Effect of anti-parallel and twisted alignment techniques on various propertiesof polymer stabilised liquid crystal (PSLC) films

Thin composite films consisting of liquid crystal (LC) domains surrounded by polymer networks, termed as polymer stabilised liquid crystals (PSLCs), were prepared by photo-polymerisation of a pre-polymer dissolved in LC. Four composite films were prepared with different rubbing directions and with and without electric field during photo-polymerisation. Morphological characterisation carried our using a polarising optical microscope (POM) and a scanning electron microscope (SEM) reveal significant changes in LC domain morphology and associated polymer networks with the application of electric field during the fabrication of the films. The electro-optic (EO) properties of PSLC films placed between two crossed polarisers were studied using a He–Ne laser under an action of externally applied electric field. It was found that the PSLC film with twisted alignment and polymerised in the presence of electric field showed better EO properties than other films. Transmittance obtained by EO studies was verified with absorbance studies using a Ultraviolet-Visible spectrophotometer. The dielectric behaviour of PSLC films in the frequency range 20–20 MHz was investigated using a precision impedance analyser. The obtained data were modelled using Debye and Cole–Cole methods to calculate relaxation time and distribution parameter. The relaxation time calculated through the Cole–Cole model is in agreement with response time.     

Dielectric relaxation and ionic transport in poly(ethylene carbonate)‐based electrolytes

To study the ion‐conductive and dielectric properties of polymer electrolytes based on poly(ethylene carbonate) (PEC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the complex permittivity and conductivity were measured using broadband dielectric spectroscopy. The temperature dependence of the relaxation frequency and ionic conductivity for PEC‐LiTFSI electrolytes (1 – 200 mol%) indicates that the segmental motion of PEC chains decreases with the addition of just 1 mol% of Li salt and increases with increasing concentration above 10 mol%. According to the Walden rule for PEC‐based electrolytes, the value of deviation from the reference line increased, and the fragility and decoupling exponents decreased with increasing salt concentration. These results indicate that there are large numbers of ion pairs and aggregated ions, which imply low ionicity and reduced fragility in highly concentrated PEC‐based electrolytes. Copyright © 2016 John Wiley & Sons, Ltd.     

Study of reorientational dynamics during real‐time crystallization of absorbable poly(p‐dioxanone) by dielectric relaxation spectroscopy

The real‐time crystallization of absorbable poly(p‐dioxanone) (PDS) was studied by dielectric relaxation spectroscopy. The dipole dynamic changes in the diminishing amorphous phase were investigated over a wide range of crystallization conditions. The location, shape, and magnitude of the α relaxation and the apparent activation energy were monitored and compared before and after the onset of crystallization. We observed no correlation between the degree of crystallinity and the location (hence, the most probable relaxation time, τ) of the α relaxation from just after the initiation up to the latest stages of the isothermal crystallization. However, an abrupt change in the intensity of the α process and the apparent activation energy allowed for the precise detection of the onset of crystallization. This was probably caused by a reorganization of dipole units occurring a few moments before the crystallization began. As crystallization proceeded, an asymmetric broadening of the α peak was observed that was directly influenced by the appearance of a new lower frequency process that originated in the highly confined amorphous portion located inside the spherulites. Finally, PDS crystallization kinetics determined from the changes of the relaxed permittivity with time are discussed and compared with calorimetric and optical microscopy data. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2436–2448, 2000     

Molecular mobility of substituted poly(p-phenylenes) characterized by a range of polymer relaxation techniques

The free volume and related mobility properties of substituted poly(p-phenylene) polymers are examined. The techniques used range from positron annihilation, dielectric relaxation, and dynamic mechanical spectroscopy to thermally stimulated currents. Fractional free volume is determined for the samples with different substituted side groups and related to the glass transition temperature. Bulkier groups lead to a greater fractional free volume and lower glass transition temperatures. Comparison of molecular relaxation times using the different characterization techniques demonstrates that there is strong coupling between motion of the main chain and the side groups, on which the dipoles reside. Intermolecular coupling between the main chains at the primary relaxation is shown in this work to be related to the nature of the side chains and resultant free volume, as are the temperature locations of local, secondary relaxations. A qualitative model describing the effect of regiochemistry on the motions and packing of these materials is also proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1465–1481, 1998     

Dielectric relaxation of LC-thermotropic poly(ester imide)s

Two different series of poly(ester imide)s, which are distinguished from each other in the orientation of the ester linkages and show well-differentiated thermotropic behavior, are investigated by means of model calculations and dielectric relaxation spectroscopy. Model calculations show that the orientation of the ester linkages has a strong influence on the rotational energy barriers. The dielectric relaxation spectra of both series shows three relaxation regions in the temperature range between 100 and 400 K that have been identified as the α-, β- and γ-relaxation processes. A difference of about two orders of magnitude between the characteristic rates of the γ-relaxation is the main feature observed in the dielectric response. However, the β-relaxation shows very similar behavior for both series. The differences in the relaxation behavior in the solid state are interpreted on the basis of the rotational barriers deduced from the model calculation results. © 1997 John Wiley & Sons, Inc.     

