Paul Archer

Electro-optic properties of polymer-stabilized ferroelectric liquid crystals before, during and after photo-polymerization

The electro-optic behaviour of polymer-stabilized polar smectic liquid crystals is studied in detail before and after photo-polymerization of a bifunctional monomer in the fluid SmA* and SmC* phases. We investigate the effect of a stabilizing, phase-separated polymer network on the electro-optic performance of smectic liquid crystals in relation to the electroclinic as well as the ferroelectric electro-optic effects. For the first time we also report time-dependent data obtained during the polymerization process, which allow the study of the dynamics of photo-polymerization in a liquid crystal host and provide evidence for how this polymerization process is accomplished and why the favourable electro-optic properties of ferroelectric liquid crystals are retained in polymer-stabilized FLCs.

Elastic coupling in polymer stabilized ferroelectric liquid crystals

The elastic coupling between a polymer network and liquid crystal has been studied in both the SmA* and SmC* phases of a polymer stabilized ferroelectric liquid crystal polymerized in the SmA* phase. Experimental electric field dependent tilt angle and polarization data are analysed through the SmA* to SmC* transition allowing the full Landau potential to be determined including the polymer network interaction parameter. The elastic coupling between the polymer network and liquid crystal director increases linearly with increasing polymer content. The effect of the network on the system is significant, accounting for approximately 10-15% of the total free energy density. To fully understand the influence of the network further into the SmC* phase, an increasing b-coefficient for increasing polymer concentration needs to be taken into account to describe the change in tilt angle for varying polymer content.

Polymer stabilisation of twisted smectic liquid crystal defect states

Polymer stabilisation of a state similar to a twist grain boundary phase is demonstrated. Layers of micrometre thick polymer networks are formed which exhibit discretely varying orientational order. The polymer network shows a high degree of orientational order with an order parameter of approximately S ≈ 0.8, which is equal for all layers formed, and equivalent to that of a polymer network formed in the smectic A* phase. The mechanism of photo-polymerisation in liquid crystalline defect phases is discussed and may offer potential for the synthesis of photonic structures in the visible and IR wavelength range.

Imaging liquid crystal defects

Polymer stabilization is used to image the director fields of a range of different s = ±1 and s = ±1/2 Schlieren and umbilical defects in nematic liquid crystals, such as radial, vortex and swirl defects of strength s = +1. We demonstrate experimentally the director field around such defects and show that they coincide with schematic representations commonly found in literature. The proposed method can be used non-invasively or invasively, depending on the resolution required.

A bent‐core dopant‐induced smectic A* twist state†

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non‐mesogenic bent‐core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent‐core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation. †. †This paper is dedicated to Sven T. Lagerwall on the occasion of his 70th birthday

Probing the material properties and phase transitions of ferroelectric liquid crystals by determination of the Landau potential

Abstract.The full Landau potential of several, widely varying ferroelectric liquid-crystalline materials has been experimentally determined. Tilt angle and polarisation data is analysed across the SmA * to SmC * transition for varying applied electric-field amplitudes, allowing the determination of all the coefficients of the generalised Landau model of ferroelectric liquid crystals. The materials investigated encompass different materials, including low-polarisation mixtures to high-polarisation single-component materials. The materials also possess a variation in the order of the SmA * to SmC * phase transition from strongly first order to strongly second order. The effects of both the polarisation and order of phase transition of the system are discussed with respect to the various terms of the generalised Landau model. Further, the mechanisms behind the difference between a first- and second-order phase transition are discussed with respect to the Landau potential and the second Landau coefficient b .

Determination of the Landau potential of chiral enantiomer ferroelectric liquid crystal mixtures

The full Landau potential was determined for mixtures of the two chiral configurations of ferroelectric liquid crystal enantiomers. Experimental temperature and electric field dependent tilt and polarisation data are analysed a multi-curve fitting procedure to determine all the parameters of the generalised Landau model for ferroelectric liquid crystals. The three Landau coefficients , and as well as the bilinear coupling, , biquadratic coupling, and dielectric susceptibility, , were obtained as a function of enantiomeric excess. The chirality dependent bilinear coupling term vanishes as the chirality of the system tends to zero on approaching the racemic mixture. All other terms remain constant within the limits of error, providing experimental evidence that the bilinear coupling term is the only chirality dependent term of the generalised Landau model.