Atomic force microscopic studies of thin oriented films of ferroelectric liquid crystalline elastomers with varying network architecture

Cross-linked ferroelectric liquid crystalline (FLC) polymers were studied by atomic force microscopy (AFM). Polysiloxane copolymers were synthesized with mesogenic and photo-cross-linkable side groups, the latter connected either directly to the backbone via a short spacer or as a terminal group on a part of the mesogens. These elastomers were prepared as thin, freely suspended films in homeotropic orientation. Topographic measurements depict plateaus separated by steps of characteristic height corresponding to the surfaces and edges of smectic layers. From the temperature behavior, as well as from the reaction to mechanical deformation, a model for the network architectures is proposed. In the first case (“intralayer cross-linking”), a predominantly two-dimensional network is formed within the backbone layers separating the smectic layers; in the second case (“interlayer cross-linking”), a three-dimensional network is established that is dependent on the mesophase present during cross-linking.     

Lattice dynamics of black phosphorus

Phonon dispersion curves of black phosphorus are calculated using the force constant model, in which the valence force model is assumed for intralayer interatomic interactions and the axially symmetric force model for interlayer ones. Ten force constants in all are determined by “least-squares” fitting to optical Г-phonons and the symmetry relations between sound velocities. The phonon dispersion curves calculated are applied successfully to interpretation of experiments on the first and second order Raman spectra, effective Debye temperature, and inelastic neutron scattering. Large discrepancy between the calculated and observed linear compressibilities exists.     

Anisotropy of dipole-dipole interactions in ferroelectric langmuir films of poly(vinylidene fluoride-trifluoroethylene)

The Lorentz tensor components L _j for poly(vinylidene fluoride-trifluoroethylene, 70/30) Langmuir-Blodgett films in the ferroelectric phase are determined experimentally. The results indicate that intralayer dipole-dipole interaction between the segments of the polymer is stronger than interlayer interaction. This conclusion agrees with the absence of the critical film thickness below which ferroelectricity would cease to exist.     

On the helical domain structure of the liquid ferroelectrics

The case when a nematic liquid crystalline phase is ferroelectric is considered. Such “liquid ferroelectric” is, surely, instable and transforms into a helical cholesteric-type structure which for this case is a variety of the ferroeltric domain structure. One can think that some cholesteric phases are liquid ferroelectrics with the helical domain structure.     

Characterization of commercial polybutylenes. part II. Crystallinity and tensile properties

A series of previously characterized polybutylenes were compression molded using various cooling conditions. Crystallinity decreased as cooling rate increased. The samples exhibited three types of tensile behavior. The crystallinity of samples containing around 5% ethylene was about 40%. These behaved as elastomers, exhibiting typical entropic elasticity. For the remaining samples, crystallinities between 55% and 70% were observed, depending on grade, and more particularly on cooling conditions. For these, lower crystallinity levels favor sample necking and stress-induced orientation, producing high elongation at break and high tensile strength values, while the higher crystallinity samples show high moduli and elasticity values coupled with uniform deformation. In the former case, significant crystalline deformation—causing alignment of the c axes of the crystallites towards the stretching direction, and thereby reinforcing the sample—is responsible for the high elongations and tensile strength encountered. In the higher-crystallinity case much of the deformation is taken up by interlamellar void generation, with little or no c-axis alignment, this elastic mechanism having been termed “hard elasticity.”     

Enhanced sensitivity to residual dipolar couplings of elastomers by higher-order multiple-quantum NMR

The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.     

脉冲射频等离子体聚合沉积乙烯基乙酸

通过脉冲射频等离子体聚合方法获得乙烯基乙酸沉积薄膜，并用红外光谱、x射线光电子能 谱、表面张力、微分扫描量热分析及扫描电子显微镜等测试方法研究聚合沉积薄膜的化学结 构、表面物理形貌与脉冲放电条件的关系. 实验结果表明，采用脉冲放电，在脉冲占空比较 低时，能够保留较多的完整的单体分子官能团. 如果脉冲占空比较高或是采用连续波放电时 ，聚合沉积薄膜的化学结构与单体相比有较大改变. 聚合沉积薄膜在放置一段时间后表面为 规整的高度交联的网络状结构. 放电形式不同，结构有所区别. 因此可以根据实际需要，选 择不同的 关键词： 等离子体聚合 脉冲射频放电 乙烯基乙酸     

Synchronization in a multiplex network of gene oscillators

Study of the synchronization in the network of gene oscillator network has vital importance in understanding of rhythmicity of molecular and cellular activities. In this paper, we analyze a network of linearly coupled genetic oscillators in a multiplex structure. The coupling strength values are changed and the coupling range is considered to be fixed, but different in the two layers. The analyses are done in two cases of periodic and chaotic oscillations. By computing the statistical measures, the interlayer and intralayer synchronization states are studied. The results show that the layer with higher coupling range has more enhanced synchrony and is less affected by the turbulent behavior of the other layer. On the other hand, the layer with lower coupling range approaches synchronization by strengthening the interlayer and intralayer couplings. The interlayer synchronization is also achieved in high coupling strength values.     

