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An infrared method to assess organoclay delamination and orientation in organoclay polymer nanocomposites.

INTRODUCTION

Smectite clays are layered, platy, hydrophilic silicate materials [1]. In the dry state, several nanosized clay layers are normally stacked on top of each other and these stacks, or tactoids, are agglomerated into particles. However, the platelets spontaneously separate from each other when dry clay powder is dispersed in water. This "delamination of layers" is at times also referred to as "exfoliation of layers". Smectite clay layers carry a net negative charge on the platelets, which is neutralized by hydrophilic metal ions that are positioned on the surfaces of the platelets. An organoclay is formed when the hydrophilic metal ions are exchanged with organic cations. Organic surface treatment is often necessary to improve the compatibility of the clay with organic systems.

Many industrial applications have emerged since the discovery of organoclays by Hauser and Jordan [2, 3]. Particularly, the ability of organoclays to swell and delaminate in organic solvents has lead to their widespread use as rheological control agents [4-6]. Delaminated organoclay layers can form a three-dimensional network and this is responsible for viscosity build and gelation of an organic system. Early on, BENTONE[R] organoclays have been used as additives in paints, greases, inks, and oil-well drilling fluids [7, 8]. During those years, organoclays were also incorporated as reinforcement/filler agents in plastics and rubbers, [9, 10] and they have been used as anti-drip additives to enhance plastics flame retardant properties [11]. Even barrier properties were improved through the incorporation of organoclay in plastics [12].

Not much attention was given to the state of organoclay dispersion in plastics during those early years. Then, researchers at Toyota showed that mechanical properties of Nylon-6 significantly improved when organoclay was completely dispersed and delaminated in the Nylon resin [13-16]. Extensive research on clay polymer composites ensued in the following years and many reviews have been published on the subject [17-20]. Most publications show that the presence of organically treated clay layers in a polymer matrix can improve barrier, mechanical, or flame retardant properties. These polymer organoclay nanocomposite benefits are typically realized when an organoclay is delaminated to the "nano" state in a polymer. It is generally accepted that, ideally, all organoclay particles are dispersed towards individual layers in a delamination process to form a polymer composite containing discrete nanosized layers. However, complete dispersion of these organoclays towards their nanosized state is not easily accomplished. Incomplete organoclay dispersion and delamination leaves microsized particles in the polymer or plastic, which may affect the properties of the resulting nanzocomposite. The degree of organoclay dispersion in plastics is extremely difficult to control, and this can result in unpredictable properties. Clay nanodispersion repeatability and reproducibility has been a long-felt need in the field of clay nanocomposites. If such repeatability and reproducibility could be achieved, polymer organoclay nanocomposites would likely become much more successful in the marketplace.

Organoclay nanocomposite characterization is the key to commercial success, and several methods are widely used to analyze the quality of nanocomposites. X-ray diffraction (XRD) is a commonly used technique that can be used to determine whether polymer has entered the organoclay interlayer region. It is assumed that all clay layers could be in an exfoliated state when no clay XRD peaks are observed in the diffraction pattern of an organoclay nanocomposite. Furthermore, XRD can fail to detect agglomerated clay particles, giving the incorrect impression that the organoclay is delaminated; therefore, care has to be taken when drawing conclusions based on XRD alone [20-22]. XRD cannot distinguish between large and small clay tactoids as the XRD signal strength depends on factors like clay loading and layer orientation, nor can XRD provide an assessment of how many exfoliated clay layers may be present in a composite. Transmission electron microscopy (TEM) is useful in obtaining visual images for improved understanding of nanocomposite structures and is often used in conjunction with XRD. However, it has some drawbacks, in that sample preparation is difficult and the TEM process is time-consuming. Other techniques such as TGA, NMR, AFM, SEM, melt rheology, and the like are utilized, but the analysis process can be laborious and these techniques are not readily applied during an actual nanocomposite production process, when organoclay is compounded with a thermoplastic in a twin screw extruder.

