Johan George Kloosterboer

Monitoring of polymerization-induced phase separation by simultaneous photo-d.s.c./turbidity measurements

Abstract Polymer—dispersed liquid crystals (PDLCs) can be used in displays. They 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 photodiode enables the simultaneous measurement of heat flux and turbidity during polymerization. The heat flux yields the rate and conversion of the polymerization process, whereas turbidity indicates the appearance of a nematic phase. In this way the influence of the LC structure and content, the rate and temperature of polymerization and the cross-linker concentration on the phase separation process have been established for a simple model system. Recording of transmission—temperature curves before and after polymerization reveals the position of cloud points or clearing points in the phase diagram. The use of special cells allows the measurement of transmission—voltage curves on the d.s.c. samples after polymerization. The morphology, which is important with respect to electro-optical performance, strongly depends on the cross-link density at phase separation. Secondary phase separation inside LC domains already separated by cooling has been observed with microscopy during fast polymerization, not during slow reaction.

Photopolymerization of Diacrylates

Experiments show that during the bulk polymerization of diacrylates at room temperature (i) the volume shrinkage cannot keep up with the chemical conversion; this explains the light intensity dependence of ultimate conversion; (ii) inhomogeneity persists far beyond the gel point; (iii) pendant double bonds initially show an enhanced reactivity due to local concentration effects; (iv) the size of domains of crystallizable monomer decreases strongly with increasing crosslink density.