Orientation of discotic and ferroelectric liquid crystals in a macroporous silicon matrix

Macroporous silicon with deep regular channels 3–4.5 μm in diameter was infiltrated with discotic and ferroelectric liquid crystals (LCs) at the temperature of the isotropic phase, and then, the system was slowly cooled to room temperature, with the liquid crystalline mesophase formed. The orientation of the LC molecules in the porous matrix was studied by FTIR spectroscopy. The alignment of LCs was ascertained by comparing the behavior of various vibrational bands of a liquid crystal introduced into the porous matrix with that for LC inside the bulk cells of planar and homeotropic alignment. The molecules of the discotic LC show a planar orientation of their column’s axis with respect to the surface of the macroporous silicon wafer; i.e., they are perpendicular to the channel axis. The long molecular axis of the ferroelectric LC is aligned with the pore walls, having homeotropic orientation with respect to the wafer surface. In a macroporous silicon matrix, both kinds of LCs show unexpected enhancement of the low-frequency vibrational bands. From Fizika Tverdogo Tela, Vol. 44, No. 6, 2002, pp. 1145–1150. Original English Text Copyright ? 2002 by Perova, Astrova, Tsvetkov, Tkachenko, Vij, Kumar. This article was submitted by the authors in English.     

Ordering of molecular dipole moments in a nanosized water cluster

A water cluster of 1000 molecules is simulated using the method of classical molecular dynamics. The calculation is performed for the “flexible” version of the TIP4P interaction potential for a constant cluster temperature of 180 K. It is found that clusters with a solid crystalline or amorphous core and a liquid-like shell are stable structures. The ferroelectric ordering of the molecular dipole moments in the central part of the cluster is found for both cluster types; this ordering is responsible for a large total dipole moment of these molecular groups. However, this effect is compensated by dipoles in the peripheral cluster layers, so that in general, the mean dipole moment of a cluster appears to be much smaller than that in the case of the random orientation of the cluster molecules.     

Electroclinic effect in free-standing smectic elastomer films

The coupling of the mesogenic tilt in smectic liquid crystals to external electric fields in the layer plane (electroclinic effect) provides an opportunity to control the smectic layer thickness with electric fields. In ordered ferroelectric smectic elastomers it is possible to achieve a macroscopic electromechanical response. The effect is particularly pronounced near the smectic A-to-smectic C^* phase-transition temperature. In this work, the electrostriction of weakly cross-linked smectic elastomers is studied by means of optical interferometry of thin films. Its observed magnitude corresponds to an induced tilt of approximately 7.7°, yielding a layer contraction of 1%, in a 3 MV/m electric field. This value is in agreement with earlier measurements of tilt susceptibility in a structurally similar polymer, but is considerably smaller than previously reported data on elastomers with a comparable chemical structure. PACS 61.30.Vx; 83.80.Va; 77.65.Bn     

Development of helical cholesteric structure in a nematic liquid crystal due to the dipole-dipole interaction

A nematic liquid crystalline phase is considered whose rod-like non-centrosymmetric molecules possess a permanent dipole moment. This phase is a “liquid ferroelectric” if all the molecules are oriented along the same “preferred” direction. It is shown that a liquid ferroelectric can not exist in a homogeneous nematic state. It is transformed into a more stable helical structure (the vector of the spontaneous polarization of such a structure rotates aroung the helical axis). There is a variety of domain structures for the specific case when the anisotropy coefficient of the polarization is equal to zero. Since each elementary dipole moment is rigidly bound to its molecule, the “preferred” alignment direction of the rod-like molecules as well as the polarization vector rotates with respect to the same axis in a helical manner. Therefore a nematic phase with a nonzero spontaneous polarization has a cholesteric structure. Its helical pitch is determined by the geometric size of the sample, the absolute value of the spontaneous polarization, and the elastic moduli. Apparently, we can consider some cholesteric phases to be liquid ferroelectrics with helical domain structure.     

Multilayer structures of the iron/chromium type with almost ideal interfaces in an external magnetic field

The magnetic phase diagram of the Fe/Cr/Fe three-layer structure with almost ideal interlayer boundaries was constructed. The effective interlayer interaction in this structure was described by the “half-angle coupling” model. Various system configurations were analyzed taking into account crystalline anisotropy, and the ground state of the system was determined. The behavior of the structure in an external magnetic field applied along easy and hard magnetic axes was studied. The magnetization curves M(H) characteristic of structures with various interface roughness parameter and interlayer exchange values were described and analyzed. The experimental situation is discussed.     

