Influence of the amphiphilicity profile of copolymers on the formation of liquid crystalline mesophases in microemulsions

In bicontinuous microemulsions, the increase of the solubilization capacity of a surfactant, e.g., by the addition of amphiphilic block copolymers (“boosting effect”), is normally accompanied by the formation of liquid crystalline mesophases. We studied a new class of amphiphilic polymers as “boosters,” so-called gradient polymers, with a gradual change from the hydrophilic to the hydrophobic part. Phase diagram measurements and static and quasielastic neutron scattering experiments reveal that the polymers cause a stiffening of the amphiphilic film while simultaneously the saddle splay modulus decreases such that the formation of liquid crystalline mesophases, normally present with diblock polymers as “boosters,” is suppressed, while bicontinuous structures are geometrically favored. In addition, these findings are supported by theoretical calculations following the works of Lipowsky. Hence, gradient amphiphilic polymers not only increase the efficiency of surfactants but simultaneously suppress the formation of liquid crystalline phases thereby greatly increasing their application potential.