Investigation of fracture mechanism on liquid crystalline epoxy networks arranged by a magnetic field

A liquid crystalline epoxy resin was cured under non‐ and 10T‐magnetic fields, and polydomain and monodomain networks were obtained, respectively. The fracture toughness of these systems was evaluated and it was clarified that the toughness of the magnetic field system showed a higher value. To investigate the toughening mechanism, polarized micro FTIR measurements were carried out. As a result, it was clarified that their mechanisms were quiet different. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1406–1412, 2006     

Polydomain–Monodomain Orientational Process in Smectic‐C Main‐Chain Liquid‐Crystalline Elastomers

The polydomain–monodomain (PM) transformation takes place when a polydomain of a smectic‐C main‐chain liquid‐crystalline elastomer (SmC MCLCE) is uniaxially stretched. We present results based on a combination of mechanical and X‐ray experiments which show how the domains initially rearrange to finally form a perfect conical layer distribution (monodomain) when the sample is fully stretched. The rearrangement and orientational process of the domains is quantified and compared to the parallel and perpendicular uniaxial stress–strain deformations of a monodomain sample. The stress–strain behaviour of the polydomain lays between the uniaxial deformations, parallel and perpendicular to the director, of the monodomain sample.

     

Reversible Photoinduced Switching of Permeability in a Cast Non‐Porous Film Comprising Azobenzene Liquid Crystalline Polymer

Permeation characteristics of an azobenzene‐containing liquid crystalline (LC) non‐porous film are investigated using a metallic corrosion method. Thin films (300 nm) are fabricated by the solution casting of an azobenzene side‐chain LC polymer on freshly polished carbon steel coupons. Coated coupons are treated under the following conditions: a) gradual annealing at a cooling rate lower than 1 °C · min⁻¹ from 150 °C (above its T_g) to room temperature, and b) irradiation at 465 nm (20 mW · cm⁻²) with either circularly polarized light (CPL) or non‐polarized light (NPL). The morphology of these films is characterized using X‐ray diffraction, polarized optical microscopy, and transmission measurements. The results suggest that the annealing treatment resulted in the formation of a polydomain structure consisting of locally ordered small smectic domains that lack mutual orientation. Ordered micro domains are surrounded by disordered phases. CPL and NPL irradiation generates a monodomain orientated structure and an isotropic liquid crystal glass, respectively. The permeability of these non‐porous films treated by CPL, NPL, and annealing are found to be 6.14 × 10⁻⁴, 1.92 × 10⁻², and 1.56 × 10⁻³ cm³ · m⁻² · d⁻¹. An orientation‐dependent structure model is constructed to explain the permeation phenomenon, considering the ordered phase is impermeable, only the disordered phase is accessible to penetrating molecules. Fast switching of gas permeation is demonstrated by alternative irradiation of the film with CPL and NPL, which results in an approximately 30‐fold difference in the permeability of the non‐porous film.

     

The kinetics of adsorption of tetracycline on chitosan particles

Experiments to monitor and characterize the kinetics of adsorption of tetracycline on chitosan particles are reported in this work. The same pseudo-order kinetics that has been widely used for describing the adsorption in systems related to wastewater purification and drug loading was used to treat the present data. As some unexpected results came out from the experiments, it was necessary a detailed deduction for this sort of kinetics to be carried out, so that approximations related to short and long times were obtained. Firstly it was shown that an apparently linear t/q(t) versus t relationship did not imply a pseudo-second-order sorption kinetics, differently of what has been repeatedly reported in the literature. It was found that this misinterpretation could be avoided by using non-linear regression. Finally, the adsorption of tetracycline on chitosan particles was analyzed, using the insights obtained from theoretical analysis, and the parameters generated were used to analyze to adsorption kinetics and to propose an adsorption mechanism.     

