One-dimensional structure of exfoliated polymer-layered silicate nanocomposites: A polyvinylpyrrolidone (PVP) case study

Applied Clay Science 47 (2010) 235–241

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l a y

One-dimensional structure of exfoliated polymer-layered silicate nanocomposites: A polyvinylpyrrolidone (PVP) case study Marek Szczerba a,⁎, Jan Środoń a, Michał Skiba b, Arkadiusz Derkowski a a b

Institute of Geological Sciences, Polish Academy of Sciences, ul. Senacka 1, 31-002 Kraków, Poland Institute of Geological Sciences, Jagiellonian University, ul. Oleandry 2A, 30-063 Kraków, Poland

a r t i c l e

i n f o

Article history: Received 19 March 2009 Received in revised form 12 October 2009 Accepted 14 October 2009 Available online 28 October 2009 Keywords: Polyvinylpyrrolidone (PVP) Smectite X-ray diffraction

a b s t r a c t Classical modeling of powdered lamellar structures by X-ray diffraction (XRD) only applies to periodic or quasi-periodic structures and another approach is needed for non-periodic structures. XRD patterns of the non-periodic structures contain a relatively small amount of information and therefore certain initial assumptions are necessary. In case of exfoliated polymer–clay mineral nanocomposites it is possible to assume the chemical composition and the structure of the clay mineral layer while the actual structure of the polymer remains unknown. This paper offers an approach which can be used to provide an approximate solution for the structure of the polymer. This new approach is based on modeling of the LpG2 factors, recorded from oriented samples in order to obtain the one-dimensional structure of the polymer. Although LpG2 factors for various smectites are quite different, in the case of polyvinylpyrrolidone (PVP) adsorbed on smectite it was found that the structure of the polymer was insignificantly affected by charge in the tetrahedral or octahedral positions of the smectite. The electron density distribution models of PVP directly adsorbed on the smectite layer suggest that PVP chains directly bound to the surface are more rigid and organized than the molecules occurring farther away. The approximate thickness and distribution of PVP layer adsorbed on the surface was calculated to be equal ca. 5–6 Å. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Polymer-layered silicate nanocomposites (PLSNs) are of great interest currently due to substantial improvement in mechanic, thermal, and other properties with respect to pure polymers (Koo et al., 2003). PLSNs consist of a polymer which is bound to a layered silicate, usually smectite. In order to explain the properties of PLSNs it is important to understand the structure of the polymer and the way it is adsorbed on the surface of a mineral. X-ray diffraction (XRD) and Infrared spectroscopy (IR) are the methods usually most helpful in such studies (e.g. Deng et al., 2006). The exact structure of polymer cannot be investigated by standard XRD techniques because PLSNs are not periodic structures in a strict crystallographic sense. Some periodicity is observed, however, as it is the case for intercalated smectites where the distances between smectitic layers are generally similar. Therefore, the distance between smectitic layers and the thickness of polymer layer can be evaluated from XRD. Most of the PLSNs are, however, not periodically intercalated structures. Often PLSNs are randomly intercalated or simply exfoliated smectites, with the polymer structure very far from a perfect order (e.g. Alexandre and Dubois, 2000). Such phases cannot be modeled using conventional

⁎ Corresponding author. Tel.: +48 12 4221910; fax: +48 12 4221609. E-mail address: [email protected] (M. Szczerba). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.10.015

