Evidence of re-entrant behavior in laponite-PEO systems
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0899-N09-03.1
Evidence of re-entrant behavior in laponite-PEO systems Hossein A. Baghdadi, Surita R. Bhatia, Elizabeth E. C. Jensen1 and Nalini Easwar1 Chemical Engineering Department University of Massachusetts – Amherst, 01003-9303, U.S.A. 1 Department of Physics Smith College, Northampton, MA 01063, U.S.A. ABSTRACT Rheology and dynamic light scattering capture re-entrant behavior of laponite-polymer systems. Neat laponite under basics conditions and concentrations of 2wt% or greater forms a viscoelastic soft glass due to electrostatic repulsions. We show that that addition of low molecular weight poly(ethylene oxide) (PEO) melts the glass due to a depletion force. The depletion force speeds up dynamics in the system resulting in a low viscosity solution. A reentrant viscoelastic solid is formed with the addition of high molecular weight PEO due to the polymer chains bridging between laponite particles. As expected the transition from a low to high viscosity solution scales with the polymer mean square end-to-end distance and gap between laponite particles. INTRODUCTION Laponite is an anisotropic colloid used to control viscosity and flow properties in different applications ranging from surface coatings, paint, personal care products, toothpaste and adhesives. It is a synthetic discotic charged clay particle with a diameter of 25 nm and height of about 1 nm. Its face has a negative charge, while the edge charge is pH dependent. When dispersed in water with a pH greater than 9 and above a concentration of 2wt% it forms a glass with an initial low viscosity. After approximately two weeks, a viscoelastic solid is formed with an elastic modulus that escalates with time. The dispersion’s viscosity and elastic modulus increase might be due to diffusion from the initial clusters and rearrangement of laponite particles, which expands the effective free volume as show in shown figure 1a,b.
(a) (b) (c) (d) Figure 1. Illustrations of (a) neat laponite initially, (b) neat laponite over time and increased effective volume, laponite with the addition of (c) low MW PEO and (d) high MW PEO
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The addition of poly(ethylene oxide) (PEO) alters the structure and rheological properties of the laponite solution. There is evidence that PEO adsorbs onto the surface of laponite in a very thin layer and either extends or wraps around the edge of the particle [1, 2]. The addition of low molecular weight (MW) PEO prevents formation of a viscoelastic solid over a long period of time [3-8]. In contrast the addition of high MW PEO showed an enhancement of the viscolesatic properties, i.e. higher elastic modulus and viscosity [9-12]. To explain this behavior we investigate the laponite-PEO system using rheology and dynamic light scattering. We show that the laponite glass is melted with the addition of low MW PEO. Above a critical molecular weight, re-entrance into a new viscoelastic solid state is achieved by the PEO bridging laponite particles. Re-entrant glass transitions have been observed with hard spheres [4, 13], block c
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