An X-ray diffraction study of the different nematic mesophases of liquid-crystalline poly(urethane-ester)s

An x-ray investigation on powder specimens and stretched oriented fibers of poly(urethaneester)s TDI-CmCn, derived from various mesogenic alkylene di[4-(ω-hydroxyalkyloxy-4-oxybenzoyl)oxybenzoate]s (CmCn; m = 2, 4, or 6, and n = 4, 6, 8, or 10) and 2, 4-toluenediisocyanate (TDI), is reported. Evidence is provided for the formation of two different nematic mesophases in the polymers, namely a cybotactic nematic and a conventional nematic mesophase. Whereas samples TDI-C2C6, TDI-C6C4, and TDI-C6C10 formed one cybotactic nematic mesophase, samples TDI-C4C6, TDI-C6C6, and TDI-C6C8 exhibited both cybotactic nematic and conventional nematic mesophases in a sequence with increasing temperature, which were connected by a first-order transition. The analysis of the various features of the small-angle x-ray diffraction patterns indicates that two structural arrangements, namely smectic C-like and conventional nematic structures, coexist inside the cybotactic nematic mesophase of these poly(urethane-ester)s. © 1995 John Wiley & Sons, Inc.     

Dielectric relaxation study of atactic poly(epichlorohydrin)/poly(3-hydroxybutyrate) blends

Binary blends of atactic poly(epichlorohydrin) (aPECH) and poly(3-hydroxybutyrate) (PHB) were investigated as a function of blend composition and crystallization conditions by dielectric relaxation spectroscopy. The quenched samples were found to be miscible in the whole composition range by detecting only one glass transition relaxation, for each composition, which could be closely described by the Gorden-Taylor equation. The cold-crystallized blends displayed two glass transition relaxations at all blend ratios indicating the coexisting of two amorphous populations: a pure aPECH phase dispersed mainly in the interfibrillar zones and a mixed amorphous phase held between crystal lamellae. The interlamellar trapping of aPECH was small and decreases with increasing the overall PHB content in the blend. At high crystallization temperatures the aPECH molecules was found to reside mainly in the interfibrillar regions due to its high mobility relative to the crystal growth rate of PHB. Our results suggest that because the intersegmental interaction in aPECH/PHB blends is weak, the mobility of the amorphous component at a given crystallization temperature decides diluent segregation.     

Molecular relaxation in the blends of polystyrene/poly(2,6-dimethyl-1,4-phenylene oxide) at low temperature

Molecular relaxation behavior in terms of the α, β, and γ transitions of miscible PS/PPO blends has been studied by means of DMTA and preliminary work has been carried out using DSC. From DSC and DMTA (by tan δ), the observed α relaxation (T_α or T_g) of PS, PPO, and the blends, which are intermediate between the constituents, are in good agreement with earlier reports by others. In addition, the β transition (T_β) of PS at 0.03 Hz and 1 Hz is observed at −30 and 20°C, respectively, while the γ relaxation (T_γ) is not observed at either frequency. The T_β of PPO is 30°C at 0.03 Hz and is not observed at 1 Hz, while the T_γ is −85°C at 0.03 Hz and −70°C at 1 Hz. On the other hand, blend composition-independent β or γ relaxation observed in the blends may be a consequence of the absence of intra- or intermolecular interaction between the constituents at low temperature. Thus it is suggested that at low temperature, the β relaxation of PS be influenced solely by the local motion of the phenylene ring, and that the β or γ relaxation of PPO be predominated by the local cooperative motions of several monomer units or the rotational motion of the methyl group in PPO. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1981–1986, 1998     

Dynamic Rheological Characterization of A Thermotropic Liquid Crystalline Poly (aryl ether ketone)

Poly(aryl ether ketone)s (PAEKs) have been widely used as advanced materials in applications because of their excellent thermal stabilities and good chemical resistance1. The high melt viscosity is one of the major drawbacks in processing them. The synthesis of thermotropic liquid crystalline PAEKs has shed light on the solving of this problem2 because they have been proved to have melt viscosities significantly lower in nematic state than those of the isotropic PAEKs and can be melt proce…     

Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

Understanding the electron spin relaxation properties of paramagnetic species is a fundamental requirement to use them as a probe to measure distances between sites in biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Even though Ni(II) ion is an essential trace element for many species, relaxation properties are not well understood. Herein, the polycrystalline sample of Ni(II) ion magnetically diluted in Zn(Pyrazole)₆(NO₃)₂ (Ni/ZPN) has been studied in detail by EPR spectroscopy to explore the electron spin relaxation time. Progressive continuous-wave (CW) EPR power saturation study on Ni/ZPN at 300 K yielded 907 mW as the P_1/2 value. The cavity constant (K_Q) has been calculated using tempol in PVA-BA glass matrix and the product of electron spin-lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for Ni/ZPN at 300 K has been reported for the first time.     

