Viscoelastic rheology of colloid-liquid crystal composites

摘要

Gelation in colloidal suspensions is mostly induced by attractive interparticle potentials. Besidethese interactions, the mechanical properties of the gel are influenced by morphological aspects likefractality. In suspensions of liquid crystal (LC) and polymeric colloids, solvent-particle interactionsdominate and can be changed when the mesogen undergoes phase transition from isotropic tonematic. In case of poly(methyl methacrylate) colloids and 4-penty1-4'-cyanobiphenyl (5CB),cooling through the isotropic-nematic phase transition results in a cellular network. Such networkformation is accompanied by a strong evolution of the mechanical properties. Shear moduli reachvalues up to 10~6Pa for temperatures of 15 K below the transition. Until now, mechanicalresponse 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 explainedsatisfactorily. We used a homemade piezorheometer to measure the complex shear modulus of thesample in parallel plate geometry. Since the applied strains are very small, only the linearviscoelastic regime was tested. This limit guarantees a high degree of reproducibility. We gainedinsight into the underlying processes by measuring the frequency response for the whole coolingprocess. Temperature and frequency showed a strong correlation allowing for a superposition of thefrequency spectra to form a single master curve similar to time-temperature-superposition. Wepropose that this superposition behavior is connected to the thermodynamics of theisotropic-nematic phase transition of 5CB located in the network walls. Additional experimentalobservations, such as hysteresis effects, support this assumption. Morphological aspects were foundto be of minor relevance.