Relationship between fracture toughness and domain size of liquid‐crystalline epoxy resins having polydomain structure

A liquid‐crystalline (LC) epoxy resin was cured at different temperatures and some types of curing systems having different phase structures (isotropic or polydomain, which have a microscopically ordered LC network structure) were obtained. The diameters of each domain in the polydomain system changed from the small to the larger size. The diameters of the LC domains were evaluated using a polarized optical microscope and the polarized microscopy FTIR mapping method. These systems were used to investigate the relationship between the network arrangement and mechanical properties. The fracture toughness of the cured systems was related to the enlargement of the ordered area in the network structures. With the toughness improvement, the meandering cracks were observed at the fracture surfaces. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 156–165, 2009     

Inducing monodomain blue phase liquid crystals by long-lasting voltage application during temperature variation

We demonstrate a new successful technique for inducing a monodomain in blue phase (BP) liquid crystals (BPLCs). By application of an AC voltage during the cooling or heating of the material into the BP temperature range, a fairly homogeneous BP area was obtained, compared to the case without voltage. We determined that applying a voltage of 3.3 V/µm was very effective in terms of obtaining a monodomain: for four of the seven different applied cooling/heating rates an area ratio of more than 99% was obtained for one particular BP orientation. This study describes a successful and replicable technique to make a BPLC monodomain, which is important for the development of BPLCs and other applications.     

Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: a step towards coarse graining

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.     

The effects of fast neutron,gamma-ray and combined radiations on the thermal decomposition of ammonium perchlorate powder-aluminum particle mixtures

The thermal decomposition kinetics of irradiated and unirradiated ammonium perchlorate and ammonium perchlorate powder-aluminum particle mixtures has been studied by determining decomposition gas pressurevs. heating time with samples at a controlled temperature Qualitatively the radiation induced changes are similar to those obtained in previous studies on ‘pure’ ammonium perchlorate. The induction period is shortened and the acceleratory and decay period rate constants are increased. The data have been analyzed using Avrami-Erofeev kinetics. The results for pure unirradiated material are in accord with published results. The activation energies for the induction, acceleratory and decay periods for pure pellets were found to be 133.5±6.7, 131.8±6.7 and 127.2±6.7 kJ·mol, respectively. Samples were exposed to either a single gamma-ray irradiation, fission neutron irradiation followed by a gamma-ray irradiation, or to a proton irradiation. When compared on an equal energy deposited basis, the fast neutron induced changes are appreciably larger than the gamma-ray changes. However, the proton induced changes are comparable or slightly more than the gamma-ray effects. Some, or all, of the fast neutron effects can be attributable to the concentrated radiation damage ‘spikes’ along the path of lattice atom recoils. It is likely that these become thermal decomposition sites when the crystals are heated. Protons create fewer spikes than fast neutrons. Overall, the results indicate that any ammonium perchlorate-aluminum propellant mixtures that may be exposed to radiation environments, such as used in this study, should be subjected to a thorough radiation effects analysis if reliable performance is required. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Heating rate effect on the DSC kinetics of oil shales

This research was aimed to investigate the combustion and kinetics of oil shale samples (Mengen and Himmetoğlu) by differential scanning calorimetry (DSC). Experiments were performed in air atmosphere up to 600°C at five different heating rates. The DSC curves clearly demonstrate distinct reaction regions in the oil shale samples studied. Reaction intervals, peak and burn-out temperatures of the oil shale samples are also determined. Arrhenius kinetic method was used to analyze the DSC data and it was observed that the activation energies of the samples are varied in the range of 22.4–127.3 kJ mol⁻¹ depending on the oil shale type and heating rate.     

