Gap discrete breathers in a two-component two-dimensional crystal in thermal equilibrium

Crystals having a gap in the phonon spectrum can maintain gap discrete breathers (DBs), i.e., nonlinear localized oscillatory modes existing in the absence of defects and having a frequency lying in the gap. The lifetime of gap DBs in a two-dimensional perfect crystal of the composition A ₃ B in thermal equilibrium has been studied by the molecular dynamics method. As was shown earlier, the existence of gap DBs in such a crystal is provided by the presence of a wide gap in the phonon spectrum if the component mass ratio m _A /m _B is sufficiently large. For comparison, a crystal with a relatively small ratio m _A /m _B is considered when the gap in the spectrum is absent and the existence of gap DBs is impossible in the case of a weak nonlinearity realized in the considered case. It has been shown that, in contrast to the opposite case, in a crystal maintaining gap DBs, long-lived localized oscillatory modes of large amplitude can emerge, whose concentration and lifetime increase with temperature.     

Preparation and Characterization of 2,2-bis[4-(3-Aminophenoxy)phenyl]propane/4,4′-Oxydianiline/4,4′-Oxydiphthalic Anhydride (3-BAPOPP/44ODA/ODPA) Polyimide Films with Various Co-monomer Ratios

A new diamine monomer 2,2-bis[4-(3-aminophenoxy)phenyl]propane (3-BAPOPP) was synthesized through two steps and its melting point was determined by differential scanning calorimetry (DSC). It was then copolymerized with 4,4′-oxydianiline (44ODA) and 4,4′-oxydiphthalic anhydride (ODPA) to obtain a series of aromatic polyimide(3-BAPOPP/44ODA/ODPA-PI) films. Fourier transform infrared (FT-IR) was used to characterize the structures of the monomer and the polyimide films. Water absorption, ultraviolet-visible (UV-Vis) spectroscopy, contact angle, DSC, and thermo-gravimetric analysis (TGA) were used to characterize the properties of 3-BAPOPP/44ODA/ODPA-P films. The results showed that the PI films had low water absorption in the range of 1.5%～1.9%, low surface energy in the range of 43.3 ～43.5 mJ/m², glass transition temperature (T_g) in the range of 192.5°C～226.1°C, and dielectric constant in the range of 2.79～3.02 at 1 MHz. The films also exhibited good thermal properties and good optical properties, with the ultra violet cut-off wavelength being in the range of 346～364 nm.     

Towards the higher solubility and thermal stability of poly(aniline-<Emphasis Type="Italic">co</Emphasis>-<Emphasis Type="Italic">m</Emphasis>-bromoaniline)

In the current study, we have described the synthesis and the physical properties of poly(aniline-co-m-bromoaniline) conducting copolymers. The copolymers of different composition are essentially obtained by varying the molar feed ratio of the two monomers. The higher solubility of the copolymers could be procured as compared to polyaniline (PA) in different solvents. The electrical conductivity has been studied by two-probe method; at room temperature, the conductivity of the copolymer decreases upon increasing the molar ratio of m-bromoaniline monomer. The introduction of bromine (–Br) group reduces the degree of conjugation in the polymer chain. Thus, conduction of electrons is prohibited along the conjugated system. In the thermogravimetric analysis (TGA), a three-stage decomposition of the copolymer has been observed. The copolymers of poly(aniline-co-m-bromoaniline) are thermally stable at high temperature. The composition of the copolymer has been confirmed from the binding energies of C–C, C–N, and C–Br in the XPS study.     

Thermal behavior of gamma-irradiated low-density polyethylene/paraffin wax blend

The thermal properties of low-density polyethylene (LDPE)/paraffin wax blends were studied using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and melt flow index (MFI). Blends of LDPE/wax in ratios of 100/0, 98/2, 96/4, 94/6, 92/8, 90/10 and 85/15 (w/w) were prepared by melt-mixing at the temperature of 150°C. It was found that increasing the wax content more than 15% leads to phase separation. DSC results showed that for all blends both the melting temperature (T_m) and the melting enthalpy (ΔH_m) decrease linearly with an increase in wax content. TGA analysis showed that the thermal stability of all blends decreases linearly with increasing wax content. No clear correlation was observed between the melting point and thermal stability. Horowitz and Metzger method was used to determine the thermal activation energy (E_a). MFI increased exponentially by increasing the wax content. The effect of gamma irradiation on the thermal behavior of the blends was also investigated at different gamma irradiation doses. Significant correlations were found between the thermal parameters (T_m, ΔH_m, T₅%, E_a and MFI) and the amount of wax content and gamma irradiation.     

