VTechWorks staff will be away for the Thanksgiving holiday beginning at noon on Wednesday, November 27, through Friday, November 29. We will resume normal operations on Monday, December 2. Thank you for your patience.

Browsing by Author "Jangu, Chainika"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Examining Structure-Morphology-Property Relationships of Novel Styrenic-Based Macromolecules for Emerging Applications
    Jangu, Chainika (Virginia Tech, 2015-09-14)
    For the first time, triblock copolymers of novel styrenic-based macromolecules were investigated in detail and examined for structure-morphology-performance relationships. We were able to design novel imidazolium- and phosphonium-containing styrenic macromolecules using controlled radical polymerization and conventional free radical polymerization strategies, for a variety of potential applications including electromechanical devices, ionic liquids, adhesives, and lithium-ion batteries. Block copolymers have a unique architecture providing physical crosslinking to behave as thermoplastic elastomers. We preferred ABA triblock copolymers as compared to random and diblock copolymers for improved mechanical performance. ABA triblock copolymers synthesized using nitroxide-mediated polymerization (NMP) of polystyrene external blocks and a charged imidazolium-containing central block, exhibited sufficient modulus and ionic conductivity for electromechanical transducers. We successfully reported the actuation behavior of triblock copolymers in the presence of added ionic liquid for the first time. We proposed that diluting the ion concentration of the ion-rich phase with neutral polymer comonomers that reduces Tg, increases ion dissociation, and potentially maximizes ionic conductivity. Tendency of ethylene-oxide units to coordinate cations, forming stable crown ether-like, multi-nuclear coordination complexes, promotes solvation and dissociation of ionic aggregates. In situ Fourier transform infrared spectroscopy (FTIR) was used to monitor the thermal polymerization in various acrylate and methacrylate monomers. It was found that acrylates have lower activation energy than methacrylates. The copolymerizations of poly(ethylene glycol)methyl ether acrylate (EG9MEA) and imidazole-containing monomer (VBIm) resulted in controlled polymerization kinetics with narrow molecular weight distributions. The control behavior of the copolymerizations is likely attributed to the observed decrease in calculated apparent rate constants for the copolymerizations with addition of VBIm as comonomer. Reversible addition fragmentation transfer (RAFT) successfully synthesized well-defined A-BC-A triblock copolymers containing a synergy of pendant ether and imidazolium sites. We demonstrated that electromechanical transducers derived from these triblock copolymer membranes with added ionic liquid showed superior actuation performance compared to a benchmark Nafion® membrane, suggesting potential for ionic polymer device applications. This was attributed to optimum modulus, improved ionic conductivity, and microphase-separated morphology of triblock copolymers. Conventional free radical polymerization and anion metathesis of 4-(diphenylphosphino)styrene (DPPS) successfully generated high molecular weight triaryl phosphine-containing copolymers. These macromolecules have no -CH2 group at the benzylic position increasing the thermal stability of the DPPS-containing polymers. Counterion exchange to fluorinated, bulkier anions broadened the library of polyelectrolytes, led to improved thermal stabilities, lower glass transition temperatures, and tunable wetting behavior. We also reported the synthesis of salt-responsive copolymers using conventional free radical polymerization. Adhesive performance measurements such as peel tests and probe tack enforced the application of these polymers as pressure sensitive adhesives. We also demonstrated the synthesis and subsequent neutralization of novel, well-defined A-BC-A triblock copolymers containing a soft central 'BC' block consisting of Sty-Tf2N and DEGMEMA with polystyrene external blocks. Sty-Tf2N monomer enables an important delocalization of the negative charge. Li+ has weak interactions with this anionic structure, consequently enabling a high dissociation level. Li+ ions are associated to the polymer chain to produce high transport numbers. Furthermore, incorporating DEGMEMA lowers the Tg of the charged block copolymers, thereby increasing the segmental mobility and thus ionic conductivity. Finally, the structure-property-morphology study of these triblock copolymers will be helpful for their use in potential applications such as ion-containing membranes, lithium-ion batteries.
