Low Molecular Liquid Research Articles

Structure, Dynamics and Properties of Materials with Polymers Having Complex Architectures

AbstractVarious non‐linear highly branched polymers such as multiarm stars, block copolymer micelles and bottlebrush‐like polymers have been studied in order to analyze their intramolecular structure and effects of spatial ordering resulting from their specific macromolecular architecture. These polymers constitute a class of compact macromolecules which, due to the high intramolecular density, interact strongly, excluding each other in space. Investigations of the structure and dynamics in such systems, using various experimental methods as well as computer simulations, have been performed. Small angle X‐ray scattering is used to characterize the structure and mechanical spectroscopy is used for the detection of the dynamic behavior of the systems. Simulations have been performed using the cooperative motion algorithm with lattice polymers equivalent to the considered macromolecules. Results of both experiments and the simulation seem to support the concept of slow structural cooperative rearrangements controlling the flow in such systems. The effects are analogous to those related to flow in low molecular liquids but take place on another size scale. The new slow relaxation processes creates new super soft‐states which are characterized by shear modulus plateau lower than 104 Pa.

Correlation between photorefractivity, photoconductivity, and dark conductivity in dye-doped, low molecular mass nematic liquid crystal cells

Abstract The photoconductivity and photorefractivity of dye-doped low molecular mass nematic liquid crystal homeotropic cells exhibit a threshold at an applied voltage of the order of 1 V (in our samples). At about the same voltage, the dark conductivity of the cells exhibits a change from a cubic current/voltage characteristic to a linear one. We explain these observations by postulating that the cell's conductivity is due to ionic charge carriers generated near the electrodes by some electrochemical processes activated by the injection of electrons and holes. Below the noted threshold, the dark current is limited by a residual ionic space–charge distribution, and grows as the cube of the applied voltage. In this regime, photogenerated charge carriers are screened and are prevented from forming the static space–charge field needed to initiate an orientational photorefractive response. Above that threshold, the residual ionic space–charge disappears, the cell is a weak ohmic electrolyte, and the photogenerated charges contribute to the photoconductivity and photorefractivity of the cell.

Organization of a Polar Molecule at the Air−Water Interface

We have studied the behavior of a low molecular mass organosiloxane ferroelectric liquid crystal at the air−water interface. Data obtained with Langmuir isotherms, surface potential measurements, and Brewster angle microscopy are presented and discussed. The compound forms films on the Langmuir trough, with phase transitions to multilayers upon compression. As seen with Brewster angle microscopy, the monolayers are optically isotropic whereas striking birefringent structures develop in the multilayer films. These periodic structures result from a surface elasticity phenomenon.

Role of network nematicity in swelling and phase equilibria of polymer networks in nematic solvents

Swelling and phase equilibria of polymer networks in a low molecular mass liquid crystal (LC) have been investigated as a function of network nematicity. LC networks with varying nematicity were prepared by copolymerization of mixtures of mesogenic monomers and non-mesomorphic (styrene) monomers with various compositions. Molar fraction of mesogenic monomer ( x) in copolymer network strongly influences the swelling behavior as well as the nematic–isotropic (N–I) transitions in both dry and swollen states. The swollen networks of sufficiently high x with strong nematicity exhibit a sharp N–I transition and simultaneously undergo a discontinuous change in gel volume, i.e. volume phase transition. Meanwhile, the swollen networks of x≤0.8 with less nematicity show a broad N–I transition, and the resulting volume change proceeds continuously over a finite temperature range. When x decreases further down to less than 0.5, the nematicity of the dry networks vanishes. The nematic ordering in the swollen copolymer networks of x<0.5 occurs at the same temperature as the N–I transition temperature of the pure nematic solvent ( T NI S), which yields the inflection in the swelling-temperature curve. A mean field theory considering network nematicity as a variable describes the effects of x on volume phase transition such as a shift of T NI G and a change in the magnitude of volume transition, apart from the emergence of continuous volume transition due to the broad N–I transition. The purely isotropic network of x=0 appreciably swells in the nematic solvent. The solvent inside the gel forms the nematic phase at the temperatures below T NI S *(=T NI S −2 ° C), which conflicts with the classical theoretical prediction that nematic ordering of LC solvent in fully swollen isotropic network never occurs.

Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic–nematic transition to higher densities but does not significantly change the position of nematic–smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase. The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.

Self-Diffusion and Viscosity of Low Molecular Weight Polystyrene over a Wide Temperature Range

The self-diffusion coefficient D and the viscosity η of low molecular weight polystyrene (1.9 × 103 g/mol) have been measured from Tg + 3 °C to Tg + 160 °C. Forward recoil spectrometry (FReS) and pulsed-gradient spin-echo NMR methods were combined to determine D values covering more than 10 orders of magnitude. Over this range, D has the same temperature dependence as T/η, to within 1 decade. The small difference can be explained by the different temperature dependences of segmental and terminal relaxation near Tg. The product of D and the terminal relaxation time is independent of temperature over the entire temperature range to an excellent approximation. In contrast, the analogous quantity for low molecular weight glass-forming liquids can increase by several orders of magnitude as Tg is approached from above. This difference is interpreted in terms of the length scale of the spatially heterogeneous dynamics which develop near Tg.

Liquid Crystal Influenced Self-assembly in Mesogenic Polymer and Liquid Crystal Blends

ABSTRACTIn this work, we found that by performing photo-polymerization of the mesogenic monomer RM257 in liquid crystals, well organized polymer walls were formed through out the cell by self-assembly of the polymer within the anisotropic host – the liquid crystal. The polymerization conditions, which were parameterized as UV intensity, photo-reactivity (characterized by photo initiator concentration), curing temperature, monomer concentration and mesophase of the liquid crystal host, were systematically varied. Different liquid crystal host also raised some difference in polymer network. We believe this kind of morphology comes from spinodal decomposition and the anisotropic elastic property of the liquid crystal host. It is found that for RM257 and low molecular weight nematic liquid crystals a monomer concentration of 5% is sufficient to use liquid crystal hosts to work as templates for well structured polymer walls.

PHYSICAL PROPERTIES OF ELECTRON BEAM-CURED TRIPROPYLENEGLYCOLDIACRYLATE AND LIQUID CRYSTAL SYSTEMS

Some salient features of physical properties of Tripropyleneglycol diacrylate (TPGDA) and the low molecular weight nematic liquid crystal E7 are discussed. A variety of experimental techniques giving complementary information of monomer and Electron Beam (EB) cured systems are employed to illustrate the effect of curing. A detailed thermophysical analysis is made first to assess the conditions of miscibility, determine the order-disorder transition and the glass transition temperature. DSC spectra yield the energy change at the transitions from which important parameters are readily extracted such as solubility limits and amount of liquid crystal dissolved in the polymer. Polarized Optical Microscopy enables us to follow phase transitions and morphology changes in different regions of the phase diagram. Consistent data are obtained from mechanical measurements in particular with regards to the variation of the glass transition temperature with liquid crystal concentration and the related plasticizing effect.

液晶アクチュエータの基礎実験(S23-4 複雑流体の流れとその応用(4),S23 複雑流体の流れとその応用)

Experimental Study on liquid crystal actuators is performed under a polarized microscope. The low molecular weight nematic liquid crystalline material is placed between parallel glass plates. The lower plate is fixed, and the upper can move freely. When 5V of electric field is imposed on the liquid crystalline material, the rod-like molecules re-align to the orientation direction perpendicular to the plates. During this re-alignment of the liquid crystalline molecules, the fluid flow, called back-flow, is induced. Because of the back-flow mechanism, the upper glass plate moves in a certain direction.

