FTIR characterization of the reactive interface of cobalt oxide nanoparticles embedded in polymeric matrices

Article abstract of DOI:10.1021/jp054469y

Fourier transform infrared spectroscopy (FTIR) was used as a novel characterization method to determine the properties of the interface that developed when cobalt oxide nanoparticles were self-assembled in a poly-(methyl methacrylate) (PMMA) matrix. The method employed the distinct changes that were observed in the infrared spectra of the polymer upon adsorption onto the cobalt oxide nanoparticles, allowing a quantitative determination of the average number of contact points that the average polymer chain formed with the surface of a cobalt oxide nanoparticle of average size. The results obtained with this method compared favorably to those obtained by the coupling of transmission electron microscopy (TEM) experiments with thermogravimetric analysis (TGA). On the basis of both methods, we concluded that the interfacial region created between the cobalt oxide nanoparticles and PMMA is extremely sensitive to the chain length, i.e., the number of anchor points and the density of the polymer layer increase with chain molecular weight. At molecular weights of ～250 000, the density of the polymer layer saturates at a value that correspond to that of very thin PMMA films.

Full text of DOI:10.1021/jp054469y

J. Phys. Chem. B 2006, 110, 2227-2232
2227
FTIR Characterization of the Reactive Interface of Cobalt Oxide Nanoparticles Embedded
in Polymeric Matrices
Rina Tannenbaum,*,† Melissa Zubris,† Kasi David,† Dan Ciprari,† Karl Jacob,‡,§
Iwona Jasiuk, and Nily Dan#
School of Materials Science and Engineering, School of Polymer, Textile and Fiber Engineering,
School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia,
Department of Mechanical Engineering, Concordia UniVersity, Montreal, Quebec, Canada, and
Department of Chemical Engineering, Drexel UniVersity, Philadelphia, PennsylVania
ReceiVed: August 10, 2005; In Final Form: December 6, 2005
Fourier transform infrared spectroscopy (FTIR) was used as a novel characterization method to determine
the properties of the interface that developed when cobalt oxide nanoparticles were self-assembled in a poly-
(methyl methacrylate) (PMMA) matrix. The method employed the distinct changes that were observed in the
infrared spectra of the polymer upon adsorption onto the cobalt oxide nanoparticles, allowing a quantitative
determination of the average number of contact points that the average polymer chain formed with the surface of
a cobalt oxide nanoparticle of average size. The results obtained with this method compared favorably to those ob-
tained by the coupling of transmission electron microscopy (TEM) experiments with thermogravimetric analysis
(TGA). On the basis of both methods, we concluded that the interfacial region created between the cobalt
oxide nanoparticles and PMMA is extremely sensitive to the chain length, i.e., the number of anchor points
and the density of the polymer layer increase with chain molecular weight. At molecular weights of ∼250 000,
the density of the polymer layer saturates at a value that correspond to that of very thin PMMA films.
1. Introduction
where the ratio between the surface area and the volume is high.
Indeed, several studies indicate that nanocomposites can display
new properties which are not present in the constituent parent
materials.1-8
Nanostructured materials consist of phases with dimensions
in the nanometer size range (1 to 100 nm). This is the range
where atomic and molecular phenomena strongly influence the
macroscopic material properties. It has been shown that the
mechanical,1-3 electronic,4-6 magnetic,6-8 and optical properties
of a material vary as a function of the nanoscale domain size,
while they are less sensitive to the size of micron-scale domains.
