Langmuir 2003, 19, 9425-9433
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Surface Characterization of Poly(E-caprolactone)-Based Nanocomposites Pascal Viville* and Roberto Lazzaroni Service de Chimie des Mate´ riaux Nouveaux (SCMN), Centre de Recherche en Sciences des Mate´ riaux Polyme` res, Universite´ de Mons-Hainaut, 20 Place du Parc, 7000 Mons, Belgium
Eric Pollet, Michael Alexandre, and Philippe Dubois 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
Gabriela Borcia and Jean-Jacques Pireaux Laboratoire Interdisciplinaire de Spectroscopie Electronique, Faculte´ s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium Received April 29, 2003. In Final Form: July 14, 2003 This work deals with the preparation and the surface characterization of biodegradable nanocomposites made of poly(-caprolactone) (PCL) and a montmorillonite-type clay. Nanocomposites with different relative compositions of PCL and montmorillonite, either natural or organo-modified by various alkylammonium cations, are prepared by melt intercalation and in situ intercalative polymerization. The goal of this study is to characterize the dispersion of the clay layers in PCL, which is a critical parameter governing the final physical properties of the obtained nanocomposites. Morphological studies of PCL nanocomposites are carried out by means of scanning probe microscopy techniques while surface analysis is performed both by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The results demonstrate that the dispersion of the clay layers is strongly dependent on both the synthetic route and the type of alkylammonium chains used for the cationic exchange. Whereas the melt-intercalation method leads to intercalated nanocomposites only after cationic exchange, the in situ polymerization method allows for reaching a much finer dispersion, even for the unmodified natural clay.
1. Introduction Polymer nanocomposites, especially polymer-layered silicate nanocomposites, represent a valuable alternative to conventionally filled polymers.1,2 Because of the dispersion of nanometer-size silicate sheets, these nanocomposites exhibit markedly improved properties when compared with pure polymers or conventional microcomposites. Polymer nanocomposites based on layered silicates (e.g., montmorillonite) are of current interest because of the fundamental questions they address and potential technological applications. Montmorillonite is a clay most commonly used in polymer nanocomposites preparation. It is a crystalline 2:1 layered clay mineral with a central alumina octahedral sheet sandwiched between two silica tetrahedral sheets. When these high-aspect-ratio nanoparticles are dispersed in a polymer, they can be either intercalated by the polymer chains or individually exfoliated within the polymer matrix. Therefore, intercalated structures show regularly alternating layered silicates and polymer monolayers in contrast to exfoliated structures in which the individual clay layers are delaminated and dispersed throughout the polymer matrix. Exfoliation of the silicate layers usually provides the nanocomposite materials with improved properties, such as a higher Young modulus and storage modulus, higher thermal stability and flame retardancy, and more efficient gas barrier properties. These two * Author to whom correspondence should be addresesd. (1) Pinnavaia, T. J., Beall, G. W., Eds. Polymer-clay nanocomposites, Wiley Series in Polymer Science; Wiley: Chichester, U.K., 2000. (2) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, R28, 1.
situations can, however, coexist in the same material, and the efficiency of the clay as a reinforcing agent to modify the physicochemical properties of the polymer is determined by the degree of its dispersion in the polymer matrix.1,2 Two main approaches can be used to prepare polymer/ clay nanocomposites: melt intercalation or in situ intercalative polymerization. In the first technique, the clay is mixed with the preformed polymer in the molten state. In the second approach, clay is dispersed in the monomer, which is then polymerized. In this study, layered silicate nanocomposites have been prepared by these two methods, as previously reported for a large variety of polymers, including thermoplastic, elastomer, and thermoset polymers. The polymer-silicate compatibility is promoted by an ion-exchange reaction of the native silicate interlayer sodium cations with alkylammonium cations.1-3 Because of rapidly increasing environmental concerns, biodegradable and biocompatible synthetic polymers such as aliphatic polyesters have been receiving steadily growing attention. The performances of these polyesters, such as poly(-caprolactone) (PCL), can be greatly enhanced by the nanodispersion of layered silicates. Recently, we reported the synthesis of PCL-based nanocomposites by both melt intercalation4,5 and in situ polymerization.6-8 These studies have highlighted the key (3) Biswas, M.; Ray, S. S. Adv. Polym. Sci. 2000, 155, 167. (4) Pantoustier, N.; Alexandre, M.; Dege´e, P.; Calberg, C.; Je´roˆme, R.; Henrist, C.; Cloots, R.; Rulmont, A.; Dubois, P. e-Polym. 2001, 009. (5) Lepoittevin, B.; Devalckenaere, M.; Pantoustier, N.; Alexandre, M.; Kubies, D.; Calberg, C.; Jeroˆme, R.; Dubois, P. Polymer 2002, 43, 4017.
