ALBERT

Notes: A theory of micellization of AB diblock copolymer molecules in a selective solvent S is developed here. The micelles are assumed to have a completely segregated core region consisting only of the A block and a shell region consisting of the solvent S and the solvent compatible B block. The theory allows one to predict the critical micelle concentration, the micelle size distribution, the average aggregation number, as well as the core radius and the shell thickness of the micelle. The novel outcome of the present theory, in contrast to the treatments of micellization pioneered by de Gennes, Leibler, Orland, and Wheeler, and Noolandi and Hong, is its prediction that the solvent compatible B block plays an important role in determining the micellization behavior. The influence of the B block becomes relatively more prominent in systems where the solvent S constitutes a very good solvent for the B block. Further, scaling relations that are not system specific have been developed relating explicitly the micellar size parameters to the characteristics of the block copolymer and the solvent.

Notes: A unified thermodynamic theory is developed here to explore the nature of association of nonionic polymer molecules with surfactant aggregates such as globular micelles, rod-like micelles, bilayers, oil in water microemulsions, and water in oil microemulsions. The association complex is visualized as consisting of the polymer molecule wrapped around the aggregate at its interface with water. The association with polymer provides enhanced shielding of the hydrophobic aggregate surface from water, increases the steric repulsions at the aggregate surface and gives rise to hydrophobic interactions between the polymer and the aggregate surface. Simple free energy models are formulated to account for these polymer-induced effects. Illustrative calculations have been carried out to examine the behavior of monoalkyl and dialkyl surfactants with nonionic, anionic, and zwitterionic polar head groups. The results show that polymer–aggregate association is almost always preferred in the case of anionic or zwitterionic surfactants. As a result of the association with the polymer, globular micelles transform into smaller globular micelles; large, polydisperse rod-like micelles change into small, monodispersed globular micelles; spherical vesicles and planar bilayers transform into smaller discoids. In all the cases, the polymer-bound aggregates are formed at a critical micelle concentration (CMC) that is significantly lower than the CMC of the polymer-free systems. In marked contrast, aggregates of nonionic surfactants may or may not associate with the polymer molecule. If association occurs, the polymer-induced changes in the aggregate morphologies are similar to those for the anionic surfactants. However, the CMC for the formation of bound aggregates is usually very close to that for the formation of the free aggregates. In many situations, association is not favored and polymer-free aggregates coexist with free polymer molecules in solution. It is found that subtle variations in the hydrophobic characteristics of the polymer molecule can tilt the equilibrium between polymer-bound and polymer-free aggregates. In the case of microemulsions, calculations show that the volume ratio of the dispersed phase to the surfactant is significantly altered by the presence of the polymer. Also,the condition for phase inversion from oil in water to water in oil type of microemulsion is modified by the presence of the polymer. The theoretical results suggest ways by which one may control the morphologies of surfactant aggregates using polymer additives.

Notes: We develop a theory of solubilization of low molecular weight species in block copolymer micelles, employing the scaling approach and building on the analogy between micelles and star polymers. Specifically, we consider spherical micelles of AB-diblock copolymers formed in selective solvents, in which the core and the shell regions are assumed to have radial concentration variations similar to those occurring in the solution state of a star polymer. Invoking the results for the conformation of a star polymer derived by Daoud and Cotton, expressions are developed for the free energy per molecule of the micelle ΔG, the core radius R, the shell thickness D, and the overall volume fraction of the polymer within the core φA as functions of the micellar aggregation number g. From the minimization of the free energy of an isolated micelle, the equilibrium values for all the micellar structural parameters and for the extent of solubilization are obtained. The results expressed as scaling relations for g, R, D, and φA are derived for various situations of interest including those of the solvent S being a good or theta solvent for the shell block B, and the solubilizate J being a good or theta solvent for the core block A.

Notes: CuCr1−xMgxO2, a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV. © 2001 American Institute of Physics.

Notes: An interfacial rheometer to measure the dynamic response of a liquid–liquid interface subjected to a small amplitude oscillatory shear stress is described. Experiments are conducted in a deep channel rheometer fitted with an oscillatory floor. This instrument is used to examine the rheological behavior of interfaces in the presence of surfactants. Rheological parameters are calculated from a hydrodynamic analysis incorporating interfacial rheological models. It is demonstrated here that an adsorbed layer of macromolecular surfactants at an oil–water interface is viscoelastic even though the adjacent phases are Newtonian.