Rheological and small-angle neutron scattering studies of aqueous solutions of branched PEO-PPO-PEO copolymers - art. no. 031505

Tetronic(R) 908 is a Four-branched copolymer comprised of polyethylene oxid e (PEO) and polypropylene oxide (PPO) blocks fixed on an aliphatic diamine. In aqueous solution, this polymer has an amphiphilic character due to the difference in PEO and PPO solubilities with increase in temperature. This s ystem presents interesting rheological properties depending on the polymer concentration in solution and temperature. At low temperatures and/or conce ntrations, the solution behaves as a Newtonian fluid. For a weight percenta ge p comprised between 2% and 22%, the viscosity of the solution goes throu gh a maximum. Small-angle neutron scattering experiments show that the incr ease in viscosity is due to progressive aggregation of chains. For weight p ercentages higher than p = 22%, the viscosity of the system diverges and th e mixture becomes gel-like. In fact, spectromechanical measurements show th at the solution behaves as an entangled polymer with a high relaxation time in the flow zone. Small-angle neutron scattering (SANS) studies of the sam e solution reveal the presence of several peaks indicating that the micelle s are organized in a cubic structure. We have developed a model that suitab ly describes the SANS curves of this system in the region where micelles ar e Formed. Our model considers that micelles are trilayered spherical entiti es that coexist with unimers by obeying an equilibrium reaction. Moreover, it allows the calculation of several parameters such as the volume fraction of micelles. their size, and composition. The evolution of these parameter s with temperature and polymer concentration is detailed. Comparisons with literature models are presented and discussed. This compilation of results has permitted the establishment of precise relations between the structure of this polymer in solution and its rheological properties.