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Polymer/Layered Silicate Nanocomposites
Smithers Information Ltd, Sep 2003, Pages: 166
Polymer/clay nanocomposites have received a lot of attention over the last decade. Companies such as Nanocor and Honeywell are already commercially exploiting nanocomposite materials. A small amount of nanodispersed filler leads to an improvement in material properties, such as modulus, strength, heat resistance, flame retardancy and lowered gas permeability. Adding clay nanofillers to biodegradable polymers has also been shown to enhance compostability.
The enhancement of material properties has been linked to the interfacial interaction between the polymer matrix and the organically modified layered silicate filler structure. The filler particles provide a very high surface area.
Montmorillonite, hectorite and saponite are the most commonly used layered silicates. For a nanocomposite to be formed successfully, the mineral must disperse into separate layers. The surface chemistry is also important - ion exchange reactions with cations (commonly alkyl ammonium or alkyl phosphonium cations) allow the silicate to be compatibilised with the polymer matrix. The strong interactions between the two materials leads to dispersion at the nanometre level.
Polymer/layered silicate nanocomposites are prepared by a variety of routes. One of the first materials, a Nylon 6 nanocomposite, was prepared by in situ polymerisation of e-caprolactam in a dispersion of montomorrillonite. The silicate can be dispersed in liquid monomer or a solution of monomer. It has also been possible to melt-mix polymers with layered silicates, avoiding the use of organic solvents. The latter method permits the use of conventional processing techniques such as injection moulding and extrusion.
Nanocomposites have been formed with a wide variety of polymers including: epoxy, polyurethane, polyetherimide, poybenzoxazine, polypropylene, polystyrene, polymethyl methacrylate, polycaprolactone, polyacrylontrile, polyvinyl pyrrolidone, polyethylene glycol, polyvinylidene fluoride, polybutadiene, copolymers and liquid crystalline polymers. Summaries of the work carried out on these different materials and references to these studies are included in this Rapra Review Report.
Many studies have been carried out to characterise different nanocomposites. Techniques in use include wide-angle X-ray diffraction and transmission electron microscopy.
Processing techniques are critical in polymer manufacturing and this holds true for nanocomposites. Several processing methods and innovative techniques are discussed. For example, Nylon 6 clay nanocomposites have been electrospun from solution, which resulted in highly aligned clay particles.
Two other types of nanofiller are briefly described here. Polyhedral oligomeric silsesquioxane (POSS) nanoparticles combine organic and inorganic segments with nanosized cage structures. Carbon nanotubes have also been examined as they offer unique mechanical and electrical properties.
This review is accompanied by around 400 abstracts from papers and books in the Rapra Polymer Library database, to facilitate further reading on this subject. A subject index and a company index are included. The majority of these references are cited in the review, which is exceptionally well referenced.
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