As a result, composite materials in which nanoparticles,6,9-16
nanofibers, or nanotubes17-20 are dispersed in a matrix of another
material, frequently display different material properties than
composites based on larger particles.21-26
The definition of nanocomposite materials encompasses a
large variety of systems made of distinctly dissimilar compo-
nents, where at least one component of the composite has
nanometer size dimensions.17-26 The general class of organic/
inorganic nanocomposite materials is a fast growing area of
research. Significant effort has been focused on the ability to
obtain control of the structure of nanoscale materials in the
composite via innovative synthetic approaches.28-34
One of the key factors influencing the macroscopic material
behavior of composites, e.g., the load transfer between matrix
and filler,35-38 is the interfacial region between these two
components. Reducing the size of filler while preserving the
filler volume fraction leads to a dramatic increase of the volume
fraction of the interfacial region (interphase). The interfacial
region in a polymer nanocomposite, depicted schematically in
Figure 1a, has distinct structural features, depending on the
strength of the interactions between the filler particles and the
polymer matrix. A weak attractive interaction will result in the
extension of the polymer chains into the bulk polymer matrix,
creating a diffuse interface, as shown in Figure 1b. Conversely,
a strong attractive interaction involves the effective adsorption
of polymer chains onto the surface of the fillers via bonding of
functional groups of the polymer with reactive sites on the
surface of the nanofillers.39 Such an interaction will result in a
flat (i.e., dense) configuration of the polymer on the surface of
the nanofillers and a compact interface, as shown in Figure 1c.
In addition, the strength of the interaction of a polymer molecule
with the surface of the filler controls also the polymer molecular
conformations and resulting interactions (e.g., entanglement
distribution) in a larger region surrounding the filler (as shown
in Figure 1a). Thus, control of the particle/polymer interface
will enable control of the mechanical and structural properties
of the nanocomposite materials.
The properties of nanocomposite materials depend not only
on the properties of their individual components but also on
their organization in the composite as well as their interfacial
characteristics. The latter is especially important in these systems
* Address correspondence to this author. E-mail: rina.tannenbaum@
mse.gatech.edu.
† School of Materials Science and Engineering, Georgia Institute of
Technology.
‡ School of Polymer, Textile and Fiber Engineering, Georgia Institute
of Technology.
As shown in Figure 1, the particle/polymer interface may be
characterized through several parameters such as the number
of “adsorbed” chains (namely, chains that have at least one
segment in contact with the particle), the thickness of the
§ School of Mechnaical Engineering, Georgia Institute of Technology.
Department of Mechanical Engineering, Concordia University
# Department of Chemical Engineering, Drexel University.
10.1021/jp054469y CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/19/2006

2228 J. Phys. Chem. B, Vol. 110, No. 5, 2006
Tannenbaum et al.
Leff
)
3
1/3
6wpolyMWclusterꢀ Dcluster
Dcluster
1
3
+ Dcluster
-
(2)
(
)
[
]
2 πFpolywclusterNA dmetal
2
Assuming that the polymer behaves as a freely jointed chain
between the contact points (i.e. bonds) with the metal cluster
surface, and neglecting bridging chains (polymer chains ad-
sorbed on multiple clusters), we can calculate the minimum
number of segments in an average chain residing in loops as
nloop
(Leff)2, and consequently, the maximum number of
contact points per chain as nanchor ) N/nloop, where N is the
average number of segments per chain.
We have found that for a given polymer molecular weight,
fewer contact points lead to a high concentration of polymer
loops and a diffuse interfacial region, while a larger number of
contact points leads to a high concentration of polymer trains
and a compact interfacial region.40 This in turn has a direct
impact on the density of the interphase and its contribution to
the mechanical properties of the nanocomposite.
The main drawback of the TEM/TGA method is the fact that
it requires some assumptions regarding the density of the
adsorbed polymer monolayer. In this paper we focus on the
development of a new characterization method of the interfacial
structure based on infrared spectroscopy. This method will allow
the direct calculation of nanchor without the need to assume the
density of the adsorbed polymer monolayer. On the contrary, it
will serve as a direct method for the determination of the density
of the adsorbed layer. We recognize that this will be possible
only in systems in which the adsorption of the polymer chains
onto the metal oxide nanoclusters generates a characteristic
infrared signature as a result of new bond formation at the
interface, i.e., due to reactive chemisorption, and cannot be
applied to physisorbed polymer chains where the adsorption
process occurs via dipole interactions.
