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Although pyridine bridged oxypolybenzimidazole (PyOPBI) membranes are considered to be promising high-temperature proton exchange membrane (HT-PEM) materials that have the potential to overcome man...

Metal organic frameworks (MOFs) have received considerable importance as proton conducting materials in recent times. However, most of the MOFs lack the ability to form film, which limits their app...

Despite several unique advantages, high-temperature proton-exchange membrane fuel cells (HT-PEMFCs) based on polybenzimidazole (PBI) membranes suffer from various drawbacks like weak chemical resis...

A facile method for the simultaneous delamination and the lateral size reduction of layered double hydroxides (LDH) is reported. This method directly resulted in the delaminated mesoporous LDH nanoplatelets (nanodot LDH) with the high specific surface area (lateral dimensions as low as 10–30 nm and featured a thickness of ∼1 nm). Such prepared LDH was used as fillers for isotactic polypropylene (iPP). For the purpose of comparison, LDH having different surface areas were also used as fillers for iPP. The incorporation of nanodot LDH showed a remarkable improvement in the polymer properties with only 1 wt % loading. The uniformly dispersed LDH particles have a significant effect on the nucleation ability, thermal stability, and mechanical properties of iPP. The nucleation ability of iPP in the presence of nanodot LDH is the best compared to other iPP nanocomposites reported using LDH as fillers in the literature. Furthermore, the microstructure of the iPP nanocomposites was systematically investigated at m...

The weak binding between carbohydrates and proteins is a major constraint toward the development of carbohydrate-based therapeutics. To address this, here we report the synthesis of glycopolymer (GP)-grafted silica nanoparticles (SiNP) by using reversible addition–fragmentation chain transfer (RAFT) polymerization through the grafting-from approach using a multistep process. GP chains of various lengths with controlled molecular weight and narrow polydispersities were grown on the RAFT agent anchored SiNP surface using mannosyloxyethyl methacrylate (MEMA) as a glycomonomer. Spectroscopic (FT-IR, NMR) and thermogravimetric studies confirmed the grafting of poly(MEMA) chains on the SiNP surface and also showed that the dry DMF is a better solvent as compared to water/ethanol mixture for carrying out the MEMA polymerization on SiNP surface. The mean diameter of the dry GP-grafted SiNPs (GP-g-SiNPs) obtained from microscopic studies was in the range 50–60 nm, whereas the hydrodynamic diameter as obtained usin...

Preparation of stable, free-standing elastic film with hydrophobic surface from water-dispersible polyurethanes (WDPUs) is a challenging task. Here, we have prepared WDPUs from polybutadiene-based diols and the resulting PU films are satisfying the requisites. Two types of diols, namely, hydroxyl-terminated polybutadiene (HTPB) and terminal-functionalized HTPB with dinitrobenzene (DNB) [HTPB-DNB], were used to make WDPUs and the effects of diol structures were investigated. The synthesized WDPUs displayed particle morphology with size in the range of ∼130–270 nm and more than 1 year storage stability. WDPUs yielded stable and free-standing films upon curing and the resulting films showed high thermal and mechanical stabilities. The contact angle of the films is in the range of ∼80–100°, attributing the hydrophobic nature of the surface. Hard segment content of the PU varied from 30 to 40% (by weight) to tune the properties of WDPUs and the resulting films.

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.

Chemical modifications of hydroxyl terminated polybutadiene (HTPB) with hydrogen bond forming functionalities were used as tactics to improve both tensile strength and elongation of polyurethanes (PUs) simultaneously. PUs were prepared using various diisocyanates with modified HTPB in which dinitrobenzene (DNB) groups are attached to terminal carbon atoms. The spectroscopic studies revealed the presence of an additional hydrogen bonding network between DNB and the urethane backbone which resulted into supramolecular cross-linking and was found to be responsible for significant improvement in mechanical properties of HTPB-DNB-PUs. Also, substantial improvement of elongation at break was observed in the case of HTPB-DNB-PUs. Small angle X-ray scattering (SAXS) and thermodynamical studies indicated a strong segmental mixing between the hard and soft segments of HTPB-DNB-PUs. Growth of partial crystalline character in HTPB-DNB-PUs was believed to be responsible for “fibrous-assembly” morphology. In summary, m...

