Dielectric relaxation in carbosilane dendrimers with cyanobiphenyl end groups

Carbosilane dendrimers of generation 1 and 2 are functionalized with mesogenic end groups (cyanobiphenyl) via spacers of 5 and 11 carbons. The dielectric relaxation is measured over broad frequency (1 Hz–1 GHz) and temperature (170–470 K) ranges. Two relaxation regimes are observed and characterized as δ and β relaxation. The δ relaxation is nearly a single Debye process and varies strongly with temperature. The S_E to S_A transition observed for the dendrimers with long spacers causes a jump in the relaxation rate of the δ process. The β process displays an Arrhenius-type temperature dependence with an activation energy of 35 kJ/mol. The relaxation time depends on spacer length. The dielectric relaxation reflects the mutual distortion of the dendrimer scaffold and the smectic layers. Received: 9 June 1999 Accepted in revised form: 21 July 1999     

Multiple short time power laws in the orientational relaxation of nematic liquid crystals

Relaxation in the nematic liquid crystalline phase is known to be sensitive to its proximity to both isotropic and smectic phases. Recent transient optical Kerr effect (OKE) studies have revealed, rather surprisingly, two temporal power laws at short to intermediate times and also an apparent absence of the expected exponential decay at longer times. In order to understand this unusual dynamics, we have carried out extensive molecular dynamics simulations of transient OKE and related orientational time correlation functions in a system of prolate ellipsoids (with aspect ratio equal to 3). The simulations find two distinct power laws, with a crossover region, in the decay of the orientational time correlation function at short to intermediate times (in the range of a few picoseconds to a few nanoseconds). In addition, the simulation results fail to recover any long time exponential decay component. The system size dependence of the exponents suggests that the first power law may originate from the local orientational density fluctuations (like in a glassy liquid). The origin of the second power law is less clear and may be related to the long range fluctuations (such as smecticlike density fluctuations)--these fluctuations are expected to involve small free energy barriers. In support of the latter, the evidence of pronounced coupling between orientational and spatial densities at intermediate wave numbers is presented. This coupling is usually small in normal isotropic liquids, but it is large in the present case. In addition to slow collective orientational relaxation, the single particle orientational relaxation is also found to exhibit slow dynamics in the nematic phase in the long time.     

Glassiness of thermotropic liquid crystals across the isotropic-nematic transition

The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.     

Broadband dielectric spectroscopy of the inter- and intramolecular dynamics of a series of random polyester copolymers

Broadband dielectric spectroscopy (1–10⁶ Hz, 183–423 K) and differential scanning calorimetry are employed to analyze the inter- and intramolecular dynamics of a series of random copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylene dimethylene terephthalate). In addition to an interfacial relaxation (α*-process), three dielectric relaxation processes are observed: The α-relaxation (“dynamic glass transition”) and two secondary relaxations (β- and β*-relaxations). The α-relaxation depends sensitively on the composition of the copolymer and shows a rapid slowing down with increasing content of cyclohexylene dimethylene (CHDM) linkages. Besides the β-relaxation, attributed to local motion of the ester group, an additional process (β*-relaxation) is observed on introducing the CHDM linkages. Increasing the content of the latter reduces the strength of the β-relaxation strongly and increases its activation energy by more than 30%. This proves that owing to interactions between the cylohexylene rings and the ester group the β-relaxation no longer has local character only. Received: 28 September 2000 Accepted: 29 January 2001     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Enthalpy relaxation in an epoxy-cycloaliphatic amine resin

Enthalpy relaxation in a system containing the diglycidyl ether of bisphenol A (DGEBA) resin and a diamine, 1,3-bisaminomethylcyclohexane (1,3-BAC) as curing agent, has been investigated by differential scanning calorimetry (DSC). Samples fully cured were annealed at temperature T_g–15 °C for periods of time from 1 h to a maximum of 168 h. The enthalpy relaxation is analyzed by the peak shift method, in which the sample is heated at 10 °C/min following cooling at various rates through the glass transition region. The key parameters of structural relaxation determined were the non-linearity parameter x=0.47 ± 0.02, the apparent activation energy Δh*=1264 ± 48 kJ/mol or Δh*/R=152 ± 6 kK and the non-exponentiality parameter β ≈ 0.3. The results, obtained by the same method, were compared with those for other systems based on fully cured DGEBA. The correlations among these parameters with the peak shift model should be considered with caution. However, the results show that a correlation between crosslink lengths and the value of Δh* can be considered. The relaxation process for DGEBA/1,3-BAC proves to be highly cooperative. Received: 28 June 2000 Accepted: 6 September 2000     

The kerr effect in the vicinity of the transition from the isotropic to smectic a phase

The equilibrium electrooptical effect in the isotropic phase of seven liquid crystalline substances (4-n-decyloxy-4′-cyanobiphenyl, carbosilane dendrimer of the fourth generation with cyanobiphenyl terminal fragments, and the fourth, fifth, sixth, seventh, and tenth members of the homologous series of 4-n-acylphenylene 4′-n-alkoxybenzoates) was studied. The smectic A phases were found to exhibit weaker divergence of the Kerr constant in the vicinity of the T _c isotropic melt-liquid crystal phase transition temperature compared with the nematic phases. The difference between T _c and the temperature T* of the virtual second-order phase transition varied from 3.2 to 19.0 K for the smectic A phases, which substantially exceeded the value (T _c-T*) ≤ 1 K for the nematic phases. A theory of the electrooptical properties of isotropic melts in the vicinity of the T _c temperature of the phase transition from the isotropic to smectic A phase is developed. An equation relating the T _c-T* difference to the phenomenological coefficients of the expansion of the Helmholtz energy of an isotropic melt into a series in powers of the coordination and orientational order parameters is obtained.     

