Micellar structures and dynamics in aqueous solutions of PEO-PPO-PEO block copolymers

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1999.Includes bibliographical references.by Isabella Goldmints.Ph.D

Quantitative description of temperature induced self-aggregation thermograms determined by differential scanning calorimetry

A novel thermodynamic approach for the description of differential scanning calorimetry (DSC) experiments on self-aggregating systems is derived and presented. The method is based on a mass action model where temperature dependence of aggregation numbers is considered. The validity of the model was confirmed by describing the aggregation behavior of poly(ethylene oxide)-poly(propylene oxide) block copolymers, which are well-known to exhibit a strong temperature dependence. The quantitative description of the thermograms could be performed without any discrepancy between calorimetric and van 't Hoff enthalpies, and moreover, the aggregation numbers obtained from the best fit of the DSC experiments are in good agreement with those obtained by light scattering experiments corroborating the assumptions done in the derivation of the new model

Micellar Dynamics in Aqueous Solutions of PEO-PPO-PEO Block Copolymers

The dynamics of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) micelle rearrangements were studied using iodine laser temperature jump experiments with light-scattering detection. Two relaxation processes were detected: the first, fast one was accompanied by an increase in scattered light intensity, while the second, slow process was accompanied by a reduction in light intensity. The Aniansson- Wall theory was used to interpret the experimental results. The equilibrium micelle structures at the start and end points of the temperature jump experiment were used as input for the Aniansson-Wall equations. The solution of these equations agrees qualitatively with experimental data and suggests the mechanisms associated with the two time constants. The first time scale, in the tens of microseconds to about 10 ms range, is attributed to unimer insertion into micelles. The other time scale, in the 1-100 ms range, is associated with the rearrangement of the micelle size distribution. It is shown that the second process is often not observed either because the unimer supply is insufficient or because the micelle number density is not temperature dependent

Microviscosity in poly(ethylene oxide)-polypropylene oxide-poly(ethylene oxide) block copolymers probed by fluorescenece depolarization kinetics

Triblock copolymers [poly(ethylene oxide) (PEC) and polypropylene oxide (PPO)], Pluronic F127 with 100 PEO blocks on each end, and 65 blocks of PPO in the center were examined in aqueous solution. The "Sol" and "gel" phase diagram was determined as a function of concentration and temperature. For further study, the concentration was fixed at 20 wt %, and the temperature dependence of the dynamic viscosity differed from the temperature dependence of fluorescence emission spectra and the microviscosity probed by the fluorescence depolarization kinetics of rhodamine 123 dye, which was dissolved in the continuous hydrophilic phase. The depolarization measurements used single-photon counting after two-photon excitation with a Ti-sapphire femtosecond laser. Although the viscoelastic modulus increased by an order of magnitude when the sol-to-gel transition was crossed, the microviscosity of the hydrophilic continuous medium showed only minor changes. At different temperatures the fluorescence lifetime was the same with a single-exponential time constant, but the fluorescence depolarization displayed a double-exponential decay. After comparison with fluorescence depolarization of the dye in PPO melt and PEO whose molecular weight and aqueous concentrations were varied, the relative proportions of faster and slower components of the fluorescence depolarization were tentatively attributed to varying ratios of the dye in free solution and associated with micelles

Temporal evolution of microstructures in aqueous CTAB/SOS and CTAB/HDBS solutions

Vesicles are formed spontaneously when aqueous solutions of cetyl trimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS), as well as CTAB and dodecyl benzene sulfonic acid (HDBS), are mixed in well-defined ratios. Microstructures in the starting solutions are composition-dependent and, in these experiments, include spherical and rodlike mieelles as well as monomers. Starting from these initial morphologies, relaxation to the equilibrium vesicle state can take several hours to months in the CTAB/SOS system, but the transition occurs within minutes in the CTAB/HDBS system at the concentrations studied. In this paper, the temporal evolution of aggregate microstructures from a range of initial states was monitored using time-resolved turbidity, dynamic light scattering, and cryogenic transmission electron microscopy (cryo-TEM). For the CTAB/SOS system, the turbidity changes slowly over a period of 2h. The rate of growth of the aggregates, measured by dynamic light scattering, was found to be independent of the specific morphology of the initial aggregates and of the added NaBr concentration. The morphologies of intermediate-state aggregates were directly identified by cryo-TEM observations of solutions quenched at different times after mixing and confirmed to be wormlike micelles, disks, and vesicles. The model that emerged for the transitions is that the micelles grow to floppy, undulating disks. The competition between the edge and bending energies drives the transition to small vesicles at a critical disk size. These vesicles then grow to the final size distribution. Varying proportions of each of these aggregates exist at all time points. In contrast to the CTAB/SOS results, both turbidity and dynamic light scattering reveal that the transition to the final size is rapid in the CTAB/HDBS system. Within the time resolution of the cryo-TEM measurements, only vesicles, and no disks are observed. These observations indicate that the bilayer bending energy dominates in this system. The solubility difference between SOS and HDBS could also play a role in the observed difference in kinetics