XRD calculations, because the polymer molecule positions, polymer layer thickness, and smectite particles orientation can be variable or are unknown. An additional complication arises if there are water molecules involved in the system, since they can affect the distance between layers and organization of the polymer molecules. This study offers a new approach which can be helpful to overcome some of the aforementioned problems. The proposed methodology was tested on polyvinylpyrrolidone (PVP), which is a polymer broadly used along with clay minerals as PLSN. PVP is also of interest to geochemists, soil scientists and clay mineralogists due to its specific interactions with clay minerals. It stabilizes colloidal clay particles (Séquaris et al., 2000), as adsorbent it is intercalated into clay mineral particles or leads to their exfoliation allowing to quantify smectite (Levy and Francis, 1975), to measure the thickness distribution of fundamental particles (Eberl et al., 1996, 1998; Uhlik et al., 2000) and to quantify the total specific surface area and content of smectite (Blum and Eberl, 2004). Modeling of the structure of PVP adsorbed on clay minerals can significantly improve the applications of these methods. PVP contains a carbonic backbone which pyrrolidine rings are attached to (Fig. 1). A segment has the following chemical composition: C6H9NO, which corresponds to the overall composition of the polymer. PVP is a nonionic polymer with amphiphilic character (Sun and King, 1996). PVP is commercially available as a hygroscopic (it absorbs atmospheric water up to 40% of its mass) white solid of

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M. Szczerba et al. / Applied Clay Science 47 (2010) 235–241 Table 1 Structural formulae of smectites used in the experiment. Sample

Mineral

Chemical composition

Kinney (Eberl et al., 1998) Garfield (Środoń and McCarty, 2008) Ferruginous Smectite (Środoń and McCarty, 2008) Black Jack (Weir and Greene-Kelly, 1962)

Montmorillonite

Na0.48(Al1.50Mg0.43Fe0.08) (Si3.92Al0.08O10)(OH)2 Na0.53(Fe1.98Mg0.03) (Si3.46Al0.54O10)(OH)2 Na0.46(Fe1.20Al0.65Mg0.13) (Si3.74Al0.26O10)(OH)2

Nontronite Intermediate nontronite– montmorillonite Beidellite

Na0.49(Al1.96Mg0.02Fe0.04) (Si3.46Al0.54O10)(OH)2

Fig. 1. The structure of polyvinylpyrrolidone (PVP).

1.2 g/cm³ density and of molar mass from 10,000 to 1,200,000, which correspond to the chain lengths of 90 to 11,000 segments. The studies on adsorption of PVP on smectites have shown that the interfacial conformation of polymer depends mainly on the chain length (Séquaris et al., 2000) and on the type of exchangeable cation (Séquaris et al., 2002). When quantifying the amount of polymer trains (parts of polymer chains that are bound directly to the surface) on the smectitic surface, higher molar mass PVP forms longer tail and loops (Séquaris et al., 2000). Blum and Eberl (2004) found that PVP forms a directly and strongly bound layer on the smectite surface in amounts ranging from 0.61 (MW of 10,000) to 0.76 g PVP per gram of smectite (MW of 360,000). Considering small differences between these values, they suggested that the coverage of the smectitic surface by the polymer remains nearly constant, indicating that the PVP chains are lying parallel to the surface. Additionally PVP adsorption is not affected by the charge density of smectitic layer, or by location of the charge (Blum and Eberl, 2004). The mass of adsorbed polymer, however, depends strongly on the concentration of smectite in dispersion and on the type of counterion on smectite (Blum and Eberl, 2004). Low enthalpy of the PVP displacement and of the adsorption on smectitic surfaces suggests that the polymer is bound to the surface by weak physisorption, largely with hydrophobic contributions (Francis, 1973; Séquaris et al., 2000). However the exact mechanism of PVP bonding to smectite is still not clear. Substantial amounts of water in the structure were found because the thickness of PVP intercalated smectite decreased after heating to 110 °C (Francis, 1973). When considering the thickness of the PVP layer on smectitic surface there is no agreement between the investigators. The molecular thickness of the PVP monomer is 5.6 Å (Francis and Levy, 1975). However, Francis (1973) calculated the thickness of PVP layer as 6 Å, Séquaris et al. (2000) obtained c.a. 7.5 Å, while Blum and Eberl (2004) suggested 11 Å. Despite the long history of the investigations of PVP adsorption on smectites, the structure and the mechanism of this adsorption still are not clear. The aim of this paper is to present a new approach, which can be used to study exfoliated PLSNs, and to give new insight into the structure and thickness of the PVP layer on the smectite basal surface. This study is based on the XRD techniques, which assume that adsorbed molecules have periodic positions, organized order, and are describable with crystallographic formalism, which contribute to the PLSNs diffraction effects. These molecules represent all the bound PVP, although other PVP molecules may exist as well between the exfoliated smectite particles. 2. Materials and methods 2.1. Smectite samples A wide range of smectites; montmorillonite, beidellite, and nontronite (Table 1) was chosen for modeling of PVP adsorption on

