Effect of clay/water ratio during bentonite clay organophilization on the characteristics of the organobentonites and its polypropylene nanocomposites.

INTRODUCTION

Polymer nanocomposites based on layered silicate reinforcing materials continue to attract considerable attention (1). The most commonly employed layered silicate for the preparation of polymeric nanocomposites has been montmorillonite, the main constituent of bentonite clays (2). Economic advantages and environmental concerns are among the reasons for the interest in bentonites, widely distributed around the world, as components for polymer-based nanocomposites. Bentonites are sedimentary rocks consisting clay minerals, such as montmorillonite (major constituent), beidellite, saponite, nontronite, hectorite, and nonclay minerals (3). Montmorillonites are 2:1 layered clay minerals with a triple-sheet sandwich structure consisting of a central, alumina-dominated octahedral sheet, bonded to two silica tetrahedral sheets by shared oxygen ions. The unit cell of this ideal structure has a composition [[Al.sub.2][(OH).sub.2]([Si.sub.2][O.sub.5]).sub.2].sub.2]] with a molar mass of 720 g/mol. Isomorphic substitution of [Al.sup.3+] in the octahedral sheets by [Mg.sup.2+], [Fe.sup.2+], [Mn.sup.2+], or [Li.sup.+], or less frequently of [Si.sup.4+] by [Al.sup.3+] in the tetrahedral sheet, results in a net negative charge that is compensated by the presence of cations, such as [Na.sup.+], [K.sup.+], [Ca.sup.2+], or [Mg.sup.2+], sorbed between the layers and surrounding the edges (4-6). These loosely held cations do not belong to the crystal structure and can be readily exchanged by other cations. The cation exchange capacity (CEC) of montmorillonite ranges from 0.8 to 1.2 meq/g (4), resulting in 0.6-0.9 exchangeable cations per unit cell. The layers organize themselves to form stacks with a regular gap between them, the interlayer space or gallery. The electrostatic and Van der Waals forces holding the layers together are relatively weak, and the interlayer distance varies depending on the radius of the cation present and its degree of hydration. A typical dry montmorillonite has a layer thickness around 0.96 nm and a lateral dimension of the order of hundreds of nanometers (7-9).

The environment in the galleries of pristine clay is highly hydrophilic and too narrow to allow the efficient penetration of polymer molecules (2). Thus, the interlayer space should be enlarged and made relatively hydrophobic or organophilic. This is usually accomplished by exchanging the loose gallery inorganic cations with organic intercalants. The most common intercalants are quaternary ammonium surfactants bearing at least one relatively long hydrocarbon chain (10). When the resulting organoclays are dispersed into a polymer matrix, further intercalation may occur, which, under ideal circumstances, lead to a complete separation of the layers or exfoliation. The end result is a very efficient reinforcing effect, because clay layers are highly anisometric and a relatively small amount of clay gives rise to a very large number of particles, with a correspondingly large surface area. Consequently, less than 5 wt% of clay can produce significant improvements in mechanical and barrier properties, fire retardancy, etc., with little adverse effect on transparency and density (1).

Surface treatment of the mineral clay is a critical step in the preparation of polymer-clay nanocomposites, to ensure the dispersion of the clay within the polymer matrix (11), (12). The structure and properties of the organoclay affect clay/polymer interactions and determine the extent of intercalation and/or exfoliation in the final nanocomposite. These complex colloidal systems were the subject of several researches (13-15).

To gain a thorough understanding of the parameters that affect the dispersion of the clay particles into the polymer matrix, the purpose of this work is to study the effect of the clay/water ratio used during bentonite clay organophilization with a quaternary ammonium salt on the structure of the resulting organobentonite and on the dispersion characteristics of polypropylene nanocomposites prepared with them. Work in progress in our laboratories will extend the investigation to other aspects of the organophilization process.