10.1021/la034723i CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003
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role of the nature of the organic modifier and the clay content on the morphological, mechanical, rheological, and thermal properties of these new materials. By melt intercalation, a microcomposite is obtained with natural clay (MMT-Na), whereas an intercalated nanocomposite is formed with organo-modified clays. On the contrary, by in situ catalyzed polymerization, an intercalated structure is obtained with MMT-Na and organo-modified clay (MMT-Alk), whereas in the presence of hydroxy-functionalized montmorillonites a covalent grafting of PCL chains occurs on the clay surface, which can lead to a large extent of exfoliation. Intercalated/exfoliated nanocomposites display improved tensile properties. Particularly, the stiffness is increased significantly even at a clay content as tiny as 3 wt %. Furthermore, a solidlike rheological behavior is observed with storage, and the loss moduli are substantially increased. A significant improvement in the thermal stability of PCL was also observed.4-8 In this study, we aim at achieving the morphological and chemical characterization of biodegradable nanocomposites based on PCL and (organo-modified) layered silicates. Nanocomposites with different relative compositions based on PCL and layered silicates (montmorillonite) either natural or modified by various alkylammonium cations were prepared by melt intercalation and in situ intercalative polymerization. Morphological studies of PCL nanocomposites were carried out by means of scanning probe microscopy (SPM) techniques while surface analysis was carried out by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy in the reflection absorption mode (FT-IRAS). 2. Experimental Section Materials. Commercial grade PCL (CAPA®650) was supplied by Solvay Chemicals sector-SBU. The number-average molar mass was 49 000 with a polydispersity of 1.4, as determined by size-exclusion chromatography. The clay minerals were supplied by Southern Clay Products (Texas, U.S.A.). The cation-exchange capacity of unmodified montmorillonite-Na (Cloisite®Na; MMTNa) is 90 mequiv/100 g. The two organo-modified montmorillonites are Cloisite®25A [MMT-Alk; modified by dimethyl 2-ethylhexyl (hydrogenated tallowalkyl) ammonium cation] and Cloisite®30B [MMT-(OH)2; modified by methyl bis(2-hydroxyethyl) (hydrogenated tallowalkyl) ammonium cation]. The content of organics is 26 and 21 wt % for Cloisite®25A and Cloisite®30B, respectively. -Caprolactone (Fluka) was dried over CaH2 and distilled under a reduced pressure prior to use. Tin(II) salt [tin(octoate), Sn(Oct2)] was purchased from Goldschmidt and diluted with dried toluene. Sn(Oct)2 solutions were stored in glass ampules under a nitrogen atmosphere. Composite Preparation by Melt Intercalation. The PCLlayered silicate composites were prepared by the mechanical kneading of PCL and a known amount of montmorillonite with an Agila two-roll mill at 130 °C for 10 min. The collected molten materials were compression-molded into 3-mm-thick plates by hot-pressing at 100 °C under 150 bar for an additional 10 s, then under 30 bar for 10 s, followed by cold pressing at 15 °C under 30 bar for 5 min. Various amounts of natural montmorillonite (MMT-Na) and montmorillonites modified by dimethyl 2-ethylhexyl (hydrogenated tallowalkyl) ammonium (MMT-Alk) and by methyl bis(2-hydroxyethyl) (tallowalkyl) ammonium (MMT-(OH)2), respectively, were dispersed in PCL, such that the final compositions were 1, 3, 5, and 10 wt % inorganic materials. The inorganic (6) Lepoittevin, B.; Devalckenaere, M.; Alexandre, M.; Pantoustier, N.; Calberg, C.; Jeroˆme, R.; Dubois, P. Macromolecules 2002, 35, 8385. (7) Lepoittevin, B.; Pantoustier, N.; Alexandre, M.; Calberg, C.; Je´roˆme, R.; Dubois, P. J. Mater. Chem. 2002, 12, 3528. (8) Pollet, E.; Paul, M. A.; Dubois, P. New aliphatic polyester layeredsilicate nanocomposites. In Biodegradable Polymers & Plastics, Kluwer Academic Plenum Press: Norwell, MA, 2003; in press.
