1584 Research Article Received: 7 November 2013 Revised: 14 January 2014 Accepted article published: 24 January 2014 Published online in Wiley Online Library: 3 March 2014 (wileyonlinelibrary.com) DOI 10.1002/pi.4715 Cationic brush-like terpolymer with pH responsive thickening behavior in a surfactant system Zhong-Xing Zhang, a* Xiping Ni a and Jun Li a,b,c* Abstract A new kind of cationic brush-like terpolymer was synthesized by copper(I)-mediated atom transfer radical polymeriza- tion (ATRP) of methyl ether poly(ethylene glycol) methacrylate (mPEGMA), poly(propylene glycol) methacrylate (PPGMA) and dimethylaminoethyl methacrylate (DMAEMA) in the presence of ethyl -bromoisobutyrate as ATRP initiator. The poly(PEG/PPG/DMAEMA) terpolymer was featured with a cationic backbone containing PDMAEMA segments and amphiphilic side chains composed of hydrophilic PEG and hydrophobic PPG short branches. The copolymer was characterized by 1 H NMR, Fourier transform IR and gel permeation chromatography. The thickening behavior of this copolymer in a surfactant system (sodium dodecyl sulfate, SDS) was extensively investigated by a rheological study. It was found that the copolymer can effec- tively thicken the SDS surfactant system at low concentrations of both copolymer and surfactant. More interestingly, the thickened SDS system by this copolymer showed dramatic pH responsive rheological behavior in the pH range 4–10. Thus the rheological behavior of the thickened SDS system was tunable by pH value. Based on our findings, a novel thickening mechanism is proposed for this kind of cationic brush-like terpolymer. This kind of cationic brush-like copolymer would be a new type of rheology modifier for advanced hair/skin care systems. © 2014 Society of Chemical Industry Supporting information may be found in the online version of this article. Keywords: cationic brush-like terpolymer; PEG/PPG/PDMAEMA; one-pot ATRP; rheology modifier; hair and skin care INTRODUCTION Aqueous-based cleansing formulations (shampoo, shower gel, body and facial washes) are usually based on concentrated sur- factant systems. Mostly, these surfactant systems are blends of anionics such as alkyl sulfates with amphoterics such as betaines, with an overall concentration of approximately 10% - 20% active surfactant. 1 Usually, electrolytes (typically NaCl) can be added to these sur- factant blends to increase the viscosity for optimum application to skin and hair. The addition of salt causes a change in the micel- lar structure of surfactant molecules from spherical to rod-shaped units. These rod-shaped micelles can grow in length, becoming ‘worm-like’ in structure, and above a critical surfactant volume fraction the worm-like micelles begin to overlap forming a ‘gel’ network. Thus, the viscosity of the surfactant solution is increased. However, addition of salt may not produce the optimum rheo- logical characteristics (viscoelasticity) for ease of application and sometimes formulations appear stringy and rubbery, giving poor sensorial effects (both visual and through touch). 1 In recent years, water-soluble associative polymers are gaining importance as thickeners or rheology modifiers in a number of waterborne applications. Associative thickeners are water-soluble polymers containing hydrophobic moieties (alkyl chains), such as hydrophobically modified ethylene oxide urethane block copolymer (HEUR), whereby alkyl chains are grafted at both ends of poly(ethylene glycol) (PEG) chains, 2 – 6 hydrophobically modified alkali-soluble emulsion 7 – 11 and hydrophobically mod- ified hydroxy ethylcellulose. 12 – 16 The macromolecules of these associative polymers can form a network-like structure when dissolved in water, thereby producing interesting rheological behavior. The formation of a network structure (gel formation) can occur at relatively lower polymer concentrations compared with the unmodified molecules. The most likely explanation of the gel formation is hydrophobic bonding (association) between the alkyl chains in the molecules. These associative structures are similar to micelles except that the aggregation numbers are much smaller. 1 Associative thickeners also show interactions with surfactant micelles. For example, rheological studies of the SDS/HEUR system have been carried out in past decades (SDS ∗ Correspondence to: Jun Li, Department of Biomedical Engineering, Faculty of Engineering, National University of Singapore, 7 Engineering Drive 1, Singa- pore 117574. E-mail: jun-li@nus.edu.sg; bielj@nus.edu.sg, Zhong-Xing Zhang, Institute of Materials Research and Engineering, A*STAR (Agency for Sci- ence, Technology and Research), 3 Research Link, Singapore 117602. E-mail: zhangzx@imre.a-star.edu.sg a Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 b Department of Biomedical Engineering, Faculty of Engineering, National Uni- versity of Singapore, 7 Engineering Drive 1, Singapore 117574 c NUS Graduate School for Integrative Sciences and Engineering (NGS), National University of Singapore, 28 Medical Drive, Singapore 117456 Polym Int 2014; 63: 1584–1592 www.soci.org © 2014 Society of Chemical Industry