Spectroscopic study of Nafion® membrane as a function of water content by Phase-Resolved Photoacoustic Method

Abstract

Nafion® membranes were developed by DuPont, a perfluorosulfonic acid/polytetrafluoroethylene copolymer. These membranes are hydrophilic and the water is preferentially sorbed into the clusters due to the hydrophobic character of fluorocarbon matrix. The sulfonic acid groups are chemically active and fixed within the Teflon polymer matrix. Thus, this first synthetic ionic polymer is chemically resistant and durable, making it useful for Proton Exchange Membrane fuel cells. In order to monitoring the groups -OH of the Nafion®, we used the Photoacoustic Spectroscopy phases. The optical absorption spectrum was determined for wavelengths of 800 to 2600 nm. The procedure was to measure the samples in different hydration times. Using the Phase-Resolved Photoacoustic method, it was possible to separate the contribution of –OH, of C=O and of methylene group. The phase lag as a water function behaviors found may be associated with structural changes induced by membrane hydration.

Thermo-Optical Properties of Perfluorinated Sulfonic Acid Membranes: An Investigation of Hydration Based on Absorption Spectra

“…1 for pristine membrane and hydrous 1 in 1 h up to 11 h and 24–25 h as reported elsewhere. 15 We found that the stretching modes and combinations O–H dominate the region of 800–2600 nm and we cataloged and categorized 13 peaks in these regions. Overall, to avoid heating and allow the cooling of the membrane, the results revealed an optimum point of the OH absorption band alongside with a higher thermal diffusivity and before saturation of the membrane, i.e., approximately 1 h of hydration.…”

“…The thermal analysis experiments were performed using the assays described in Table I after PA results were obtained elsewhere. 15 The samples were discs measuring 3 mm diameter. The TG-DSC experiments were carried out in the following conditions: heating rate of 20 C min À1 , alumina (Al 2 O 3 ) crucibles of open capsule type, under argon atmosphere at 30 cm 3 min À1 , temperature range from 30 C (the membranes were maintained at 30 C for 300 s) to 700 C. The thermal analyzer was calibrated using nickel as standard (Curie temperature 354 C).…”

“…Overall, to avoid heating and allow the cooling of the membrane, the results revealed an optimum point of the OH absorption band alongside with a higher thermal diffusivity and before saturation of the membrane, i.e., approximately 1 h of hydration. 15 Accordingly, the monitoring of the hydration process can be performed by checking the absorption bands peaks related from around 1450-1490 nm (NIR) and 1800-1900 nm (mid-IR) once the OH vibrational modes provide information on the structure and concentration evolution of the water component.…”

“…Employing the phase-resolved photoacoustic method (PRPA), 21 it is possible to characterize and distinguish between two (or more) absorbing centers present in the same matrix. Using the PRPA as reported elsewhere, 15 we separated the band OH combinations of terminal methylene groups and C¼O stretches from around 1850-2100 nm (Fig. 1b), while from around 1450-1490 nm (Fig.…”

“…13,14 The experiments were carried out in the range of 1300-2300 nm, since overtones and combinations of hydroxyl bonds were easily defined in this spectral region. 15 Thermal diffusivity measurements 15,16 using the open photoacoustic cell technique (OPC) [16][17][18] were compared to another photothermal technique, 16 thermal lens spectroscopy (TLS), to evaluate the structural changes in the swelling and hydration-dehydration processes of the pristine membrane. Thermal analysis using TG and DSC has been used to check the stability, crystallization transition, and water retention of membranes.…”