Preparation and assessment of piezo- and pyroelectric poly (vinylidene fluoride-hexafluoropropylene) copolymer films

Films of poly(vinylidene fluoride-hexafluoropropylene) copolymer [P(VDF-HFP)] were cast from a dimethylsulfoxide (DMSO)/acetone solution of Solef?85-15 P(VDF-HFP) copolymer powder grade 21508. Undrawn and uniaxially drawn cast copolymer films were investigated with respect to their piezo- and pyroelectric properties. Quasistatic charge integration was employed for the determination of the poling-field dependence of the piezoelectric d₃₁ and pyroelectric p₃ coefficients. In addition, the thermal stability of the pyroelectric effect was studied with a combination of thermally stimulated discharge (TSD) and temperature-modulation techniques. Cast copolymer films could withstand electrical poling fields of up to 400 MV/m. The maximum values of d₃₁=30 pC/N and p₃=49 μC/(m²K) for uniaxially drawn samples are similar to those found on commercial PVDF films and much higher than those on pure PVDF films cast from solution. Samples kept for 5 min at 150 °C still exhibit 30–40% of the initial pyroelectric effect [up to around 20 μC/(m²K)]. After this annealing step, no further decay of the pyroelectric coefficient could be observed during storage at 120 °C for several hours. Received: 5 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001     

Cure Behavior and Thermal Properties of Poly (Amide-Amidic Acid) Modified Diglycidyl Ether of Bisphenol-A/4,4′-Diaminodiphenylsulfone

Poly (amide-amidic acid) (PAA) was selected to modify diglycidyl ether of bisphenol-A (DGEBA)/4,4′-diaminodiphenylsulfone (DDS). The cure behavior was studied by means of nonisothermal differential scanning calorimeter (DSC) analysis, indicating that PAA played a role of catalyst during the process of the curing reaction. Results of Fourier transform infrared spectroscopy (FT-IR) analysis showed that the PAA acted as a co-curing agent when the PAA content was 3.2–38.4 phr and also as a modifier when the PAA content was 12.8–38.4 phr. The glass transition temperature (T_g ) decreased with the increase of PAA content. The thermal stability improved when the PAA content was 3.2–6.4 phr because of the catalytic effect of PAA. The flexural strength improved for the varying PAA content studied in this work, with the highest flexural strength being obtained when the PAA content was 6.4 phr. The fracture surface morphology was observed using scanning electron microscopy (SEM); the morphologies varied with changing content of PAA.     

Effect of Zn–Ti double substitution on the electrical properties of Bi4V2O11

New samples of the Bi₂Zn_0.1–xTi_xV_0.9O_5.35+x; 0.02 ≤ x ≤ 0.08 system have been synthesized through a standard solid-state reaction route. XRD analysis and differential thermal analysis have been used to characterize the phase structure of samples. The γ′ phase is stabilized to room temperature in all investigated samples. The electrical properties of the BIZNTIVOX system have been studied by using AC impedance spectroscopy. An AC impedance response as a function of frequency (20 Hz–1 MHz) has been used to investigate the electrical conductivity and the dielectric permittivity in the temperature range of 150 °C–700 °C. In this temperature range, the phase transition γ′ to γ has been observed in all the compositions studied. AC impedance spectroscopy indicates that the resistance of samples decreases with increase of temperature. The ionic conductivity of samples appeared as a two-line region in Arrhenius dependence. At 300 °C, the highest ionic conductivity is shown by the composition x = 0.05 (σ₃₀₀ = 1.35 × 10^?4 S cm^?1).     

Gap discrete breathers in two-component three-dimensional and two-dimensional crystals with Morse interatomic potentials

The properties of gap discrete breathers in three-dimensional and two-dimensional crystals of the composition A ₃ B with interatomic bonds described by the Morse potential have been investigated by the molecular dynamics method for different ratios between the masses of components m _A /m _B . The transition to a thermal equilibrium from a state far from equilibrium has been studied for the two-dimensional crystal. In this case, a short-wavelength phonon vibrational mode is excited in the crystal. This mode appears to be modulationally unstable for not too small amplitudes. During the transition to the state characterized by a uniform energy distribution between all vibrational modes of the crystal, the energy is localized in the form of gap discrete breathers, which exist in time intervals that exceed their oscillation period by several orders of magnitude.     

Thermal and Tribological Properties of Fullerene‐Containing Composite Systems. Part 3. Features of the Mechanism of Thermal Degradation of Poly‐(N‐Vinyl‐Pyrrolidone) and Its Compositions with Fullerene C60

By mass‐spectrometric thermal analysis (MTA) the thermochemical features of poly(N‐vinyl pyrrolidone) (PVP) and its compositions with fullerene C₆₀ were studied. The mechanism of PVP thermal degradation was investigated; in particular the nature of the low‐temperature degradation (between 75 and 300°C) accompanied by output of pyrrolidone was explained as well as the influence of fullerene C₆₀ on this mechanism. It was shown that during thermal degradation of copolymer PVP‐C₆₀, there is a disappearance of the low‐temperature peaks of the output of pyrrolidone that is interpreted as an increase of the thermal stability of N‐vinyl‐pyrrolidone fragments in this product in comparison with their thermal stability in pure PVP.     