  • Thumbnail Image
    Imidazole-containing triblock copolymers with a synergy of ether and imidazolium sites
    Jangu, Chainika; Wang, Jing-Han Helen; Wang, Dong; Fahs, Gregory B.; Heflin, James R.; Moore, Robert Bowen; Colby, Ralph H.; Long, Timothy E. (The Royal Society of Chemistry, 2015-03-06)
    Reversible addition-fragmentation chain transfer (RAFT) polymerization enabled the synthesis of well-defined A-BC-A triblock copolymers containing a synergy of pendant ether and imidazolium sites. The soft central BC block comprises low Tg di(ethylene glycol) methyl ether methacrylate (DEGMEMA) and 1-(4-vinylbenzyl) methyl imidazolium units. External polystyrene blocks provide mechanical reinforcement within a nanoscale morphology. Dynamic mechanical analysis (DMA) of the A-BC-A triblock copolymers exhibited a plateau region, which suggested the formation of a microphase-separated morphology. Atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) collectively probed the morphology of the A-BC-A triblock copolymers, revealing long-range order at the nanoscale dimensions. Dielectric relaxation spectroscopy (DRS) examined the ion-transport properties of ionomeric A-BC-A triblock copolymers and random copolymers with different compositions. The role of morphology was demonstrated with block copolymer nanoscale structures providing superior ionic conductivity and mechanical performance compared to random copolymers. Under a 4 V direct current (DC) applied voltage, electromechanical transducers derived from these triblock copolymer membranes with added ionic liquid showed superior actuation performance compared to a benchmark Nafion[registered sign] membrane, suggesting potential for ionic polymer device applications. This was attributed to optimum modulus, improved ionic conductivity, and microphase-separated morphology of triblock copolymers.
  • Thumbnail Image
    Phosphonium-containing diblock copolymers from living anionic polymerization of 4-diphenylphosphino styrene
    Schultz, Alison R.; Fahs, Gregory B.; Jangu, Chainika; Chen, Mingtao; Moore, Robert Bowen; Long, Timothy E. (The Royal Society of Chemistry, 2015-11-20)
    Living anionic polymerization of 4-diphenylphosphino styrene (DPPS) achieved well-defined homopolymers, poly(DPPS-b-S) styrenic block copolymers, and poly(I-b-DPPS) diene-based diblock copolymers with predictable molecular weights and narrow polydispersities. In situ FTIR spectroscopy monitored the anionic polymerization of DPPS and tracked monomer consumption for kinetic analysis. Post-alkylation enabled controlled placement of phosphonium functionality in poly(I-b-DPPS) diblock copolymers, producing well-defined phosphonium-containing block copolymers with low degrees of compositional heterogeneity. Incorporating phosphonium charge disrupted the lamellar bulk morphology of the neutral diblock precursor and provided morphologies with interdigitated packing of alkyl chains on the phosphonium cation.
  • Thumbnail Image
    Thermal and Living Anionic Polymerization of 4-Vinylbenzyl Piperidine
    Schultz, Alison R.; Jangu, Chainika; Long, Timothy E. (The Royal Society of Chemistry, 2014-07-02)
    Elevated temperatures that are often required for controlled radical polymerization processes lead to the thermal autopolymerization of 4-vinylbenzyl piperidine. In situ FTIR spectroscopy monitored 4-vinylbenzyl piperidine autopolymerization, and pseudo-first-order thermal polymerization kinetics provided observed rate constants (kobs). Arrhenius analysis determined the thermal activation energy (Ea) for 4-vinylbenzyl piperidine, revealing an activation energy requirement 80 kJ mol_1 less than styrene due to the presence of the piperidine ring. The similarities in chemical structure of styrene and 4-vinylbenzyl piperidine suggested a thermally initiated polymerization according to the Mayo mechanism; however, the piperidine substituent enabled a proposed cationic polymerization to enhance overall polymerization rates. In the absence of thermal polymerization, living anionic polymerization of 4-vinylbenzyl piperidine provided a viable strategy for achieving piperidine-containing polymers with predictable molecular weights and narrow polydispersities. This study also reports piperidine-containing polymeric precursors for subsequent alkylation to form novel piperidinium ionomers and polyelectrolytes.