The effect of amphiphilic polymers with a continuous philicity profile on the membrane properties in a bicontinuous microemulsions studied by neutron scattering

Wherever surfactants are applied, it is of general interest to use as little surfactant as possible. However, in e.g. microemulsion systems increasing the solubilization capacity of an amphiphilic mixture is always accompanied by the formation of liquid crystalline mesophases. Integrating amphiphilic block copolymers (so called efficiency boosters) into the amphiphilic film leads to a considerable increase of the efficiency. This effect is mainly due to an increase of the bending rigidity of the amphiphilic film due to the presence of polymer domains on either side of the film. The formation of liquid crystalline mesophases was found to be suppressed at low concentrations of blockcopolymers, while it was enhanced at high concentrations. To break this trade-off between surfactant efficiency and the stabilization of liquid crystalline phases we studied a new class of amphiphilicpolymers - so called tapered polymers - following the amphiphilicity profile going from diblock via triblock to continuously tapered [1]. In contrast to the commonly used diblock or triblock copolymersthe molecular structure of tapered polymers gradually changes from hydrophilic to hydrophobic. The influence of this new class of polymers on the properties of microemulsion systems was investigatedby systematic phase behaviour studies, SANS (small angle neutron scattering) and NSE (neutron spin echo) experiments. These measurements reveal that the polymers cause a stiffening of the amphiphilicfilm while simultaneously the saddle splay modulus increases considerably less such that the formation of liquid crystalline mesophases is suppressed while bicontinuous structures aregeometrically favored. In addition, these findings are supported by theoretical calculations following the works of Lipowsky [2]. Hence tapered amphiphilic polymers not only increase the efficiency ofsurfactants, but simultaneously suppress the formation of liquid crystalline phases, and, thereby, greatly increasing their application potential.The underlying NSE experiments for this interpretation rely on smallest changes of the relaxation curves (of ca. 1% steps) for still small changes of the bending rigidity (of ca. 10% steps). This high reliability of the experiments conducted at the SNS-NSE displays the accuracy of the instrument itself and the latest developments of the evaluation software, which were necessary to interpret such tiny changes of the bending rigidity reliably.[1] H.F.M. Klemmer, J. Allgaier, H. Frielinghaus, O. Holderer, Soft Matter (submitted) 2016.[2] C. Hiergeist, R. Lipowsky, Journal de Physique II, 6, 1465-1481 (1996