistance than PA amorphous regions.Permittivity of layered silicate is about twice larger thanthat of polyamide itself. A hypothetical model for expla-nation of PD resistance of this material is considered as

Žfollows. PD will concentrate on the nanofillers about.twice on the surface of a nanocomposite specimen due to

the difference in permittivity, but the nanofillers are moreresistant against PD than the PA matrix. Spherulites mighthelp to resist against PD, either. PD might be faint on theamorphous regions that are less resistant against PD. Thisexplanation supports the experimental fact thatPArlayered silicate nanocomposite is more resistantagainst PD than pure PA. How PDs degrade the

Figure 5. Relation between average surface roughness andw xnanofiller content. V s 6 kV, t s 1 and 48 h 8 .

Figure 6. Schematic illustration of possible mechanisms for PD re-Ž .sistance of polyamiderlayered silicate nanocomposite hypothesis .

nanocomposite is illustrated in Figure 6, too. PDs concen-trate on the layered silicates and shift toward polymer re-gions because charge has been built up on the surface ofthe layered silicates. PDs crawl selectively into the amor-phous regions that are less resistant against PD thanspherulite regions.

Nevertheless, it is still uncertain how PD resistance willbe affected by nanometric spherulites and ror ‘‘interac-tion zones’’ around nanofillers.

Results obtained are summarized as follows:Ž .i Polyamide nanocomposites exhibited much stronger

PD resistance than pure polyamide.Ž .ii Surface erosion due to PD was 5 times shallower

for polyamide nanocomposites than for pure polyamideunder a certain condition.Ž .iii High PD resistant layered silicate is considered to

play a significant role in PD resistance of PArlayered sili-cate nanocomposite.Ž .iv Roles of spherulites and interaction zones remain

still unsolved.

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Figure 7. Frequency dependence of dissipation factors for poly-imide without fillers, and with 5 wt% 3�m microfillers and 5 wt% 40

w xnm nanofillers 9 .

4.2 ELECTRICAL CONDUCTION INPOLYIMIDE/SILICA

[ ]NANOCOMPOSITES 6Thermosetting polyimide with 2�10 wt% nanofillers

were prepared by the solvent cast method. Silica was usedas nanofillers. Their size was estimated to be 40 nm. Poly-imide films were cured into free standing films of approxi-mately 25 to 50 �m in thickness. Aluminum electrodes of1.27 cm in diameter were deposited onto both surfaces ofthe films by sputtering prior to electrical tests. The elec-trical properties of polyiminde-based nanocompositeswere investigated by several ways such as isothermal steadystate current, dielectric spectroscopy, and thermally stim-ulated current measurements.

The effect of nanofillers on the dielectric properties wasevaluated via interfacial polarization and electrical con-duction mechanism investigation. As shown in Figure 7,

Ž .polyimide with micrometer size 3 �m fillers has a losspeak at about 1 kHz, while pure polyimide has no peakaround the frequency. This peak is attributed to theMaxwell-Wagner interfacial polarization. This peak ismuch reduced for nanocomposites with 40 nm fillers. Thereduction of interfacial polarization was explained in termsof the field mitigation with the reduction of filler dimen-sion.

The electrical conduction of polyimide nanocompositeswas found to be fairly consistent with the space charge

Ž .limited current SCLC mechanism. Polyimide with 2 wt%nanofillers showed some reduction in bulk electrical con-ductivity compared to pure material and 10wt% micro-filler polyimide at elevated temperatures. Thermally stim-

Ž .ulated current TSC peak shifted from 185�C to 200�C orhigher temperatures. Results obtained are summarized asfollows:

Ž .i Peak dissipation factor of PI nanocomposite wassmaller at 1 kHz than that of PI microcomposite.

Ž .ii dc current of PI nanocomposite was smaller at hightemperatures than that of pure PI and PI microcomposite.Ž .iii A TSC peak shifted from 185�C for pure PI to 200

�C or higher temperatures for PI nanocomposite.The first phenomenon was understood in terms of in-

terfacial polarization. The second and third phenomenawere correlated to the introduction of deeper carrier traps.It was emphasized that the nature of such traps and theinteraction between filler and matrix would deserve fur-ther investigation.

4.3 THERMAL AND MECHANICALPROPERTIES OF POLYIMIDE

[ ]NANOCOMPOSITES 11Commercially available nanoparticles were dispersed

evenly into standard thermosetting polyimide enamel af-ter proper surface treatment or coating of nanoparticles.Nanoparticles of 40 nm in thickness were added by 1 to10wt%. All the materials were solvent cast and cured intofree standing films of approximately 50 to 100 �m inthickness. Tensile strength, scratch hardness and thermalconductivity were measured for these films. By focusingon the relationship between nanoparticles and polymermatrices inside nanocomposites, the following were ob-tained:Ž .i Elongation and strength to failure improved if PI was

converted into PI nanocomposite.Ž .ii Scratch hardness of PI nanocomposite was larger

than that of PI microcomposite.Ž .iii Tensile modulus of PI nanocomposite showed no

significant change between pure PI and PI nanocompos-ite.Ž .iv Thermal conductivity was enhanced for PI

nanocomposite filled with coated nanoparticles as com-pared to pure PI and PI microcomposite.

