Micellization and Phase Separation for Triblock Copolymer 17R4 in H2O and in D2O

The reverse Pluronic triblock copolymer 17R4 is formed from poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO): PPO14-PEO24-PPO14, where the subscripts denote the number of monomers in each block. In water, 17R4 shows both a transition to aggregated micellar species at lower temperatures and a separation into copolymer-rich and copolymer-poor liquid phases at higher temperatures. For 17R4 in H2O and in D2O, we have determined (1) the phase boundaries corresponding to the micellization line, (2) the cloud point curves marking the onset of phase separation at various compositions, and (3) the coexistence curves for the phase separation (the compositions of coexisting phases). In both H2O and in D2O, 17R4 exhibits coexistence curves with lower consolute temperatures and compositions that differ from the minima in the cloud point curves; we take this as an indication of the polydispersity of the micellar species. The coexistence curves for compositions near the critical composition are described well by an Ising model. For 17R4 in both H2O and D2O, the critical composition is 0.22 +/- 0.01 in volume fraction. The critical temperatures differ: 44.8 degrees C in H2O and 43.6 degrees C in D2O. The cloud point curve for the 17 R4/D2O is as much as 9 degrees C lower than in H2O.