Equilibrium and kinetic surface segregation in binary alloy thin films

A general theoretical analysis of the effect of film thickness on equilibrium and kinetic surface segregation in binary alloy thin films is presented. In this analysis, a constrained condition that represents the finite size of thin film system has been introduced to the modified Darken model, which has been used to describe both equilibrium and kinetic surface segregation in bulk materials. Simulation of surface segregation for alloy thin films can be carried out for all composition ranges and all film thicknesses if only knowing the surface segregation parameters for bulk materials. Simulations of equilibrium and kinetic surface segregation in Cu(1 1 1)Ag binary alloy thin film are presented.     

Ethylene–propylene copolymerization with bis(β‐enaminoketonato) titanium complexes activated with modified methylaluminoxane

Ethylene–propylene copolymerization, using [(Ph)NC(R₂)CHC(R₁)O]₂TiCl₂ (R₁ = CF₃, Ph, or t‐Bu; R₂ = CH₃ or CF₃) titanium complexes activated with modified methylaluminoxane as a cocatalyst, was investigated. High‐molecular‐weight ethylene–propylene copolymers with relatively narrow molecular weight distributions and a broad range of chemical compositions were obtained. Substituents R₁ and R₂ influenced the copolymerization behavior, including the copolymerization activity, methylene sequence distribution, molecular weight, and polydispersity. With small steric hindrance at R₁ and R₂, one complex (R₁ = CF₃; R₂ = CH₃) displayed high catalytic activity and produced copolymers with high propylene incorporation but low molecular weight. The microstructures of the copolymers were analyzed with ¹³C NMR to determine the methylene sequence distribution and number‐average sequence lengths of uninterrupted methylene carbons. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5846–5854, 2006     

Synthesis and characterization of poly(arylene ether)s containing 6‐(4‐hydroxyphenyl)pyridazin‐3(2H)‐one or 6‐(4‐hydroxyphenyl)pyridazine moieties

A series of novel soluble pyridazinone‐ or pyridazine‐containing poly(arylene ether)s were prepared by a polycondensation reaction. The pyridazinone monomer, 6‐(4‐hydroxyphenyl)pyridazin‐3(2H)‐one ( 1 ), was synthesized from the corresponding acetophenone and glyoxylic acid in a simple one‐pot reaction. The pyridazinone monomer was successfully copolymerized with bisphenol A (BPA) or 1,2‐dihydro‐4‐(4‐hydroxyphenyl)phthalazin‐1(2H)‐one (DHPZ) and bis(4‐fluorophenyl)sulfone to form high‐molecular‐weight polymers. The copolymers had inherent viscosities of 0.5–0.9 dL/g. The glass‐transition temperatures (T_g's) of the copolymers synthesized with BPA increased with increasing content of the pyridazinone monomer. The T_g's of the copolymers synthesized from DHPZ with different pyridazinone contents were similar to those of the two homopolymers. The homopolymers showed T_g's from 202 to 291 °C by differential scanning calorimetry. The 5% weight loss temperatures in nitrogen measured by thermogravimetric analysis were in the range of 411–500 °C. 4‐(6‐Chloropyridazin‐3‐yl)phenol ( 2 ) was synthesized from 1 via a simple one‐pot reaction. 2 was copolymerized with 4,4′‐isopropylidenediphenol and bis(4‐fluorophenyl)sulfone to form high‐T_g polymers. The copolymers with less than 80 mol % pyridazinone or chloropyridazine monomers were soluble in chlorinated solvents such as chloroform. The copolymers with higher pyridazinone contents and homopolymers were not soluble in chlorinated solvents but were still soluble in dipolar aprotic solvents such as N‐methylpyrrolidinone. The soluble polymers could be cast into flexible films from solution. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3328–3335, 2006     

Synthesis and characterization of polyimides derived from (3‐amino‐2,4,6‐trimethylphenyl)‐(3′‐aminophenyl)methanone and aromatic dianhydrides

