Search

In this report, non-hydrogen bonded, segmented polyurethane (SPU) of ferrocene functionalized polybutadiene diol has been synthesized and studied in depth to understand the role of ferrocene moiety in the structure-property relationship of SPU. Interactions among hard segments of SPU through the ferrocene moiety played a vital role in the separation of hard and soft segments. Ferrocene attached hydroxyl terminated polybutadiene (HTPB) diol was polymerized with isophorone diisocyanate followed by curing at 70 °C to obtain polyurethane (PU). Dynamic mechanical analysis (DMA) and an in-depth small angle X-ray scattering (SAXS) studies revealed the presence of a strong microphase separation between hard and soft segments which resulted in the formation of nano hard segment domain (HSD) in PU and thus yielded SPU. Though spectroscopic studies of SPU could not identify any hydrogen bonding between the PU chains but due to the presence of ferrocene moiety on the adjacent carbon of urethane group, favorable electrostatic interaction between the hard segments has been identified. This interaction was found to be the driving force for the formation of micrometer scale assemblies of nano HSD, as clearly seen in scanning electron microscopy, and the size of nano HSD assembly increases with increasing ferrocene content in the HTPB. This dependency confirming that the presence ferrocene functional moiety is responsible for formation of assembly of nano HSD which in turn enhanced tensile strength and aggregation induced emission intensity of the resulting SPU.

In the present work, proton exchange membrane (PEM) based on series of polybenzimidazole (PBI) composites are prepared with acidic surfactant like molecules (ASMs) with an objective to improve properties of PEM especially proton conductivity. Composites are obtained by homogenizing poly (4, 4′-diphenylether-5, 5′-bibenzimidazole) (OPBI) in three different ASMs namely camphorsulfonic acid (CSA), p-toluenesulfonic acid (PTSA) and mono-n-dodecyl phosphate (MDP). FT-IR and solid-state NMR studies indicate the presence of interactions between OPBI and ASMs which are necessary for obtaining homogeneous composite membranes. Mechanical reinforcement is observed in case of composite membranes and storage modulus increases with increasing ASM loading in the composite. The detailed thermal analysis shows that phosphoric acid loaded (PA) composite membranes have higher thermal stability than the PA loaded pristine OPBI, it increases with increasing loading of ASM and it largely depends upon the type of ASM in the composite. Though OPBI is an amorphous polymer but ASMs self-organizes themselves in the polymer matrix owing to the strong interaction between OPBI and ASMs. As a result composite membranes display the crystalline character which in turn significantly influences the morphological features of the composites. Fibrillar to porous morphology are observed in composites depending on the type and loading of ASM. This morphological features and the crystalline nature of the composites are found to be responsible for mechanical reinforcement and significant increase in PA loading. The PA doped pristine OPBI, OPBI/CSA-20%, OPBI/PTSA-20% and OPBI/MDP-20% composite membranes proton conductivities are 8.6 × 10−2 S/cm, 2.82 × 10−1 S/cm, 1.71 × 10−1 S/cm and 1.43 × 10−1 S/cm, respectively at 180 °C. OPBI/ASM composite membranes also display very low acid leaching in comparison to pristine OPBI owing to the formation of strong interaction between PA and polymer chains through ASMs.

Series of metal based polyurethanes (PUs) have been synthesized from hydroxyl terminated polybutadiene (HTPB) with combination of a potential energetic material, 2,4-dinitrobenzene (DNB) and burn rate (BR) enhancer, ferrocene with two objectives: (1) exerting flexibility into otherwise brittle PU film and (2) increasing BR of composite solid propellant (CSP). HTPB was first functionalized with DNB at the terminal carbons and then ferrocene was grafted radically as poly (vinyl ferrocene) (PVF) chain onto the pendant vinyl bond of HTPB. The degree of PVF grafting was altered by appropriate reaction recipes to find out the effect of Fe content on various physical properties including fluidity of the HTPB. Density function theory (DFT) calculation showed that the dominating intra-chain interactions over inter-chain owing to the strong interactions between NO2 of DNB and cyclopentadiene of ferrocene are the driving force for improvement in various physical properties of PVF-grafted-HTPB-DNB. Cyclic voltammetry (CV) measurement showed one electron reversible redox behavior of the grafted PVF polymer chain with slow electron transfer process. Further the mechanical stability, thermal stability and properties of PUs have been studied thoroughly; the results indicated a strong influence of DNB and ferrocene in the chains on the physical properties. All the DNB modified PU membranes displayed exceptionally enhanced flexibility along with much lower Tg value compared to neat PVF-g-HTPB-PU. The presence of DNB at the chain end of soft segment (SS) causes strong segmental mixing between SS and hard segment (HS) domains which helps in enhancing the elasticity of SS chain by increasing the inter polymer chain distance. Morphology of the hard segment domains formation has been probed by small angle X-ray scattering (SAXS) and further confirmed by FESEM. Burn rate of composite solid propellant made from the HTPB-DNB-g-PVF binder is found to be ~18% larger than the HTPB-DNB.

Despite the presence of bulk literature on polybenzimidazole (PBI), unavailability of readily soluble and processable PBI remains as the tallest challenge for the end-use. N-alkyl PBIs (N-PBIs) were synthesized by grafting the alkyl chain in the imidazole backbone to resolve this key constrain. The chain length of substituted alkyl groups was varied to evaluate the influence of chain size on the structures and properties of N-PBIs. Significant enhancement of solubility of N-PBIs compared to parent PBI in formic acid offered the opportunity to fabricate the homogeneous mechanically tough membranes with minimal efforts. The substituted long alkyl chains pushes apart the PBI chains and hence increases the face-to-face packing distance by breaking the self-association between the chains; resulted into less rigid highly soluble N-PBIs. Alkyl chain length dependent weight loss at ∼300 °C, presence of two glass transition temperatures and peculiar deep rubbery modulus in storage modulus vs. temperature plots resembled the copolymer molecular structure of N-PBIs. Hydrophobic character of alkyl chains and loosely packed structure of N-PBIs facilitated decrease in water uptake and swelling of the N-PBI membranes compared to parent PBI membrane. The temperature dependent proton conductivity of PA loaded N-PBIs membranes were found to be satisfactory.

Search of thermo-chemically and mechanically stable PEM with high proton conducting ability for the use in high temperature fuel cell motivated us to develop PEM from the blend of polybenzimidazole (PBI) with poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The formation of blend (miscibility of the two polymeric components) is confirmed by studying varieties of spectroscopic and thermodynamical techniques. The results obtained from these studies attribute the presence specific interactions and partial miscibility between the two polymers. The hydrophobicity of PVDF-HFP controls the amount of phosphoric acid (PA) loading and is found to be the responsible for lower water uptake of the blend membrane compared to pristine PBI. The blending significantly reduces the increase in thickness and swelling ratio of the membranes upon doping with PA. The presence of fluorine atoms of PVDF-HFP in the blend membranes helps to obtain significantly higher proton conductivity than the native PBI and enhance the oxidative stability substantially. PA doped blend membranes display superior mechanical stability compared to neat PA doped PBI. Hence the current blend membranes bear all the essential physical characteristics which are required for the PEM to be use in fuel cell.