smectites. The <0.2 μm fraction was separated to avoid contamination with phases other than smectite. Data for the Kinney smectite were taken from Eberl et al. (1998) for comparison. 2.2. XRD patterns PVP-smectite exfoliated samples were prepared by a solution intercalation procedure. Diluted dispersions (2.5 mg clay mineral/ 1 ml of distilled water) of Na+ saturated smectites, were mixed with aqueous solutions of PVP (molecular weight = 55,000; 5 mg PVP/1 ml of distilled water; source of reagent: Sigma-Aldrich) in the proportion of 25 mg PVP to 2.5 mg smectite (modified after Dudek et al., 2002). The dispersions were sonified for approximately 1 min using a TECHPAN UD II ultrasonic probe and were stored overnight. A small aliquot of each dispersion was deposited on 2 cm · 4 cm polished Siwafer cut perpendicularly to (100) and dried at 60 to 90 °C. Then the samples were dried at 200 °C for 1 h to remove the adsorbed water. During preliminarily studies it was found that the system PVP+ smectite slowly readsorbed atmospheric water. Samples were analyzed from 2 to 40° 2θ using a Thermo ARL XRD system, CuKα radiation source, and a Peltier-cooled solid-state detector. The tube current and voltage were 45 mA and 35 kV, respectively. The following slit sizes from tube to detector were used; 0.9 mm (0.645°) Soller, 1.05 mm sample, 1.0 mm Soller, 0.3 mm sample. The step size was 0.05° and the counting time was 30 s per step. To remove the background resulting from the excess PVP and the Siwafer the LpG2 factor for PVP-smectite was calculated as a difference between the XRD patterns for PVP-smectite and for PVP on the Si substrate (after Eberl et al., 1998), the latter scaled so that the resulting LpG2 intensities at higher 2θ angles (>36° 2θ) reached zero. 2.3. Computer program A Java computer program was written to calculate the one-dimensional structure of the polymer on the surface of smectite. The program uses genetic algorithms (GA) as an error minimization procedure (e.g. Koza, 1992), to optimize the structure by minimizing the differences between the experimental LpG2 factor and the theoretical X-ray pattern of smectite layer with a hypothetical structure of polymer on its basal surfaces. The theoretical one-dimensional X-ray pattern is calculated as follows (Moore and Reynolds, 1997): N

J

Ið2θÞ = Lpð2θÞ × ∑n = 1 ∑j

j j = 1 Pn ðzn Þfn ð2θÞ cosð4πzn

sin 2θ = λÞ

ð1Þ

where: Lp(2θ) is the Lorentz-polarization factor, Pn(zjn) is the number of atoms n in a plane located at the position zjn along z direction perpendicular to the smectitic layer, fn(2θ) is the atomic factor of the atom n at 2θ angle corrected for thermal vibrations, λ—X-ray wavelength. The first sum was taken over all the atoms (N), the second overall J positions of the atom n. The zn values for the atoms of the

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silicate layer were taken from Moore and Reynolds (1997). The atomic factor, fn(2θ), was calculated as follows: −Bðsin2θ = λÞ

