Two-phase nanoscale morphology of polymer/LC composites

Abstract

Small-angle X-ray scattering and high-resolution scanning electron microscopy (SAXS/HRSEM) were utilized to examine the nano (1–100nm) and meso (100–1000nm) scale morphology of polymer dispersed liquid crystal (PDLC) films of varying liquid crystal (LC) concentration. In contrast to the conventional PDLCs derived from photo-initiated step-growth polymerizations, these PDLC films were formed using photo-irradiation of initially homogeneous syrup comprised of highly functional free-radical monomer and liquid crystal, resulting 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 existed on the fracture surface. In contrast, SAXS reveals increasing nanoscale heterogeneity with increasing LC content. The scattering behavior is consistent with a structure factor derived from an Ornstein–Zernicke model indicating that composition fluctuations frozen by network formation exist at the lowest LC concentrations. At higher LC concentrations, a discontinuous LC phase is observed which coalesce into a co-continuous polymer/LC phase between 35 and 40% LC. Above this regime, aggregated beads of polymer form whose size and uniformity steadily increase with concentration. These morphological 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 importance of the polymerization mechanism in controlling the two-phase morphology in PDLCs.