New molecular scale insights into the α-transition of Nafion® thin films from variable temperature ATR-FTIR spectroscopy.

Ultrathin films of Nafion® have attracted significant attention in recent years owing to their application in a variety of electrochemical devices. For improved mechanical integrity, the films are often subjected to a thermal treatment step. In this paper, we report a variable-temperature Fourier transform infrared spectroscopy study of ultra-thin (14 to 660 nm) Nafion films on SiO2/Si substrates in the attenuated total reflection mode. A key finding is the dramatic increase in the intensity of the 636-624 cm-1 doublet band, generally associated with CF2 wagging or helix-reversal defect, observed at around 110 °C during in situ heating. This onset temperature provides a molecular link to the α-transition temperature of bulk Nafion and is the first time such an observation has been made using infrared spectroscopy. Smaller changes in the intensity and/or a shift of the other peaks in the region 1300-900 cm-1 were noted.