Detection of the Direct Effect of Clay on Polymer Dynamics: The Case of Spin-Labeled Poly(methyl acrylate)/Clay Nanocomposites Studied by ESR, XRD, and DSC

Abstract

Nanocomposites of poly(methyl acrylate) (PMA) with synthetic fluoromica (Somasif) as the inorganic component were studied as a function of clay content by the spin-label electron spin resonance (ESR) technique, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). PMA was modified by attachment of nitroxide radicals and trimethylammonium chloride (TMC) groups in low concentrations (≈ 1 mol %). Ion exchange between the TMC moieties as anchoring groups and the Somasif surface allowed the preparation of exfoliated nanocomposites even in the absence of surfactants. The presence of nitroxide labels and the absence of surfactants allowed the detection of the direct effect of the clay on polymer dynamics. The properties of these materials were compared with those of the nanocomposites prepared with the clay modified by surfactants. Structural data from XRD were combined with ESR results in order to assess the extent and intensity of polymer−clay interactions at the interface. ESR spectra indicated that the mobility of PMA chains in the nanocomposites is constrained due to the interactions in the interface region. The average thickness of the rigid interface in the nanocomposites prepared without surfactants was estimated to be in the range 5−15 nm, based on the deconvolution of ESR spectra measured as a function of temperature into slow (S) and fast (F) components. The average interface thickness decreased with increasing Somasif content, most likely because of overlap between Somasif platelets. In nanocomposites prepared in the presence of surfactants, the interaction between PMA and Somasif was reduced, and the mobility of PMA was enhanced. No significant effect on polymer dynamics was detected in conventional composites, in which the clay was dispersed on a scale siginificantly larger than the nanoscale.