Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 159–164 Mixed solutions of hydrophobically modified graft and block copolymers Maria Karlberg ∗ , Krister Thuresson, Lennart Piculell, Björn Lindman Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden Received 23 June 2003; accepted 28 January 2004 Abstract Phase separation temperatures and viscosities of dilute and semidilute aqueous mixtures of hydrophobically modified graft and block copolymers have been studied. The graft copolymer was hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), and the block copolymers were hydrophobically modified poly(ethylene glycol) (HM-PEG) with diblock (DB) or triblock (TB) structures, i.e., modified at one or both ends, respectively. For comparison, viscosity measurements were made also on mixtures of TB with HM-HEC, and on systems where one of the polymers was unmodified (i.e. HM-EHEC/PEG or TB/EHEC). All results indicate mixed hydrophobic associations between graft and block copolymers. Moderate viscosity enhancements were found on addition of HM-PEG to HM-EHEC, and the effects were larger for TB than for DB. The viscosity changes were quite sensitive to the concentration of HM-EHEC. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrophobically modified copolymers; Graft copolymer; Block copolymer 1. Introduction Hydrophobically modified water-soluble polymers (HMP) are polymers containing a small fraction of strongly hydrophobic groups, hydrophobes, grafted onto the hy- drophilic main chain. Mixed solutions of HMP and surfac- tants have been intensely studied during the last decades [1,2]. A reason for this is that such mixtures have found use as viscosity modifiers in commercial products such as water borne paints and skin care products. Other suggested uses are in applications such as enhanced oil recovery [3–5]. Here, we take a different but related approach, investigating mixtures of two different types of HMP. Although this type of mixture is used commercially, for example in paints, in- vestigations concerning these systems are rare and it seems that only mixtures containing two ionic or one ionic and one non-ionic hydrophobically modified polymers have been investigated before [6,7]. In the latter studies, it was shown that mixed aggregates formed and that most mixtures ex- hibited higher viscosities than the solutions of either of the polymers alone. ∗ Corresponding author. Fax: +46-46-2224413. E-mail address: maria.karlberg@fkem1.lu.se (M. Karlberg). We have chosen to base the present investigation on aqueous solutions of a cellulose derivative, hydrophobi- cally modified ethyl(hydroxyethyl)cellulose (HM-EHEC). Like the parent polymer, EHEC, HM-EHEC has a reversed solubility in water, i.e., heating leads to phase separa- tion. Polymers with this type of behaviour are referred to as LCST polymers since they have a lower critical so- lution temperature. EHEC has this property in common with some other water soluble polymers, for example poly(N-isopropylacrylamide) and poly(ethylene glycol) (PEG). In previous investigations, the influence on the phase separation temperature, often called the cloudpoint temper- ature, T cp , by adding a third component has been followed [8–12]. In dilute aqueous solutions, intra-molecular associations of the hydrophobes of HMP are known to contract the polymer coils [4,13,14], while in solutions with concen- trations around or above the overlap concentration, c ∗ , inter-molecular associations are common. The latter will give rise to an infinite network throughout the solution [13,15]. This network formation will have consequences for thermodynamic properties (phase behaviour) as well as for the dynamic properties (molecular diffusion and viscosity) of the solution. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.01.035