EXPERIMENTAL

Materials

A sodium bentonite clay (Argel 35), code AN, provided by Bentonit Uniao Nordeste, Campina Grande- PB, Brazil, in powdered form, with particle size [less than or equal to]74 [micro]m, was used in this work as received. The CEC of this bentonite was determined by the method described by Phelps and Harris (16) and found to be 0.92 meq/g. The most important clay mineral present in the Argel bentonite is a sodium montmorillonite (around 63%) as determined by XRD-6000 software. It also contains a minor fraction of nonlayered minerals such as quartz, kaolinite, and carbonates. The quaternary ammonium salt, cetyl trimethyl ammonium bromide, [C.sub.16][H.sub.33][([CH.sub.3]).sub.3]NBr, commercially known as cetrimide, from Vetec (Brazil), was used to modify the bentonite.

Polybond 3200 from Crompton Corporation (USA), a maleated polypropylene (PP-g-MA) resin, with a melt flow index 115 dg/min (190[degrees]C/2.16 kgf). density 0.91 g/[cm.sup.3], melting point 160-170[degrees]C, and with 1 wt% maleic anhydride content, was used as polymer matrix for nanocomposites preparation.

Clay Modification

Natural bentonite AN was organically modified by ionexchange reaction with the cetrimide using 16 g of the natural bentonite clay and 7.5 g of cetrimide mixed with different amounts of distilled water. Volumes between 300 and 1600 mL were used, resulting in bentonite concentrations (x) varying between 5.33 and 1.00 g clay per 100 mL water. The same bentonite/cetrimide ratio was maintained for all samples. The content of surfactant (cetrimide) was 140% of the mineral CEC.

After 30 min mechanical stirring at 75[degrees]C [+ or -] 5[degrees]C, the suspension was allowed to stand for 24 h at ambient temperature before filtering and washing with distilled water to remove excess bromine anions; dried at 60[degrees]C for another 24 h, the clay agglomerates were broken down with a mortar and pestle and sifted through an ISO 200 screen. Samples of the natural bentonite, AN, and the organophilized bentonites prepared with different amounts of water, ANO-x, where x was the approximate bentonite concentration in % w/v, were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TG), and X-ray diffraction (XRD).

Preparation of the Polymer Nanocomposites

PP-g-MA/organobentonite hybrids, containing 2 wt% organically modified bentonite, were processed in laboratory batch mixer (Haake Rheomix 600) operating with roller type rotors at 170[degrees]C and 60 rpm for 10 min. Before compounding, PP-g-MA was dried in a vacuum oven at 80[degrees]C for 24 h. Hybrids prepared with ANO-x organobentonite samples were coded ANOP-x. Immediately after processing, hybrids for XRD characterization were molded in a hot press at 180[degrees]C for 2 min as dise-shape samples with 25 mm diameter and 1 mm thickness.

Characterization of the Natural and Organically Modified Bentonites and Nanocomposites

Infrared Spectroscopy. IR characterization was conducted using a Nicolet Avatar TM 360 Fourier transform infrared spectrometer operating in the range of 400-4000 [cm.sup.-1]. KBr/bentonite pressed disks were used for bentonite and organobentonite characterization.

Thermogravimetry. TG characterization of bentonite and organobentonites were performed on a Shimadzu TG apparatus, SIHA model, with samples of about 15 mg of material. The samples were heated at 10[degrees]C/min under an atmosphere of air, 50 mL/min. Weight loss curves were recorded from 30 to 900[degrees]C.

X-Ray Diffraction. XRD analysis was conducted at room temperature on a Shimadzu XRD-6000 X-ray generator, operating at 40 kV and 30 mA, and the X-ray beam was monochromatized to [lambda] ([CuK.sub.[alpha]]) = 0.154056 nm wave length. The samples were scanned for an interval of 20 between 1.5 and 10.0[degrees], at a rate of 2[degrees] per minute. XRD plots were used to determine the mean interlayer spacing of the basal plane ([d.sub.001]) of natural and organophilic bentonites, and in the PP-g-MA hybrids using Bragg's law:

[d.sub.00n] = [[n[lambda]]/[2sin[theta]]], (1)

where n is an integer, [theta] is the angle of incidence (or reflection) of the X-ray beam, and [lambda] is the X-ray wavelength (0.154 nm). For the principal reflection, n = 1.

