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484 NANOCOMPOSITES, METAL-FILLED Vol. 10
NANOCOMPOSITES, METAL-FILLEDIntroduction
Atomic clusters of transition metals embedded into polymeric
matrices repre-sent a very attractive class of materials which
combine properties belongingto the nanometer-sized metal phase
(such as certain magnetic, electronic, opti-cal, or catalytic
properties) and the polymer phase (such as processability
andlm-forming properties) (1). By the careful selection and
combination of bothcomponents, tailor-made functional materials
exhibiting unique characteristics(2) can be obtained, with the
advantage of using the well-established, low costtechnologies
available for polymer processing, such as printing, spraying,
orspin-coating.
Polymeric dispersions of nanometer-sized metal particles offer
the possibil-ity of functionalizing the polymer by properties
coming from the large numberof surface atoms (3) and the
quantum-size effects (4). Nanometric metals showproperties that
differ signicantly from that of bulk metals, which makes
thesenanocomposite systems intriguing for scientic study and
potentially useful for anumber of technological applications (512).
The control of nanoparticle morphol-ogy becomes a very important
aspect, since morphology profoundly inuences thematerial
performance. As a long-term goal the ability to control and vary
particlesize, distributions, shapes, and composition independently
from one another isvery desierable, in order to allow the tuning of
nanocomposite properties. A broadarea of nanoparticle features,
ranging from nanosized single crystals to somewhatlarger (in the
range of about 100200 nm) yet well-stabilized nonagglomerates,gain
signicant technological importance.
Polymer-embedded nanostructures are potentially useful for a
num-ber of technological applications, especially as advanced
functional materials(eg, high energy radiation shielding materials,
microwave absorbers, optical lim-iters, polarizers, sensors,
hydrogen storage systems) (512). In addition to theintrinsic
nanoscopicmaterial properties, the presence of a very large
interface areain these polymer-based nanocomposites can affect
signicantly polymer charac-teristics (eg, glass-transition
temperature, crystallinity, free volume content, igni-tion
temperature), allowing the appearance of further technologically
exploitablemechanical and physical properties (eg, re-resistance,
low gas diffusivity).
Historical Background
The fundamental knowledge on the preparation and nature of
metal/polymernanocomposites looks back at a long history which is
connected to the namesof many illustrious scientists (2).
The oldest technique for the preparation of metal/polymer
nanocompositesthat can be found in the literature was described in
detail in an abstract in 1835(13). The original article appeared in
1833 (J. Erdmann, p. 22). In an aqueoussolution, a gold salt was
reduced in the presence of gum-arabic, and subsequentlya
nanocomposite material was obtained in the form of a purple solid
simply bycoprecipitation with ethanol.
Encyclopedia of Polymer Science and Technology. Copyright John
Wiley & Sons, Inc. All rights reserved.
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 485
Around 1900, widely forgotten reports indicated the preparation
of polymernanocomposites with uniaxially oriented inorganic
particles, and their remark-able optical properties (14,15).
Dichroic plants and animal brils (eg, linen, cotton,spruce, or
chitin, amongst others) were prepared by impregnation with
solutionsof silver nitrate, silver acetate, or gold chloride,
followed by reduction of the cor-responding metal ions under the
action of light (16).
Dichroic lms were also obtained using gold chloride treated
gelatin whichwas subsequently drawn, dried, and nally exposed to
light (17). Similar resultswere obtained when gelatin was mixed
with colloidal gold before drying and draw-ing (18). The gold or
silver content in such systems amounted typically to ca 1%by weight
(19).
In 1904, Kirchner and Zsigmondy (Nobel Laureate in Chemistry,
1925) re-ported that nanocomposites of colloidal gold and gelatin
reversibly changed colorfrom blue to red upon swelling with water
(20). In order to explain the mechanismof nanocomposite color
change, they suggested that the material absorption mustalso be
inuenced by the distance between the embedded particles. In
addition,around the same time, the color of gold particles embedded
in dielectric matriceswas subject of detailed theoretical analyses
by Maxwell Garnett who explainedthe color shifts upon variation of
particle size and volume fraction in a medium(21,22).
