Physical properties of mixtures of low molar mass nematic liquid crystals with photochromic fulgide guest dyes

Abstract

This paper considers how a low concentration (> 2 per cent w/w) of a photochrome dissolved in a liquid crystal affects the physical properties of the host mesogen. A novel class of photochromes, the fulgide based dyes, have been used to give a new approach in this area of study. Unlike many other light active dyes, the fulgides exhibit only a relatively small shape change. Because of this, the influence of the changes in the physical properties of the guest (for example, the dipole moment) are readily studied. Both thiophene and furan fulgide doped liquid crystal systems were studied in the irradiated and non-irradiated states and as a function of concentration. The changes in phase transition temperature, refractive index and order parameter for these various states as compared to the pure host liquid crystal have been measured to investigate the way in which the dye alters the mesomorphic properties of the host, on a molecular level.     

Graph-theoretical analysis of molecular properties. Isomeric variations in nonanes

Paths of length two and three appear to dominate variations in isomers of alkanes when various physical and chemical molecular properties are compared. The regularities previously observed for octanes have been fully examined for the set of 35 nonane isomers C₉H₂₀. Such examinations are facilitated by construction of appropriate grid graphs with paths of length two and three, respectively, representing molecules and connecting such points along the grid axes. By ordering structures in the two-dimensional coordinate plane, various trends and regular changes in the relative magnitudes for the selected thermodynamic properties have been traced to molecular connectivity and topology.     

Electron impact mass spectrometry of alkanes in supersonic molecular beams

The electron impact mass spectrometry of straight chain alkanes C₈H₁₈-C₄₀H₈₂, squalane, methylstearate, 1-chlorohexadecane, 1-bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams. A fly-through Brink-type electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences between the kinetic energy of the supersonic beam species and that of thermal molecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by a pronounced or dominant molecular weight peak together with all the fragment ions normally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST library of most of these molecules did not show any molecular weight peak. By eliminating tile intramolecular thermal vibrational energy we gained control over the degree of molecular ion fragmentation by the electron energy. At an electron energy of 18 eV the molecular ion dissociation was further reduced considerably, with only a small absolute reduction in the peak height by less than a factor of 2. The effect of vibrational cooling increased with the molecular size and number of atoms. Pronounced differences were observed between the mass spectra of the straight chain triacontane and its branched isomer squalane. Similar mass spectra of octacosane (C₂₈H₅₈) achieved with 70-eV EI in a supersonic molecular beam were obtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV and an ion source temperature of 150 °C. However, this ion source temperature precluded the gas chromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes was studied with an ion trap gas chromatograph-mass spectrometer at an ion source temperature of 230 °C. Thermal peak tailing was observed for C₂₀H₄₂ and heavier alkanes, whereas for C₂₈H₅₈ and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. In contrast, no peak tailing existed even with C₄₀H₈₂ for GC-MS in supersonic molecular beams. The minimum detected amount of eicosane (C₂₀, H₄₂) was shown to be 60 fg. This was demonstrated by using single ion monitoring with the quadrupole mass analyzer tuned to the molecular weight peak of 282 u. The coupling of electron impact mass spectrometry in supersonic molecular beams with hyperthermal surface ionization and a fast GC-MS inlet is briefly discussed.     

Effects of solvent density on retention in gas-liquid chromatography. I. Alkanes solutes in polyethylene glycol stationary phases

Gas-liquid chromatographic columns were prepared coating silica capillaries with poly(oxyethylene) polymers of different molecular mass distributions, in the range of low number-average molar masses, where the density still varies significantly. A novel, high-temperature, rapid evaporation method was developed and applied to the static coating of the low-molecular-mass stationary phases. The analysis of alkanes retention data from these columns reveals that the dependence of the partition coefficient with the solvent macroscopic density is mainly due to a variation of entropy. Enthalpies of solute transfer contribute poorly to the observed variations of retention. Since the alkanes solubility diminishes with the increasing solvent density, and this variation is weakly dependent with temperature, it is concluded that the decrease of free-volume in the liquid is responsible for this behavior.     