XRD measurements were also used to calculate the degree of exfoliation ([X.sub.E]) and the average number of layers (m). According to Ishida et al. (17) and Utracki (4), the degree of exfoliation can be obtained by comparing the peak area (A) of the hybrid to the peak area of the corresponding organoclay ([A.sub.0]):

[X.sub.E] = 1 - [A/[A.sub.0]], (2)

Utracki (4) quotes an expression to relate, in a semiquantitative way, the peak broadening, measured by the peak width at half-height, [[DELTA][[theta].sub.1/2], in radians, to the average thickness h of the tactoids (multilayered clay particles):

h = [[k[lambda]]/[[DELTA][0.sub.[1/2]]cos[theta]]], (3)

where [theta] is the peak location, [lambda] is radiation wavelength, and k [approximately equal to] 0.9. Eliminating [lambda] between Eqs. 1 and 3 we obtain:

h = [[2k[d.sub.001]tan[theta]]/[[DELTA][[theta].sub.[1/2]]]]. (4)

For small angles, the average number of layers m in a tactoid is given by:

m = 1 + [h/[d.sub.001]][approximately equal to]1 + [1.8/[[DELTA][[theta].sub.[1/2]]]]. (5)

Thus, m depends only on peak width.

RESULTS AND DISCUSSION

Infrared Spectroscopy

Figure 1 shows the typical IR spectrograms in the range of 4000-400 [cm.sup.-1] wavenumber for the surfactant (cetrimide), natural bentonite, AN, and organically modified bentonite samples, prepared with 1, 2, and 4% clay in water, coded ANO-1, ANO-2, and ANO-4, respectively.

[FIGURE 1 OMITTED]

All the samples show an absorption band near 3630 [cm.sup.-1] attributed to the OH stretching, corresponding to structural hydroxyl groups of the clay, and a 3432 [cm.sup.-1] band of the OH stretching, corresponding absorbed water. The organic salt cetrimide, used in the preparation of the organobentonites, shows characteristic absorption bands at 3020 [cm.sup.-1], corresponding to CH stretching; at 2920 and 2856 [cm.sup.-1], corresponding to asymmetric and symmetric vibration modes of the [CH.sub.2] group; and at 1480 [cm.sup.-1],corresponding to asymmetric vibration modes of the group CH (18), (19). The negligible absorption band at 2920 [cm.sup.-1] present in the AN sample may be attributed to organic contaminants (20). The significant "organic groups" bands present in the modified bentonites strongly suggest that the organic cation was successfully incorporated to the bentonite, and that organobentonites were in fact prepared for all bentonite/water ratios studied.

Thermogravimetry

Figure 2 shows the typical TG plots of weight loss as a function of temperature in the range of 30-1000[degrees]C for samples of unmodified bentonite (AN) and organically modified bentonites, ANO-l, ANO-2, and ANO-4. Decomposition steps for these and other samples are shown in Table 1.

[FIGURE 2 OMITTED]

TABLE 1. Thermal decomposition steps for unmodified bentonite (AN) and
organically modified bentonites (ANO-x).

                                    Dehydration

Sample          x (g/100 mL)  [T.water] ([degrees]C)  Water (wt%)

AN                    -                 74.0             6.67
ANO-1               1.00                53.4             2.21

ANO-2               2.00                42.5             1.11

ANO-3.2             3.20                48.8             2.83

ANO-3.6             3.56                40.2             2.35

ANO-4               4.00                63.7             3.77

ANO-4.6             4.60                51.0             2.35

ANO-5.3             5.33                58.8             3.33

Pure cetrimide

                         Surfactant decomposition

Sample        x      [T.sub.0]    [T.sub.max]  Organic  Dehydroxylation
           (g/100  ([degrees]C)  ([degrees]C)    part     [T.sub.OH]
             mL)                                (wt%)    ([degrees]C)

