LIQUID-CRYSTAL PHASE-TRANSITIONS IN DISPERSIONS OF ROD-LIKE COLLOIDALPARTICLES

The isotropic-nematic (I-N) phase transition in dispersions of sterica lly stabilized rod-like boehmite (AIOOH) colloids is studied. We have examined the influence of the steric stabilizer, the dispersion medium and the presence of non-adsorbing polymer on the phase transition pro cess. Dispersions in cyclohexane show an I-N phase separation that pro ceeds by a slow sedimentation of a pinned structure, shrinking from th e meniscus, finally forming the nematic phase after weeks or months, d epending on the steric stabilizer used. In toluene the onset of the I- N has shifted to higher volume fractions where individual nematic drop lets grow and sediment, forming the nematic phase after one week. By a dding non-adsorbing polymer to dispersions in cyclohexane the onset of the I-N phase separation shifts to lower colloid volume fractions. At polymer concentrations just above the phase boundary the same scenari o as in the toluene dispersions without added non-adsorbing polymer is observed. At slightly higher polymer concentrations an abrupt change in scenario occurs. Now an interconnected network is formed which star ts to sediment resembling the process in pure cyclohexane dispersions. Clearly small variations in colloidal interactions engendered by the changes in dispersion characteristics considered in this work, have a strong influence on the I-N phase transition process.