Gel point behavior of colloidal unimolecular polymer (CUP) particles

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Gel point behavior of colloidal unimolecular polymer (CUP) particles Minghang Chen & Cynthia Riddles & Michael Van De Mark

Received: 16 April 2013 / Revised: 13 June 2013 / Accepted: 15 July 2013 / Published online: 8 August 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract The gel point of a colloidal unimolecular polymer (CUP) aqueous suspension was determined. The zero-shear viscosities of suspensions were measured by capillary viscometer and cone-and-plate rheometer. The relative were h viscosities i fit to the Krieger-Dougherty equation ηr ¼ 1− ϕϕ

−½ηϕmax

max

to

determine the maximum packing volume fraction ϕmax. The gel point was found to be much lower than the anticipated random close packing (~0.63) or hexagonal packing (~0.74). The gel point was attributed to the effect of surface water, and subsequently the thickness and density of water layer were calculated. The CUP results were compared with the rheological behavior of commercial waterborne suspension with particle size 25 and 77 nm. The packing volume fraction of colloids with different particle sizes was predicted considering the effect of surface water. Keywords CUP . Gel point . Surface water . Random close packing . Hexagonal packing

Introduction In the waterborne paint industry, the percent solids of paint have always been a focus from both the quality and the price/ cost point. For a regular latex paint, manufactures find it hard to increase the percent solids of paint higher than 60 % without the paint becoming too thick to use. For waterborne urethane, the maximum percent solid is even lower. Formulators have noticed that the solid components of waterborne paint, i.e., resin particles or pigments, increase the viscosity as a function of solids. How the resin or pigment interacts with

M. Chen : C. Riddles : M. Van De Mark (*) Coating Institute of Missouri University of Science and Technology, Missouri, USA e-mail: [email protected]

water and each other is therefore of critical interest. But since latex or waterborne urethane resins contain surfactant or emulsifiers to keep the dispersion stable, it is difficult to apply physical science. The additives in the resins may interfere with any study of the bound or free water. The interaction of a particle with the water produces a layer of water on the particle surface which is different from bulk or free water. The bound or surface water has been suggested to be as thick as from a few water molecules to several dozens of water molecules depending on the surface properties of particles [1–3]. Colloidal unimolecular polymer particles (CUP) are a new type of colloid that are formed by the effect of hydrophilic/ hydrophobic interaction of the polymer with hydrophilic groups and hydrophobic backbone in the solvent [4]. The formation of CUP particles is driven by the polymer–polymer interaction being greater than the polymer–solvent and entropically favored by release of water analogously to micelle formation with hydrophilic or charged groups creating the shape. Figure

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