Rheological Characterization of Poly(3,3′-Diaminodiphenylsulfone Terephthaloylchloride) Solutions in Dimethylsulfoxide

Steadyshear and oscillatory shear rheological measurements were performed to characterize the solution rheological behavior of poly(3,3′-diaminodiphenylsulfone terephthaloylchloride) (P(3,3′-DDS-TPC)) in dimethyl sulfoxide (DMSO). The effects of temperature, concentration, and weight-average molar mass () on the rheological properties were investigated. From the temperature dependence of zero-shear viscosity, the flow activation energies, E_η, of P(3,3′-DDS-TPC)/DMSO solutions were calculated. Both the overlap concentration, C*, and the entanglement concentration, C_e, were determined from the concentration dependence of the specific viscosity η_sp. All the P(3,3′-DDS-TPC) solutions, we studied, can be separated into three regimes: the dilute, semidilute-unentangled, and entangled regime with slopes of 1, 1.3, and 3.9, of concentration versus η_sp plots, respectively, which are consistent with scaling predictions for flexible polymers in a good solvent.     

Influence of sintering conditions on structural,thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Na_0.5Bi_0.5TiO₃ ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ? and dielectric loss tanδ, a shift of T_m and T_d and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.     

Correlation between phase structure and electrical conduction in BISNVOX system for intermediate temperature solid oxide fuel cells (IT-SOFC)

Samples of Sn⁴⁺-substituted bismuth vanadate, formulated as Bi₄Sn _x V_{2? x} O_{11?( x /2)? δ} in the composition range 0.07 ≤ x ≤ 0.30, were prepared by standard solid-state reactions. Sample characterization and the principal phase transitions (α ? β, β ? γ and γ′ ? γ) were investigated by FT-IR spectroscopy, X-ray powder diffraction, differential thermal analysis (DTA) and AC impedance spectroscopy. For composition x = 0.07, the α ? β and β ? γ phase transitions were observed at temperatures of 451 and 536°C, respectively. DTA thermograms and Arrhenius plots of conductivities revealed the γ′ ? γ phase transition at 411 and 423°C for x = 0.20 and 0.30, respectively. AC impedance plots showed that conductivity is mainly due to the grain contribution, which is evident in the enhanced short-range diffusion of oxide ion vacancy in the grains with increasing temperature. The highest ionic conductivity (5.03 × 10^?5 S cm^?1 at 300°C) was observed for the x = 0.17 solid solution with less pronounced thermal hysteresis.     

Preparation and Characterization of Poly (Acrylonitrile-co-1-Acryloylpyrrolidine- 2-Carboxylic Acid) with High Molecular Weight

1-acryloylpyrrolidine-2-carboxylic acid (APCA) monomer was copolymerized with acrylonitrile (AN) by aqueous suspension polymerization. High molecular weight (HMW) copolymers of AN and APCA [poly(AN-co-APCA)] with different copolymer composition were successfully prepared by employing azobisisobutyronitrile (AIBN) as initiator and polyvinyl alcohol (PVA) as dispersant in a H₂O/N,N-dimethylformamide (DMF) mixture at 60°C. The PAN homopolymer and copolymers were characterized by elemental analysis (EA), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The EA results indicated that the content of oxygen increased significantly in PAN copolymers with increasing APCA content. The APCA copolymer composition calculated from the EA was higher than that from ¹H NMR spectra. The FTIR spectra of PAN and poly(AN-co-APCA) with different monomer ratios confirmed that the contents of APCA units in the copolymer chains increased with increasing APCA content in the feed. The DSC exotherms revealed that copolymerization with APCA could slow the rate of the exothermic reactions during the heat-treatment processes. The XRD results indicated that the PAN homopolymer and copolymers poorly crystallized and the crystallinity decreased with increasing APCA contents.     

