Salt influence upon the structure of aqueous solutions of branched PEO-PPO-PEO copolymers

Triblock copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), present an amphiphilic character in aqueous solution. Since PPO is less hydrophilic than PEO and since their solubilities decrease when the temperature increases, the copolymers self-assemble spontaneously, forming micelles at moderate temperature. For higher temperature or concentration, these solutions can form lyotropic liquid crystalline phases. In order to improve their properties for particular applications, these solutions are often used in the presence of salts. The influence of the salts nature is qualitatively well described for linear copolymer solutions of PEO-PPO-PEO type. The presence of a 'salting out' electrolyte shifts the cloud point and the critical micelle temperature to lower temperature while 'salting in' electrolyte induces opposite effects. In this article, we study the influence of the presence of sodium chloride (NaCl) on the structure of aqueous solutions of a four-branched copolymer. First, we show by viscosimetric studies that NaCl has a 'salting out' effect with these star copolymers as with theirs linear counterparts. Secondly, small angle neutron scattering studies make it possible to determine quantitatively the influence of the NaCl amount on the equilibrium unimers-micelles. These experiments allow obtaining the radius of micelles, their volume fraction and their aggregation number. These informations are compared with the data obtained with salt-free solutions. SANS results are also used to demonstrate that the micelles form bcc polycrystals in a significant zone of the phase diagram. The cell parameter of these crystals is determined according to the salinity of the solution. At last, we observe that the solution presents at high temperature a phase separation. Our results show clearly that this phenomenon is due to the dehydration of the PEO external layer of the micelles.