Order and dynamics inside H-PDLC nanodroplets: an ESR spin probe study

We have performed a detailed study of the order and dynamics of the commercially viable BL038 liquid crystal (LC) inside nanosized (50–300 nm) droplets of an actual reflection-mode Holographic-PDLC (H-PDLC) device where the nanodroplets are located in layers alternated to polymer layers forming a diffraction grating. We have determined the macroscopic configuration of the LC local nematic domain director and we have derived a model of the nanodroplet organization inside the layers. To do this we have taken advantage of the high sensitivity of the electron spin resonance (ESR) spin probe technique, not previously used to study these materials. The spectroscopic analysis was conducted at a series of temperatures ranging from the nematic to the isotropic phase of the LC. In conjunction with SEM images of the H-PDLC cross-section, which provide additional information on the nanodroplet size and shape distribution, the observed director configuration has been modeled as a bidimensional (cylindrical) distribution of elongated nanodroplets (prolate ellipsoid) whose long axis is, on the average, parallel to the layers and whose internal director configuration is a quasi-monodomain (“stretched” bipolar) aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep an average orientation parallel to the nanodroplet layers even when these are perpendicular to the magnetic field, suggesting that the molecular organization is dictated mainly by the confinement. This result might explain, at least in part, (i) the need for switching voltages significantly higher and (ii) the observed faster turn-off times in H-PDLCs compared to standard PDLC devices.