During the following three decades, dichroic bers were prepared
with manydifferent elements (ie, Os, Rh, Pd, Pt, Cu, Ag, Au, Hg, P,
As, Sb, Bi, S, Se, Te,Br, I) (2327). The dichroism was found to
depend strongly on the employed ele-ment, and optical spectra of
dichroic nanocomposites, made of stretched poly(vinylalcohol) lms
containing gold, silver, or mercury, were presented in 1946
(however,the preparative scheme used is not really clear) (28). It
was assumed already inthe early reports that dichroismwas
originated by the linear arrangement of smallparticles (29) or by
polycrystalline rod-like particles (30) located in the
uniaxiallyoriented spaces present in the bers. An electron
micrograph depicted in 1951showed that tellurium needles with
typical dimensions of ca 5 50 nm werepresent inside a dichroic lm
made of stretched poly(vinyl alcohol), however alsoin this case,
just a limited number of details were given about the technique
usedfor sample preparation (31).
In 1910, Kolbe was the rst to prove that dichroic nanocomposite
samplesbased on gold contained the metal indeed in its zero-valence
state. Such afrma-tion was conrmed a few years later by X-ray
scattering. In particular, it wasshown that zero-valence silver and
gold were present in the respective nanocom-posites made with
oriented ramie bers, and the ring-like interference patternsof the
metal crystallites showed that the individual primary crystallites
were notoriented (32). Based on Scherrers equation, which was
developed just in this pe-riod, the average particle diameter of
silver and gold crystallites was determinedin bers of ramie, hemp,
bamboo, silk, wool, viscose, and cellulose acetate to bebetween 5
and 14 nm (33).
Basic Concepts
A nanocomposite is a material made of two or more phases one of
which has atleast one dimension in a nanometric size range.
Metal/polymer nanocomposites
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486 NANOCOMPOSITES, METAL-FILLED Vol. 10
Fig. 1. Structure of atomic clusters of metals: (a) elemental
cluster, (b) small cluster, (c)large cluster, (d) thiol-derivatized
cluster, and (e) alloyed cluster.
are made of a continuous polymeric matrix embedding nanosized
metal do-mains. Thermoplastic polymers, elastomers, and
thermosetting resins can be po-tentially used as matrix, but
amorphous linear polymers, like optical plastics(eg, polystyrene,
poly(methylmethacrylate)) and conductive polymers are
themostfrequently used. About the metallic ller, very small
clusters and nanoparticleswith a pseudospherical shape are
frequently employed (see Fig. 1). The use ofnanorods and nanowires
has also been described (2).
In these materials, the polymer has the function of protecting
the nanostruc-tures and allowing their manipulation, whereas the
nanosized ller provides thepolymericmatrixwith unique properties
coming from small-size effects (ie,meso-scopic properties). To
allow the appearance of mesoscopic properties, the nanopar-ticle
size is required to be very small; usually a dimension less than 30
nm isnecessary for most metallic materials. In particular, the
metal domain size, whereelectrons move, needs to be comparable to
the critical lengths of physical phe-nomena (eg, wavelength of the
electrons at the Fermi edge, mean free paths ofelectrons or
phonons, coherency length, screening length). Such phenomenon
isnamed electron connement (4). In addition, on this size scale,
also phenomenarelated to the high percentage of surface atoms
appear in the material (3).
Different topologies can result in the preparation of
polymer-embeddedmetalclusters. To prepare materials characterized
by the properties of surface atoms orwith characteristics coming
from the connement effect, a contact-free disper-sion of clusters
must result in the polymer matrix, since only in this case thelarge
amount of surface atoms present allows the surface properties of
matter toprevail on that of bulk. Both regular (eg, uniaxially
oriented pearl-necklace typeof arrays of nanoparticles) and
irregular metal cluster distributions are used intechnological
applications.
The properties of nanometric particles strictly depend on their
microscopicalstructure (ie, chemical composition, shape, size,
percentage of defects, microstrainconcentration, etc). For example,
the characteristic surface plasmon absorption ofa system of metal
nanoparticles dispersed into a dielectric matrix is related to
theparticle shape and size (34). To prepare a color lter, identical
particles should beused, otherwise the material will appear black.
The presence of a single type ofmicroscopic structure allows each
particle to provide the same contribution to thecomposite
properties. From a theoretical point of view, an ideal
nanostructuredcomposite should be made of identical metal domains
uniformly dispersed intothe polymeric matrix. However, since it is
very difcult to prepare a sample of
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 487
identical metal grains, the only practically possible situation
is a system of singlecrystals with a narrow size distribution.
