Studies on curing kinetics of a novel combined liquid crystalline epoxy containing tetramethylbiphenyl and aromatic ester‐type mesogenic group with diaminodiphenylsulfone

The curing kinetics of a novel liquid crystalline epoxy resin with combining biphenyl and aromatic ester‐type mesogenic unit, diglycidyl ether of 4,4′‐bis(4‐hydroxybenzoyloxy)‐3,3′,5,5′‐tetramethyl biphenyl (DGE‐BHBTMBP), and the curing agent diaminodiphenylsulfone (DDS) was studied using the advanced isoconvensional method (AICM). DGE‐BHBTMBP/DDS curing system was investigated the curing behavior by means of differential scanning calorimetry (DSC) during isothermal and nonisothermal processes. Only one exothermal peak appeared in isothermal DSC curves. A variation of the effective activation energy with the extent of conversion was obtained by AICM. Three different curing stages were confirmed. In the initial curing stage, the value of E_a is dramatically decreased from ～90 to ～20 kJ/mol in the conversion region 0–0.2 for the formation of LC phase. In the middle stage, the value of E_a keeps about ～80 kJ/mol for cooperative effect of reaction mechanism and diffusion control. In the final stage, a significant increase of E_a from 84 to 136 kJ/mol could be caused by the mobility of longer polymer chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3922–3928, 2007     

Highly magnetic latexes from submicrometer oil in water ferrofluid emulsions

The synthesis of functionalized submicrometer magnetic latex particles is described as obtained from a preformed magnetic emulsion composed of organic ferrofluid droplets dispersed in water. Composite (polystyrene/γ‐Fe₂O₃) particles were prepared according to a two‐step procedure including the swelling of ferrofluid droplets with styrene and a crosslinking agent (divinyl benzene) followed by seeded emulsion polymerization with either an oil‐soluble [2,2′‐azobis(2‐isobutyronitrile)] or water‐soluble (potassium persulfate) initiator. Depending on the polymerization conditions, various particle morphologies were obtained, ranging from asymmetric structures, for which the polymer phase was separated from the inorganic magnetic phase, to regular core–shell morphologies showing a homogeneous encapsulation of the magnetic pigment by a crosslinked polymeric shell. The magnetic latexes were extensively characterized to determine their colloidal and magnetic properties. The desired core–shell structure was efficiently achieved with a given styrene/divinyl benzene ratio, potassium persulfate as the initiator, and an amphiphilic functional copolymer as the ferrofluid droplet stabilizer. Under these conditions, ferrofluid droplets were successfully turned into superparamagnetic polystyrene latex particles, about 200 nm in size, containing a large amount of iron oxide (60 wt %) and bearing carboxylic surface charges. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2642–2656, 2006     

Syntheses, structure and intercalation properties of low-dimensional phenylarsonates, A(HO3AsC6H5)(H2O3AsC6H5) (A = Tl, Na, K and Rb)

Four new low-dimensional phenylarsonates, A(HO₃AsC₆H₅)(H₂O₃AsC₆H₅) (A = Tl(1), Na(2), K(3) and Rb(4)), have been synthesized and characterized by X-ray diffraction, spectroscopic and thermal studies. They crystallize in triclinic unit cells and have approximately planar arrangement of A⁺ ions, coordinated to oxygen atoms of phenylarsonates, on both sides. Structure of thallium phenylarsonate as determined by single crystal X-ray diffraction, is one-dimensional, whereas those of alkalimetal analogues are two-dimensional. Successful intercalation reactions of compounds1 and2 with primaryn-alkyl amines have been demonstrated. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Hybrid organic/inorganic films of conducting polymers modified with phthalocyanines. I—Film preparation and voltammetric studies

Conducting polymers were deposited on the surface of platinum and glassy carbon electrodes. The monomers used were N-methyl pyrrole and 3-methyl thiophene. The electrochemical synthesis of the polymer was achieved using constant applied potential or cyclic polarization techniques in acetonitrile as a solvent and tetra-alkyl ammonium salts as supporting electrolyte. The resulting conducting polymeric film was modified with an inorganic metal complex, namely, Cu–phthalocyanine or Co–phthalocyanine. Two different approaches were adopted for the modification: (1) the first was to directly apply the metal–phthalocyanine layer on the surface of the polymer, and (2) the second was by the inclusion of the metal–phthalocyanine in a sol–gel matrix that was in turn applied to the conducting polymer film. In the first part of this work, we studied the effect of changing the type of polymer matrix and the central metal of the inorganic complex on the electrochemical behavior of the resulting film. We also found that changing the method of metal–phthalocyanine application to the polymer film affected the electrochemical response and kinetics at the electrode surface. The new electrode was tested for the reduction of hydrogen peroxide and showed better conversion efficiency compared to conventional surfaces, which suggests its use in fuel cell applications.     

