Tuning the Glass Transition of and Ion Transport within Hydrogen-Bonded Layer-by-Layer Assemblies
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The influence of pH and ionic strength on the structure and properties of hydrogen-bonded layer-by-layer (LbL) assemblies of polyethylene oxide) (PEO) and poly(acrylic acid) (PAA) is explored. The degree of inter- and intramolecular hydrogen bonding is estimated from Fourier-transform infrared spectroscopy, the glass transition temperature is measured using differential scanning calorimetry of bulk free-standing films, and ionic conductivity is studied using electrochemical impedance spectroscopy. Results indicate that (PEO/PAA) LbL films assembled without added salt are sensitive to pH, with a T g decrease (59-26C) and intermolecular hydrogen bonding increase (27 to 51% COOH groups bonding with PEO) with increasing assembly pH (2 to 3). Films assembled in the presence of 0.1 M lithium triflate exhibit properties independent of assembly pH (T g 48C and 12% COOH groups bonding with PEO), presumably due to the "screening" of hydrogen bonds. Ionic conductivity is found to range from 10 -6 to 10 -10 S cm -1, depending on humidity, plasticization, and salt content. 2007 American Chemical Society.