Polyethylene/Poly(Butyl Methacrylate-co-Styrene) Copolymers Interpenetrating-Like Networks. I. Influence of the Copolymer Composition on Interpenetrating Polymer Network Properties

Abstract

IPN-like systems, consisting of low-density polyethylene (PE) and poly[butyl methacrylate (BMA)-co-styrene (S)] copolymer networks, were prepared in situ by a procedure described in previous papers. The initial PE/copolymer molar ratio was kept equal to 1 for all samples. The initial molar BMA/S ratio of the copolymer was varied over a composition range from pure S to pure BMA in order to investigate its influence on IPN properties. The samples obtained were analyzed by DSC; WAXS; tensile mechanical, dynamic-mechanical, and impact tests; swelling in CCl₄; and by some optical and electronic observations. The materials, became more and more brittle after undergoing mechanical and impact tests, even in the rubbery state, with increasing copolymer BMA content. The morphology consisted of a two-phase system. Good optical properties were obtained for the transparent specimens at relatively high BMA molar contents in the copolymer (70–80% BMA). Reversible changes of the optical properties were induced by temperature variations. A matching/mismatching of the refractive indexes of PE and the copolymer was found to be the cause of the optical behavior of these materials. Work is in progress to improve the mechanical behavior of these systems.     

Two unusual mesophases in chiral side chain polymers

Abstract

We have reported in a previous paper the synthesis, characterization, structural and electro-optic properties of a new family of chiral mesomorphic side chain polyacrylates. The most salient finding was that many of these polymers presented two S*_C phases of similar structure, but differing in their switching properties. In addition, two unusual mesophases called U1 and U2 were also discovered in this family. We describe here in more detail the symmetries and molecular organizations for these U1 and U2 phases. We also discuss their relation, on the one hand, to the newly discovered antiferroelectric chiral S*_CA and S*_o phases, and, on the other hand, to the 2-dimensional fluid smectic à and smectic °C phases displayed by strongly polar low molar mass mesogens. The occurrence of chevron-like ordering in the U1 and U2 phases may give a clue to an understanding of this peculiar S*_C polymorphism.     

Shape variation of cross-linked liquid-crystalline polymers by electric fields

Abstract

First experiments show that cross-linked liquid-crystalline polymers, which are swollen with nematic low molar mass liquid crystals and freely dispersed in this liquid crystal, change their shape if an electic field is applied. This shape variation depends on the degree of swelling.     

Tunable self-organization in n-type liquid crystalline dibenzocoronene tetracarboxdiimides for high photoconductivity

ABSTRACT

A comprehensive study on the self-organised mesomorphism and charge-transport properties of three n-type liquid crystalline (LC) dibenzocoronene tetracarboxdiimide derivatives (Dibenzo-CDI 1, 2, 3) bearing branched alkyl chains with varied side-chain length and branching point (α or β site) has been carried out. All three Dibenzo-CDI compounds possessed high solubility in common organic solvents and low-lying lowest unoccupied molecular orbital energy levels below ?4.0 eV indicating their air-stable electron-transport capacity under ambient conditions. It is discovered that the side-chain length has a moderate influence on the molecular arrangement and thus leads to only limited variation of electron-transport properties. In contrast, the branching position plays a critical role on tuning molecular packing and orientation in the bulk mesophases, thereby resulting in dramatic differences in electron mobilities. Remarkably, Dibenzo-CDI 3 demonstrates good photoconductivity by time-of-flight method with high electron mobility up to 0.075 cm²/Vs, close to the highest value reported in certain n-type LC derivatives. All these results reveal that LC Dibenzo-CDI derivatives are potential candidates for high-performance solution-processable electronics, such as air-stable n-type organic field-effect transistors (OFETs) and solar cells.     

Analogy between liquid crystal side chain polysiloxanes and liquid crystal twins with siloxane spacers

Abstract

The goal of this present work was to compare the properties of dimers with a siloxane spacer to those of side chain polysiloxanes including the same mesogenic groups and the same dilution with dimethylsiloxane segments. A systematic study of the influence of the dilution parameter on the mesomorphic and structural properties of dimers and diluted side chain polysiloxanes, as well as on the behaviour of their solutions with low molar mass liquid crystals is reported.     

