Low Molar Mass Materials Research Articles

Phenylethenyl substituted 10-alkylphenoxazines as new electroactive materials for organic light emitting diodes

Phenylethenyl substituted 10-alkylphenoxazines were synthesized by a multi-step synthetic route. The derivatives were characterized by thermo-gravimetric analysis, differential scanning calorimetry, electron photoemission and time of flight technique. The materials were found to show very high thermal stability having initial thermal degradation temperatures in the range of 338–354 °C. Glass transition temperatures of the low molar mass materials were in the rage of 41–49 °C. The electron photoemission spectra of thin layers of the derivatives showed ionization potentials in the range of 5.16–5.3 eV. Hole drift mobility in thin layers of these derivatives ranged from 6 × 10−4 cm2V−1s−1 to 4.5 × 10−3 cm2V−1s−1 at high electric fields at room temperature. The synthesized materials have been tested as hole transporting layers in electroluminescent bilayer organic light emitting diodes with tris(quinolin-8-olato)aluminium as an emitter. The device with hole transporting layer comprised of 3,7-di(2-phenylethenyl)-10-(2-ethylhexyl)phenoxazine exhibited the best overall performance with low turn-on voltage of 2.8 V, a maximum photometric efficiency of about 3 cd/A and maximum brightness of 2160 cd/m2. Taking the device at 1000 cd/m2, the photometric efficiency was increased by 25%, as compared with that of PEDOT:PSS-based device.

Chemical characterization of high-molar-mass fractions in a Norway spruce knotwood ethanol extract

The low-molar-mass (LMM) fraction, only, i.e., the GC-eluting compounds, which are mainly lignans, has been characterized in Norway spruce knotwood hydrophilic extracts previously. Of this fraction, many lignans and sesquilignans and all GC peaks supposedly representing dilignans remain unidentified. In this work, dilignans and the GC non-eluting compounds (the high-molar mass fractions, HMM) were characterized in a 7-hydroxymatairesinol-reduced knotwood ethanol extract of Norway spruce by using several fractionation and analytical techniques. A methyl tert-butyl ether (MTBE) insoluble fraction of the extract contained mainly HMM material, of which the main part was shown to consist of lignan oligomers. The oligolignans (with a molar mass up to approximately 3700 Da) seemed to be linked by 55′ bonds, some of them containing one or two guaiacylglycerol ether units linked to the lignan by βO4 or β5 bonds. Several oligolignans were identified or tentatively identified. The MTBE soluble fraction, which accounted for the major part (81%) of the extract, contained mainly LMM material (lignans, sesqui- and dilignans). The part of the HMM material in the MTBE soluble fraction that was easily isolable (2%) seemed to contain polymers of fatty acids and alcohols, resin acids, and sterols.

Determination of the degree of polymerisation of fructans from ryegrass and chicory using MALDI-TOF Mass Spectrometry and Gel Permeation Chromatography coupled to multiangle laser light scattering

This study is concerned with the determination of the degree of polymerisation (DP) of fructans from chicory and rye-grass (Lolium perenne L.) using Gel Permeation Chromatography coupled to multiangle laser light scattering and refractive index detectors (GPC/MALLS) and Matrix-Assisted Laser Desorption/Ionisation Time of Flight Mass Spectrometry (MALDI-TOF MS). The results show that fructans isolated from ryegrass have a DP in the range 2–>100 and the commercially produced fructans derived from chicory have a DP in range 2–61. It has been demonstrated that MALDI-TOF MS is particularly effective at determining the DP of low molar mass material but is less effective for detecting the presence of high DP molecules. On the other hand GPC/MALLS is able to provide a much broader range of DP values although it is less sensitive at very low DP. It has been shown that the two techniques give complementary information thus providing a more accurate estimate of the overall DP of the fructan molecules.

Synthesis and Phase Transitional Behavior of Dimer-like Optically Active Liquid Crystals

