Nanocomposites of Microbial Polyglutamic Acid and Nanoclays Compatibilized by Organophosphonium Surfactants

Abstract

Ionic coupling of bacterial poly(γ-glutamic acid) (PGGA) with alkyltrimethylphosphonium surfactants is reported to render comb-like PGGA complexes with a biphasic-layered structure displaying periodicity at the nanoscopic scale. In this work, nanocomposites made of PGGA and montmorillonite, and covering a wide variety of compositions, are prepared by using either dodecyl- or eicosyl-trimethylphosphonium surfactant as a third component with a double purpose, that is, ionic complexation of PGGA and organo-modification of the nanoclay. Thermogravimetric analysis ascertains the compositions of these three-component nanocomposites and evidences their excellent thermal stability. The nanocomposite structure with PGGA and clay more or less intercalated according to composition and thermal history is evidenced by both X-ray diffraction and transmission electron microscopy. Simultaneous thermal SAXS/WAXS analysis at real time reveals extensive intermixing of the two phases that become notably enhanced by heating treatment. Thermal transition characteristics of the surfactant–PGGA complex are not significantly altered in the nanocomposites, but elastic moduli and strength to yield are found to increase proportionally to the content of clay.