Surface modification or coating effect of the aboveŽ .statement iv was found to increase thermal conductivity

as shown in Figure 8. It is interesting to cite that surfacetreatment of nanofillers seemed to improve the thermalconductivity of PI nanocomposite, which indicated thatfiller-matrix interactions functioned for performance im-provement. It was emphasized that polymer chains wouldinteract with the surface of nanofillers to alter variousproperties such as crystallinity, cross-link density, mobilityand conformation to change mechanical and thermalproperties in the end.

4.4 ELECTRICAL PROPERTIES OFPOLYMER NANOCOMPOSITES BASED

UPON ORGANOPHILIC LAYERED[ ]SILICATES 12

Ž .Ethylenervinylacetate EVA and isotactic polypropy-Ž .lene iso-PP nanocomposites were prepared from

organophlic layered silicates. Synthetic fluorohectorite wasmodified by means of cation exchange of their interlayer

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Figure 8. Thermal conductivity vs. filler content characteristics forŽpure PI, PI microcomposite, and PI nanocomposites nanoparticles

. w xuncoated and coated 10 .

sodium cations for hydrophobic alkyl ammonium cationsin order to expand the interlayer distance and to facilitateintercalation and exfoliation during processing. Non-polarpolymers such as iso-PP needed compatibilizers and highshearing stress during processing, while polar polymerssuch as EVA did not. In case of iso-PP, the addition ofmaleic anhydride grafted PP promoted exfoliation andin-situ formation of nanocomposites

Fluorohectorite, modified by means of cation exchangeŽ .with protonated octadecylamin ODA , was used as nan-

nofillers with different concentration, i.e., 2, 3 and 6 wt%.Specimens used were prepared in a two step process. In

Ž .the first step, the iso-PP PP HC 001 A-B1 from Borealiswas melt-blended in conjunction with the PP-g-MA com-

Ž .patibilizer Licomont AR 504 from Clariant and theŽorganohectorite Somasif ME 100 from CO-OP Chemical

.Japan , modified via ODA cation exchange, using a coro-Ž .tation twin-screw extruder Collin; ZK 25T at a tempera-

ture of 220�C. Similar procedure was used to prepare EVAŽ .ESCORENE ultra UL 00012 from Exxon Chemicalsnanocomposites. In the second step, nanocomposite filmswere prepared using the same extruder with a chill role

Ž .unit Collin; CR 72T . Obtained films were as thick asabout 400�m. Nanofillers in the films were checked byTEM.

Space charge, conduction current and dielectric strengthmeasurements were performed for polymer nanocompos-ites described above. The comparison among unfilled andnanofilled materials, with different filler concentrations,showed that space charge accumulation phenomena wereconsiderably affected by the presence of nanofillers. Re-sults are summarized as follows:Ž .i Space charge decreased at medium high electric field

by introduction of nanofillers for EVA and PP, while itincreased at low fields.

Figure 9. Comparison of space charge values at poling field of 40Ž . w xkVrmm for EVA and PP base and nanofilled 6 wt% 11 .

Ž .ii Space charge inception threshold shifted to lowerŽ .values e.g. from 14 to 5 kVrmm for PP for both EVA

and PP, if nanofillers were added. It decreased if nanofillercontent increased.Ž .iii Depolarization charge was faster in its decay rate

for nanocomposites than for base EVA and PP. EspeciallyPP with 6 wt% ODA showed a marked fast decay rate.Ž .iv dc conduction current increased for nanocompos-

ites.Ž .v dc breakdown strength increased for nanocomposite

PP, while it did not change significantly for EVA

Space charge accumulated at a probable design stresssuch as 40kVrmm was found to be smaller for nanocom-posites than for pure or base polymers, as shown in Figure9. This deviation was more evident as nanofiller contentincreased up to a level of about 10%. Further increase ofnanofiller concentration worsened electrical properties. Itwas considered from space charge characteristics, depo-larization and dc current measurements that ionic carrierswere available and that carrier trap distribution was sig-nificantly modified likely due to introduction of shallowtraps.