Viville et al. content of each composite was analyzed by thermogravimetry (TGA) under air flow and calculated from the residues left at 600 °C. Composite Preparation by in Situ Intercalative Polymerization. Composites containing 3 wt % (organo-modified) montmorillonite were prepared by in situ intercalative polymerization. Before polymerization, the organo-modified montmorillonites were dried in a ventilated oven at 70 °C for 1 night, and the unmodified natural montmorillonite was dried at 100 °C under a vacuum for the same amount of time. Then, the desired amount of layered silicate was further dried in a glass tube under a vacuum at 70 °C for 3 h. A given amount of -caprolactone was then added under nitrogen, and the reaction medium was stirred at room temperature for 1 h. Sn(Oct)2 was used as the activator of the -caprolactone ring-opening polymerization. A predetermined volume of the Sn(Oct)2 solution was added to the suspension, and the polymerization was allowed to proceed in the bulk at 100 °C for 24 h. In this case, the inorganic content was 3 wt % natural (MMT-Na) or organo-modified montmorillonite [MMT-Alk and MMT-(OH)2]. The inorganic content of each composite was checked by TGA. XPS. Surface chemical characterization was carried out by XPS. Spectra were recorded on a HP 5950A spectrometer using monochromatic Al KR radiation (1486.6 eV) under a vacuum of about 10-9 Torr at an electron takeoff angle of 51.5°. An X-ray power of 600 W was used during analysis. The high-resolution spectra were taken in the constant analyzer energy mode with a 120-eV pass energy. The value of 285.0 eV of the hydrocarbon C(1s) core level was used as a calibration of the energy scale. The peak envelopes were fitted by mixed Gaussian-Lorentzian component profiles. XPS analysis was performed on two series of samples. First, thin films of the composites were prepared by spin-coating, at ambient conditions, using 5 mg/mL solutions in toluene, yielding films with a thickness of a few micrometers. Typically, 20 µL of the solution was spin-coated on a 1 × 1 cm2 piece of gold-coated silicon. Samples were analyzed after complete evaporation of toluene at room temperature. A second series of spectra was obtained for solid samples, consisting of thin sections of bulk composites. IR Spectroscopy. The characterization of the surface chemical groups have been carried out by FT-IRAS. Analysis was performed using a Biorad FTS-60A FT-IR spectrometer. FTIRAS spectra were recorded between 4000 and 500 cm-1 by coadding 300 scans (background and sample) at a resolution of 2 cm-1 with an 85° angle of incidence, using a nitrogen-cooled mercury cadmium telluride broadband detector. Samples were analyzed as thin films prepared as described previously. SPM Characterization. Prior to atomic force microscopy (AFM) characterization, the initial compression-molded samples were microtomed using an Ultracut FC4E microtome from Reichert-Jung. This yields a pyramidal sample with a very smooth ∼2 × 2 mm2 surface, which is more suitable for an AFM study. The microscope was operated in the tapping mode (TM) to minimize the sample distortion due to mechanical interactions between the AFM tip and the surface. The images were recorded in an ambient atmosphere at room temperature with a Nanoscope IIIa (Veeco Instruments, Santa Barbara, CA). The probes were commercially available silicon tips with a spring constant of 2452 N/m, a resonance frequency lying in the 264-339 kHz range, and a typical radius of curvature in the 10-15 nm range. Both the phase and the topography images were recorded with the highest sampling resolution, that is, 512 × 512 data points. In this work, TMAFM was used to characterize both melt-intercalated and in situ polymerized PCL-based composites. The technique provides high-resolution imaging of the clay dispersion at the surface of the different composites. In particular, phase imaging is well-adapted to distinguish between the polymer and the clay areas on the basis of their local mechanical properties. To further differentiate the clay and polymer areas, on a semiquantitative level, we also measured approach-retract curves based on a procedure described by Kopp-Marsaudon et al.10 (9) Davis, C. H.; et al. J. Polym. Sci., Part B: Polymer. Phys. 2002, 40, 2661.