fn ð2θÞ = fCM e

ð2Þ

where: B is the Debye–Waller temperature factor, and the atomic factor fCM was calculated from Cromer and Mann (1968). The following values of B for clay layer were accepted after Moore and Reynolds (1997): 1.5 for metals and Si of silicate layer and 2.0 for oxygen of the silicate layer. For the polymer the value was set to 11.0, which is the value accepted for ethylene glycol and water molecules in the interlayer region (Moore and Reynolds, 1997). For Na+ ions the value of B was set to 6.0, assuming that Na+ cation is less flexible than atoms of the polymer but also more flexible than the atoms of clay layer. This was confirmed by preliminary molecular dynamics calculations (not presented here). The Lorentz-polarization factor was calculated from the equation of Reynolds (1986; Eq. 17) which incorporates the values of σ * which is the mean degree of particle orientation and degrees of primary and secondary soller slit opening. The values of σ * were assumed to equal 4, 6 or 12 (after: Reynolds, 1986). It was found that the best solution for σ * was 12. The properties of soller slits were taken from the known geometry of the X-ray diffractometer. The theoretical patterns were corrected for intensity loss at low diffraction angles caused by limited sample length. The length of irradiated area can be calculated from the following equation (Moore and Reynolds, 1997): LB =

R0 α sinθ

ð3Þ

where: R0 is the goniometer radius and α is the angular aperture of the divergent slit. For the angles at which LB exceeded the length of sample, the intensities were multiplied by the sample length and divided by LB. Because the samples were not infinitely thick the theoretical patterns were also corrected for the absorption. The intensities were multiplied by (Moore and Reynolds, 1997):

⌊


 −2μ*g 1− exp sinθ

⌋

ð4Þ

where: μ * is the sample mass absorption coefficient, g is the sample mass in grams per square centimeter. μ * was calculated according to

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Klug and Alexander (1974) from the average chemical formula of smectite and PVP, and from known proportions of these two compounds in the XRD samples. g was obtained from the known volume and density of solution that was settled on the wafer (see Moore and Reynolds, 1997). The atomic scattering factors for oxygen, nitrogen and carbon have very similar 2θ dependances, so it is difficult to independently calculate the numbers of all these atoms (Pn in Eq. (1)) from the LpG2 factor. Oxygen and hydrogen atoms can originate either from the PVP polymer or from water molecules. The quantity of water, however, is difficult to constrain because some amount of atmospheric water can still be adsorbed by PVP. Moreover, PVP can have preferred orientation with respect to the surface, which also can affect the distribution of different atoms at particular distances from the surface. Therefore the polymer layer was calculated only by the sum of all these atoms at distances z (in Å) from the centre of the smectite layer (PC(z)). Carbon was selected as a representative atom of PVP and water in this distribution function because of its dominance in the structure. Therefore in Eq. (1) fn(2θ) of carbon and PC(z) was used as a representation of the polymer. Thus PC(z) corresponds to the number of atoms of carbon representing the same diffracting power as the molecules of PVP and water occupying a given volume. As discrete zn values cannot be used to characterize the polymerwater layer, PC(z) has to be treated as a distribution. The programming requires the distance in the z direction to be discontinuous. It was found that the shape of X-ray pattern changes insignificantly if the distribution of atoms is divided into values of 0.2 Å or smaller, therefore, 0.2 Å values were used for the PC(z) calculations. For simplicity the distribution of atoms was calculated as a sum of Gaussian distributions located at integer distances from the smectite surface using the following equation:

U

PC ðzÞ = ∑u

= u1 Au exp

−ðz−uÞ2 2σ 2

! ð5Þ

where: Au is the participation in the total sum of a particular Gaussian distribution, located at the integer distance u, with the variance of σ2; u1 is the position of the centre of the Gaussian distribution that is the closest to the smectitic surface; U is the position of the farthest centre of the Gaussian distribution from the smectitic surface. The value of

Fig. 2. Distribution of atoms of PVP (black squares and lines) along the z direction on both sides of a smectite layer, calculated as a sum of Gaussian distributions of atoms (grey square and lines) located at integer distances. A distribution of atoms is divided into ranges of 0.2 Å. Lines are shown only to guide the reader.

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M. Szczerba et al. / Applied Clay Science 47 (2010) 235–241

Fig. 3. Experimental LpG2 factors for PVP-smectites: Garfield, Ferruginous Smectite, Black Jack and Kinney (the latter from Eberl et al., 1998), respectively.