Some apparent contradiction should be pointed out asfor dc current for future investigation. Namely dc currentdecreased, when some nanofillers were added to PI, as

w xthe reference 9 indicated, while increased when layeredsilicates were dispersed in EVA and PP, as in referencew x w x w x11 . In both papers 9 and 11 , ionic carriers and carriertraps were proposed for electrical conduction, but it wasnot revealed whether the traps were associated with ionicconduction or electronic conduction.

4.5 EPOXY WITH METAL-OXIDE FILLERS[ ]13

Ž . Ž .Titanium dioxide TiO micro 1.5 �m fillers and2Ž .nanofillers 38 nm were dispersed in Bisphenol-A epoxy

Ž .Vantico CY1300 q HY956 . Such composites were me-chanically stirred and molded into films of 500 to 750 �m

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in thickness. In case of nanofiller dispersion, the compos-ites were subjected to large shear force in the mixing pro-cess to obviate unwanted clustering or agglomeration of

Žnanofillers it was pointed out that nanoparticles wouldtend to agglomerate to make dispersion in resins quite

.difficult . For most electrical characterization, the castfilms with 1, 10 and 50 wt% micro- and nano-particleswere provided with evaporated 100 nm aluminum elec-trodes. Results can be summarized as follows.Ž . Ž .i Glass transition temperature Tg decreased or re-

mained unchanged when an epoxy resin was filled bynanocomposite particles, while Tg increased when it wasmicrocomposited.Ž .ii Permittivity and loss tangent exhibited different

characteristics depending on voltage frequency and tem-perature.Ž .iii Space charge behaved in a similar way as the base

resin for nanocomposites, while substantial space chargewas generated internally for microcomposites.Ž .iv Decay of charge in nanocomposites was very rapid.

It was stated from Tg data that nanoparticles might be-have in a similar way to filtered plasticizers rather than as‘‘foreign’’ substances creating a macroscopic interfaces.Dielectric spectra were virtually indistinguishable betweenmicrocomposites with 10 wt% particulates and the baseresin in the low frequency range, because only the chargeat the electrode was involved. On the contrary, a markeddifference was observed at low frequencies and high tem-peratures between base resin and nanocomposites with 10wt% nanofillers. The nanocomposites showed a flat tan�

Ž .response at low frequencies. In addition to iii , the decayof charge was very rapid in nanocomposites compared tomicorcomposites. It was emphasized from the above re-sults and some consideration that nanometric fillers wouldmitigate the interfacial polarization characteristic of con-ventional materials with a reduction in the internal fieldaccumulation. Hypothetical ‘‘interaction zone’’ was de-fined here as a short-range highly immobilized layers thatmight develop near the surface of nanofillers to explainmajor phenomena observed in nanocomposites. This willbe discussed later.

w xThe continuation of the same research, 14 , consideredŽ .uniform field breakdown BDV specimens, prepared by

the use of a mold having multiple spherical protrusions soas to form a plurality of recessed specimens in one opera-tion. Divergent filed specimens used for electroclumines-

Ž .cence EL measurements were created by molding aroundelectrolytically etched tungsten needles having a well-characterized tip radius of about 4 �m. Laminar moldedspecimens were subjected to thermally stimulated currentŽ .TSC measurements. Experimental results are as follows:Ž .i BDV remained almost same up to 10% nanoparticle

loading, while it decreased significantly for 10% micropar-ticle loading.

Figure 10. Electroluminescence onset field as a function of TiO2w xloading. a, 38 nm; b, 1.5 �m 13 .

Ž .ii EL onset field was higher for nanocomposites thanfor base resins and microcomposites.

Ž .iii The �-TSC peak seemed to shift upward in tem-perature for nanocomposites. Microcomposites exhibiteda significant �-TSC peak of interfacial polarization above100 �C, while nanocomposites had no peak at all.

As shown in Figure 10, EL onset field was a function offiller loading with a peak at 10wt% loading for nanocom-posites. Based on experimental results obtained above, in-terfacial space charge or interfacial polarization was con-sidered crucial and was associated with Maxwell-Wagnereffect. Then, it was emphasized that nanoparticles mightmitigate effects of trapped entanglement on the Maxell-Wagner effect.

4.6 STUDY OF THE PROPERTIES OF[ ]RTV NANOCOMPOSITE COATINGS 15

Specimens used for tracking tests were prepared andŽdivided in three groups, that consisted of Group 1 virgin

.room temperature vulcanized silicone rubber, i.e., RTV ,Ž .Group 2 RTV filled with 40 wt% ATH and Group 3

Ž .RTV with 5 wt% layered silicate nanocomposite . Sur-face SEM observations confirmed that layered nanosili-cates were randomly exfoliated and dispersed in RTV. Itwas considered that layered nanosilicates were coveredwith organophilic cations when pristine layered silicateswere processed by intercalation, so that the surface en-ergy of normally hydrophilic silicates was lowered to makelayered nanosilicates compatible with RTV coatings.Specimens of 120 mm � 50 mm � 6 mm in size weresubjected to a standard tracking test. As shown in Table 6,the addition of nanoparticles to RTV silicone rubber im-proved their tracking performances comparable to that ofATH filled RTV. In addition to that, the maximum ero-sion depth was much smaller for RTV nanocompositesthan for virgin RTV and ATH filled RTV. Experimentsconfirmed a slight increase of tan � , a slight decrease of� and a little change of � .r v

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Results are summarized as follows:

Ž .i Flame retardation of RTVrlayered silicatenanocomposite was highly improved.

