Mechanical properties of thermo-reversible colloid liquid crystal composites

The mechanical properties of concentrated dispersions of sterically-stabili sed colloidal particles in nematic hosts are explored using a combination o f optical microscopy and viscoelastic shear measurements. Starting from an initially homogeneous dispersion in an isotropic host, it is found that the kinetics of the isotropic-nematic transition lead to the formation of a pe rcolated particle network that imparts surprising mechanical rigidity to th e resulting colloid/liquid crystal composite. Specifically, the viscoelasti c storage modulus G, is observed to rise by approximately 5 orders of magni tude within a few degrees of the bulk isotropic-nematic transition temperat ure. Using 4-4'pentyl cyano-biphenyl as the solvent, the composite at room temperature is a self-supporting soft solid over a wide range of particle c oncentrations. The particle microstructure depends sensitively on thermal t reatment and particle concentration and it is found that faster cooling enh ances the visoelastic moduli of the composite. Also, several classes of net work topology have been identified. In all cases studied to date, the parti cles can be redispsersed upon heating the solvent through the nematic-isotr opic transition. The formation of the composite is therefore thermally reve rsible. A rigorous theory to account for the observed mechanical properties is not currently available.