In this paper we chose to focus on poly(methyl methacrylate)
(PMMA) as the polymeric matrix, with the reinforcing nano-
particles being cobalt oxide nanoparticles that were formed in
situ in the polymer matrix from homogeneously dispersed
organometallic cobalt precursors. The guiding principle for the
choice of this nanoparticle/polymer system is the distinct
changes that are observed in the infrared spectra of the polymer
upon adsorption onto the cobalt oxide nanoparticles, allowing
a quantitative determination of the average number of contact
points that the average polymer chain forms with the cluster
surface. Finally, we will compare the two characterization
systems and evaluate the implications regarding the structure
of the polymer-metal cluster interfacial region.
Figure 1. Schematic representation of a metal-polymer nanocom-
posite: (a) The various components in a metal-polymer nanocomposite
highlighting the presence of the extended interfacial region, i.e., the
interphase. (b) A detailed schematic view of the adsorption character-
istics on the surface of the cobalt oxide clusters of a weakly binding
polymer, in which most of the segments reside in loops. (c) A detailed
schematic view of the adsorption characteristics on the surface of the
cobalt oxide clusters of a strongly binding polymer, in which most of
the segments reside on the surface.
adsorbed layer, and the number of contacts between the particle
and the polymer. We have previously developed a method that
uses transmission electron microscopy images (TEM) coupled
with thermal gravimetric analysis (TGA) data to characterize
the average thickness of the adsorbed polymer chains on a
nanoparticle surface, Leff (as in Figure 1b,c), and subsequently
determine the number of contact points (anchors) between the
adsorbed polymer and the nanoparticle.40 The method consists
of the determination of the average cluster diameter, D, from
TEM images, and the calculation of Leff from TGA measure-
ments of the coated nanoparticles (after all physisorbed polymer
chains have been removed by repeated washing). The average
volume of the monolayer of chemisorbed polymer chains on
each cluster is given by Vpoly ) msamplewpoly/NclusterFpoly, where
wpoly is the mass fraction of the polymer moiety in the sample
as determined from TGA measurements, Ncluster is the total
number of nanoclusters in the sample, and Fpoly is the density
of a thin film of the polymer matrix used in a given nanocom-
posite system.35 Since the volume of a polymer-coated nano-
cluster, Vtot, is the sum of the volumes of the metallic and
polymeric moieties, the following expression can be developed:
2. Experimental Methods
2.1. Synthesis of the Nanocomposites. Forty five milliliters
of a 2 wt % solution of poly(methyl methacrylate), PMMA (Alfa
Aesar), of Mh w ) 30 000, 60 000, 120 000, 250 000, or 330 000
(first two with PDI ) 1.04, and the last three with PDI ) 1.12)
in chlorobenzene was added to a three-neck, jacketed reaction
flask, followed by the addition of 45 mL of a 1 × 10-2
M
3
D + 2Leff
4π
3
solution of Co2(CO)8 in chlorobenzene. The combined solution
had final PMMA and Co2(CO)8 concentrations of 1 wt % and
5 × 10-3 M, respectively. The reaction flask was flushed with
dry N2 and an aliquot was removed for Fourier transform
infrared spectroscopy (FTIR) analysis. The decomposition was
carried out at 90 °C with constant stirring for ca. 10 h. The
reaction was deemed complete when all infrared carbonyl
absorption bands of Co2(CO)8 have disappeared. This process
Vtot ) Vcluster + Vpoly
)
(1)
(
)
2
The expression D + 2Leff denotes the diameter of a polymer-
coated nanoparticle as shown by the dotted red circle in Figure
1a. Therefore, it is possible to calculate Leff, the effective
thickness of the polymer layer, as follows:

Co Oxide Nanoparticles Embedded in Polymeric Matrices
J. Phys. Chem. B, Vol. 110, No. 5, 2006 2229
TABLE 1: Average Sizes of the Cobalt Oxide Nanoparticles
Formed in Solutions of Poly(methyl methacrylate) of Various
Polymer Molecular Weights
molecular weight of PMMA
av particle diameter (nm)
30 000
60 000
120 000
250 000
330 000
67 ( 12
50 ( 8
28 ( 8
27 ( 5
31.5 ( 5
resulted in the formation of Co2O3 and some CoO nanoparticles
suspended in a poly(methyl methacrylate) solution. The average
particles sizes of cobalt oxide nanoparticles formed in solutions
of PMMA with different molecular weights are summarized in
Table 1. To form nanocomposite films, a small amount of
polymer solution containing the already-formed nanoparticles
was deposited on a glass slide and the solvent was allowed to
evaporate slowly in a temperature-controlled oven at 90 °C for
a week. The volume fraction of the oxide nanoparticles in the
nanocomposite films was (1.6 ( 0.3)%.
