Manohar V. Badiger

Porogens in the preparation of microporous hydrogels based on poly(ethylene oxides)

To enable the synthesis of hydrogels containing microporosity, a family of pore generating additives (porogens) were synthesized from poly(ethylene glycols) (PEGs) with different molecular weights using naphthyl acetic acid. The products formed were characterized by UV-vis, differential scanning calorimetry and solution nuclear magnetic resonance techniques. Subsequently, poly(ethylene oxide)-urethane hydrogels were synthesized incorporating the above mentioned PEG porogens to modify the structure of the hydrogel. These highly water soluble PEG porogens were inert and extracted out in water. The hydrogels obtained exhibited significant increase in the equilibrium water uptake. This was attributed to the formation of a microporous structure in the hydrogel. It was also evidenced by the observed increase in the diffusion coefficients of the drugs proxyphylline and vitamin B12 through this hydrogel. The proportional increase was greater for the higher molecular weight vitamin B12 than for the proxyphylline. These results may be useful in developing porous hydrogels for controlled release technology.

Thermodynamics of hydrogen-bonded polymer gel-solvent systems

A statistical thermodynamic theory, which accounts for hydrogen-bonding interactions between polymeric gels and solvents, is developed. The theory is shown to provide quantitative predictions of swelling behavior of poly(ethylene oxide) gels in chloroform and water and qualitative predictions of thermoreversible volume transitions of poly(N-isopropyl acrylamide) (PNIPA) gel in water. At the LCST of PNIPA gel, the theory predicts a sharp increase in the number of hydrogen-bonds formed between polymer molecules of the gel and a sharp decrease in the hydrogen-bonds formed between polymer molecules and water molecules. The predictions of this theory can have significant implications in designing smart gels based on hydrogen-bonding interactions. Such gels have applications in separations and in biomedical technology.

Unusual retardation and enhancement in polymer dissolution : role of disengagement dynamics

This work addresses the issue of resolution of some unusual observations in the swelling-dissolving problem in polymers. There are observations showing several fold increase in the rate of dissolution when the residual solvent content is increased only marginally. Additionally, by using an in situ NMR technique to study dissolution, we have found the existence of an intermediate plateau in the dissolution curve indicating an unusual retardation in the rate of dissolution under certain conditions. Interestingly, this plateau disappears on either increasing the molecular weight of the polymer or on doing dissolution under shearing rather than under stagnant conditions. An improved model is proposed to explain these results, which cannot be explained by the existing models. The new feature is the incorporation of a kinetic model to relate the disengagement rate to the swelling rate, through the changing mobility of the disengaging macromolecule at the gel-liquid interface. The present study thus enables a clearer elucidation of the role of disengagement dynamics in the dissolution process.

Design of a high performance thin all-solid-state supercapacitor mimicking the active interface of its liquid-state counterpart.

Here we report an all-solid-state supercapacitor (ASSP) which closely mimics the electrode-electrolyte interface of its liquid-state counterpart by impregnating polyaniline (PANI)-coated carbon paper with polyvinyl alcohol-H2SO4 (PVA-H2SO4) gel/plasticized polymer electrolyte. The well penetrated PVA-H2SO4 network along the porous carbon matrix essentially enhanced the electrode-electrolyte interface of the resulting device with a very low equivalent series resistance (ESR) of 1 Ω/cm(2) and established an interfacial structure very similar to a liquid electrolyte. The designed interface of the device was confirmed by cross-sectional elemental mapping and scanning electron microscopy (SEM) images. The PANI in the device displayed a specific capacitance of 647 F/g with an areal capacitance of 1 F/cm(2) at 0.5 A/g and a capacitance retention of 62% at 20 A/g. The above values are the highest among those reported for any solid-state-supercapacitor. The whole device, including the electrolyte, shows a capacitance of 12 F/g with a significantly low leakage current of 16 μA(2). Apart from this, the device showed excellent stability for 10000 cycles with a coulombic efficiency of 100%. Energy density of the PANI in the device is 14.3 Wh/kg.

Molecular tailoring of thermoreversible copolymer gels: Some new mechanistic insights

We earlier reported the role of hydrophobic and hydrogen bonding interactions on the transition temperatures of thermoreversible copolymer gels. We show here that the chemical structure of the hydrophobe and its concentration determine the transition temperatures [lower critical solution temperature (LCST)] and the heat of transition of new hydrophobically modified poly(N-isopropyl acrylamide) [PNIPAm] copolymer gels. The gels, prepared by copolymerizing NIPAm monomer with hydrophobic comonomers containing increasing lengths of alkyl side groups and a terminal carboxyl acid group, showed lower LCST and lower heat of transition when compared to pure PNIPAm gel. The experimental results were also compared with theoretical calculations based on a lattice-fluid-hydrogen-bond [LFHB] model. We show experimentally and theoretically that a linear correlation exists between the transition temperature and length of the hydrophobic alkyl side group. Also, in apparent contradiction to previous work, we found a reduct...