AN         -                                     0           720

ANO-1      1.00         244           273       20.46        633

ANO-2      2.00         237           268       20.92        634

ANO-3.2    3.20         249           280       20.03        643

ANO-3.6    3.56         239           270       21.25        640

ANO-4      4.00         248           281       21.18        643

ANO-4.6    4.60         240           271       21.67        637

ANO-5.3    5.33         246           276       21.48        637

Pure                                  283
cetrimide

Unmodified bentonite displays two thermal degradation transitions. The first one ([TH.sub.2]O) occurred at a temperature of 74[degrees]C and is attributed to the volatilization of both the free water (i.e., the water sorbed on the external surfaces of crystals) and the water residing inside the interlayer space, which form hydration spheres around the exchangeable cations. The second transition ([T.sub.OH]) took place at higher temperature (720[degrees]C) and is attributed to the loss of structural water resulting from the dehydroxylation of clay (21-24).

Unmodified bentonite contains a large quantity of water in the interlayer spacing because of the strongly hydrated sodium cations intercalated within the clay layers. The amount of water in the galleries is reduced in the organobentonites, reflecting the weaker hydration of the organic cations. The temperature [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] required for the complete volatilization of this water is reduced upon organophilization and is not affected by the clay/water ratio utilized during the preparation of the organobentonites (Table 1). The alkylammonium cation lowers the surface energy of the inorganic material, converting the hydrophilic silicate surface to an organophilic one (4), (20).

Organically modified bentonites display an additional transition at intermediate temperatures, attributed to the decomposition of organic matter, in this case the organic surfactant cations. Table 1 shows the onset decomposition temperature [T.sub.0] of the organophilized bentonites prepared with different amounts of water. [T.sub.0] is defined as the temperature at which 2% mass loss occurs. [T.sub.0] values (~240[degrees]C) were practically unchanged for all clay/water concentration studied.

The initial surfactant decomposition temperature is an important parameter when polymer nanocomposites are prepared with these organoclays by a melt intercalation process. If the processing temperature is higher than the thermal stability of the organoclay, salt decomposition will occur, reducing the affinity between the polymer and the filler surface. The weak interactions between these two components may influence the thermomechanical properties of the hybrids (25), (26).

The temperature [T.sub.max] at the maximum rate of weight loss (peak in DTG) is also shown in Table 1. The thermal stability of organobentonites was not affected by the clay/ water concentration utilized during their preparation.

The decomposition of the intercalated alkylammonium occurs at around 275[degrees]C, and the fraction total of the organic part in the organobentonites, determined by the total weight loss from 200 to 500[degrees]C, was 20-21%, resulting in a surfactant concentration equivalent to 95-100% of the clay CEC. The decomposition occurred in one step. According to Marras et al. (21), this indicates that the surfactant is mainly adsorbed inside the interlayer spaces through cation exchange process only.

As expected, by comparison with the natural clay, organobentonites displayed a lower dehydroxylation temperature ([T.sub.OH]), which is attributed to the lower relative amount of inorganic material in the organobentonite (21).

X-Ray Diffraction

Figure 3 shows the typical diffractograms with 2[theta] in the range of 1.5[degrees]-10[degrees] for samples of unmodified bentonite (AN) and organobentonites, modified as 1, 2, and 4% w/v clay/water suspensions (ANO-1, ANO-2, and ANO-4). Values of the interlayer spacing ([d.sub.001]) for all samples prepared are displayed in Table 2.

[FIGURE 3 OMITTED]

TABLE 2. XRD interlayer spacing for unmodified bentonite (AN) and
organically modified bentonites (ANO-.x).

