Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Calorimetric study of the cubic mesogen,ACBC(16)

The heat capacity of the cubic mesogen ACBC(16) was measured between 16 and 500?K by adiabatic calorimetry. As well as the known condensed phases, a new crystalline phase was found to undergo a glass transition at around 165?K. Phase transitions between crystal, SmC, cubic, and isotropic liquid phases took place at 399.16, 431.15, and 474.30?K, respectively. As in the case of ANBC, a broad hump was observed in the heat capacity of the isotropic liquid phase. The first order nature of the SmC–cubic phase transition was confirmed for the first time by the observation of supercooling of the cubic phase. The broad hump in the isotropic liquid phase was shown to extend to a low temperature side if the isotropic liquid was supercooled, suggesting that the event occurring at the hump is not directly related to the cubic–isotropic liquid phase transition.     

甲基纤维素/二氯乙酸液晶溶液的相转变

本文研究了甲基纤维素/二氯乙酸液晶溶液液晶相与各向同性相之间相互转变的过程。在连续升温过程中,由于液晶相内部有序度的非均一性,升温速率对液晶相织构随温度的变化有较大的影响。在等温相转变过程中,无论是从各向同性态转变为液晶态,还是其逆过程,从液晶态转变为各向同性态,相转变曲线均具有指数方程特征。利用与 Avrami 方程相似的指数方程进行处理,在从各向同性态转变为液晶态时,指数 n 大部分约为1。但液晶相向各向同性相在低于 Tc 时的相转变中,n 均小于1。相转变速率受溶液浓度和温度的影响。在质量分数为0.259的浓度时,液晶相向各向同性相的转变在低于 Tc 时39.5℃进行得最快。     

Calorimetric study of the cubic mesogen, ACBC(6)

The heat capacity of the cubic mesogen ACBC(16) was measured between 16 and 500 K by adiabatic calorimetry. As well as the known condensed phases, a new crystalline phase was found to undergo a glass transition at around 165 K. Phase transitions between crystal, SmC, cubic, and isotropic liquid phases took place at 399.16, 431.15, and 474.30 K, respectively. As in the case of ANBC, a broad hump was observed in the heat capacity of the isotropic liquid phase. The first order nature of the SmC-cubic phase transition was confirmed for the first time by the observation of supercooling of the cubic phase. The broad hump in the isotropic liquid phase was shown to extend to a low temperature side if the isotropic liquid was supercooled, suggesting that the event occurring at the hump is not directly related to the cubic-isotropic liquid phase transition.     

Possibility of isostructural condensed states of matter in the D phase of ANBC and the cubic mesophase of BABH: heat capacity of 4'-n-octadecyloxy-3'-nitrobiphenyl-4-carboxlic acid, ANBC(18)

Heat capacity measurements have been made on ANBC(18) at temperatures from 8 to 490 K by adiabatic calorimetry. All known phases were detected. The temperatures, enthalpies and entropies of transition were determined for the phase transitions observed. On the basis of the entropy of transition to the SmC phase from the D or cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is shown that the arrangement of 'molecular' cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase, whereas the terminal alkoxy chains are more disordered in the isotropic phases than in the SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per methylene group. The inverted phase sequence in ANBC (SmC D on heating) and BABH (cubic SmC) can be accounted for in terms of the competing roles in the entropy between the molecular core and the chains.     

Simulation of scattering and phase behavior around the isotropic-nematic transition of discotic particles

The detailed study of the isotropic-nematic phase transition in a system of discotic particles of aspect ratios L/D≤0.1 presented here is relevant to a broad range of colloidal suspensions of chemically modified clay particles. Using Monte Carlo simulation techniques the equation of state, radial distribution functions, structure factors and normalized scattering intensities are calculated for each phase. The results are interpreted and related to previously reported free energy calculations [Fartaria and Sweatman, Chem. Phys. Lett. 478 (2009) 150], suggesting a nearly continuous isotropic-nematic transition for lower aspect ratios. Given this behavior we examined the structural information for each phase to determine how experimental scattering data might be used to distinguish the two phases. The radial distribution functions in each phase depend strongly on aspect ratio, and for larger aspect ratios a dramatic increase in the local ordering of discotic particles (represented here as cut-spheres) is observed just before the phase transition. However, this nearest-neighbor ordering seen in g(r) around r/D=0.1 would hardly be discernible in experimental scattering data subject to usual statistical errors. The structure factors and scattering intensities were calculated for L/D=0.1, 0.04 and 0.01 for the isotropic and nematic phases at and away from the isotropic-nematic transition. While the isotropic-nematic phase transition can be detected from the height and shape of the first scattering peak around 7QD for larger aspect ratios, this feature becomes much less discriminatory with decreasing aspect ratio. Instead, scattering intensities at low scattering vector amplitudes (Q→0) can be used for detection of the phase transition at low aspect ratios. These results provide useful insight to guide interpretation of X-ray and light scattering measurements for colloidal dispersions of thin platelets undergoing isotropic-nematic transitions.     

