Kaolinite--poly(methacrylamide) intercalated nanocomposite via in situ polymerization.(Report)

Introduction

A variety of organic--inorganic nanohybrid materials have been prepared and characterized in recent years (1). Among them, polymer nanocomposites, such as polymer--layered silicate (PLS), have become an effective alternative to conventional polymer composites in many applications. They exhibit enhanced mechanical and thermal properties, decreased gas permeability, improved flame-retardation properties, and increased solvent resistance (2,3). Smectites have been widely used for the preparation of PLS nanocomposites. In general, two types of polymer--layered clay nanocomposites can be obtained, intercalates and exfoliates (4--7). Melt intercalation is the most common method of synthesis of PLS nanocomposites, as it is applicable to a wide range of polymers. Other strategies include in situ polymerization of a previously intercalated monomer and solution polymerization.

Smectites are the most used class of phyllosilicates for the elaboration of nanohybrid and nanocomposite materials due to their swelling properties, their high exchange capacity, and surface area. On the other hand, the interlayer chemistry of kaolinite (Kao), one of the most abundant clay minerals on Earth, is much less developed.

Kaolinite is a 1:1 layered dioctahedral aluminosilicate, characterized by the chemical composition [Al.sub.2][Si.sub.2][O.sub.5](OH)4 (8a). The individual layers are linked together through hydrogen bonds between the aluminol groups of the octahedral sheet on one side, and the siloxane macrorings of the tetrahedral sheets on the other side. They are also linked by the strong dipole interactions between the non-centrosymmetric layers (8b). These structural characteristics result in interlayer spaces that are not easily accessible by guest molecules. Consequently, kaolinite is not yet commonly used as the inorganic component of nanocomposites.

However, a few examples of intercalated kaolinite-polymer nanocomposites have been reported so far (9--18a). Direct intercalation from the melt was achieved in a few cases (9, 13). Other kaolinite--polymer nanocomposites were obtained either by the intercalation of the monomer followed by polymerization (10, 11, 14, 15, 17) or by direct intercalation of the polymer using a guest-displacement method (16).

In this paper, the synthesis of a new kaolinite-methacrylamide intercalation compound was achieved by direct treatment of an intermediate with an aqueous solution of methacrylamide. The surface modification of kaolinite particles by (trimethoxysilyl)propylmethacrylate (A174) followed by copolymerization with methacrylamide acid monomers has been reported (18b). However, in this work, for the first time, a kaolinite--poly(methacrylamide) intercalated nanocomposite was synthesized by in situ polycondensation of the pre-intercalated methacrylamide monomer. Methacrylamide (Chart 1) was first intercalated into kaolinite by a guest-displacement method, using kaolinite--dimethylsulfoxide (Kao--DMSO) as an effective intermediate. The intercalated monomer adopted a monolayer arrangement between the layers, confirmed by XRD analysis. Free-radical polymerization of methacrylamide was reported in the interlamellar spaces of Na-montmorillonite (19). Herein, the polymerization was achieved by thermal treatment of kaolinite-- methacrylamide intercalation compound at ~150 [degrees]C for 2 h in air atmosphere. [sup.13]C MAS NMR analysis confirmed the polymerization with the presence of residual monomeric species in the interlamellar space. The polymerization product had a high thermal stability, compared with the kaolinite-methacrylamide intercalation compound, as evidenced be thermogravimetric analysis. Analyses results confirmed the feasibility of the synthesis of an intercalated Kao--polymer nanocomposite, using in situ polymerization strategy.

Experimental

Chemicals and reagents

All chemicals used were of reagent-grade quality and were not further purified. Methacrylamide ([C.sub.4][H.sub.7]NO, 98%) was obtained from Sigma-Aldrich and was used without further purification. Well-crystallized kaolinite from Georgia (labeled KGa-1b) was obtained from the Source Clays Repository of the Clay minerals Society (Purdue University, West Lafayette, Indiana, USA). This was purified according to previously reported sedimentation techniques (20), and the < 2 [micro]m size fraction was used.

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Analysis and characterization …