Volume 14, Issue 4

In recent years, extensive research on the preparation and properties of proton exchange membranes (PEMs) has been realized. This article focusses on the recent studies on new PEM materials based on aromatic hydrocarbon polymers with sulfonated groups as hydrophilic domains and fluorinated groups as hydrophobic domains as alternatives to conventional perfluorinated polymers. It is necessary to improve the proton conductivity especially under low-humidity conditions and at high operating temperatures to break through the current aromatic PEM system. Hence, there is a need to develop new high-conductivity fuel cell ionomers with improved thermal, chemical, and electrochemical stability by designing a suitable polymer structure for PEM application.

In this work, the authors studied the effects of electron beam (EB)-induced changes in stress-strain characteristics, heat shrinkage stresses, and structure characteristics on polypropylene (PP) composites containing bisphenol-A-dimethacrylate (BPDMA) as the radiation sensitizer and different contents of multi-walled carbon nanotube (CNT) filler (0–2 wt.%). The effect of stearic acid (SA) as the surface modifier on the improvement of CNT dispersion in the PP matrix was also studied. Initially, PP blends with different contents (up to 10 wt.%) of BPDMA were prepared to determine the effective concentration of the sensitizer for the modification of the PP matrix. PP/BPDMA composites and blends filled with CNTs were irradiated up to 25–50 kGy of radiation doses. The properties of unirradiated compositions and those modified by EB, were compared. Radiation-induced changes were confirmed by gel fraction and by the changes in Fourier transform infrared spectroscopy spectra. The results showed an increase in Young’s modulus, yield strength, and thermal-relaxation stresses for the irradiated PP/CNT compositions grafted with 3 wt.% of BPDMA and compatibilized with SA.

Flexible ethylene propylene diene monomer (EPDM)-titania nanocomposites of different compositions were prepared by room temperature mixing using both neat and annealed titania. All these composites showed composition-dependent dielectric and mechanical properties, and composites with controlled dielectric properties could be made through judicial adjustment of the composition. The effect of moisture/filler heat treatment was also studied and found that composites with annealed titania showed lower dielectric constant than composites with normal titania. There was a significant improvement in mechanical properties, where composites with 60 parts per hundred parts of titania gave the optimum tensile strength. The particle size of titania particles was analyzed by high-resolution transmission electron microscopy (HRTEM) and a dynamic light scattering technique. The morphology and dispersion of titania particles in the EPDM matrix were studied by field emission scanning electron microscopy and HRTEM. Finally, different dielectric models were compared with experimental data, and the best match was achieved by the Lichtenecker model, which can be used as a predictive rule for different volume contents of titania filler in the EPDM matrix.

Monocyclopentadienyl titanium complexes of the type Cp*TiX 2 L (Cp*=C 5 Me 5 , 1: L=C 6 F 5 S, X=OMe; 2: L=C 6 F 5 S, X=Cl; 3: L=2, 6-Me 2 -C 6 H 3 S, X=OMe) have been synthesized with high yields. These complexes are used as the catalysts for the syndiospecific polymerization of styrene. High catalytic activity is observed, and a polymer with high molecular weight and narrow molecular weight distribution is obtained. It is concluded that not only the sulfur-containing donor ligand but also the anion ligand strongly affects the polymerization activity and the molecular weight of the resultant polymer.

Poly(3-hydroxybutyrate) (PHB)/poly(propylene carbonate) (PPC) blends containing various amounts of plasticizer propylene carbonate (PC) were prepared, and the toughness of the blends as a function of temperature was studied. It was found that the brittle-ductile transition temperature ( T BD ) of PHB toughened by PPC decreased from 60°C to 10°C with the increase in PC content. As PC is the plasticizer of PPC, the mechanical properties, such as Young’s modulus of plasticized PPC with different PC contents, were also studied. Sequentially, the relationship between T BD and the ratio of the Young’s modulus ( E 1 ) of the PHB matrix to that of the plasticized PPC elastomer ( E 2 ) was investigated. It was found that the Young’s modulus of plasticized PPC should be considerably lower than that of the PHB matrix in order to obtain ’the tough PHB/plasticized PPC blends at a lower temperature. In contrast, the morphology of the dispersed particles also had an important influence on the toughness. When the plasticizer content was more than 20 wt.%, the PPC particles became more spherical and smaller. The toughness could be considerably improved accordingly.

In this study, hydrogels of poly(2-hydroxyethyl methacrylate) with different cross-link density were prepared by the free-radical polymerization method. l -Cystine, which acts as a cross-linker, was prepared at different concentrations, ranging from 0.02 to 0.08 mol/l, to identify the concentration that provided the highest mechanical strength and healing efficacy. Healing of the hydrogels was achieved by heating above their glass transition temperature. Intermolecular diffusion of the dangling chain or chain slippage led to the healing of the gels. Results showed that 0.04 mol/l of l -cystine in poly(2-hydroxyethyl methacrylate) hydrogels provided the highest ultimate tensile strength (0.780 N/mm 2 ) and healing recovery (92%). This healing capability was also observed using optical microscopy.