Unusual crystallization behavior of organoclay reinforced poly(L-lactic acid) nanocomposites

The effect of organically modified montmorillonite clay addition on crystallization of a polymer matrix has been studied. Two types of commercially available organoclays with different extent of miscibility with a polymer matrix were employed, leading to fully exfoliated (high miscibility) and intercalated (low miscibility) nanocomposite morphologies. The nanocomposites were fabricated via the exfoliation-adsorption technique with the biodegradable polyester matrix polymer poly(L-lactic acid) (PLLA). As-cast nanocomposite films were melted and isothermally crystallized at different temperatures. Bulk kinetics studies and radial spherulite growth rates indicate that when a high degree of filler-polymer matrix miscibility is present, nucleation properties of the organoclay are low relative to the less miscible organoclay. Therefore, the overall bulk crystallization rate was increased in the intercalated system and somewhat retarded in the exfoliated system. Surprisingly, spherulite growth rates were significantly increased relative to the bulk in the fully exfoliated nanocomposite. This phenomenon, linked with less effective nucleation properties, resulted in significantly bigger spherulite sizes in the exfoliated nanocomposite. The overall percent of crystallinity and the size of crystalline domains decreased by addition of organoclays and are the lowest in the fully exfoliated case.