A series of new polyimides were prepared via the polycondensation of (3‐amino‐2,4,6‐trimethylphenyl)‐(3′‐aminophenyl)methanone and aromatic dianhydrides, that is, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA), 4,4′‐oxydiphthalic anhydride, 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride, and 2,2′‐bis(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride. The structures of the polyimides were characterized by Fourier transform infrared and NMR measurements. The properties were evaluated by solubility tests, ultraviolet–visible analysis, differential scanning calorimetry, and thermogravimetric analysis. The two different meta‐position‐located amino groups with respect to the carbonyl bridge in the diamine monomer provided it with an unsymmetrical structure. This led to a restriction on the close packing of the resulting polymer chains and reduced interchain interactions, which contributed to the solubility increase. All the polyimides except that derived from BPDA had good solubility in strong aprotic solvents, such as N‐methyl‐2‐pyrrolidinone, N,N′‐dimethylacetamide, N,N‐dimethylformamide, and dimethyl sulfone, and in common organic solvents, such as cyclohexanone and chloroform. In addition, these polyimides exhibited high glass‐transition values and excellent thermal properties, with an initial thermal decomposition temperature above 470 °C and glass‐transition temperatures in the range of 280–320 °C. The polyimide films also exhibited good transparency in the visible‐light region, with transmittance higher than 80% at 450 nm and a cutoff wavelength lower than 370 nm. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1291–1298, 2006     

Highly substituted poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives having bulky phenyl and fluorenyl pendant groups: Synthesis,characterization, and electro‐optical properties

Two series of poly(2,3‐diphenyl‐1,4‐phenylenevinylene) (DP‐PPV) derivatives containing multiple bulky substituents were synthesized. In the first series, two different groups were incorporated on C‐5,6 positions of the phenylene moiety to increase steric hindrance and to obtain blue‐shifted emissions. In the second series, bulky fluorenyl groups with two hexyl chains on the C‐9 position were introduced on two phenyl pendants to increase the solubility as well as steric hindrance to prevent close packing of the main chain. Polymers with high molecular weights and fine‐tuned electro‐optical properties were obtained by controlling the feed ratio of different monomers during polymerization. The maximum photoluminescent emissions of the thin films are located between 384 and 541 nm. Cyclic voltammetric analysis reveals that the band gaps of these light‐emitting materials are in the range from 2.4 to 3.3 eV. A double‐layer EL device with the configuration of ITO/PEDOT/P4/Ca/Al emitted pure green light with CIE′1931 at (0.24, 0.5). Using copolymer P6 as the emissive layer, the maximum luminescence and current efficiency were both improved when compared with the homopolymer P4. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6738–6749, 2006     

Investigation of sulfonated poly(ether ether ketone sulfone)/heteropolyacid composite membranes for high temperature fuel cell applications

The sulfonated poly(ether ether ketone sulfone) (SPEEKS)/heteropolyacid (HPA) composite membranes with different HPA content in SPEEKS copolymers matrix with different degree of sulfonation (DS) were investigated for high temperature proton exchange membrane fuel cells. Composite membranes were characterized by Fourier transfer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). FTIR band shifts suggested that the sulfonic acid groups on the copolymer backbone strongly interact with HPA particles. SEM pictures showed that the HPA particles were uniformly distributed throughout the SPEEKS membranes matrix and particle sizes decreased with the increment of copolymers' DS. The holes were not found in SPEEKS‐4/HPA30 (consisting of 70% SPEEKS copolymers with DS = 0.8 and 30% HPA) composite membrane after composite membranes were treated with boiling water for 24 h. Thermal stabilities of the composite membranes were better than those of pure sulfonated copolymers membranes. Although the composite membranes possessed lower water uptake, it exhibited higher proton conductivity for SPEEKS‐4/HPA30 especially at high temperature (above 100 °C). Its proton conductivity linearly increased from 0.068 S/cm at 25 °C to 0.095 S/cm at 120 °C, which was higher than 0.06 S/cm of Nafion 117. In contrast, proton conductivity of pure SPEEKS‐4 membrane only increased from 0.062 S/cm at 25 °C to 0.078 S/cm at 80 °C. At 120 °C, proton conductivity decreased to poor 0.073 S/cm. The result indicated that composite membranes exhibited high proton conductivity at high temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1967–1978, 2006     