Ž .ii The needed amount of nano-layered silicate wasonly about one tenth of that of the conventional ATHfiller.

Ž .iii Properties such as tan � , � and � remained, af-r vter the addition of the nanofiller, satisfactory.

4.7 EFFECT OF SPACE CHARGE IN[ ]NANOCOMPOSITE OF LDPE/TIO 162

Ž .Low-density polyethylene LDPE r TiO nanocompos-2ites were prepared by the solution blending method using

Žnanoparticles of TiO purity: 99.5 % or larger, average2diameter 30 nm, specific surface 150� m2rg, loose den-

3.sity: 0.05�0.06 grcm . Detailed procedure is described inw xliterature 12 . Specimens for experiments were 30 �m in

thickness. The disperse performance of TiO in LDPE was2Ž .observed with the scanning electron microscope SEM .

TiO was dispersed uniformly in LDPE up to 5wt%2through controlling temperature and content of the solu-tion of LDPE; the largest conglomeration diameter ofTiO in LDPE was less than 80 nm. Space charge distri-2bution of such specimens with and without nano TiO was2

Ž .measured by the pulsed electroacoustic PEA method.The following results were obtained:

Ž .i Hetero-polar space charge near electrodes was muchless in LDPEr TiO nanocomposites than that in pure2

Ž . ŽLDPE under lower direct current dc stress no more than.40 kVrmm .

Ž .ii Space charge inside the nanocomposites was muchmore uniform than that in pure LDPE. Thus electricalstress concentration was improved under dc stress in thenanocomposites.

Ž .iii Decay rate of the remnant of space charge in LDPEspecimens containing TiO increased with increasing of2the TiO , when short-circuited after pre-stress at 502kVrmm for 1 h.

4.8 EFFECT OF � AND PHASE NANOAl O ON MECHANICAL PROPERTIES OF2 3

[ ]EPDM 17Nano Al O in � and phases was mechanically2 3

Žblended with sulphurized agent dicumyl peroxide or DCP:. Ž .2 wt% and 1010 type of antioxidant 0.2 wt% into ethy-

Ž .lene-propylene-diene methylene linkage rubber EPDM .Both kinds of compounds were cross-linked and moldedinto specimens under the same conditions. SEM observa-tions indicated that Al O in phase was dispersed more2 3uniformly than Al O in � phase. The following results2 3were obtained:

Ž .i Tensile strength and elongation of EPDM contain-ing nano Al O in phase were better than those of2 3EPDM containing nano Al O in � phase.2 3Ž .ii The smaller diameter of the Al O nanoparticles2 3

gave the larger improvement in mechanical properties.

Ž .iii Surface pretreatment of Al O nanoparticles2 3helped improve the mechanical properties.

4.9 SYNTHESIS ANDCHARACTERIZATION OF

POLYIMIDE/SILICA NON-CLUSTERED[ ]COMPOSITES 18

Ž .A new class of PD partial discharge resistant filmsŽ .made of ‘‘nanocluster-trapped’’ polyimide PI rsilica

Ž .SiO nanocomposites that were synthesized by the sol-gel2reaction was obtained by hydrolysis and poly-con-

Ž .densation of tetraethoxysilane TEOS ceramic precursorŽ .or methyl-triethoxysilane MTEOS ceramic precursor in

Ž .the solution of polyamic acid PAA dissolved in N,N-Ž .dimethyl-acetamide DMAc , followed by heating. Some

w xmore detailed description is given in literature 17 . Thechemical surface and the surface morphology of the com-posite films were characterized by using Atomic Force

Ž .Microscope AFM and Fourier Transform Infrared Spec-Ž .troscope FTIR . Size of silica particles ranged from 127

to 506 nm, and MTEOS gave smaller sizes than TEOS.PD resistance of the composite films was tested under highvoltage using rod-plate electrode to derive PD lifetime. Asshown in Figure 11, PD lifetime increases with the in-crease of silica content based on MTEOS. The same be-havior was shown by TEOS, with life increasing almostlinearly with silica content. PD lifetime was longer forMTEOS than for TEOS. Therefore, life was consideredto be affected not only by the type of precursor, but alsoby agglomeration extent of inorganic particles. The resultsobtained are summarized as follows:

Ž .i PD resistance was larger for PIrsilica nanocompos-ites than for pure PI specimens.

