Polymer/layered silicate nanocomposites: A review from preparation to processing

Abstract

A review is given of the academic and industrial aspects of the preparation, characterization, materials properties, crystallization behavior, melt rheology, and processing of polymer/layered silicate nanocomposites. These materials are attracting considerable interest in polymer science research. Hectorite and montmorillonite are among the most commonly used smectite-type layered silicates for the preparation of nanocomposites. Smectites are a valuable mineral class for industrial applications because of their high cation exchange capacities, surface area, surface reactivity, adsorptive properties, and, in the case of hectorite, high viscosity and transparency in solution. In their pristine form they are hydrophilic in nature, and this property makes them very difficult to disperse into a polymer matrix. The most common way to remove this difficulty is to replace interlayer cations with quarternized ammonium or phosphonium cations, preferably with long alkyl chains. A wide range of polymer matrices is covered in this review, with special emphasis on biodegradable polymers. In general, polymer/layered silicate nanocomposites are of three different types, namely (1) intercalated nanocomposites, for which insertion of polymer chains into a layered silicate structure occurs in a crystallographically regular fashion, with a repeat distance of few nanometers, regardless of polymer to clay ratio, (2) flocculated nanocomposites, for which intercalated and stacked silicate layers flocculated to some extent due to the hydroxylated edge-edge interactions of the silicate layers, and (3) exfoliated nanocomposites, for which the individual silicate layers are separated in the polymer matrix by average distances that depend only on the clay loading. This new family of composite materials frequently exhibits remarkable improvements of material properties when compared with the matrix polymers alone or conventional micro- and macro-composite materials. Improvements can include a high storage modulus, both in solid and melt states, increased tensile and flexural properties, a decrease in gas permeability and flammability, increased heat distortion temperature, an increase in the biodegradability rate of biodegradable polymers, and so forth. © 2003 Elsevier Ltd. All rights reserved.