Controlled polymer grafting on single clay nanoplatelets

Article abstract of DOI:10.1021/ja048657y

We report on the controlled chemical grafting of well-defined polymer chains onto individual montmorillonite-type clay nanoplatelets and the direct visualization of the formed hybrid material at the nanoscale level. Our approach is based on the use of a surfactant mixture that contains varying proportions of hydroxyl-substituted alkylammonium and unsubstituted alkylammonium cations to exchange the initial Na⁺ counterions of the natural montmorillonite. This allows for the exchange of Na⁺ by a tunable amount of hydroxyl functions at the surface of the clays. Those functions are then derivatized into aluminum alkoxides in order to initiate the ring-opening polymerization of ε-caprolactone directly from the clay surface that was swollen in an organic solvent. Atomic force microscopy measurements on the resulting polymer-grafted nanoplatelets demonstrate the strong dependence of the coating of the individual clay particles with the composition of the surfactant mixture used for the cationic exchange. This allows for the generation of a range of morphologies varying from polymer islands distributed over the clay surface to homogeneous polymer layers thoroughly coating the platelets. Finally, the control that is achievable over the synthesis of this new family of organic-inorganic nanohybrid materials has been extended to the surface grafting of semicrystalline poly(ε-caprolactone)-poly(lactic acid) diblock copolymers with defined compositions.

Full text of DOI:10.1021/ja048657y

Published on Web 07/03/2004
Controlled Polymer Grafting on Single Clay Nanoplatelets
Pascal Viville,*,† Roberto Lazzaroni,† Eric Pollet,‡ Michae¨l Alexandre,‡ and
Philippe Dubois‡
Contribution from the SerVice de Chimie des Mate´riaux NouVeaux (SCMN), Centre de
Recherche en Sciences des Mate´riaux Polyme`res, and SerVice des Mate´riaux Polyme`res et
Composites (SMPC), Centre de Recherche en Sciences des Mate´riaux Polyme`res, UniVersite´ de
Mons-Hainaut, 20, Place du Parc 7000, Mons, Belgium
Received March 9, 2004; E-mail: pascal@averell.umh.ac.be
Abstract: We report on the controlled chemical grafting of well-defined polymer chains onto individual
montmorillonite-type clay nanoplatelets and the direct visualization of the formed hybrid material at the
nanoscale level. Our approach is based on the use of a surfactant mixture that contains varying proportions
of hydroxyl-substituted alkylammonium and unsubstituted alkylammonium cations to exchange the initial
Na+ counterions of the natural montmorillonite. This allows for the exchange of Na+ by a tunable amount
of hydroxyl functions at the surface of the clays. Those functions are then derivatized into aluminum alkoxides
in order to initiate the ring-opening polymerization of ꢀ-caprolactone directly from the clay surface that was
swollen in an organic solvent. Atomic force microscopy measurements on the resulting polymer-grafted
nanoplatelets demonstrate the strong dependence of the coating of the individual clay particles with the
composition of the surfactant mixture used for the cationic exchange. This allows for the generation of a
range of morphologies varying from polymer islands distributed over the clay surface to homogeneous
polymer layers thoroughly coating the platelets. Finally, the control that is achievable over the synthesis of
this new family of organic-inorganic nanohybrid materials has been extended to the surface grafting of
semicrystalline poly(ꢀ-caprolactone)-poly(lactic acid) diblock copolymers with defined compositions.
1. Introduction
enhanced, which could make them good competitors for
commodity materials such as polyolefins, while being com-
pletely biodegradable.
A very promising alternative to conventional filled polymers
consists of the preparation of polymer-layered silicate nano-
composites. The dispersion of nanometer-size silicate nano-
platelets within a polymer matrix leads to nanocomposites
exhibiting markedly improved physicochemical properties, such
as higher Young’s modulus and storage modulus, higher thermal
stability and flame retardancy, and more efficient gas barrier
properties, compared to pure polymers or conventional micro-
composites.1-10 Because of environmental concerns, this ap-
proach, applied to biodegradable and biocompatible synthetic
aliphatic polyesters such as poly(ꢀ-caprolactone) (PCL) or poly-
(lactic acid) (PLA), has been receiving growing attention.11,12
Indeed, the performance of these polymers can be strongly
It is well documented that the efficiency of the clay as a
reinforcing agent is strongly determined by its degree of
dispersion in the polymer matrix. Two main approaches can be
used to prepare polymer/clay nanocomposites: either melt
intercalation, for which the clay is mixed with the preformed
polymer in the molten state, or in situ intercalative polymeri-
zation, where the clay is dispersed in the monomer, which is
then polymerized. Depending on the nature of the clay and the
preparation conditions, these two methods can lead either to
intercalated or exfoliated morphologies or to a coexistence of
the two morphologies.