Microphase separation at two length scales

The possibility of microphase separation at two different length scales in monodisperse AB block copolymer melts consisting of a homopolymer A block and either a linear alternating AB copolymer block (poly(A)_m-block-poly(B-alt -A)_n) or an AB comb copolymer block poly(A)_m-block-poly(A-graft-B)_n, is investigated. An analysis of the structure factor reveals that in the parameter space of n and m three different cases can be distinguished: I) The structure factor has only one minimum corresponding to the short length scale (i.e. the characteristic length of the repeating unit of the alternating or comb block). II) The structure factor has only one minimum corresponding to the long length scale (the characteristic length of the blocks). III) Two minima are present leading to a competition between microphase separation at the short and the long length scale. Depending on the choice of n and m, one of these three possibilities will occur. Received 25 August 2000     

Critical temperature of infinitely long chains from Wertheim's perturbation theory

Wertheim's theory is used to determine the critical properties of chains formed by m tangent spheres interacting through the pair potential u(r). It is shown that within Wertheim's theory the critical temperature and compressibility factor reach a finite non-zero value for infinitely long chains, whereas the critical density and pressure vanish as m ^-1.5. Analysing the zero density limit of Wertheim's equation or state for chains it is found that the critical temperature of the infinitely long chain can be obtained by solving a simple equation which involves the second virial coefficient of the reference monomer fluid and the second virial coefficient between a monomer and a dimer. According to Wertheim's theory, the critical temperature of an infinitely long chain (i.e. the Θ temperature) corresponds to the temperature where the second virial coefficient of the monomer is equal to 2/3 of the second virial coefficient between a monomer and dimer. This is a simple and useful result. By computing the second virial coefficient of the monomer and that between a monomer and a dimer, we have determined the Θ temperature that follows from Wertheim's theory for several kinds of chains. In particular, we have evaluated Θ for chains made up of monomer units interacting through the Lennard-Jones potential, the square well potential and the Yukawa potential. For the square well potential, the Θ temperature that follows from Wertheim's theory is given by a simple analytical expression. It is found that the ratio of Θ to the Boyle and critical temperatures of the monomer decreases with the range of the potential.     

Law of corresponding states and thermal-diffusion factor of Ne---Kr and Ar---Kr mixtures

New data of the thermal diffusion factor, α_T, for five Ne---Kr and four Ar---Kr mixtures in the low temperature range are given. It is shown that the dependence of α_T on the composition of these mixtures together with the thermodynamic and first-order transport properties of the noble gases can be interpreted consistently using the law of corresponding states proposed by Kestin, Ro and Wakeham. Moreover it is suggested that the thermal diffusion factor of these mixtures could also be interpreted by this law.     

Hole-transporting property of a chemically hybridized poly(vinylcarbazole)-fullerene

We synthesized a star copolymer of poly(vinylcarbazole) (PVK) branched from a fullerene (C₆₀) center, and investigated its optical absorption and photoluminescence properties. The chemically hybridized PVK-C₆₀ was then employed as a hole-transporting layer of the electroluminescent device with a poly(9,9^′-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene) (CPDHFPV) emitting layer. The ITO/PVK-C₆₀/CPDHFPV/LiF/Al device showed a strong electroluminescence quenching due to a direct contact of the PVK-C₆₀ and the CPDHFPV layers. In contrast, when an additional PVK layer was introduced between the two layers, the electroluminescence was largely enhanced. The emitted light power of the ITO/PVK-C₆₀/PVK/CPDHFPV/LiF/Al device was improved by 3 times compared with the device without the PVK-C₆₀ layer.     

On the electronic structure and conduction properties of polymeric quasi-one-dimensional model superlattices: Study of Type I and Type II-staggered superlattices

The electronic spectrum of the various quasi-one-dimensional model compositional superlattices (copolymers) (A_mB_n)_x belonging to class of Type I and Type II-staggered superlattices have been calculated in the tight binding approximation using the direct numerical approach. The trends in their electronic structure and conduction properties as a function of (i) composition (m/n), (ii) block sizes m and n and (iii) arrangement of the blocks in the copolymer chain are discussed. The results obtained are important guidelines for designing copolymers with tailor made conduction properties.     

Preparation and characterization of anhydrous polymer electrolyte membranes based on poly(vinyl alcohol-<Emphasis Type="Italic">co</Emphasis>-ethylene) copolymer

Anhydrous polymer electrolyte membranes with cross-linked structure have been prepared based on poly(vinyl alcohol-co-ethylene) (PVA-co-PE) copolymer. The PVA units of copolymer served to induce thermal cross-linking with 4,5-imidazole dicarboxylic acid (IDA) via esterification while PE units controlled the membrane swelling and the mechanical properties of films. Upon doping with phosphoric acid (PA, H₃PO₄) to form imidazole-PA complexes, the proton conductivity of membranes continuously increased with increasing PA content. As a result, proton conductivity reached 0.01 S/cm at 100 °C under anhydrous conditions. X-ray diffraction analysis revealed that both the d-spacing and crystalline peak of membranes were reduced upon introduction of IDA/PA due to the cross-linking effect. The PVA-co-PE/IDA/PA membranes exhibited good mechanical properties, e.g., 150 MPa of Young’s modulus, as determined by a universal testing machine. Thermal gravimetric analysis also represented that the thermal stability of membranes was increased up to 200 °C upon introduction of IDA/PA.