Tushar Jana

Polybenzimidazole/silica nanocomposites: Organic-inorganic hybrid membranes for PEM fuel cell

Despite the myriad studies on polybenzimidazole based polymer electrolyte membranes for fuel cells operating above 100 °C, the development of membranes with higher proton conductivity without compromising mechanical stability has continued to be the prime challenge. In this article, organic/inorganic hybrid nanocomposites of poly (4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) were prepared with surface functionalized silica nanoparticles to address this key issue. Structural and morphological studies probed by SAXS, WAXD and TEM, respectively revealed the formation of self-assembled clusters of nanoparticles when the OPBI nanocomposites were made with amine modified silica (AMS) whereas a well dispersed structure was obtained for OPBI and the unmodified silica (UMS) composite. The OPBI/AMS nanocomposites displayed a significant enhancement in the thermal stabilities compared to the pristine OPBI and OPBI/UMS nanocomposite. The OPBI/AMS nanocomposite membranes exhibited a larger mechanical reinforcement than the pristine OPBI and OPBI/UMS nanocomposite. The formation of nanoparticle clusters in the OPBI matrix in the case of OPBI/AMS was found to be the driving force for the higher thermal and mechanical stability of OPBI/AMS than those of OPBI/UMS. The incorporation of AMS in the OPBI matrix shielded the polymer chains from the attack of oxidative radicals, resulting in a huge enhancement of oxidative stability of the nanocomposite membranes compared to the pure OPBI membrane. The OPBI/AMS nanocomposite membranes have significantly higher phosphoric acid (PA) loading compared to the pure OPBI membrane which resulted in the higher proton conductivities of the former. The self-assembled clusters of AMS in the OPBI matrix facilitated the proton transport process.

Photonic crystal hydrogel material for the sensing of toxic mercury ions (Hg2+) in water

We report the development of a new photonic crystal hydrogel for the sensing of highly toxic mercury ion (Hg2+) in water. This new sensing material optically reports the Hg2+ concentration in watervia diffraction of visible light from polymerized crystalline colloidal array (PCCA). The PCCA consists of light diffracting crystalline colloidal array (CCA) of monodisperse, highly charged polystyrene particles, which are polymerized within the polyacrylamide hydrogel. The changes in hydrogel surroundings trigger the volume change of hydrogel, which alters the lattice spacing of CCA and hence shifts the diffraction wavelength of light. The ions responsive urease coupled PCCA (UPCCA) hydrolyzes the urea and produces the HCO3− and NH4+ ions inside the hydrogel. These ions induce the charge-screening of the polyacrylamide carboxylates by decreasing the electrostatic repulsion between carboxylates and the polyacrylamide backbone relaxes, causing the shrinkage of hydrogel. Hence the UPCCA exhibits the blue shift of the diffracted wavelength. Hg2+ being the principal inhibitor of the urease-urea hydrolysis perturbs urea hydrolysis by the UPCCA when UPCCA is exposed to Hg2+ along with urea and hence suppresses the production of ions. This intervenes the shrinkage of hydrogel and does not allow the hydrogel to shrink as it shrinks in absence of Hg2+. Therefore the PCCA net blue shift decreases in the presence of Hg2+ along with urea compared to only urea. The extent of this hydrogel volume change is a function of Hg2+ concentrations. This UPCCA photonic crystal sensor detects ultra low (1 ppb) concentration of Hg2+ in water, exhibits reversibility and displays very high selectivity towards Hg2+. The uncompetitive inhibition nature of the urease enzyme, when it is covalently attached in the polymer hydrogel backbone, is the driving force for the very high selectivity and reversibility of the UPCCA sensor.

