Journal of Colloid and Interface Science 274 (2004) 458–464 www.elsevier.com/locate/jcis Supramolecular association of a triblock copolymer in water Giacomo Gente, Alessandro Iovino, and Camillo La Mesa ∗ Dipartimento di Chimica, Università “La Sapienza,” P. le A. Moro 5, 00185 Rome, Italy Received 23 April 2003; accepted 26 February 2004 Available online 10 April 2004 Abstract Solutions of a poly(oxyethylene)–poly(oxypropylene)–poly(oxyethylene) triblock copolymer, Pluronic F 68 , were investigated in isother- mal and isopleth mode. Surface tension, σ , dynamic shear viscosity, η(ω), QELS experiments, and volumetric, colligative, and refractive index measurements characterize the system behavior in a wide range of compositions and temperatures. The thermodynamic properties associated with micelle formation, above the critical micellar temperature, were determined by different experimental methods. The large entropic contributions to the system stability are ascribed to significant dehydration of the oxypropylene portion in the copolymer, conse- quent to micelle formation. Temperature has a pronounced effect on the association features of F 68 . It gives rise to abrupt changes in QELS and rheological properties when the critical micellar temperature is approached. Such effects are explained in terms of thermally driven micellization processes and interconnection between micelles.  2004 Published by Elsevier Inc. Keywords: Block copolymers; Solutions; Micelle formation; Thermodynamics; Surface activity; Rheology; Critical micellar temperature 1. Introduction Block copolymers, BC, are the subjects of fundamental and technological research [1–3]. The reasons for so much interest arise from their ability to form micelles, macro- or microemulsions, and several liquid crystalline phases [4–7]. Applications are focused on the cosmetics, detergent, and paint industries [8]. BC also find use in the preparation of biocompatible matrices [9] as solubilizers of peptide-based vaccines, in toothpaste formulation, in materials for bone hemostasis, and so forth. Biomedical applications require products available in large amounts, chemically inert toward tissues and biological fluids, with significant solvent capabil- ity, good solubility in water, controlled osmotic properties, well-defined surface activity, and peculiar rheological be- havior. Block copolymers of the Pluronics family fulfill the above requirements. Pluronics BC consist of a linear poly/oxyethylene, poly/ oxypropylene, poly/oxyethylene sequence. Their hydro- philic–lipophilic balance, HLB, depends on the oxyethylene, PEO, to oxypropylene, PPO, mass ratio [10,11]. The rich polymorphic behavior of Pluronics [6] is modulated by their * Corresponding author. Fax: +39-06-49913707. E-mail address: camillo.lamesa@uniroma1.it (C. La Mesa). HLB. High percentages of oxyethylene groups, for instance, imply significant aqueous solubility. Several studies have been dealing with the above field. To clarify in more detail still unknown aspects of such block copolymers in solution, we report on some physicochemical properties of F 68 , a member of the Pluronics family. Sur- face activity, differential scanning calorimetry (DSC), and QELS properties of the above compound in solution have been reinvestigated. In addition, volumetric, refractive index (n), rheological, and colligative properties give further in- formation on the solution behavior of F 68 in a wide range of composition and temperature, up to the limits of existence of the liquid crystalline phase [12]. The results indicate the occurrence of changes in the so- lution properties at the critical micellar temperature (CMT), the threshold above which micelles begin forming when the copolymer content is kept constant [13]. Micelle formation in block copolymers is obtained by raising the concentration, the temperature and both. From such a viewpoint, Pluronics are lyotropic and thermotropic associating species. The purpose of this contribution is to give informa- tion on the water–F 68 system in a wide range of compo- sitions and temperatures. Changes in thermodynamic and transport properties associated with micelle onset are de- scribed. 0021-9797/$ – see front matter  2004 Published by Elsevier Inc. doi:10.1016/j.jcis.2004.02.064