Thermoreversible hydrogel based on radiation induced copolymerisation of poly(N-isopropyl acrylamide) and poly(ethylene oxide)

Thermoreversible copolymer hydrogel based on poly(ethylene oxide) and poly ( N -isopropyl acrylamide) has been prepared by γ-radiation technique. The utility of 13 C n.m.r. spectroscopy in elucidating the structure and copolymer composition has been demonstrated. The volume transition as a function of temperature in these copolymers has been studied by swelling ratio measurements. Unlike poly(ethylene oxide) homopolymer gel, the copolymer gels show first order volume transition in the temperature range of 35–40°C. These gels are easy to synthesise in any shape and size and are found to be having good mechanical strength even in the fully swollen state. They can have potential applications in controlled drug delivery, bioseparations and biomedical fields.

Concentration of macromolecules from aqueous solutions: A new swellex process

A new swellex process (extraction through swelling polymers) to concentrate macromolecules from aqueous solutions has been demonstrated. Dilute solutions of biological macromolecules such as proteins have been concentrated with good recoveries and high selectivity. The selectivity was controlled by the extent of crosslinking in the gels. Electrolyte solution when pulsed through the column, causes the collapse of the gel. The reversible volume phase transition has been exploited for the regeneration of the gels.

High-Performance Flexible Solid-State Supercapacitor with an Extended Nanoregime Interface through in Situ Polymer Electrolyte Generation

Here, we report an efficient strategy by which a significantly enhanced electrode-electrolyte interface in an electrode for supercapacitor application could be accomplished by allowing in situ polymer gel electrolyte generation inside the nanopores of the electrodes. This unique and highly efficient strategy could be conceived by judiciously maintaining ultraviolet-triggered polymerization of a monomer mixture in the presence of a high-surface-area porous carbon. The method is very simple and scalable, and a prototype, flexible solid-state supercapacitor could even be demonstrated in an encapsulation-free condition by using the commercial-grade electrodes (thickness = 150 μm, area = 12 cm(2), and mass loading = 7.3 mg/cm(2)). This prototype device shows a capacitance of 130 F/g at a substantially reduced internal resistance of 0.5 Ω and a high capacitance retention of 84% after 32000 cycles. The present system is found to be clearly outperforming a similar system derived by using the conventional polymer electrolyte (PVA-H3PO4 as the electrolyte), which could display a capacitance of only 95 F/g, and this value falls to nearly 50% in just 5000 cycles. The superior performance in the present case is credited primarily to the excellent interface formation of the in situ generated polymer electrolyte inside the nanopores of the electrode. Further, the interpenetrated nature of the polymer also helps the device to show a low electron spin resonance and power rate and, most importantly, excellent shelf-life in the unsealed flexible conditions. Because the nature of the electrode-electrolyte interface is the major performance-determining factor in the case of many electrochemical energy storage/conversion systems, along with the supercapacitors, the developed process can also find applications in preparing electrodes for the devices such as lithium-ion batteries, metal-air batteries, polymer electrolyte membrane fuel cells, etc.

PREDICTION OF RE-ENTRANT SWELLING BEHAVIOR OF POLY(N-ISOPROPYL ACRYLAMIDE)GEL IN A MIXTURE OF ETHANOL-WATER USING LATTICE FLUID HYDROGEN BOND THEORY

The re-entrant volume phase transition of poly(N-isopropyl acrylamide) gel in ethanol–water mixtures has been predicted by using the extended lattice-fluid-hydrogen bond (LFHB) theory. In our calculations we do not make any arbitrary assumptions for the polymer–solvent interaction parameters. Instead, we determine the interaction parameters by fitting the LFHB theory to the swelling data of the gel in each of the solvents. In addition to predicting the re-entrant transition, the theory predicts selective absorption of ethanol over water, particularly by the collapsed gel. Simultaneously, the hydrogen bonding between water and ethanol is predicted to be enhanced in the presence of the gel. The interpolymer hydrogen bonds increase during the gel collapse region. The polymer–ethanol hydrogen bonds increase and the polymer–water hydrogen bonds decrease continuously with increasing ethanol composition in the outside phase. These predictions are in qualitative agreement with experimental observations and overcome the empiricism in previous theoretical work. A variety of qualitatively different swelling behaviors of gels in mixed solvents is also predicted for varying hydrophilic–hydrophobic balance in the chemical structure of the gels.

Proton magnetic resonance imaging in hydrogels: volume phase transition in poly(N-isopropylacrylamide)

Proton Magnetic Resonance (PMR) Imaging has been used to study the volume-phase-transition in a thermoreversible gel. This is demonstrated in the LCST polymer poly(N-isopropylacrylamide), swollen in water. The volume-phase-transition is conveniently monitored through one-dimensional proton images. PMR images, derived from relaxation and diffusion weighted planar spin-echo experiments, provide insights on the hydration state of water and the motional state of polymer, in the fully swollen and collapsed states of the gel.