Joannis K. Kallitsis

Development and Characterization of Acid-Doped Polybenzimidazole/Sulfonated Polysulfone Blend Polymer Electrolytes for Fuel Cells

Polymeric membranes from blends of sulfonated polysulfones (SPSF) and polybenzimidazole (PBI) doped with phosphoric acid were developed as potential high-temperature polymer electrolytes for fuel cells and other electrochemical applications. The water uptake and acid doping of these polymeric membranes were investigated. Ionic conductivity of the membranes was measured in relation to temperature, acid doping level, sulfonation degree of SPSF, relative humidity, and blend composition. The conductivity of SPSF Was of the order of 10 3- S cm 1 . In the case of blends of PBI and SPSF it was found to be higher than 10 -2 S cm -1 . Much improvement in the mechanical strength is observed for the blend polymer membranes, especially at higher temperatures. Preliminary work has demonstrated the feasibility of these polymeric membranes for fuel-cell applications.

Comparison of compatibilizer effectiveness for PET/HDPE blends

Abstract The efficiency of four different compatibilizers melt-mixed with a poly(ethylene terephthalate)/high density polyethylene (PET/HDPE) blend was investigated. The compatibilizers studied were an ethylene-glycidyl methacrylate copolymer (E-GMA), an ethylene ethylacrylate glycidyl methacrylate terpolymer (E-EA-GMA), a hydrogenated styrene-butadiene-styrene copolymer grafted with maleic anhydride (SEBS-g-MA) and a MA-modified ethylene-methyl acrylate copolymer (E-MeA-g-MA). The techniques applied were dynamic mechanical analysis, tensile and impact testing, optical and electron microscopy, thermal analysis and Fourier transform infra-red spectroscopy. On the basis of morphological evidence and tensile testing, which proved most discriminating, the compatibilizing effectiveness was found to decrease in the sequence, E-GMA > E-EA-GMA > SEBS-g-MA > E-MeA-g-MA. These results are explained on the basis of differing reactivities of the GMA vs. the MA functionality contained in these compatibilizers.

A novel experimental model of cervical spondylotic myelopathy (CSM) to facilitate translational research

Cervical spondylotic myelopathy (CSM) is the most common form of spinal cord impairment in adults. However critical gaps in our knowledge of the pathobiology of this disease have limited therapeutic advances. To facilitate progress in the field of regenerative medicine for CSM, we have developed a unique, clinically relevant model of CSM in rats. To model CSM, a piece of synthetic aromatic polyether, to promote local calcification, was implanted microsurgically under the C6 lamina in rats. We included a sham group in which the material was removed 30s after the implantation. MRI confirmed postero-anterior cervical spinal cord compression at the C6 level. Rats modeling CSM demonstrated insidious development of a broad-based, ataxic, spastic gait, forelimb weakness and sensory changes. No neurological deficits were noted in the sham group during the course of the study. Spasticity of the lower extremities was confirmed by a significantly greater H/M ratio in CSM rats in H reflex recordings compared to sham. Rats in the compression group experienced significant gray and white matter loss, astrogliosis, anterior horn cell loss and degeneration of the corticospinal tract. Moreover, chronic progressive posterior compression of the cervical spinal cord resulted in compromise of the spinal cord microvasculature, blood-spinal cord barrier disruption, inflammation and activation of apoptotic signaling pathways in neurons and oligodendrocytes. Finally, CSM rats were successfully subjected to decompressive surgery as confirmed by MRI. In summary, this novel rat CSM model reproduces the chronic and progressive nature of human CSM, produces neurological deficits and neuropathological features accurately mimicking the human condition, is MRI compatible and importantly, allows for surgical decompression.

A Quasi-Direct Methanol Fuel Cell System Based on Blend Polymer Membrane Electrolytes

On the basis of blend polymer electrolytes of polybenzimidazole and sulfonated polysulfone, a polymer electrolyte membrane fuel cell was developed with an operational temperature up to 200°C. Due to the high operational temperature, the fuel cell can tolerate 1.0-3.0 vol % CO in the fuel, compared to less than 100 ppm CO for the Nafion-based technology at 80°C. The high CO tolerance makes it possible to use the reformed hydrogen directly from a simple methanol reformer without further CO removal. That both the fuel cell and the methanol reformer operate at temperatures around 200°C opens the possibility for an integrated system. The resulting system is expected to exhibit high power density and simple construction as well as efficient capital and operational cost.

Blends of poly(ethylene terephthalate) with unmodified and maleic anhydride grafted acrylonitrile-butadiene-styrene terpolymer

Abstract Tensile, dynamic mechanical, thermal properties and morphology features of poly(ethylene terephthalate) (PET) blends with the acrylonitrile-butadiene-styrene (ABS) terpolymer were examined at up to 25 wt% content of ABS. Both unmodified and maleic anhydride grafted ABS (ABS-g-MA) were used. PET/ABS blends quenched from the melt and tested shortly after, show good mechanical properties—the result of component miscibility in the melt predicted by the theory and providing temporal component adhesion in the solid state. After storage, properties deteriorate due to ABS particle debonding and a polymer alloy is obtained with poor tensile properties. This property deterioration is not observed in PET/ABS-g-MA blends due to the grafting of the modified ABS particles onto the PET matrix. These findings are supported by the optical and mainly by the scanning electron microscopy examination of cryofractured and etched blends.

