Aqueous Colloidal Dispersions with Grafted Poly(ethylene Oxide) Chains: Synthesis, Microstructure, and Rheology

Abstract

Water-based coatings are an environmentally benign alternative to thesolvent-based systems traditionally used in industry. Water-based systems containcolloidal particles dispersed in an aqueous medium and are typically stabilizedthrough electrostatic means. Widespread acceptance of water-based coatings hasbeen hindered by their low solids content, which leads to slow drying rates andinconsistent quality. This study focuses on developing a model coating systembased upon a novel poly(ethylene oxide) (PEO)-based macromer that can be graftedto the surfaces of the colloidal particles, stabilizing the particles against flocculationand allowing the coating to be processed at a high solids content. The initial phaseof this study involves the synthesis of colloidal polystyrene (PS) latex dispersionssterically stabilized with these PEO-based macromers, and the identification of theeffects of synthesis parameters on molecular and structural properties. Electronmicroscopy reveals the spherical, submicron morphology of the latex particles, andproton nuclear magnetic resonance data confirms a direct correlation between theamount of macromer in the reaction feed and the amount grafted to the PS particles.In the second phase of the research, rheological techniques are used to investigateflow behavior and interparticle interactions for latex dispersions with differentamounts of grafted macromer. Dynamic rheological experiments reveal that, at high macromer concentrations, the PEO surface layer effectively shields theattractive interactions between the core PS particles that lead to flocculation.However, at low macromer concentrations, strong interactions are seen even at lowparticle weight fractions, indicating the presence of a flocculated system. Steadyshear rheological evaluations show that the latex systems possess suitable flowbehavior for coating applications, even at high particle contents. Steady shear data,in conjunction with the Krieger-Dougherty model, confirm that the stabilizing layerthickness decreases as particle concentration increases, indicating a compressiblesystem. In addition, the relationship between the strength of interparticleinteractions and PEO graft density is gauged by a power law model relating theelastic modulus to particle concentration. The third phase of the study examines theeffects of temperature on interparticle interactions and dispersion stability.Dynamic rheological experiments reveal a sol-gel transition as temperature isincreased, with the transition temperature being dependent on the PEO graftdensity. Dynamic light scattering measurements show a change in apparent particlesize at the sol-gel transition. The transition is postulated to be due to the collapseof the PEO chains as temperature is increased (i.e., solvent quality is decreased),which allows the attractive forces between the PS particles to dominate the stericrepulsion provided by the stabilizing layer. The PEO chain collapse is attributed tothe disruption of the structured orientation of the hydrogen-bonded water moleculesthat surround the PEO.