Tetraamine 2,6-bis(3′,4′-diaminophenyl)-4-phenylpyridine (Py-TAB) was found to be an efficient, readily accessible, inexpensive monomer for synthesizing pyridine bridge polybenzimidazoles (Py-PBIs). The Py-TAB monomer replaced the frequently and conventionally used 3,3′,4,4′-tetraaminobiphenyl (TAB) monomer for synthesizing polybenzimidazoles (PBIs) structure. Py-TAB monomer showed wider scope for synthesis of PBIs with various dicarboxylic acids. Py-PBIs displayed superior solubility in low boiling solvent and hence eliminated the processability issues of PBIs which often restricted the large scale use. Comparable and in some instances superior thermal, mechanical and oxidative stability of Py-PBI proved that the Py-TAB is the most attractive alternative tetraamine monomer to TAB for synthesis of PBIs. The membranes obtained from Py-PBIs displayed significantly lower water uptake and swelling than the conventional PBIs due to the presence of bulky Py-TAB monomer. The presence of pyridine moiety in Py-PBI...

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.

Polymer nanocomposites of poly (4,4′-diphenylether-5,5′-bibenzimidazole) (OPBI) were prepared with two structurally different organoclays, namely, montmorillonite (OMMT) and kaolinite (OKao), to evaluate the effect of clay structures on the nanocomposites morphology, structure, and the properties. Solid State 13C cross-polarization magic-angle spinning nuclear magnetic resonance, small-angle X-ray diffraction, and transmission electron microscopy studies suggested the formation of exfoliated structure for OPBI/OMMT, whereas and intercalated structure was obtained for OPBI/OKao. Both the nanocomposites displayed significant enhancement in the thermal stabilities compared to the pristine OPBI, and the higher thermal stability of OPBI/OMMT than that of OPBI/OKao was attributed to the higher degree of dispersion of the nanoclay into the OPBI matrix owing to the exfoliated structure of the former. Both the nanocomposites membranes exhibited large mechanical reinforcement by the clays and the superior mechanica...

The aggregation behavior of poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) in polar aprotic (dimethyl acetamide, DMAc) and protic (formic acid, FA) solvents is studied as a function of the polymer concentration and solution temperature. The effects of solvent protic character on the aggregation behavior of OPBI are elucidated. The photophysical studies suggest that the OPBI chains form aggregated structures in both DMAc and FA solutions when the OPBI concentration is increased. The dependences of the emission spectra on the polymer concentrations in two solvents are not similar in nature, indicating that in both of the solvents the aggregations are intermolecular processes, though their mechanisms are different owing to the polyelectrolytic nature of OPBI in FA medium. The triexponential decay profiles obtained from the time-resolved fluorescence study for the concentrated solutions (both in DMAc and FA) display a negative fractional coefficient and longer excited state lifetime, providing support for the aggregations at higher concentration. The temperature dependence emission spectra suggest that the aggregations in both of the solvents destabilize with increasing temperature. The higher activation energy of aggregation (E(A)) in DMAc (5.62 KJ/mol) compared with that in FA (3.07 kJ/mol) reveals that the aggregation formation pathways are different in two solvents and stronger aggregates are formed in the former solvent. The dilute solution viscometry (DSV) studies demonstrate that the OPBI chains adapt a bigger extended conformation in FA compared with DMAc owing to the stronger intramolecular chain repulsion in FA arising due to the polyelectrolyte nature of OPBI in this solvent. A conformation transition of OPBI chains from compact collapsed to extended conformer is observed in DMAc solvent with increasing concentration, whereas any such transition is absent in FA medium. Transmission electron microscope (TEM) images and circular dichroism (CD) spectra are also in agreement with the presence of a conformational transition in DMAc and the absence of it in FA. The temperature dependent DSV studies further support the disruption of aggregated structure with increasing temperature in both of the solvents. DSV studies exhibit that the deaggregation is driven by a conformation transition (extended to compact collapsed) in DMAc, whereas in FA the disruption happens without conformational transition.

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 theCF(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.

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.