Molecular electronegative distance vector (MEDV) related to 15 properties of alkanes

Several quantitative structure-property relationship (QSPR) models between 15 basic physical properties or thermodynamic functions of alkanes and their molecular electronegative distance vectors (MEDV) are developed. For six of the properties-boiling point (BP), density (D) at 25 degrees C, refraction index (RI) at 25 degrees C, critical temperature (CT), critical pressure (CP), and surface tension (ST) at 20 degrees C-logarithmic models are found to give better results than conventional (linear) models since the values of these properties all tend to a limit with increasing carbon chain length. All models are created using multiple linear regression (MLR). Conventional models are proposed for the remaining nine physical properties or thermodynamic functions: molar volume (MV) at 20 degrees C, molar refraction (MR) at 20 degrees C, heat capacity (HC) at 300 K, enthalpy (E) at 300 K, heats of vaporization (HV) at 25 degrees C, heat of atomization (HA) at 25 degrees C, standard heat of formation (HF) at 25 degrees C, heat of formation in liquid (HFL) at 25 degrees C, and heat of formation in gas (HFG) at 25 degrees C.     

Correlations between topological features and physicochemical properties of molecules

The article reviews in brief, thede novo group additivity approach and, at length, the different topological approaches to obtain predictive and internally consistent correlations between various properties and structural features of molecules. The stress has mainly been on the molecular connectivity method. A new rational scheme for nomenclature of connectivity indices of different orders and types is introduced. The concept of the perturbation connectivity parameter developed by us recently has been applied to obtain correlations for molar refraction, boiling point, molar volume, heat of atomisation, heat of combustion, heat of vaporisation, magnetic susceptibility and critical constants of alkanes, alcohols and alkylbenzenes. A comparative study of various approaches reveals that the present perturbation topological approach has an edge over other similar methods.     

Vapor–liquid critical properties of multi-component mixtures containing carbon dioxide and n-alkanes

Vapor–liquid critical temperatures, pressures and densities of multi-component mixtures containing CO₂ and n-alkanes (C4–C7) were measured in a high-pressure view cell by direct visual observations. The molar ratio of alkanes was fixed during the experiment while the composition of CO₂ was varied over the whole range. The critical loci show type I fluid phase behavior if the n-alkanes mixture is treated as a pseudo-continuous component, while correspondingly, CO₂ is a discrete component. The critical properties were calculated by Redlich–Kwong–Soave equation of state (SRK) combined with a renormalization group correction (RG). The predictions of critical properties by SRK + RG are in good agreement with the experimental results.     

Modeling of ion complexation and extraction using substructural molecular fragments

A substructural molecular fragment (SMF) method has been developed to model the relationships between the structure of organic molecules and their thermodynamic parameters of complexation or extraction. The method is based on the splitting of a molecule into fragments, and on calculations of their contributions to a given property. It uses two types of fragments: atom/bond sequences and "augmented atoms" (atoms with their nearest neighbors). The SMF approach is tested on physical properties of C2-C9 alkanes (boiling point, molar volume, molar refraction, heat of vaporization, surface tension, melting point, critical temperature, and critical pressures) and on octanol/water partition coefficients. Then, it is applied to the assessment of (i) complexation stability constants of alkali cations with crown ethers and phosphoryl-containing podands, and of beta-cyclodextrins with mono- and 1,4-disubstituted benzenes, and (ii) solvent extraction constants for the complexes of uranyl cation by phosphoryl-containing ligands.     