Infrared (IR) spectroscopy is a versatile analytical tool and this technique can be adapted for in-line monitoring of the extrusion process when compounding plastics [23]. Furthermore, polymer composite (film) samples can be prepared off-line and analyzed with modern transmission or reflectance FTIR equipment, which can measure IR absorbance with high sensitivity and accuracy. In the past, IR spectroscopy has been utilized to follow organoclay dispersion in mineral oil when producing grease [24]. It was noted that the clay silicon-oxygen (Si-O) bond infrared absorption bandwidth reduces as the organoclay becomes more dispersed. Furthermore, a combination of IR trichroic analysis and TEM has been used to determine the orientation distribution function of montmorillonite platelets in Nylon-6 nanocomposite films [25]. Johnston and coworkers [26, 27] extensively studied the infrared characteristics of single clay layers obtained via Langmuir-Blodget methods. We have utilized clay (dichroic) infrared absorption to assess the degree of organoclay dispersion and to compare clay layer orientation in organoclay nanocomposites. A patent application has been filed for this unique characterization method.

EXPERIMENTAL

Materials

Highly refined hectorite and montmorillonite clay and other organoclay products were obtained from Elementis Specialties, Inc. BENTONE HC is a sodium hectorite clay. BENTONE 108 organoclay is a hectorite clay modified with a dimethyl dihydrogenated tallow quaternary ammonium compound. Anhydrous propylene carbonate was used as a polar dispersant to delaminate organoclay in mineral oil. Caprolactam, mineral oil, and other common chemicals were obtained from Aldrich. A general grade LDPE is used for all polyethylene composites.

Materials Preparation

Dispersions of bentonite and hectorite clay in water were prepared by dispersing 3 wt% sodium clay in demineralized water, with the aid of a Cowles high shear mixer. Typically, 5 wt% BENTONE 108 organoclay mineral oil dispersions were made by first allowing the organoclay to wet out before treatment with a dispermat high shear mixer. The slurry is then treated with an IKA ultra-turrax rotor-stator mixer, followed by a pass through an APV Gaulin homogenizer. Ultra-turrax and APV Gaulin shear was also applied after polar dispersant (amounts based on organoclay weight) is added to the organoclay dispersion. Melt processed composites were prepared with a Brabender mixer or twin screw extruder. Solvent cast derived and other composites were prepared using procedures described in the literature. All composites contained about 5 wt% organoclay.

Equipment

The infrared spectra shown in this study were collected on a Thermo Electron Corp. Nicolet Avator 370 system. The spectra were collected at a 1.0 [cm.sup.-1] resolution, using a DTGS KBr detector. Each spectrum shown is an average of 64 individual scans. Nonpolarized light was used unless otherwise specified. Attenuated total reflectance (ATR) spectra were collected using a Nicolet Smart Orbit accessory equipped with a single bounce diamond crystal at a 45[degrees] angle. Thermo Electron Corp supplied the polarizer used to study dichroic ratios. The crystal was manufactured using a holographic technique, which deposits an array of parallel aluminum conductors at a spacing of 4000 lines per inch on a Zinc Selenide substrate. The crystal was mounted on a rotary holder calibrated with 2[degrees] divisions. Spectra collected at different tilt angles were made with the sample mounted in a Brewster's Angle sample holder manufactured by Harrick Scientific Corp. The sample holder was calibrated in 1[degrees] increments. Spectral curve fitting was performed using the Omnic software package version 7.1a. This software package provides a convenient method of fitting the clay Si-O absorption bands to a Gaussian-Lorentzian peak shape. When necessary, curve subtraction was used to remove polymer IR contribution. The bandwidth is reported as the full width at half maximum (FWHM) of the absorption band. Absorption band area and intensity are also calculated by using the software. The polymer films for infrared analysis were prepared by hot pressing the polymer at temperatures at or above the melt point of the polymer. Film thickness was controlled to obtain an in-plane peak height of 0.7 absorbance units or less.

Wide angle XRD patterns were recorded on a Scintag XDS2000 Theta/Theta diffractometer equipped with 2 mm divergence/4 mm scatter slits and 0.3 mm receiving/0.5 mm scatter slits. X-rays were generated from a standard focus Cu X-ray tube operated at 45 kV/40 mA. Measurements were made with an intrinsic germanium solid state detector operated at liquid [N.sub.2] temperature. Samples were run in continuous mode at 1[degrees]/min, with a data point every 0.02[degrees]. Composite samples were first injection-molded to fit in the sample holder and then shimmed to the correct height. Organoclays were analyzed as powders.