Recently, we reported the synthesis of PCL/montmorillonite
nanocomposites (PCL/MMT) by those two approaches (melt
intercalation13,14 and in situ polymerization15-17). Those studies
have highlighted the key role of (i) the synthetic path used, (ii)
the nature of the organic modifier, and (iii) the clay content on
† Service de Chimie des Mate´riaux Nouveaux (SCMN).
‡ Service des Mate´riaux Polyme`res et Composites (SMPC).
(1) Polymer-clay nanocomposites; Pinnavaia, T. J., Beall, G. W., Eds.; Wiley
Series in Polymer Science; Wiley: New York, 2000.
(2) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, R28, 1.
(3) Biswas, M.; Ray, S. S. AdV. Polym. Sci. 2000, 155, 167.
(4) Giannelis, E. P. AdV. Mater. 1996, 8, 29.
(5) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito,
O. J. Polym. Sci., Part A.: Polym. Chem. 1993, 31, 983.
(6) Gilman, J. W. Appl. Clay Sci. 1999, 15, 31.
(13) Pantoustier N.; Alexandre, M.; Dege´e, P.; Calberg, C.; Je´roˆme, R.; Henrist,
C.; Cloots, R.; Rulmont, A.; Dubois, P. e-Polym. 2001, 009.
(7) Strawhecker K. E.; Manias, E. Chem. Mater. 2000, 12, 2943.
(8) Krishnamoorti, R.; Vaia, R. A.; Giannelis, E. P. Chem. Mater. 1996, 8,
1728.
(9) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1993, 5, 1064.
(10) Sinha Ray, S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539.
(11) Sinha Ray, S.; Yamada, K.; Okamoto, M.; Fujimoto, Y.; Ogami, A.; Ueda,
K. Polymer 2003, 44, 6633.
(12) Chang, J.-H.; An, Y. U.; Sur, G. S. J. Polym. Sci., Part B.: Polym. Phys.
2003, 41, 94.
(14) Lepoittevin B.; Devalckenaere, M.; Pantoustier, N.; Alexandre, M.; Kubies,
D.; Calberg, C.; Je´roˆme, R.; Dubois, P. Polymer 2002, 43, 4017.
(15) Lepoittevin B.; Devalckenaere, M.; Alexandre, M.; Pantoustier, N.; Calberg,
C.; Je´roˆme, R.; Dubois, P. Macromolecules 2002, 35, 8385.
(16) Lepoittevin, B.; Pantoustier, N.; Alexandre, M.; Calberg, C.; Je´roˆme, R.;
Dubois, P. J. Mater. Chem. 2002, 12, 3528.
(17) Pollet, E.; Paul, M. A.; Dubois, P. New aliphatic polyester layered-silicate
nanocomposites. In Biodegradable Polymers and Plastics; Chiellini, E.,
Solaro, R., Eds.; Kluwer Academic/Plenum Publishers: New York, 2003.
9
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J. AM. CHEM. SOC. 2004, 126, 9007-9012
9007

A R T I C L E S
Viville et al.
Scheme 1. Chemical Structure of the Ammonium Cations Used to
Organo-Modify the Natural Sodium Montmorillonite Cloisite-Na
(MMT-Na)
the morphological, mechanical, rheological, and thermal proper-
ties of the materials. In situ intercalative polymerization was
shown to be the most efficient synthetic path since it allows
the generation of completely exfoliated morphologies, leading
to PCL/MMT nanocomposites with much-improved properties.
