Membrane decoration by amphiphilic block copolymers in bicontinuous microemulsions

The effect of various amphiphilic block copolymers of different molar masses on the structure and phase behavior of ternary amphiphilic systems (water, oil, and nonionic surfactant) is investigated. Small amounts of PEP-PEO block copolymer lead to a dramatic increase in the volumes of oil and water, which can be solubilized in a bicontinuous microemulsion. High-precision neutron scattering experiments with a sophisticated contrast variation technique demonstrate that the polymers form uniformly distributed mushroom conformations on the surfactant membrane. Based on these observations, we propose a universal mechanism for the swelling behavior, which is due to the variation of the membrane curvature elasticity.

Topology, phase instabilities, and wetting of microemulsion networks

We predict theoretically the gradual formation of fluctuating, connected microemulsion networks from disconnected globules as the spontaneous curvature is varied, in agreement with recent direct measurements of these topological transitions. The connectivity induced instability together with emulsification failure of the network relate the ultralow tensions and wetting transition to the changing microstructure.

Doppler shift anisotropy in small angle neutron scattering

The two-dimensional patterns in our small angle neutron scattering (SANS) experiments from rapidly moving aerosols are anisotropic. To test the kinematic theory of two-body scattering that describes the anisotropy, we conducted SANS experiments using a constant source of D2O aerosol with droplets moving at approximately 440 m/s, and varied the neutron velocity from 267 to 800 m/s. The theoretically predicted anisotropy of the laboratory scattering intensities agrees well with the experimental results. Based on an analysis of the scattering intensity in the Guinier region, we also determined the particle velocity. The results are in very good agreement with independent velocity estimates based on supersonic flow measurements.

Periodicity-dependent stiffness of periodic hydrophilic-hydrophobic heteropolymers

From extensive Monte Carlo simulations of a Larson model of perfectly periodic heteropolymers (PHP) in water, a striking stiffening is observed as the period of the alternating hydrophobic and hydrophilic blocks is shortened. At short period and low temperature needlelike conformations are the stable conformations. As temperature is increased thermal fluctuations induce kinks and bends. At large periods compact oligomeric globules are observed. From the generalized Larson prescription, originally developed for modeling surfactant molecules in aqueous solutions, we find that the shorter the period is the more stretched the PHP is. This novel effect is expected to stimulate polymer synthesis and trigger research on the rheology of aqueous periodic heteropolymer solutions.