Polymer, Vol.42, No.3, 1055-1065, 2001
Two-phase nanoscale morphology of polymer/LC composites
Small-angle X-ray scattering and high-resolution scanning electron microscopy (SAXS/KRSEM) were utilized to examine the nano(1-100 nm) and meso (100-1000 nm) 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 Omstein-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.
Keywords:polymer dispersed liquid crystal;polymerization-induced phase separation;small angle X-ray scattering