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Figure 1. TMAFM phase images comparing the surface morphologies of pure PCL (left, 5 × 5 µm2) and pure montmorillonite (right, 1.5 × 1.5 µm2). In addition to TMAFM, we also used scanning thermal microscopy (SThM) to characterize the composites. SThM is a technique derived from AFM consisting of a special heated probe that simultaneously measures the topography and the temperature of the sample surface. In its classical scanning mode, SThM provides simultaneous images of the surface thermal conductivity and topography, whereas in the so-called “local thermal analysis” (LTA) mode, the probe is positioned over selected regions of the surface and a predefined temperature scan is applied. The vertical deflection of the cantilever is then monitored as the temperature is increased. This method is known to allow identification of different components at the surface of heterogeneous materials such as, for instance, polymer blends. In this work, we thus exploit this technique to further identify the clays and the polymer on the basis of their thermal transitions.
3. Results and Discussion Study of Reference Samples. To follow the evolution in surface morphology and surface chemical composition of the different PCL-based composites, we first studied reference samples of pure PCL and montmorillonite. In this first section, we thus describe the TMAFM measurements and XPS analysis performed on thin films of these two references samples. For AFM imaging, thin films of pure PCL were obtained by spin-coating a CHCl3 solution on a mica surface, whereas thin films of the clay were obtained by dispersing few milligrams of the montmorillonite powder in tetrahydrofuran and by casting 20 µL of the suspension on mica. Figure 1 shows TMAFM phase images comparing the surface morphologies of pure PCL and pure montmorillonite. As expected, the two morphologies are very different. On one hand, PCL (left image) exhibits a spherolitic morphology depicting the semicrystalline character of PCL, within which the internal fibrillar microstructure of the spherulites is clearly identified. On the other hand, the montmorillonite thin film consists of a homogeneous “carpet” of clay sheets deposited flat on the surface. (10) Kopp-Marsaudon, S.; Lecle`re, P.; Dubourg, F.; Lazzaroni, R.; Aime´, J. P. Langmuir 2000, 16, 8432.
Table 1. Binding Energies and Relative Peak Areas of the PCL C(1s) and O(1s) Fitted Peaks C(1s) BE (eV) area (%)
O(1S)
1
2
3
4
1
2
285.0 51
285.5 18
286.6 17
289.1 14
532.2 51
533.5 49
In Figure 2, we present, as a reference, the reconstructed C(1s) and O(1s) core-level photoemission spectra of pure PCL, together with the chemical structure of the monomer unit. The C(1s) spectrum for PCL consists of four distinct contributions, and the O(1s) envelope decomposes into two distinct components (Figure 2, Table 1). Spectra were fitted on the basis of reference measurements.11 The relative elemental composition for PCL is 3.1:1.0 C/O, in very good agreement with the stoichiometric formula, which predicts a 3:1 ratio. The XPS spectra of aluminosilicate clays, as montmorillonites, present distinct features that allow for the identification of those clays when present in various composites. The energetic range situated between about 65-165 eV, with the four Si and Al peaks present, can be considered a “fingerprint” of these materials (Figure 3). We mention that the Au(4f) doublet is due to the goldcoated silicon substrate because clays dispersed in solution do not form uniform films; this allows for a very good calibration for the energy scale. The measured values for the binding energy of Si [Si(2s), 154 eV; Si(2p), 103 eV) and Al (Al(2s), 120 eV; Al(2p), 75 eV) are characteristic for aluminosilicate species. The relative elemental composition calculated from this spectral “fingerprint” shows 2.4:1.0 Si/Al for all the montmorillonite samples, both natural and modified. For the ammonium cation-modified clays, nitrogen was detected in traces of about 1:12 N/Si. The characterization of the different PCL-based composites, described hereafter, is interpreted on the basis of the results obtained for these two reference samples. (11) High-resolution XPS of organic polymers; Beamson, G., Briggs, D., Eds.; The Scienta ESCA300 Database; John Wiley & Sons: New York, 1992.
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Figure 2. C(1s) and O(1s) spectra for a PCL thin film.
Figure 3. “Fingerprint” XPS spectrum of montmorillonite clay.