The results of our detailed studies pertaining to a relatively new class of low molar mass mesogens are presented. Four homologous series of optically active dimer-like mesogens, comprising cholesterol as the conventional pro-mesogenic core covalently tethered to a nonmesogenic salicylaldimine segment through a flexible spacer of varying length and parity, have been synthesized and evaluated for their thermal, electrical switching, electrochemical, and gelation properties. The thermal behavior, being the prime focus of this study, has been characterized by optical, calorimetric, X-ray diffraction, and electrical switching studies. In each series, the length of the even/odd-parity spacer is held constant, while the length of the terminal N-alkyl tail attached to nonmesogenic salicylaldimine core has been varied to gain insight into the fundamental correlation between structure and phase transitional properties. These compounds, with a few exceptions, exhibit liquid crystal phase(s); the identified phases are chiral nematic (N*), twist grain boundary (TGB), chiral smectic A (SmA), and chiral smectic C (SmC*) phases. The selective reflection property of the N* phase and ferroelectric behavior of the SmC* phase have been ascertained for some selected members. In general, the phase behavior shows a dependence on the length and parity of the central spacer as well as the length of the terminal tail. The odd-even effect has been prominently found in the clearing temperatures; the even-parity dimer-like compounds belonging to three different series exhibit higher values when compared to members of an odd-parity series. Thus, our study demonstrates that these new class of low molar mass materials behave analogous to liquid crystal dimers. Electrochemical behavior and gelation ability have been demonstrated for some selected materials.

ESC resistance of commercial grade polycarbonates during exposure to butter and related chemicals

Three commercial grades of polycarbonates (Lexan ® 144, Lexan ® 104 and Makrolon Rx1805) were studied with respect to resistance to environmental stress cracking (ESC) when exposed to butter and related chemicals. The polycarbonates (PCs) were extensively characterised to determine whether differences in ESC resistance could be related to their structural or chemical properties. MALDI-TOF mass spectrometry revealed that Makrolon Rx1805 contains a low molar mass material characterised as poly(propylene glycol)p, which was confirmed by ATR-FTIR and 1H NMR. Some “non-absorbing” chemicals, such as butter, cause the PCs to be less resistant to ESC under stress. The reason for this is that these chemicals and the PCs have sufficiently similar Hansen solubility parameters to allow surface conformational changes even though absorption is non-existent or extremely small. ATR-FTIR was used to detect changes in molecular structure in the PC surfaces.

Comparison of the Effiency of Aluminium and Ferric Sulphate in the Removal of Natural Organic Matter During Drinking Water Treatment Process

The removal of natural organic matter by coagulation in the drinking water treatment train was studied for a period of two years. In the middle of the study, the coagulation/flotation process was modified by replacing the aluminium sulphate by ferric sulphate. At the same time, the filtration unit was enhanced by adding a sand filter unit before the activated carbon filtration and by changing new carbons on to the activated carbon filters. A special aim was to compare the efficiency of the aluminium and ferric coagulants in the organic matter removal by several methods, including high-performance size-exclusion chromatography. A comparison of quantity and characteristics of organic matter in treated water before and after treatment process modification provided an insight into process performance. Approximately 95% of high molar mass organic substances were removed in the process with both coagulation agents. The greatest difference between the coagulants occurred in the removal of organic matter having molar masses of 1000-4000 g mol−1. These intermediate molar mass organic compounds were removed 25 % more efficiently with iron-based coagulant than with aluminium-based. Low molar mass material was poorly removed regardless of the coagulant. On average, only 10 % of this fraction was removed. According to different measuring techniques used in the study, the ferric sulphate coagulation was 10 % more efficient in the overall organic matter removal compared to the aluminium sulphate coagulation. Turbidity removal was more efficient with aluminium than with ferric sulphate. Turbidity even increased during winter in ferric sulphate coagulation. However, turbidity was effectively removed in filtrations.

Field-induced director dynamics in a nematic liquid crystal: A molecular site dependence?

Studies of the field-induced alignment of the nematic director for low molar mass materials using vibrational spectroscopy have revealed some fascinatingly unexpected results. Several studies have found that the relaxation time for the director alignment is dependent on the group in the molecule used to monitor the director orientation. Seemingly this undermines the basic concept of the Leslie–Ericksen hydrodynamic theory of nematics. Here we report complementary studies using deuterium NMR spectroscopy of perdeuteriated 4-pentyl-4′-cyanobiphenyl where the director is aligned by a magnetic or an electric field. This particular technique was chosen because the spectral peaks associated with each rigid group in the molecule are clearly resolved and of comparable intensity. We have investigated the director alignment using different temperatures, electric field strengths and angles between the magnetic and electric fields, each of which influences the director relaxation time. For all of the experiments we find that the relaxation times are independent of the group used to determine the director orientation during the alignment process.

