Characterization of the Interphase in Model Associative Polymer/Titanium Dioxide and Polystyrene Latex Systems

The adsorption behavior of model associative polymers in aqueous titanium dioxide (TiO2) and polystyrene latex (PS) particle dispersions has been studied using liquid-state Nuclear Magnetic Resonance (NMR) and a modified serum replacement technique (SRT). The structure of the associative polymer is a linear, water soluble, A-B-A block copolymer with a polyethylene oxide backbone and hydroxyl (H) or hexadecyl (C16H33) endgroups.Absolute intensity NMR measurements allow the determination of the adsorbed polymer configuration at the solid-solution interface and adsorption isotherms for comparison to the SRT results. Both the SRT and NMR results demonstrate that the adsorption/desorption of the C16 and H polymers is strongly affected by the substrate. For the polymer/PS particle systems, there is good agreement in complete surface saturation values for the SRT and NMR methods. In the polymer/TiO2 systems, the H polymer plateau as determined by NMR differs slightly from that determined by SRT. For the C16/TiO2 system the interpretation is less straightforward. Either the NMR saturation value is at least double that of the SRT value or the C16 and H polymers have very similar isotherms by NMR. This would suggest that on TiO2 the effect of the hydrophobe is minimal. For all the polymers the polymer/particle interaction on PS is strong and desorption is difficult while on TiO2 it is weak and completely reversible. A pancake-to-brush conformation change on the PS surface is evident for the C16 polymers and probably for the H polymer as well. The H polymer seems to form a multi-layered structure on PS from the NMR and laser light scattering results. The contribution of the hydrophobe and the effect of increasing the backbone molecular weight are clearly illustrated.