Biomedical Engineering Reference
In-Depth Information
of condensation (approximately -53 kJ/mol of water). This is a clear indication of
an endothermic contribution arising from rearranging and expanding the polymer
structure. Clearly, it has been established that Nafion can have three different types
of water (Escoubes and Pineri, 1982; Yoshida and Miura, 1992; Davis et al., 1997).
type 1: water closely bound to the ions
type 2: water weakly bound to the ions (or the polymer—possibly ether oxygens)
type 3: free water
According to the differential scanning calorimetry (DSC) studies, the presence of
several types of water structures within the ionic polymer is more plausible (Escoubes
and Pineri, 1982; Yoshida and Miura, 1992; Davis et al., 1997). The heat absorbed
(desorbed) by a sample as a function of temperature is monitored. Figure 3.7 illustrates
60
40
20
4
8
H
2
O/SO
3
−
FIGURE 3.6
O absorption as a function of the water content (Nafion).
(Escoubes, M. and M. Pineri. 1982. In
Enthalpy of H
2
Perfluorinate Ionomer Membranes
, ed. A. Eisenberg
and H. L. Yeager. Washington, D.C.: American Chemical Society.)
H
2
O peak
Peak 1
Exotherm
Peak 2
~20 H
2
O/SO
3
−
~6 H
2
O/SO
3
−
153
183
213
243
273
Te m p ( K )
FIGURE 3.7
DSC data for cooling of Nafion (Li
form). (Yoshida, H. and Y. Miura. 1992.
+
J. Membrane Sci
. 68:1-10.)