Because of the high surface chemical reactivity, metal clusters
can be easilyoxidized or contaminated by nucleophilicmolecules. The
surface reactivity of smallclusters is so high that even noble
metals (eg, Pt, Pd, Ag) can be oxidized by air.Since small
molecules present in the air (eg, oxygen, water, sulfur dioxide)
aredissolved in polymers and can diffuse through it, the polymeric
matrix cannotprevent metal from surface reactions. Finally, the
polymer matrix has only thefunction to freeze particles, avoiding
their diffusion and aggregation by sintering.For such a reason the
nanoparticles need to be passivated before their embeddingin
polymers. The surface treatment (eg, thiol-derivatization, see Fig.
1d) preventsparticle aggregation by short-range steric repulsion
and stabilizes the metal coreprincipally by electronic effects.
Surface passivation offers also the possibility todisperse ller
into most of liquid monomers and organic solvents where polymersare
soluble.
Classication
Nanocomposites are biphasic materials that can be classied on
the basis of theirmicrostructure by the self-connectivity concept,
that is the number of space di-rections (X, Y, Z) in which each
phase inside the composite physically contactsitself. Composite
classication based on self-connectivity has been proposed forthe
rst time by Newnham in 1978 (35). Because of the discontinuous
nature ofmost nanocomposite materials, the self-connectivity
concept needs to be appliedto a limited but representative
composite portion (ie, a local self-connectivity def-inition should
be used). In particular, like in macro- and microcomposite
systems,each phase in a nano-composite material can be locally
self-connected in zero, one,two, or three dimensions. It is natural
to conne the attention to three perpen-dicular axes since all
property tensors are generally referred to such a system.
Ingeneral, for a n-phase system the number of possible
self-connectivity patternsis given by (n + 3)!/3!n!. Therefore,
inside a biphasic system (n = 2) there are10 different
self-connectivity patterns: (00), (10), (20), (30), (11),
(21),(31), (22), (32), and (33). The rst number in the notation
represents thephysical connectivity of one of the two phases and
the second number refers toconnectivity of the other one. A
schematic representation of these 10 types of self-connectivities
is given in Figure 2, using a cube as the basic building block.
Arrowsare used in Figure 2 to indicate the connected directions,
and an asterisk has beenintroduced in the symbol to indicate the
local connectivity.
Properties of Nanosized Metals
Nanosized metals are characterized by novel thermodynamic,
chemical, catalytic,optical, magnetic, and transport properties
which are much different from thoseof corresponding massive metals.
For this reason a 3-D periodic table of ele-ments has been
frequently proposed by chemists working in this material scienceeld
(36).
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488 NANOCOMPOSITES, METAL-FILLED Vol. 10
Fig. 2. The connectivity patterns of a biphasic solid (35).
The thermodynamic properties of matter are classically described
as naturalconstants; however, they change signicantly when the
dimension approaches afew nanometers (37). The rst small-size
effect which has been observed for met-als is the change in the
melting point (3843). When a solid is heated, it melts, andthe
melting temperature is normally considered one of the most
important charac-teristics of a material. Nanometer-size crystals
melt at much lower temperaturesthan extended ones. This can be
understood in two ways. First, the cohesive en-ergy of a crystal
arises by the sum of all pairwise interactions between the atoms.In
a very small crystal the number of surface atoms is large, and
therefore the co-hesive energy per atom has not yet converged on
the bulk value. A second pictureis more thermodynamic: as a solid
is heated, melting takes place at the temper-ature where the
chemical potentials of solid and liquid are equal. In addition
tothe usual term in the chemical potential, a very small crystal
also has a term init for the surface energy. In general, the
surface energy of liquids is less than thatof solids, because a
liquid can readily assume the lowest surface area shape, asphere.
As a consequence, the smaller the crystal the lower is the melting
temper-ature, and the reduction in the melting temperature is
proportional to the surfaceto volume ratio, or inversely
proportional to the nanocrystal radius. This scalinglaw for melting
temperature reduction has been veried in many metals (37).
Several interesting chemical properties arise as the grain size
of a metal isdecreased to a nanometer-size range (eg, enhanced
reactivity, stoichiometric be-havior of heterogeneous reactions,
new reaction routes). Theoretical computations
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 489
(37) showed that the fraction of metal atoms residing on the
surface of a 100-nmgrain increases from 12% up to 8595% for grain
sizes in the range of 12 nm.The importance of this observation is
that the chemistry of such surfaces is muchdifferent from that of
the same atoms contained within larger grains. If the size ofa
cubic crystal is decreased from 100 nm down to the 35 nm range, the
fractionof atoms contained in edge sites as compared to those
contained in basal planesincreases to 70%. The consequence of this
observation is that as the crystal sizedecreases, the fraction of
atoms located in low coordination sites increases sharply,which
inherently imparts a higher chemical reactivity to suchmaterials
(activatedmetals). High surface areas and intrinsically high
surface reactivities of hyper-ne reactants allow surface reactions
to approach stoichiometric conversion. Withthe development of
hyperne powders, the heterogeneous phase reaction can becarried out
much faster and at lower temperatures (44,45). Finally, gassolid
re-actions and liquidsolid reactions take on a new dimension, and
hyperne solidsenter now in the chemists arsenal as novel chemical
reagents.