Surface ordering at the air–nematic interface. Part 2. A spectroscopic ellipsometry study of orientational order

Spectroscopic ellipsometry has been used to measure enhanced orientational ordering at the nematic–air interface of 8CB as the smectic A phase was approached by cooling from the isotropic phase. The depth profile of the orientational order has been estimated by calculating the ellipsometric parameters for a homeotropic uniaxial surface film on a uniaxial sub‐phase using the Abelès matrix method. This showed that the depth of the enhanced orientationally ordered region was ～10 nm at 0.5°C above the nematic–smectic A transition. This is substantially less than the thickness of the region with surface enhanced smectic order as determined by neutron reflection and a model of the surface structure consistent with both sets of results is proposed.     

Dielectric spectroscopy of liquid crystalline dispersions

We describe dielectric spectroscopy measurements on dispersions of two thermotropic liquid crystals (5CB and 8CB) in a poly(dimethylsiloxane) matrix. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the interval from 100 to 1000 Hz, with relaxation times that depend strongly on whether the dispersed phase is isotropic, nematic, or smectic. The dielectric relaxation times also depend on the viscosity of the matrix fluid. These results suggest a coupling between the electric field and the mechanics of the interface that affects the spectrum of the dispersed phase and shifts the Maxwell-Wagner interfacial polarization peak.     

NMR determination of smectic ordering of probe molecules

The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.     

Liquid Crystalline Organisation and Reaction Rate in the Formation of Liquid Crystal Thermoset Networks from Diepoxides and Dicyanates

Summary: The influence of the type of mesophase on the rate of reaction in amine curing or anionic polymerisation of mesogenic diepoxides, and in the cyclotrimerisation of mesogenic dicyanates has been investigated by isothermal DSC and IR-spectroscopy. Epoxide/amine systems were found to react faster in the nematic than in the isotropic phase. Anionic polymerisation of epoxides is an example where increase of the reaction rate occurs with the transition from a mesophase of higher order (smectic) to phases with lower order (nematic, isotropic). For triaromatic dicyanates with one or three methyl groups at the central ring cyclotrimerisation is faster in the isotropic than in the nematic phase showing an increase of activation energy in the nematic phase with the number of methyl groups. A pronounced rate increase has been observed in the smectic phase as compared to the nematic phase for the unsubstituted triaromatic dicyanate. In this case activation energies in the two phases are comparable with those of non liquid crystalline dicyanates.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

Effect of terminal perfluorocarbon chain containing mesogens on phase behaviors of chiral comb-like liquid crystalline polymers

A novel perfluorinated liquid crystal 4′-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)biphenyl-4-yl undec-10-enoate (PFOBU) was synthesized, which exhibited smectic C phase. Several liquid crystalline polymers (PI–PVI) were synthesized by use of poly(methylhydrogeno)siloxane, PFOBU, and cholesteryl 3-(4-allyloxy-phenyl)-acryloate. The chemical structures and liquid crystalline (LC) properties of the monomers and polymers, and some ferroelectric properties of the chiral smectic C (S_C^*) phase were characterized by use of various experimental techniques. The effect of perfluorocarbon chains on phase behaviors of the fluorinated LC polysiloxanes was studied as well. PI and PII showed single chiral nematic (N^*) mesophase when they were heated and cooled, but PIII, PIV, PV, and PVI containing more perfluorocarbon chain units exhibited S_C^* phase besides N^* mesophase. Introduction of perfluorocarbon chain containing mesogens to the chiral cholesteryl-containing polymer systems resulted in a S_C^* mesophases, indicating that the fluorophobic effect could lead to microphase segregation and modifications of smectic mesophases from the chiral nematic phase.     

Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C^* _A, and smectic Q. In the other system the phase sequence is smectic C^*, smectic C^* _A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.     

Structural and dynamic properties of a new type of discotic nematic compounds

Thermodynamic, structural and dynamical properties of a new type of discotic compounds, a hydrocarbon without any heteroatoms, displaying a nematic discotic phase have been investigated by means of X-ray diffraction, electro-optical relaxation, and calorimetric studies. Of particular interest are the strength of the first order nematic—isotropic phase transition and the nature of the orientational fluctuations in the isotropic phase. The short range positional order was found to be biaxial in both the isotropic and the nematic phase. The isotropic phase displays strong pretransitional effects originating from orientational fluctuations in the neighbourhood of the transition to the nematic phase. The character of these pretransitional effects differs from that found for calamitic systems in that the number of correlated molecules g₂ is extremely large, of the order of 600 at the clearing temperature and the electro-optical relaxation time is very large, caused by the large value of g₂.     