Chiral nematic octasilsesquioxanes

The octasilsesquioxane cage has been successfully derivatised by platinum-catalysed hydrosilylation reaction of the alkene functionality of appropriate laterally substituted, chiral mesogenic moieties to yield monodisperse multipodal octamers. The liquid-crystalline silsesquioxanes were characterised by multinuclear MR, SEC, TGA, POM and X-ray diffractometry. The giant supermolecular materials display remarkably wide temperature range enantiotropic chiral nematic phases and glass transitions near to room temperature. One of the materials is iridescent and has properties similar to a low molar mass material even though it has a size approaching that of a small globular protein. Thus these observations raise questions as to whether the materials act as singular molecular entities or as spherical side chain polymers.

Mobility of mesogenic side-chains in liquid crystalline polymers based on the Baylis-Hillman reaction

The hydrodynamic and electro-optical (Kerr effect) properties of two side-chain liquid crystalline (SCLC) polymers have been investigated in benzene and chloroform solution. The polymers are unique in structure and have been prepared using the Baylis-Hillman methodology. The polymers contain (a) a polar group (CN or COCH3) along the polymer backbone and (b) a hydroxyl group on the spacer group which is adjacent to the point of attachment of the spacer/mesogenic side groups to the polymer backbone. Molecular dimensions and the Kerr constants were determined for two SCLC polymers and for three low molar mass materials, the n-alkoxycyanobiphenyls (nOCB), which are structurally similar to the side chain used in the SCLC polymers. These results are discussed in relation to the chemical structure and dipole moment of the monomer unit. Dielectric and electro-optical properties of the SCLC polymer containing the —CN group along the polymer backbone were found to be similar to the properties of the nOCB compounds. The difference in the mesogenic side-chain mobility of the polymers was related to the dipole-dipole interactions of the polar groups along the polymer backbone.

Mobility of mesogenic side-chains in liquid crystalline polymers based on the Baylis-Hillman reaction

The hydrodynamic and electro-optical (Kerr effect) properties of two side-chain liquid crystalline (SCLC) polymers have been investigated in benzene and chloroform solution. The polymers are unique in structure and have been prepared using the Baylis-Hillman methodology. The polymers contain (a) a polar group (CN or COCH 3 ) along the polymer backbone and (b) a hydroxyl group on the spacer group which is adjacent to the point of attachment of the spacer/mesogenic side groups to the polymer backbone. Molecular dimensions and the Kerr constants were determined for two SCLC polymers and for three low molar mass materials, the n-alkoxycyanobiphenyls (nOCB), which are structurally similar to the side chain used in the SCLC polymers. These results are discussed in relation to the chemical structure and dipole moment of the monomer unit. Dielectric and electro-optical properties of the SCLC polymer containing the -CN group along the polymer backbone were found to be similar to the properties of the nOCB compounds. The difference in the mesogenic side-chain mobility of the polymers was related to the dipole-dipole interactions of the polar groups along the polymer backbone.

Novel Vitrified Liquid Crystals and Potential Applications

AbstractAn overview is presented for a generic approach to low molar mass liquid crystalline materials capable of vitrification into morphologically stable, optically anisotropic thin films. Potential applications as polarization control devices, optical notch filters, and luminescent elements will be explored. Preliminary data for defect formation and annihilation will also be presented to illustrate an advantage of low molar mass materials over polymer analogs.

Thermoreversible Supramolecular Networks with Polymeric Properties

Hydrogen bonding between carboxylic acids and pyridines can be used as a means of chain extension. However, the reversibility of hydrogen bond formation does not allow the chain-extended structure to be fixed, and a temperature-dependent chain length results. Complexation between a tetrapyridyl species and a diacid is shown to counteract the reversibility of the hydrogen bonding. These complexes form thermoreversible three-dimensional supramolecular networks which have properties typical of low molar mass materials at high temperatures but polymeric properties at low temperatures. The complexes exhibited a glass transition and could form fibers from the melt yet exhibited a low-viscosity isotropic liquid at elevated temperatures. Atypically, a complex incorporating a 2,2',6,6'-tetrasubstituted biphenyl moiety was found to be liquid crystalline.

Dimesogenic liquid crystalline oligosiloxanes

Abstract In this paper, the syntheses, phase properties and ferroelectric properties of new liquid crystalline oligosiloxane dimesogens are described. Smectic phases are dominant in this class of materials. Spontaneous polarizations are in the range of 10 to approximately 100 nCcm−2. The materials show interesting properties for display applications. Although offering higher mechanical stability than low molar mass materials, their switching is significantly faster (switching times in the range of some 10−5s) than is the case for similar side group polymers (typically some 10−3s). Therefore these new oligosiloxane derivatives may be ideal for building flexible displays. In addition, mixtures of oligosiloxane dimesogens were compared with copolymers and with siloxane-free low molar mass materials bearing the same mesogenic groups. The results support the idea of a ‘virtual polymer backbone’, which is formed by a microphase separation phenomenon. Dimesogenic siloxane mixtures and copolymers behave quite similarly, while the siloxane-free mixtures show a different phase behaviour.