The properties of nanosized metals in heterogeneous catalysis
are well es-tablished, since heterogeneous catalysts represent one
of the pioneering elds ofnanotechnology (3). The increasing portion
of surface atomswith decreasing parti-cle size, comparedwith
bulkmetals,makes smallmetal particles as highly reactivecatalysts,
since surface atoms are the active centers for catalytic elementary
pro-cesses. Among the surface atoms, those sitting on the crystal
edges and corners(see Fig. 3) are more reactive than those on basal
planes. The percentage of edge
Fig. 3. SEM micrograph of 3-D self-organized poly(methyl
methacrylate) particles. Ifbeads are considered as atoms, the
picture can be used to visualize the different catalyticsites on
the surface of a metal crystal.
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490 NANOCOMPOSITES, METAL-FILLED Vol. 10
and corner atoms also increases with decreasing size and this is
the reason forthe enhanced catalytic activity (ie, supercatalytic
effect) and different selectivityof very small metal catalysts.
If metal particles become very small, reaching the
nanometer-size scale, acolor may occur. This is a typical
phenomenon of nanometric metals. Actually, op-tical absorption may
result in the ultraviolet or visible part of the spectrum, andthis
arises from a surface plasmon resonance. This is due to a
collective electronplasma oscillation (plasmon) that is coupled to
an external transverse electro-magnetic eld through the particle
surface. It is possible to quantitatively relatethe absorption
coefcient to the wavelength of the exciting radiation by the
Mietheory for spherical inclusions in a dielectric matrix (34).
Far-IR luminescence isanother optical phenomenon frequently
observed with nanosized metals (46).
New magnetic properties appear in metals when the size
approaches thenanometric regime (47). The so-called oddeven effect
is a phenomenon observedwhen diamagnetic metals are reduced to a
nanometric-size regime. In particular,diamagnetic materials have
only spin-paired electrons. However, in practice itcannot be
assumed that a macroscopic piece of a diamagnetic metal does
nothave one or more unpaired electrons. This cannot be measured
because of theeffectively innite number of atoms and electrons.
However, if the particle sizeis small enough to make one unpaired
electron measurable, the oddeven effectshould become visible. Among
small diamagnetic metal particles there should bean equal
percentage of odd and even numbers of electrons.
For magnetic materials such as Fe, Co, and Ni, the magnetic
properties aresize-dependent (47). In particular, the coercivity
force Hc needed to reverse aninternal magnetic eld within the
particle changes with particle size and it ismaximum for
single-domain particles. Further, the strength of the internal
mag-netic eld of a single particle can be size-dependent.
Giant magnetostriction, magnetoresistivity, and magnetocaloric
effects rep-resent further examples of new properties arising from
the small size of magneticdomains (48).
Also transport properties, which are strictly related to the
electronic struc-ture of a metal particle, critically depend on
size (37). For small particles, theelectronic states are not
continuous, but discrete, because of the connement ofelectron wave
function. Consequently, also properties like electrical and
thermalconductivity may exhibit quantum-size effects.
Most of these unique chemical and physical characteristics of
nanosized met-als can be used for the functionalization of plastic
materials.
Preparation Methods
A limited number of methods have been developed for the
preparation ofmetal/polymer nanocomposites. Usually, such
techniques consist of highly spe-cic approaches, which can be
classied as in situ and ex situ methods. In the insitu methods two
steps are needed: rstly, the monomer is polymerized in solu-tion,
with metal ions introduced before or after polymerization.
Secondly, metalions in the polymer matrix are reduced chemically,
thermally, or by UV/ - irradia-tion. In the ex situ processes, the
metal nanoparticles are chemically synthesized,
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 491
and their surface is organically passivated. The derivatized
nanoparticles are dis-persed into a liquid monomer which is then
polymerized.