Such a situation is typically obtained when hydroxyl-substituted
alkylammonium cations are used to exchange the initial Na+
counterions of the natural clay. By adequate activation of the
hydroxyl groups into metal (Sn or Al) alkoxides, the hydroxyl-
substituted organo-modified clays (MMT-OH) can then play
the role of co-activators for the bulk (i.e., in the absence of any
solvent) polymerization of ꢀ-caprolactone, eventually leading
to PCL chain grafting onto the clay surface. Therefore, it appears
that one key parameter yielding fully exfoliated nanocomposites
with much-enhanced material performances is the covalent
bonding between the in situ-grown polyester chains and the
ammonium cations covering the clay surface.15-18
from Fluka and diluted with dried toluene. AlEt3 solutions were stored
in glass ampules under a nitrogen atmosphere. The handling of AlEt3
required the work to be performed in an inert atmosphere, free of
oxygen and water.
The unmodified montmorillonite-Na (Cloisite-Na, MMT-Na),
characterized by a cation-exchange capacity (CEC) of 92 mequiv/100
g, was supplied by Southern Clay Products (Gonzales, TX). 1-Iodo-
hexadecane (95%), N,N-dimethylethanolamine (99%), and trimethyl-
amine (33 wt % solution in ethanol) were purchased from Aldrich and
used without any further purification.
In that context, it is of particular interest to get better spatial
control over the polymer grafting reaction, the growth of
polymer material on the silicate layer surface, and the resulting
clay delamination. For that purpose, an original synthetic
strategy has been designed which consists of (i) controlling the
density of the hydroxyl functions covering the surface of the
organo-clay, (ii) activating them into aluminum alkoxides,
known for their high and selective activity in controlled lactone
polymerization, (iii) swelling the so-organo-modified clay in
an organic solvent like toluene, and (iv) promoting ꢀ-caprolac-
tone (CL) polymerization in toluene, as initiated from the clay
in suspension.
The first step is achieved in the following way: instead of
using a surfactant containing 100% OH-functionalized groups,
we use different ratios of hydroxyl-terminated alkylammonium
(OH) and unsubstituted alkylammonium (Alk) surfactants to
exchange the interlayer sodium cations of the natural clay. This
particular cationic exchange is expected to disperse a controlled
amount of hydroxyl functions at the surface of the clays prior
to polymerization. As a consequence, the polymerization of CL
and the subsequent PCL chain growth are confined to the OH-
bearing sites at the surface of the clay, which can significantly
lower the grafting density, compared to an exchange for which
the clay is modified by a 100% OH surfactant. Atomic force
microscopy (AFM) is then used for direct visualization of the
polymer grafted at the surface of single clay platelets. This
allows us to follow the gradual coating of individual clay
platelets by grafted PCL as the (OH)/(Alk) ratio is gradually
increased.
2.2. Synthesis of Ammonium Cations. 2-Hydroxyethyl(hexadecyl)-
dimethylammonium iodide (CH3)2(C16H33)N+(CH2CH2OH) was pre-
pared by a quaternization reaction as follows. 1-Iodohexadecane (7.75
g, 0.022 mol) was dissolved in ethanol (200 mL) and reacted with N,N-
dimethylethanolamine (2.0 mL, 0.020 mol) at 70 °C for 20 h. After
solvent evaporation, the reaction product was precipitated in diethyl
ether, purified by recrystallization in acetone, and recovered as a white
powder that was dried under vacuum at ambient temperature for 24 h.
Melting point, 86 °C. 1H NMR (CDCl3) 0.9 ppm, CH3; 1.2-1.4 ppm,
(CH2)13; 1.8 ppm, N-CH2-CH2; 3.4 ppm, (CH3)2N; 3.6 ppm, N-CH2;
3.8 ppm, N-CH2CH2OH; 4.15 ppm, CH2OH.
Hexadecyltrimethylammonium (CH3)3N+(C16H33) iodide was simi-
larly prepared by reaction of 1-iodohexadecane (9.86 g, 0.028 mol)
with trimethylamine (6.0 mL, 0.025 mol) in 200 mL of ethanol. The
product was purified by recrystallization in ethanol to yield a white
powder. Melting point, 118 °C. 1H NMR (CDCl3) 0.9 ppm, CH3; 1.2-
1.4 ppm, (CH2)12; 1.8 ppm, N-CH2-CH2-CH2; 2.35 ppm, N-CH2CH2;
3.4 ppm, (CH3)2N; 3.55 ppm, N-CH2.