                      Interlayer spacing [d.sub.001] (nm)

Sample   x (g/100mL)  Main peak  Shoulder 1  Shoulder 2

AN            -         1.34          -           -
ANO-1       1.00        1.85          -           -
ANO-2       2.00        1.89        2.81        1.46
ANO-3.2     1.20        1.91        2.81        1.45
ANO-3.6     3.56        1.87        2.80        1.42
ANO-4       4.00        1.91        2.87        1.43
ANO-4.6     4.60        1.90        2.96        1.45
ANO-5.3     5.30        1.85        2.70        1.41

According to Fig. 3 and Table 2, the interlayer spacing increased from the 1.34 nm in the neat bentonite (AN) to 1.85-1.91 nm in the modified bentonites, an increase of ~40%. This is taken as an indication that the interaction between clay and surfactant depends on the proportion surfactant present in the interlayer space (21). The clay/water ratio utilized during the preparation of the organobentonite scarcely affected the amount of surfactant taken up by the bentonite, since the [d.sub.001] values for all organobentonites were practically the same. These results agree with TG data (Table 1), showing that the total amount of cetrimide cation incorporated into the clay is fairly constant for all clay/water concentrations investigated.

Our data also show that for bentonite concentration (x) above 1.00 g/100 mL, the main peak of organobentonite samples is flanked by two weaker shoulder peaks (Fig. 3 and Table 2). The basal spacing corresponding to the shoulder peak I was 2.70-2.96 nm, about 50% larger than the spacing corresponding to the main peak, whereas that of the shoulder peak 2 was 1.41-1.46 nm, about 23% lower than the main peak. Again, the interlayer spacing ([d.sub.001]) remained almost constant for all clay/water ratios utilized during the preparation of the organoclays.

This constancy, however, does not extend to the area under the peaks. Peak areas were estimated using an apparent baseline as in the example shown in Fig. 4 and noted as [A.sub.0] (main peak), [A.sub.1] (shoulder peak at higher [d.sub.001]), and [A.sub.2] (shoulder peak at lower [d.sub.001]). Areas were measured in arbitrary but consistent units; values for several organobentonite samples (ANO-x) are included in Table 3. It was found that the areas under the peaks increase with clay concentration in water.

[FIGURE 4 OMITTED]

TABLE 3. XRD peak areas of organicallv modiiied henlonite samples.

                      Area under peak (arbitrary units)

Sample   x (g/100mL)  Main peak  Shoulder 1  Shoulder 2

ANO-1       1.00          628        0.0         0.0
ANO-2       2.00          592        5.4         7.0
ANO-3.2     3.20          514       21.0        14.7
ANO-3.6     3.56         1412       26.7        35.2
ANO-4       4.00         1231       27.9        16.2
ANO-4.6     4.60         1278       27.6        39.5
ANO-5.3     5.30         1327       31.9        27.5

In summary, the degree of swelling of the organobentonites--reflected by the basal spacing, as shown in Table 2--is practically independent of the clay/water ratio, within the range investigated in this work, but the areas under the peaks are significantly affected by clay concentration.

Figure 5 displays the areas versus the bentonite concentration x (g bentonite/100 mL water) for the main peak (a) and the shoulder peaks (b).

[FIGURE 5 OMITTED]

Because the total amount of cetrimide cation incorporated into the clays is fairly constant, as shown by the TG results and the same applied to the spacing between the clay layers, as seen by XRD measurements, the decrease in area may be attributed to structural changes in the organically modified bentonites. According to the literature (13-15), the decrease in XRD peak area indicates a lower number of platelets per tactoid as the clay concentration decreases. Our data are in agreement with the thermodynamic predictions.

The disorderly structure and partial exfoliation of the clay in water dispersion is retained as the organobentonite is dried, as evinced by the data in Table 3 and Fig. 5, taken on dry samples, stored sometimes for months. These results agree with the data reported by Benna et al. (27). To check if the different structures in the organobentonite have some effect on hybrids prepared with these materials, they were compounded with maleated polypropylene in a batch mixer. Compounds, with 2% organobentonite nominal content, derived from organically modified bentonites (ANO-x) were coded as ANOP-x Figure 6 shows the XR diffractograms of PP-g-MA hybrids made with organobentonites prepared at different clay/water concentration, i.e., 1, 2, and 4 g bentonite in 100 mL water (ANOP-1, ANOP-2, and ANOP-4).