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

The schematic structure of a smectite clay layer [1] is shown in Fig. 1. One can distinguish "in-plane" Si-O bonds, which are oriented parallel to the clay layer, and "out-of-plane" bonds, which have a direction roughly perpendicular (normal) to the clay layer [28, 29]. The ATR--FTIR absorption spectra for a Wyoming montmorillonite clay powder in the 950-1150 [cm.sup.-1] region is shown in Fig. 2a. One broad band is observed in the region where clay Si-O bonds typically absorb infrared radiation. When the clay powder is dispersed in water and the layers are in a delaminated state, several absorption bands are resolved under these conditions (see Fig. 2b). These bands can be attributed to the vibrations of different Si-O bonds in the clay structure. The absorption bands around 1021 and 1042 [cm.sup.-1] are ascribed to absorption by the in-plane Si-O bonds, whereas the band at 1087 [cm.sup.-1] is attributed to the out-of-plane silicate bonds. Our results are comparable with those obtained by Johnston et al. [29] for single montmorillonite layers obtained via Langmuir-Blodget methods. A comparable experiment was carried out for hectorite clay. Similarly, a broad band is observed for hectorite clay powder (Fig. 3a). When this commercial sodium hectorite clay (BENTONE HC) is dispersed in water and the clay layers are delaminated, two absorption bands are resolved (Fig. 3b). The in-plane absorption band emerges around 1004 [cm.sup.-1] and the out-of-plane absorption band appears around 1080 [cm.sup.-1]. Hectorite clay is a so-called trioctahedral clay and the silicon oxygen framework deviates less from the ideal hexagonal symmetry when compared with that of montmorillonite, which is a dioctahedral clay. Consequently, hectorite clay has fewer distinct lattice vibrations that give rise to infrared absorption, and therefore, the resulting infrared absorption spectra is much simpler when compared with that of montmorillonite [28]. Deconvolution of the in-plane absorption band gives an absorption FWHM value of 28.9 [cm.sup.-1] for delaminated hectorite clay layers that are dispersed in water. The FWHM bandwidth for the out-of-plane absorption bands is 22.2 [cm.sup.-1]; however, we have found that this band is more difficult to deconvolute as the intensity significantly changes with layer orientation, and we have observed differences in the values obtained for various samples. Therefore, we prefer to use the in-plane absorption band to monitor clay layer dispersion state.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The optical properties (position, relative intensity, shape, and bandwidth of absorption bands) of a material in the infrared region are dependent on the size and shape of the (agglomerated) particles and on the nature of the matrix in which the material is embedded. More specifically, infrared light absorption is related to the dielectric constant of the absorbing medium, and its value depends on the aggregation state of the particles [30]. On the basis of the results described in the previous section, we postulate that agglomeration of clay layers (stacking) leads to bandwidth broadening of the silicon-oxygen absorption band. When clay layers are agglomerated into stacks (tactoids) and particles thereof, bandwidth broadening occurs and the in-plane and out-of-plane absorption bands overlap into one broad band. Conversely, the absorption bands narrow as the clay platelets become increasingly delaminated and absorption bands that initially overlap may become better resolved. The Si-O stretching absorption bands do not resolve further, nor is a further reduction in absorption bandwidth observed, once all layers are completely isolated from other clay platelets.

Organoclays are much more compatible with organic systems than unmodified clays. However, compared to the spontaneous exfoliation of clay in water, an organoclay does not readily delaminate in organic solvents such as toluene or mineral oil. The assistance of a "polar dispersant" is often necessary to completely delaminate the organically treated layers in the organic systems [3-8, 31]. According to gelation studies, the clay electrical double layer becomes more diffuse through interaction with the polar disperant promoting organoclay layer delamination [31]. Small polar compounds like methanol, acetone, or propylene carbonate are generally excellent organoclay dispersants.

Figure 4a shows the transmission FTIR absorption spectra for BENTONE 108 organoclay powder dispersed, but not delaminated, in mineral oil. As seen before with the non-dispersed hectorite clay, only one very broad absorption band is observed in the Si-O stretch region. The broadness of this band does not change when the dispersion is treated with varying levels of shear to break up organoclay agglomerates. However, the exact bandwidth measured at this point depends on how the organoclay was prepared, as our bandwidth broadening is a layer agglomeration effect. Interestingly enough, the type of organic treatment on the organoclay has only a minor influence on bandwidth unless very large interlayer distances are achieved. When the organoclay is completely delaminated in mineral oil, through the addition of a quantity of propylene carbonate polar dispersant, absorption bands resolve and deconvolution of the in-plane absorption band gives a FWHM of about 28.5 [cm.sup.-1] (Fig. 4b). This bandwidth is the same as we observed for hectorite clay delaminated in water. We conclude that the organic treatment of the clay layers has no influence on the observed bandwidth for delaminated layers, nor does the embedding matrix, mineral oil instead of water, have a significant effect on the absorption bandwidth of the delaminated layers (28.5 versus 28.9 [cm.sup.-1]). Thus, when hectorite clay layers are delaminated, one can expect to find an absorption bandwidth between 28.5 and 29 [cm.sup.-1] for the in-plane absorption band. One could derive a similar inference for montmorillonite clay, except that the IR spectra for this clay is much more complicated as several clay absorption bands overlap.

