DCS Study of Smectic Phase Transitions

A previously described technique for measuring the order of phase transitions^l-3 using DSC has been applied to the liquid crystalline transitions found in TBBA. All of the values obtained were in agreement with those that have been measured by other techniques, and also with what would be anticipated from the nature of the transition. This study has shown that the technique is applicable to smectic-smectic transitions in liquid crystals.     

Neutron Scattering by Copolymers

Abstract

Neutron scattering is a very efficient tool for studying polymers in bulk as well as in solution. This is mainly due to the difference of scattering length between hydrogen and deuterium which allows the possibility of changing the contrast without too serious alteration in the conformation and thermodynamics of the systems.

In order to take full advantage of this possibility, one has to remember the general rules which allow the evaluation of the total scattering function S(q) as a function of the partial scattering functions S _pq (q) which describes the contributions of the interferences between scattering center of species p and q.^1.2 In a first part we shall briefly establish these rules for incompressible systems introducing what has been called by J. Koberstein³ the molecular contrast and the phase contrast.

In the second part of this talk we shall discuss the problem of copolymers. S(o) the intensity scattered at zero angle depends on the heterogeneity in composition of the sample,⁴ for homogeneous composition S(o) = o. This gives the possibility of studying trans-esterification in polyester, for example.

At finite q for a monodisperse system, S(q) is not zero and it will be shown how it depends on the length of the sequences and the architecture of the copolymers. If the number of blocks is large S _pq (q) is practically not affected by the total length of the polymer.⁵

The condition I ^?1(q) = o allows one to write the equation of a q dependent generalized spinodal and therefore to determine the parameters which govern the stability of a multiphase system.

In the last part we shall discuss the problem of the application of this formalism to networks and gels made of different types of units. It will be shown that the scattering is mainly sensitive to the local conformation and that the introduction of translational order between the units does not affect significantly the scattering curves but can explain the occurrence of secondary maxima which have been observed in polyurethane and ionomers.⁶     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Studies on the Smectic Phase of Some Schiff's Bases With a Terminal Trifluoromethyl Group

Some terminally substituted trifluoromethylbenzylideneanilines have been synthesized with a view to investigate an unidentified smectic phase which exhibits an unusual texture as observed under the polarizing microscope. This phase was first reported by Griffin et al.² for the compound N-(4-n-decyloxybenzylidene)-4'-trifluoromethylaniline. In a comparable compound but with reversed imine linkage the nature of this smectic phase remains unchanged. Also if the terminal n-decyloxy group is replaced by an n-decyl group the nature of the smectic phase remains the same except that it is observed at a considerably lower temperature. We observe that this smectic phase has uniaxial symmetry when studied with a polarizing microscope as well as with NMR techniques; the latter showing an order parameter S = 0.8 for this phase. X-ray diffraction patterns show this phase to be crystal smectic B.     

Order Parameters and Distribution Function of 4'-(N-(p-Pentyloxy Carbonyloxy-Benzylidene)Amino) Valerophenone in Smectic Phase from X-ray Diffraction Measurements

Abstract

X-ray diffraction studies of 4'-(N (p-pentyloxycarbonyloxy-benzylidene)amino)valerophenone (PCBAV in short) oriented by a magnetic field is reported here. The compound has a smectic A phase which is identified by X-ray studies and texture studies. Orienta-tional order parameters (p ₂) and (P ₄) have been determined from the calculation of f(β), the orientational distribution function. An approximate expression for the angular part of the potential of the mean field is obtained from the temperature variation of the orientational distribution function. The value of d, the layer thickness, and D, the intermolecular distance, have been calculated. An approximate value for the translational order parameter τ₁ and average value of the root mean square displacement ?z²?½ are given. We could not get an accurate estimate of the translational order parameter due to the inaccuracy in the measurement of the intensity of the inner ring.     

2H-NMR Spectral Patterns in Magneto-Aligned Smectic E and G Phases

Unique ²H-NMR spectral patterns can be obtained from biaxially ordered smectic E and G phases, where only one director of the phase is aligned by the magnetic field. Two effects on the shape of the deuterium spectral pattern are demonstrated: (1) The effect of a motionally induced asymmetry parameter in the time averaged deuterium quadrupole interaction, and (2) the effect when the principal z-axis of the time averaged interaction is not parallel to the aligned director. Spectral patterns for the S_G phase of the compound 4-n-pentoxybenzylidene-4-n'-heptylaniline selectively deuterated on one of the aromatic rings (50.7-d ₄) were obtained for both unaligned and magneto-aligned samples. Both of these spectral patterns were consistent with that expected for an asymmetry parameter, n < 0.2. Spectral patterns in the S_G phase of the compound 50.6 (24.7 wt.%) + 90.4-αd ₂-γd ₂ (75.3 wt.%) were studied showing a different behavior with respect to the spectral patterns obtained in 50.7. We believe that a shift in the orientation of the z principal axis relative to the sample c axis has occurred in this S_G phase. Spectral patterns were also recorded for the S_E phase in magneto-aligned samples of the mixture of compounds 4-cyano-4'-octyloxybiphenyl (80CB) with 50 wt.% 40.8-d ₄. These data also show a small asymmetry parameter but, in addition, a distribution in the orientation of the principal z-axis indicating that the time scale of the motion of the long molecular axis is in a regime where it is of the order of the ²H-NMR measurement. Models for the smectic E and G phases based on these observations are discussed.     

