Counterion Dynamics in Perfluorosulfonate Ionomer Membranes Measured by Quasielastic Neutron Scattering

 

Kirt A. Page1

J.K. Park and Robert B. Moore2

Victoria Garcia-Sakai3

 

1Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899

2The University of Southern Mississippi, Department of Polymer Science, Hattiesburg, MS 39406

3NIST Center for Neutron Research, Gaithersburg, MD 20899

 

The overall program of this research is to gain a fundamental understanding into the molecular origins of thermally induced morphological relaxations and dynamics of alkylammonium forms of Nafion® membranes.  To probe these dynamics, we employ a variety of techniques including variable temperature small-angle x-ray scattering (SAXS), solid-state 19F NMR spectroscopy, dynamic mechanical analysis (DMA), dielectric relaxation spectroscopy (DRS), and quasi-elastic neutron scattering (QENS).  The current study focuses on our recent results using QENS to probe the dynamics of the counterions in tetramethylammonium (TMA) and tetrabutylammonium (TBA) neutralized forms of this perfluorosulfonate ionomer.  Using the back-scattering spectrometer at NIST, the elastic scattering intensity over scattering vector (Q)  range of Q = 0.25 Angstroms^-1 to Q = 1.75 Angstroms^-1 and a temperature range of 50 K to 600 K was measured.  The scattering was shown to have a strong dependence on the counterion used for neutralization and the scattering vector probed.  At a Q = 0.25 Angstroms^-1, a drop in the elastic scattering was observed at 513 K and 373 K for TMA-Nafion and TBA-Nafion, respectively.  This drop in intensity correlates well with the alpha relaxation observed in DMA and the variable temperature SAXS data.  At higher Q values, two transitions are observed and are currently being investigated.  Preliminary dynamic scans of TBA-Nafion at 450 K reveal an elastic incoherent structure factor that can be described by diffusion within concentric spheres.  Debye-Waller (DW) analysis shows a distinctly different behavior for the two systems.  More specifically, a break-down in the DW analysis occurs at different temperatures depending on the counterion used.  These data are being used to further support and understand the dynamics of the ion-hopping process previously reported.

 

Kirt A. Page

NRC Postdoctoral Fellow
Multivariant Measurement Methods Group

Polymers Division, MSEL

National Institute of Standards and Technology

Bldg. 224/Room B220
100 Bureau Drive MS8542
Gaithersburg, MD  20899-8540
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Non-member

Materials

Mentor: Michael J. Fasolka