Rheology of lyotropic liquid crystals obtained from self-assembly of amphiphilic block copolymer at water-oil interface*

At relatively high concentrations of amphiphilic block copolymer, micelles form ordered lyotropic liquid crystals (LLCs) with the length scale in the range of 2-50 nm. Block copolymers in the presence of two selective solvents provide more flexibility in designing different LLCs at a constant temperature and block copolymer composition but different solvent ratios. In this work, we study the rheological characteristics of a series of LLCs with lamellar, normal hexagonal, and reverse hexagonal structures. The flow behavior of LLCs indicates that they have yield stress. We suggest that at relatively low frequencies, defects control the rheological behavior of LLCs, while at high frequencies, the contributions in micellar scale are dominant. Applying high strains on the LLCs decreases the storage modulus in the whole frequency range and decreases the loss modulus in small-frequency regime with negligible effect at high frequencies. The decrease in moduli is reversible and the system relaxes back to its original elastic modulus at rest. The observed behavior can be attributed to the elimination of defects under high strains and their re-formation during long enough rest times.