Pluronic L64 micelles near cloud point: investigating the role of micellar growth and interaction in critical concentration fluctuation and percolation.

The structure and the properties of the (Ethylene Oxide)(13)(Propylene Oxide)(30)(Ethylene Oxide)(13) (Pluronic L64, MW approximately 2900) micelles have been studied in the aqueous medium by small angle neutron scattering (SANS), dynamic light scattering (DLS) and viscometry measurements. The aqueous solutions of this triblock copolymer are unique among the pluronic solutions in showing critical concentration fluctuations and a concomitant enhancement in viscosity on approaching their cloud point. So far these results have been attributed solely to the presence of attractive interaction between the spherical L64 micelles. Recent theoretical studies, on the other hand, suggest that L64 micelles prefer a prolate ellipsoidal structure to sphere (Bedrov et al. Langmuir 2007, 23, 12032) and have a predominantly repulsive intermicellar interaction. A comparative analysis of our SANS data based on the spherical and prolate ellipsoidal structure shows that the L64 micelles can be best described by a prolate ellipsoidal structure, the aspect ratios of which increase progressively with increase in temperature. This, together with our viscosity and DLS studies, suggests that the enhanced viscosity of the copolymer solution near the cloud point arises largely due to the anisotropic growth of the micelles to the worm-like structures. The role of the intermicellar attractive interaction has thus been limited to the observed critical concentration fluctuation, and its effect decreases progressively with increase in copolymer concentration. It has also been shown that the water structure making salts like NaCl reduces the micellar growth temperature and helps in forming worm-like micelles at the room temperature. These studies thus identify the effect of micellar growth and interaction in determining the properties of the copolymer solution near the cloud point, which are the first of its kind in the case of the pluronics.