Effects of nanoscale dispersion in the dielectric properties of poly(vinyl alcohol)-bentonite nanocomposites María C. Hernández, 1, * N. Suárez, 1 Luis A. Martínez, 1 José L. Feijoo, 2 Salvador Lo Mónaco, 3 and Norkys Salazar 4 1 Departamento de Física, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, Venezuela 2 Departamento de Ciencias de los Materiales, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, Venezuela 3 Instituto de Ciencias de la Tierra, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47586, Caracas 1041-A, Venezuela 4 Departamento de Química, Universidad Simón Bolívar, Apartado 89000, Caracas 1080-A, Venezuela Received 16 November 2007; revised manuscript received 20 February 2008; published 2 May 2008 We investigate the effects of clay proportion and nanoscale dispersion in the dielectric response of polyvi- nyl alcohol-bentonite nanocomposites. The dielectric study was performed using the thermally stimulated depolarization current technique, covering the temperature range of the secondary and high-temperature relax- ation processes. Important changes in the secondary relaxations are observed at low clay contents in compari- son with neat polyvinyl alcoholPVA. The high-temperature processes show a complex peak, which is a combination of the glass-rubber transition and the space-charge relaxations. The analysis of these processes shows the existence of two segmental relaxations for the nanocomposites. Dielectric results were comple- mented by calorimetric experiments using differential scanning calorimetry. Morphologic characterization was performed by x-ray diffraction XRD and transmission electron microscopy TEM. TEM and XRD results show a mixture of intercalated and exfoliated clay dispersion in a trend that promotes the exfoliated phase as the bentonite content diminishes. Dielectric and morphological results indicate the existence of polymer-clay interactions through the formation of hydrogen bounds and promoted by the exfoliated dispersion of the clay. These interactions affect not only the segmental dynamics, but also the secondary local dynamics of PVA. DOI: 10.1103/PhysRevE.77.051801 PACS numbers: 82.35.Np, 77.84.Lf, 77.22.Ej, 64.70.Nd I. INTRODUCTION Polymer-inorganic nanocomposites have attracted great interest due to their improved properties in comparison with neat polymer or microscale composites like polymer- polymer blends or networks. There is a broad diversity in the inorganic nanofillers used depending on the intended appli- cation. This variety ranges from metal to insulators and in- cludes spherical, cylindrical, and flake shapes 1–3. Due to the nanoscale dispersion of the inorganic phase, very small filler content is enough to affect several properties on the polymer matrix. In polymer-clay nanocomposites the nano- filler is a smectite-type layer silicate with an interlayer spac- ing between individual sheets of about 1 nm thick. These individual silicate sheets with lateral dimension of about 1 m are piled up parallel to each other and coupled with weak electromagnetic forces of dipolar or van der Waals ori- gin. The interlayer coupling in natural smectites usually in- volves inorganic cations such as Li + , Na + ,K + , Ca 2+ , and Mg 2+  attached to the negatively charged silicate surface 4. Depending on the experimental technique used, as well as on the compatibility between the clay and the polymer matrix, three basic types of clay dispersion are possible: a a microphase-separated compound with tactoids formation, b a clay-polymer intercalation with an increase in the interlayer spacing, and c an exfoliated morphology with individual silicate sheets dispersed within the polymer matrix 4. In order to provide detailed microstructural information about clay dispersion in polymer nanocomposites, a combination of x-ray diffraction XRD and transmission electron micros- copy TEM is needed 5,6. Different experimental tech- niques have been developed to produce intercalated and ex- foliated polymer-clay morphologies. Typically the first step involves the improvement of clay-polymer compatibility, usually through organic modification of the clay via a cation- exchange process in which the inorganic cation is replaced with a higher-molecular-weight organic cation 7. The next step for nanocomposite preparation is related to the clay blending in the polymer through monomer polymerization, melt blending, or solvent casting procedures 5–8. In water- soluble polymers such as polyethylene oxide or polyvinyl alcohol, it is possible to obtain a good clay dispersion with- out the organic modification of the clay through the solvent casting method in water because of the hydrophilic character of most natural smectites 9. Earlier studies of polymer-clay nanocomposites have focused on preparation, structural in- formation, and thermal or mechanical improvements of the polymer matrix 4. Over the last years attention has centered on understanding how the polymer-clay interaction and mor- phology correlate with the dynamic behavior of the polymer matrix at different scales. Because of the high surface-to- volume ratio of clays, it is expected that even for low filler content, a highly dispersed configuration allows a high frac- tion of polymer-clay interphase, and depending on the inter- molecular interactions, a modification of the polymer mo- lecular mobility appears 10. For intercalated morphologies the modification in chain dynamics is associated with nano- confinement caused by dimensional restrictions of nanomet- ric scale 11,12. As a consequence of the cooperative nature of the glass-rubber transition, usually the study of macromol- ecules dynamics in polymer-clay nanocomposites is re- * mahernan@usb.ve PHYSICAL REVIEW E 77, 051801 2008 1539-3755/2008/775/05180110 ©2008 The American Physical Society 051801-1