Single molecule spectroscopy of conjugated polymer chains in an electric field-aligned liquid crystal

Using single molecule polarization spectroscopy, we investigated the alignment of a polymer solute with respect to the liquid crystal (LC) director in an LC device while applying an external electric field. The polymer solute is poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (or MEH-PPV), and the LC solvent is 5CB. The electric field induces a change in the LC director orientation from a planar alignment (no electric field) to a perpendicular (homeotropic) alignment with an applied field of 5.5 x 103 V/cm. We find that the polymer chains align with the LC director in both planar and homeotropic alignment when measured in the bulk of the LC solution away from the device interface. Single molecule polarization distributions measured as a function of distance from the LC device interface reveal a continuous change of the MEH-PPV alignment from planar to homeotropic. The observed polarization distributions are modeled using a conventional elastic model that predicts the depth profile of the LC director orientation for the applied electric field. The excellent agreement between experiment and simulations shows that the alignment of MEH-PPV follows the LC director throughout the LC sample. Furthermore, our results suggest that conjugated polymers such as MEH-PPV can be used as sensitive local probes to explore complex (and unknown) structures in anisotropic media.     

The director distribution in a spinning nematic mesophase subject to a statis magnetic field

We have determined the probability distribution function for the director in a spinning nematic mesophase subject to a static magnetic field using electrons resonance spectroscopy. For low spinning speeds the director is found to be inclined to the magnetic field as predicted by a continum theory analysis of the system; there is however a small spread in the angle made by the director with the field. This is inhomogeneity in the director alignment increases smoothly as the angular velocity of the mesophase is increased until at high speeds the director adopts an isotropic distribution mesophase which account for the dispersion in the director produced by sample rotation.     

Linear electromechanical effect in a S*c polymer liquid crystal

A linear electromechanical effect, similar to that observed in low-molecular weight S*_c liquid crystals, is reported for a S*_c polymer liquid crystal. A 20 μm thick film of LCP1 (polymer liquid crystal, S*_c, supplied by BDH) was prepared that could be aligned by shearing. An alternating electric field was applied perpendicular and the lateral oscillating motion of the cover glass was measured and analysed. In addition to a linear response, vibrations with the frequency of the applied field, a quadratic response was also observed. The frequency, temperature and applied voltage dependences were measured. The response depends strongly on the alignment. The qualitative analysis of the effects indicates that the oscillations are due to viscous coupling between director reorientation and flow.     

Twin nematic phenylbenzoates in a.c. electric fields

The orientational behaviour of nematic compounds having twin phenylbenzoate mesogens was examined under a wide range of a.c. electric fields (0-2 V mum-1 and 10Hz-50 kHz). For this study, crossed polarizing optical microscopy (POM) and real-time X-ray diffraction (RTXRD) measurements were employed to investigate optical and orientational response. These nematic compounds have a positive dielectric anisotropy and a relatively low epsilon// relaxation frequency which allowed study in both homeotropic and planar orientations over a controllable frequency range. The optical behaviour and X-ray results corresponded well, providing a tool for understanding the orientational behaviour of these liquid crystals. For homeotropic alignment, an electric field of over 1 V mum-1 was required in order to obtain good orientation. However, homeotropic orientation depended on a delicate balance between thermal fluctuations and dielectric torque imposed by the electric field, which are both strongly related to the elasticity of the LC domains. Due to this effect, the highest orientation parameter achieved for homeotropic orientation was only 0.48, which indicated that this state was still non-equilibrium. On the other hand, for planar orientation, a uniform texture with orientation parameter of 0.65 was easily obtained even at electric fields as low as 0.2 V mum-1. The application of an electric field stronger than 1 V mum-1 induced a distortion in the texture, and reduced the orientation parameter to 0.45 for planar alignment.     