2.3. Preparation of Organophilic Clays. The different organo-
modified montmorillonites used in this work were obtained by
exchanging the natural Cloisite-Na with varying ratios of two different
surfactants, 2-hydroxyethyl(hexadecyl)dimethylammonium and hexa-
decyltrimethylammonium cations, referred to here as (OH) and (Alk),
respectively (Scheme 1), in different molar ratios. The resulting
exchanged montmorillonites (called MMT-(Alk/OH) hereafter) thus
contain varying proportions of (OH) groups.
The mixtures used for the co-exchange reaction contained 10, 25,
50, or 75% OH-substituted cations. The exchange reaction was
performed in water at 85 °C for 24 h and led to organo-clays coined
MMT x% (OH), with x ) 10, 25, 50, and 75. The modified clays were
extensively washed with hot water and collected by freeze-drying. The
intercalated ammonium compositions were determined from 1H NMR
analysis of the aqueous solution left after clay intercalation. The results
show that the two ammonium species intercalate according to their
relative concentration in the starting solution (within a relative error
of (4%). Intercalated clays were analyzed by X-ray diffraction (XRD),
with the basal spacing increasing from 1.21 nm for MMT-Na to ca.
1.90 nm for the organo-clays, independent of the composition of the
mixture of intercalated ammonium cations. The organic content of all
the organo-modified montmorillonites was in the 18-24 wt % range,
as determined by thermogravimetric analysis (TGA). This is consistent
with the complete exchange of the Na+ cations on the basis of the
MMT-Na CEC value of 92 mequiv/100 g.
The controlled polymerization/grafting process is then ex-
tended to the synthesis of semicrystalline P[CL-b-LA] (LA )
L,L-lactide) diblock copolyesters, again surface-anchored on the
organo-clay. Beyond the aim of understanding the interfacial
aspects in exfoliated nanocomposites, this work thus paves the
way to a new family of organic-inorganic nanohybrid materials.
2. Experimental Section
2.1. Materials. ꢀ-Caprolactone (Fluka) was dried over CaH2 and
distilled under reduced pressure prior to use. LA was purchased from
Boehringer Ingelheim and recrystallized three times in dried toluene
(20 wt %/vol) before use. Triethylaluminum (AlEt3) was purchased
2.4. Polymerization/Grafting onto Organo-Clays. 2.4.1. PCL
Surface Grafting. Before polymerization, the organo-modified MMT-
(Alk/(OH)) clays were dried in a ventilated oven at 70 °C for one night.
A desired amount of organo-clay was further dried in a round-bottom
(18) Viville, P.; Lazzaroni, R.; Pollet, E.; Alexandre, M.; Dubois, P.; Borcia,
G.; Pireaux, J.-J. Langmuir 2003, 19, 9425.
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Polymer Grafting on Single Clay Nanoplatelets
A R T I C L E S
Table 1. (Co)Polymerization Time for Synthesizing the PCL (and
PCL-b-PLA)-Grafted Clay Nanohydrids with Defined Inorganic
Content
Scheme 2 . Synthetic Pathway to the Synthesis of
Clay-Surface-Grafted (Co)Polyesters
polymerization
time (hours)
clay contenta
(wt % in inorganic)
MMT x% (OH)
10
25
50
75
100
168
72
24
58
52
50
57
3
24
1/168b
a Determined by thermogravimetric analysis (TGA). b Block copolym-
erization between CL and LA: values for PCL and PLA blocks, respectively
(noted PCL/PLA).
flask in a vacuum at 70 °C for 3 h. The dried clay was then allowed
to swell in 100 mL of dry toluene for 1 h at 70 °C under a nitrogen
atmosphere. The intercalated hydroxylated ammonium cations were then
derivatized into aluminum-alkoxide-active species by reaction with
triethylaluminum, AlEt3, ([OH]/[Al] ) 3) for an additional hour at 70
°C. A given amount of ꢀ-caprolactone (CL) was then added under
nitrogen, and the reaction was allowed to proceed at 70 °C for 1-7
days, depending on the type of organo-clay. The polymerization reaction
was stopped by the addition of diluted HCl, and the resulting PCL-
grafted nanohybrids were recovered after solvent and residual monomer
removal, performed by drying the mixture at 85 °C under reduced
pressure for 24 h.
of 24-52 N/m, a resonance frequency in the 264-339 kHz range, and
a typical radius of curvature in the 10-15 nm range. Unfiltered TM
topography images were recorded with the highest sampling resolution
available, i.e., 512 × 512 data points.
