Mesoscopic Simulations of the Phase Behavior of Aqueous EO 19 PO 29 EO 19 Solutions Confined and Sheared by Hydrophobic and Hydrophilic Surfaces Hongyi Liu, †,‡ Yan Li, § Wendy E. Krause, † Melissa A. Pasquinelli,* ,† and Orlando J. Rojas* ,†,⊥,∥ † Fiber and Polymer Science Program and Department of Textile Engineering, Chemistry and Science and ⊥ Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695, United States § The KAUST-Cornell Center for Energy and Sustainability (KAUST-CU), Cornell University, Ithaca, New York 14853, United States * S Supporting Information ABSTRACT: The MesoDyn method is used to investigate associative structures in aqueous solution of a nonionic triblock copolymer consisting of poly(propylene oxide) capped on both ends with poly(ethylene oxide) chains. The effect of adsorbing (hydrophobic) and nonadsorbing (hydrophilic) solid surfaces in contact with aqueous solutions of the polymer is elucidated. The macromolecules form self-assembled structures in solution. Confinement under shear forces is investigated in terms of interfacial behavior and association. The formation of micelles under confinement between hydrophilic surfaces occurs faster than in bulk aqueous solution while layered structures assemble when the polymers are confined between hydrophobic surfaces. Micelles are deformed under shear rates of 1 μs −1 and eventually break to form persistent, adsorbed layered structures. As a result, surface damage under frictional forces is prevented. Overall, this study indicates that aqueous triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) (Pluronics, EO m PO n EO m ) act as a boundary lubricant for hydrophobic surfaces but not for hydrophilic ones. KEYWORDS: triblock nonionic polymers, pluronics, MesoDyn, lubrication, boundary layer, spherical micelles, cylindrical micelles, wormlike micelles ■ INTRODUCTION Polymers are often used to adjust the functional and interfacial properties of surfaces by adsorption and surface assembly. 1,2 For example, nonionic triblock copolymers comprising ethylene oxide (EO) and propylene oxide (PO) blocks (Pluronics, EO m PO n EO m ), have received increased attention as a modifier of solid surfaces by physical adsorption. 3−5 One of the advantages of EO m PO n EO m is their amphiphilic properties that endow molecular constructs with tailorable surface affinities, 6 depending on the adsorbing surface and the surrounding medium. For example, these materials are of interest as drug-delivery vehicles because in aqueous solution their micelles contain a hydrophilic corona and a hydrophobic core within which drugs can be solubilized and transported. 7,8 Pluronic triblock copolymers have also been successfully used as surfactants, emulsifiers, stabilizers, and food additives. 7−13 The interfacial behaviors of EO m PO n EO m solutions in the presence of solid surfaces, both mineral and polymeric, have been investigated by a number of authors. 4,14−26 For example, by using atomic force microscopy, Brandani and Stroeve 24 reported on the formation of a uniform and monolayer-like adsorbed structure of EO m PO n EO m copolymers on hydro- phobic surfaces. Rojas and co-workers 4 investigated the adsorption of EO 37 PO 56 EO 37 triblock copolymer on different types of surfaces, namely, polypropylene, poly(ethylene terephthalate), nylon, graphite, cellulose and silica. They found that the hydrophobic mineral surfaces adsorbed molecules of EO 37 PO 56 EO 37 as a monolayer, whereas spherical micellar structures were observed on the hydrophilic ones. Li et al. studied the effect of molecular weight as well as the EO/PO molar ratio on polymer-surface interactions. 27 A further understanding of the interfacial behaviors and self- assembly processes is required; however, such endeavors are limited by the experimental conditions. Therefore, we utilized mesoscale modeling to provide details about the morphology and dynamics of molecular assemblies of EO m PO n EO m in aqueous solutions and their adsorption on hydrophobic and hydrophilic surfaces. Mesoscopic dynamics (MesoDyn) 28−35 is a simulation method that treats the polymer chains at the coarse-grained level by grouping atoms together up to the equivalent length of the polymers. It is based on the dynamic mean field density functional theory 28,36 in which the phase separation dynamics are described by Langevin-type equations for polymer diffusion and the thermal fluctuation are added as Received: July 12, 2011 Accepted: December 2, 2011 Published: December 2, 2011 Research Article www.acsami.org © 2011 American Chemical Society 87 dx.doi.org/10.1021/am200917h | ACS Appl. Mater. Interfaces 2012, 4, 87−95