[FIGURE 6 OMITTED]

The presence of multiple peaks in XR diffractograms is quite common and often originates from different organoclay structures and incomplete exchange during incorporation in a polymeric matrix (4), (23). Interlayer spacing [d.sub.001] obtained with Bragg's law, area under the main peak, and peak width at half-height are shown in Table 4. Peak areas are measured in the same (arbitrary) units as the areas under the peaks in the organobentonites XRD (Table 3), based on an apparent baseline that was also used to determine peak heights and thus peak widths at half-height.

TABLE 4. XRD peak characteristics of PP-g-MA/organobentonite hybrids.

Sample  x (g/l00     Interlayer      Area under peak   Peak width at
           mL)         spacing      (arbitrary units)   half-height
                  [d.sub.001] (nm)                      ([degrees])

ANOP-1    1.00          4.20                91             0.64
ANOP-2    2.00          3.15               242             0.50
ANOP-4    4.00          3.77               263             0.41

Comparison of hybrid and organobentonite diffractograms shows that hybrids have shorter, broader peaks with significantly larger and more variable interlayer spacing. These facts indicate a significant intercalation of polymer molecules in the space between the bentonite layers, and perhaps an incipient exfoliation of the tactoids. Figure 7 shows the X-ray diffractograms for ANO-4 and ANOP-4 samples, smoothed and plotted in the same scale.

[FIGURE 7 OMITTED]

The important point is the trend toward larger interlayer spacing, smaller areas, and relatively broader peaks as the clay/water ratio used to prepare the organobentonites diminishes. XRD peak broadening has many, complex explanations (4); in this case, it probably reflects the disorder of the crystalline structure of the clay.

Values of the degree of exfoliation and the average number of layers per tactoid computed based on the data in Tables 3 and 4 using Eqs. 2 and 5 are presented in Table 5. There is a clear trend toward a better dispersion of organobentonites prepared with a lower clay/water ratio.

TABLE 5. Degree of exfoliation and number of layers per tactoid for PP-
g-MA/organobentonite hybrids estimated from XRD.

Sample  x (g/l00 mL)  Degree of exfoliation  Number of layers per
                           ([X.sub.E])            tactoid (m)

ANOP-1      1.00              0.86                   163
ANOP-2      2.00              0.59                   207
ANOP-4      4.00              0.78                   254

The subtle structural changes observed in the organobentonites prepared in systems with different bentonite concentrations (but with the same bentonite/organic salt ratio) carry on and are more evident in the hybrids made with these organobentonites, resulting in better dispersion for lower clay/water ratios. These structural changes are not reflected in the amount of cation substitution (all samples show almost complete saturation) or the interlayer spacing in the organobentonites (virtually uniform), but in something that may be associated with a permanent (or at least durable) disorder of the clay layered structure.

CONCLUSIONS

In this work, it was found that the clay/water ratio employed during bentonite organophilization affects the structure of the organobentonites produced. The use of lower clay/water ratio (1 g bentonite/100 mL water) during the organophilization process of bentonite clay with cetrimide resulted in more disorderly structures. At higher water content, the diluted suspension probably has particles almost exfoliated into isolated layers. Better clay dispersion was obtained in composites of PP-g-MA prepared with these organobentonites.

ACKNOWLEDGMENTS

The authors thank Bentonit Uniao Nordeste (BUN) for the donation of the bentonite clay and Crompton Corporation for the donation of the maleated polypropylene.

REFERENCES

(1.) K.C. Cole, Macromolecules, 41, 834 (2008).

(2.) V. Mittal, J. Colloid Interface Sci., 315, 135 (2007).

(3.) M. Onal and Y. Sarikaya, Powder Technol., 172, 14 (2007).

