Clay-polyvinylalcohol nanocomposites: Competitive adsorption of polyvinylalcohol and plasticizers onto Na-bentonite.

Competitive adsorption of poly(vinyl alcohol) (PVOH) and plasticizers (PEG Mw 600 and 2000, M600 and quaternized M600) onto Na-bentonite has been studied. Preliminary experiments showed that as the concentration of plasticizers or PVOH increased, the amount adsorbed by clay increased and followed a Langmuir-type adsorption. The order of affinity to clay was PVOH, PEG2000, M600 and PEG600. Step changes occurred in the gallery with increasing amounts of plasticizer offered as it expanded from a depleted single layer through to a full single layer, then a combined single and bilayer system, and finally a full bilayer system. For PVOH, multi-layers were observed, as well as mostly exfoliated structures when the PVOH concentration was > 75 wt%. The extent of change in the d-spacings of PEG600/clay and PVOH/clay systems were shown to be greatly affected by thermal treatment and exposure to different relative humidities at low organic loadings (23 wt%). The thermal stability of PEG600 or PVOH in clay was dependant on the loading and so related to the type of structure. Desorption of organic from clay by washing up to ten times with water showed that not all organic was removed since the d-spacing of organic/clay did not collapsed to that of clay alone.In the competitive adsorption studies, both plasticizers (PEG600 or M600) and PVOH were present within the clay gallery, the amounts of plasticizer and PVOH present increased with the amount of plasticizer and PVOH offered. The amount of PVOH adsorbed by clay was slightly increased by the presence of 1-30pph PEG600, but reduced in the presence of 30 pph M600 and 1, 10 and 30 pph QM600, whilst the amount of PEG600 or M600 adsorbed by clay was reduced when the amount of PVOH offered was increased. Samples prepared by mixing PEG600 with clay first before adding PVOH consistently exhibited slightly higher d[001]-spacings and peak intensities when compared to the corresponding samples prepared by either mixing PEG600 and PVOH before adding clay or mixing PVOH with clay first before adding PEG600. The differences are assigned to their molecular arrangements rather than quantities adsorbed. The tensile storage modulus, &, alpha temperature and E[a] of PVOH/clay systems increased as the amount of clay increased whilst the opposed trend was observed for the toughness and damping properties. Introduction of clay into PVOH resulted in a reduced water permeability in the nanocomposites since an 8 times reduction was observed when clay content was 50 wt%. The opposite trend was observed when 20 wt% PEG600 was introduced to the PVOH since it increased the water permeability by 4.4 times. Introduction of clay to PVOH/PEG600 reduced the water permeability and -23 wt% clay was needed to return the WVTR value to that of PVOH alone. Besides clay, the crystallinity of PVOH was shown to reduce the water permeability of the nanocomposites. The levels of PVOH crystallinity were identified using FTIR from the intensity ratio of the bands at 1142 cm[-1] and 2942 cm[-1]; the ratio increased with heating temperature and clay content but decreased with time when exposed to 85% relative humidity. In addition, the intensity of the XRD peak at ≈ 19 °20, which relates to PVOH crystallinity increased with temperature and decreased with exposure time for all films. The sample crystallinity was greatly affected by water content, however, the induced crystallinity was not completely lost after being maintained at 23 °C and 85 % RH for 51 days.