Ž .ii Both agglomeration of particles and the type of pre-cursor seemed to affect PD lifetime.

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Figure 11. PD lifetime vs. silica content for MTEOS PIrsilica com-w xposite 17 .

4.10 PD-RESISTANCE OF POLYIMIDE[ ]NANOCOMPOSITES 19

w xSpecimens same as those used in the paper 18 werew xprepared. Nanocluster-trapped PI rsilica composites 18

were renamed as polyimide-nano inorganic composites inw x19 . Particle size was presumed to be in the range 127 to506 nm. PI nanocomposite films were subjected to surfacepartial discharges according to IEC-343 and ASTM-2275.PD breakdown time was measured for 30�m films. PDbreakdown time was 3-12 times longer for nanocompositePI than for pure PI. Silica loading was optimum at 8 wt%for PD resistance. Dielectric spectroscopy, such as C -pand tan �- characteristics, was investigated both beforeand after PD aging, and the surface morphology was char-

Figure 12. Temperature dependence of permittivty and tan � forw xpure and nanocomposite epoxy resin 19 .

acterized by AFM. Dielectric spectroscopy data were ob-tained at 0, 30, and 45 minutes of aging time. Results re-garding capacitance were uncertain. Permittivity and thepeak value of tan � decreased steadily as aging time in-creased, while the frequency at tan� peak shifted upwardfrom 1.16 to 1.27 MHz for pure PI and from 0.67 to 1.99MHz for nanocomposite PI.

The following results were obtained:

Ž .i PD resistance was stronger for nanocomposites thanfor pure PI.

Ž .ii Silica loading of 8 wt% gave the best PD resistance.

Ž .iii Capacitance at tan� peak in the frequency depen-Ždence decreased with aging time from 169 to 179 pF and

.from 194 to 274 pF .

Ž .iv The frequency corresponding to the peak value oftan � shifted to higher values as aging time increased.

Ž . Žv Such frequency shift was larger from 0.67 to 1.99. Ž .MHz for nanocomposite PI than from 1.16 to 1.27 MHz

for pure PI.

4.11 EPOXY-ORGANICALLY MODIFIEDLAYERED SILICATE NANOCOMPOSITES

[ ]20Epoxy-silicate nanocomposites were prepared by dis-

persing synthetic layered silicates modified with alkyl-am-Žmonium ions in an epoxy resin diglycidyl ether of bisphe-

nol-A, DGEBA, Epikote 828, epoxide equivalent weights 184 to 194 or diglycidyl ether of bisphenol-F, DGEBF,Epikote 807, epoxide equivalent weight s 160 to 175,

.Japan Epoxy Resin Co. . Two kinds of organically modi-Ž .fied silicates STN and SEN provided by Co-op Chemical

Co. were used. In the dispersing process, the organicallymodified layered silicates were mixed in epoxy resin withshearing, and aggregations of the silicates were removedby centrifugal separation after mixing epoxy resin and sili-cates. Micrographs taken by transmission electron mi-

Ž .croscopy TEM indicated that the nanocomposites had amixed morphology including both parallel silica layersŽ .0.1�0.5 �m, 5�15 layers and exfoliated silica layersŽ .nano-scale dispersion area. Epoxy nanocomposite with3wt% DGEBArSEN showed similar mechanical strengthto that with 6 wt% DGEBArSTN. As shown in Figure 12,the permittivity and tan � were smaller for nanocompos-ite epoxy especially at high temperatures than for pureepoxy. Marked improvement was confirmed for tan � attemperatures above 120�C.

The following results were obtained:

Ž . Ž .i A glass transition temperature T of the nanocom-gŽ .posite shifted to a higher temperature q20�C than pure

epoxy.

Ž .ii Mechanical strength improved by layered silicateŽ .addition 3 to 6 wt% .

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Ž .iii Permittivity and tan � were smaller for nanocpm-posite epoxy than for pure epoxy.Ž .iv Dispersion of modified silicate prevented relative

Ž . Ž .permittivity � and dielectric loss tan � from increas-ring at high temperatures above the glass transition tem-perature.

