The influence of surfactant in reflective HPDLC gratings

The use of highly functional acrylate monomers (large number of reactive bonds) as the precursor to the polymer-rich phase of holographic polymer-dispersed liquid crystals (HPDLC) is well established as providing the best balance between fast polymerization, gelation, and vitrification. Monomers with >3 double bonds (typically between 4 and 5) yield films with the largest diffraction efficiency, all other things being equal. A consequence of this chemistry is that the HPDLC films formed require very high switching fields to dynamically varying their optical properties. These high switching fields are unappealing for many commercial applications. A potential solution has been to add a non-reactive surfactant-like molecule to the reactive syrup. A substantial lowering of the switching field has been attributed to a modification of the anchoring properties of the LC molecules and the polymer host. The dynamics of switching seem to support this supposition as much longer relaxation times are observed. We investigate differences in the polymerization kinetics using differential photocalorimetry (DPC) and relate these results to differences in HPDLC evolution using real-time scattering experiments. Herein, we present large differences in the final morphology of reflection gratings which are supported by optical measurements. We also indirectly show that the surfactant molecules reside near or at the polymer interface by examining contamination of PDLC droplets using polarized optical microscopy and differential scanning calorimetry.