In situ Methods. A simple, direct, and versatile in situ method
was pro-posed by Watkins and McCarthy (49). In this method, an
organometallic precursoris dissolved in supercritical uid (SCF)
carbon dioxide and infused into a solidpolymer as SCF solution.
Chemical or thermal reduction of the precursor to thezero-valence
metal either in presence of SCF or subsequent to its removal
pro-duces metal domains within the solid polymer matrix. Such
technique has beenused in the synthesis of nanoscale platinum
clusters embedded into poly(4-methyl-1-pentene) and
poly(tetrauoroethylene), using dimethyl(cyclooctadiene)Pt(II)
asmetal precursor. There are multiple advantages associated with
this approach:rst, the high permeation of CO2 in virtually all
polymers and the wide range oforganic and organometallic reagents
which are soluble in CO2 render this tech-nique a generally useful
scheme for the synthesis of polymer composites. Neitherthe polymer
substrate nor the reaction product needs to be soluble in CO2.
Second,the sorption of CO2 is a very fast process which signicantly
enhances the kinet-ics of the penetrant absorption. The degree of
polymer swelling, diffusion rateswithin the substrate, and the
partitioning of penetrates between the SCF andthe swollen polymer
can be controlled by density mediated adjustments of
solventstrength via changes in temperature and pressure. Coupled
with manipulation ofreaction rates, SCFs offer unprecedented
control over composite composition andmorphology. In addition, SCFs
such as CO2 are gases at ambient conditions andthe solvent
dissipates rapidly upon the release of pressure. In the case of
metala-tion using dimethyl(cyclooctadiene)Pt(II), the process
efuent consists of CO2 andlight hydrocarbons (methane and
cyclooctane) derived from the precursor ligands.
Bronstein and co-workers (50) reported an in situmethod for the
preparationof polymer-cobalt nanocomposites by mixing CO2(CO)8 with
a polyacrylonitrilecopolymer or an aromatic polyamide in
dimethylformamide (DMF). The cobaltcarbonyl interactswithDMFgiving
the complex [Co(DMF)6]2+[Co(CO)4]2 , whichis then converted to
nanodispersed Co particles by thermolysis.
Analogously, silver/polyimide nanocomposites have been prepared
by Fra-gala` and co-workers (51) by thermolysis of a mixture of a
metallorganic silver com-plex and polyamidic acid (PPA). The PAA
precursor transforms in polyimide (PI)at a curing temperature
compatible with the metallorganic precursor reductiontemperature,
so that at the same time the precursor decomposes, giving
metallicsilver particles, and the PAA transforms itself in PI,
allowing the silver particlesto remain trapped in the polymeric
cage.
Copper/polymer nanocomposites have been prepared in the solid
state byLyons and co-workers (52). A soluble precursor was
synthesized by complexingpoly(2-vinylpyridine) with copper formate
(ie, Cu(HCO2)2) in methanol. The ther-mal decomposition of the
complex results in a redox reaction whereby Cu(II) isreduced to
copper metal and the formate anion is oxidized to CO2 and H2. By
incor-porating a reducing agent into the complex, the thermal
decomposition reactionis not diffusion-limited, and nanocrystalline
copper particles can be prepared inthe solid state.
Chen and co-workers (53) describe the synthesis of iron
nanoparticles dis-persed in poly(4-vinylpyridine) homopolymer and
vinylstyrene-4-vinylpyridinecopolymers by chemical reduction of
thin lms of iron chloride/polymer
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492 NANOCOMPOSITES, METAL-FILLED Vol. 10
nanocomposite precursors swollen by DMF. Idrazine was used as
reducing agent.Such approach can be used because the metal ions
have a high tendency to formionic clusters in a polymeric matrix.
The produced microscopic phases, which areseparated by the
hydrophobic portions of the polymer, have a diameter rangingfrom 2
to 10 nm. The chemical reduction of the dispersed ionic metal
domainsproduces ultrane metal particles.
Huang and co-workers (54) prepared copper/poly(itaconic
acid-co-acrylicacid) nanocomposites via in situ chemical reduction
of the Cu2+polymer com-plex by hydrazine hydrate aqueous
solution.
Metal/polymer nanocomposites were prepared by Chen and
co-workers (55)using dispersion of metal chlorides in polyurethane.
Both polyurethane and metalsalts were dissolved in
N,N-dimethylacetamide, followed by lm casting and re-duction of
themetal salts by sodiumborohydrate. Themetal particle size
dependedon the type of metal salt used and on its
concentration.
