Study on the structure of polymer electrolyte membrane using small angle X-ray scattering and positron annihilation spectroscopy

Abstract

The structure of poly (styrenesulfonic acid)- grafted poly(ethylene-co- tetrafluoroethylene) polymer electrolyte membrane (ETFE-PEM) was investigated by comparison with those of precursor original ETFE film and styrene-grafted films (grafted-ETFE) using positron annihilation lifetime spectroscopy (PALS) and small angle Xray scattering (SAXS). PALS of these samples indicated that there were two lifetime components, which were assigned to the annihilation of ortho- Positronium (o-Ps). The two types of o-Ps relate to the free volume at sub nanoscale locating in the crystalline and amorphous phases. SAXS profiles of these samples indicated the presence of lamellar stacks andlamellar grains. Sizes of lamellar stacks and lamellar grains varied significantly during the graftingprocess, whereas only change a little bit at sulfonation process and at wet state. A compination of PALS and SAXS allowed us to investigate the structures of ETFEPEMs with scale ranging from several Ångströms to micrometer.

Characterization of graft-type polymer electrolyte membranes at low grafting degrees for fuel cells

“…As shown in the DTG curves, the decomposition temperature at 242 • C of the original ETFE and grafted film relates to the intrinsic crystallinity of the ETFE base film 30 . Namely, ETFE possesses a lamellar crystal with a crystallinity of 35%, as determined by small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) 13,17,20,[24][25][26][27] . Steps 2 and 3 are attributed to the decomposition of the polystyrene graft and the pristine ETFE backbone, respectively 25 .…”

“…Note that in our previous works, the grafting was already confirmed using Fourier transform infrared spectroscopy (FT-IR) 12,29 and X-ray photoelectron spectroscopy (XPS) 30 . Moreover, the presence of polystyrene grafts in the grafted-ETFE films and final membranes (ETFE-PEMs) was also affirmed using ultrasmall and small-angle X-ray scattering (USAXS/SAXS) 14,21,25,27,28 and positron annihilation lifetime spectroscopy (PALS) 24,27 .…”

“…These hierarchical microstructural features strongly relate to the mechanical integrity of the membranes under severe operation conditions (high temperature and high relative humidity) of fuel cells 14 . However, previous reports mainly represented the results of ETFE-PEMs having a grafting degree (GD) higher than 19% [14][15][16][17][18][20][21][22][23][24][25][26][27][28][29][30][31] . The higher the GD is, the higher the conductance, but the lower the mechanical integrity and the higher the swelling [7][8][9]11,17,25,29 .…”

Effects of source correction on positron annihilation lifetime spectroscopic analysis of graft-type polymer electrolyte membranes

“…This assumption is used for many PAL spectroscopic analyses of polymers 13 . The relationship between the hole radius and the lifetime of ortho-positronium trapped in the holes is shown by the Tao-Eldrup (TE) model, which is represented by the following formula 10,25 :…”

“…where τ i (ns) (i = 3, 4) is the lifetime component, r i is the pore radius (nm), and ∆R (≈ 0.166 nm) is the thickness of the electron layer. The nanohole volume, V i , is determined by the equation 10,25 :…”