Melt-Intercalated Composites. We first investigate the surface of a composite obtained by melt intercalation with a natural (unmodified) MMT-Na montmorillonite. In this case, the microtomed sample surface exhibits a large number of micrometer-size particles, such as that shown in the two AFM images of Figure 4. These particles are embedded in a homogeneous PCL matrix and, most probably, correspond to aggregates of stacked clay sheets. Under low applied force conditions between the AFM tip and the surface, the contrast observed in the phase image (right) can be correlated with the local stiffness.10 The higher phase lag over the clay particle clearly indicates that it is stiffer than the surrounding matrix. To confirm this result, we also carried out SThM measurements. We performed a LTA on both the particle and the matrix. The LTA results are shown at the bottom of Figure 4. The curves, describing the change of the cantilever deflection as the tip temperature is increased, highlight two very different thermal behaviors. On one hand, on the polymer matrix (for instance, on the dark circle in the right phase image), we observe that the vertical position of the tip remains unchanged up to a temperature around 60 °C, above which the lever deeply
penetrates into the material. This behavior allows for unambiguous identification of PCL because, in this area, the penetration of the tip occurs at the melting temperature of PCL. On the other hand, the lever response on the particle is completely different. First of all, no indentation occurs around 60 °C; as the temperature is raised, we observe an upward bending of the lever up to a temperature around 150 °C. This type of evolution is often observed and is classically attributed to the thermal expansion of the material below the heated probe. At 150 °C, we then observe a further upward bending of the lever. This bending depicts a dilation of the particle upon heating. Note that this dilation is not observed on the PCL areas and occurs at a temperature much higher compared to that of the thermal transition observed on the PCL areas. Above 150 °C, the tip finally penetrates into the material, probably because heat dissipation has led to the softening of the surrounding PCL regions. Our combined AFM/ LTA approach thus allows for the identification of the clay and the PCL areas on this composite. The results also point to the fact that the meltintercalation procedure of natural MMT-Na does not lead to intimate intercalation of the clays, forming a nanocomposite, because all the clay particles observed in this case are in the micrometer size range. In contrast, composites prepared by melt intercalation of PCL with MMT-Alk and MMT-(OH)2 present a completely different morphology. The AFM images are shown in Figure 5. In contrast to microcomposites showing micrometer-size aggregates, we observe here individual clay sheets and nanometer-size stacks of these sheets embedded in a highly textured PCL matrix. They appear as well-defined bright objects inserted in a darker matrix (see, for instance, the white arrows in Figure 5a). Once again, in the conditions chosen for AFM imaging (low applied force between the tip and the surface), this phase contrast indicates that the clay areas (bright appearance) present a higher stiffness than the surrounding PCL matrix (darker). The lamellae-like texture of the PCL matrix observed in Figure 5b most probably corresponds to the crystallization of the PCL around the clays.
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Figure 4. Top: topographic (left) and phase (right) 5 × 5 µm2 TMAFM images of a 10 wt % MMT-Na (cloisite Na)/PCL composite prepared by melt intercalation. Bottom: LTA curves recorded on the PCL matrix (dots) and the particle (squares).
On the basis of these AFM images, we do not observe clear differences between the composites prepared with MMT-Alk and MMT-(OH)2; the two materials exhibit clay particles well-dispersed into the polymer matrix and, locally, stacks of clay sheets. Also, the apparent crystallinity of the polymer, as revealed by the regular texture of the matrix, is observed both with MMT-Alk and MMT(OH)2. The coexistence of stacks of clay sheets and the polymer matrix, as shown in Figure 5b, were exploited to perform approach-retract curves with AFM. For that purpose, the tip was successively positioned over the clay stack (the white diamond in Figure 5b) and over the surrounding PCL matrix (the dark circle). The variation of the cantilever oscillation amplitude was then measured as a
function of the tip-sample distance, during numerous approach-retract cycles. The results shown in Figure 6 correspond to the average of the approach curves taken on the clay stack and on the surrounding polymer matrix. When the tip is far away from the surface, it oscillates freely with a large amplitude (right part of the curves). As the tip-surface distance is reduced, the probe starts to feel the attractive interaction with the surface and the oscillation amplitude increases (here, around D ) 20 nm). Upon further approach, the amplitude gradually decreases as the oscillation is damped by the repulsive interaction with the surface. The curves recorded on the clay and on the polymer both exhibit a classical decrease of the oscillation amplitude as the tip approaches the sample surface. However, this reduction is very different depend-
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Figure 5. Typical TMAFM phase images of the surface of MMT-(CH2CH2OH)2/cloisite 30B (a) and MMT-C8H17/cloisite 25A (b) composites prepared by melt intercalation. The image sizes are 500 × 500 nm2 for both images; the vertical grayscale is 15°. Table 2. XPS-Measured Al/C Atomic Ratios for the Melt-Intercalated Compositesa melt intercalation (wt %)
MMT-Na
MMT-Alk
MMT-(OH)2
1 3 5 10
1:98 1:54
1:228 1:108 1:56
1:229 1:122 1:52
a When no value appears, the ratio was too small to be determined accurately.