Zhu and co-workers (56) developed a method for the preparation
ofmetal/polymer nanocomposites based on -irradiation. In this
method, the metalsalt was dissolved in the organic monomer, and the
formation of nanocrystallinemetal particles andmonomer
polymerizationwas obtained simultaneously in solu-tion by
irradiation with a 60Co -ray source, leading to a homogeneous
dispersionof nanocrystalline metal particles in the polymer matrix.
Also UV-irradiation hasbeen used for the reduction of metal ions
dispersed in polymer matrices (57).
Wizel and co-workers (58) have described the use of ultrasounds
in thein situ synthesis of composite materials made of polystyrene
and iron. The prop-agation of ultrasound waves through a uid causes
the formation of cavita-tion bubbles. The collapse of these
bubbles, described as an implosion in thehot-spot theory, is the
origin of extreme local conditions: high temperatures(500025,000 K)
and high pressures (1000 atm). The cooling rates obtained duringthe
bubble collapse are greater than 107 K s1. These high cooling rates
havebeen utilized in the sonication of Fe(CO)5 as a neat liquid or
in solution, to produceamorphous iron nanoparticles. In addition,
ultrasound radiation has been widelyused for the preparation of
polymers without the use of initiators (59). Thesetwo chemical
processes have been combined for the fabrication of
metal/polymernanocomposites by sonication of styrene solution of
iron carbonyl.
Ex Situ Methods. Because of the high optical purity that can be
achievedin the nal product, the ex situ synthesis of metal/polymer
nanocomposites is avery attractive technique, especially in the
preparation of materials for opticalapplications. The particulate
material of the required size, as obtained by a so-lution chemistry
route, is stabilized by legand chemisorption (eg, thiols) in
orderto reduce their surface reactivity and tendency to
agglomeration, and then it isincorporated into a castable polymeric
matrix. Usually, the passivated nanoparti-cles are dispersed into a
liquid monomerinitiator mixture (eg, styrene or methylmethacrylate
activated by benzoyl peroxide), which is then thermally
polymer-ized. Because a little amount of ller is required, the
polymerization behavior ofthemonomer is not signicantly inuenced by
the presence of passivated nanopar-ticles (the reaction rate is
only slightly decreased (60)). Gonsalves and co-workers(60,61) have
prepared surface-derivatized gold particles by phase-transfer
reac-tion of gold ions with dodecanthiol and used these particles
for the synthesis ofpoly(methyl methacrylate) based nanocomposites.
Carotenuto (62) has developed
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 493
a new technique for the preparation of bulk quantities of atomic
clusters of gold,derivatized by thiols by a modication of the
well-known Polyol Process technique(63). The ex situ approach has
been used also in the synthesis of
silver/polyethylenenanocomposites (64,65). A variety of high
nuclearity metal cluster compoundswhich can be used for the
polymeric nanocomposite preparation have been re-cently synthesized
and studied (6676).
Clusters of noble and easily reducible metals (eg, Ag, Au, Pd)
can be ob-tained by alcoholic reduction of metal ions in presence
of polymeric stabilizers.Poly(vinylpyrrolidinone), poly(vinyl
alcohol), poly(methylvinyl ether), etc are usu-ally utilized. High
quality polymer-protected metal clusters can be obtained atthe end
of reaction simply by addition of a nonsolvent liquid which causes
thenanocomposite separation from the reactive mixture
(coprecipitation) (7779).However, the use of such nanocomposites is
limited because of their excessivemoisture sensitivity.
Characterization Techniques
For the comprehension of mechanisms involved in the appearance
of novel prop-erties in polymer-embedded metal nanostructures,
their characterization repre-sents the fundamental starting point.
The microstructural characterization ofnanollers and nanocomposite
materials is performed mainly by transmissionelectron microscopy
(TEM), large-angle X-ray diffraction (XRD), and optical
spec-troscopy (UVvis). These three techniques are very effective to
determine particlemorphology, crystal structure, composition, and
grain size (48).
Of the many techniques which have been used to study the
structure ofmetal/polymer nanocomposites, TEM has undoubtedly been
the most useful. Thistechnique is currently used to probe the
internal morphology of nanocomposites.As visible in Figure 4, high
quality images can be obtained because of the presence,in the
sample, of regions that do not allow the high voltage electron beam
passage(ie, the metallic domains) and region perfectly transparent
to the electron beam(ie, the polymeric matrix). High resolution TEM
(HRTEM) allows morphologicalinvestigations with a resolution of
0.1nm, and thus this technique makes possibleto accurately image
nanoparticle sizes, shapes, and in some cases even inner atoms(80)
(see Fig. 5).