Molecular dynamics in fluorinated side-chain maleimide copolymers as studied by broadband dielectric spectroscopy

A series of alternating maleimide (MI) copolymers with fluorinated side chains have been investigated using broadband dielectric spectroscopy. The side chains consist of fluoroalkane (–C _x F_2x+1, x=1, 7, 9) end groups connected to the main chain via methylene spacers. The experiments were carried out in a frequency range of 0.1 Hz to 10 MHz and at temperatures between 120 K and 500 K. The fluorinated MI copolymers show a fast sub-T _g (β) relaxation characterized by an Arrhenius-type temperature dependence with activation energy in the range of 30–37 kJ/mol. Two more processes (α and δ-like) are observed, corresponding to independent relaxations of the main chain and the fluoroalkane domains respectively. For shorter side chains, the δ-like process is not observed but instead another relaxation process, α _S , occurs at temperatures higher than either the α and δ-like processes. When compared with unfluorinated MI copolymers, the fluorinated MI copolymers show the δ-like process and a slower β-relaxation unlike their unfluorinated counterparts. A model to explain the molecular origin of the four processes is proposed, supplemented by differential scanning calorimetry and published WAXS/SAXS data.     

Investigation of orientational behaviour of nematic and smectic a compounds using the Raman microprobe

The Raman microprobe technique, has been used to probe orientational order in oriented samples of nematic CB₅ and nematic and smectic CB₈ versus spatial localization of the analysed microvolume. In the nematic state, the influence of long-wavelength director fluctuations is pointed out and an elastic constant derived.     

The Hydrolysis of AuCl<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> and the Stability of Aquachlorohydroxocomplexes of Gold(III) in Aqueous Solution

The equilibria AuCl₄⁻+jOH⁻+kH₂O⇌AuCl_4−j−k (OH) _j (H₂O) _k ^k−1+(j+k)Cl⁻, β _jk (0≤j,k≤4) have been studied spectrophotometrically at 20 °C in aqueous solution. For I=2 mol⋅dm⁻³(HClO₄) the conventional constants, β _i ^*, of the equilibria, Au^*+iCl⁻ ⇌AuCl _i ^*, are equal to log ₁₀ β ₁^*=(6.98±0.08); log ₁₀ β ₂^*=(13.42±0.05); log ₁₀ β ₃^*=(19.19±0.09); and log ₁₀ β ₄^*=(24.49±0.07), where [AuCl _i ^*]=∑[AuCl _i (OH) _j (H₂O)_4−i−j ] at i=const. The hydrolysis and other transformations of AuCl₄⁻ in aqueous solution are discussed. On the basis of new and known data, a full set of equilibrium constants, β _jk , or their estimates has been obtained.     

2H NMR measurements of the molecular orientational ordering in a reentrant liquid-crystalline mixture

Deuterium NMR measurements of the molecular orientational ordering (nematic order) are reported for a reentrant binary mixture of some alkoxy-eyanobiphenyls (nOCB) to which small amounts of perdeuterated p-xylene have been added. The results indicate that the degree of orientational order is enhanced at the smectic A to reentrant nematic phase transition. This effect is shown to be in agreement with the predictions of a Landau-type theory of the reentrant phase transition where-by this phase transition is explained as the result of a coupling between the smectic and the nematic order parameters.     

Investigation of molecular order and phase transitions of smectic poly(ester imide)s by means of small-angle X-ray scattering

The molecular order and phase transitions of two smectic poly(ester imide)s based on aminobenzoic acid trimellitimide (PEI 1) or aminocinnamic acid trimellitimide (PEI 2) and α,ω-dihydroxydodecane were investigated by X-ray scattering. During cooling, the polymers pass through monotropic smectic liquid-crystalline (LC) phases (S_A, S_C), which transform into higher-ordered smectic-crystalline phases (S_E, S_H). The smectic layer structure of about 3 nm gives rise to a sharp reflection at 2θ ≅ 3°. Peak shape analysis and analysis of the interface distribution function revealed long-range longitudinal correlation among the mesogens in the LC phase but short-range lateral correlation. The development of a broad reflection in the small-angle X-ray scattering (SAXS, 2θ < 1°) indicates the formation of a lamellar two-phase system. The long-period changes reversibly between 10 and 30 nm with increasing temperature. The crystalline lamellae comprise a number of smectic-crystalline layers with packed mesogens, while the noncrystalline interlamellar regions keep their smectic-LC order. In the metastable S_B phase, formed during annealing of quenched PEI 1, the diffuse SAXS indicates a random distribution of small, probably fringed, crystals with hexagonal-packed mesogens. In the lamellar S_E and S_H phases, tie molecules play an important role, but chain folding cannot be excluded. Received: 16 July 1999/Accepted: 28 April 2000