Elsevier

Applied Clay Science

Volume 47, Issues 3–4, February 2010, Pages 235-241
Applied Clay Science

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

https://doi.org/10.1016/j.clay.2009.10.015Get rights and content

Abstract

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.

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 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, Eberl et al., 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 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.

Section snippets

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.

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

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 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

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), with higher concentrations of atoms near the silicate surface, in the

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

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 their help in optimization protocol. The manuscript benefitted from constructive reviews by Victor Drits and by Dennis D. Eberl and from comments of Gerhard Lagaly. We are also grateful to Stephan Hlohowskyj for checking the English.

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