Two-phase nanoscale morphology of polymer/LC composites

Small-angle X-ray scattering and high-resolution scanning electron microsco py (SAXS/KRSEM) were utilized to examine the nano(1-100 nm) and meso (100-1 000 nm) scale morphology of polymer dispersed liquid crystal (PDLC) films o f varying liquid crystal (LC) concentration. In contrast to the conventiona l PDLCs derived from photo-initiated step-growth polymerizations, these PDL C films were formed using photo-irradiation of initially homogeneous syrup comprised of highly functional free-radical monomer and liquid crystal, res ulting in rapid molecular weight increase and network formation prior to or in conjunction with phase separation. Two-phase morphology observable with HRSEM was absent below 20% LC, although fine, small modulation features ex isted on the fracture surface. In contrast, SAXS reveals increasing nanosca le heterogeneity with increasing LC content. The scattering behavior is con sistent with a structure factor derived from an Omstein-Zernicke model indi cating that composition fluctuations frozen by network formation exist at t he lowest LC concentrations. At higher LC concentrations, a discontinuous L C phase is observed which coalesce into a co-continuous polymer/LC phase be tween 35 and 40% LC. Above this regime, aggregated beads of polymer form wh ose size and uniformity steadily increase with concentration. These morphol ogical observations are consistent with analysis of the SAXS data via a two component Debye-Bueche model at low q. The nanoscale features of the PDLCs formed from highly functional free-radical monomers underscore the importa nce of the polymerization mechanism in controlling the two-phase morphology in PDLCs. (C) 2000 Published by Elsevier Science Ltd.