Dielectric studies of the nematic mixture E7 on a hydroxypropylcellulose substrate

To study the influence of a polymeric substrate, hydroxypropylcellulose (HPC), on the dynamics of the nematic mixture E7, the real ( ε') and imaginary ( ε" ) parts of the complex dielectric permittivity of the unaligned composite system (polymeric substrate covered with liquid crystal) were measured as a function of frequency and temperature, and compared with those of unaligned pure E7. The temperature range was extended down to the supercooled region. Three superimposed processes were detected, related to different alignment states in relation to the electric field: (1) a main relaxation mechanism due to the hindered rotation of molecules about their short axis, corresponding to the case where the director is parallel to the electric field in an oriented sample, (2) a low frequency process attributed to molecular aggregates, and (3) a high frequency process due to the tumbling of molecules. Processes (2) and (3) correspond to the case when the director is perpendicular to the electric field in an oriented sample. In the composite system the main relaxation mechanism results in a faster process but the other two processes almost superimpose in the frequency window, the high frequency process being much more intense relative to pure E7. The enhancement of the high frequency process in HPC+ E7 can be interpreted as an increase in the number of dipoles whose director has a component aligned perpendicular to the electric field, due to surface effects. The temperature dependence of the main and high frequency relaxation mechanisms obeys the VFT law, which is a feature of glass-like systems.     

Electric-field driven director oscillations in a nematic liquid crystal: a NMR investigation

We have investigated the oscillatory behavior of the nematic director for 4-pentyl-4'-cyanobiphenyl (5CB) when it is subjected to a static magnetic field and a sinusoidal electric field. In these experiments the two fields were inclined at about 50 degrees and the frequency of the electric field was varied from several hertz to approximately 1000 Hz. The director orientation was measured using time-resolved deuterium NMR spectroscopy since this has the advantage of being able to determine the state of director alignment in the sample. In fact, for all of the frequencies studied the director is found to remain uniformly aligned. Since the diamagnetic and dielectric anisotropies are both positive the director oscillates in the plane formed by the two fields. These oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The maximum and minimum angles made by the director with the magnetic field were determined, as a function of frequency, from the NMR spectrum averaged over many thousand cycles of the oscillations. At low frequencies (several hertz) these limiting angles are essentially independent of frequency but as the frequency increases the two angles approach each other and become equal at high frequencies, typically 1000 Hz. Our results are well explained by a hydrodynamic theory in which the sinusoidal time dependence of the electric field is included in the torque-balance equation. This analysis also shows that, for a range of frequencies between the high and low limits, these NMR experiments can give dynamic as well as static information concerning the nematic phase.     

The influence of electric field-induced orientation on the retraction of a liquid crystal droplet immersed in a flexible polymer matrix

We measured the apparent interfacial tension between a liquid crystal and a flexible polymer by deformed droplet retraction method. An external electric field is applied to change the director orientation in liquid crystal droplet. The deformation and recovery of a single liquid crystal droplet dispersed in a polydimethylsiloxane (PDMS) matrix were realized by a transient shear flow and observed by polarized optical microscope. In order to control the director orientation in LC droplet, the electric field is applied perpendicular and parallel to the flow field, respectively. The different orientation induced by electric field in liquid crystal droplet has different behavior during droplet retraction and affect the apparent interfacial tension between liquid crystal and flexible polymer.     

Director distribution in field-induced undulated structures of cholesteric liquid crystals

Structures with a periodic in-plane liquid crystal director field modulation induced by an electric field are studied in cholesteric liquid crystals (CLCs). A phenomenon of the electric-field-induced instability in a planarly aligned cholesteric cell is used to create these undulated structures. The initial field-off state is planarly aligned with the cholesteric helix axis oriented perpendicular to the cell substrates. The interaction of the CLC with an electric field results in modulation of the refractive index, which is visualised as stripe domains oriented either along or perpendicular to the rubbing direction at cell alignment surfaces. The threshold electric field for the undulation appearance and a period of stripes are measured experimentally for three Grandjean zones (ratio d/p ~ 0.5, 1.0, and 1.5, where d is a cell thickness and p is the natural cholesteric pitch). For the zone with d/p ~ 1.0 using numerical simulations, we describe in detail the director distribution at an applied electric field. It is found that the in-plane undulated structure is characterised by a conical director rotation on moving along the alignment direction. The conical axis is tilted with respect to the alignment axis. The sign of the tilt angle depends on the handedness of CLC.     

