Oriented Thin Films of the Low-Band-Gap Polymer PTB7 by Friction Transfer Method

Low-band-gap polymers are promising materials for organic photovoltaic application. We prepared an oriented thin film of a low-band-gap polymer, PTB7, by friction transfer method. The oriented film showed a strong dichroic absorption in the whole visible range. The orientation of molecular chain was characterized by two-dimensional grazing incident X-ray diffraction (2D-GIXD) with synchrotron radiation. 2D-GIXD results showed PTB7 molecular plane parallel to the substrate plane, i.e., “face on orientation.”     

State of Order in Liquid Crystalline Elastomers

The orientation behavior of crosslinked liquid crystal-line side chain polymers is determined by IR-dichroism measurements. In stretched samples the conformation of the polymer main chain and the chemical constitution of the rodlike side chain determine the position of the director in relation to the axis of stress. The order parameter S has the same temperature dependence and is of the same magnitude as in the corresponding uncrosslinked liquid crystalline polymers. The order parameter and the nematic to isotropic phase transformation temperature are independent of the applied mechanical stress under the experimental conditions (elongation up to 50%).     

Tolanes derivatives

The phase behavior for each of two linear thermotropic liquid crystalline polyesters with a corresponding low molecular weight liquid crystal of similar structure have been investigated. Two distinct types of phase diagrams have been obtained for binary mixtures of a polymer, one containing an in-chain azoxybenzene moiety mixed with para-azoxyanisole and one containing a naphthalene unit mixed with a dimethylester liquid crystal. These phase diagrams were studied by DSC, polarized light microscopy and x-ray diffraction. From this understanding of the physical mixtures a transesterification reaction incorporating the bifunctional low molecular weight liquid crystal by a solid-state reaction into the naphthalene-containing polyester was undertaken. This reaction may be viewed as a potential new processing technique for polymers. The thermal stability of an interreacted blend was shown to be superior to that of either individual component by thermogravimetric analysis. Thus, the potential new processing technique features a lower melting, lower viscosity blend followed by the incorporation of the dimethylester liquid crystal into the polyester main chain producing a more rigid, more stable polymer backbone.     

Orientation of Guest Molecules and Formation of Mesoporous Silica Induced by Layered Silicate-Organic Interactions

Previous work has suggested that the molecular ordering in some main-chain thermotropic polymers is biaxial nematic. In one, a 60/40 copolyester of p-hydroxybenzoic acid and ethylene terephthalate residues, there is evidence for a temperature range where molecular rotation about the local chain axes occurs without an accompanying loss of overall chain segment alignment. The present paper reports a theoretical and experimental investigation of whether a magnetic field, applied to the polymer when held at such temperatures, can favour the formation of optical monodomains which are large enough to give an unambiguous uniaxial or biaxial interference figure.     

Modification of montmorillonite by quaternary polyesters

In this paper, we report on the preparation of organically modified montmorillonite (OMM) and its properties, using quaternary polyesters. The polyester was synthesized from N-methyldiethanolamine and succinic acid anhydride, converted to the quaternary polyester using benzyl bromide, and characterized by nuclear magnetic resonance (NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Quaternary polyesters with a quaternization degree of 0.25 and 0.5 were used to modify montmorillonite (MMT). The intercalation was determined by X-ray diffraction. Interlayer spacing increased with decreased MMT/polyester ratio and with increased degree of quaternization. The thermal stability of polyester bound to MMT greatly increased.     

Microwave power absorption analysis in the devitrification process of Co-based amorphous ribbons

Melt-spun Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu soft magnetic ribbons were devitrified at low annealing temperatures (623 K), for times 5–20 min. Microwave power absorption measurements at 9.4 GHz (X-band) were carried out in two geometries. In geometry 1, the ribbon’s plane was oriented parallel to AC magnetic field. For the orientation 2, the ribbon’s plane was normal to the AC magnetic field. In both cases, the ribbon’s axis was parallel to the DC magnetic field. For both orientations, two absorptions were observed: the first corresponds to a low field microwave absorption (LFA) centered in zero dc magnetic field, and a higher field absorption corresponding to the ferromagnetic resonance (FMR). In the geometry 1, a single FMR spectrum was observed for all the samples, with a shift in resonant field as annealing increased. For geometry 2, evidence of the superposition of two FMR signals was observed. FMR spectra are therefore due to a combination of two different magnetic phases corresponding to the amorphous matrix and nanocrystallites. Deconvolution calculations were carried out on FMR spectra to separate the contributions. Their behavior as a function of annealing time was in good agreement with the magnetic softening, also obtained with LFA results. The differences in microwave absorption, for both geometries, can be explained by differences in the electromagnetic wave propagation volume.     