Swelling and Deswelling of Poly (n-butylacrylate) Networks in Isotropic and Nematic Solvents

We report swelling/deswelling data of Poly(n-butyl acrylate) networks in isotropic and anisotropic solvents. The investigated systems are prepared via polymerization induced phase separation processes of initial mixtures composed of a reactive polymer precursor, a cross-linker, a photoinitiator and a low molecular weight solvent by ultraviolet (UV) radiation. A comparison of data obtained from isotropic organic solvents and a low molecular weight nematic liquid crystal is made in terms of temperature and cross-linking density of the network. Hexanediol-diacrylate (HDDA), a di-functional monomer is used as a cross-linker and the conditions of preparation are set in order to obtain well defined model networks. Effects of temperature and composition are evaluated by measuring the size via microscopy and the mass of dry and swollen networks. Simple modelling is used to rationalize the data.

Preparation of photo-crosslinked films of new Chalcone-based side chain type liquid crystalline polymers and its application to alignment film

We synthesized two kinds of side chain type polymers containing chalconyl moieties to investigate the alignment's capability of photo-crosslinked polymer films. One is methacrylate type polymer having the chalconyl moiety in the side chain, and the other is imide type polymer. A smectic phase was exhibited for the methacrylate type polymers, while no mesophases were observed for the imide type polymer. These polymer films were irradiated with linearly polarized UV (LPUV) light (350 nm) under different conditions. The liquid crystalline (LC) cell was fabricated from the polymer film irradiated with LPUV light to examine the alignment properties of low molecular liquid crystal on the film. A mixture of a low molecular liquid crystal and a dichroic dye was filled in the LC cells. Alignment behavior of the low molecular liquid crystal was discussed by the dichroic ratio of the dichroic dye in the LC cell.

片側パターン電極における低分子液晶系ER流体のER効果の発現(アクチュエータの機構と制御2)

片側パターン電極における低分子液晶系ER流体のER効果の発現(アクチュエータの機構と制御2)

Morphology control of liquid crystalline composite gels based on molecular self-assembling kinetics

Liquid crystalline composite gels consisting of a low molecular mass gelator and a low molecular mass liquid crystal were prepared by two types of gelation method (continuous cooling and isothermal gelating), which provide different molecular self-assembling kinetics of the low molecular mass gelator as gelation proceeds. Optical microscopy and atomic force microscopy revealed that numerous fine strands of the one-dimensionally assembled low molecular mass gelators were formed in the composite gels for both the continuous cooling method and the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the isothermal gelation product at an appropriate temperature, than in the continuous cooling process. This difference in dispersion state of the strands was shown (by polarizing optical microscopy) to have a significant influence on the molecular alignment of the low molecular mass liquid crystal in the liquid crystalline composite gel. The electro-optical response and light scattering–transmitting switching, of the liquid crystalline composite gel in an applied electric field was extremely dependent on the morphology of the gelators. High contrast light switching was achieved for the composite prepared by isothermal gelation. The response time of electro-optical switching was less than 100 µs under 30 Vrms.

Theory of the viscosity of supercooled liquids and the glass transition: fragile liquids.

A statistical mechanical theory is presented for viscosity of relatively low molecular weight organic liquids which are supercooled down to the glass transition temperature. In this theory a relation resembling the Stokes-Einstein relation between the viscosity and self-diffusion coefficient of supercooled liquids and an expression for the self-diffusion coefficient are augmented by a suitably constructed semiempirical generic van der Waals equation of state that makes it possible to calculate the free volume. The theory accounts in excellent accuracy for viscosities and self-diffusion coefficients of fragile liquids over the entire range of temperature experimentally investigated. According to the theory, vitrification occurs when the free volume available for translational molecular motion falls below a critical value.