Dielectric relaxation studies and electro‐optical measurements in poly(triethylene glycol dimethacrylate)/nematic E7 composites exhibiting an anchoring breaking transition

Electric field driven anchoring breakage in poly(triethylene glycol dimethacrylate)/nematic E7 composites was studied using dielectric spectroscopy and transmittance measurements. The transmittance hysteresis observed on increasing and decreasing an applied electric field, associated with different alignment states of the liquid crystal (LC), was monitored through dielectric loss. Essential changes are felt mainly in the δ‐peak, i.e. the dielectric response of the nematic when the director lies parallel to the applied electric field. An irreversible effect persists after the field had exceeded a critical value, which was manifest in a higher transmittance and a higher dielectric strength of the δ‐peak in the OFF state. The initial scattering/opaque state of the sample can only be recovered by heating to the clearing temperature of the nematic LC. The effect referred, commonly called memory effect, is rationalized in terms of anchoring breakage of the LC at the polymer–LC interfaces. The electro‐optical response was tested for different poly(triethylene glycol dimethacrylate)/nematic E7 composites in different composition ratios prepared by polymerisation‐induced phase separation. The lowest threshold field was observed for the 30:70 composite.     

Dielectric study on the flow alignment in 4-n-pentyl-4'-cyanobiphenyl

Dielectric permittivity and loss are measured under steady shear flow as functions of temperature, shear rate, electric field frequency, and electric field strength in the nematic (N) and the isotropic (I) phases of 4-n-pentyl-4'-cyanobiphenyl. In the N phase, the dielectric permittivity in the quiescent state is largely modified if the steady shear flow is applied. These behaviors are discussed based on the Leslie-Ericksen theory [Q. J. Mech. Appl. Math. 19, 357 (1966); Arch. Ration. Mech. Anal. 4, 231 (1960)], showing that the dielectric properties under the shear flow are consistently interpreted in terms of the flow alignment of the director, a unit vector specifying the orientation of the liquid crystals. It is also suggested that the behaviors of dielectric permittivities are similar to those of the viscosities.     

Tunable polarised fluorescence of quantum dot doped nematic liquid crystals

The concentration, excitation photon wavelength, and polarisation dependent fluorescence of quantum dot (QD)–liquid crystal (LC) mixtures has been studied at room temperature using high-resolution, steady-state fluorescence spectroscopy. The fluorescence of QD–LC mixture increases with increasing QD’s concentration but the spectral red shift of ~10 nm relative to the stock QD–Toluene solution remains independent of concentration. In vertical switching (VS) cells, an external electric field changes the LC alignment direction and enhances the apparent fluorescence intensity. The apparent fluorescence anisotropy compared to that at zero applied electric field monotonically increases up to ~27% at an applied electric field of 0.6 V/µm. These results are consistent with the formation of disc-like assemblages of QDs oriented on planes perpendicular to the director of the nematic liquid crystal (NLC). These findings have important utility in polarisation sensitive photonic devices.     

The mechanical and magnetic alignment of liquid crystalline epoxy thermosets

The processing of a new series of liquid crystalline (LC) epoxy networks was evaluated. Above the glass transition temperature, the low crosslink density networks could be mechanically aligned. The mechanically oriented networks readily lost orientation upon heating. Highly anisotropic liquid crystalline (LC) epoxy networks were also prepared by aligning the mesophase of the prepolymer during the curing process under the influence of a magnetic field. Orientation parameters (f) of 0.13 to 0.57 were achieved by these processes as determined by x-ray diffraction analysis. The ability of the magnetically aligned networks to retain their orientation above the glass transition temperature was determined by time-resolved x-ray diffraction. The stability of the alignment of these networks was found to depend on crosslink density. The effect of the anisotropy of these networks was investigated by measuring the coefficient of thermal expansion (CTE). In the aligned networks, there was a substantial reduction in CTE parallel to the direction of the applied field compared to the randomly oriented networks. © 1992 John Wiley & Sons, Inc.     

Flexoelectric response at defect sites in nematic inversion walls

Electric field experiments have been carried out on +1 and ?1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non‐singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c‐director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro‐optic switching characterized by an azimuthal angle variation is observed. When the c‐director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field‐induced structural distortions observed at a ?1 defect having a combination of radial and tangential c‐director fields.     

Director reorientation in nematics studied by microwave dielectrometry

A new experimental method is presented which is a useful approach in studying the reorientational dynamics in liquid crystals by means of dielectric measurements at microwave frequencies. The theoretical model is developed to describe the motion of the director when it is driven by two orthogonal electric or magnetic fields. A specific set up for the experimental apparatus is described. Experiments in fairly good agreement with the theoretical model are given for a nematic mesophase at different temperatures and field values. The capabilities of the method in monitoring the slow reorientational properties of collective molecular motions in liquid crystals are demonstrated. Possible refinements of the experimental apparatus to allow more quantitative measurements of the different physical parameters of anisotropic media are indicated.     