σ 2 was set 0.5, which was constrained by trial and error method to obtain the most reliable PC(z) distributions. It was assumed that this distribution should not be ragged, σ2 too small, and should not overlap too much, σ2 too large. An example of such a distribution is presented in Fig. 2. The distribution of Na+ ions on the smectitic surface was calculated as the sum of Gaussian distributions, but the range of u values was limited only to the closest distances from the smectitic surface. The program optimizes the values of Au using genetic algorithms to minimize the difference between the experimental LpG2 factors and the one-dimensional theoretical X-ray pattern of smectite with a hypothetical structure of PVP on its surface. The genetic algorithms are based on concepts inherited from evolutionary biology. The parameters undergoing optimization are denoted as genes in the populations of chromosomes that reproduce, crossover and undergo mutations (e.g. Koza, 1992). In this study, genes are the values of Au. In order to check the quality of the results (genes), a phenotype function F is defined:

During preliminary calculations it was found that quite acceptable fits between the theoretical and the experimental X-ray patterns for particular ranges of 2θ are possible for different distributions of atoms along the z direction. These distributions can differ by the calculated mass of PVP per gram of smectite. This parameter can be measured experimentally, which offers a chance to constrain the model. To account for the constrained mass of polymer per mass of smectite, the phenotype function was redefined to: 2

F = 1000 = ð1 + SÞ–Dðm−MÞ

where: D is a factor which defines how far m should be from M for the best solution (e.g. if D is large m should be relatively close to M), M is the constrained mass of polymer per mass of smectite and m is the mass of polymer per mass of smectite calculated from the theoretical distribution of atoms:

m= F = 1000 = ð1 + SÞ

ð6Þ

where: I

2

S = ∑ ðti −yi Þ i=1

ð7Þ

ti is a particular point of the experimental X-ray pattern, and yi—a particular point of the theoretical X-ray pattern. The sum is taken over selected range of points (I) corresponding to selected range of 2θ of theoretical and experimental diffractograms. This is simply a converse of the function used by the least squares method. For the best solutions, with the theoretical X-ray pattern close to the experimental one, the value of S tends to zero, and the function F reaches the maximum close to 1000. The intensities of the theoretical X-ray pattern were multiplied by an additional optimized parameter to obtain the best fit with the experimental pattern.

ð8Þ

∑zz =

z1 PC ðzÞ × AW C N ∑n = 1 ∑Jj = 1 Pn ðzjn Þ × AW n

ð9Þ

where: AWn is the atomic mass of n atom (in the numerator C corresponds to carbon); z1 is the position of the closest atoms of polymer to the smectitic surface, Z is the position of the farthest atoms of polymer from the smectitic surface. In the numerator the sum was taken over all atoms of polymer, while in the denominator over all atoms of smectite plus Na+ ions. 3. Results 3.1. Experimental LpG2 factor for PVP-smectites The overall shapes of LpG2 for smectite with PVP on its surface are quite similar for all the samples (Fig. 3) and they correspond to the data of Eberl et al. (1998). The pattern of PVP-Kinney was scaled to obtain similar intensities as the rest of PVP-smectites. The first peak visible at ca. 10° 2θ seems to arise from LpG2, because exfoliation

Fig. 4. Comparison of the experimental (thick black line) and the theoretical (thick grey line) LpG2 factors for a) Black Jack, b) Ferruginous and c) Garfield smectites with PVP on basal surfaces. LpG2 factors for smectites without PVP are also shown (thin grey lines). The distributions of PVP with respect to the smectite layer surfaces are presented in the small windows (see: Fig. 2). The grey thick line corresponds to the distribution of Na+.