Figure 2. (a) The infrared absorption bands of PMMA in the carbonyl
spectral region at 1743, 1702, and 1674 cm-1, corresponding to the
stretch of the CdO groups, the stretch of the hydrogen-bonded COOH
groups, and the asymmetric stretch of the COO- groups, respectively.
(b) The changes of the relative intensities of the 1241 and 1271 cm-1
infrared bands, corresponding to the cooperative symmetric and
antisymmetric stretches of the C-C-O groups, respectively, indicating
changes in the conformation of the polymer at the surface of the cobalt
oxide clusters. The spectrum of a very thin spin-coated PMMA layer
on Au is used as a reference.
2.2. Characterization of the Nanocomposites. Thermo-
gravimetric analysis (TGA) was conducted to measure the
thickness of the polymer interphase for the PMMA nanocom-
posites. TGA samples were prepared by centrifuging an aliquot
of a previously reacted solution containing cobalt oxide nano-
clusters capped with PMMA, with Mh w ) 120 000, 250 000, and
330 000, using an Eppendorf Concentrator 5301 Centrifuge, at
12 000-17 000 rpm. This separated excess solvent from the
polymer-coated nanoclusters. The supernatant chlorobenzene
solution was removed, and the remaining particles were washed
with both chlorobenzene and hexane to remove any excess
unbound polymer or partially reacted cobalt carbonyl fragments.
The suspension was centrifuged again and the process was
repeated three times. Upon completion, a fraction of the isolated
PMMA-coated particles were placed into a TGA pan and the
data were collected with a TA Instruments TGA Model 50.
Transmission electron microscopy (TEM) was performed to
allow the determination of the average size of the cobalt
nanoparticles formed and provide the basis for the TEM/TGA
analysis of the interfacial structure of the PMMA nanocom-
posites and the calculations of the adsorbed PMMA layer
density. TEM samples were obtained by placing a small droplet
of the reacted solution containing the polymer-coated cobalt
particles onto a Formvar-coated copper TEM grid from Ted
Pella. The grid rested on a thin piece of tissue paper so that the
liquid will drain into the paper leaving a very thin film on the
grid itself. The TEM analysis was performed on a JEOL 4000EX
high-resolution electron microscope with an operating voltage
of 200 keV.
gold surfaces were used as a reference, representing a nonre-
active interface. The details of the preparation of these samples
have been previously described elsewhere.41-43
3. Results and Discussion
The nanocomposites were synthesized via an in situ synthesis
consisting of the thermal decomposition of organometallic
precursors in the presence of polymer as previously de-
scribed.39,40 This method allows the control of particle size via
the variations of polymer concentration in the original reaction
solution. The system examined consisted of cobalt oxide
nanoclusters formed in the presence of PMMA. It has been
previously shown39 that under the reaction conditions, PMMA
interacts vigorously with the surface of the cobalt clusters,
resulting in a strong adsorption. This interaction occurs in two
steps, as shown in eq 3.
