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.     

Blue phase III widening in CE6-dispersed surface-functionalised CdSe nanoparticles

The phase transition behaviour of the chiral liquid crystal CE6 doped with spherical surface-functionalised CdSe nanoparticles has been examined by means of high-resolution adiabatic scanning calorimetry and polarising microscopy. The addition of nanoparticles results in an essentially stabilised blue phase III. The phase diagram is displayed upon heating and cooling and the enthalpy changes involved in the conversion between the blue phases are determined. The dispersion of functionalised nanoparticles is prominent for the stabilisation of blue phase III, which is potentially useful for applications, especially if applied on liquid crystals that exhibit blue phases close to room temperature.     

Thermodynamic, structural and morphological studies on liquid-crystalline blue phases

This article describes the progress in experimental studies of liquid-crystalline blue phases during the past 5 years. Additionally, these results are compared critically with the predictions of theories of the blue phases. Areas considered in this review include (i) the thermodynamic stability of the three polymorphic blue phases and the influence of the cholesteric pitch on this; (ii) the phase diagrams of blue phases; (iii) electric field effects on the blue phase structure and stability; (iv) the morphology and growth of liquid single crystals of blue phases; (v) the symmetry and structural properties of the blue phases.     

Induced phase separation in low-ionic-strength cellulose nanocrystal suspensions containing high-molecular-weight blue dextrans

Spontaneous entropic phase separation phenomena occur in a wide range of systems containing highly anisotropic colloidal particles. Among these are aqueous suspensions of negatively charged cellulose I nanocrystals produced by sulfuric acid hydrolysis of native cellulose, which phase separate into isotropic and chiral nematic liquid-crystalline phases. Phase separation of an isotropic phase from a completely ordered nanocrystal suspension may be induced by the addition of salts or nonadsorbing macromolecules. In previous work (Edgar, C. D.; Gray, D. G. Macromolecules 2002, 35, 7400-7406), an isotropic phase was found to form over a period of several days when blue dextran (a sulfonated triazine dye, Cibacron blue 3G-A, covalently attached to high-molecular-weight dextran chains) was added to initially ordered suspensions. Here we report work showing that the observed phase separation was associated with the charged dye molecules attached to the dextran. The Cibacron blue 3G-A dye attached to blue dextran was found to induce greater phase separation than free (unbound) dye; at increasing ionic strength, depletion attractions due to the blue dextran increasingly contribute to the phase separation.     

Thermodynamic,structural and morphological studies on liquid-crystalline blue phases

Abstract

This article describes the progress in experimental studies of liquid-crystalline blue phases during the past 5 years. Additionally, these results are compared critically with the predictions of theories of the blue phases. Areas considered in this review include (i) the thermodynamic stability of the three polymorphic blue phases and the influence of the cholesteric pitch on this; (ii) the phase diagrams of blue phases; (iii) electric field effects on the blue phase structure and stability; (iv) the morphology and growth of liquid single crystals of blue phases; (v) the symmetry and structural properties of the blue phases.     

Blue phases and twist grain boundary phases (TGBA and TGBC) in a series of fluoro-substituted chiral tolane derivatives

A series of fluoro-substituted tolane derivatives: ( R )-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.     

Measurement of the Kerr effect in cholesteric blue phases

The electric field induced birefringence within the blue phases and the isotropic phase of several different liquid crystals has been measured. The temperature dependences within the blue phases are found to be strong, characteristically different for each phase, and unlike what is predicted by simple theoretical arguments.     

Study of polymer-stabilised blue phase liquid crystal on a single substrate

One outstanding feature of the polymer-stabilised blue phase (PSBP) is that it is unnecessary to form an alignment film, which requires a high-temperature baking process. Therefore, PSBPs may enable flexible liquid crystal displays (LCDs) on plastic substrates. In this study, polymer stabilisation of a blue phase (BP) on a single substrate was performed without using a conventional sandwich-type cell, and the electro-optical properties are demonstrated to be similar to those of a sandwich-type PSBP LCD cell. It was experimentally shown that the oxygen which inhibits radical polymerisation is required to be excluded in order to complete the polymer stabilisation in blue phase.     