Synthesis of core―shell polystyrene nanoparticles by surfactant free emulsion polymerization using macro-RAFT agent

Novel approach for the synthesis of core-shell polystyrene nanoparticles by living hydrophilic polymer consisting of thiocarbonyl thio end group is reported. The surfactant free emulsion polymerization of styrene in the presence of macro-RAFT (reversible addition fragmentation chain transfer) agent is carried out to synthesize stable latex particles with smaller particle size. A macro-RAFT agent is prepared by homopolymerization of sodium styrene sulfonate (NaSS) in aqueous phase by using dithioester as chain transfer agent. This synthesized polystyrene sulfonate-sodium (PSS-Na) based macro-RAFT agent, which is essentially water soluble macromolecular chain transfer agent used for the surfactant-free batch emulsion polymerization of styrene. Transmission electron microscopy (TEM) study of the synthesized colloids shows the narrow particle size distribution with core-shell morphology.

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.

Proton Exchange Membrane Developed from Novel Blends of Polybenzimidazole and Poly(vinyl-1,2,4-triazole)

In continuation (J. Phys. Chem. B2008, 112, 5305; J. Colloid Interface Sci. 2010, 351, 374) of our quest for proton exchange membrane (PEM) developed from polybenzimidazole (PBI) blends, novel polymer blend membranes of PBI and poly(1-vinyl-1,2,4-triazole) (PVT) were prepared using a solution blending method. The aim of the work was to investigate the effect of the blend composition on the properties, e.g., thermo-mechanical stability, swelling, and proton conductivity of the blend membranes. The presence of specific interactions between the two polymers in the blends were observed by studying the samples using varieties of spectroscopic techniques. Blends prepared in all possible compositions were studied using a differential scanning calorimetry (DSC) and exhibited a single T(g) value, which lies between the T(g) value of the neat polymers. The presence of a single composition-dependent T(g) value indicated that the blend is a miscible blend. The N-H···N interactions between the two polymers were found to be the driving force for the miscibility. Thermal stability up to 300 °C of the blend membranes, obtained from thermogravimetric analysis, ensured their suitability as PEMs for high-temperature fuel cells. The proton conductivity of the blend membranes have improved significantly, compared to neat PBI, because of the presence of triazole moiety, which acts as a proton facilitator in the conduction process. The blend membranes showed a considerably lower increase in thickness and swelling ratio than that of PBI after doping with phosphoric acid (PA). We found that the porous morphology of the blend membranes caused the loading of a larger amount of PA and, consequently, higher proton conduction with lower activation energy, compared to neat PBI.

Polystyrene-graphene oxide (GO) nanocomposite synthesized by interfacial interactions between RAFT modified GO and core-shell polymeric nanoparticles.

Here we report simple and robust one-pot method for the preparation of polystyrene (PS)/graphene oxide (GO) nanocomposite using reversible addition fragmentation chain transfer (RAFT) modified GO in surfactant free emulsion polymerization (SFEP). The results suggested that ionic comonomer, styrene sulfonate sodium salt (SS-Na), concentration plays vital role in forming PS/GO nanocomposite. X-ray and electron diffraction studies suggest that there is no recombination of GO sheets when moderate SS-Na concentration is used, resulting complete exfoliation of GO sheets in the PS/GO nanocomposite. The formation of core-shell particles in which PS is the core and polystyrene sulfonate sodium salt (PSS-Na) is the shell, and the specific interactions between functional groups of GO and PSS-Na are attributed as the driving forces for the PS/GO nanocomposite formation.

Polybenzimidazole Block Copolymers for Fuel Cell: Synthesis and Studies of Block Length Effects on Nanophase Separation, Mechanical Properties, and Proton Conductivity of PEM

