Coupling of mutual diffusion to viscoelasticity in moderately concentrated polyisobutylene solutions

The normalized intensity autocorrelation function g⁽²⁾(t) were obtained by dynamic light scattering for moderately concentrated entangled solutions of polyisobutylene in n-heptane at 25.0 °C and in isoamyl isovalerate (IAIV) at 25.0 °C (Θ). The obtained data have been successfully analyzed by the ‘procedure X’ familiar for determination of mechanical relaxation spectra on the basis of the recent theory for g⁽²⁾(t). The results have shown that while the mutual diffusion coefficient D increases in the n-heptane solutions and decreases in the IAIV solutions with increasing polymer mass concentration c, the friction coefficient ζ for both solutions increases with c showing the same power-law behavior irrespective of the weight-average molecular weight M_w and solvent quality. It has been found that the instantaneous longitudinal modulus L₀ for n-heptane solutions increases in proportion to c², obeying the familiar relation for the plateau value (4/3)G_N of the longitudinal stress relaxation modulus, but L₀ for the IAIV solutions becomes progressively smaller than the values predicted from the relation with decreasing c. The terminal relaxation time τ_m has been found to follow the power-law τ_m∝M_w^3.4 established by rheological measurements.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.     

Direct electrochemistry and electrocatalysis of cytochrome c based on chitosan-room temperature ionic liquid-carbon nanotubes composite

A robust and effective composite film combined the benefits of room temperature ionic liquid (RTIL), chitosan (Chi) and multi-wall carbon nanotubes (MWNTs) was prepared. Cytochrome c (Cyt c) was successfully immobilized on glassy carbon electrode (GCE) surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Cyt c were investigated in detail. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.1 mol L⁻¹ pH 7.0 phosphate buffer solution (PBS), indicating the Chi-RTIL-MWNTs film showed an obvious promotion for the direct electron transfer between Cyt c and the underlying electrode. The immobilized Cyt c exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0 × 10⁻⁶ to 2.6 × 10⁻⁴ mol L⁻¹, with a detection limit of 8.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The apparent Michaelis-Menten constant (K_m) was calculated to be 0.45 ± 0.02 mmol L⁻¹. Moreover, the modified electrode displayed a rapid response (5 s) to H₂O₂, and possessed good stability and reproducibility. Based on the composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.     

Transparent polycrystalline MgAl2O4 ceramic fabricated by spark plasma sintering: Microwave dielectric and optical properties

The optical properties and microwave dielectric properties of transparent polycrystalline MgAl₂O₄ ceramics sintered by spark plasma sintering (SPS) through homemade nanosized MgAl₂O₄ powders at temperatures between 1250 °C and 1375 °C are discussed. The results indicate that, with increasing sintering temperatures, grain growth and densification occurred up to 1275 °C, and above 1350 °C, rapid grain and pore growth occurred. The in-line light transmission increases with the densification and decreases with the grain/pore growth, which can be as high as 70% at the wavelength of 550 nm and 82% at the wavelength of 2000 nm, respectively. As the sintering temperature increases, Q×f and dielectric constant ε_r values increase to maximum and then decrease respectively, while τ_f value is almost independent of the sintering temperatures and remains between −77 and −71 ppm/°C. The optimal microwave dielectric properties (ε_r=8.38, Q×f=54,000 GHz and τ_f=−74 ppm/°C) are achieved for transparent MgAl₂O₄ ceramics produced by spark plasma sintering at 1325 °C for 20 min.     

Effects of heating rate on microstructures and microwave dielectric properties of (1 − x)ZnAl2O4–xTiO2 (x = 0.21) ceramics

(1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) ceramics were synthesized at 1500 °C for 3 h using the solid-state reaction at a heating rate from 1 to 7 °C/min. The effects of heating rate on the microstructure, phase composition and oxidation state of titanium in the ceramics were investigated. The XRD results show that this system is composed of two phases, i.e. ZnAl₂O₄ spinel and rutile. The “black core” phenomenon resulting from reduction of Ti⁴⁺ ion valence appears after the ceramics are sintered at the speed of 1 and 3 °C/min. As the heating rate increases, the density and quality factor (Q·f) increase initially and reach the maximum value when the heating rate is 5 °C/min, and then reduce quickly to the minimum, while the dielectric constant (?_r) and temperature coefficient of resonator frequency (τ_f) nearly do not change. The optimal microwave dielectric properties can be achieved in (1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) ceramics sintered at a heating rate of 5 °C/min with an ?_r value of 11.6, a Q·f value of 74,000 GHz (at about 6.5 GHz), and a τ_f value of −0.4 ppm/°C.     

