Addressing the interface in polymer-clay nanocomposites by electron paramagnetic resonance spectroscopy on surfactant probes

The structure and dynamics of the surfactant layer in organoclays prepared from synthetic clays (Laponites, expandable fluoromica) and in their composites with polystyrene are studied in order to understand the influence of the interface layer on composite properties and composite formation from melt or solution. Electron paramagnetic resonance spectroscopy of nitroxide-labeled surfactants is used to examine length scales between a few Angstroms and a few nanometers and time scales between 10 ps and 1 mus. The nitroxide labels are stable up to at least 433 K, so that dynamics can be studied at temperatures relevant for melt intercalation. For organoclays prepared with an excess of surfactant with respect to cation exchange capacity, two or three fractions of surfactants with different mobility exist, the number of fractions, their relative amount, and the mobility differences being determined by the type of clay. Dynamics close to the region of the anchor groups depends on the type of clay and, for large clay particles, on the presence and molecular weight of polystyrene. For dispersions of organoclay in toluene-d(8), deuterium electron spin echo envelope modulation evidences penetration of solvent molecules into the surfactant layer down to the region of the anchor groups. Changes in spatial distribution of the anchor groups caused by intercalation can be detected. (C) 2004 Society of Plastics Engineers.