Tuning the glass transition of and ion transport within hydrogen-bonded layer-by-layer assemblies

The influence of pH and ionic strength on the structure and properties of hydrogen-bonded layer-by-layer (LbL) assemblies of poly(ethylene 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 (59-26 degrees C) 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 similar to 48 degrees C 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.