Stimulation of the Electrohydrodynamic Instability in the Smectic a Phase

The electrooptical properties of 4-n-octyl-4′-cyano-biphenyl (8CB) in the smectic A phase were investigated. The electrohydrodynamic instability in the smectic A phase was observed, when a small quantity of any homologue (C₇, C₈, C₉) of 4-n-alkoxyben-zoic acid was adzed to this material. After the electrical field is turned off, the stable focal-conic texture is formed. A reverse focal-conic to homeotropic transition can be as well induced by means of an electric field.     

The Dielectric Anomaly Near the Transition from the Smectic A* to Smectic C* Phase and Visco-Elastic Properties of Ferroelectric Liquid Crystals

A pyro-electric technique is developed which allows the measurement of the dielectric response near the A*-C* phase transition in ferroelectric liquid crystals. The temperature dependence of the elastic modulus K _θ(T) corresponding to the molecular tilt in smectic layers is calculated from the experimental data. Direct pyro-electric measurements of the relaxation time for the spontaneous polarization P _s and the data on K _θ(T) allow us to calculate the temperature behaviour of the twist-viscosity γ₁(T) for the smectic C* phase. The curves γ₁(T) are compared for the smectic C* and the nematic phase of the same compound and the dependence of the twist viscosity on the molecular tilt angle in the C* phase is investigated. The results of the dielectic measurements are discussed in terms of the mean-field approximation.     

Induction of B1, B6 and Biaxial Smectic A Phases in Binary Mixtures of Compounds with Rod-Like and Bent-Core Molecules

Abstract

We have studied a binary mixture of compounds, one of which has rod-like molecules and the other bent-core molecules. The rod-like molecules are biphilic in nature with a long alkoxy chain attached to an aromatic moeity only at one end, and exhibit the bilayer SmA₂ phase, even though they do not have the highly polar cyano or nitro groups. The compound with bent-core molecules exhibit the B₂ phase which has polarized layers with tilted molecules. Three liquid crystalline phases are induced in the binary mixtures, depending on the concentration: the 2-dimensionally ordered B₁ phase for compositions with 15 to 63 mol% of the rod-like molecules and the B₆ phase for 63 to 87 mol%. The sequence obtained on increasing the concentration of rod-like molecules is the same as that seen on shortening the chain length of the BC molecules. Between 87 to 95.5 mol% of the rod-like molecules, a new biaxial smectic A₂ (SmA_2b ) phase is induced. On the basis of several observations, we argue that it corresponds to a structure in which the BC molecules are reoriented with their arrow directions pointed along the layer normal. The SmA₂ to SmA_2b transition corresponds to an orientational transition of the bent-core molecules in the anisotropic SmA₂ background medium.     

Mathematical Description of Strongly Nonlinear Pressure Dependence of Phase Transition Temperatures

In order to describe mathematically the pressure dependence of phase transition temperatures we have derived an equation using the following assumption: i. The transition enthalpy Δ_tH = const. ii. The transition volume Δ_tV depends on the pressure p according to: The modified Simon-Glatzel-equation T = T₀(1 + p/b)^a exp (cp) allows to calculate the transition temperatures T also in cases of strongly non-linear relations.     

Proton Spin-Lattice Relaxation Dispersion Study of the Ferroelectric Smectic Phases in Dobambc

The frequency dispersion of the proton spin-lattice relaxation rates T^?1 ₁ in the ferroelectric smectic phases of DOBAMBC has been determined in the MHz region and analyzed in terms of the order fluctuations, self-diffusion, and rotational contributions to T^?1 ₁. In the smectic C* phase the main rate determining contribution are order fluctuations with a T^?1 ₁ ∝ ν^?1/2 _L frequency dependence, whereas in the smectic H* phase the translational diffusion, which is slower than in the smectic H* phase, determines the dispersion of T^?1 ₁.     

Experimental Evidence for a Hyperscaling Relation at a Nematic to Smectic A Phase Transition

A hyperscaling relation v_″ + 2v_┴ = 2 – α is examined for the nematic to smectic A transition of 4-octyloxy-4′-cyanobiphenyl (8OCB) and 4-octyl-4′-cyanobiphenyl (8CB). From the heat capacity results, 2-α is analyzed to be 1.84 ± 0.03 for 8OCB and 1.75 ± 0.02 for 8OCB. On the other hand v_″ + 2v_┴ is calculated from the results reported by X-ray experiments to be 1.87 ± 0.12 for 8OCB and 1.69 ± 0.11 for 8CB. From the above coincidences the hyperscaling relation is found to be applicable to the nematic to smectic A transition.     