Large-angle X-ray powder diffraction (XRD) has been one of the
most ver-satile techniques utilized for the structural
characterization of nanocrystallinemetal powders. The modern
improvements in electronics, computers, and X-raysources have
allowed it to become an indispensable tool for identifying
nanocrys-talline phases as well as crystal size and crystal strain
(see Fig. 6). The comparisonof the crystallite size obtained by the
XRD diffractogram using the Scherrer for-mula with the grain size
obtained from the TEM image allows to establish if thenanoparticles
have a mono- or polycrystalline nature (11).
Metal clusters are characterized by the surface plasmon
resonance, which isan oscillation of the surface plasma electrons
induced by the electromagnetic eld,and consequently their
microstructure can be indirectly investigated by
opticalspectroscopy (UVvis spectroscopy). The characteristics of
this absorption (shape,intensity, position, etc) are strictly
related to the nature, structure, topology, etc
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494 NANOCOMPOSITES, METAL-FILLED Vol. 10
Fig. 4. TEM micrograph of metal gold clusters embedded in
poly(vinylpyrrolidinone).
of the cluster system. In fact, the absorption frequency is a
ngerprint of the par-ticular metal, the eventual peak splitting
reects aggregation phenomena, theintensity of the peak is related
to the particle size, the absorption wavelengthis related to the
particle shape, the shift of the absorption with increasing
oftemperature is indicative of a cluster melting, and so on. For
bimetallic particlesinformation about inner structure
(intermetallic or core/shell) and compositioncan be obtained from
the maximum absorption frequency (81). Differently fromout-line
techniques (eg, TEM, XRD) this method allows on-line and in situ
clustersizing and monitoring of morphological evolution of the
system (see Fig. 7). Thismethod has been used also in the study of
cluster nucleation and growth mech-anisms (82,83). In addition to
multielectron transition (ie, plasmon absorption)also
single-electron transition (interband transition) can be detected
and studiedby optical spectroscopy in order to obtain important
microstructural information.
This outline of the principal characterization techniques for
nanocompositematerials and nanosized metal llers is far from being
complete. Advances inRaman spectroscopy, energy dispersive
spectroscopy, infrared spectroscopy, andmany other techniques are
of considerable importance as well. In fact, the successthat
nanostructured materials are having in the last few years is
strictly relatedto the advanced characterization techniques which
are available today.
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 495
Fig. 5. HRTEM image of a very small metal cluster.
Fig. 6. XRD pattern of nanosized gold clusters.
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496 NANOCOMPOSITES, METAL-FILLED Vol. 10
Fig. 7. UVvis spectra of a colloidal gold suspension during the
cluster growth. Temper-ature = 60C; 5 min, 10 min, - - - - 15
min.
Applications
Applications of metal/polymer nanocomposites have already been
made in differ-ent technological elds. However the use of a much
larger number of devices basedon these materials is predicted for
the near future.
Because of the plasmon surface absorption band, atomic clusters
of metalscan be used as pigments for optical plastics. The color of
the resulting nanocom-posites is light-fast and intensive, and
these materials are perfectly transparent,since the cluster size is
much lower than light wavelength. Gold, silver, and coppercan be
used for color lter application. Also UV absorbers can be made for
exampleby using Pd clusters. The plasmon surface absorption
frequency is modulated bymaking intermetallic particles (eg, Pd/Ag,
Au/Ag) of adequate composition.
As shown in Figure 8, polymeric lms containing uniaxially
oriented pearl-necklace type of arrays of nanoparticles exhibit a
polarization-dependent and tun-able color (64,65,8486). The color
of these systems is very bright and can changestrongly, modifying
the light polarization direction. These materials are obtainedby
dispersing metal nanoparticles in polymeric thin lms and
subsequently reor-ganizing the dispersed phase into pearl-necklace
arrays by solid-state drawing attemperature below the polymer
melting point. The formation of these arrays inthe lms is the cause
of a strong polarization-direction-dependent color which can
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 497
Fig. 8. (a) UVvis spectra of drawn polyethylene/silver
nanocomposites containing 4 wt%dodecanethiol coated silver
nanoparticles in linearly polarized light. (b) Absorption spectraof
drawn polyethylene/silver nanocomposite lm annealed for 15 h at
180C. The anglebetween the polarization direction of the light and
the drawing direction of the lm isgiven (64).
be used in the fabrication of liquid-crystal color display (see
Fig. 9) and specialelectrooptical devices (see Fig. 10).