Investigating the Calculation of Rotational Viscosity of the Mixture Comprising Different Kinds of Liquid Crystals:Molecular Dynamics Computer Simulation Approach

Molecular dynamics (MD) computer simulation techniques, as a powerful tool commonly utilized by the liquid crystal display (LCD) community, usually are employed for computing the equilibrium and transport properties of a classical many body system, since they are very similar to real experiments in many respects. In this paper we present molecular dynamics computer simulation results taken for a mixture of the two different kinds of nematic liquid crystals (LCs). We calculated rotational viscosity from Brownian behavior with friction of the mean director of the mixture comprising pentylcyanobiphenol (5CB) and decylcyanobiphenol (10CB) by using molecular dynamics computer simulation, where intermolecular potential parameter is Generalized AMBER force field (GAFF). Our computed results show a good agreement with the experimental results.     

Molecular dynamics and conduction mechanism of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) terpolymer containing ionic liquid

Dielectric spectroscopy (DS) measurements were performed to probe the segmental dynamics and ion mobility of poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate) terpolymer dopped with different amounts of tetrabutylammonium tetrafluoroborate ([TBA] [BF₄]) ionic liquid (IL). Differential scanning calorimetry (DSC) was also employed to trace the change in the glass transition temperature (T_g) at different loads of IL. The DSC measurements revealed a remarkable reduction in the PVVH T_g from 344 to 310 K just by adding 20 wt% of IL. The DS measurements revealed three relaxation processes named α, β₁, and β₂. The α-process is related to the segmental motion of PVVH while the β₁ and β₂ are due to the restricted local dynamics of side chains. The segmental relaxation times (α-relaxation) speed up with increasing the concentration of IL due to the plasticization effect of IL on polymer chains. The temperature dependence of α-relaxation follows the Vogel-Fulcher-Tammann (VFT) relation with dynamic glass transition between 323 and 294 K in agreement with the DSC measurements. The β₁ and β₂-relaxations have an Arrhenius temperature dependence. The temperature dependence of ionic conductivity obeys the VFT behavior indicating the coupling between the segmental motion of PVVH chains and ion transport. Polaronic tunneling is the predominant conduction mechanism in PVVH and its composites. The specific capacitance increases with increasing both the temperature and IL concentration.     

Poly(vinyl chloride)‐based miscible blends: Changes induced in the structural characteristics and in dielectric α relaxation of pure poly(vinyl chloride)

In this work we investigate by means of dielectric relaxation spectroscopy how segmental motions occurring in poly(vinyl chloride) (PVC) are modified by blending of PVC with small amounts of two different homopolymers: crystalline poly(ϵ‐caprolactone) (PCL) and glassy syndiotactic poly(methylmethacrylate) (sPMMA). The dynamics of the α relaxation of PVC is severely changed by blending it with PCL or sPMMA becoming faster or slower, respectively. Simultaneously, the shape of the relaxation function is being importantly altered. It shows a stronger non‐Debye character being broader and strongly temperature‐dependent. This fact leads us to calculate distributions of relaxation times for the blends that are wider in comparison to the one obtained for pure PVC. Complementary X‐ray diffraction measurements have been performed in order to assure the absence of crystallinity in the blends, and some small variations can be deduced at the level of interchain structural correlations of PVC. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 234–247, 2000     

Molecular dynamics of poly(ATRIF) homopolymer and poly(AN-co-ATRIF) copolymer investigated by dielectric relaxation spectroscopy