A Generalized High-Elasticity Model to Describe the Stress-Strain Dependence for Polyurethane Elastomers When Stretched at a Constant Rate

The stress versus strain relationships for cross-linked polyetherurethane and polybutadieneurethane elastomers were investigated on samples when simply stretched at a constant rate. Use of the generalized approach earlier developed for equilibrium strain, with related improvements taking into account the relaxation nature of the elastomers, permitted successfully describing the investigated dependences.     

Dielectric properties of side chain liquid crystalline elastomers: influence of crosslinking on side chain dynamics

Dielectric response, from 1 Hz to 10 MHz, of liquid crystalline side chain elastomers has been compared to the one of analogous uncrosslinked materials. Results are discussed in terms of mobility of the mesogens. Two different systems have been investigated: smectic A elastomers allow to determine the influence of crosslinking on the relaxation (i.e. the reorientation of the whole mesogen around the chain), a S _C ^* elastomer shows the drastic influence of the polymer network on the Goldstone mode. Received 12 October 1998     

Softening of magnetorotons in a semi-infinite Fibonacci superlattice

Calculations of interlayer and intralayer screening of the Coulomb interaction on the softening of bulk and surface magnetoroton modes are presented for density and position modulations of the two-dimensional (2D) electron gas (EG) layers of a semi-infinite quasiperiodic superlattice. It is shown that the softening of these modes is due to an increase in the screening by all other layers of the effective intralayer Coulomb interaction. Numerical results are obtained for variable thickness of a 2DEG layer, the separation between layers and the distance between the surface layer and the top metal gate. The critical values of the structure parameters, determining the interlayer and intralayer screening of the Coulomb interaction, are obtained and used in constructing the phase diagrams showing the separation between the quantum fluid and charge-density wave phases.     

External field-induced switching in nematic elastomers: A Monte Carlo study

We present a Monte Carlo study of external field-induced switching in nematic elastomers, employing a coarse-grained shearable lattice model. In large enough systems a full-wavelength Fréedericksz effect is observed --as opposed to the half-wavelength effect seen in ordinary nematics-- that clearly reflects in simulated polarized light textures, as well as in deuterium magnetic resonance spectra. The reorientation of mesogenic units is accompanied by pronounced shear deformations.     

Modification induced by gamma irradiation in Bayfol CR 1-4 polycarbonate

Bayfol CR 1-4 polycarbonate is a class of polymeric solid state nuclear track detector which has many applications in various radiation detection fields. Samples from sheets of Bayfol have been irradiated with gamma doses ranging from 100 to 620 kGy. The structural modifications in the gamma-irradiated Bayfol samples have been studied as a function of dose, using different characterization techniques such as X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, intrinsic viscosity and refractive index. The results indicate that the carbonyl group (C?O) degraded under irradiation up to 200 kGy. This degradation, reported by FTIR spectroscopy enhanced the degree of ordering in the degraded samples as revealed by the XRD technique. Above 200 and up to 620 kGy, cross-linking is achieved, leading to an increase in the intrinsic viscosity from 0.41 to 0.78 at 35°C, indicating an increase in the average molecular mass. On the other hand, the resultant effect of gamma irradiation on the thermal properties of Bayfol has been investigated using thermo-gravimetric analysis, results indicating that the gamma irradiation in the dose range 200–620 kGy led to a more compact structure of Bayfol polymer, which resulted in an improvement in its thermal stability with an increase in the activation energy of thermal decomposition due to cross-linking. In addition, the V–I characteristics of the polymer samples were performed, results indicated that at higher voltage, the conduction mechanism of Bayfol CR 1-4 was identified as the Poole–Frenkel type.     

结晶型PEO8∶NaPF6聚电解质中晶区链段的运动

利用固体NMR 研究了高度结晶的聚氧乙烯(PEO)/六氟磷酸钠(NaPF6)（按照氧钠摩尔比8∶1 描述为PEO8∶NaPF6,分子量Mw = 1 000 和6 000 g/mol）固体聚电解质晶区链段的结构和运动．对于纯PEO 来说,晶区链段的构象交换或大角度再取向促使其¹³C 粉末线形从低温的非轴对称(δ₃₃ <δ₂₂ <δ₁₁)变成高温的轴对称线形(δ₁₁ =δ₂₂ >δ₃₃)．通过变温的¹³C 粉末线形和243 K 下的二维交换谱,PEO8∶NaPF6 晶区链段同样存在大角度再取向,且开启温度也很低(~243 K)与PEO 接近．这种长程的运动使得PEO8∶NaPF6 从低温的类轴对称(δ₃₃ <δ₂₂ <δ₁₁)变成高温的轴对称线形(δ₃₃ >δ₂₂ =δ₁₁),高温线形是PEO 高温线形的翻转．与其它PEO/Na(Li)固体聚电解质不同,PEO8∶NaPF6 中晶区链段与Na+络合后仍具有很高的运动性（与纯PEO 链段的运动性相当）,这种高分子链段和Na+协同运动促使Na+沿PEO 分子链轴向迁移,提高电导率．