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air-water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 degrees C, is constructed by measuring the surface pressure (pi)-time (t) adsorption isotherms. Below 25.8 degrees C, each of the pi-t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the pi-t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G-LE (liquid expanded) phase transition, which is followed by another first-order LE-I phase transition. The second-order phase transition is an I-LC phase transition. Contrary to these results, at 36 degrees C both the pi-t measurements and the BAM observation present only two first-order phase transitions, which are G-LE at zero surface pressure and LE-LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 degrees C to an elongated at 24 degrees C and finally to a circular shape at 36 degrees C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.     

Bulk and interfacial wetting behavior of binary mixtures induced by associating between unlike-pair molecules

The statistical associating fluid theory of Wertheim is applied to describe binary mixtures with associating between unlike-pair molecules. The phase behavior of this binary mixture would fall into five different types (I, II, III, V, and VI) of the classification scheme of van Konynenburg and Scott by varying the associating strength and the energy parameters. Both interfacial wetting behavior and wetting transitions are carefully examined in all the vapor-liquid-liquid (gamma-beta-alpha) three-phase-coexisting regions of the binary mixtures. The global wetting behavior and wetting transitions are delineated by scanning the parameter space. In certain regions, the middle beta phase exhibits interfacial phase transitions sequentially, nonwetting --> partial-wetting --> nonwetting, at the interface separating lower alpha and upper gamma phases along with increasing temperature.     

Light induced shifts of ferroelectric mesophase transitions

Several series of recently synthesized chiral azobenzene liquid crystals exhibit transitions driven by both illumination and temperature: some compounds can be 'melted' from the SmC* phase to the isotropic phase by increasing illumination only; according to the phase sequences, SmA, TGBA, N*, and blue phases are also involved in such behaviour. The observed phenomena are reversible and transition temperatures are reproducible in identical light conditions. Some specific transitions have been studied. Their temperatures can be lowered to 10 degrees below the dark transition values by increasing the illumination of samples. Under illumination, those transitions exhibit first order behaviour.     

Molecular dynamics simulations of nanoparticles in dense isotropic nematogens: the role of matrix-induced long-range repulsive interactions

We have carried out molecular dynamics simulation studies of binary mixtures of spherical nanoparticles (NPs) in a matrix of dense isotropic rod-shaped nematogens, with the size of the nematogen length being similar to that of the NP diameter. NPs at even low concentrations were found to shift the isotropic-nematic (I-N) transition significantly to higher pressure at a given temperature, indicative of long-range perturbation of the nematogenic matrix by the NPs. The NPs were found to be dispersed in the dense isotropic nematogenic matrix over a wide range of NP concentrations due to long-range (compared with the molecular size of the nematogens) repulsion caused by NP-induced local order fluctuations and reduced local orientational correlation in the isotropic nematogenic matrix, in contrast to the phase separation predicted and observed in other studies where the particles were much larger or smaller than the nematogens. Furthermore, since the repulsion observed in the NP-nematogen mixtures is only microscopically long range (on the order of about ten molecular lengths of the nematogens), globally ordered clustering observed in mixtures of colloidal particles in nematic matrices resulting from macroscopically long-range interaction is not observed in our simulations.     

Solid/solid phase transitions in confined thin films: a zero temperature approach

We report a density functional theory study of confinement induced solid/solid phase transitions in a thin film (modeled as methane) at T=0. The solid film is confined by two graphite surfaces represented by a mean-field potential. As the wall separation is varied the structure of the confined film changes, which influences its density and the solvation force. Using the directly accessible grand canonical potential density we determine the stable phases and calculate the exact location of the phase transitions. We observe a series of phases having square and triangular symmetry. At low wall separations we find zig-zag buckling and an asymmetric buckled phase, whose structure is consistent with the strongest buckling instability of a triangular monolayer predicted by Chou and Nelson [Phys. Rev. E 48, 4611 (1993)] but, to our knowledge, has not been observed as a stable phase before. We find that the two-dimensional order parameters Psi(4) (square symmetry) and Psi(6) (triangular symmetry) show unphysical behavior in the transition region between square and triangular symmetry. Thus, in the present model they fail to predict the right location of the phase transitions.     

