Browsing by Author "Fahs, Gregory B."

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    Double helical conformation and extreme rigidity in a rodlike polyelectrolyte
    Wang, Ying; He, Yadong; Yu, Zhou; Gao, Jianwei; ten Brinck, Stephanie; Slebodnick, Carla; Fahs, Gregory B.; Zanelotti, Curt J.; Hegde, Maruti; Moore, Robert Bowen; Ensing, Bernd; Dingemans, Theo J.; Qiao, Rui; Madsen, Louis A. (Nature Publishing Group, 2019-02-18)
    The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we report a double helical conformation in the densely charged aromatic polyamide poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~ 1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of other 1D-reinforced composites. © 2019, The Author(s).
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    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.
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    Nucleobase-Functionalized ABC Triblock Copolymers: Self-assembly of Supramolecular Architectures
    Zhang, Keren; Fahs, Gregory B.; Aiba, Motohiro; Moore, Robert Bowen; Long, Timothy E. (The Royal Society of Chemistry, 2014-06-24)
    RAFT polymerization afforded acrylic ABC triblock copolymers with self-complementary nucleobase-functionalized external blocks and a low-Tg soft central block. ABC triblock copolymers self-assembled into well-defined lamellar microphase-separated morphologies for potential applications as thermoplastic elastomers. Complementary hydrogen bonding within the hard phase facilitated self-assembly and enhanced mechanical performance.
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    Nucleobase-functionalized acrylic ABA triblock copolymers and supramolecular blends
    Zhang, Keren; Aiba, Motohiro; Fahs, Gregory B.; Hudson, Amanda G.; Chiang, William D.; Moore, Robert Bowen; Ueda, Mitsuru; Long, Timothy E. (The Royal Society of Chemistry, 2015-01-30)
    Reversible addition-fragmentation chain transfer (RAFT) polymerization afforded the unprecedented synthesis of well-defined acrylic ABA triblock copolymers with nucleobase-functionalized external blocks and a central poly(n-butyl acrylate) (PnBA) block. Size exclusion chromatography (SEC) confirmed the molecular weight and molecular weight distribution of the central block. 1H NMR spectroscopy revealed the successful chain extension of the PnBA macro-chain transfer agent (CTA) using adenine or thymine-functionalized acrylic monomers. The acrylic monomer with a flexible spacer to the pendant nucleobases promoted intermolecular recognition of nucleobases and long range segmental motion of polymer main chains. The external block glass transition temperatures (Tg's) of thymine (T) and adenine (A) functionalized blocks were 52 °C and 76 °C, respectively. Thermomechanical and morphological analysis revealed the effect of processing conditions on self-assembly and microphase-separated morphology of nucleobase-functionalized ABA copolymers. Thymine and adenine-functionalized ABA triblocks formed a thermodynamically stable, hydrogen-bonded complex upon blending. The supramolecular blend exhibited a cylindrical microphase-separated morphology with an extended plateau window compared to the individual block copolymers. The complementary hydrogen bonding between adenine and thymine formed a thermally labile, physically crosslinked, network that exhibited enhanced mechanical performance with melt processability. Thus, these ABA nucleobase-functionalized block copolymers demonstrate potential as thermoplastic elastomers for hot melt adhesives and coatings.
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    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.