Figure 6. Averaged approach curves in approach-retract measurements carried out over the PCL matrix (black dots) and over a clay sheet (grey dots). S denotes the slope of the curve. The dashed line corresponds to the case of a model infinitely stiff surface.
ing on whether the tip is located above the clay or above the polymer. While the amplitude decrease is quite gradual on the polymer area, it is more brutal for the clay area with a value of the slope close to 1. This behavior is attributed to the indentation of the tip in the polymer. For this reason, for a given tip-sample approach distance, the amplitude of oscillation always shows at a higher value when the approach-retract cycle is performed over the polymer. In other terms, the slope of the curves is sensitive to local mechanical properties. Because no indentation can occur into the clay sheets, a slope close to 1 is measured, as is the case for surfaces characterized by an infinite stiffness (such as silicon). The AFM images shown in Figure 5 indicate that the melt intercalation of PCL with the MMT-Alk and MMT(OH)2 organo-modified clays favors the formation of nanocomposites because nanometer-size clays are found dispersed in the PCL matrix for both types of organomodified clays. Let us recall here that this situation completely contrasts with that of the microcomposites obtained by melt intercalation of unmodified clays.
In addition to AFM imaging, we then analyzed the meltintercalated composites by means of XPS and FT-IRAS. For each melt-intercalated composite, prepared both as films and as thin slices, the polymer and montmorillonite characteristic elements (C, O, Si, and Al) were identified on the wide-energy-scan XPS spectra. The deconvolution and quantification of the spectra were performed on the basis of the above results for pure polymer and clay samples. It turned out that several XPS spectra for both thin films and solid composite samples showed a marked silicon surface contamination, at levels of about 1:20 Si/C, the silicon content presenting no correlation with the composition of the samples. A possible source for this contamination could be residual silicon grease used to tightly seal the glass reactor and desiccators. Nevertheless, because the measured Al content relates only to the clay present within the composites, the Al/C ratios can be used for characterization of the chemical composition (Table 2); note, however, that values are missing for those samples with a low clay content (1 wt %), where the Al content is hardly measurable with reasonable accuracy. The results clearly indicate a very good correlation between the chemical composition of the samples and the amount of clay introduced in the composites. For example, we calculate an increase by a factor of about 2 between the composites with 5 wt % clay and 10 wt %, respectively. The results also indicate that the chemical composition shows no dependence on the nature of the cation included in the montmorillonite. The increased sensitivity of the FT-IRAS technique allowed the detection of specific features related to the
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Figure 7. FT-IRAS spectra of PCL-based MMT-Alk composite thin films.
clay presence in the composites. An example of IR spectra is given in Figure 7 for the PCL-based MMT-Alk thinfilm composites. The assignment of the bands was based on previous assignments in the literature. The principal bands of polyesters, as PCL, are12 3000-2800 cm-1 (CH2 stretching vibrations), 1740 cm-1 (CdO stretching vibrations), 12751050 cm-1 (CsOsC aliphatic ether stretching vibrations), 730 cm-1 (CH2 long chain rocking motion vibrations), and 1450 and 1380 cm-1 (CH2 and CH bending vibrations). All these characteristic bands are identified on our spectra. The principal bands of montmorillonite-type clays (aluminosilicates) that appear in the IR range can be separated into two categories.13-16 There are important bands due to the water contained in the clay: 3700-3600 cm-1 (OsH stretching vibrations), 3400 cm-1 (OsH stretching vibrations), and 1600 cm-1 (HsOsH bending vibrations). The characteristic bands for aluminosilicate structures can usually be observed between 1200 and 400 cm-1, among which the most intense is situated in the region 1040-990 cm-1, due to Si-O stretching vibrations. However, for the PCL-based composites studied here, the characteristic bands of aluminosilicates (assigned to groups containing Si or Al atoms) cannot be identified because the polymer has multiple bands in the same IR spectral range and is by far the major component of the system. The spectral elements that can be associated with the clay presence in the composites are the bands at 37003600 cm-1 and 3400 cm-1, respectively, assigned to OsH stretching vibrations. These bands are absent on the pure PCL spectrum and show an increased intensity correlated with the increasing clay content in the composite sample. A marked modification is observed for certain bands assigned to vibrations within the PCL chain. The bands situated at 3000-2800 cm-1 (CH2 stretching vibrations) and 1730 cm-1 (CdO stretching vibrations) appear as shifted and deformed for the composites containing at least 5 wt % montmorillonite. Moreover, the multiple bands situated in the region 1600-600 cm-1 appear with strongly attenuated features as compared to those of pure PCL. This behavior is observed for the composites prepared by melt intercalation with ammonium cation-modified mont(12) Ponchert, C. J. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Chemical Co.: Milwaukee, WI, 1981. (13) Bora, M.; Ganguli, J. N.; Dutta, D. K. Thermochim. Acta 2000, 346 169. (14) Salil, M. S.; Shritvastava, J. P.; Pattanayak, S. K. Chem. Geol. 1997, 136, 25. (15) Vicente-Rodriguez, M. A.; et al. Spectrochim. Acta, Part A 1996, 52, 1685. (16) Farmer, V. C. Spectrochim. Acta, Part A 2000, 56, 927.