Fracture toughness and fracture mechanism of liquid‐crystalline epoxy resins with different polydomain structures

Liquid‐crystalline (LC) epoxy resins were cured at different temperatures to obtain polydomain LC phase–cured resins. The cured resins had polydomain structures with a nematic LC phase and their domain diameters differed depending on the curing temperatures. The relationship between the domain diameter and fracture toughness of the diglycidyl ether of terephthalylidene‐bis‐(4‐amino‐3‐methylphenol) (DGETAM)/m‐phenylenediamine (m‐PDA) systems with the nematic phase and the previously reported smectic LC phase structures was investigated. It was clarified that the highly ordered LC structure (smectic phase) in each domain could improve the fracture toughness. In addition, the changes in the network orientation of the DGETAM/m‐PDA systems were evaluated by a mapping of the microscopic infrared dichroism in the fracture process and their toughening mechanism was suggested. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Fracture mechanism of liquid‐crystalline epoxy resin systems with different phase structures

A liquid‐crystalline epoxy resin was cured at two different temperatures. The phases of the cured systems clearly showed isotropic and nematic polydomain structures, which depended on the curing temperature. The fracture toughness of the systems was measured, and the fracture mechanism was investigated with polarized IR measurements. The nematic polydomain structure system showed considerably higher fracture toughness than the isotropic structure. Moreover, both systems exhibited a reorientation of the network chains near the fracture surface during the fracture process, and the region of the network reorientation in the nematic polydomain structure system was larger than that in the isotropic structure system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4044–4052, 2004     

Influence of strain rate and temperature on necking transition in a polydomain smectic main chain elastomer

Smectic main‐chain liquid crystalline elastomers (MCLCE) with polydomain morphology are rare examples of elastomers that can form a neck and undergo cold drawing under tension. However, not all previous studies of the mechanical behavior of smectic MCLCE reported neck formation. The mechanical response of a polydomain smectic MCLCE has therefore been characterized by elongation at varying strain rates and temperatures to identify factors favoring mechanical instability. Yielding and neck formation are increasingly favored as the strain rate increases at constant temperature, or as the temperature decreases toward T_g. As cold drawing proceeds, significant creep occurs continuously within the neck, in contrast to the behavior of certain linear polymers that exhibit a “natural” draw ratio. Thermal imaging during elongation indicates that viscous heating is not a prerequisite for neck formation. Rather, inherent softening of the material during yielding due to morphological changes leads to an enhanced rate of deformation and contraction at the neck. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Origin of strong chiroptical activities in films of nonafluorenes with a varying extent of pendant chirality

Novel nonafluorenes with a varying extent of pendant chirality were synthesized for an investigation of the origins of chiroptical activities in neat films. Thermal annealing of 4-microm-thick sandwiched films and of 90-nm-thick spin-cast films, all on surface-treated substrates, produced monodomain glassy films characterized as a right-handed cholesteric stack with a helical pitch length ranging from 180 to 534 nm and from 252 to 1151 nm, respectively. The observed strong circular dichroism (CD) and g(e) as functions of helical pitch length in single-substrate monodomain glassy cholesteric films were quantitatively interpreted with a circularly polarized fluorescence theory accounting for light absorption, emission, and propagation in a cholesteric stack. Although intertwined molecular helices were likely to be present, cholesteric stacking of rodlike molecules seemed to be the predominant contributor to the strong chiroptical activities. All the cholesteric stacks comprising a polydomain glassy film on an untreated substrate were found to contribute to CD and g(e) largely to the same extent as in a monodomain film. A circularly polarized blue organic light-emitting diode containing a nonafluorene film resulted in a g(e) of 0.35 with a luminance yield of 0.94 cd/A at 20 mA/cm(2), the best performance to date.     

The influence of photodegradation and weathering on the water vapour sorption kinetic behaviour of scots pine earlywood and latewood

The effect of weathering or accelerated photodegradation on the water vapour sorption kinetics of Scots pine (Pinus sylvestris) has been studied. The sorption kinetics were analysed using the parallel exponential kinetics (PEK) model and the parameters obtained used as input into a Kelvin-Voigt thereby providing values for the wood cell wall stiffness and viscosity. Differences in kinetic behaviour were found between earlywood and latewood samples, and between samples which had been photodegraded or weathered when compared to unexposed samples. The values for the cell wall stiffness are in general agreement with values previously published in the literature.     

