Formation of self-supporting reversible cellular networks in suspensions of colloids and liquid crystals.

In mixtures of thermotropic liquid crystals with spherical poly (methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transformation. To characterize the process of network formation in terms of morphology, phase transformation kinetics, and mechanical properties, we have combined data from polarization and laser scanning confocal microscopy with calorimetric, NMR, and rheological results. Our data suggest that the mechanism of network formation is dominated by a broadened temperature and time interval of phase transformation rather than by particle size or concentration. The observation that the width of the transformation interval strongly depends on sample preparation supports the hypothesis that a third component, most likely alkane remnants slowly liberated from the particles, plays a crucial role. In addition, calorimetric findings for liquid crystal/colloid mixtures, heated and cooled up to 13 times, point to separation of the liquid crystal into two compartments with different phase transformation kinetics. This could be explained by redistribution and enrichment of alkane in the particle-composed network walls. A further increase of the storage modulus, G', and incomplete dissolution of the networks in the isotropic state indicate that the formation of two compartments during repeated temperature cycles stabilizes the network and confers strong memory effects.