Voltage‐induced resistivity relaxation in a high‐density polyethylene/graphite nanosheet composite

The resistivity relaxation behavior under applied voltages in a high‐density polyethylene/graphite nanosheet composite was investigated. The influence of applied voltages on the resistivity relaxation was measured by the collection of the electric current passing through the sample and the increasing temperature of the sample. With increments in the voltage, three distinguishable relaxation curves corresponding to different dominating mechanisms were observed. The sawed curve, corresponding to the application of a high voltage, could be attributed to the reorganization of conductive particles induced by the electric field and the destruction of the conductive network due to the thermal expansion of the high‐density polyethylene matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 860–863, 2007     

Crystallization behavior of poly(ε‐caprolactone)/multiwalled carbon nanotube composites

Differential scanning calorimetry (DSC), polarized optical microscopy, and X‐ray diffraction methods were used to investigate the isothermal crystallization behavior and crystalline structure of poly(?‐caprolactone) (PCL)/multiwalled carbon nanotube (MWNT) composites. PCL/MWNT composites were prepared via the mixing of a PCL polymer solution with carboxylic groups containing multiwalled carbon nanotubes (c‐MWNTs). Both Raman and Fourier transform infrared spectra indicated that carboxylic acid groups formed at both ends and on the sidewalls of the MWNTs. A transmission electron microscopy micrograph showed that c‐MWNTs were well separated and uniformly distributed in the PCL matrix. DSC isothermal results revealed that introducing c‐MWNTs into the PCL structure caused strongly heterogeneous nucleation induced by a change in the crystal growth process. The activation energy of PCL drastically decreased with the presence of 0.25 wt % c‐MWNT in PCL/c‐MWNT composites and then increased with increasing MWNT content. The result indicated that the addition of c‐MWNT to PCL induced heterogeneous nucleation (lower total activation energy) at a lower c‐MWNT content and then reduced the transportation ability of polymer chains during crystallization processes at a higher MWNT content (higher total activation energy). A correlation between the crystallization kinetics, melting behavior, and crystalline structure of PCL/c‐MWNT composites was also discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 598–606, 2006     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Epoxies toughened with triblock copolymers

Three different commercial triblock copolymers from Arkema were evaluated as potential toughening agents for two different lightly crosslinked epoxies. It was found that the plane strain fracture toughness, K_IC, was on the order of 3.0 MPa√m for 10 parts per hundred resin (phr) of NanoStrength™ E20 resin (a styrene–butadiene–methylacrylate, SBM, type triblock copolymer) in epoxies cured with either aminoethylpiperazine or piperidine. In contrast, 10 phr NanoStrength E40 resin (also an SBM type triblock copolymer) was ineffective in toughening such epoxies. The difference in toughening effectiveness was attributed to the smaller amount of polybutadiene present in the E40 resin. The third toughening agent from Arkema was NanoStrength M22 resin, which is a symmetric triblock copolymer consisting of side blocks of PMMA surrounding a center block of poly(butyl acrylate) and is designated as MAM. At 10 phr MAM both lightly crosslinked epoxies exhibited improvements in toughness. Morphologies were characterized using TEM and toughening mechanisms revealed using SEM and TOM. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1470–1481, 2007     

Mechanical properties and morphology of homoblends of a poly(ethyl acrylate) ionomer having one ion pair per ionic repeat unit and a poly(ethyl acrylate) ionomer having two ion pairs

The mechanical properties and morphology of homoblends of poly(ethyl acrylate‐co‐acrylate) (PEAA) having one ion pair per ionic monomer repeat unit and poly(ethyl acrylate‐co‐itaconate) (PEAITA) having two ion pairs were investigated. It was found that the compositional variation in the ionomer homoblends did not affect the matrix or cluster glass transition temperatures of the two ionomers of the homoblends. It was also observed that the ionomer homoblends showed two ionic plateaus and that the changes in the two ionic moduli were directly related to the relative amounts of the two ionomers. The ionic moduli calculated with the model for filler‐dispersed materials were found to fit the experimental data to a great extent. Therefore, it was suggested that the PEAITA/PEAA ionomer homoblends were filler‐containing composite materials rather than miscible blends. In the X‐ray scattering study, it was observed that the morphology of the ionomer homoblends was not affected by mixing. The results obtained in this work might be useful for the modification of the storage moduli of copolymers in a certain temperature range without the alteration of their processing temperatures. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1045–1052, 2007.