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

In Part I of this thesis, the structure and dynamics of micelles in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers were investigated. The structure of micelles was studied using small-angle neutron scattering (SANS). A model-independent contrast variation method was used to obtain the aggregation number in solutions at lower concentrations, when the Guinier approximation can be used. To determine the structure of the micelles (core and corona radii), a number of spherical core-corona models were explored. It was found that a model allowing for different water contents in the core and corona described the experimental data well for all temperatures and concentrations. However, at lower concentrations the model fits were non-unique, and equally good fits could be obtained for widely varying core and corona water contents. At higher concentrations, steric interactions between micelles are present. These interactions were described using a hard sphere interaction potential with the assumption that the interaction radius is equal to the corona radius. Only with this constraint did the model become sufficiently sensitive to the fitted parameters to allow us to extract meaningful information on the water content in the core and the corona. It was conjectured that these conclusions could be extrapolated to lower concentrations. However, the details of the internal structure could not be determined because the scattering length densities (SLD) of PEO and PPO are very close, making it impossible to distinguish experimentally between the PPO and PEO regions within the micelle. To verify the model and obtain information on the internal structure of the micelles, a copolymer with deuterated PEO blocks was used. The different SLDs of the d-PEO and PPO blocks make it possible to distinguish between these blocks. By using a range of D20 to H2 0 ratios in the solvent to highlight different regions of the micelle, PPO and PEO regions can be studied independently. The assumption of equality of the corona and the hard-sphere interaction radii was not required. We used only dilute solutions for which the contributions from the structure factor to the scattering were negligible. The size of the PPO core was obtained directly from the form factor, which is very sensitive to the core radius. An analysis of the sensitivity 2 of the model to all fitting parameters was performed. It was found that the model is very sensitive to the aggregation number and the core radius, and less sensitive to the corona radius. The effect of micelle polydispersity was studied. It was shown that the standard deviation of the aggregation number is at most 25%. The results of the experiments were used to verify the validity of the previously developed corecorona model for lower concentrations. The average aggregation number was found to increase with increasing temperature with a corresponding decrease in the micellar water content. The number density of the micelles was found to decrease with temperature at higher concentrations. Now that the validity of the model has been established, it can be used to describe the micelle structure of commercial protonated copolymers. The dynamics of block-copolymer micelle rearrangements were determined by following the relaxation from one equilibrium state to another after creating a temperature perturbation using a 2 ps iodine laser pulse. The relaxation of the system was monitored by measuring the change in intensity of scattered light at 90'. For many polymers, two time scales were identified. The first, fast relaxation process was accompanied by an increase in the light scattering intensity, while the second, slow relaxation process was accompanied by a decrease in the light scattering signal at temperatures close to the CMT (critical micellization temperature). The second relaxation process was detected only in a certain concentration and temperature range. The Aniansson-Wall theory of stepwise micellization was used to interpret the experimental results. A numerical solution of the Aniansson-Wall equations was obtained for the first time with experimentally measured initial conditions. The numerical solution of the Aniansson-Wall equations agrees with experimental data and suggests the mechanisms associated with the two time constants. The first time scale, in the tens of microseconds range, is attributed to monomer insertion into the micelles. The second time scale, in the millisecond range, is associated with the rearrangement of the micelle size distribution accompanied by a decrease in the micelle number density. For the second process, the Aniansson-Wall theory predicts a negative amplitude of the light scattering signal in the certain concentration and temperature range, and it explains the lack of it at other conditions. In Part II of the thesis, the process of spontaneous vesicle formation in mixtures of cationic and anionic surfactants was studied. It was found that aggregate growth during the first three hours after mixing the initial solutions is independent of the presence of mixed micelles in the initial solutions and of added salt concentration. Viscosity and dynamic light scattering measurements suggest that the intermediate aggregates are nearly neutral and are non-equilibrium vesicles or mixture of vesicles and disk-like structures. Vesicles formed in the mixture were imaged by cryo-TEM. Thesis Supervisor: T. Alan Hatton Title: Ralph Landau Professor of Chemical Engineering Practice Thesis Supervisor: Kenneth A. Smith Title: Edwin R. Gilliland Professor of Chemical Engineering