Stable responsive diblock copolymer micelles for rheology control

Stable block copolymer micelles with crosslinked rubbery cores serve as model soft colloids with tunable rheology. We demonstrate this by studying systematically two such micelles: one with a small polyisoprene core and long polystyrene hairs that dominate the response of the system, and another with a larger polybutadiene core and shorter polystyrene hairs, exhibiting colloid-like behavior. In addition to mass concentration, we use temperature as a means to vary the volume fraction and show that achieving the same volume fraction by these two different routes yields different macroscopic properties. The number density of the particles and osmotic pressure effects are the origin of the difference. The response of these micelles to varying solvent quality conditions has unexpected rheological consequences: remarkably, for concentrated colloid-like micelles, the viscosity exhibits a non-monotonic temperature dependence. This is attributed to particle swelling and changing particle interactions with temperature. It is thus evident that such systems provide ways for tailoring the flow of soft colloids

Kinetics of Light-Induced Concentration Patterns in Transparent Polymer Solutions

When exposed to weak visible laser light, solutions of common polymers like poly­(isoprene) and poly­(butadiene) respond by local concentration variations, which in turn lead to refractive index changes. Various micropatterns have been recently reported, depending mostly on the solvent environment and the irradiation conditions. Here, we focused on the simpler case of single polymer-rich filaments and we employed phase contrast microscopy to systematically investigate the influence of laser illumination and material parameters on the kinetics of the optically induced local concentration increase in the polydiene solutions. The refractive index contrast of the formed filaments increased exponentially with the laser illumination time. The growth rate exhibited linear dependence on the laser power and increased with polymer chain length in semidilute solutions in good solvents. On the contrary, the kinetics of the formed filaments appeared to be rather insensitive to the polymer concentration. Albeit the origin of the peculiar light field-polymer concentration coupling remains yet elusive, the new phenomenology is considered necessary for the elucidation of its mechanism