An investigative study of the interactions of poly(vinylamine)s with montmorillonite
The intent of this study was to probe the conditions under which the balance of enthalpic adsorption interactions and loss of conformation entropy that will allow penetration of polymers into the galleries and exfoliation of montmorillonite. While these types of systems have been described by their mechanisms of interaction, few studies have been geared towards evaluating and actually defining these interactions. Poly(vinylamine) was chosen for these studies because the simple structure of this cationic polyelectrolyte combined with the ability to manipulate charge density simply by changing pH of the system makes it a good model for the study of interactions with montmorillonite clay. In addition, the amino functionality of poly(vinylamine) is particularly well suited to achieve partial functionalization by coupling with electrophilic reactants to tailor the hydrophobic nature of the polymer. Poly(vinylamine) was modified with a short chain hydrophobe (p-methoxybenzoic acid) at 10-12% incorporation on the polymer backbone. This hydrophobe was chosen due to good retention of water solubility even at relatively high incorporation on the backbone. The precursor polymer to poly(vinylamine), poly(N-vinylformamide), was also studied as a system control. By combining the results from the following experiments it was possible to determine the mechanism of interaction for each of the polymer/montmorillonite systems at pH 3.0, 7.0 and 10.0. The adsorption capacity of each polymer was determined by constructing adsorption isotherms. Poly(N-vinylformamide) conformation when it adsorbed on the surfaces was determined by 13 C NMR line broadening experiments. The sedimentation volume was evaluated by statistically designed phase diagrams as a function of polymer and montmorillonite concentrations, and the rheological response of these systems were measured. These experiments gave insight into the phase behavior and mechanism of interaction in the polymer/montmorillonite systems. FTIR spectroscopy verified which surface of the clay was involved in adsorption along with the polymer moiety bound to the surface. The conditions under which poly(vinylamine) and its selected derivatives would penetrate the gallery spacing of stacked montmorillonite platelets were determined by X-ray diffraction and transmission electron microscopy studies. In general, hydrophobic modification of the poly(vinylamine) backbone was found to promote intercalation and exfoliation of the montmorillonite platelets.