Synthesis and characterization of single-source molecular precursors for the preparation of metal chalcogenides

Metal chalcogenides constitute an important family of functional materials. Subtle changes in shape, size and phase of these materials result in variations in physical properties (e.g. electronic and optical), which can be exploited for various technological applications. Several strategies have evolved recently for controlling shape, size and phase of these materials. This work discusses design and synthesis of single-source molecular precursors for the preparation of metal chalcogenides both in bulk and nano-size regime. Precursors for palladium chalcogenides, indium sulphides andII–VI materials are presented. Synthesis of a variety of palladium(II)/platinum(II) complexes with internally functionalised chalcogenolate ligands, selenocarboxylates; gallium and indium dithiolate complexes and zinc/cadmium/ mercury complexes with N,N′-dimethylaminoalkylselenolate ligands and their characterization by NMR and X-ray crystallography are also discussed. Data on thermal behaviour of a few representative complexes, [Pd(SeCOAr)₂(PR₃)₂], [PdCl(E∩N)(PR₃)], [InMe₂(S∩S)], [In(S∩S)₃] and [M(E(CH₂)_nNMe₂)₂] (M = Zn, Cd, Hg;n = 2 or 3) are presented.     

Manipulation of Molecular Weight Distribution Shape as a New Strategy to Control Processing Parameters

Molecular weight and dispersity (Ð ) influence physical and rheological properties of polymers, which are of significant importance in polymer processing technologies. However, these parameters provide only partial information about the precise composition of polymers, which is reflected by the shape and symmetry of molecular weight distribution (MWD). In this work, the effect of MWD symmetry on thermal and rheological properties of polymers with identical molecular weights and Ð is demonstrated. Remarkably, when the MWD is skewed to higher molecular weight, a higher glass transition temperature (T _g), increased stiffness, increased thermal stability, and higher apparent viscosities are observed. These observed differences are attributed to the chain length composition of the polymers, easily controlled by the synthetic strategy. This work demonstrates a versatile approach to engineer the properties of polymers using controlled synthesis to skew the shape of MWD.     

Size-exclusion chromatography of perfluorosulfonated ionomers

A size-exclusion chromatography (SEC) method in N,N-dimethylformamide containing 0.1 M LiNO₃ is shown to be suitable for the determination of molar mass distributions of three classes of perfluorosulfonated ionomers, including Nafion^®. Autoclaving sample preparation is optimized to prepare molecular solutions free of aggregates, and a solvent exchange method concentrates the autoclaved samples to enable the use of molar-mass-sensitive detection. Calibration curves obtained from light scattering and viscometry detection suggest minor variation in the specific refractive index increment across the molecular size distributions, which introduces inaccuracies in the calculation of local absolute molar masses and intrinsic viscosities. Conformation plots that combine apparent molar masses from light scattering detection with apparent intrinsic viscosities from viscometry detection partially compensate for the variations in refractive index increment. The conformation plots are consistent with compact polymer conformations, and they provide Mark–Houwink–Sakurada constants that can be used to calculate molar mass distributions without molar-mass-sensitive detection. Unperturbed dimensions and characteristic ratios calculated from viscosity–molar mass relationships indicate unusually free rotation of the perfluoroalkane backbones and may suggest limitations to applying two-parameter excluded volume theories for these ionomers.     

Liquid crystal/liquid-crystalline network composite systems Structure formation and electro-optic properties

Abstract

The supermolecular structure and the electro-optic properties of a composite system consisting of a dense liquid-crystalline network and a low molar mass liquid crystal embedded in the network have been characterized. The composite systems were obtained by a photopolymerization of mixtures of a rigid mesogenic spacerless diacrylate and a low molar mass liquid crystal. They are characterized by a homogeneous distribution of the low molar mass liquid crystal in the network. The network has a cocontinuous periodic spinodal-type structure having a characteristic length scale of the order of 10 nm, if the low molar mass liquid crystal is removed or replaced by other solvents. The electro-optic switching properties are characterized by the fact that the active switching time and the width of the Frederiks transition are shifted to larger values as compared to those of the pure liquid-crystalline state. The decay times, on the other hand, are strongly reduced in the network.     

Equilibrium properties in the liquid state from interactions of walks

Abstract

When molecules of chemical compounds are represented by graphs, thermodynamic properties may be connected with numbers of certain classes of walks on graphs. The basic proposition of summing up contributions resulting from interactions of walks is related to fluid distribution functions. The principle of corresponding volumes and structural schemes are discussed. Exemplary calculations for normal alkanes and their binary mixtures give in many cases values within limits of the experimental accuracy