Niraj Kumar Vishwakarma

Synthesis and Characterization of Stereocontrolled Poly(N-isopropylacrylamide) Hydrogel Prepared in the Presence of Y(OTf)3 Lewis Acid

Macroporous poly(N-isopropylacrylamide) (PNIPAM) hydrogels have been prepared in methanol-water (1:1, v/v) mixture in the presence of 0, 0.05, 0.1, 0.15, and 0.2 M Y(OTf)(3) Lewis acid concentrations. Synthesis of the corresponding linear PNIPAM homopolymers in the absence of a cross-linker keeping all other conditions the same shows that the isotacticity (meso dyad, %) and the cloud point temperature of the resulted in polymers increases and decreases, respectively, with the increase in the concentration of the Lewis acid. SEM micrographs reveal that the resulted hydrogels are highly porous. Swelling ratios of all the hydrogels in water decrease with the increase in the temperature. Moreover, swelling ratios of all the hydrogels in different methanol-water mixtures pass through a minimum in the co-nonsolvency zone, and the co-nonsolvency zone shifts toward the lower methanol-content solvent mixture with gradual increase in the Lewis acid concentration. Deswelling rate of the hydrogel prepared in methanol-water (1:1, v/v) mixture is much faster than that of the conventional hydrogel prepared in water. Moreover, the deswelling rate slightly increases with the hydrogels prepared with the increasing concentrations of Lewis acid. But, the reswelling rate of the hydrogels follows almost the reverse order. All these results have been explained on the basis of the formation of highly porous hydrogels with higher isotactic PNIPAM chain segment owing to the faster polymerization rate in the methanol-water mixture in the presence of Lewis acid and the co-nonsolvency behavior of the methanol-water (1:1, v/v) mixture toward PNIPAM chain segment in the PNIPAM hydrogel.

Synthesis and study of the properties of stereocontrolled poly(N-isopropylacrylamide) gel and its linear homopolymer prepared in the presence of a Y(OTf)3 Lewis acid: effect of the composition of methanol-water mixtures as synthesis media.

Poly(N-isopropylacrylamide) (PNIPAM) hydrogels and the corresponding linear homopolymers were synthesized in different methanol-water mixtures (x(m) = 0, 0.13, 0.21, 0.31, 0.43, 0.57, and 0.76, where x(m) is the mole fraction of methanol) in the presence of 0.1 M Y(OTf)(3) Lewis acid. The isotacticity (meso dyad (m), %) and cloud-point temperature of these homopolymers were gradually increased and decreased, respectively, with the increase in the x(m) values of the synthesis solvent mixtures. Moreover, the corresponding linear PNIPAM homopolymers prepared in the absence of Y(OTf)(3) showed an almost constant isotacticity of m = 45% and a cloud-point temperature of 33.0 °C. A SEM study revealed that the resulting hydrogels were highly porous except for the gels prepared at x(m) = 0 and 0.76. The swelling ratios of these hydrogels in water at different temperatures and in different methanol-water mixtures at 20 °C and the deswelling rate and the reswelling rate of these hydrogels were studied. All of these swelling results were compared with that of the corresponding gels prepared in the absence of a Lewis acid (Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Bhimireddi, R.; Rai, R.; Ray, B. J. Appl. Polym. Sci.2012, DOI: 10.1002/app.36318) and explained on the basis of the porosity of the gel, the state of aggregation and isotacticity of the PNIPAM chain segment, and the cononsolvency of the methanol-water mixture toward the PNIPAM chain segment.

Synthesis, characterization, and solution behavior of well-defined double hydrophilic linear amphiphilic poly ( N -isopropylacrylamide)- b -poly ( ε -caprolactone)- b -poly ( N -isopropylacrylamide) triblock copolymers

Well-defined linear dihydrophilic amphiphilic ABA-type triblock copolymers of ε-caprolactone (CL) and N-isopropylacrylamide (NIPAAm) have successfully been synthesized with a high yield by combining the ring opening polymerization (ROP) and xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization methods. The resulted block copolymer shows the formation of micelles in water as supported by light scattering. The critical micelle concentration (cmc) value of the micelle increases with the increase in the chain length of the poly (N-isopropylacrylamide) (PNIPAAm) block. Cloud point of the block copolymers decreases with the decrease in the PNIPAAm chain length. The TGA analysis shows a one-step degradation and a lower thermal stability of the triblock copolymer than the PNIPAAm. The DSC analysis of the triblock copolymer shows the lowering of glass transition temperature (Tg), and melting temperature (Tm) peaks possibly due to the partial miscibility of the poly (ε-caprolactone) (PCL) block with the amorphous PNIPAAm block through the interaction of ester groups of PCL with the amide groups of PNIPAAm. The XRD pattern of the triblock copolymer shows a broad peak due to the suppression of the crystallization of PCL block owing to the mixing of PNIPAAm block with the PCL block.

Nanochannel conduction in piezoelectric polymeric membrane using swift heavy ions and nanoclay

A poly(vinylidene fluoride) nanohybrid membrane has been prepared using high energy swift heavy ions (SHI) followed by chemical functionalization. Through-channels were generated by selective etching of the amorphous latent track created during irradiation of SHI, whose average dimensions are varied from 52 nm in pure PVDF to 40 nm for the nanohybrid in the presence of nanoclay, indicating suitable membrane formation using the nanohybrid, as compared to pure PVDF. The nanochannels were suitably grafted with conducting polymer (pyrrole) using the active free radicals caused by SHI irradiation. Spectroscopic and gel permeation chromatographic experiments have been performed to understand the effect of irradiation on filled polymer and subsequent functionalization. Doping and sulphonation on the grafted species has been done to introduce a variety of functionalities. The effect of fluence on the channel width and functionalization study indicates a larger diameter of the channels and formation of a greater ionomer at higher fluence, leading to enhancement of the bulk electrical conductivity up to 10−3 S cm−1 (12 orders higher in magnitude than pure PVDF). The activation energy of the modified nanohybrid membrane is calculated to be 26.9 kJ mol−1, compared to 10.6 kJ mol−1 of pure PVDF, indicating superior stability of the nanohybrid membrane at higher temperature as compared to pure PVDF or functionalized PVDF under similar conditions. The nanohybrid conducting membrane produces a current of 46 nA compared to a value of almost zero for pristine PVDF or NH measured through I–V characteristic curves against a bias voltage of 10 V, making the nanohybrid suitable for electrochemical applications.

Synthesis of fluorescence poly(N-vinylpyrrolidone) via click chemistry using azide-terminated xanthate mediator (S)-2-(4-azidobutyl propionate)-(O-ethyl xanthate)

ABSTRACT Azide-terminated xanthate RAFT agent (S)-2-(4-azidobutyl propionate)-(O-ethyl xanthate) has been synthesized and used for the controlled radical polymerization of N-vinylpyrrolidone (NVP). Kinetics study showed the pesudo first-order kinetics along with gradual increase in molecular weight (Mn) of the resulted polymer up to 69% conversion. Chain-end analysis of the resulted polymer by 1H NMR showed the presence of the fragments of xanthate mediator at both chain ends. Successful chain extension has also been performed via the click reaction of alkyne-terminated PNVP with azide-terminated PNVP. Moreover, fluorescence pyrene-tagged PNVP has successfully been made via the click reaction of alkyne-functionalized pyrene with azide-terminated PNVP. GRAPHICAL ABSTRACT