4.12 CO-CONTINUOUSNANOSTRUCTURED POLYMER BY

[ ]REACTIVE BLENDING 21Polymer blends consist of a particulate minority phase

dispersed in a matrix. There are several methods to blendtwo homopolymers in continuous wt%: the ‘‘compatibi-lizer’’ method, and the block copolymerrhomopolymerrhomopolymer blend method. A stable co-continuouscopolymer was obtained on the basis of the graft polymerblend method. Then, a nanostructured polyethyleneŽ . Ž .PE -polyamide PA copolymer, especially with co-con-tinuous morphology, was produced through a self-assem-bly process by reactive blending. PE and PA were func-tionalized to drive reactive blending. The functionalizedPE used for the backbone chain was a random co-polymer

Ž .of ethylene, ethylacrylate and maleic anhydride MAH .PA6 synthesized by polycondensation, was terminated atone end by the reactive functional group NH . It exhib-2ited excellent mechanical properties over conventional PEand even classical PErPA blends, as well as a nearly con-stant elastic modulus of 10 MPa between 100�C and 200�C, high yield stress and high strain tensile behavior. It didnot creep, when heated above the PE melting point.

Results obtained for mechanical properties of a newPErPA blend are as follows:Ž .i It exhibited lower creep and greater heat resistance.Ž .ii It remained stable at much higher temperatures and

was provided with better mechanical properties than clas-sical blends.

Ž .iii This technology would point the way towards thedesign of stable co-continuous structure over a wide rangeof compositions and polymer types.

4.13 NEW EPOXY RESINS WITHCONTROLLED HIGH ORDER

[ ]NANOSTRUCTURE 22, 23Epoxy resins with mono and twin mesogens were for-

mulated. Mesogens were highly ordered in nanoscalethrough their self-assembly process, possibly resulting inimprovement of thermal conductivity by reducing as it isscattering. They would form nanometric liquid crystal re-gions that were 5 to 30 nm for mono mesogens, and in therange of 500 nm for twin mesogens. Thermal conductivitywas obtained to be 0.33 WrmK for mono mosogens, and0.85 � 0.96 WrmK for twin mesogens. These values shouldbe compared to 0.17 WrmK for conventional epoxy. Asshown in Figure 13, epoxy resins with nano-ordered struc-tures were obtained to have better thermal conductivitythan conventional epoxy resins.

Results obtained are summarized as follows:

Ž .i Epoxy resins exhibited higher thermal conductivity,if mesogens were highly ordered in nanometric scale.

Ž .ii Thermal conductivity was 5 times higher for epoxyresins with twin mesogens than for conventional epoxyresins.

w xHigh thermal conductivity epoxy explained in 19 wasaimed at low thermal expansion and low water absorp-tion, as well as high Young’s modulus even at high tem-peratures, for use in microelectronics printed circuitboards. Some other methods have even been developed toincrease thermal conductivity of epoxy resins. One is toform smectic liquid crystal structure in epoxy by using largemesogens, and the other is to increase unidirectional ther-

w xFigure 13. Thermal conductivity of various polymers for commercial use 21, 22 .

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mal conductivity by driving liquid crystal regions towardanisotropic orientation under high magnetic field, such as10 Tesla.

5 DISCUSSION5.1 DISCUSSION ON ELECTRICAL

PROPERTIES

5.1.1 EFFECT OF NANOMIZATION ONPERMITTIVITY AND TAN �

Ž . w x Ž .i Polyimidersilica nanocomposites 6 , Irwin GE :

Value of tan � tends to decrease in the order of purePI, PIrsilica microcomposite and PIrsilica nanocompositeat the low frequency region up to about 200 Hz between60 Hz and 1 MHz. A peak appears at about 1 kHz in the

Ž .middle frequency region between 60 Hz and 1 MHz incase of PIrsilica microcomposite. This peak is reducedfairly much in case of PIrsilica nanocomposites. The peakis ascribed to the Maxwell-Wagner interfacial polariza-tion. This reduction might be caused by the mitigation ofthe field around fillers due to size difference.

Ž . w xii EpoxyrTiO nanocomposites: Permittivity 13 , Nel-2Ž .son RPI :

Above 50 or 60 Hz, the dielectric spectra are virtuallyindistinguishable among base epoxy, epoxy microcompos-ite and epoxy nanocomposite. This very low frequencyprocess might not be due to particulates in bulk, but tocharges at the electrodes. The microcomposite materialexhibits two peaks of tan � that are observed at very lowfrequencies, such as 8 mHz and 100 mHz at two differenthigh temperatures for the microcomposite, respectively.Values of tan � at intermediate frequencies, around 100Hz, are smaller for nanocomposite than for microcompos-

w xite. This trend is similar to the data 9 obtained forPIrsilica composite. This might be related to the functionof nanofillers to immobilize polymer chains in what arecalled ‘‘interaction zones’’.

Ž .iii RTV silicone rubberr layered silicate nanocompos-w x Ž .ites: 15 Lan Wuhan

Nanocomposites exhibit a slight increase of tan � and aslight decrease of � at the commercial frequency. No dig-rital data are available as for tan � , although some figuresare given.

