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.
Abstract 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. magnified image Thermoreversible gelation of polybenzimidazole in phosphoric acid and the membrane produced from the gel.
Abstract Progress in using biomimetic micropatterning of surfaces for tuning of wetting and adhesion properties of polymeric materials is reported. Surface patterning with low aspect ratios and low surface energy leads to superhydrophobicity and low adhesion while surface patterning with high aspect ratios leads to high adhesion. The condition of high aspect ratio and/or high number density creates the risk of fibril bundling owing to the adhesion between fibrils. A thin film‐terminated structure can help overcome this problem and increase adhesion. In contrast to the spatular contacts, the conformal adhesion at interface can be enhanced significantly by creating micropatterns; the enhancement monotonically increases with the aspect ratios, where the side‐wall friction plays an important role. magnified image
Autonomous self-healable crosslinked materials designed with built-in ability to repair physical damage and cracks can prevent catastrophic failure and thus extend the lifetime of materials. They also retain their dimensional stability, mechanical strength, thermal stability, and solvent resistance. These features promote the development of effective self-healing materials for various applications. This review summarizes recent advances in the development of novel self-healable polymeric materials, both through extrinsic methods involving the encapsulation of extra healing agents in microcapsules and through intrinsic methods utilizing the formation of reversible chemical or physical crosslinks. Further, the outlook is briefly discussed on the important aspects for the current and future development of self-healable materials.
This chapter contains sections titled: Introduction Polymer Nature of Paper Functional Polymers and Sizing Agents Used in Papermaking Polymer Adhesion and the Formation of Paper Polymer Adhesion Measurement Summary and Perspectives References
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.
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cg id='b'%3e%3cpath id='a' stroke='%23000' stroke-width='2' d='M-6-25H6m-12 3H6m-12 3H6'/%3e%3cuse xlink:href='%23a' y='44'/%3e%3c/g%3e%3cpath stroke='%23fff' d='M0 17v10'/%3e%3ccircle r='12' fill='%23cd2e3a'/%3e%3cpath fill='%230047a0' d='M0-12A6 6 0 0 0 0 0a6 6 0 0 1 0 12 12 12 0 0 1 0-24z'/%3e%3c/g%3e%3cg transform='rotate(-123.69)'%3e%3cuse xlink:href='%23b'/%3e%3cpath stroke='%23fff' d='M0-23.5v3M0 17v3.5m0 3v3'/%3e%3c/g%3e%3c/svg%3e)