[FIGURE 4 OMITTED]

Figure 5 shows the change in bandwidth for the in-plane Si-O clay bond when controlled amounts of polar dispersant (propylene carbonate) are added to a dispersion of BENTONE 108 in mineral oil. Increasingly, more layers become delaminated when more polar dispersant is added to the clay-mineral oil system. The in-plane Si-O absorption bandwidth does not reduce any further when more than 30 wt% polar dispersant is added to the clay dispersion. Apparently, all clay layers are delaminated at this point. A maximum viscosity build is also noted at this same point, indicating again that all clay layers are delaminated. We use this plot as a calibration curve to assess degree of organoclay layer delamination in polymer systems.

Assessing the Degree of Organoclay Dispersion in Polymer Nanocomposites

A Nylon organoclay nanocomposite was prepared and analyzed to demonstrate how infrared absorption measurements can provide information about the organoclay dispersion state. For organoclay polymer composites, the less complex absorption spectra of hectorite clay is particularly preferred when polymer absorption bands coincide with the region of interest, and need to be subtracted or deconvoluted from the absorption pattern to isolate the clay Si-O stretching contribution. Nylon-6 nanocomposites prepared from aminododecanoic acid-exchanged clay and caprolactam are well-studied and understood. It is generally accepted that the aforementioned organoclay will delaminate when polymerizing the caprolactam monomer [13-16, 25]. The XRD pattern of our Nylon 6/12 composite prepared with protonated 12-aminododecanoic acid hectorite clay shows a marginal amount of polymer intercalated organoclay (Fig. 6). The in-plane IR absorption band shown in Fig. 7 deconvolutes at 29 [cm.sup.-1], indicating that a substantial amount of organoclay is delaminated. According to the curve shown in Fig. 5, about 80% of all clay layers are delaminated. The IR technique is not very sensitive at these very high degrees of organoclay delamination, but the result correlates well with the observed XRD signature, and we can now, for the first time, estimate how many layers have been delaminated in a Nylon organoclay nanocomposite.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The IR absorption method is more sensitive at lower degrees of organoclay delamination. Three BENTONE 108-polyethylene (LDPE) composites were made; one melt compounded and two solvent cast prepared under different conditions. XRD results shown in Fig. 8 indicate that the melt compounded composite is not intercalated with polyethylene, whereas the solvent cast composites are intercalated. Inspection of the in-plane absorption bandwidths illustrated in Fig. 9 shows values of 60.3, 41.4, and 36.9 [cm.sup.-1], respectively, which, according to Fig. 5, corresponds to about 5, 25, and 35% organoclay exfoliation. We use this method of bandwidth analysis to fine tune twin screw extruder processing parameters when compounding organo-clays in plastics, and the details will be presented in a future publication.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Layer Orientation in Organoclay Nanocomposites

Vibrations in a molecular bond give rise only to an infrared light absorption if the dipole moment of the bond changes during vibration [32]. The dipole moment is related to, but does not necessarily coincide with, the direction of the bond. The absorption intensity will be greatest when infrared light is polarized in the same direction as the oscillating dipole, but will diminish when the light is polarized at an angle towards the dipole. The absorption intensity depends on the square of the cosine of the angle between the absorbing dipole and the polarization direction of the infrared light beam. Thus, for a clay polymer composite sample with aligned clay layers, clay silicate bonds will absorb infrared light that is polarized in one direction, with a different efficiency than light polarized in a different direction. This absorption anisotropy, or dichroism, can be used to calculate a dichroic ratio, which can be correlated to the orientation of an absorbing bond. This approach can be used to estimate the average degree of clay layer alignment in clay containing systems. Loo and Gleason [25] used a combination of IR trichroic analysis and TEM to determine the orientation distribution function of montmorillonite platelets in Nylon-6 nanocomposite films.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Dichroic values can also be used to compare the relative degrees of clay platelet alignment for different samples, without assigning absolute values to the degree of orientation. We illustrate this approach with our second solvent cast BENTONE 108-LDPE composite, in which the in-plane absorption bandwidth was 36.9 [cm.sup.-1], indicating about 35% exfoliated clay layers. The remaining clay layers are still stacked in tactoids and agglomerates. To obtain dichroic information for the in-plane and out-of-plane Si-O bonds, we measure the absorption of infrared light polarized in one direction, and then we measure the absorption of light that is polarized in a perpendicular direction. The absorption ratio is the dichroic ratio. If a clay composite sample has layer alignment, the dichroic ratio will depend on how the sample is oriented with respect to the incident beam. Hence, a unique dichroic ratio can be derived for any sample tilt angle. However, if a sample has randomly oriented layers, then the dichroic ratio equals unity and is invariant with sample position.