Ž . w x Ž .iv PIrsilica nanocomposites: 19 , Lei Harbin

Nanocomposite PI gives lower permittivity than pure PIin the frequency region between 100 kHz and 10 MHz.ŽNote: This appears to be contradictory to the corre-

.sponding figures.

Ž . w xvi Epoxyrlayered silicate nanocomposites: 20 , ImaiŽ .Toshiba

Permittivity and tan � are smaller for nanocompositeepoxy than for base epoxy.

Interpretation of dielectric spectra seem to be compli-cated when comparing base resin, microcomposites, andnanocomposites. It is questionable whether or not thepermittivty and tan� are reduced by nanomization at thecommercial frequency. Some data indicate a certain re-duction, but some other data do not, resulting in apparentcontradictions. This may depend on the way nanofillersinteract with companion polymers. These results requirefurther investigation, considering also methods of dispers-ing fillers in polymer matrices. It is certainly crucial toprevent nanofillers from agglomeration or to dispersefillers homogeneously in polymer matrices for obtainingreproducible and reliable data.

5.1.2

dc ELECTRICAL CONDUCTIVITY

Ž . w x Ž .i Polyimidersilica nanocomposites 6 , Irwin GE :dc current at low field decreases at high temperatures

by nanomization.

Ž . w xii PP and EVArlayered silicate nanocomposites 12 ,Ž .Montanari Bologna :

Ž .dc current at high field 30kVrmm increases at roomtemperature by nanomization. Thus apparently oppositedata were obtained. But this will depend on the modifica-tion of shallow trap depth levels introduced by nanomiza-tion.

5.1.3 SPACE CHARGE, TSC AND ELBEHAVIORS

Ž .i Space Charge

� Space charge increases at low field and decreases atw w xhigh field due to nanomizationy PP and EVArLS 12 ,

Ž .xMontanari BLN .

� Space charge inception field decreases due to nano-w w x Ž .xmization PP and EVArLS 12 , Montanari BLN .

� Space charge is generated internally by nanomizationw w x Ž .xEpoxyrTiO 13 , Nelson RPI .2

� wCharge decay time decreases due to nanomization PPw x Ž .xand EVArLS 12 , Montanari BLN .

Charge decay time decreases due to nanomizationw w x Ž .xEpoxyrTiO 13 , Nelson RPI .2Ž .ii TSC

� A TSC peak shifts toward higher temperatures due tow w x Ž .xnanomization PIrSilica 6 , Irwin GE .

� A TSC peak shifts toward higher temperatures due tow w x Ž .xnanomization EpoxyrTiO 14 , Nelson RPI .2

Ž .iii Electroluminescence

� EL onset field increases due to nanomizationw w x Ž .xEpoxyrTiO 14 , Nelson RPI .2

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Modification of trap depth distribution due to the intro-duction of nanofillers, particularly in reference to deeptraps, might explain space charge, TSC and EL character-istics as described above. Fast charge decay and otherspace charge characteristics seem to be contradictory withthe deep trap theory and certainly need some other con-sideration.

5.1.4 DIELECTRIC BREAKDOWN STRENGTH

� BDV increases for PP due to nanomization, whilew w xEVA decreases a bit PP and EVArLS 12 , Montanari

Ž .xBLN .

� BDV remains same up to 10wt% loading, and tendsw w xto decrease for more loading EpoxyrTiO2 14 , Nelson

Ž .xRPI .

As indicated above, dielectric breakdown strength mightnot be greatly affected by nanomization under small load-ing of nanofillers and proper dispersion conditions. Favor-able results are obtained in some cases.

5.1.5 PD RESISTANCE

� wPD resistance improves due to nanomization PArLSw x Ž .x10 , Kozako Waseda .

� PD resistance improves due to nanomizationw w x Ž .xPIrSilica 18, 19 , Zhang Harbin .

It would be probable that nanomization improves PDresistance of polymers, which certainly depends on hownanofillers are dispersed in polymer matrices and arecompatible with them.

5.1.6 TRACKING RESISTANCE

� Flame retardancy improves due to nanomizationw w x Ž .xRTVrLS 15 , Lan Wuhan .

5.2 RELATED PROPERTIES5.2.1 GLASS TRANSITION TEMPERATURE

� Tg decreases or remains unchanged due to nanomiza-w w x Ž .xtion EpoxyrTiO 13 , Nelson RPI .2

� w w xTg increases due to nanomization EpoxyrLS 20 , ImaiŽ .xToshiba .

It is possible to increase Tg by nanomization. It shouldbe noted that it would require homogeneous dispersion ofnanofillers for that purpose.

5.2.2 THERMAL CONDUCTIVITY

� Thermal conductivity enhances due to nanomizationw w x Ž .xPIrSilica 11 , Irwin GE .

