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            The aim of the research has been to determine the behavior of block copolymers that form micelles when those block copolymers are in solvents that show liquid-liquid miscibility gaps.  We studied both micelles in one-component solvents and in two-component solvents.  For two-component solvents, studies of the block copolymer in each solvent alone preceded studies in the solvent mixtures.

Polybutadiene-b­-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and methanol + cyclohexane:      We studied the assembly behavior of a polybutadiene-b­-poly(ethylene oxide) diblock copolymer, PB₈₉-b-PEO₁₃₂ (subscripts indicate the number of monomers in each block), in methanol, in cyclohexane, and the corresponding partially miscible binary solvent mixture.  Dynamic light scattering indicated that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents.  In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures.  Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature.  At the critical solvent composition, micelles form in the single phase region above the critical temperature.  Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase, and that the preference becomes more pronounced as temperature decreases.

            Micellization of PB₈₉-b-PEO₁₃₂ was then studied by small angle neutron scattering (SANS) in deuterated methanol and in deuterated cyclohexane, in order to clarify the morphology of the micelles.  In deuterated methanol, spherical micelles occur and change little in size or shape over a 50 ^oC temperature span.  In deuterated cyclohexane, the copolymer forms flexible, cylindrical micelles below 40 ^oC, and these micelles become spherical in shape at higher temperatures.  Thus the micellization in the pure solvents is changed by the deuteration of the solvents -- an important point since it is often assumed that deuteration of a solvent does not affect properties.

 Poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide) in H₂O and in D₂O:    We then studied the behavior of the triblock copolymer Pluronic 17R4, poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(propylene oxide), PPO₁₄PEO₂₄PPO₁₄, in H₂O and in D₂O.  This fascinating system, first studied in H₂O by Zhou and Chu [Macromolecules, 27, 2025 (1994)], 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 the H₂O part of the study, we collaborated with Professor Donald T. Jacobs at The College of Wooster.  For the D₂O work, we collaborate with B. Hammouda at the National Institute of Standards and Technology. 

          We used visual observations, SANS, and dynamic light scattering to characterize the behavior of 17R4 in H₂O and in D₂O.  We have determined (1) the phase boundaries corresponding to the micellization lines, (2) the cloud point curves marking the onset of phase separations, and (3) the coexistence curves for the phase separations.  In both H₂O and in D₂O, the coexistence curves have lower consolute temperatures and compositions that differ from the minima in the cloud point curves, which is an indication of the polydispersity in the aggregated 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 ºC in H₂O and 43.6 ºC in D₂O.  The cloud point for the 17R4/D₂O is as much as 9 ºC lower than in H₂O.  Scattering techniques indicate the presence of a network, or clustering, of unimers in the low temperature phase.  We do see aggregates above the micellization lines, but we are still analyzing those data.

 Pluronic P85 in D₂O:    The last study was a "side project" between graduate student B. Clover and B. Hammouda at NIST.  In dilute solutions of Pluronic P85 (PEO₂₆PPO₄₀PEO₂₆) in D₂O, transitions between clustered unimers, spherical micelles, cylindrical micelles, and lamellar micelles are observed with increasing temperature.  The effect of pressure on this system was examined by SANS techniques.  At temperatures above 95 ^oC, a new phase of “demixed lamellae” was observed.