3. Results and Discussion
3.1. Synthesis of Clay-Surface-Grafted (Co)Polyesters. The
grafting of PCL chains onto single clay platelets has been
performed according to an original three-step synthetic strategy.
The first step relies upon the organo-modification of sodium
montmorillonite by a mixture of two alkylammonium cations,
one of them being functionalized by a primary hydroxyl group.
The cationic exchange reaction was readily carried out in water
at ca. 85 °C (see Experimental Section).15,16 The efficiency of
the exchange reaction was evidenced by both XRD and TGA
experiments, while the relative composition in alkylammonium
cations, OH-functionalized or not, was attested by 1H NMR and
proved to strictly fit the impregnation bath composition at the
start. The second and third steps consist of selectively activat-
ing the OH functions into aluminum alkoxides and initiating
the polymerization/grafting of CL from the activated clay
surface, respectively. This synthetic strategy is represented in
Scheme 2.
Aluminum alkoxides are well known for their high efficiency
to promote the controlled ring-opening polymerization of CL
under very mild conditions.19 Accordingly, triethylaluminum has
been reacted with the surface OH functions of the organo-clay
swollen in toluene. The evolution of ethane as a volatile
byproduct attests that the aluminum-alkoxide-active species is
indeed formed. CL polymerization has then been carried out
by adding the desired amount of lactone monomer. As reported
in the Experimental Section, several organo-clays have been
tested with relative OH function concentration ranging from 10
to 100%. In all cases, polymerization occurred smoothly and
reached a PCL-to-clay composition close to 50 wt %, i.e., an
ideal composition for allowing the visualization of numerous
individual clay nanoplatelets and the polymer grafting onto their
surface.
2.4.2. P[CL-b-LA] Diblock Copolymer Surface Grafting. Poly-
[CL-b-LA] diblock copolymers were synthesized via sequential
copolymerization of CL and LA monomers, conducted in solution (dry
toluene). The drying, swelling, and derivatization processes of the
organo-clay were performed as described above. A given amount of
CL was then added under nitrogen, and the reaction was carried out at
40 °C for 1 h. Then, part of the reaction medium was set aside for
further characterization. A desired amount of LA, previously dissolved
in hot, dry toluene, was then added to the reaction medium, and the
polymerization was allowed to proceed at 70 °C for 7 days. The
polymerization reaction was stopped by the addition of diluted HCl,
and the resulting copolymer-grafted nanohybrids were immediately
recovered by precipitation in cold methanol, filtration, and drying in a
vacuum until the weight remained constant.
2.5. Nanocomposite Characterization. TGA was performed under
an air flow (75 cm3/min) at a heating rate of 20 K/min from room
temperature to 600 °C with a Hi-Res TGA 2950 from TA Instruments.
The clay content of each composite was assessed by TGA as the residue
left at 600 °C. A previous comparative TGA study was carried out in
triplicate on a separate sample, attesting to the excellent reproducibility
in terms of temperature ((2 °C) and weight loss ((1 wt %). The
inorganic content of the obtained nanocomposites is reported in Table
1. Nanohybrids containing between 50 and 60 wt % of inorganic matter
were prepared by the aforementioned method. This composition range
was selected in order to favor the AFM visualization of a large number
of individual clay platelets.
The thermal behavior was analyzed by differential scanning calo-
rimetry (DSC), using a DSC 2920 from TA Instrument at a heating
rate of 10 K/min from -100 to 200 °C under a flow of nitrogen. The
reported values were recorded during the second heating scan.
The morphological analyses by XRD were performed on a Siemens
D 5000 powder diffractometer using the Cu KR radiation (wavelength
) 1.5406 Å) at room temperature in the range of 2θ ) 1.5-30° with
a scanning rate of 2°/min.
