Rheology control by modulating hydrophobic and inclusion associations in modified poly(acrylic acid) solutions Xuhong Guo a, * , Ahmed A. Abdala a,b , Bruce L. May c , Stephen F. Lincoln c , Saad A. Khan d , Robert K. Prud’homme a, * a Department of Chemical Engineering, Princeton University, Princeton, NJ 08544, USA b Department of Chemical Engineering and Petroleum Refining, Faculty of Petroleum and Mining Engineering, Suez Canal University, Suez, Egypt c Departmant of Chemistry, University of Adelaide, Adelaide, SA 5005, Australia d Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA Received 9 January 2006; received in revised form 24 February 2006; accepted 3 March 2006 Abstract The rheology of modified poly(acrylic acid) (PAA) solutions can be tuned by controlling the inclusion interactions between a-cyclodextrins and alkyl hydrophobes. We demonstrate three modes of control: (1) using free cyclodextrins (CD) to displace hydrophobe–hydrophobe association in hydrophobically modified poly(acrylic acid) (HMPAA) polymers—which reduces fluid viscosity, (2) using competitive inclusion interactions where stronger SDS:CD binding can be used to ‘unmask’ CD:hydropobe inclusion interactions—which increases viscosity, and (3) employing HMPAA inclusion interactions with CD groups grafted to PAA chains (CDPAA)—which produces higher viscosities than purely hydrophobic association systems at the same concentration. The inclusion association between alkyl side-group in HMPAA and CD, either free or grafted onto PAA, obeys a 1-to-1 stoichiometry at low polymer concentrations (!1 wt%). In contrast to purely hydrophobically associating polymers, the CD:hydrophobe interaction is only binary, and, therefore, these associated networks should be ideal model systems to test theoretical predictions for associative fluids. q 2006 Elsevier Ltd. All rights reserved. Keywords: Poly(acrylic acid); Rheology; Inclusion association 1. Introduction Interactions among polymer chains give the ability to control solution rheology [1]. For example, hydrophobically modified water soluble polymers (HMP) are extensively used as rheology modifiers in paints, cosmetics, pharmaceuticals, foods, enhanced oil recovery, water treatment and controlled release of bioactive materials [2–4]. The hydrophobic interactions, and thus the solution viscosities, can be controlled by the type or density of hydrophobic groups [5–7], or the addition of salts or surfactants [8–11]. However, hydrophobic interactions do not provide the specificity to enable ‘triggered’ changes in rheology by minor changes in composition. The inclusion or ‘host–guest’ interaction between a cylcodextrin (CD) host and a hydrophobic guest is an ideal reversible, specific, physical interaction that can be exploited to modulate the rheology of polymer solutions. Cyclodextrins are cyclic oligosaccharides containing 6, 7 or 8 glucose rings, which are called a-, b- or g-cyclodextrin, respectively. The internal cavity of a CD is hydrophobic and can accommodate suitable hydrophobic groups to form an inclusion complex [12–13]. For example, free CDs have been shown to reduce viscosity and viscoelas- ticity of HMP solutions by screening hydrophobic interactions [14–16]. More interestingly, if CDs are grafted onto polymer chains, novel polymer networks can be generated by host– guest interactions between polymers with hydrophobes and those with CD side-groups [17–20] as shown schematically in Fig. 1. The inclusion association between a CD and a hydrophobic guest group normally obeys a 1-to-1 stoichi- ometry. The theory of associative polymers has advanced significantly over the last decade [21–25]. However, the theories address only binary interactions, such that the associating network connectivity is independent of polymer concentration. The more advanced theories of associating Polymer 47 (2006) 2976–2983 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2006.03.006 * Corresponding authors. Tel.: C1 609 258 4577; fax: C1 609 258 0311. E-mail addresses: xguo@princeton.edu (X. Guo), prudhomm@princeton. edu (R.K. Prud’homme).