Deuteron Magnetic Resonance Study of Order and Dynamics in Liquid Crystal Polymers

Thermotropic nematic polyesters, specifically deuterated at different positions of the polymer chain are studied by multiple pulse dynamic NMR. Analysis of the NMR experiments is achieved, employing a comprehensive model, based on the stochastic Liouville equation. Computer simulations provide the orientational distributions and conformations of the polymer chains and the correlation times of the various motions. In the anisotropic melt the correlation times for chain reorientation and trans-gauche-isomerization are in the range of 0.1–10 ns. Decreasing the temperature of the solid polymer first freezes the intermolecular motions. Thus, below the glass transition only intramolecular motions such as trans-gauche isomerization and ring flips can be detected. The chain order parameter of the nematic melt is S(ZZ) = 0.85. In addition, the chains adopt a highly extended conformation, evidenced by a trans population of n(t) = 0.8 throughout the entire spacer. This microorder is retained, when the polymer is cooled below the melting point and glass transition, respectively.     

Investigation of Crystalline Polymeric Solids at Reduced Beam Damage in the High Voltage Electron Microscope

A special technique was used for investigating polymers in the electron microscope at essentially reduced radiation damage. The method is based on a high voltage electron microscope (accelerating voltage of 1 MV) and the use of highly sensitive X-ray films. Compared with the conventional transmission electron microscopy this technique reduces the specimen damage by a factor of 50 … 100. Diffraction contrast can therefore be used to investigate the structure of semi-crystalline polymers. Using samples of polyethylene, the arrangement of the lamellae was determined by bright field and dark field images and diffraction patterns. There are further advantages concerning the improved possibility of investigating very thick specimens (thicknesses above 5 μm). Thus, a better stereoscopic analysis of extended structures is possible, and in-situ deformation tests of polymers can be performed in the electron microscope.     

Kinematical limit in the theory of X-ray magnetic reflectivity

A kinematical approximation has been derived from the general formulas for calculating the reflectivity from a multilayer anisotropic structure. An explicit form of reflection matrices at the boundary of layers and the refraction tensors has been obtained for a particular case where the layer magnetization is oriented in the surface plane and for an arbitrary form of X-ray susceptibility tensors. Calculations of the reflectivity from a Nb(4 nm)/[Dy(5 nm)/Lu(3 nm)]₄₂₀ model structure with helicoidal ordering of Dy magnetic layers show that kinematical approximation can be applied for grazing angles that are larger than the critical total-reflection angle and that the “magnetic” refraction must be taken into account.     

Mesophase Formation in Polymers With Anisometric Side – Groups in Relation to Their Chemical and Stereochemical Structure

The influence of the flexibility of the polymer main chain on the smectic ordering has been investigated, by means of optical, diffractometric and calorimetric techniques, in several polymers carrying rigid anisometric side-groups. The effect of the chemical structure of the sidegroups has been examined throughout: atactic polymers with biphenylyl side-groups may be amorphous or mesomorphous depending on the nature of the groups linking the biphenylyl moiety to the backbone chain. Correlations between the molecular weight and the stereochemical structure of the macromolecules and the occurrence of a thermotropic smectic phase in a series of polyacrylates and polymethacrylates, have been reported.     

A Polyester Forming a Thermotropic Cholesteric Phase

Mesomorphic polyesters were synthesized from 4, 4′-dihydroxy-α-methylstilbene and adipic acid (P-6) or (+)-3-methyl adipic acid (P-6M). P-6 forms a thermotropic nematic phase and P6-M a thermotropic cholesteric phase. The nematic phase of P-6 could easily be identified by optical microscopy. For both polymers we observed a biphasic region in which the isotropic and liquid crystalline phases coexist. Bright colors were obtained by increasing the pitch of P6-M by admixture with either a low molecular weight nematogen or with polymer P-6, and also by synthesizing a copolymer containing the two dibasic acids. The copolymeric cholesteric phase, which is stable between 199 and 282[ddot]C, had a predominately planar texture, and these features could be retained in the solid state by quenching to produce a film having a deep blue color at room temperature. The role of the degree of polymerization upon the development of organization, and parameters, of the mesophase is discussed.     

Texture growth and screw dislocations in smectic A phases

The formation of the textures of smectic A phases was investigated. In planar oriented samples the A phases grow in the shape of rods without defects. The velocity of growth is strongly anisotropic with its maximum normal to the director orientation. In non oriented samples the smectic A texture is growing in batonnets with defect structure to give focal conic textures. The maximum growth velocity of the batonnets is parallel to the director orientation. Since this would contradict to the Wulff theorem we assume a growth mechanism with a screw dislocation.     

Neutron nano-spin-echo spectrometer based on magnetic nanostructures

A neutron spin-echo spectrometer based on spin precessors in the form of magnetic layered nanostructures is described. A model of a spin-echo spectrometer is developed on beam no. 9 in the IBR-2 reactor. In this model, spin precession occurs during motion of neutrons in a magnetic field and their double reflection from Al(30 nm)/Fe(15 nm)/Al(120 nm)/Cu(150 nm) magnetic layered structures. The obtained spectrometer parameters make it possible to investigate excitations in films with a wave vector oriented along the neutron beam direction in the range from 10^?3 to 10^?1 Å^?1 and perpendicularly to the beam in the range from 10^?4 to 10^?5 Å^?1.     