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was confirmed by the lack of peaks at angles lower than 5° 2θ. For nontronites the reflection at 16° 2θ is less intense than the peak at 27° 2θ, which is opposite to what was observed for other smectites. The optimization of LpG2 was performed using the range from 6 to 50° 2θ. At lower angles the background is relatively high, and there can be strongly influenced by randomly thick supercrystallites (Plançon, 2002), or peaks from the interference function (Blum and Eberl, 2004). The range below 6° 2θ was checked if the shape of the theoretical LpG2 corresponds approximately to the experimental LpG2. 3.2. Optimization of the PVP-smectite structure based on XRD patterns From preliminarily molecular dynamics calculations (not presented here) it was found that if there is no water in the system when Na+ ions are located near the silicate surface in the hexagonal cavities of the tetrahedral sheet. If a larger amount of water (a monolayer or more) is present in the system, then Na+ ions are usually taken out of these cavities but not farther than 1–2 Å from the surface. This corresponds to the results of Skipper et al. (2006), who found the first layer of hydrated Na+ at 1.7 Å distance from the smectite surface. Francis (1973) found that PVP can partially displace cations on the surface of smectite. Therefore, the relative concentration of Na+ ions on the smectite surface can be difficult to constrain. Assuming that there could be water in the system (Francis, 1973) the amounts of Na+ were optimized in the range of 0–2 Å from the smectitic surface. This takes into account that the ratio between Na+ and the smectites should not exceed the ratio derived the chemical formula of each particular mineral (Table 1). These distributions are not very well constrained because changes in concentration of Na+ ions in the assumed ranges have only slight influence on the LpG2. For the purpose of calculations it was assumed that the adsorption density of PVP on smectite 001 surfaces (M in Eq. (8)) is 0.61 g of PVP per gram of smectite. Blum and Eberl (2004) obtained 0.61 g (MW of 10,000), 0.72 g (MW of 55,000) and 0.76 g (MW of 360,000) per gram of smectite. However for longer chain length polymers, longer tails and loops were found not always located on the 001 surface. Moreover, it can be assumed that some part of polymer can be extended or wrapped over the edges, as it is the case for polyethylene oxide (Nelson and Cosgrove, 2004a,b). Therefore, the value of 0.61 g/g was selected as the most probable value. The program optimized the values of Au along the z direction to minimize the difference between the experimental and the theoretical X-ray pattern of smectite with a hypothetical structure of PVP on its surface. The 4 to 20 Å range of individual Gaussian distribution locations (value u from Eq. (5)) for PVP was used in the optimization procedure. The resulting distributions of PVP on the surfaces of particular smectites, along with comparison between theoretical and experimental LpG2 factors for σ * are 12 (Fig. 4). At the initial step of optimization it was assumed that M equals 0.61 g/g and D (Eq. (8)) was set to be higher than zero. But in the next step of optimization D was set to zero, which indicates that there were no assumptions on the amount of PVP on the smectitic surface. For the final solutions presented in Fig. 4, the following values of adsorption density were obtained: 0.60 for Garfield, 0.69 for Ferruginous Smectite and 0.72 g/g for Black Jack smectites. 4. Discussion 4.1. PVP structure on smectites Although LpG2 factors of smectites are quite different, the polymer exhibits a relatively similar structure (Fig. 4). Thus, the differences in chemical composition of the smectitic layers, mainly in the presence of iron in the octahedral sheet are the main factor responsible for the differences in LpG2 factors. The thickness of the PVP layer is found to be generally about 5–6 Å (half-width of the distributions in Fig. 4),

Fig. 5. A representation of possible distribution of PVP between adjacent smectitic layers.

