MONITORING OF POLYMERIZATION-INDUCED PHASE-SEPARATION BY SIMULTANEOUSPHOTO-DSC TURBIDITY MEASUREMENTS/

Polymer-dispersed liquid crystals (PDLCs) can be used in displays. The y can be made by polymerization-induced phase separation in a mixture of monomer and LC. Modification of a photo-d.s.c. with a laser and a p hotodiode enables the simultaneous measurement of heat flux and turbid ity during polymerization. The heat flux yields the rate and conversio n of the polymerization process, whereas turbidity indicates the appea rance of a nematic phase. In this way the influence of the LC structur e and content, the rate and temperature of polymerization and the cros s-linker concentration on the phase separation process have been estab lished for a simple model system. Recording of transmission-temperatur e curves before and after polymerization reveals the position of cloud points or clearing points in the phase diagram. The use of special ce lls allows the measurement of transmission-voltage curves on the d.s.c . samples after polymerization. The morphology, which is important wit h respect to electro-optical performance, strongly depends on the cros s-link density at phase separation. Secondary phase separation inside LC domains already separated by cooling has been observed with microsc opy during fast polymerization, not during slow reaction. Copyright (C ) 1996 Elsevier Science Ltd.