A series of meta-polybenzimidazole-block-para-polybenzimidazole (m-PBI-b-p-PBI), segmented block copolymers of PBI, were synthesized with various structural motifs and block lengths by condensing the diamine terminated meta-PBI (m-PBI-Am) and acid terminated para-PBI (p-PBI-Ac) oligomers. NMR studies and existence of two distinct glass transition temperatures (Tg), obtained from dynamical mechanical analysis (DMA) results, unequivocally confirmed the formation of block copolymer structure through the current polymerization methodology. Appropriate and careful selection of oligomers chain length enabled us to tailor the block length of block copolymers and also to make varieties of structural motifs. Increasingly distinct Tg peaks with higher block length of segmented block structure attributed the decrease in phase mixing between the meta-PBI and para-PBI blocks, which in turn resulted into nanophase segregated domains. The proton conductivities of proton exchange membrane (PEM) developed from phosphoric acid (PA) doped block copolymer membranes were found to be increasing substantially with increasing block length of copolymers even though PA loading of these membranes did not alter appreciably with varying block length. For example when molecular weight (Mn) of blocks were increased from 1000 to 5500 then the proton conductivities at 160 °C of resulting copolymers increased from 0.05 to 0.11 S/cm. Higher block length induced nanophase separation between the blocks by creating less morphological barrier within the block which facilitated the movement of the proton in the block and hence resulting higher proton conductivity of the PEM. The structural varieties also influenced the phase separation and proton conductivity. In comparison to meta-para random copolymers reported earlier, the current meta-para segmented block copolymers were found to be more suitable for PBI-based PEM.

Urease immobilized polymer hydrogel: Long-term stability and enhancement of enzymatic activity

A method has been developed in which an enzyme namely urease was immobilized inside hydrogel matrix to study the stability and enzymatic activity in room temperature (∼27-30°C). This urease coupled hydrogel (UCG) was obtained by amine-acid coupling reaction and this procedure is such that it ensured the wider opening of mobile flap of enzyme active site. A systematic comparison of urea-urease assay and the detailed kinetic data clearly revealed that the urease shows activity for more than a month when stored at ∼27-30°C in case of UCG whereas it becomes inactive in case of free urease (enzyme in buffer solution). The aqueous microenvironment inside the hydrogel, unusual morphological features and thermal behaviour were believed to be the reasons for unexpected behaviour. UCG displayed enzyme activity at basic pH and up to 60°C. UCG showed significant enhancement in activity against thermal degradation compared to free urease. In summary, this method is a suitable process to stabilize the biomacromolecules in standard room temperature for many practical uses.

High temperature d.c. conductivity of sulfonic acid doped thermoreversible polyaniline gels

Thermoreversible polyaniline gels in four different sulfonic acids 2,7-dinonylnapthalene-4-sulfonic acid (DNNSA), 2,7-dinonylnapthalene-4,5-disulfonic acid (DNNDSA), (±) camphor-10-sulfonic acid (CSA) and n-dodecyl-hydrogen sulfate (DHS) are prepared for different weight fractions of polyaniline (PANI). The high temperature d.c. conductivity of these gels is measured. Near the gel melting temperature a two/three order drop in d.c. conductivity is observed. Also metal like behaviour of conductivity with temperature is observed in some of these gels. Phase diagrams are drawn from these results and they almost coincide with those determined by DSC. The drop in conductivity is explained using a parameter defined as gel structure factor (F).

Tunable core–shell nanoparticles: macro-RAFT mediated one pot emulsion polymerization

Simple and robust one-pot synthesis of polystyrene (PS) nanoparticles composed of a poly(hydroxyethyl methacrylate) (PHEMA) based macro-RAFT (reversible addition fragmentation chain transfer) agent has been developed. Initially generated hydrophilic PHEMA macro-RAFT agents were chain-extended in situ with a hydrophobic monomer (styrene) to form nanoparticles with core–shell morphology in which the PS core is covered with the PHEMA shell. Nanoparticle sizes and the dimensions (thickness) of core and shell were readily tuned by varying hydrophilic monomer (HEMA) concentration and polymerization time for the formation of a PHEMA based macro-RAFT agent. Field emission scanning electron microscopy showed the influence of molecular weight of the hydrophilic macro-RAFT agent on polymer particle size and self-assembly. The results suggested that unreacted HEMA during the nucleation process has a significant influence on the polymer morphology. In summary, we demonstrated that the current strategies can be effectively used to generate core–shell particles along with dimension tuning in a one-pot process.