It is worth pointing out that the PCL grafting onto the organo-
modified montmorillonite is demonstrated by the impossibility
For AFM imaging, the as-synthesized nanohybrids were dissolved
in toluene to reach a final concentration of 0.1 wt %, and the solutions
were deposited by casting on mica substrates. The images were recorded
in tapping mode (TM) in ambient atmosphere at room temperature with
a Nanoscope IIIa microscope (Veeco Inst., Santa Barbara, CA). The
probes were commercially available silicon tips, with a spring constant
(19) Mecerreyes, D.; Je´roˆme, R.; Dubois, P. Novel Macromolecular Architectures
Based on Aliphatic Polyesters: Relevance of the “Coordination-Insertion”
Ring-Opening Polymerization. In AdVances in Polymer Science; Hilborn,
J. G., Ed.; Springer: New York, 1999; Vol. 147, p 1 and references therein.
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A R T I C L E S
Viville et al.
Figure 1. Top: Topographic (500 × 500 nm2) TMAFM images showing the surface morphology of an individual clay platelet for a PCL-MMT nanohybrid
in which MMT is exchanged by 10% (OH)-containing surfactant (a) and by 25% (OH)-containing surfactant (b). The vertical grayscale is 5 nm. Bottom:
Cross-sectional analysis of image (a).
to separate the polyester chains from the clay by tentative
solubilization in good solvents, such as toluene, THF, or
chloroform. Even in the presence of an excess of lithium
chloride salt, known to cationically exchange with the interca-
lated ammonium cations,15-17 recovery of the PCL chains
(covalently attached to these ammonium cations through an ester
bond) proved to be extremely difficult, which most often
prevented the molecular characterization by size exclusion
chromatography (SEC). Interestingly, one SEC analysis could
be performed on the extracted PCL chains that were polymerized
from the clay surface modified with the lowest content in OH
functions, i.e., 10%. A number average molecular weight (Mn)
of 2900 g/mol has been determined; such a value is in very
good agreement with the molecular weight expected on the basis
of the monomer-to-hydroxyl molar ratio and the CL monomer
conversion (Mn,theor ) 3000 g/mol).
Another piece of evidence for the control achievable over
the polymerization/grafting reaction is the sequential block
copolymerization of CL and LA monomers. As expected for a
controlled process, P[CL-b-LA] diblock copolymers were
generated and surface-grafted onto the organo-clay (see Table
1). Their relative comonomer composition agrees with the
starting composition, and each polyester block is shown to
crystallize, with melting temperatures at 53 and 150 °C for PCL
and PLA blocks, respectively. It has also been possible to readily
tune up the crystallinity of each polyester block by controlling
the surface-grafted copolyester composition and molecular
weight, as evidenced by DSC characterization. Clearly, the
possibility to graft such diblock copolyesters onto organo-clays
is a strong indication that the (co)polymerization grafting
reaction is indeed controlled. The complete characterization of
these surface-grafted diblock copolyesters is beyond the scope
of this paper and will be the topic of a forthcoming communica-
tion.
3.2. Visualization of PCL Chain Grafting onto Single Clay
Nanoplatelets. In this section, we present the tapping mode
atomic force microscopy (TMAFM) results for thin deposits of
the different nanohybrids as the proportion of (OH) functions
in the clay is gradually increased. As a reference system, the
organo-modified clays, prior to polymerization, were first
investigated (results not shown here). In this case, we observe
smooth and featureless individual clay platelets that show a
constant height profile of 1.2 ( 0.2 nm, independent of the
relative amounts of the two ammonium ions. We never observe
the presence of impurities that has been recently reported in
the atomic force microscopy (AFM) imaging of layered clay
materials,20 most probably because our clay is from a different
origin. The height of the platelets observed in this case
corresponds to the value expected for uncoated clay platelets
(i.e., clay + alkylammonium surfactant without PCL).
The results for nanohybrids in which MMT is modified by a
surfactant mixture containing 10 and 25% (OH) groups are
shown in Figure 1. The TMAFM topographic images of these
two nanohybrids reveal isolated clay platelets lying flat on the
substrate surface, with their geometrical contours being well
contrasted with respect to the substrate surface. In contrast to
the featureless platelets observed before polymerization, they
are now partially coated either by round-shaped particles (left
image, for 10% (OH)) or by more spread out domains (right
image, for 25% (OH)).
