Biodegradable polylactide and its nanocomposites: Opening a new dimension for plastics and composites

The academic and industrial aspects of the preparation, characterization, mechanical and materials properties, crystallization behavior, melt reheology, and foam procesing of pure polylactide (PLA) and PLA/layered silicate nanocomposites are described in this feature article. Recently, these materials have attracted considerable interest in polymer science research. PLA is linear aliphatic theromplastic polyester and is made from agricultural products. Hectorite and montmorillonite are among the most commonly used smectite type layered silicated for the preparation of nanocomposites. Smectites are a valuable mineral class for industrail 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 overcome this difficulty is to replace interlay cations with quaternized ammonium or phosphonium cations, preferably with long alkyl chains. In general, polymer/layered silicate nanocomposites are of three different types; (1) intercalated nanocomposites, in which insertion of polymer chains into the layered silicate structure occurs in a crystollographically regular fashion, regardless of polymer to layered silicate ratio, with a repeat distance of few nanometer; (2) flocculated nanocomposites, in which intercalated and stacked silicate layers are sometimes flocculated due to the hydroxylated edge-edge interactions between the silicate layers; (3) exfoliated nanocomposites, in which individual silicate layers are uniformly distributed in the polymer matrix by average distances that totally depend on the layered silicate loading. This new family of composite materials frequently exhibits remarkable improvements in its material properties when compared with those of virgin PLA. Improved properties can include a high storage modulus both in the solid and melt states, increased flexural properties, a decrease in gas permeability, increased heat distortion temperature, an increase in the rate of biodegradability of pure PLA, and so forth.