� Thermal conductivity enhances by introduction ofwnannometric liquid crystal polymers EpoxyrnanoLC poly-

w x Ž .xmer 22, 23 , Takezawa Hitachi .

Nanoparticles and nanomteric crystal parts have higherthermal conductivity than base polymers, so that mixturesmight be more thermally conductive than the base resins.It was suggested that ‘‘interaction zones’’ might have asgood heat transfer as possible.

6 CONCLUSIONPolymer nanocomposites could be advantageous over

traditional filled polymers in electrical and thermal prop-erties as well as mechanical properties from the stand-point of dielectrics and electrical insulation. This featurewill technologically result in compact design of electricalequipments with high reliability and thereby in significantcost reduction for system integration and maintenance.Since this feature is originated from mesoscopic charac-teristics of interaction zones between polymer matricesand nanofillers, it will open a new academic arena for di-electric and electrical insulation that will need quantummechanics as well. Such interaction zones might be re-lated to free volume and charge carrier trap distributionŽ .shallow and deep traps , which should be further ex-plored. In order to obtain excellent but low-cost polymernanocomposites, existing material processing technologiesshould be more advanced so as to match dielectrics andelectrical insulation. Results are summarized as follows:

6.1 EFFECTS OF NANOMIZATIONŽ .1 dc conductivity increases and decreases depending onmeasurement conditions. Introduction of deep traps aresuggested.

Ž .2 Interfacial polarization can be reduced compared tomicrocomposites.

Ž .3 There seems to be a certain reduction of permittivitydue to nanomization. But change of permittivity as well astan� is complicated, and not conclusive. Manufacturingprocesses should be more investigated for homogenousdispersion of nanofillers.

Ž .4 Space charge, TSC and EL also give complex resultsin their threshold field and quantity. Introduction of addi-tional levels of shallow and deep traps, as well as increaseof trap density, might be involved. These might be deeplyrelated to ‘‘interaction zones’’. It is therefore necessary tocharacterize the interaction zones between nanofillers andpolymer matrices chemically and physically.

Ž .5 PD and tracking resistance improve. It is most proba-ble. Role of nanofillers and interaction zones should bemore clarified.

Ž .6 Thermal conductivity and glass transition temperaturecould be increased by proper methods.

6.2 PROPERTIES OF POLYMERNANOCOMPOSITES

Ž .1 Electrical and thermal properties as well as mechani-cal properties could be improved by nanomization of poly-

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mers. Polymer nanocomposites are advantageous overconventional filled polymers, because a small amount ofnanofillers might not modify the characteristics of basepolymers considerably.

Ž .2 Intercalation methods, sol-gel method, molecularcomposite formation method and nanofiller direct disper-sion method are the major processing technologies, andshould be more developed for better and cheaper materi-als with excellent interaction zones.

Ž .3 Polymer matrices, nanofillers, and interaction zonesbetween them are three major parts of nanocomposites.Their respective roles should be investigated based onmaterial characteristics of interest.

Ž .4 Especially interaction zones should be characterizedchemically and physically.

Ž .5 Deep and shallow traps should be investigated andcorrelated with physical and chemical characteristics ofinteraction zones. Especially a theory for trap level anddensity modification should be established.

Ž .6 Interaction zones are mesoscopic in nature. This willopen a completely new aspect of dielectrics and insulationstudies, which need consideration based on quantum andstatistical mechanics, too.

Ž .7 Heat resistant thermoplastic nanocomposites are en-vironmentally benign because of their recyclability. Thesecould replace thermoset resins that cannot be recycled.

Ž .8 Biodegradable polymers such as polylactic acid can befilled with nanofillers. PLA nanocomposites are expectedto be used for eco-friendly electrical insulation.

7 APPLICATIONSŽ .1 Polymer nanocomposites have been investigated forfuture use of electrical insulation for power apparatus,power cables, outdoor insulators, and insulated wires forelectric power technologies as well as printed circuitboards for electronics.Ž .2 Insulation could be more compact by using polymernanocomposites, resulting in overall cost reduction in ap-paratus and installation.Ž .3 Reliability could be improved by using polymernanocomposites, resulting in lower maintenance cost.Ž .4 Polymer nanocomposites will give much innovation indielectric and insulation technologies.

ACKNOWLEDGEMENTThis work was in part supported by Grand-in-Aid

Ž .Fundamental Research B-14350171 for Scientific Re-search from Japan Society for the Promotion of Science,Project Creative Energy and Environments of WasedaUniversity Advanced Research Institute for Science and

Ž .Technology, the Deutsche Forschungsgeminschaft DFGand the ‘‘Sonderforschungsbereich SFB428’’, to which theauthors are deeply indebted.

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