Light induced shifts of ferroelectric mesophase transitions

Several series of recently synthesized chiral azobenzene liquid crystals exhibit transitions driven by both illumination and temperature: some compounds can be 'melted' from the SmC* phase to the isotropic phase by increasing illumination only; according to the phase sequences, SmA, TGBA, N*, and blue phases are also involved in such behaviour. The observed phenomena are reversible and transition temperatures are reproducible in identical light conditions. Some specific transitions have been studied. Their temperatures can be lowered to 10 degrees below the dark transition values by increasing the illumination of samples. Under illumination, those transitions exhibit first order behaviour.     

Density functional theory study of the nematic-isotropic transition in an hybrid cell

We have employed the density functional theory formalism to investigate the nematic-isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyze the behavior of the capillary transition as a function of the fluid-substrate interactions and the pore width. In addition to the usual capillary transition between isotropiclike to nematiclike states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interfacelike states which allow us to continuously transform the nematiclike phase to the isotropiclike phase without undergoing a sharp phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematiclike state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematiclike state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematiclike states.     

Simulation of multiple ordered phases in C23 n-alkane

Normal alkanes display multiple ordered phases, including an orthorhombic crystal (X) and two partially ordered rotator phases (RI and RII). The rotator phase transitions X-RI and RI-RII are of interest because they are weakly first-order, and because experiments suggest that crystalline polyethylene may nucleate via a metastable rotator phase. We have performed heating and cooling scans of all-atom NσT (isothermal, isostress) simulations of a pure C(23) solid. We find a sequence of phases, transition temperatures, structural and thermodynamic properties, all reasonably consistent with experiment, except that a monoclinic crystal is more stable in our simulations than the experimental orthorhombic structure. We find that the RI phase is well described as an orthorhombic crystal disordered by random ±90° rotations of molecules about their stem axis, and the RII phase can be represented as a loose hexagonal packing of parallel chain stems, which tend to orient with the in-plane projection of C-C bonds pointing between neighbors. To measure local orthorhombic, RI, or RII order, we define Potts- and Ising-like order parameters, from which global order parameters and correlation functions can be computed. We observe modest pretransitional fluctuations of local RI order in the RII phase near T(RI-RII), characteristic of this weakly first-order transition.     

Orientational ordering in hard rectangles: The role of three-body correlations

We investigate the effect of three-body correlations on the phase behavior of hard rectangle two-dimensional fluids. The third virial coefficient B3 is incorporated via an equation of state that recovers scaled particle theory for parallel hard rectangles. This coefficient, a functional of the orientational distribution function, is calculated by Monte Carlo integration, using an accurate parametrized distribution function, for various particle aspect ratios in the range of 1-25. A bifurcation analysis of the free energy calculated from the obtained equation of state is applied to find the isotropic (I)-uniaxial nematic (N(u)) and isotropic-tetratic nematic (N(t)) spinodals and to study the order of these phase transitions. We find that the relative stability of the N(t) phase with respect to the isotropic phase is enhanced by the introduction of B3. Finally, we have calculated the complete phase diagram using a variational procedure and compared the results with those obtained from scaled particle theory and with Monte Carlo simulations carried out for hard rectangles with various aspect ratios. The predictions of our proposed equation of state as regards the transition densities between the isotropic and orientationally ordered phases for small aspect ratios are in fair agreement with simulations. Also, the critical aspect ratio below which the N(t) phase becomes stable is predicted to increase due to three-body correlations, although the corresponding value is underestimated with respect to simulation.     

Phase diagram of hard snowman-shaped particles

We present the phase diagram of hard snowman-shaped particles calculated using Monte Carlo simulations and free energy calculations. The snowman particles consist of two hard spheres rigidly attached at their surfaces. We find a rich phase behavior with isotropic, plastic crystal, and aperiodic crystal phases. The crystalline phases found to be stable for a given sphere diameter ratio correspond mostly to the close packed structures predicted for equimolar binary hard-sphere mixtures of the same diameter ratio. However, our results also show several crystal-crystal phase transitions, with structures with a higher degree of degeneracy found to be stable at lower densities, while those with the best packing are found to be stable at higher densities.     

Surface-induced phase behavior of polymer/nanoparticle blends with attractions

In an athermal blend of nanoparticles and homopolymer near a hard wall, there is a first order phase transition in which the nanoparticles segregate to the wall and form a densely packed monolayer above a certain nanoparticle density. Previous investigations of this phase transition employed a fluids density functional theory (DFT) at constant packing fraction. Here we report further DFT calculations to probe the robustness of this phase transition. We find that the phase transition also occurs in athermal systems at constant pressure, the more natural experimental condition than constant packing fraction. Adding nanoparticle-polymer attractions increases the nanoparticle transition density, while sufficiently strong attractions suppress the first-order transition entirely. In this case the systems display a continuous transition to a bulk layered state. Adding attractions between the polymers and the wall has a similar effect of delaying and then suppressing the first-order nanoparticle segregation transition, but does not lead to any continuous phase transitions.