Highly branched polymer/montmorillonite clay nanocomposites

The aim of this thesis has been to assess the potential of branched polymers as novel exfoliants for layered silicates and their application in polyurethanes. Three different types of –OH functional branched polymers have been studied: dendrimers, hyperbranched polymers (HBPs) and star branched polymers (SBPs), with a range of structures and molecular weights. The dendrimers and their HBP analogues were synthesized in-house, while the other HBPs and SBPs were obtained from commercial suppliers. Various types of montmorillonite (MMT) layered aluminosilicate clays have been used as modifiers. The different branched polymers were either melt processed or solution processed with the MMT, using water and THF as dispersants. Whether intercalated or exfoliated nanocomposites were obtained depended on the choice of MMT, the presence of solvent during processing as well as the characteristics of the interface between the polymer and the MMT, with exfoliation being favored when the polymer-MMT interactions were strong. Very high degrees of exfoliation were obtained from dendrimers and HBPs processed in water at loadings as high as 20 wt% unmodified Na+MMT, with intercalation only becoming dominant at higher loadings. The MMT layer spacing in the intercalated HBP/Na+MMT nanocomposites depended directly on the pseudo-generation number, that is, on the size of the HBP, at intermediate loadings, consistent with adsorption of a monolayer of unperturbed HBP molecules at the layer surface in aqueous suspension. Dispersions of the HBPs with a range of organically modified MMTs in THF, on the other hand, showed mainly intercalation on drying, with layer spacings of the order of 3.8 nm at intermediate clay contents, which were apparently independent of the pseudo-generation. The difference between the final MMT layer spacing and its value prior to mixing was found to increase significantly with the polarity of the organic modifier. The contrasting behavior of Na+MMT and the organically modified MMTs was partly attributed to the tendency of Na+MMT to exfoliate in aqueous dispersion. The HBPs and dendrimers are then able to coat the individual MMT layers and stabilize the exfoliated structure during drying. The organically modified MMTs, on the other hand, did not exfoliate in THF dispersions and intercalation consequently dominated. Predominantly intercalated nanocomposites were also obtained with the SBPs, although increasing degrees of exfoliation were observed as the strength of the interactions between the SBP and the clay increased, as a result of systematic modification of the polarity of the SBP. The rheological properties of a range of exfoliated and unexfoliated highly branched polymer/MMT nanocomposites were investigated in small strain oscillating shear. The exfoliated nanocomposites showed a sharp increase in shear viscosity at very low MMT contents, accompanied by a transition from Newtonian behavior to solid-like behavior, characterized in terms of a li