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morillonite [MMT-Alk and MMT-(OH)2]. These composites, prepared by melt intercalation, lead to intercalationofPCLchainswithintheorgano-modifiedaluminosilicate layers accompanied by some extent of exfoliation. This might suggest that chemical interaction between functional groups of the polymer and surface-organo-modified silicate layers, which improves the penetration of the polymer into the interlayer space of the modified clay, is responsible for the changes observed in the IR signature of the polymer. In strong contrast, no modifications are observed on the IR spectra of the composites prepared with natural MMT-Na. In that case, melt blending of PCL together with natural montmorillonite only yield microcomposites without any trace of intercalation. When these IR results are considered, it appears that the clay modification (with ammonium cations) plays an important role in the formation of nanocomposites, allowing thus a better dispersion of the montmorillonite layers within the polymer material. These results are in very good agreement with those obtained by AFM in this work and by X-ray diffraction and transmission electron microscopy in previous works.5,6,9 In Situ Intercalative Polymerized Composites. In a second series of measurements, we investigated nanocomposites prepared by in situ polymerization again, using natural MMT-Na and the two organo-modified clays MMT-Alk and MMT-(OH)2. One typical AFM phase image of a nanocomposite prepared by in situ polymerization with MMT-Na is shown in Figure 8a. The presence of the clay is clearly identified in this sample. These clay sheets are of nanometer size and are only visible at a high magnification; they sometimes form stacks, but no micrometer-size aggregates are observed. In good agreement with the X-ray diffraction data obtained on the same sample,6-8 our AFM results indicate that in situ polymerization of PCL with natural MMT-Na leads to intercalated structures. It is important to note here that this completely contrasts with the melt-intercalation process using the same clay, which leads to microcomposites. The dispersion of the clay is thus strongly dependent on the synthetic route used, the in situ polymerization method being in this case a major improvement compared to the melt-intercalation process. The in situ polymerized nanocomposite with the MMTAlk organo-modified clay (result not shown here) shows the same type of morphology because we also observe stacks of clays organized flat to the surface, such as those shown in Figure 8a. This also suggests an intercalation process of the clays. In contrast, the AFM data suggest that exfoliated nanocomposites are formed when hydroxyl-containing alkylammonium MMT-(OH)2 is used. This is illustrated by the image of Figure 8b in which we can distinguish isolated clay sheets lying parallel to the surface. In this sample, no clay stacks were observed, contrasting with the two previous clays. These results point to a better clay dispersion, that is, the dissociation of the clay stacks. According to recent publications,6-9 the exfoliation of the clay is achieved in this case because the alcohol-bearing organo-modified clay acts as a co-initiator for the polymerization reaction. In these conditions, the grafting of PCL chains is thus observed, leading to a more efficient dispersion of the clay sheets in the PCL matrix. Actually, it is worth pointing out that complete exfoliation of the clay platelets has also been evidenced by more conventional methods such as X-ray diffraction and transmission electron microscopy, as recently reported.7,8 As for the melt-intercalated composites, the XPS data acquired for the “in situ” polymerized sample indicate no
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Figure 8. Typical TMAFM phase images of the surface of MMT-Na/cloisite Na (a) and MMT-(CH2CH2OH)2/cloisite 30B (b) composites prepared by in situ polymerization. The image sizes are 750 × 750 nm2 for part a and 600 × 600 nm2 for part b; the vertical grayscale is 15° for both images. Table 3. XPS-Measured Al/C Atomic Ratios for the in Situ Polymerized Compositea in situ intercalative polymerization (wt %) 3
MMT-Na
MMT-Alk
MMT-(OH)2
1:220
1:216
a
When no value appears, the ratio was too small to be determined accurately.