Effect of an electric field on defects in a nematic liquid crystal with variable surface anchoring

We report experimental studies on defects in a nematic liquid crystal with negative dielectric anisotropy mounted in a cell with perfluoropolymer-coated surfaces. The sample exhibits a discontinuous anchoring transition from planar to homeotropic on cooling at zero or a small electric field, and above a cross-over voltage a continuous ‘inverse Freedericksz transition’, at which the director starts tilting in opposite directions at the two surfaces. Defects of strength ±1/2 are either annihilated or expelled when the director tilts. On the other hand, disclination lines of ±1 which end in partial point defects (boojums) at the surfaces in the planar alignment regime acquire point defects of strength ±1 at the midplane of the cell when the director tilts. At a low enough temperature, the homeotropic anchoring becomes strong, and an electric field above the Freedericksz threshold generates the usual umbilic defects, which follow the dynamic scaling laws found in earlier studies.     

Cone motion viscosity and optical second harmonic generation of ferroelectric liquid crystalline dendrimers

We report second harmonic generation in a ferroelectric liquid crystalline trimer and ferroelectric liquid crystalline dendrimers of first, second and third generation. Thin cells were filled with the compounds by capillary forces at elevated temperature, and cooled from the surface stabilized ferroelectric state to below the glass transition temperature, while kept in an electric field. The cone motion viscosity and the threshold electric field for unwinding of the helix axis of the chiral tilted smectic mesophases were studied separately at elevated temperature, and these data were used to optimize the preparation of the films. The measured response time was between 0.3 and 3ms, which corresponds to a cone motion viscosity between 0.5 and 50 Pa s. Second harmonic generation was studied both at elevated temperature with an electric field and at room temperature with and without electric field. The first generation dendrimer exhibited a strong increase in the second order non-linear optical response with time at room temperature. The d₂₃-coefficient of this dendrimer was approximately four times larger than for the other macromolecules and was 0.045 pm V^-1. The relatively large d-coefficient of the first generation dendrimer is ascribed to crystallization, which improved the orientation of the molecular dipoles.     

Thermotropic homopolyesters. IV. Study of fiber formation

We report a melt spinning and viscosity study of two semiflexible homopolyesters containing both rigid and flexible segments in the repeating unit. Single filaments of the polyesters formed from 4,4′-diacetoxybiphenyl and azelaic acid (PB7) and sebacic acid (PB8), and from 4′-hydroxyphenyl-4-hydroxycinnamate and azelaic acid (C7), were spun at temperatures between 205 and 255°C. The temperature dependence of the Newtonian melt viscosity of PB7 and C7 was investigated, and a range of molecular weights was studied for PB7. The spinning parameters, fiber characteristics, and viscosity-temperature behavior are related to the type of mesophase formed. The mechanical properties of fibers spun from both the nematic and smectic phases of these semiflexible chain polymers were poor. Increasing the polymer molecular weight or extrusion rate only afforded a modest improvement in fiber properties. Most polymers could not be spun at temperatures corresponding to the existence of the single mesophase. Hence the low viscosity typical of the nematic mesophase is not necessarily an advantage in fiber formation from the melt. It appears from these results that this type of polyester does not possess adequate chain extension to develop ultrahigh-modulus properties. The director, which describes the local orientation of molecules within the mesophase, may undergo more frequent variations than is the case for rigid chain polyesters. Mechanisms relevant to flexible polymers may contribute to the development of orientation for this class of nematogenic melts.     

On the parallel-perpendicular transition for a nematic phase at a wall

We use an Onsager-level density functional theory to investigate the behaviour of the nematic phase in contact with a solid wall. The nematic consists of hard rigid rods having perfect uniform alignment and uniform spatial density. In the absence of any particle-wall interactions besides excluded-volume forces, we predict a director orientation parallel to the wall. We show that this preference for parallel alignment is due to the entropy associated with the larger volume available to the particles in their parallel orientation. An adsorption energy favouring normal alignment gives rise to a transition from a high temperature parallel orientation to a low temperature normal orientation. We derive expressions for the temperature of this transition, relating it explicitly to the wall adsorption energy, particle axial ratio, and nematic density. Effects such as layering near the wall and imperfect nematic order are argued not to be necessary for the existence of this transition.