The structure of poly(γ-glutamic acid)/nanoclay hybrids compatibilized by alkylammonium surfactants

Stoichiometric ionic complexes of bacterial poly(γ-glutamic acid) with the cationic N-octadecyl and N-docosyl-N,N,N-trimethylammonium surfactants were mixed with alkylammonium modified bentonite (Cloisite 30B) to generate hybrid compounds covering a wide range of clay/polymer compositions. The structure of these compounds was examined by XRD and cryoTEM, which indicated extensive penetration of the biopolymer into the nanoclay galleries with widening or even destruction of the layered arrangement of the complex and the clay. Simultaneous SAXS/WAXS analysis in real time showed that hybrids with moderate inorganic contents displayed melting/crystallization upon heating/cooling within the 40–70 °C interval with concomitant expansion/contraction of the intersheet spacing. 13C CP-MAS NMR revealed that an anti-to-gauche conformational interconversion of the alkyl chains was involved in the melting/crystallization process. The results provided by energy-based dynamical simulations proved the feasibility of the polyacid and the nanoclay to be homogenized in the hybrid due to the compatibilizing effect of the surfactant.