(20) Piner, R. D.; Terry, T. X.; Fisher, F. T.; Qiao, Y.; Ruoff, R. S. Langmuir
2003, 19, 7995.
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A R T I C L E S
The profile analysis performed on those layers indicates that
the brighter domains on top of the platelets are 2.5 nm high
with respect to the substrate surface, while the flat, uncovered
areas of the platelets are about 1.4 nm high. TMAFM imaging
under much higher loading force conditions was carried out to
test the mobility of those domains. Typically, this was realized
by modifying the ratio (R) between the setpoint amplitude (Asp)
and the amplitude of the free-oscillating cantilever (A0): R )
Asp/A0. The reduction of this ratio leads to the increase of the
loading force on the sample. A semiquantitative description of
the loading force relies on the typical instrument settings used;
the amplitude of the oscillation was lowered as much as possible
(down to R ) 0.5), meaning high loading forces. Under these
conditions, for which the imaging conditions were still correct,
it was found that the position of the grafted domains did not
change.
The positional stability of those particles, their absence in
the nonpolymerized system, and the evolution of the morphology
with the amount of (OH)-containing surfactants (see below)
clearly indicate that the particles we observe here correspond
to the domains of grafted PCL.
Figure 2. Top: Topographic (550 × 225 nm2) TMAFM image showing
the surface morphology of individual clay platelets for a PCL-MMT
nanohybrid in which MMT is exchanged by 50% (OH)-containing surfactant
(the vertical grayscale is 25 nm). Bottom: Cross-sectional analysis.
For the 10% (OH) system, most of these PCL-grafted domains
are isolated. This allows for estimation of their volume and,
consequently, for calculation of the number of PCL chains
contained in a single domain. As an example, the area pointed
to by the black arrow in Figure 1a is characterized by a shape
corresponding to a short cylinder with an apparent volume of
about 1400 nm3. Considering the PCL density (1.07) and molar
mass (2900 g/mol), we estimate that the number of PCL chains
in that domain lies between 150 and 300, depending on the exact
size of the tip (AFM imaging of very small features can indeed
lead to an overestimation of the lateral size of those features
because of the convolution of the topographic profile by the
profile of the tip21,22).
Considering the size of the PCL chains (approximately 30
units for a 2900 g/mol PCL), and the fact that, in the extreme
case where they are fully extended, those PCL chains would
be 25 nm long, any polymer domain observed in the AFM
images is no farther than 25 nm from hydroxyl-substituted
ammoniums ions. This means that the polymer domains can be
used as markers for the position of OH-substituted ammoniums.
On that basis, the “spotty” morphology observed in Figure 1a
is an indication of phase separation between OH-substituted and
unsubstituted ammonium ions. Each polymer spot lies over an
area with a large number of OH-substituted ammonium ions,
while the large uncovered areas bear the unsubstituted am-
monium ions. Along that line, it is noteworthy that the PCL
domains in Figure 1a occupy ∼15% of the nanoplatelet surface,
which is in rather good agreement with the 10/90 OH/Alk ratio
used to prepare the clay.
sites at the surface of the clay. This thus demonstrates that the
amount of grafted polymer strongly depends on the proportion
of hydroxyl groups present at the surface.
The situation where the clay is exchanged with a surfactant
mixture containing higher amounts of (OH) functions, for
instance 50%, is illustrated in Figure 2. The surface of single
clay platelets appears again to be featureless. However, the
section analysis indicates that the platelets are, in this case,
significantly thicker than their pristine counterparts: 5 ( 1 nm
vs 1.2 ( 0.2 nm. This thickening demonstrates that the platelets
are now entirely and homogeneously covered by PCL, consistent
with the increase in the grafting density. Consistently, at this
point, any further increase of the OH proportion in the surfactant
mixture leads to additional thickening of the platelets. For
instance, for 75% (OH) content, the height of individual layers
with respect to the substrate surface increases into the 6-8 nm
range. Clearly, this evolution in the platelet thickness is coherent
with the increase in the amount of PCL present on the platelets
and with the increase in the number of grafting sites.