Molecular and lamellar structure of an extrusion‐grade medium‐density polyethylene for gas distribution

AbstractAn extrusion‐grade medium‐density polyethylene was fractionated by size exclusion chromatography and selective solvent extraction. The fractions were studied by infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Infrared spectroscopy showed that chain branching was concentrated to the low molar mass material. Transmission electron microscopy showed the presence of roof‐shaped crystals in the low molar mass samples and curved S‐or C‐shaped lamellae in the high molar mass samples. These features could be used for identification of different molar mass species in the unfractionated sample.

Mean field lattice equations of state

Abstract In preceding publications in this series, the Mean Field Lattice Gas (MFLG) model has been shown to produce a useful description of the pVT behavior of both polymers and low molar mass materials. Here we introduce a method, referred to as use of the Bondi constraint, by which the number of adjustable parameters required to apply the model can be reduced from five to three. This is accomplished by taking advantage of the general observations that (1) the parameter which represents the number of segments per molecule (or polymer repeat unit), m1, varies linearly with the van der Waals volume as calculated by Bondi and (2) the ratio of the molecular surface areas of a compound and a reference fluid, calculated from the MFLG parameters m1 and γ1, can be equated to the analogous ratio of surface areas calculated using Bondi's group contribution methodology. We show that this predictive method for derivation of the parameters m1 and γ1, produces a pVT description for ethane and ethylene which is comparable to that produced using the standard five-parameter fit. Thus the Bondi constraint is a useful means of generating material parameters that are necessary to apply the model to compounds for which few volumetric data are available.

Structure formation in doped discotic polymers and low molar mass model systems

Abstract Doping of low molar mass materials or polymers, possessing disc-like units, with electron acceptors leads to the stabilization of columnar discotic phases or even to the induction of such phases in compounds which either display a nematic discotic phase or only an amorphous phase in the absence of the electron acceptor. The induced columnar phase corresponds frequently to a hexagonally ordered one. We have observed, however, in addition the induction of new columnar phases such as the rectangularly ordered (Dro) and the columnar nematic phase (Nc). The enhancement of the tendency towards the formation of columnar phases is a consequence of electron acceptor—electron donor complex formation. Using a model proposed by de Jeu to describe the induction of smectic phases by complex formation we are able to account qualitatively for the experimental findings.

Insoluble monolayers of weakly ionising low molar mass materials and their deposition to form Langmuir-Blodgett multilayers

The properties of insoluble monolayers of weakly ionising materials e.g. carboxylic acids and amines are sensitive to the pH of the subphase and to the type and concentration of added electrolyte. The effects which these variables have on the π-A isotherms and on the structural parameters deduced from X-ray and neutron reflectivity data are dicussed in terms of the ionisation (and ion incorporation) within the monolayer. Of particular importance for the successful deposition of such monolayers onto solid substrates (as Langmuir-Blodgett films) is the requirement that they be maximally stable. We discuss how the stability of the floating monolayer (with respect to collapse or dissolution) is affected by changes in pH, surface pressure and electrolyte addition and, via analysis of the deposited multilayers, conclude that maximum monolayer stability occurs when the film is roughly half-ionised. The mechanisms of the successive transfer of monolayers from water surfaces to solid substrates remain poorly understood. We review the main findings of such studies to date, focussing in particular on the understanding to be gained from measuring the dynamic contact angle and transfer ratio during deposition. The problem involves, inter alia, understanding the nature of the forces between approaching surfaces and the properties of the thin liquid film between them. Interesting results from force-distance measurements between mica surfaces are discussed in this context.

Effects of Molecular Structure on the Properties of Terminally Cyano-Substituted Side Chain Liquid Crystalline Polysiloxanes

Abstract The synthesis and properties of a range of liquid crystalline side chain polysiloxanes are described. The effects of changes in chemical structure, such as lateral substitution, on the properties of these materials are discussed, and comparisons are made with the effects of similar structural changes in low molar mass materials. By selecting suitable side chains for homopolymers and copolymers, room temperature smectic, nematic and cholesteric (chiral nematic) polysiloxanes of positive dielectric anisotropy have been prepared, including the first example of a nematic siloxane homopolymer containing a cyano-group.