Novel Hybrid Materials Consisting of Regioregular Poly(3‐octylthiophene)s Covalently Attached to Single‐Wall Carbon Nanotubes

Facile routes for the synthesis of hybrid materials consisting of regioregular poly(3-octylthiophene)s covalently attached to single-wall carbon nanotubes are presented for the first time. These materials are easily processable using common organic solvents, and at the same time combine the properties of regioregular poly(3-alkylthiophene)s with those of single-wall carbon nanotubes. Moreover, studies of the properties of these materials have provided strong evidence for an electron transfer from the regioregular poly(3-octylthiophene) to the single-wall carbon nanotube.

Cross-linked high temperature polymer electrolytes through oxadiazole bond formation and their applications in HT PEM fuel cells

Chemical cross-linking of linear, high molecular weight polymer electrolytes has been performed herein, using thermally stable cross-linking bonds. Aromatic copolymeric or terpolymeric polyethers combining polar main chain pyridine units with side cross-linkable carboxylic acid groups were employed. The selected cross-linking chemistry was that of oxadiazole ring formation due to the synthetic versatility and the exceptional thermal and chemical stability of the formed linkages. Membranes of increased robustness and thermal stability were obtained after cross-linking that moreover presented higher doping levels than their linear analogues. Selected cross-linked membranes were formulated into membrane electrode assemblies (MEAs) that were tested in a single cell at temperatures up to 220 °C for 350 hours showing stable operation and high ionic conductivities close to 10−1 S cm−1. These facts point to the superiority of the cross-linked materials and their great potential as PEMs for fuel cells operating well above 180 °C.

Synthesis of alternating polystyrene/poly(ethyleneoxide) branched polymacromonomers

Newly designed PS/PEO alternating branched polymacromonomers have been obtained by polycondensation of alpha-dicarboxy-functionalized polystyrene and alpha-dihydroxy-functionalized polyethyleneoxide. 4-[3,5-Bis(methoxycarbonyl)phenoxymethyl]benzyl bromide was used as atom-transfer radical polymerization (ATRP) initiator for the synthesis of alpha-dicarboxy functionalized polystyrenes. These macromonomers possess low polydispersities and molecular weights in the range of 7000 to 100,000, as proved by gel permeation chromatography (GPC) and 1H NMR. Alpha-dihydroxy functionalized polyethyleneoxide (PEO) was synthesized by treatment of monofunctionalized PEO with 3,5-bis(benzyloxy)benzoyl chloride. Polycondensation of the alpha-dicarboxy PS with the alpha-dihydroxy PEO in solution or in bulk resulted in alternating PS/PEO polymacromonomers, which were effectively purified from the unreacted macromonomers and characterized by using 1H NMR, GPC, thermal analysis, and optical microscopy. Light-scattering measurements in organic solvents like THF or dioxane have shown that these polymacromonomers form stable micelles.

Study of hybrid solar cells made of multilayer nanocrystalline titania and poly(3-octylthiophene) or poly-(3-(2-methylhex-2-yl)-oxy-carbonyldithiophene)

Hybrid solar cells have been constructed by using nanocrystalline titania and hole-transporting polymers. Titania was deposited on fluorine-doped tin-oxide transparent electrodes in three layers: a blocking layer and two nanostructured layers, giving densely packed or open structures. Open structures produced higher currents due to better polymer penetration and larger oxide-polymer interface. Cells based on the dithiophene-unit-containing polymer gave higher open-circuit voltage. Efficient cells could be made only in the presence of a dye sensitizer and a lithium salt. Cells were neither sealed nor encapsulated and their components were deposited under ambient conditions except for the metal back electrode, which was deposited under vacuum. Cells demonstrated a transient behavior in two stages: initially an increase of both current and voltage followed by an increase in voltage and a drop in current. Both quantities were stabilized at values approximately established within a few days. These values remained stable for several months when the cells were stored in the dark.

Formation of hybrid wormlike micelles upon mixing cetyl trimethylammonium bromide with poly(methyl methacrylate-co-sodium styrene sulfonate) copolymers in aqueous solution.

The association of cetyltrimethylammonium bromide, CTAB, with a series of P(MMAx-co-SSNa) random copolymers of sodium styrene sulfonate (SSNa) with methyl methacrylate (MMA) was explored in aqueous solution as a function of the MMA molar content, x, of the copolymers. The polyelectrolyte/surfactant complexation in aqueous solution was verified through pyrene fluorescence probing. In addition, turbidimetry studies in dilute or more concentrated aqueous solutions elucidated the phase separation behavior of the P(MMAx-co-SSNa)/CTAB systems as a function of the copolymer composition x and the surfactant to polyelectrolyte mixing charge ratio. It is found that practically phase separation is completely suppressed within the studied mixing range when the MMA content of the copolymers is ∼30-40 mol%. For lower MMA contents the polyelectrolyte/surfactant complex separates out from water, while for higher x values the solubility limits of the copolymers in water are attained. For the intermediate MMA contents, viscoelastic systems are obtained in more concentrated polymer/surfactant solutions provided that the polyelectrolyte is fully complexed with the cationic surfactant ((1)H NMR results). Moreover, the (1)H NMR studies indicate that hybrid P(MMAx-co-SSNa)/CTAB wormlike micelles are formed in water under these conditions. Finally, it is shown that addition of salt prevents syneresis problems and facilitates the rheological investigation.