Lyotropic liquid crystalline phase behavior of amphiphilic monomers and polymers having a rod-like hydrophobic moiety

Distinct changes in the lyotropic mesophase behaviour of nonionic polyethyleneglycol containing amphiphiles are observed, when rigid, rod-like molecular units are introduced into the hydrophobic part of these amphiphiles, or when the amphiphiles are added as side chains to poly(methylhydrogene siloxane), yielding amphiphilic side chain polymers. Low molar mass amphiphiles with a dominating rigid, rod-like hydrophobic unit form planar-like micelles which aggregate into lamellar mesophases. Compared with their corresponding low molar mass amphiphiles, amphiphilic side chain polymers show a distinct stabilization of mesophases composed of anisometric micelles, i.e. hexagonal H₁-phases (built up by rod shaped micelles), and lamellar L -phases (built up by planarshaped micelles). In addition, several of the polymeric amphiphiles exhibit the formation of a lyotropic nematic N_c-phase regime. Optically isotropic I₁-phases composed of approximately spherical-like micelles are, on the other hand, disfavoured by the polymerfixation. Investigations of monodisperse amphiphilic side chain oligomers reveal a linear relationship between the thermal stability of the individual mesophases and the logarithm of the degree of polymerization. Changes in mesophase behaviour, due to the introduction of rigid, rod-like molecular units into the hydrophobic part of the low molar mass amphiphiles, are ascribed to packing restraints of the amphiphiles in the micelles. Alterations in mesophase behaviour which occur as a consequence of polymerfixation are discussed in terms of changes in micellar kinetics, packing restraints, and changes in the interaction between adjacent hydrophobic layers of neighbouring micelles.