Colloidal silver nanoparticles modified electrode and its application to the electroanalysis of Cytochrome c

A colloidal silver nanoparticles (CSNs) chemically modified electrode was prepared and its application to the electroanalysis of Cytochrome c (Cyt. c) was studied. The CSNs were prepared by reduction of AgNO₃ with NaBH₄, and were stabilized by oleate. They could be efficiently immobilized on the surface of a silver electrode. The result showed that the CSNs could clearly enhance the electron transfer process between Cyt. c and the electrode compared with bulk silver electrode. Linear sweep voltammetric measurement of Cyt. c at the chemical modified electrode indicated that the oxidative peak current of Cyt. c was linear to its concentration ranging from 8.0 nmol L⁻¹ to 3.0 μmol L⁻¹ with the calculated detection limit was about 2.6 nmol L⁻¹. The direct electrochemistry of Cyt. c was also studied by cyclic voltammetry.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

Adjustable dielectric properties of Li2CuxZn1−xTi3O8 (x=0 to 1) ceramics with low sintering temperature

Single-phase dielectric ceramics Li₂Cu_xZn_1−xTi₃O₈ (x=0–1) were synthesized by the conventional solid-state ceramic route. All the solid solutions adopted Li₂MTi₃O₈ cubic spinel structure in which Li/M and Ti show 1:3 order in octahedral sites whereas Li and M are distributed randomly in tetrahedral sites with the degree of Li/M cation mixing varying from 0.5 to 0.3. The substitution of Cu for Zn effectively lowered the sintering temperatures of the ceramics from 1050 to 850 °C and significantly affected the dielectric properties. As x increased from 0 to 0.5, τ_f gradually increased while the dielectric constant (ε_r) and quality factor value (Q×f) gradually decreased, and a near-zero τ_f of 1.6 ppm/°C with ε_r of 25.2, Q×f of 32,100 GHz could be achieved for Li₂Cu_0.1Zn_0.9Ti₃O₈ ceramic sintered at 950 °C, which make it become an attractive promising candidate for LTCC application. As x increases from 0.5 to 1, the dielectric loss significantly increases with AC conductivity increasing up to 2.3×10⁻⁴ S/cm (at 1 MHz).     

Effect of chemisorbed carbon monoxide on Pt/C electrode on the mechanism of the hydrogen oxidation reaction

The influence of poisoning of Pt catalyst by CO on the kinetics and mechanism of H₂ oxidation reaction (HOR) at Pt/C electrode in 0.5 mol dm⁻³ HClO₄, saturated with H₂ containing 100 ppm CO, was examined with rotating disc electrode (RDE) at 22 °C. Commercial carbon black, Vulcan XC-72 was used as support, while Pt/C catalyst was prepared by modified polyol synthesis method in an ethylene glycol (EG) solution. The kinetically controlled current (I_k) for the HOR at Pt/C decreases significantly at CO coverage (Θ_CO) > 0.6. For Θ_CO < 0.6 the HOR takes place through Tafel-Volmer mechanism with Tafel reaction as rate-determining step at the low CO coverage, while Volmer step controls the overall reaction rate at the medium CO coverage. When CO coverage is higher then 0.6, Heyrovsky-Volmer mechanism is operative for the HOR with Heyrovsky as the rate-determining step (rds).     

Crystallization behavior of poly(l-lactic acid)

The crystallization behavior of poly(l-lactic acid) was studied in the range of 80-160 °C. The peak crystallization time (τ_p) was defined and obtained from the crystallization isotherm measured with a differential scanning calorimeter (DSC). Isothermal crystallization temperature (T_c) dependence of log(τ_p) discretely changed at 113 °C (= T_b). The linear growth rate of spherulite, G, was measured with a polarizing microscope. The T_c dependence of G and the size of the spherulite also discretely changed at T_b. Crystal structures for samples isothermally crystallized at temperatures which were higher and lower than T_b were orthorhombic (α-form) and trigonal (β-form), respectively. The discrete change of the crystallization behavior was explained by the formation of different crystal.     

