Tuning the molecular properties of polybenzimidazole by copolymerization

In the present work, a series of novel random polybenzimidazole (PBI) copolymers consisting of m- and p-phenylene linkages are synthesized from various stoichiometric mixtures of isophthalic acid (IPA) and terephthalic acid (TPA) with 3,3',4,4'-tetraaminobiphenyl (TAB) by solution copolycondensation in polyphosphoric acid (PPA). The resulting copolymers are characterized by different techniques to obtain their molecular properties parameters. The monomer concentration in the polymerization plays an important role in controlling the molecular weight of the polymer. Surprisingly, a simple change in the dicarboxylic acid architecture from meta (IPA) to para (TPA) increases the molecular weight of the copolymers, which is maximum for the para homopolymer. The low solubility of TPA in PPA is found to be the dominating factor for obtaining the higher molecular weight polymer in the case of the para structure. FT-IR study shows that the introduction of the para structure enhances the conjugation along the polymer chain. The positive deviation of the copolymer composition from the feed ratio is due to the higher reactivity ratio of TPA than IPA, which is obtained from proton NMR studies. The incorporation of the para structure in the chain enhances the thermal stability of the polymers. The para homopolymer shows 59 degrees C lower glass transition temperature compare to the meta homopolymer indicating enhancement of the flexibility of the polymer chain due the introduction of the p-phenylene linkage in the backbone. The T(g) of the copolymers shows both positive and negative deviation from the expected T(g) calculated by the Fox equation. The enhanced conjugation of the polymer chains also influences the photophysical properties of the polymers in solution. All the PBI polymers exhibit strong fluorescence in dimethylacetamide solution. As expected, that all the polymers are amorphous in nature reveals that the copolymerization does not influence the packing characteristics of the PBI chains.     

Thermodynamic, kinetic and electrical switching studies on Si15Te85−xInx glasses: Observation of Boolchand intermediate phase

An interesting topic for quite some time is an intermediate phase observed in chalcogenide glasses, which is related to network connectivity and rigidity. This phenomenon is exhibited by Si-Te-In glasses also. It has been addressed here by carrying out detailed thermal investigations by using Alternating Differential Scanning Calorimetry technique. An effort has also been made to determine the stability of these glasses using the data obtained from different thermodynamic quantities and crystallization kinetics of these glasses. Electrical switching behavior by recording I-V characteristics and variation of switching voltages with indium composition have been studied in these glasses for phase change memory applications.     

Blends of polybenzimidazole and poly(vinylidene fluoride) for use in a fuel cell

We report a new blend system consisting of an amorphous polymer polybenzimidazole (PBI) and a semicrystalline polymer poly(vinylidene fluoride) (PVDF). A systematic investigation of the blend pair in various compositions using Fourier transform infrared (FT-IR) spectroscopy provides direct evidence of specific hydrogen bonding interaction involving the N-H groups of PBI and the >CF(2) groups of PVDF. Blending shows a maximum 30 cm(-1) frequency shift in the N-H stretching band of PBI and also the existence of a partial double bond character in the PVDF chain. Differential scanning calorimetry (DSC) study proves the miscibility of these polymers in a wider composition range. The decrease of the T(g) with increasing PVDF in the blend and also the decrease of both the T(m) and T(c) with increasing PBI in the blend attribute the miscibility of the blend systems. The PA doping level of the blend membranes improves significantly as a result of the hydrophobic nature of the PVDF component.     

Thermoreversible Gelation of Polybenzimidazole in Phosphoric Acid

Summary: We report a study of thermoreversible gelation of polybenzimidazole (PBI) in phosphoric acid (PA). The PBI gel in PA exhibits fibrillar network morphology and reversible first order phase transition. The gelation rate is measured by the tube tilting method and found to depend both upon gelation concentration and gelation temperature. The UV‐vis study demonstrates that the gelation process is a two‐step process: conformational transformation and aggregation which produces crystallites for gel formation. The WAXS study supports the presence of crystallites in the gel. The PA doping level of the membrane increases significantly because of gelation.

Thermoreversible gelation of polybenzimidazole in phosphoric acid and the membrane produced from the gel.     

Charged polystyrene nanoparticles: role of ionic comonomers structures

Emulsion copolymerizations of styrene were carried out with four structurally different ionic comonomers namely acrylic acid (AAc), methacrylic acid (MAA), 2-hydroxyethyl methacrylate (HEMA), and sodium styrene sulfonate (NaSS) to study the effect of monomer structure on the copolymerization kinetics and size, morphology, charge density, and the self-assembly of the particles. The copolymerization kinetics was found to be highly dependent upon the ionic comonomer structure, and the nature of this dependence altered from homogeneous to micellar nucleation regime. The decrease in particle size (D) with increasing surfactant concentration (S) was observed in all the cases; however, the exponents of D vs. S were not similar for all the cases. In the homogeneous nucleation regime, exponents followed the order as AAc (0.446) > MAA (0.396) > NaSS (0.252) > HEMA (0.241), whereas the order was almost reversed in the micellar nucleation regime as NaSS (0.406) > HEMA (0.228) > AAc (0.206) > MAA (0.172). The hydrophobic/hydrophilic character and the steric factors were found to be the driving force for the variation in D vs. S exponents with ionic comonomer structure. The presence of charges on the particle surface contributed by the ionic comonomers triggered the self-assembly of the particles upon sedimentation and diffracted visible light obeying Bragg's law.