Phase behaviour and structure of polymer surfactants in aqueous solution. The occurrence of lyotropic nematic phases

Abstract

AbstractThe binary phase diagrams are analyzed in aqueous solutions of monomeric, non‐ionic surfactants and the corresponding polysiloxanes having attached the surfactants as side chains via the hydrophobic parts of the amphiphiles. In contrast to common surfactants these hydrophobic parts additionally contain a rigid, mesogenic biphenyl moiety. While the polymeric surfactants exhibit hexagonal and lamellar lyotropic liquid crystalline phases, similarly to the low‐molar‐mass surfactants, two important results have to be emphasized: (i) The liquid crystalline phase regions are markedly extended with respect to concentration and temperature compared with the corresponding monomers, as shown in a similar way for thermotropic polymer liquid crystals. (ii) The phase sequence of the polymorphic liquid crystalline phases can be changed by polymerization. While for the systems under investigation the monomeric surfactant exhibits a cubic phase, the polymeric surfactant shows no cubic but a nematic phase in front of the hexagonal phase at low surfactant concentration. The nematic phase of the polymeric surfactant is the first example for a nematic phase, which is formed for binary mixtures of non‐ionic surfactants and water.