Magnetic nanostructures in liquid and amorphous materials as probed by small angle neutron scattering

Small angle neutron scattering using polarized neutrons is introduced as a special type of contrast variation for magnetic systems. Examples of diluted magnetic systems are reviewed where low magnetic contrasts had to be analyzed beside strong nuclear contributions or vice versa. In Ferrofluids magnetic core–shell composite particles and magnetic aggregates could be precisely evaluated beside non-magnetic micelles and free surfactants of similar sizes. In more concentrated Co-Ferrofluids an external magnetic field induces a pseudo-crystalline hexagonal ordering which coexists with chain like arrangements of particles. In magnetic glasses magnetization density profiles have been monitored. Magnetically inactive deadlayers or weakly magnetic interfaces separate nanocrystalline ferromagnetic particles from amorphous (para- or ferromagnetic) matrices in soft magnetic materials based on Fe₃O₄-glass ceramics, Fe–Si–B and Fe–Nb–B alloys. Nanosized inclusions have been detected and identified to be at the origin of the hard magnetic properties of Nd based alloys.     

Study of magnetoimpedance effect of Co-based amorphous ribbons after current annealing at various kinds of ambient pressure

The physical properties of magnetic amorphous alloys can be improved by using different annealing processes. In this paper, for annealing purposes, different driving currents were flowed through Co-based magnetic amorphous ribbons at different ambient air pressures between 5 × 10^?5 mbar and 10³ mbar. The magnetoimpedance effect in the annealed samples was studied at a frequency interval between 250 kHz and 10 MHz. Magnetic properties and microstructures of samples were investigated by means of alternative gradient force magnetometers and X-ray diffraction. Regarding the giant magnetoimpedance (GMI) effect, annealing at different ambient pressures with the same current can lead to various responses. For annealing at higher pressures such as in air, higher current is necessary for crystallization of samples which results in the development of greater transverse magnetic anisotropy.     

Monte Carlo Simulation of the Liquid-Crystalline Ordering of Semiflexible Polymers

We present results from Monte Carlo simulations of the liquid-crystalline ordering in the athermal systems of partially flexible polymers in a two- and three-dimensional continuum. Any two non-bonded chain monomers interact via a hard spher potential. The chains are placed in a periodic box consistent with the number density C. It is observed that the mean dimensions of chains and the orientational order parameters increase sharply with increasing C. In other words, we observe elongation and stiffening of the chains. In contrast to semiflexible polymers, the mean dimensions of flexible chains are reduced as the density is increased.     

Optical properties of azo-based poly(azomethine)s with aromatic fluorinated fragments,ether linkages and aliphatic units in the backbone

Abstract

The aim of this work is to study the optical properties of the core-fluorinated azo-containing polyazomethines with 1,4-tetrafluorobenzene or 4,4′-octafluorobiphenylene dioxyphenylene units and aliphatic fragments in the chain. Due to the reorganization process at the nanometer-scale under UV/Vis irradiation, azo-containing polymers (APs) have attracted much attention as materials for biology, optoelectronics, nano-manipulation field, etc. Moreover, the introduction of azomethine group into these polymers enhances not only their optical properties but also expands their practical application. APs polymers have the useful combination of chemical and physical properties such as liquid crystalline states, high nonlinearity and an ability to form metal complexes. It was shown that the optical properties of such polymers could be improved by the inclusion of fluorine. Fluorinated polymers demonstrate high thermal stability and have low dielectric constant and dielectric losses. It should be noted that the presence of flexible moieties in a polymer backbone allows regulating the different properties (solubility, thermostability, optical and liquid crystalline properties) of the final materials.     

Polariton spectrum of a bounded antiferromagnet with a center of antisymmetry in an external electric field oriented normally to the surface

The features of transmission of TE and TM polaritons through an interface between magnetic and nonmagnetic media and their localization at the interface in an external dc electric field have been studied. This field is directed along the hard magnetization axis. The magnetic medium is an easy-plane tetragonal antiferromagnet; it is odd with respect to inversion. A magnetic plate and a 1D magnetic photonic crystal in an external dc electric field, oriented normally to the interface, are considered.     

On Mechanism of Oxide Formation in Electrodeposited Co-based Films with Columnar Structure

An investigation of the Co oxides formation and distribution dependent on preparation conditions during electrodeposition of hcp Co-based films with perpendicular magnetic anisotropy has been made. It has been shown that the [100] oriented columnar crystallites promote some regularity in impurity distribution and orientation in the film plane, whereas the [001] oriented ones- non-regular distribution. Such a behaviour is associated with the difference in the impurity incorporation for these two cases: preferably crystalline and intergranular, respectively, and atomic bonds saturation, and influence on the mechanical and magnetic properties of the investigated films.