Time dependent growth of the block copolymer P123 micelles near cloud point: employing heat cycling as a tool to form kinetically stable wormlike micelles.

The sphere-to-rod growth behavior of the triblock copolymer EO(20)PO(70)EO(20) (P123) micelles has been studied in an aqueous medium by dynamic light scattering (DLS), viscometry, and small angle neutron scattering (SANS) techniques. Unlike the other aqueous pluronic systems, the P123 solutions show a time dependent sphere-to-rod micellar growth in the aqueous medium on approaching their cloud points. The rate of micellar growth increases with increase in temperature, but quite interestingly, it improves rather dramatically when the copolymer solutions are subjected to heat cycling, i.e., heating them to the phase separation and subsequently cooling them back to below their cloud points. The observed kinetically restricted micellar growth has been attributed to the slow dynamics of the micellar restructuring processes essential to arrive at the temperature dependent equilibrium structure. It has been suggested that the improvement in the micellar growth rate upon heat cycling is due to overcoming of the activation energy associated with the micellar restructuring process. In the presence of water-structure-making salts like NaCl, such heat cycling produces kinetically stable wormlike micelles at room temperature, which is observed for the first time in the aqueous pluronic systems.