Viscoelastic rheology of colloid-liquid crystal composites

Gelation in colloidal suspensions is mostly induced by attractive interparticle potentials. Beside these interactions, the mechanical properties of the gel are influenced by morphological aspects like fractality. In suspensions of liquid crystal (LC) and polymeric colloids, solvent-particle interactions dominate and can be changed when the mesogen undergoes phase transition from isotropic to nematic. In case of poly(methyl methacrylate) colloids and 4-pentyl-$4′$-cyanobiphenyl (5CB), cooling through the isotropic-nematic phase transition results in a cellular network. Such network formation is accompanied by a strong evolution of the mechanical properties. Shear moduli reach values up to $106Pa$ for temperatures of 15 K below the transition. Until now, the mechanical response of the gel was attributed to the elastic interactions of the LC with the colloids. However, the dynamic viscoelastic stiffening with decreasing temperature could not be explained satisfactorily. We used a homemade piezorheometer to measure the complex shear modulus of the sample in parallel plate geometry. Since the applied strains are very small, only the linear viscoelastic regime was tested. This limit guarantees a high degree of reproducibility. We gained insight into the underlying processes by measuring the frequency response for the whole cooling process. Temperature and frequency showed a strong correlation allowing for a superposition of the frequency spectra to form a single master curve similar to time-temperature-superposition. We propose that this superposition behavior is connected to the thermodynamics of the isotropic-nematic phase transition of 5CB located in the network walls. Additional experimental observations, such as hysteresis effects, support this assumption. Morphological aspects were found to be of minor relevance.