Multi-month thermal aging of electro-optic polymer waveguides: Synthesis, fabrication, and relaxation modeling

The preparation of polyimides containing side-chain chromophores and the long-term aging performance of poled films are described. These materials were compared to guest-host polycarbonate films. Mach-Zehnder optical interferometers were fabricated from these polymers that contained CLD- and FTC-type chromophores. Changes in optical properties were monitored for months at four temperatures ranging from ambient to 110 °C. The isothermal relaxation data were modeled using both a stretched exponential equation and a power law in time equation. The temperature dependency of the time constants of these equations was modeled using a new activation-energy equation: ln(τ/τ_p) = E_R(1 + tanh[(T_c − T)/D])/2RT + E_p/RT where T_c is the central temperature of the transition zone, D is the breadth of the zone, and Es are the activation energies of rigid and pliable materials. Multi-year high-temperature stability of the poled guest-host and side-chain materials was predicted.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures

The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (T_g^PLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (T_c), the glass transition temperature of PS (T_g^PS), the peak melting point of PLLA crystals (T_m^PLLA), and the end melting point of PLLA crystals (T_m,end^PLLA). When annealed at (T_c=) 80 °C (T_c < T_g^PS < T_ODT, order-disorder transition temperature), 123 °C (T_g^PS < T_c < T_m^PLLA < T_ODT), 165 °C (T_g^PS < T_m^PLLA < T_c < T_m,end^PLLA < T_ODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization.     

Lyotropic liquid crystal formation of polystyrene polymacromonomers in dichloromethane

Liquid crystallinity of dichloromethane (DCM) solutions of five samples of polymacromonomer F65 consisting of 65 styrene residues in each side chain was studied by birefringence observation and phase separation experiments at different temperatures in a molecular weight range from 9.4 × 10⁵ to 4.1 × 10⁶. Dilute-solution characterization was also made by light scattering and viscometry in DCM at 20 °C. The polymer concentration c_I on the phase boundary between the isotropic and biphasic regions was lower than the previously determined c_I for a polymacromonomer with a shorter side-chain length of 33 styrene residues at the same molecular weight, reflecting the higher chain stiffness and larger diameter of the F65 polymer. The molecular weight dependence of c_I for the two polymacromonomers (at 20 °C) was explained almost quantitatively by the scaled-particle theory for worm-like cylinders with the model parameters (the Kuhn length, the linear mass density, and the chain diameter) describing gyration radius and intrinsic viscosity data in DCM but without chain-end effect. It was concluded that this theory is capable of predicting the phase boundary concentration of brush-like polymers with essentially the same degree of accuracy as that known for linear, stiff chains.     

Electrospinning fabrication of partially crystalline bisphenol A polycarbonate nanofibers: The effects of molecular motion and conformation in solutions

Partially crystalline bisphenol A polycarbonate (BPAPC) nanofibers were successfully fabricated using a combination of a centrifugal field (1800 rpm) and an electrostatic field (25 kV). The BPAPC solution properties are key factors for adequately electrospinning the partially crystalline BPAPC nanofibers. The correlation times (τ_c) of methyl (τ_c = 9.3 ns) and of benzene-ring (τ_c = 15.3 and 15.8 ns) motions in the 14 wt.% BPAPC/THF solution were longer than in CH₂Cl₂ and CHCl₃, as determined by NMR. The distribution-peak maximum of the hydrodynamic radius of BPAPC in the 14 wt.% THF solution (R_h = 15 Å) was higher than in CH₂Cl₂ (R_h = 9.2 Å) and CHCl₃ (R_h = 7.9 Å), as evidenced by DLS data. We conclude that the BPAPC assumed a denser, more worm-like chain conformation in THF solvation.     

Impedance analysis of Sr-substituted CePO4 with mixed protonic and p-type electronic conduction

Members of the solid-solution series Ce_1−xSr_xPO_4−δ (x = 0, 0.01, 0.02) with mixed protonic and electronic transport have been synthesized by a nitrate-decomposition method followed by sintering at 1450 °C. Impedance spectroscopy is employed to estimate the bulk electrical conductivity in wet (∼0.03 atm) and dry atmospheres of O₂ and 10%H₂:90%N₂. Conductivity increases with dopant concentration (x), oxygen partial pressure (pO₂) and water vapour partial pressure (pH₂O) reaching ∼3.5 × 10⁻³ S cm⁻¹ at 600 °C for x = 0.02 in wet O₂. Activation energies (E_a) for the bulk conductivity of Ce_0.98Sr_0.02PO_4−δ below 650 °C are 0.44 and 0.78 eV for wet oxidising and wet reducing conditions, respectively. A moderate but positive pO₂⁺ⁿ power-law dependence (n < 1/10) of conductivity is exhibited in the pO₂ range 10^−2.5 to 10⁻¹ atm, consistent with mixed ionic and p-type electronic transport. Thermogravimetric analysis indicates that the Sr-doped materials are stable in a CO₂ atmosphere in the temperature range 25–1200 °C.     