Interactions between flow and superstructure of liquid crystalline polymers

The monodomain equation for rigid rodlike polymers in the nematic phase is here extended, in a simple form, to the polydomain situation. The mathematical problem is further simplified by considering a two-dimensional restriction. Preliminary results are presented for simple shear flows, showing the formation of a superstructure along the sample thickness.     

Small-angle neutron scattering from a hexagonal phase under shear

Summary The shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.     

The Morphology of Liquid-Crystalline Polymers and the Possible Consequences for their Rheological Behaviour

Abstract

Polymer liquid crystals can occur as polydomain materials where the domain size may be tens of microns. While the material within each domain may be characterized by a common order parameter, the directors of the domains can be more or less randomly distributed. Since the transition from polydomain to monodomain material only involves the removal of grain boundaries and the alignment of directors, the free energy change must necessarily be small. Such a transition can readily be achieved, therefore, by the action of any external field: electrical, magnetic, stress or surface. In this work optical photomicrographs of polymeric liquid crystals with widely varying and in some cases well controlled morphologies are presented. Probable dependence of rheological behaviour on morphology is also discussed. Such dependence is expected to be considerable under certain conditions. Due to experimental and sample limitations, however, direct correlations of rheology and morphology are sparse. Morphological consequences for the rheology of liquid-crystalline materials can be exemplified by the following possibilities. In contrast to the case of isotropic melts, wall effects can be non-negligible. Zero shear rate rheological parameters are not expected to be uniquely defined quantities since the domain sizes are large and the director may not be effectively averaged over typical sample dimensions. Non-zero shear-rate measurements of rheological parameters is effected by the propensity of: (1) individual domain directors to align under the influence of a stress field and (2) flow alignment to dominate surface-induced alignment above some critical shear rate. The effects might be manifested by a non-newtonian regime as well as yield stress behaviour and thixotropy. The kinetics of relaxation from mono- to poly-domain material has implications for the dynamic response and rheological hysterises of the material.     

Combustion characteristics of lignite and oil shale samples by thermal analysis techniques

In this research thermal analysis and kinetics of ten lignite's and two oil shale samples of different origin were performed using a TA 2960 thermal analysis system with thermogravimetry (TG/DTG) and differential al analysis (DTA) modules. Experiments were performed with a sample size of ~10 mg, heating rate of 10°C min^-1. Flow rate was kept constant (10 L h^-1) in the temperature range of 20-900°C. Mainly three different reaction regions were observed in most of the samples studied. The first region was due to the evaporation of moisture in the sample. The second region was due to the release of volatile matter and burning of carbon and called as primary reaction region. Third region was due to the decomposition of mineral matter in samples studied. In kinetic calculations, oxidation of lignite and oil shale is described by first-order kinetics. Depending on the characteristics of the samples, the activation energy values are varied and the results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Long‐range orientation correlations and molecular alignment in sheared thermotropic copolyester. In situ light and X‐ray scattering

Orientation correlations induced by shear flow and their relaxation were investigated using in situ small‐angle light scattering (SALS) in the thermotropic random copolyester of 60 mol% hydroxybenzoic acid (B) and 40 mol% ethylene terephthalate (ET). B‐ET displays a nematic polydomain texture, the SALS and wide‐angle X‐ray scattering (WAXS) patterns are amorphous and isotropic. Shear flow produced optical defect multiplication with the consequent reduction of the micro–domains size. However, SALS detected long‐range spatial correlations within the optically chaotic texture, the SALS patterns showed bimodal orientation of defects. After cessation of shear the orientation correlation rapidly relaxed back to a polydomain and the SALS pattern became again isotropic. Above a threshold shear rate of about the SALS pattern showed unimodal orientation arising from line defects oriented nearly orthogonal to the velocity axis. Strikingly, the texture relaxation now showed the well known “banded texture”. The threshold shear rate coincided with a significant increase in the degree of molecular alignment as determined from in situ X‐ray scattering. This technique also showed that shear flow always oriented the molecular chains along the flow direction regardless of the shear rate. Copyright © 2007 John Wiley & Sons, Ltd.