Pitch Behavior of the Cholesteric Helix at the Phase Transition Point Cholesteric/Smectic A up to 2500 Bars. Study of Some Cholesteryl n-Alkanoates

Experimental evidence is given, for the first time, for a finite pitch of the cholesteric helix at the phase transition point cholesteric/smectic A (Ch/S_A). This finite pitch changes into an infinite one at a given point on the phase equilibrium line Ch/S_A. Therefore this point is called the pitch infinity point (PIP).

With increasing n-alkyl chain length of the investigated cholesteryl n-alkanoates the pressure coordinate of the PIP is shifted to higher values up to a chain length of 13 C atoms. For the alkanoates with 13,15 and 17 C atoms the same PIP pressure of about 1000 bars was determined.

The Ch/S_A phase equilibrium line of the cholesteryl esters could at best be followed up to 2600 bars. The respective transition enthalpies were measured at normal pressure and the corresponding changes in volume calculated by means of the Clausius-Clapeyron equation.     

Structural Phase Transition of an Intercalation Compound Mn1/4NbS2

Abstract

A structural phase transition of an intercalation compound Mn_1/4NbS₂ has been investigated by X-ray diffraction at high temperatures. The lattice parameter c exhibited a discontinuous change at 640K. The superlattice reflections observed below 640K disappeared suddenly above 640K. The phase transition at 640K took an aspect of the first-order phase transition. The precise structure analyses were performed at various temperatures above and below the phase-transition temperature. It was revealed that Mn atoms were arranged in disorder in the high-temperature phase, while the Mn atoms were ordered forming the 2a ₀ × 2a ₀ × c ₀ superlattice in the low-temperature phase. The Nb and S atoms around the ordered Mn atoms slightly shifted from the high-symmetry position in the low-temperature phase. The order parameters were the degree of order of the Mn atoms and the degree of displacement of the Nb and S atoms.     

Low‐Temperature Raman Scattering Spectra of GaSexS1‐x Layered Mixed Crystals

Raman scattering has been used to study the vibrational spectra of GaSe_xS_1‐x layered mixed crystals at 10 K. We report the frequency dependencies of different modes on composition x, with particular emphasis on A′₁⁽²⁾ (A_1g¹) and A′₁⁽⁴⁾ (A_1g²) intralayer compressional modes having low dispersion in the Brillouin zone. The appearance of additional bands is attributed to multimode behavior typically exhibited by mixed crystals of anisotropic compounds.     

Neutron Magnetic Scattering of Intercalation Compounds Fex TiS2

Abstract

Neutron magnetic scattering of intercalation compounds Fe^xTiS² has been measured systematically as a function of Fe concentration x. In the x = 0.15 and 1/4 samples, so-called small-angle scattering was observed associated with a spin-glass transition. In the x = 1/2 sample, clear magnetic Bragg reflections were observed. The position of the magnetic peak with the smallest Q number was not commensurate with the nuclear lattice-unit. The magnetic structure of Fe^1/2TiS² was found not to be a simple ferromagnetic structure as was ever suggested but a long-period magnetic structure. In the x = 1/3 sample, broad magnetic diffuse peak whose Q number was also not commensurate with the nuclear lattice-unit was observed. The origin of the magnetism of Fe^1/3TiS², what is called a cluster-glass, seems the short-range ordered magnetic-clusters with a long-period magnetic structure.     

Solid-Solid Phase Transitions of Long-Chain n-Alkyltrimethylammonium Halides

Transition properties of some compounds of the type, n-C _n H_2n+1N⁺(CH₃)₃X_-, with n = 10, 12, 14, 18, 22, and X = Cl, Br, and I, were investigated. All these compounds show a solid-solid phase transition with a large enthalpy change in the range of temperature 350 ～ 400K. The transition behaviors were investigated by the aid of DSC, TG, X-ray, and optical microscope. The solid-solid phase transition is regarded to be caused by melting of the layer of hydrocarbon chains of the compounds, while the rigid ionic layer retains the regular arrangement. Some relationships between the physical properties and the structure of the compounds are discussed.     

On the Phase Transition Behaviour of Binary Systems: Various Non-Nematic Solutes in Nematic Solvent (EBBA)

Phase transition behaviour of the systems of EBBA with methyl alcohol (MeOH), 1.2-dibromo-ethylene (DBE), o-xylene, m-xylene, and p-xylene has been studied. The phase diagrams for each system have been determined by the polarizing microscope. The slopes, β_n and B_i, of the nematic and the isotropic phase boundary lines in each system have been qualitatively described by the molecular field treatment proposed by Humphries. The order parameters for both cis-and trans-DBE in EBBA have been obtained from analyses of ¹H-NMR spectra. In the nematic phase, they decrease with increase in temperature. In the nematic/isotropic coexisting phase (N/I phase), the order parameter for CIS-DBE is almost independent of temperature. The proton spin-lattice relaxation times (T₁) in the MeOH/EBBA system have been measured by a pulse NM R technique. Little temperature dependence of T¹ has been found in the N/1 phase region.c