Surface plasmon resonance has been used to produce awide variety
of opticalsensors, eg, systems which are able to change their color
in presence of specicanalytes. These devices can be used as sensors
for immunoassay, gas, and liquid(see Fig. 11) (48).
Metals are characterized by ultrahigh/low refractive indices and
thereforethey can be used to modify the refractive index of optical
plastics (8789). Ultra-high/low refractive index optical plastics
can be used in the waveguide technology(eg, planar waveguides and
optical bers).
Plastics doped by atomic clusters of ferromagnetic metals show
magneto-optical properties (ie, when subject to a strong magnetic
eld, they can rotatethe vibration plane of a plane-polarized light)
and therefore they can be used
Fig. 9. Schematic representation of (a) a conventional color
twisted nematic liquid-crystaldisplay; and (b) display set-up
containing a color polarizing lter (64).
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498 NANOCOMPOSITES, METAL-FILLED Vol. 10
Fig. 10. Twisted nematic liquid-crystal cell equipped with a
polyethylene/silver nanocom-posite lter (65).
as Faraday rotators (90,91). These devices have a number of
important opticalapplications (eg, magneto-optic modulators,
optical isolators, optical shutters).
Nanosized metals (eg, gold, silver) have attracted much interest
because ofthe nonlinear optical polarizability, which is caused by
the quantum connementof the metals electron cloud (92) (see
NONLINEAR OPTICAL PROPERTIES). When ir-radiated with light above a
certain threshold power, the optical polarizabilitydeviates from
the usual linear dependence on that power. By incorporating
theseparticles into a clear polymeric matrix, nonlinear optical
devices can be made ina readily processable form (61). These
materials are used to prepare a number ofdevices for photonics and
electrooptics (see ELECTROOPTICAL APPLICATIONS).
Contact-free dispersion of noble metal clusters in polymer can
be also used asnonporous catalytic membranes (9395). Traditionally,
nonporous catalytic sep-aration layers are composed of palladium or
palladium alloy foils. Reduction in
Fig. 11. Nanosized gold-based optical sensor for biological
assay.
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Vol. 10 NANOCOMPOSITES, METAL-FILLED 499
thickness by metal lm deposition on special supports increases
the hydrogenpermeability and reduces the costs for the precious
metal. Polymeric matrix lledby a catalytic nanosized metal offers
in addition to the above advantages, thepossibility of using the
catalytic properties of nanoscale metal catalysts.
One of the most important characteristics of polymers lled by
metal mi-croclusters is their ability to absorb microwave
radiation, producing heat (96).These plastic materials can be
processed by using new technologies based onthe microwave heating
and can be used for food packaging, microwave shielding,RADAR
camouage, plastic welding by microwaves, etc.
Metal/polymer nanocomposites can have many other important
applications.For example, nanoparticles embedded into
poly(vinylpyrrolidinone) can be usedfor the electroless plating of
polymeric, ceramic, and semiconductor substrates(9398). These
materials have also been used for the preparation of smart sys-tems
that experience a reversible alteration of their properties upon
exposure tolight. They are used as infrared barriers against
exposures to intense solar lightor res (99).
Summary
Metal clusterpolymer systems are receiving increased attention
because of theinteresting properties and large potentialities for
technological applications. Inparticular, these nanocomposite
systems can provide simple, low cost options forobtaining tailored
materials with high promise for various catalytic, optical,
mag-netic, and electronic applications. Usually, the control of the
nanoparticle mor-phology, ie, particle size, size distribution,
shape, and composition, is of main in-terest. Nanoparticle features
can be varied by selection of preparation method andvariation of
experimental conditions. In addition to the versatility of the
nanopar-ticle features and the polymer morphologies, options can be
provided to tailor thetopology of the metalpolymer systems. This
includes for example the uniaxiallyoriented pearl-necklace type
distribution of metal nanoparticles within the poly-mer matrix. For
a variety of applications special topologies are important
anddetermining factors for tuning the composite material
performance.
In conclusion, further research activity in this eld is
extremely importantfor the development of advanced devices for
functional applications based on thesematerials.
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G. CAROTENUTOL. NICOLAISInstitute for the Composite Material
Technology,National Research Council
NANOCOMPOSITES, POLYMERCLAY. See Volume 3.