Dynamics and microstructure of the surfactant layer in organically modified silicates and their composites with polystyrene were studied as a function of surfactant loading and temperature in the range relevant for melt intercalation. Site selectivity was achieved by using specifically headgroup-and tail-labeled surfactant EPR spin probes and by applying 31 P MAS NMR to phosphonium surfactants. Bimodal dynamics is observed over a broad range of surfactant loadings and temperatures in both the absence and presence of polymers and correlates with a bimodal distribution of surfactant headgroup distances from the silicate surface. Excess surfactant with respect to the cation exchange capacity of the silicate plasticizes the surfactant layer. Electron spin echo envelope modulation on nanocomposites with ammonium surfactant and deuterated polystyrene demonstrates close contact between the polymer and surfactant tail ends. Surfactant dynamics changes strongly during microcomposite formation, i.e., by embedding stacks of organoclay platelets in a polymer matrix, even if no intercalation takes place.

Clays used in polymer‐layered silicate nanocomposites are heterogeneous. To improve insight into structure and dynamics of the interface, the iron‐free and structurally homogeneous layered silicate magadiite was synthesized. Morphology of the magadiite and the extent of intercalation in melt‐prepared composites were characterized by SEM and WAXS. Carbonyl groups in the main chain of the polymer appear to facilitate intercalation. Surfactant motion was studied by 2H NMR and CW EPR. Nanocomposite formation with PCL enhances and microcomposite formation with PS diminishes motion. The activation energies change at the glass transition temperature of PS and the melting temperature of PCL.magnified image

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 Ångstrøms and a few nanometers and time scales between 10 ps and 1 μs. 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‐d8, 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. Polym. Eng. Sci. 44:1112–1121, 2004. © 2004 Society of Plastics Engineers.

No abstract

Polymer–clay nanocomposites exhibit much improved mechanical, physical, and chemical properties compared to the pure polymer. The interaction of polymer and organically modified silicates is mainly influenced by the surfactant layer in the system. To investigate the structure and dynamics of this surfactant layer, various electron paramagnetic spectroscopy (EPR) techniques were applied. Continuous wave EPR experiments showed a temperature-dependent heterogeneous mobility of the surfactant layer in organoclay as well as a difference in dynamics along the alkyl chain. Intercalation of polystyrene causes a significant slowdown in surfactant dynamics. Electron spin echo envelope modulation indicates a closer contact of the polymer with the mid of the surfactant tail than with the end of the tail. From the obtained data the picture of flatly lying surfactants on clay platelets with a mobility gradient along their alkyl chains can be drawn.