Order and Dynamics Inside H-PDLC Nanodroplets: An ESR Spin Probe Study

Abstract

We have performed a detailed study of the order and dynamics of the commercially available BL038 liquid crystal (LC) inside nanosized (50-300 nm) droplets of a reflection-mode holographic-polymer dispersed liquid crystal (H-PDLC) device where LC nanodroplet layers and polymer layers are alternately arranged, forming a diffraction grating. We have determined the configuration of the LC local director and derived a model of the nanodroplet organization inside the layers. To achieve this, we have taken advantage of the high sensitivity of the ESR spin probe technique to study a series of temperatures ranging from the nematic to the isotropic phase of the LC. Using also additional information on the nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross section, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi-monodomain aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep their average orientation even when the layers 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.

Prediction of EPR Spectra of Lyotropic Liquid Crystals using a Combination of Molecular Dynamics Simulations and the Model‐Free Approach

“…EPR with nitroxide spin probes has proven to be a particularly effective technique in the study of the dynamic behaviour of various soft matter systems including lyotropic liquid crystals . EPR has the ability of directly resolving molecular reorientational dynamics on the sub‐nanosecond time scale .…”

“…Both systems are industrially ubiquitous, have well definede xperimentalp hase diagrams exhibitingm ultiple lyotropic phases including micellar,r od/hexagonal and lamellar phases,a sw ell as intermediate mixed structures. [4,15,16] EPR with nitroxide spin probesh as proven to be aparticularly effective technique in the study of the dynamic behaviour of variouss oft matter systems [17][18][19][20] includingl yotropic liquid crystals. [3,4,6,21] EPR has the ability of directly resolvingm olecular re-orientational dynamics on the sub-nanosecond time scale.…”

Chapter 2. Computational approaches for simulating motional EPR spectra

“…[51][52][53][54] For instance, Zanonni and co-workers have employed the SLE method for the fitting of EPR spectra of n-cyanobiphenyl (nCB) nematic liquid crystals doped with the cholestane (CLS) and methyl-5-DOXYL-stearate (5DS) spin probes. 28,53,55 This has allowed the authors to extract rotational diffusion coefficients of molecules along different directions and also to investigate the effects of the dispersed nanometric size silica nanoparticles on the director distribution of the mesogens as well as molecular behaviour in polymer-dispersed LC (PDLC) composites. More recently, Polimeno and co-workers have studied nematic liquid crystalline environments by performing SLE simulations of the EPR spectra of the prototypical nitroxide probes 4-hexadecanoyloxy-2,2,6,6-tetramethylpiperidine-1-oxy and pmethylthiophenylnitronyl nitroxide in isotropic and nematic phases of nematic liquid crystals.…”

Optical properties of composite heterophase objects with liquid crystal material for different display applications

“…Many of such systems belong to heterophase objects. There are porous systems made of polymers or solid crystals with the LC incorporated into the pores or confined space: polymer dispersed liquid crystals (PDLC), [6][7][8][9][10][11] polymer stabilzed liquid crystals, 12 fibers, 13 porous silicon, 14,15 or porous anodized aluminum oxide. 16,17 Recently, many developments are described with mesogens comprising microparticles and nanoparticles for reduction of operation voltage or switching time.…”