Non-Covalent Interactions in Block Copolymers:
Synergy of Hydrogen Bonding and Ionic Associations
Brian D. Mather and Timothy E. Long
Department of Chemistry, Virginia Tech, Blacksburg VA 24061
Non-covalent interactions enable the development of novel supramolecular structures from
functional polymeric precursors.1 Both ionic interactions and hydrogen bonding provide
important routes to self assembly. Ionic interactions result from strong electrostatic
attractions, which persist to elevated temperatures and lead to the formation of ionic
aggregates in the solid state.2 In contrast, hydrogen bonding interactions exhibit greater
thermoreversibility and specificity,3 allowing reversible attachment of guest molecules.
Due to the thermal integrity of ionic aggregates, processing of ionic polymers requires
high energy input. Current research efforts involve combining hydrogen bonding
interactions with ionic interactions to benefit from the advantages of both association
modes. Our strategy involves the introduction of ionic hydrogen bonding guest molecules
to reversibly attach ionic guests to the hydrogen bonding elastomeric triblock
copolymers. Potential applications include elastomers, as well as ion-conducting
materials for fuel cells.
Hydrogen bonding block copolymers consisting of DNA nucleobase (thymine and adenine)
functionalized polystyrene outer blocks and poly(n-butyl acrylate) rubber block sequences
were synthesized via nitroxide mediated polymerization from a novel difunctional alkoxyamine
initiator based on DEPN nitroxide (Figure 1). The nucleobase functional block copolymers
exhibited novel microphase separated surface textures via AFM. Hydrogen bonding
interactions were studied using variable temperature AFM and solution rheology.
Reversible attachment of complementary nucleobase functional phosphonium ionic guests
was studied. The presence of the ionic guest resulted in increased modulus but improved
processability above 150 oC due to screening of hydrogen bonding interactions. Changes
in morphology as well as decreased solution viscosity were also observed.
Figure 1. Synthesis of adenine containing elastomeric block copolymers. Tapping mode AFM
phase image of a blend of adenine containing triblock copolymer (1.5K-16.5K-1.5K) with a
uracil functional trioctylphosphonium salt.
References
1. Mather, B.D.; Lizotte, J.R.; Long, T.E. Macromolecules 2004, 37, 9331.
2. Eisenberg, A.; Hird, B.; Moore, R. B. Macromolecules 1990, 23, 4098.
3. Yamauchi, K.; Lizotte, J. R.; Hercules, D. M.; Vergne, M. J.; Long, T. E. J Am Chem Soc
2002, 124, 8599.
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