with higher concentrations of atoms near the silicate surface, in the first 3 Å of the layer. This corresponds to the results of Francis (1973), Francis and Levy (1975) and Séquaris et al. (2000), who found that the PVP layer thickness is 6 to 7.5 Å. The decrease in the apparent concentration of atoms at distances farther from the smectitic surface could be interpreted in two ways; 1) The concentration of atoms is higher at the part of the PVP layer closest to the smectite surface, decreasing farther from the smectitic surface. 2) The thermal vibrations of atoms in the outer polymer layers are relatively intense, i.e. the atoms of polymer directly bound to the silicate surface are more rigid (more ordered), than the outer parts of the polymer density function. The second explanation, however, seems to be more reasonable. There is definitely a large quantity of the polymer present at distances farther than 12–13 Å, in exfoliated PLSNs. Lower quantities of the polymer at z higher than 13 Å suggest that the polymer affects the diffraction profile of PVP-smectite much less than at closer distances due to its lower order of distribution (Fig. 4). The excess polymer occurs in a random distribution and is responsible for contribution to the background. Polymer trains adsorbed to the surface may at least partially increase the concentration of atoms at distances immediate to the clay mineral layer (denser packing of the PVP molecules). The outer, more flexible parts of the polymer may contribute less to the diffraction effect, thus producing lower concentration of atoms along the z axis. A schematic model showing possible distribution of PVP between adjacent layers is presented in Fig. 5. Similarity of the one-dimensional structures of PVP on different samples of smectites confirms the results of Blum and Eberl (2004), who found that PVP adsorbs in similar way on smectites of different layer charge. The configuration of PVP on surface is not affected by the location of this charge in the tetrahedral or octahedral positions. Thus, the generalized configuration of PVP from Fig. 4 can be used as a model for the other surfaces of smectitic character, e.g. surface of illite fundamental particles (c.f. Eberl et al., 1998). This is very remarkable that the quantity of PVP corresponding to ∼11 Å in the periodic structure (Blum and Eberl, 2004) is distributed over a much longer range in the exfoliated structure. The electron density distribution of PVP adsorbed between two smectite layers (as producing 23.5 Å periodicity, Blum and Eberl, 2004) should have a bimodal shape, assuming two monolayers of PVP at ∼ 6 Å each.

M. Szczerba et al. / Applied Clay Science 47 (2010) 235–241

Smectitic charge provided from both sides of the PVP component must result with a high ordering of PVP molecules. In case of exfoliation, the electron density distribution yields a lognormal shape because the influence of the opposite, parallel smectite surface is negligible due to too high interparticle distance. The model presented covers only the strongly bound PVP molecules which are ordered in a crystallographically-describable structure and produce diffraction effects. Theoretically, some molecules occurring between the exfoliated smectite particles, beyond the modeled XRD electron density function, may still be considered as “adsorbed”, depending on the influence of smectitic charge on their position and orientation. As the operational definition, all the weakly and non-clay mineral-bound PVP molecules can be removed by centrifugation at 10,000 rpm for 4 h in water, and they do not contribute to the PLSNs diffraction pattern (Blum and Eberl, 2004). 4.2. Remarks on methodology The presented methodology can be applied to any PLSN which can be obtained by the exfoliation of smectite and for which the oriented specimens can be prepared. It has some advantages over classical Xray diffraction of intercalated nanocomposites. The approximate layer thickness of the adsorbed polymer can be obtained without any assumptions about thickness, or using the diffraction effect from various populations of interference function values. As a disadvantage the LpG2 factor for exfoliated nanocomposites contains a relatively small amount of information. Thus some assumptions about the structure of the clay mineral layer, the polymer, and the average polymer/smectite mass ratio should be made. In spite of these disadvantages the presented methodology can provide valuable information about the structure of a polymer on the smectite surface. Many PLSNs do not display periodicity in the crystallographic sense, and the XRD structure solution cannot be applied. This methodology can be modified easily to be applied to any other layered mineral with an adsorbed polymer on its surface. 5. Conclusions In this study an alternative approach to the structural investigations of exfoliated polymers was developed. This method is based on modeling of the LpG2 factors, recorded from oriented samples. It allows solving an approximate one-dimensional structure of polymer adsorbed on the surface of smectite, but requires an approximate assumption about the polymer/smectite mass ratio. In this experiment PVP was shown to form a layer of about 5–6 Å thickness on the 001 surface of smectite with higher concentrations of atoms in the first 3 Å of the layer. PVP chains that are directly bound to the surface appear to be more rigid, while the outer parts are more flexible. This effect may enhance of the concentration of atoms at close distances to the clay mineral layer. Acknowledgments This paper reports a part of the PhD thesis of Marek Szczerba directed by Jan Środoń. The authors thank Victor Drits for his very helpful comments and to Kamil Szastak and Sebastian Szastak for

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