Forced translocation of polymer chains through a nanotube: A case of ultrafiltration

Translocation of polymer chains under the application of an external force has been studied through coarse-grained Monte Carlo simulations. The chains are pulled through a nanotube of finite length and diameter and their translocation times measured. The average translocation time, τ follows a scaling relation involving the chain length, N and applied force, F as, τ ∼ N^ν′F^−μ, where ν′ and μ are two different exponents (ν′ = 0.674, and μ = 0.95 ± 0.05). The scaling law is closely similar to the nanopore translocation scaling law reported by Milchev et al. [Ann N Y Acad Sci 2009;1161:95]. Characteristic signatures of the chain escape time have been exhibited by the square of end-to-end distance R², axial radius of gyration R_g−x and other constituent properties. The behavior of the linear polymers under the application of a pulling force has been exploited to gain insights into the ultrafiltration process of unentangled polymers in dilute solution. The generic pulling force-translocation time (F, τ) data obtained through simulation can be matched reasonably well with the hydrodynamic force-critical macroscopic flow time (f_h, Q_c⁻¹) data and also with the hydrodynamic force-reduced critical microscopic flow time (f_h, q_c⁻¹) data obtained in the ultrafiltration experiment on long linear polystyrene chains in cyclohexane, as recently reported by Ge et al. [Macromolecules 2009;42:4400] The simulation technique reported here may be extended to study biomolecular transports occurring in long protein channels, as studied experimentally through current-time or voltage-time traces.     

Effect of pyridine in electrolyte on the current-voltage characteristics in dye-sensitized solar cells

This study aimed to investigate the effect of pyridine in electrolyte on the photocurrent density-photovoltage (J-V) characteristics in dye-sensitized solar cells. When the concentration of pyridine (c_p) in an electrolyte (0.1 M LiI and 0.05 M I₂ in acetonitrile) is increased from 0 to 0.4 M, the open-circuit voltage (V_oc) increased by 0.14 V, whereas the short-circuit photocurrent density (J_sc) decreased by 3.0 mA cm⁻². The optimum c_p for the maximum cell performance was around 0.1-0.2 M. The model calculation predicted that by increasing c_p from 0 to 0.4 M, the electron diffusion coefficient (D) decreases by more than one order of magnitude and the lifetime of conduction band-free electrons (τ) increases by more than one order of magnitude, implying that the electron diffusion length () increases by 31%, whereas the electron injection flux from dye molecules to TiO₂ nanoporous films (Φ) decreases by 28%.     

Gelation behaviors of cellulose solution dissolved in aqueous NaOH/thiourea at low temperature

Cellulose was dissolved rapidly in 9.5 wt% NaOH/4.5 wt% thiourea aqueous solution pre-cooled to −5 °C, as a result of the formation of an inclusion complex (IC) associated with cellulose, NaOH and thiourea, which could bring cellulose to the aqueous system. To clarify the rheological behaviors of the system dissolved at low temperature, this cellulose solution was investigated by dynamic viscoelastic measurement. The shear storage modulus (G′) and loss modulus (G″) as a function of the angular frequency (ω), concentration (c), temperature (T) and weight-average molecular weight (M_w) were analyzed and discussed. The results revealed that gels could form in the cellulose solution at either high temperature or low temperature, or for longer time. Interestingly, 4 wt% cellulose solution having cellulose M_w of 12.0 × 10⁴ remained at liquid state for longer time (12 days) at the temperature ranging from 0 to 5 °C. The gels already formed at elevated temperature were irreversible, i.e., after cooling to lower temperature including the temperature of cellulose dissolution (−5 °C), they could not be dissolved to become liquid. The Arrhenius analysis of the temperature dependence of viscosity in the cellulose solution indicated that a high apparent activation energy (E_a) occurred at 0 to −5 °C, suggesting the relatively stable IC structure. However, the viscosity of the cellulose solution increased slowly with an increase in the temperature at 0-40 °C, leading to the negative E_a values. The results suggested that the cellulose solution in NaOH/thiourea system is complex to differ from normal polymer systems.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.