A thin film of the BENTONE 108-LDPE composite was melt pressed. The majority of the clay layers are assumed to be aligned to some extent in this sample, as it is known that clay layers preferentially align themselves during film pressing. Next, infrared absorption spectra were measured for this sample, using light polarized along the y-axis and then the z-axis. The y,z plane of IR polarization in this dichroism experiment initially coincides with the plane of the film sample (at 0[degrees] rotation) to establish the initial relative intensities of the in-plane and out-of-plane absorption bands. The film is then rotated around the y axis to observe the changes in band intensity for z- and y-polarized IR radiation. The sample was tilted 0[degrees], 15[degrees], 30[degrees], 45[degrees], or 60[degrees] around the y-axis. Figure 10 shows the geometric set-up for these polarized experiments. The absorption spectra for polarized light in the y- and z-axis directions are shown, respectively, in Figs. 11 and 12. Tilting of the sample while the light is polarized in the y-direction does not notably alter the infrared absorption as the sample orientation changes. However, when the light is polarized around the z-axis and the film orientation is incrementally changed from 0[degrees] to 60[degrees] a decrease in absorption intensity is noted for the in-plane absorption band (around 1000 [cm.sup.-1]), while an increase in absorption is observed for the out-of-plane absorption band (around 1080 [cm.sup.-1]). This absorption anisotropy is ascribed to clay layer orientation within the polyethylene composite film. Note that the Si-O stretch bandwidth does not vary with the light polarization direction or with the sample orientation, since the bandwidth depends only on the degree of clay nanodispersion, and this does not change when tilting the sample.

Some of the same LDPE composite material was ground to a fine powder and then mixed with KBr and pressed into a pellet. A minimum amount of KBr was used to prepare a self supporting pellet with a thickness so that the resulting Si-O bands have an absorption intensity of 0.7 absorbance unit or less. This pellet sample is assumed to have clay layers that are randomly oriented. As before, the absorption spectra were recorded using polarized light while tilting the sample, and the dichroic ratios were calculated from the intensities. Figure 13 shows dichroic ratios for the in-plane (1000 [cm.sup.-1]) absorption band as a function of the sample tilting angle for both the aligned and the random sample. Figure 14 shows this for the out-of-plane (1080 [cm.sup.-1]) absorption band.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

As expected, the dichroic ratio for the randomly oriented sample does not change much from unity when tilting the sample, particularly for the 1000 [cm.sup.-1] band. The 1080 [cm.sup.-1] absorption band does show some degree of orientation, as the dichroic ratio changes a little upon sample tilting. However, the changes in the dichroic ratio are far larger for the aligned clay layer sample, indicating that this sample has a much higher degree of layer alignment. Dichroic ratios calculated from absorption band areas rather than absorption intensities show similar results. Aforementioned plots can be used to assess relative clay layer alignment differences between samples. We have seen interesting and unexpected results when analyzing clay layer orientation via this method.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

CONCLUSIONS

A quick and easy to use analytical method that is able to measure the degree of organoclay delamination and layer alignment in a polymer--organoclay nanocomposite is very desirable. By applying the IR bandwidth and dichroism methods described in this paper to the in-plane and out-of-plane Si-O vibrations of polymer-clay nanocomposites, it is possible to estimate the degree of clay exfoliation and compare clay layer alignment in a simple manner [33]. Furthermore, these methods may allow the practitioner to tune process parameters to obtain optimum quality and consistent results when compounding organoclay polymer nanocomposites.

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Wouter L. IJdo, Steven Kemnetz, Daphne Benderly

Elementis Specialties, Inc., 329 Wyckoffs Mill Road, Hightstown, New Jersey 08520

Correspondence to: Wouter L. IJdo; e-mail: wouter.ijdo@elementis-na.com
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Date:Aug 1, 2006
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