Aiming to develop new dielectric polymers containing CN and F groups with strong dipole moments, a novel copolymer of acrylonitrile (AN) and 2,2,2-trifluoroethyl acrylate (ATRIF) was synthesized in acetonitrile by free radical process as well as the respective homopolymer (poly(ATRIF)). The copolymer’s composition and microstructure were analyzed by FTIR, ¹H and ¹³C NMR spectroscopy and SEC. The molar incorporation of AN determined in the copolymer by NMR was 58 mol%. Thermogravimetric analysis of poly(AN-co-ATRIF) copolymer showed good thermal stability comparatively to the fluorinated homopolymer.Both copolymer, poly(AN-co-ATRIF), and homopolymer, poly(ATRIF), were dielectrically characterized over a frequency range from 10⁻¹ to 10⁶ Hz, and in a temperature range from 223 to 393 K. The dominating relaxation process detected in both materials is the α-relaxation, associated with the dynamic glass transition. A VFTH temperature dependence of the relaxation times (τ) was found for both materials, as characteristic of cooperative processes, from which the respective glass transition temperatures (T_g(τ = 100 s)) were estimated, which differ ∼40 K, the one of the copolymer being higher (307 K) in accordance to the calorimetric analysis. This effect was attributed to a higher stiffness of the backbone in the copolymer originated by the inclusion of the acrylonitrile groups. Both relaxation functions have the same breath of relaxation times allowing constructing a single master curve, indicating similar non-exponential character. A less fragile behavior was found for the copolymer. This was rationalized in a more straightforward way by the free volume approach instead from a correlation between fragility and intermolecular coupling. It was found that in the copolymer the free volume increases at a lower rate with the temperature increase. It was inferred from the VFTH temperature dependence of the dc conductivity and low values of the decoupling index that ion motion is significantly influenced by the dynamics of the α-process.     

Temperature and frequency dependencies of the complex dielectric constant of poly(ethylene oxide) under hydrostatic pressure

Electrical impedance measurements have been made in the frequency range 5 Hz to 10 MHz in pure poly(ethylene oxide) having a molecular weight of 600,000 from 12 K nearly up to the melting point of the crystalline phase (about 330 K). A pronounced relaxation peak in the dielectric loss and a corresponding step in the dielectric constant have been observed at about 240 K, which can be readily related to the glass-rubber transition in the amorphous region of the polymer. As the temperature approaches the melting point there are large increases in the real ϵ′ and imaginary e′ parts of the dielectric constant. The frequency dependence of ϵ′ is characterized by a primary relaxation process, whose frequency increases with increasing temperature as a consequence of decrease of the average structural relaxation time. There is strong evidence that this low-frequency dispersion arises mainly from the diffusive transport of ionic charge carriers rather than a purely orientation relaxation process. In addition, the effects of hydrostatic pressures (0–0.25 GPa) on the frequency dependencies of the real ϵ′ and imaginary ϵ′ parts of the dielectric constant have been measured in the temperature range from 254 to 329 K. An advantage of applying pressure is that it shifts the α_𝒶 relaxation peak into an experimentally accessible frequency window of the equipment; the lowering of frequency results from a decrease in the relaxation volume and a consequent reduction in the mobility of the molecular units. Results are discussed in terms of theoretical models of the effect of pressure on the glass transition, providing information on the cooperative dynamics. © 1996 John Wiley & Sons, Inc.     

Mesomorphic phase formation of poly (macromonomer)s of polystyrene macromonomers

The liquid crystalline phase formation of poly(macromonomer)s associated with the specific multibranched architecture of high branch density was investigated. The poly(macromonomer)s were prepared by radical chain polymerizations of ω‐methacryloyloxyethyl polystyrene macromonomers. It was confirmed that the mesomorphic phase formation depended on the branching architecture, where sufficient length of the branch chains as well as the backbone chain is crucial for the formation of the mesomorphic phase. Formation of the optically anisotropic mesophase also depended on the nature of solvent. The mesophase was observed in the cast films prepared from p‐xylene, toluene, tetrahydrofuran, carbon disulfide and chloroform but not observed for cyclohexane. The effects of the branched structure and the solvent nature were explained by repulsive interaction between the polystyrene branch chains of high branch density. The repulsive interaction increases the chain stiffness of the central backbone and also prevents the interpenetration of the polystyrene branches of different molecules in solution, which allow poly(macromonomer) molecules to arrange with the orientational order. Copyright © 2000 John Wiley & Sons, Ltd.     

多面体碳烷的分子轨道方法研究

利用INDO自洽场方法和Edmiston-Ruedenberg定域化方法,计算了多面体碳烷C_2nH_2n(n=2,3,4,5和10)及其骨架C_2n,讨论了它们的电子结构、稳定性和化学键性质。     

Molecular structure of paramagnetic <Emphasis Type="Italic">bis</Emphasis>-diisobutyl dithiophosphinate complexes of lanthanides(III) with 1,10-phenanthroline in solution by the data of NMR relaxation spectroscopy

NMR relaxation spectroscopy (RS) is used to examine the molecular structure of paramagnetic bis-diisobutyl dithiophosphinate complexes of lanthanides (by the example of europium and lutetium) with 1,10-phenanthroline in CDCl₃ solution. The results obtained are consistent with the results of studying the molecular structure of bis-diisobutyl dithiophosphinate complexes of yttrium with 1,10-phenanthroline in the crystalline phase by XRD.