(4.) L.A. Utracki, Clay-Containing Polymeric Nanocomposites, Vol. 1, RAPRA Technology, Shawbury, UK (2004).

(5.) X. Peng, Z. Luan, F. Chen, B. Tian, and Z. Jia, Desalination, 174, 135 (2005).

(6.) F. Salles, O. Bildestein, J.M. Douillard, M. Jullien, and H.V. Damme, J. Phys. Chem. C. 111, 13170 (2007).

(7.) W. Lertwimolnun and B. Vergnes, Polymer, 46, 3462 (2005).

(8.) S. Kaufhold, Appl. Clay Sci., 34, 14 (2006).

(9.) H.H. Murray, Appl. Clay Sci., 17, 207 (2000).

(10.) C. Manzi-Nshuti and C.A. Wilkie, Polym. Degrad. Stab., 92, 1803 (2007).

(11.) J. Grandjean, J. Bujdak, and P. Komadel, Clay Miner., 38, 367 (2003).

(12.) L.Q. Wang, J. Liu, G.J. Exarhos, K.Y. Flanigan, and R. Bordia, J. Phys. Chem. B. 104, 2810 (2000).

(13.) A. Shalkevich, A. Stradner, S.K. Bhat, F. Muller, and P. Schurtenberger, Langmuir, 23, 3570 (2007).

(14.) D.L. Ho. R.M. Briber, and C.J. Glinka, Chem. Mater., 13, 1923 (2001).

(15.) J.C.P. Gabriel, C. Sanchez, and P. Davidson, J. Phys. Chem., 100, 11139 (1996).

(16.) G.W. Phelps and D.L. Harris, Am. Ceram. Soc. Bull., 47, 1146 (1968).

(17.) H. Ishida, S. Campbell, and J. Blackwell, Chem. Mater., 12, 1260 (2000).

(18.) J. Madejova, Vib. Spectrose., 31, 1 (2003).

(19.) M. Kozak and L. Domka, J. Phys. Chem. Solids, 65, 441 (2004).

(20.) N.B. Colthup, L.H. Daly, and S.E. Wiberley, Introduction to Infrared and Raman and Spectroscopy, Academic Press, New York (1990).

(21.) S.I. Marras, A. Tsimpliaraki, I. Zuburtikudis, and C. Panayiotou, J. Colloid Interface Sci., 315, 520 (2007).

(22.) B. Bolto, D. Dixon, R. Eldridge, and S. King, Water Res., 35, 2669 (2001).

(23.) W. Xie, Z. Gao, W.P. Pan, D. Hunter, A. Singh, and R. Vaia, Chem, Mater., 13, 2979 (2001).

(24.) F.G. Ramos Filho, T.J.A. Melo, M.S. Rabello, and S.M.L. Silva, Polym. Degrad. Stab., 89, 383 (2005).

(25.) A. Pozsgay, T. Frater, L. Szazdi, P. Muller, L Sajo, and B. Pukanszky, Eur. Polym. J., 40, 27 (2004).

(26.) S.I. Marras, I. Zuburtikudis, and C. Panayiotou, Eur. Polym. J., 43, 2191 (2007).

(27.) M. Benna, N. Kbir-Ariguib, C. Clinard, and F. Bergaya, Appl, Clay Sci., 19, 103 (2001).

Suedina M.L. Silva, (1) Paolo E.R. Araujo, (1) Kaline M. Ferreira, (1) Eduardo L. Canedo, (1) Laura H. Carvalho, (1) Claudia M.O. Raposo (2)

(1) Department of Materials Engineering, Federal University of Campina Grande, Campina Grande 58109-970, PB, Brazil

(2) Department of Mining and Geology, Federal University of Campina Grande, Campina Grande 58109-970, PB, Brazil

Correspondence to: Suedina Silva: e-mail: [e-mail]

Contract grant sponsors: RENAMI, CNPq (Pronex FAPESQ/MCT/CNPq), PIBIC, PQ.

DOI 10.1002/pen.21399