Aggregation of an amphiphilic rod-coil diblock copolymer, poly(n-hexyl isocyanate)-b-poly-(ethylene glycol) (PHIC-b-PEG), in toluene, a selective solvent for the rodlike PHIC block, was studied using light scattering and polarized optical microscopy. Static and dynamic light scattering data indicate the formation of cylindrical micelles with a hydrodynamic radius of 279 nm in very dilute solutions (weight fraction of polymer, w, from 0.0002 to 0.008) at 25°C. The cylindrical micelles were determined to be disklike (planar), 0.9 µm in diameter and 20 nm in thickness, based on a comparison between the experimental data and the theory of scattering and self-diffusion of ellipsoids. Planar aggregates are expected if nematic interactions of the coronal rod block are dominant. The aggregate size decreases with increasing temperature. The micelles were observed visually in solution with the help of a polarized optical microscope due to the optical anisotropy of the PHIC block. Disklike objects with diameters in the micron range were observed to undergo translational and rotational motion in and out of the focal plane of the microscope.
Small-angle neutron scattering experiments were conducted on a series of off-critical binary polymethylbutylene/polyethylbutylene (PMB/PEB) blends over a wide range of blend compositions, component molecular masses, temperatures, and pressures. The blends become more immiscible with either decreasing temperature or increasing pressure. A simple extension of the Flory-Huggins theory that accounts for finite volume changes of mixing (∆V) is presented. Our extension demonstrates the validity of the usual mean-field theory of scattering from polymer mixtures based on the random phase approximation (RPA) at elevated pressures. We use this framework to analyze the temperature and pressure dependence of the small-angle neutron scattering profiles obtained from binary PMB/PEB blends. We propose that the volume change of mixing is a linear response to the repulsive interactions between monomers. We demonstrate that off-critical PMB/PEB blends can be undercooled or superpressurized deep into the metastable two-phase region (e.g., up to 50°C undercooling) without detectable signs of phase separation. The parameters and the statistical segment lengths obtained by fitting the data obtained in the metastable region are within experimental error of those determined from stable, singlephase PMB/PEB blends well-removed from a phase boundary. This indicates that the concentration fluctuations in the metastable region of the phase diagram have a mean-field character similar to those in stable, single-phase blends that have been extensively characterized by the RPA-based theory.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.