Interfacial interactions in polymer-layered silicate nanocomposites

Interactions between sodium montmorillonite (Na-MMT) and a variety of probes, some of which are intended to model components of a polyurethane system, have been studied. Particular attention was given to the effect of preadsorbed water on the adsorption behavior of the probes. Flow microcalorimetry (FMC), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and wide-angle X-ray scattering (WAXS) were used to monitor the adsorption process. The probe set included alcohols, amines, ethers, poly(propylene glycol) monobutyl ethers (PPG), and 4-ethylphenyl isocyanate (4-EPI). FMC revealed that the preadsorbed water molecules on undried Na-MMT hindered the adsorption of alcohol and ether probes, but had little effect on the adsorption of amines. Drying of Na-MMT to less than 0.3% w/w H2O led to an increase in heat of adsorption and generally greater retention of the probes. PPG showed strong interaction with Na-MMT due to multipoint adsorption. With dried Na-MMT, WAXS revealed that PPG of molecular weight (MW) 1000 was partly intercalated into the gallery while lower molecular weight PPG (MW 340) did not intercalate the Na-MMT. DRIFTS spectra of 4-EPI adsorbed on undried Na-MMT revealed urea linkages, indicating formation of N,N'-bis(4-ethylphenyl) urea. In contrast, with dried Na-MMT the 4-EPI formed a urethane linkage with hydroxyl groups present at the edges of the silicate platelets.