We report a study of aggregation behavior of polybenzimidazole (PBI) in polar aprotic solvents such as dimethylacetamide (DMAc). The photophysical studies of the PBI solution at various concentrations show concentration quenching and reveal that aggregated structures are formed when the polymer concentration is increased. The decay profiles obtained from time-resolved fluorescence study for low (0.00154 g/dL) and high (0.154 g/dL) concentrations of PBI in DMAc solution fit into a triexponential decay, surprisingly high concentration shows a growth (negative pre-exponential factor) in the decay profile, providing a support for excimer formation. The excited-state life time for the aggregated/excimer structure is found to be 4.14 ns, longer than that for the free polymer chains for which the life time is 502 ps. The concentration dependence emission spectra attribute that the aggregation/excimer formation is an intermolecular process. An abrupt decrease of Huggins constant and reduced viscosity with increase in concentration indicate the conformational transition of polymer chains of PBI from compact coil to an extended helical rodlike structure. The NMR and viscosity studies demonstrate that the intra- and intermolecular interactions (interchain hydrogen bonding) play an important role for the conformational transition and aggregation process. Transmission electron microscope images support the conclusion drawn from other studies; show helical rods for high concentration and featureless morphology for low concentration. The circular dichroism spectrum is also in agreement with the helical characteristics of aggregated structure. The temperature-dependent NMR and viscosity studies show that the disruption of interchain hydrogen bonding with increasing temperature destabilizes the aggregated structure at higher temperature.

The surface morphology of the thermoreversible polyaniline (PANI) gels prepared with dinonylnaphthalenesulfonic acid (DNNSA), dinonylnaphthalenedisulfonic acid (DNNDSA), (±)-camphor-10-sulfonic acid (CSA), and n-dodecyloxy sulfonic acid (DOSA) are studied using atomic force microscopy (AFM) for 15% PANI concentration (w/w). The AFM study clearly reveals the formation of lamellar morphology in the gel. X-ray scattering experiments of these gels also support the lamellar structure formation, and the lamellar thickness measured from it remains invariant with composition for the PANI−DNNSA and PANI−DNNDSA systems. In PANI−CSA and in PANI−DOSA systems the lamellar thicknesses vary with PANI concentration. Both X-ray diffraction and electron diffraction experiments on the gels of different compositions of PANI−DNNSA and PANI−DNNDSA systems indicate new spacings of lower dhkl values which are invariant with composition. Similar observations are also found in PANI−CSA and PANI−DOSA systems. These results indicate...

Thermoreversible gelation behavior of polyaniline (PANI) in the presence of varying concentrations of sulfonic acids are studied. Four different sulfonic acids are used, e.g., dinonylnaphthalene sulfonic acid (DNNSA), dinonylnaphthalene disulfonic acid (DNNDSA), (±)-camphor-10-sulfonic acid (CSA), and n-dodecyloxo sulfonic acid (DOSA). The composition range studied here is W PANI = 0.05-0.80 (W PANI is the weight fraction of PANI in the gel). It has been found from the SEM study that for all the sulfonic acids in the composition range W PANI = 0.05-0.40, fibrillar network is present. The TEM study also supports the above viewpoint. The thermodynamic study of the gels has been done by DSC-7, and for all the systems broad peaks consisting of two fused gel melting/gel formation peaks are observed. After proper deconvolution of the two peaks the gel melting/gel formation temperatures are measured. When they are plotted with W SO3H (weight fraction of sulfonic acid), typical phase diagrams consisting of two almost parallel curves are found. They are explained by considering the lamellar model where the bilayer and monolayer portions of the surfactant form different crystalline domains. The higher melting point curve is due to the former and the lower one is due to the later in each phase diagram. The lowering of gel melting/gel formation points with increase in PANI concentration of each curve has been attributed to the dilution effect of PANI in the gel. The corresponding enthalpy values (ΔH) also show similar decrease with increasing PANI concentration and are explained also from the dilution effect of PANI. The conductance of these gels varies with weight fraction of PANI (W PANI ), showing a maximum, and it has been explained by considering that conductivity in the gel is due to both intrachain and interchain contributions. Attempt is made to discuss the above results by using a molecular model of the PANI-DOSA system with the help of MMX program.

Regioregular poly(3-hexylthiophene) (P3HT) produces thermoreversible gel in xylene. The gel is brownish-red in color. SEM and TEM studies indicate the presence of fibrillar network. WAXS and electron diffraction pattern indicate the presence of P3HT crystallites in the gel. The gels exhibit a first-order phase transition when heated in DSC. A time-dependent UV−vis study indicates that gelation in this system is probably accompanied by two different processes, e.g., (1) coil-to-rod transformation and (2) aggregation of rods to form the crystallites producing the gel. The gelation rate (tgel-1) measured from the test tube tilting method is analyzed using the equation tgel-1 = f(C) f(T), where f(c) = φn, φ being the reduced overlapping concentration and n is an exponent. The average “n” value determined is 0.52, which indicates that three-dimensional percolation is a suitable model for this gelation. The gelation rate is analyzed according to the Flory−Weaver theory of coil-to-helix transition, and the free ...