Viscometric study of gelatin in dilute aqueous solutions

The viscosities of aqueous solutions of gelatin at different temperatures were carefully measured in a common glass‐capillary Ubbelohde viscometer at dilute to extremely dilute concentrations. The adsorption effect that occurred in the viscosity measurements was theoretically analyzed and discussed. A theory based on Langmuir isotherms could adequately describe the existing data. Some structural information was obtained by the use of an iterative fitting procedure to treat the reduced viscosity data, which disclosed that individual gelatin chains underwent a coil‐to‐helix transition as the solution cooled from 40 to 15 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1804–1812, 2006     

Crystal structure and morphology of phenyl‐capped tetraaniline in the leucoemeraldine oxidation state

A series of crystals of phenyl‐capped tetraaniline in the leucoemeraldine oxidation state were obtained at different isothermal temperatures and were observed directly under transmission electron microscope. The crystals obtained at higher temperatures exhibit more perfect structures than those obtained at lower temperatures. Both the lamella thickness and the crystal size increase with crystallization temperature. The tetraaniline is apt to form larger scale crystals under lower degree of supercooling. However, their crystal structures keep steady with the crystallization temperature. The tetramer was found to adopt a monoclinic lattice with unit cell parameter of a = 13.93 Å, b = 8.82 Å, c = 23.20 Å, and β = 95.03°, as determined using electron diffraction tilting method combined with wide‐angle X‐ray diffraction experiment. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 764–769, 2006     

Meshless simulation of equilibrium swelling/deswelling of pH‐sensitive hydrogels

Hydrogels have been widely used in microelectromechanical systems (MEMS) and Bio‐MEMS devices. In this article, the equilibrium swelling/deswelling of the pH‐stimulus cylindrical hydrogel in the microchannel is studied and simulated by the meshless method. The multi‐field coupling model, called multi‐effect‐coupling pH‐stimulus (MECpH) model, is presented and used to describe the chemical field, electric field, and the mechanical field involved in the problem. The partial differential equations (PDEs) describing these three fields are either nonlinear or coupled together. This multi‐field coupling and high nonlinear characteristics produce difficulties for the conventional numerical methods (e.g., the finite element method or the finite difference method), so an alternative—meshless method is developed to discretize the PDEs, and the efficient iteration technique is adopted to solve the nonlinear problem. The computational results for the swelling/deswelling diameter of the hydrogel under the different pH values are firstly compared with experimental results, and they have a good agreement. The influences of other parameters on the mechanical properties of the hydrogel are also investigated in detail. It is shown that the multi‐field coupling model and the developed meshless method are efficient, stable, and accurate for simulation of the properties of the stimuli‐sensitive hydrogel. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 326–337, 2006     

Melting behaviors,crystallization kinetics,and spherulitic morphologies of poly(butylene succinate) and its copolyester modified with rosin maleopimaric acid anhydride

This article investigated the melting behaviors, crystallization kinetics, and spherulitic morphologies of poly(butylene succinate) (PBS) and its copolyester (PBSR) modified with rosin maleopimaric acid anhydride, using wide‐angle X‐ray diffraction, differential scanning calorimeter (DSC), and polarized optical microscope. Subsequent DSC scans of isothermally crystallized PBS and PBSR exhibited two melting endotherms, respectively, which was due to the melt‐recrystallization process occurring during the DSC scans. The equilibrium melting point of PBSR (125.9 °C) was lower than that of PBS (139 °C). The commonly used Avrami equation was used to describe the isothermal crystallization kinetics. For nonisothermal crystallization studies, the model combining Avrami equation and Ozawa equation was employed. The result showed a consistent trend in the crystallization process. The crystallization rate was decreased, the perfection of crystals was decreased, the recrystallization was reduced, and the spherulitic morphologies were changed when the huge hydrogenated phenanthrene ring was added into the chain of PBS. The activation energy (ΔE) for the isothermal crystallization process determined by Arrhenius method was 255.9 kJ/mol for PBS and 345.7 kJ/mol for PBSR. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 900–913, 2006