The important result to stress here is that, for OH proportions
g50%, all the platelets are completely and homogeneously
covered. This raises the question of whether the phase separation
between OH-containing and unsubstituted ammonium ions
mentioned above occurs during the cationic exchange or during
the polymerization process. In our opinion, the observation of
half-coated platelets in the 50% (OH) system would have
suggested that the phase separation takes place during the
cationic exchange, with polymer chains growing only over the
OH-containing domains. In contrast, the homogeneous coating
observed here clearly points to a homogeneous mixing of the
two ammoniums at the platelet surface.
The morphology of the system containing 25% (OH)-
substituted ammonium ions is illustrated in Figure 1b. In this
case, instead of being dispersed into individual domains, the
grafted PCL always forms more spread out patches, partly
covering the platelets. The height of the PCL-covered areas with
respect to the substrate surface lies within the 2.5-3 nm range,
as in the case of the 10% (OH) system. This higher coverage
clearly originates in the presence of a larger number of grafting
Therefore, the coexistence of PCL domains and uncovered
areas that is observed in the 10 and 25% OH systems (instead
of a thinner homogeneous polymer coating) probably means
that the OH-containing ions, which initially were homoge-
neously dispersed on the platelet surface, gather during the
(21) Keller, D. J. Surf. Sci. 1991, 253, 353.
(22) Bustamante, C.; Keller, D. J. Phys. Today 1995, 48, 32.
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A R T I C L E S
Viville et al.
Scheme 3 . Sketch for the Polymer Surface Grafting onto Individual Clay Platelets and Concommitant Phase Separation
polymerization process via lateral mobility (Scheme 3). Most
probably, the driving force for such an assembly is the aggre-
gation of the growing PCL chains to form polymer domains.
This lateral mobility is made possible because the ammonium
ions interact electrostatically with the clay surface; positional
interchange of ammonium ions is therefore relatively easy and
is promoted by the affinity between neighboring grafted PCL
chains, furthermore, solvated by toluene, i.e., the solvent in
which the polyester chains are growing during the polymeri-
zation.
the polymer deposit is not simply a continuous film growing in
thickness with increased OH content. Instead, separate polymer
islands are formed in the low-OH-content systems, probably as
a result of a phase separation process between the ammonium
ions induced by the polymerization reaction. Homogeneous
coverage and subsequent thicknening only take place from 50%
OH content. When this situation is achieved, adjacent platelets
become fully independent of each other (because they are fully
covered by the polymer), which greatly favors exfoliation.
This work also highlights the possibility of producing
organic-inorganic nanohybrids, where the nanoscale compo-
nents are linked to each other through electrostatic interactions.
We are now investigating how the control achievable over the
(co)polymerization, the grafting density, the composition, and
the crystalline structure of the grafted (co)polymer chains, as
well as over the mobility of the ammonium cations to which
the polymer is covalently anchored, can be exploited to tune
the properties of those hybrid materials.
4. Summary
The chemical grafting of polymer chains onto clay nano-
platelets is obtained by a coordination-insertion polymerization
of ꢀ-caprolactone in toluene with a montmorillonite-type clay
that is previously exchanged by varying proportions of hydroxyl-
substituted alkylammonium and unsubstituted alkylammonium
cations. The resulting hydroxylated clay particles play the role
of polymerization initiators, leading to the grafting and growing
of PCL chains directly at the surface of the clay platelets. The
polymerization is well controlled, as evidenced by the unique
possibility to grow semicrystalline diblock copolymers between
CL and LA, the composition of which is predictable, from the
organo-clay surface.
The direct visualization of the polymer grafted onto the clay
platelets brings important new information on the structure of
those nanohybrid materials: first, the grafting density drastically
increases as the proportion of OH-substituted alkylammonium
cations used to organo-modify the clay is increased. Second,
Acknowledgment. We are much indebted to the Re´gion
Wallonne and the Fonds Social Europe´en for support in the
frame of Phasing out- -Hainaut: Materia Nova. This work was
partly supported by the Re´gion Wallonne Programme WDU-
TECMAVER and by the Belgian Science Policy Programme
“Poˆle d’Attraction Interuniversitaire en Chimie Supramole´culaire
et Catalyse Supramole´culaire” (PAI 5/3). SMPC thanks Ce´cile
Delcourt for high-quality technical assistance.
JA048657Y
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9012 J. AM. CHEM. SOC. VOL. 126, NO. 29, 2004