Structure, Physical Properties, and Transport of Multiblock Ionomers

Date of Completion

January 2012

Keywords

Chemistry, Polymer|Engineering, Chemical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Molecule transport and physical properties in block ionomer membranes are dependent upon molecular weight, ion concentration as well as the ionic interactions within functional groups. The micro phase separation of block copolymers has the potential of attaining higher proton conductivity and desirable physical properties. A series of sulfonated poly(arylene ether sulfone) multiblock ionomers (BPSH-BPS) and poly((t-butyl styrene)-b-(ethylene propylene)-b-(styrene-co-styrene sulfonate)- b-(ethylene propylene)-b-(t-butyl styrene)) (PBC) ionomers were studied to evaluate their morphology, physical, and transport properties. ^ Structure, viscoelastic and transport properties of BPSH-BPS were investigated as a function of block length and temperature using several analytical techniques. The results revealed the long range ordering lamellar morphology was formed in BPSH-BPS. Relaxation time and glassy state modulus decreased with increasing ionic block length, while the larger ionic domain size was observed in BPSH-BPS with longer block length. An expansion in ionic domain size resulted in lower fractional free volume (FFV) and gas permeability in BPSH-BPS (15k:15k). ^ Another block copolymer of PBC ionomers (PBC-XX-Y) were studied by varying ion concentration (IEC) and ionic interaction (Y). Ordering structure, which was observed in unsulfonated PBC (PBC-0.0), was disrupted by ionic interaction in sulfonated PBC copolymers. Slight difference in gas permeability P, diffusivity D and solubility coefficient S were observed when IEC differed in a range of 0.0-2.0 mequiv./g. Further, the percolation threshold for proton transport was observed at IEC=1 .0 mequiv./g. ^ In addition, ionic interactions between the sulfonate groups of PBC ionomers coordinated with various metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ba2+) were investigated. Properties of PBC were dependent upon metal-ion cationic radius that influenced thermal stability, liquid swelling properties, and transport of gas and liquids, while polymer structure was less affected by metal ions. Further, the choice of solvents used for casting membranes significantly impacted PBC ionomer morphology and then their transport properties. Good solvent enhanced the segment motion and their ability to self-assembly into ordering structure. Fundamental knowledge from this work is expected to advance the development of block ionomer copolymers and their use in gas separations and polymer fuel cells. ^

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