Effects of nanoscale dispersion in the dielectric properties of poly(vinyl alcohol)-bentonite nanocomposites

Abstract

We investigate the effects of clay proportion and nanoscale dispersion in the dielectric response of poly(vinyl 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 relaxation processes. Important changes in the secondary relaxations are observed at low clay contents in comparison 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 complemented 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.

Revisiting the thermal relaxations of poly(vinyl alcohol)

“…Primary, secondary, and crystalline relaxations have been reported by DS6, 13, 21 and DMA in pristine PVA and PVA systems 16, 17. Secondary relaxations in PVA have been shown to be related to the rotation of OH side groups and local twisting motions about the main chain, as observed by mechanical and dielectric measurements 22. The mechanical results include additional processes, named γ modes, often observed around −70°C and associated with localized rotations of hydroxyl groups or absorbed molecules 9, 22.…”

“…Secondary relaxations in PVA have been shown to be related to the rotation of OH side groups and local twisting motions about the main chain, as observed by mechanical and dielectric measurements 22. The mechanical results include additional processes, named γ modes, often observed around −70°C and associated with localized rotations of hydroxyl groups or absorbed molecules 9, 22. Also, important effects on amorphous relaxations induced by changes in the crystalline–amorphous phases have been reported 22, 23.…”

PVA / Montmorillonite Nanocomposites: Development and Properties

“…As a consequence of these bonding interactions, water is capable to destroy inter-and intra-chain hydroxyl bonds, affecting PVA's crystalline regions and acting as a plasticizer, increasing the free-volume of the amorphous phase (Hodge et al 1996). On the other hand, in PVA/MMT nanocomposites, the interactions of the surface of the clay with the polymer can be attributed to the formation of hydrogen bonds between the PVA hydroxyl groups and the negatively charged clay surface (Grunlan et al, 2004), (Hernández et al, 2008). Thus, in previous studies, the authors trying to maximise the polymer-clay interactions, utilized mainly fully hydrolysed PVA.…”

Materials, preparation, and characterization of PVA/MMT nanocomposite hydrogels: A review

“…In the polymer nanocomposites of PVA and MMT as a novel materials, the hydrogen bonding between the silanol groups and negatively charged on the surface of the MMT and hydroxyl groups of the PVA has the most important role to the surface interactions of MMT with the PVA chains . In addition, because of the existence of metal ions in the MMT lattice, interactions of the MMT layers with the acetoxy groups in the PVA chains will be increased causing strong PVA‐MMT interactions that lead to enhancement of the MMT layers dispersion into the PVA matrix .…”