Growth of Polystyrene Domains in Isotropic, Nematic and Smectic Phase of 8CB Liquid Crystal

A small-angle light-scattering (SALS) technique is performed to investigate the phase separation in the films of flexible polymer (polystyrene, PS) mixed with low molecular weight thermotropic liquid crystal LC (4-cyano-4‘-n-octylbiphenyl, 8CB). The growth of isotropic (polymer) domains is studied in both isotropic and anisotropic (nematic or smectic phase) LC matrices, as a function of time, film thickness, and film composition. The size of the domains, L(t), grows algebraically with time as L(t) ∼ tβ. For 70/30 wt % of 8CB/PS, we found the diffusion growth in isotropic (β = 0.25) and smectic (β = 0.28) matrices, independent of the film thickness. In the nematic matrix, β changes from 0.33 to 0.47 as we change the thickness of the sample from 120 to 10 μm. We think that the change of β is due to the attractive forces between the polymer domains which follow from the elastic deformations of the nematic matrix caused by the glass surfaces and polymer domains. In every matrix and for different thicknesses of a sample, scaling is observed in this growth regime. At longer times, there is a crossover from the diffusion growth to the hydrodynamic fast-mode growth, characteristic for systems in which one of the components wets the confining walls. In this regime, we do not observe the scaling; i.e., there is more than one characteristic length scale in the system, and β ranges from 1 to 3/2. Considering extremely viscous systems (50/50 wt % of 8CB/PS), we also find the diffusion growth but with smaller exponent β < 0.2. In this case, we observe two peaks in the scattering intensity S(q,t). One of them is the surface and one the bulk peak. For systems with small amount of a polymer (90/10 wt % of 8CB/PS), the process of growth is very fast, and β = 0.4. In this case, the bulk peak is very quickly covered by the peak coming from the growth of the domains at the surface. To perform the measurements in anisotropic LC systems, we had to eliminate the multiple scattering of light coming from the large difference in ordinary and extraordinary refractive indexes of LC.

Calorimetric studies of poly (2-phenoxyethylacrylate)/5CB mixtures

The thermophysical properties of mixtures of poly (2-phenoxyethylacrylate) and 4-cyano-4′-pentyl-biphenyl, 5CB, are investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The polymer has a molar mass M w = 181 000 g mol -1 the low molecular mass liquid crystal exhibits a nematic to isotropic transition at 35.3°C and crystallizes below 23°C. The phase diagram exhibits miscibility gaps in certain regions of temperature and composition where coexisting nematic and isotropic phases are found. From a practical point of view when considering the electro-optical applications of these systems, it proves to be useful to know precisely the amount of small liquid crystal molecules dissolved in the polymer matrix and the concentration of polymer in the nematic phase. The former quantity has a mechanical impact due to a plasticizing effect, an optical impact since it changes the polymer refractive index, while the polymer in the nematic phase shifts the transition temperatures influencing the electro-optical response of the liquid crystal. The present work addresses these important aspects using POM and DSC.

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.

Noise Effects on the Transient Shear Flow of Tumbling Nematic Liquid Crystals

This paper describes a stochastic approach to nematodynamics in a specific case. We start from the director equation derived from the Leslie-Ericksen theory for a monodomain and we assume that the shear rate is a random function of time. This assumption is a simple way to account for the occurrence of noise in the velocity field produced by any thermal or mechanical fluctuations. We show that this approach provides a new framework allowing for the description of transient rheology upon flow inception in low molecular weight nematic liquid crystals. A bidimensional model in which the director stays in the shear plane is developed and compared to experimental data for the tumbling nematic formed by the dissolution of a side-chain polymer in 5CB.

Thermal glass transition beyond the Vogel-Fulcher-Tammann behavior for glass forming diglycidylether of bisphenol A.

For the low molecular weight fragile liquid diglycidyl ether of bisphenol A we report, based on Brillouin and dielectric spectroscopy, on a thermal glass transition where the relaxation time of the alpha process does not go to infinity. Instead, the structural alpha relaxation disappears spontaneously at the transition point. That discontinuity in relaxation time coincides with a kink in the longitudinal hypersonic velocity and determines unambiguously the transition from the liquid to the glassy state.

Stationary statistical size distribution of nematic droplets in the course of the isotropic liquid-nematic phase transition

The statistical size distribution of nematic droplets in the course of the phase transition isotropic liquid-nematic has been analysed using polarizing optical microscopy and described by the model of reversible aggregation. The materials under investigation are low molecular mass nematics, polymeric nematics and polymer dispersed liquid crystals observed at their clearing temperatures in the stationary time regime.