Functional Block Copolymers via Anionic Polymerization for Electroactive Membranes

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Date
2013-06-17
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Virginia Tech
Abstract

Ion-containing block copolymers blend ionic liquid properties with well-defined polymer architectures. This provides conductive materials with robust mechanical stability, efficient processability, and tunable macromolecular design. Conventional free radical polymerization and anion exchange achieved copolymers containing n-butyl acrylate and phosphonium ionic liquids. These compositions incorporated vinylbenzyl triphenyl phosphonium and vinylbenzyl tricyclohexyl phosphonium cations bearing chloride (Cl), or bis(trifluoromethane sulfonyl)imide (Tf2N) counteranions. Differential scanning calorimetry and dynamic mechanical analysis provided corresponding thermomechanical properties. Factors including cyclic substituents, counteranion type, as well as ionic concentration significantly influenced phosphonium cation association.

1, 1'-(1, 4-Butanediyl)bis(imidazole) neutralized NexarTM sulfonated pentablock copolymers and produced novel electrostatically crosslinked membranes. Variable temperature FTIR and 1H NMR spectroscopy confirmed neutralization. Atomic force microscopy and small angle X-ray scattering studied polymer morphology and revealed electrostatic crosslinking characteristics. Tensile analysis, dynamic mechanical analysis, thermogravimetric analysis, and vapor sorption thermogravimetric analysis investigated polymer properties. The neutralized polymer demonstrated enhanced thermal stability, decreased water adsorption, and well-defined microphase separation. These findings highlight NexarTM sulfonated pentablock copolymers as reactive platforms for novel, bis-imidazolium crosslinked materials.

4-Vinylbenzyl piperidine is a novel styrenic compound that observably autopolymerizes. In situ FTIR spectroscopy monitored styrene and 4-vinylbenzyl piperidine thermal polymerizations. A pseudo-first-order kinetic treatment of the thermal polymerization data provided observed rate constants for both monomers. An Arrhenius analysis derived thermal activation energy values. 4-Vinylbenzyl piperidine exhibited activation energy 80 KJ/mol less than styrene. The monomer differs from styrene in its piperidinyl structure. Consequently, in situ FTIR spectroscopy also monitored styrene thermal polymerization with variable N-benzyl piperidine concentrations. Under these circumstances, styrene revealed activation energy 60 KJ/mol less than its respective bulk value. The similarities in chemical structure between styrene and 4-vinylbenzyl piperidine suggested thermally initiated polymerization occurred by the Mayo mechanism.  The unique substituent is proposed to offer additional cationic effects for enhancing polymerization rates.

Living anionic polymerization of 4-vinylbenzyl piperidine achieved novel piperidinyl-containing polymers.  Homopolymer and copolymer architectures of this design offer structural integrity, and emphasize base stability.  Sequential anionic polymerization afforded a 10K g/mol poly(tert-butyl styrene-co-4-vinylbenzyl piperidine) diblock and a 50K poly(tert-butyl styrene-co-isoprene-co-4-vinylbenzyl piperidine) triblock. Alkylation studies involving a phosphonium bromide salt demonstrated the future avenues for piperidinium based polymer designs. These investigations introduce piperidinyl macromolecules as paradigms for a new class of ammonium based ionic materials.

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Keywords
water purification, random copolymer, ionic liquids, anionic polymerization, NexarTM, thermomechanical property
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