Polymer-layered silicate nanocomposites: an overview

Abstract

An overview of polymer–clay hybrid nanocomposites is provided with emphasis placed on the use of alkylammonium exchanged smectite clays as the reinforcement phase in selected polymer matrices. A few weight percent loading of organoclay in nylon 6 boosts the heat distortion temperature by 80°C, making possible structural applications under conditions where the pristine polymer would normally fail. A similar loading of clay nanolayers in elastomeric epoxy and polyurethane matrices dramatically improves both the toughness and the tensile properties of these thermoset systems. Glassy epoxy nanocomposites exhibit substantial improvement in yield strength and modulus under compressive stress–strain conditions. The latest development in polypropylene hybrids have yielded nanocomposites with improved storage moduli. Polyimide hybrids in thin-film form display a 10-fold decrease in permeability toward water vapor at 2 wt.% clay loading. In situ and melt intercalation processing methods are effective in producing reinforced polystyrene hybrids. Nitrile rubber hybrids show improved storage moduli and reduced permeabilities even toward gases as small as hydrogen. Poly(ε-caprolactone)–clay nanocomposites prepared by in situ polymerization of ε-caprolactone in organoclay galleries show a substantial reduction in water adsorption. Polysiloxane nanocomposites produced from poly(dimethylsiloxane) and organoclay mixtures have improved in tensile properties, thermal stability and resistance to swelling solvents. Organoclay-poly(l-lactide) composite film was obtained by solvent casting technique. Clay nanolayers dispersed in liquid crystals act as structure directors and form hybrids composites that can be switched from being highly opaque to highly transparent by applying an electric field of short duration.

Introduction

Composites that exhibit a change in composition and structure over a nanometer length scale have been shown over the last 10 years to afford remarkable property enhancements relative to conventionally-scaled composites (Schmidt, 1985; Novak, 1993; Mark, 1996). Layered silicates dispersed as a reinforcing phase in an engineering polymer matrix are one of the most important forms of such “hybrid organic–inorganic nanocomposites” (Okada and Usuki, 1995; Giannelis, 1996; Ogawa and Kuroda, 1997). Although the high aspect ratio of silicate nanolayers is ideal for reinforcement, the nanolayers are not easily dispersed in most polymers due to their preferred face-to-face stacking in agglomerated tactoids. Dispersion of the tactoids into discrete monolayers is further hindered by the intrinsic incompatibility of hydrophilic layered silicates and hydrophobic engineering plastics. However, as was first demonstrated by the Toyota group more than 10 years ago (Fukushima and Inagaki, 1987), the replacement of the inorganic exchange cations in the galleries of the native clay by alkylammonium surfactants can compatibilize the surface chemistry of the clay and the hydrophobic polymer matrix. ε-Caprolactam was polymerized in the interlayer gallery region of the organoclay to form a true nylon 6–clay nanocomposite (Usuki et al., 1993a, Usuki et al., 1993b). At a loading of only 4.2 wt.% clay, the modulus doubled, the strength increased more than 50%, and the heat distortion temperature increased by 80°C compared to the pristine polymer (see Table 1). They also demonstrated that organoclays exfoliated in a nylon 6 polymer matrix greatly improved the dimensional stability, the barrier properties and even the flame retardant properties (Kojima et al., 1993a, Kojima et al., 1993b; Gilman et al., 1997). More significantly, these composites have been in use in under-the-hood applications in the automobile industry (Okada and Usuki, 1995).

The use of organoclays as precursors to nanocomposite formation has been extended into various polymer systems including epoxys, polyurethanes, polyimides, nitrile rubber, polyesters, polypropylene, polystyrene and polysiloxanes, among others. For true nanocomposites, the clay nanolayers must be uniformly dispersed (exfoliated) in the polymer matrix, as opposed to being aggregated as tactoids or simply intercalated (see Fig. 1). Once nanolayer exfoliation has been achieved, the improvement in properties can be manifested as an increase in tensile properties, as well as enhanced barrier properties, decreased solvent uptake, increased thermal stability and flame retardance (Okada and Usuki, 1995; Giannelis, 1996).

The complete dispersion of clay nanolayers in a polymer optimizes the number of available reinforcing elements for carrying an applied load and deflecting cracks. The coupling between the tremendous surface area of the clay (∼760 m²/g) and the polymer matrix, facilitates stress transfer to the reinforcement phase, allowing for such tensile and toughening improvements. Conventional polymer–clay composites containing aggregated nanolayer tactoids ordinarily improve rigidity, but they often sacrifice strength, elongation and toughness. However, exfoliated clay nanocomposites, such as those that have been achieved for nylon 6 and epoxy systems, have to the contrary shown improvements in all aspects of their mechanical performance. High aspect ratio nanolayers also provide properties that are not possible for larger-scaled composites. The impermeable clay layers mandate a tortuous pathway for a permeant to transverse the nanocomposite (Fig. 2). The enhanced barrier characteristics, chemical resistance, reduced solvent uptake and flame retardance of clay–polymer nanocomposites all benefit from the hindered diffusion pathways through the nanocomposite.