evolution of the surface composition depending on the nature of the clay (Table 3). The Al/C ratios are almost the same for the MMT- Alk and the MMT-(OH)2, 1:220 and 1:216 Al/C, respectively. If we compare these results with those acquired for the melt-intercalated samples, it also appears that the surface composition, for the same amount of clay, does not depend on the synthetic route. Indeed, for 3 wt % MMT-Alk or MMT-(OH)2 introduced in the composite, we observe the same Al/C ratio for the melt intercalation and the in situ intercalative polymerization case. Finally, let us mention that the IR spectra of the samples prepared by in situ intercalative polymerization also show no montmorillonite-specific features, but this could be due to the low amount of clay (3 wt %). 4. Conclusions We investigate the dispersion of clay layers in nanocomposites made of PCL and a montmorillonite-type clay. Surface morphology studies of the obtained PCL-based nanocomposites are carried out by means of SPM techniques while surface analysis is performed by both XPS and FT-IR. This work particularly highlights the influence of different parameters, which are (i) the nature of the ammonium cation surfactant used as an organic modifier of the silicate layers, (ii) the chosen synthetic route, and (iii) the clay content. For the melt-intercalation process, the results demonstrate that the nature of the surfactant plays an important role in the nanocomposite formation. While microcomposites are always formed with natural unmodified MMTNa, SPM measurements clearly show that intercalation
of PCL chains in montmorillonite is obtained when the clay is modified by either alkylammonium surfactants or hydroxylammonium surfactants. The influence of the various alkylammonium cations on the dispersion of clays within PCL is also emphasized on the IR spectra obtained in the reflection absorption mode. The deformation and the shift of specific PCL bands suggest that chemical interactions take place between the polymer and the clay cation-modified functional groups. Such interactions are not evidenced in the case of the nonmodified (sodium) montmorillonite. A very good correlation between the XPS spectra and the amount of clay in the PCL-layered silicate composites is finally observed by monitoring the energetic “fingerprint“ of the montmorillonite in all melt-intercalated nanocomposites. The surface chemical composition is instead found to be independent of the type of clay used. In contrast, the in situ polymerization technique allows the formation of intercalated nanocomposites even with unmodified MMT-Na. The best dispersion (complete exfoliation) of the clay layers is observed in the AFM imaging when the clay is modified by alcohol-bearing alkylammonium chains [MMT-(OH)2]. According to recent publications, exfoliation is reached because the hydroxyl functions act as co-activators of the polymerization, allowing the grafting of PCL chains onto the clay layers. XPS study confirms that the surface chemical composition also does not depend on the clay type used for the in situ polymerized composites. It also demonstrates that the surface composition does not vary as a function of the synthetic route used for an identical proportion of clay in the composite. In future studies, we plan to confirm the role of the ammonium cation and the preparation route in the success of the nanocomposite formation. In particular, new AFM experiments are in progress to visualize the grafting process directly at the surface of the clays. Acknowledgment. SMPC is much indebted to the Re´gion Wallonne and the Fonds Social Europe´en for support in the frame of Objectif 1-Hainaut: Materia Nova.
Poly(-caprolactone)-Based Nanocomposites
This work was partly supported by the Re´gion Wallonne Programme WDU-TECMAVER and by the Belgian Federal Government Office of Science Policy (SSTC) “Poˆle d’Attraction Interuniversitaire en Chimie Supramole´culaire et Catalyse Supramole´culaire” (PAI 5/3). SMPC thanks Ce´cile Delcourt for high quality technical as-
Langmuir, Vol. 19, No. 22, 2003 9433
sistance. SCMN thanks Ce´dric Calberg from CERM (University of Lie`ge, Ulg) for providing the microtomed samples for AFM analysis. R.L. is Directeur de Recherches du FNRS (Belgium). LA034723I