Electric field-induced blue phases in liquid-crystalline systems of high chirality and negative dielectric anisotropy

Electric field effects on liquid-crystalline blue phases (BP) of high chirality and negative dielectric anisotropy have been studied by polarizing microscopy and reflection spectrometry. Temperature-electric field phase diagrams are presented and selective reflections of different blue phases in electric fields are shown. In systems showing only BP I and BP III but lacking a zero field BP II an electric field-induced blue phase was observed in the temperature region of BP III and was identified as a BP II.     

Red and blue pulse timing control for pulse width modulation light dimming of light emitting diodes for plant cultivation

A pulse width modulation (PWM) light dimming system containing red and blue light emitting diodes was designed and constructed. Cultivation of the plant Arabidopsis thaliana under various light dimming wave patterns was compared. Control of the pulse timing (phase of wave pattern) between red and blue light in PWM light dimming was examined. Different plant growth was obtained by changing the phase of red and blue pulses. Pulse timing control of PWM light dimming for plant cultivation has the potential to act as a method for probing photosynthesis.     

A chiral material with a new phase sequence: Twist grain boundary smectic A phase—blue phases

A new phase sequence: twist grain boundary S_a phase to blue phases was observed in a chiral compound (R)- or (S)-1-methylheptyl 3'-fluoro-4'-(3'-fluoro-4'-octadecyloxybenzoyloxy)tolan-4-carboxylate. It is the first time that blue phases have been found to occur above the TGBA phase in the absence of a cholesteric phase. These phases were characterized by microscopic observation, differential scanning calorimetry, the contact method and study of the racemic system.     

Investigation of the chirality dependence of thermal properties in chiral-racemic mixtures of the cholesteric ester CE6

A scanning adiabatic calorimetric technique has been used to study the thermal properties of the chiral and racemic liquid crystal CE6 and the phase diagram covering the cholesteric phase, the three blue phases and the isotropic phase. The purpose of this investigation is to study thermal properties of liquid crystals as a function of chirality, while all other parameters remain constant. Results for the temperature and the chirality dependence of the enthalpy and of the heat capacity are reported. The latent heats between the cholesteric phase and BPI and between the different blue phases change slightly as a function of the chirality. The total heat of transition at the isotropic phase boundary is independent of the chirality, but with decreasing chirality, we observe a large increase in the latent heat and, correspondingly, a decrease in the pretransitional contribution. These experimental facts are in qualitative agreement with the predictions of a Landau-de Gennes theory for blue phases.     

Blue phase stability of n-OCB homologue chiral nematic liquid crystal mixtures

Blue phase (BP) stability of a chiral nematic liquid crystal (LC) mixture is dependent upon chemical structure as well as physical properties. In this study, the blue phase temperature range dependent on alkyl chain length was investigated in order to evaluate the relationship between blue phase stability and the molecular structures of four kinds of 4-n-alkyloxy-4'-cyanobiphenyl (n-OCB) homologue chiral nematic LC mixtures composed of rod-like nematic LCs. It was confirmed that the blue phase temperature range was strongly dependent upon the molecular parity, K ₃₃/K ₁₁ and the helical twist power of the n-OCB homologues chiral nematic LC mixtures.     

An order-disorder transition in the conjugated polymer MEH-PPV

The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature T(c) of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π-system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature T(c), the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character.     