In the first step, a fraction of the ester bonds in the polymer
are hydrolyzed in the presence of the cobalt oxide nuclei to
form carboxylic acid groups.39,41,42 In the second step, some of
these carboxylic groups undergo dissociation to form the
carboxylate anionic group. The formation of the carboxylate
anion has been shown to occur only upon formation of a bond
(anchoring point) at the surface of the cobalt oxide nanoparticle,
and hence it can be considered as a quantitative measure of the
number of anchoring points per polymer chain.39,43 The con-
centration of the carboxylate anion in a given sample can be
obtained via the evolution of the 1674 cm-1 infrared absorption
band, corresponding to the asymmetric stretch of the COO- group,
as shown in Figure 2a. It is important to remember that in the
case of PMMA, the strong interactions with the cobalt oxide
surface will promote extensive bonding, which will result in
the loss of entropy and changes in the conformation of the poly-
mer due to chain confinement on the surface. The changes in the
Finally, another fraction of the isolated PMMA-coated
particles (described previously) was mixed with a drop of Nujol
mull to form a thick paste, cast on an NaCl circular crystal,
and analyzed by means of transmission FTIR, using Nicolet
Nexus 870, at a resolution of 2 cm-1 and 3000 scans. The
infrared spectra of very thin films of PMMA spin-coated on

2230 J. Phys. Chem. B, Vol. 110, No. 5, 2006
Tannenbaum et al.
TABLE 2: Calculations of the Thickness of the Adsorbed Polymer Layer and the Resulting Number of Polymer Anchoring
Points for the System of Co2O3 Nanoclusters Embedded in a Poly(methyl methacrylate) Matrix, Based on Experimental Results
Obtained from FTIR Measurements and the TEM/TGA Method for Selective Systems
no. of anchoring
points/chain by
FTIR ((35)
no. of anchoring
points/chain by
TEM/TGA ((40)
mol wt of
PMMA
no. of polymer
chains in sample
E1674/E1743
E1241/E1271
×
no. of
reacted groups
30 000
60 000
120 000
250 000
330 000
2.1 × 1018
1.0 × 1018
5.2 × 1017
2.5 × 1017
1.9 × 1017
0.061
0.091
0.270
0.252
0.282
1.9 × 1019
2.8 × 1019
8.0 × 1019
7.8 × 1019
8.7 × 1019
9.2
27.5
155.7
315.7
466.1
N/A
N/A
160.1
289.0
414.8
conformation of the polymer at the surface can be directly ob-
served by following the changes of the relative intensities of
the 1241 and 1271 cm-1 infrared bands, corresponding to the co-
operative symmetric and antisymmetric stretches of the C-C-O
group, respectively, and shown in Figure 2b. The characteristic
changes in the infrared spectrum of PMMA upon its reactive
adsorption on the surface of the cobalt oxide nanoclusters41-43
allow the quantitative analysis of the extent of this adsorption
based solely on spectral information, without the need of prior
assumptions. The ratio of the 1241 and 1271 cm-1 infrared
absorption bands gives a direct indication as to the fraction of
functional groups that have been affected by the conformational
changes of the polymer chain due to adsorption. Multiplying
this ratio by the absorbance intensity of the 1674 cm-1 infrared
absorption band, and by the number of monomers in the sample,
and normalizing the result by the intensity of the initial adsorp-
tion of the CdO bond at 1743 cm-1 (acting as an internal stan-
dard), will give the total number of PMMA ester groups that
have undergone hydrolysis followed by dissociation and anchor-
ing. The quotient of the total number of adsorbed carboxylate
the effective thickness of the adsorbed layer, Leff, and the average
particle size, Dcluster, via the expression
3
3
D
Dcluster
cluster + Leff
-
4π
3
Vpoly
)
(
[
)
(
)
]
2
2
We see that the density of the adsorbed layer (Figure 3b)
increases with the chain length, or with the number of anchor
points per chain (Figure 3a). This is as expected. If all segments
would have adsorbed, then the density should have reached that
of bulk polymer. Thus, it is not surprising that increasing the
number of anchoring points (through increase in chain length)
would lead to an increase in the interphase density.
However, we find that even though only a small fraction of
the chains is anchored (e20%), in the limit of high molecular
weight chains, the density of the interphase saturates at a value
of 1.26 g/cm3. This value is consistent with the previously
measured density of very thin PMMA films (<20 nm), 1.28
g/cm3.45 The fact that the density of thin films is higher than
the density of bulk PMMA (1.19 g/cm3) is most likely an
overestimate due to the sharp transition between the density of
the adsorbed PMMA film and that of the Co2O3 nanocluster,
as observed on silica substrates.45
-
groups, NCOO , and the total number of chains in the sample,
N
chain, shown below in eq 4, will give the average number of
anchoring points per chain, as summarized in Table 2.
