AmericanChemicalSociety.com

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

We have first studied the self-assembly of a polybutadiene-b­-poly(ethylene oxide) diblock copolymer in methanol, in cyclohexane, and in the partially miscible binary mixture of methanol + cyclohexane.  Molecular probe experiments indicated that PB₈₉-b-PEO₁₃₂ (subscripts indicate the number of monomers in each block) forms micelles with PEO cores and PB coronas in pure cyclohexane, and micelles with PB cores and PEO coronas in pure methanol.  In both pure solvents, dynamic light scattering indicated that the copolymer forms coexisting spherical and cylindrical micelles.  In the binary solvent mixture, only spherical micelles are observed.  In the methanol-rich phase, spherical micelles form over a wide range of temperatures.  In the cyclohexane-rich phase, spherical micelles are present only near the upper critical solution temperature of the mixture.  At the critical solvent composition, spherical micelles form in the single-phase region above the critical temperature.  Size exclusion chromatography showed that for the binary solvent mixture, the copolymer distributes mostly into the methanol-rich phase, and that this preference becomes more pronounced as the temperature decreases.  This work has resulted in:
(1)  Master's Thesis:  Christopher D. Ploetz, "Micelles of polybutadiene-b­-poly(ethylene oxide) in a binary solvent system," Department of Chemical and Biomolecular Engineering, The University of Maryland, College Park, May 2008. 
(2)  An invited conference presentation: 17th Symposium on Thermophysical Properties, National Institute of Standards and Technology, Boulder, CO, June 22-26, 2009.
(3)  A refereed paper that has been accepted for publication in the ACS journal Langmuir.

We are now studying 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 is a fascinating system, first studied in H₂O by Zhou and Chu [Macromolecules, 27, 1994, 2025], that shows a phase separation with a lower critical solution point.  The "one-phase" region below the critical point shows two regions: the solutions are cloudy at low temperatures and concentrations, and then become clear at higher temperatures and concentrations, with a "transition line" separating these two parts of the phase diagram.  Zhou and Chu report some small assembles (due to the presence of larger copolymers in the polydisperse sample) in the cloudy region, and the formation of true, spherical micelles in the clear region.

For the H₂O part of the study, we collaborate 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.  Preliminary results were presented in two contributed presentations at the 17th Symposium on Thermophysical Properties, National Institute of Standards and Technology, Boulder, CO, June 22-26, 2009.

We have used visual observations, small angle neutron scattering (SANS), and dynamic light scattering (DLS) to characterize the morphologies assumed by 17R4 in D₂O.  It is important to note that D₂O and H₂O are different solvents, and that the behavior of the copolymer can differ between the two solvents.  We do observe the same three regions of the phase diagram (clear, cloudy, two-phase), at roughly the same temperatures (the critical point is a few degrees lower).  The cloudy, low-temperature one-phase region of 17R4 in D₂O shows the presence of  networks of unimers.  The clear, one-phase region shows the presence of aggregates, but the morphology of these aggregates is not yet clear, and there are indications that the spherical micelles proposed by Zhou and Chu in H₂O are not formed in D₂O.  We are also studying the nature of the copolymer in the coexisting phases at higher temperatures.

In H₂O, Dr. Jacobs finds coexistence curves that shift with initial copolymer concentration, perhaps indicating very long times for micellar equilibration.