Investigation of the chirality dependence of thermal properties in chiral-racemic mixtures of the cholesteric ester CE6

Abstract

A scanning adiabatic calorimetric technique has been used to study the thermal properties of the chiral and racemic liquid crystal CE6 and the phase diagram covering the cholesteric phase, the three blue phases and the isotropic phase. The purpose of this investigation is to study thermal properties of liquid crystals as a function of chirality, while all other parameters remain constant. Results for the temperature and the chirality dependence of the enthalpy and of the heat capacity are reported. The latent heats between the cholesteric phase and BPI and between the different blue phases change slightly as a function of the chirality. The total heat of transition at the isotropic phase boundary is independent of the chirality, but with decreasing chirality, we observe a large increase in the latent heat and, correspondingly, a decrease in the pretransitional contribution. These experimental facts are in qualitative agreement with the predictions of a Landau–de Gennes theory for blue phases.     

Shapes of blue phase crystals grown in electric fields

By breaking spatial isotropy, an applied electric field modifies the equilibrium shapes of crystals. Only those crystal shapes whose symmetry is compatible with this reduced spatial symmetry grow. The effect is small in atomic crystals but observable in blue phase crystals which have millions of molecules per unit cell. an analysis is presented discussing electric field-induced modifications to cubic blue phase crystals. Specific examples are shown for BP II.     

Blue phase mixtures exhibiting low fractions of a chiral compound experimental observation of some unusual properties

We have used chiral compounds with very high helical twisting power as components in induced-cholesteric mixtures in order to obtain new blue phase systems. In one of these mixtures an unusual large blue phase temperature range of more than 10 K was observed. The lattice constant of the BPI in this system increases strongly with increasing temperature, a behaviour observed for the first time. The threshold voltage for the field-induced BP/cholesteric phase transition in this mixture decreases with increasing temperature. Thus the reentrant phase sequence cholesteric phase-blue phase-cholesteric phase-isotropic phase can be observed with increasing temperature if an electric field is applied to the sample.     

Shapes of blue phase crystals grown in electric fields

By breaking spatial isotropy, an applied electric field modifies the equilibrium shapes of crystals. Only those crystal shapes whose symmetry is compatible with this reduced spatial symmetry grow. The effect is small in atomic crystals but observable in blue phase crystals which have millions of molecules per unit cell. an analysis is presented discussing electric field-induced modifications to cubic blue phase crystals. Specific examples are shown for BP II.     

Fine tuning the purity of blue emission from polydioctylfluorene by end-capping with electron-deficient moieties

We propose a simple way to achieve pure blue emission and improved device efficiency via capping poly(9,9-dioctylfluorene) (PFO) with electron-deficient moieties (EDMs, such as oxadiazole (OXD) and triazole (TAZ)), which can induce a minor amount of long conjugating length species (regarded as beta phase) to control extents of energy transfer from amorphous matrix to the beta phase. The device efficiency of PFO end-capped with TAZ is higher than that with para-tert-butyl phenyl (TBP) by a factor of 2 (with CsF/Al as cathode), and its electroluminescent spectrum remains stable and with pure blue emission during cyclic operations (C.I.E. color coordinates x = 0.165, y = 0.076, independent of operating voltage and within the limit for pure blue emission x + y < 0.30). The improvement of device efficiency is dependent on the structure of EDM, such as size and planarity. The deep blue emission is originated from the incomplete energy transfer from amorphous matrix to the beta phase induced by the end-cappers.     

Moving blue band caused by cooperation of diffusion and phase separation

Diffusions of Cu(2+) and Fe(3+) in gelatin generate a moving blue band. It is formed by a diffusion of Cu(2+) and a phase separation of gelatin with diffusing Fe(3+). The diffusing Fe(3+) forms Fe(OH)(3) colloids and gathers gelatin molecules from the surroundings. The diffusion of gelatin molecules generates the concentration gradient, resulting in a gel/sol transition in the dilute phase. In the region where the concentration of Fe(3+) is high enough, the gel remains hard, while a sol phase appears under the hard gel. The absorption spectrum of Cu(2+) depends on the concentration ratio of Cu(2+) to gelatin. As a consequence, we can see a blue band in the restricted region between the diffusing front of Cu(2+) and the phase separation front. The movement of the blue band is caused by a coupling of a simple diffusion and the phase separation.