The sharp increase in the number of anchoring points per
chain with molecular weight in PMMA differs from the case
of PS, where the number of anchoring points per chain increases
linearly with molecular weight.40 To understand the difference
between the two polymers we must refer to the parameters
controlling polymer adsorption onto solid substrates:44 The
formation of an anchor point between a chain in solution and a
solid surface reduces the chain translational entropy, while
gaining in interaction energy. Further adsorption of segments
from that same chain would reduce the conformational entropy
of the chain, while further increasing the gain due to interaction
energy. In the case of PS, the energetic gain due to PS/Co
association is small. As a result, the adsorbed polymer chains
do not deform significantly and the number of anchor points is
proportional to their length. In the case of PMMA, the gain
associated with anchoring is high, so that chains that are already
confined to the particle surface deform to form even more
contacts, leading to the increased sensitivity of anchor points
to chain length. In the limit where the molecular weight of the
PMMA chains is high, however, the particle surface becomes
saturated (as given by the high interphase density), which leads
to a leveling off as seen in Figure 3c.
NCOO
E1674 E1241 Nmonomers
-
no. of anchors/chain )
)
(4)
Nchains E1743 E1271 Nchains
When comparing the number of anchoring points for the
samples containing PMMA with Mh w ) 120 000, 250 000, and
330 000 obtained from infrared intensities with the number of
anchoring points of the same samples that were calculated using
the combination of TEM and TGA, summarized in Table 2 as
well, the results seem to be in very good agreement.
As previously observed with the PS-cobalt system, in which
the polymer is weakly bonded to the cluster surface via dipole
interactions,40 the extent of polymer adsorption on the cluster
surface is dependent on the molecular weight of the polymer.
The same is also true in the case of PMMA, as shown in Figure
3a. As the molecular weight of the polymer chain increases, so
does the number of anchor points per chain However, the
number of anchor points per chain increases more rapidly with
PMMA molecular weight, and so the fraction of adsorbed
segments increases as well. It is interesting to note that in the
limit of high molecular weight the effect of chain length
becomes more or less linear, so that the fraction of adsorbed
segments is similar for molecular weights where Mh w > 120 000.
The use of the infrared spectroscopic method for the
characterization of the structure of the cluster-polymer inter-
facial region allows the independent determination of the density
of the adsorbed polymer layer, as shown in Figure 3b. The
density is calculated by dividing the mass of the adsorbed
polymer layer (obtained from the number of reacted segments)
by the volume of the adsorbed polymer layer (obtained from
4. Summary
In this paper we have shown that in the case of strongly
binding polymers, such as PMMA, FTIR may be used as an
independent method for the characterization of the interfacial
structure, provided that the new bonds that developed at the
interface between the polymer and the metal oxide cluster
surface gave rise to a characteristic new infrared absorption

Co Oxide Nanoparticles Embedded in Polymeric Matrices
J. Phys. Chem. B, Vol. 110, No. 5, 2006 2231
is in contrast to the case of weakly adsorbing PS, where the
number of anchor points increases linearly with chain length.40
The increase in the number of PMMA anchor points correlates
to an increase in the interphase density, which saturates at a
value that corresponds to a thin PMMA film (Figure 3b). The
effect of polymer chain length on the structure of the interphase
layer can be understood in terms of an adsorption model, where
the number of anchor points and density of the adsorbed layer
are a function of the strength of interactions between the
nanoparticle and the polymer.
Acknowledgment. This work was supported by the Petro-
leum Research Fund, administered by the American Chemical
Society, PRF #40014-AC5M.
Note Added after ASAP Publication. This manuscript was
originally published on the Web January 19, 2006 with an
incorrect value in Table 1 due to production error. The corrected
version of this manuscript was reposted January 20, 2006.
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