Studying polymer-dispersed liquid-crystal formation by FTIR spectroscopy. 2. Phase separation and ordering

Polymerization-induced phase separation to form polymer-dispersed liquid cr ystals (PDLCs) is a complex process involving simultaneous curing and phase separation to form nematic microdomains. Real-time FTIR spectroscopy has b een used to simultaneously observe and quantify the curing process using a model system-a fast photocurable matrix (NOA65) and a liquid crystal (E7). While the role of FTIR spectroscopy in monitoring chemical changes is well recognized and reinforced for PDLCs, it is demonstrated in this paper that it is also a powerful tool to monitor physical changes (phase separation an d nematic ordering). Phase separation was detected during the curing proces s by a scattering-induced change in the absorbance spectrum of the sample. Nematic ordering could be observed and quantified based on a change in a ch aracteristic band of the liquid crystal. Moreover, the phase separation and the onset of nematic ordering are temporally resolved. The conversion at p hase separation decreases strongly with an increase in liquid-crystal conte nt, while the conversion required for phase separation increases with incre asing temperature. Isotropic droplets are formed followed by nematic orderi ng in the domains at lower E7 concentrations, while the processes are simul taneous for higher concentrations. Temperatures close to and above the LC t ransition temperature also lead to isotropic domains, which form nematic do mains upon cooling. The fraction of liquid crystal present as nematic dropl ets and total fraction of nematic domains in the PDLC are quantified based on changes in the vibrational spectrum of E7. On the basis of mass balances applied to the closed process, the phase diagram of the system could be de termined as a function of curing temperature. Solubility limits obtained by this method agree well with results obtained by other researchers. The con cept of a spectroscopic composite plot that completely describes the format ion process in a PDLC is proposed.