All issues / Volume 5 (2011) / Issue 6 (June)
This is an editorial article. It has no abstract.
Epoxidized soybean oil (ESO) was successfully thermal-cured by using methylhexahydrophthalic anhydride (MHHPA) curing agent, in the presence of tetraethylammonium bromide (TEAB) catalyst of varied concentration (0.3–0.8 phr). The polyesterification process of ESO thermoset was proven and supported by Fourier transforms infrared spectroscopy (FTIR) and gas chromatography-mass spectroscopy analysis (GC-MS). A possible chemical reaction of the MHHPA, TEAB and ESO was proposed based on the experimental work. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) revealed that there is a positive relationship between the degree of conversion and crosslink density of ESO thermoset with TEAB concentration. The kinetics of water absorption of the ESO thermoset were found to conform to Fickian law behavior.
Electroactive random copolymers of 3,4-Propylenedioxythiophene (ProDOT) and N-Phenylsulfonyl Pyrrole (PSP) were electrochemically synthesized on single carbon fiber microelectrode (SCFME) by cyclic voltammetry (CV). Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) measurements indicate the inclusion of PSP into the copolymer structure. The influence of feed ratios on the copolymers was studied by CV and electrochemical impedance spectroscopy (EIS) and equivalent circuit modelling (ECM). The morphologies and film thicknesses of copolymers were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results have shown that the principal changes in morphology, conductivity, porous nature and thickness of Poly(ProDOT-co-PSP) film depend on the concentration of PSP. The strong electron-withdrawing sulfonyl group substitution on PSP significantly inhibited electrochemical copolymerization. Semicircular characteristics at Nyquist plots reflected an increasing trend with the increase of PSP concentration in the feed at high frequency. The semicircular characteristic of the copolymer film is useful for the bioelectrochemical sensor applications.
This study aims to evidence the formation of stable polyelectrolyte complex particles as colloidal dispersions using some weak polyelectrolytes: chitosan and poly(allylamine hydrochloride) as polycations and poly(acrylic acid) (PAA) and poly(2-acrylamido-2-methylpropanesulfonic acid – co – acrylic acid) (PAMPSAA) as polyanions. Polyelectrolyte complex particles as colloidal dispersion were prepared by controlled mixing of the oppositely charged polymers, with a constant addition rate. The influences of the polyelectrolytes structure and the molar ratio between ionic charges on the morphology, size, and colloidal stability of the complex particles have been deeply investigated by turbidimetry, dynamic light scattering and atomic force microscopy. A strong influence of polyanion structure on the values of molar ratio n–/n+ when neutral complex particles were obtained has been noticed, which shifts from the theoretical value of 1.0, observed when PAA was used, to 0.7 for PAMPSAA based complexes. The polyions chain characteristics influenced the size and shape of the complexes, larger particles being obtained when chitosan was used, for the same polyanion, and when PAMPSAA was used, for the same polycation.
Multi-walled carbon nanotubes (MWCNTs) were functionalized by ozone (O3) and then melting mixed into polycarbonate (PC) matrix. The effect of O3 functionalization on the mechanical performance was systematically studied. It was found that the modified MWCNTs exhibited improved dispersion and interfacial adhesion to the matrix as evidenced by scanning electron microscope and micro-Raman spectrometer, which were believed to contribute to the enhanced yield strength of MWCNTs/PC nanocomposites.
Binary polyvinylidene fluoride/barium titanate (PVDF/BaTiO3) and its nanographite (GN) doped ternary nanocomposites were fabricated using a simple solution casting process followed by compression molding. The dielectric behavior of such hybrids over a wide frequency range was studied. Additions of GN with contents close to the percolation threshold were found to be very effective to enhance dielectric permittivity of the PVDF/BaTiO3 nanocomposites. In this regard, the electrical behavior of ternary PVDF/BaTiO3/GN hybrids can be explained in terms of the percolation theory. Furthermore, both dielectric constant and electrical conductivity of hybrids were found to be strongly frequency and temperature dependent.
Reversible addition-fragmentation chain transfer (RAFT) polymerization was adopted to synthesize starch-based conjugates that possessed controllable architecture and properties. Starch-based xanthate agent was prepared and applied as chain transfer agent to conduct the living/controlled polymerization (LCP) of vinyl acetate, which generated tailor-made conjugates of starch and well-defined poly(vinyl acetate) (SVAc). The relevant derivatives, conjugates of starch and chain length-controlled poly(vinyl alcohol) (SVA), were obtained subsequently. Various characterizations such as Fourier transform infrared spectra (FTIR), ultraviolet-visible spectroscopy (UV), proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), and dynamic mechanical thermal analysis (DMTA) were performed to examine the structure of intermediates and the starch-based conjugates. Static contact angle measurements revealed that the hydrophilic character of starch-based conjugates was tunable. Well-defined SVAc was amphiphilic and it was able to self-assemble into size controllable micelles, which was verified by contact angles, transmission electron microscopy (TEM) and dynamic light scattering (DLS) tests. SVA exhibited much higher capability to form physically cross-linked hydrogel than starch did. Both the characteristic of SVAc and SVA were chain length-dependent.
Biomimetic mineralization was performed on a large scale by a rapid and efficient approach. Chitosan scaffolds were placed in a mixed solution of urea, ethanol and distilled water, followed by the introduction of dibasic sodium phosphate (0.1M) and calcium chloride (0.1M) with the molar ratio of 1.67. These mixed solvents was then adjusted to weakly alkaline by adding sodium hydroxide solution. Finally the reaction mixture was sealed and kept at 80ºC for predetermined time. The composition and morphology of the apatite and the hybrid scaffolds were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier Transform Infrared spectroscopy (FTIR) and environmental scanning electron microscopy (ESEM). The mechanism of nucleation and growth of crystals was discussed as well. The results revealed that chitosan scaffolds improved the crystallinity of hydroxyapatite (HAP) crystals. With the extension of mineralization time, the mineral layers on the outer surface and inner section of chitosan scaffolds increased as well. Furthermore, the compressive strength and modulus of the HAP-chitosan scaffolds biocomposites increased to 0.55±0.003 and 29.29±1.25 MPa respectively. Such one-pot approach may be extended to the mineralization of other materials and will have a broad application in the future.
New composites with improved thermal stability, mechanical and dielectric properties were developed, which consist of 2,2'-diallylbisphenol A (DBA)/4,4'-bismaleimidodiphenylmethane (BDM) resin and a new kind of organic/inorganic mesoporous silica (MPSA). Typical properties (curing behavior and mechanism, thermal stability, mechanical and dielectric properties) of the composites were systematically investigated, and their origins were discussed. Results show that MPSA/DBA/BDM composites have similar curing temperature as DBA/BDM resin does; however, they have different curing mechanisms, and thus different crosslinked networks. The content of MPSA has close relation with the integrated performance of cured composites. Compared with cured DBA/BDM resin, composites with suitable content of MPSA show obviously improved flexural strength and modulus as well as impact strength; in addition, all composites not only have lower dielectric constant and similar frequency dependence, more interestingly, they also exhibit better stability of frequency on dielectric loss. For thermal stability, the addition of MPSA to DBA/BDM resin significantly decreases the coefficient of thermal expansion, and improves the char yield at high temperature with a slightly reduced glass transition temperature. All these differences in macro-properties are attributed to the different crosslinked networks between MPSA/DBA/BDM composites and DBA/BDM resin.