Stable Blue Phase Research Articles

Two-Photon Polymerization of Polydiacetylene

We show that visible light can polymerize diacetylene monomers into polydiacetylene (PDA) in a two-photon process. We monitor the process by measuring Raman intensities of PDA using a Raman laser at 633 nm with variable intensity I and show that the Raman cross section at short times increases as I3, corresponding to a two-photon process. The process generates a relatively stable blue phase PDA, in contrast with UV polymerization that leads to a fast blue to red phase transformation.

Stabilizing the blue phases

We present an investigation of the phase diagram of cholesteric liquid crystals within the framework of Landau-de Gennes theory. The free energy is modified to incorporate all three Frank elastic constants and to allow for a temperature dependent pitch in the cholesteric phase. It is found that the region of stability of the cubic blue phases depends significantly on the value of the elastic constants, being reduced when the bend elastic constant is larger than splay and when twist is smaller than the other two. Most dramatically we find a large increase in the region of stability of blue phase I, and a qualitative change in the phase diagram, in a system where the cholesteric phase displays helix inversion.

Polymer-stabilized liquid crystal blue phases.

Blue phases are types of liquid crystal phases that appear in a temperature range between a chiral nematic phase and an isotropic liquid phase. Because blue phases have a three-dimensional cubic structure with lattice periods of several hundred nanometres, they exhibit selective Bragg reflections in the range of visible light corresponding to the cubic lattice. From the viewpoint of applications, although blue phases are of interest for fast light modulators or tunable photonic crystals, the very narrow temperature range, usually less than a few kelvin, within which blue phases exist has always been a problem. Here we show the stabilization of blue phases over a temperature range of more than 60 K including room temperature (260-326 K). Furthermore, we demonstrate an electro-optical switching with a response time of the order of 10(-4) s for the stabilized blue phases at room temperature.

Interferometric Determination of Electrooptical Effects in Liquid‐Crystalline Blue Phases

AbstractIn cholesteric liquid crystals with high chirality up to three thermodynamically stable blue phases (BPI, BPII and BPIII) occur in a narrow temperature range below the isotropic phase. The BPI and BPII are characterized by a three‐dimensional helicoidal structure of cubic symmetry with lattice constants of the order of several hundred nanometers.An applied electric field causes a i) change of the BP lattice constants (electrostrictive effect) and ii) a field‐induced birefringence (electrooptical effect). At high fields, new field‐induced phases with different symmetries compared to the zero field blue phases were observed.To get further informations about the molecular arrangement within the blue phases, the electrooptical effect has been studied with a modified microscope‐interferometer in BPs of systems with positive and negative dielectric anisotropy. For the first time an electrooptical effect has been observed also in field‐induced blue phases.

The influence of pretransitional phenomena on blue phase range

Abstract The particular phase in which a liquid crystal system will exist is that which has the lowest free energy at a certain temperature. The free energy may depend on variables such as temperature, pressure, chirality, etc. One way in which the stability of a particular thermodynamic phase, relative to its neighbours, would be manifest is in its temperature range. The effect of chirality, in particular, on the temperature range or stability of blue phases has been well-studied both experimentally and theoretically. To date these studies assume that chirality is the only parameter which will influence the existence of blue phases. However, blue phases with relatively low chirality and broad range, which should in principle only show very narrow blue phases, have been reported. This suggests that factors other than chirality are involved in blue phase stability. In this paper we investigate the phase stability of various blue phase mixtures containing equal amounts of a chiral dopant, via their blue phase temperature range. Correlation between blue phase stability and the elastic constants k 2 and k 22, molecular length of the nematic host, and the order parameter at the blue phase to cholesteric transition is discussed. We have confirmed that for our mixtures the total blue phase temperature range may be related equally to the elastic constant k 22 and the chirality. We also present the first data displaying an odd-even effect in blue phases. Finally, we have found an apparent correlation between the stability of the blue phases and the magnitude of the orientational order parameter of the cholesteric phase at the cholesteric to blue phase transition.

Heat capacity peaks observed at the blue phase transitions in compounds of different purities

Abstract Latent heat measurements, determined by high resolution and slow temperature scanning calorimetry (10−3o C/min), in three compounds of differing purity—where any enantiomer of opposite handedness to that of the predominant enantiomer is included in the impurity count—suggest a correlation between blue phase stability and material purity when chiral excess is included in estimates of impurity, the data suggest that materials with less than ∼ 1.3 per cent impurities present will not exhibit blue phases even though their pitch is small enough to support blue phases. One class of compounds is discussed where this condition may indeed apply. Furthermore, the magnitude of the total latent heat of blue phase transitions was observed to be sensitive to scan rates: a significantly smaller latent heat was measured at the slowest scan rates compared to faster scan rates and the magnitude of the difference was largest for the isomerically purest compound.

Investigation of the blue phase in a compensable cholesteric mixed system

Pitch dependence of cholesteric blue phase stability has been investigated in a compensable mixed system of cholesteryl nonanoate/chloride. Two critical pitches above which no blue phase exists were found depending on the structure of the liquid crystal. The clearing enthalpy does not exhibit discontinuities over the whole mixing range.