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/ODA(1/1) displayed much better water stability than NTDA-ODADS
/BAPF(1/1). This clearly indicates that the stability of polyimide membranes not
only depends on the flexibility of polymer chains and the IEC but also depends on
other factor(s). A common structural feature between ODADS and BDSA is that
the sulfonic acid groups are directly attached to the phenyl rings to which the
amino groups are attached, and here they are noted as “type 1” sulfonated dia-
mines. Unlike the case of ODADS and BDSA, the sulfonic acid groups of
BAPFDS are attached to the bridged phenyl rings, and it is noted as “type 2” sul-
fonated diamine (Scheme 2). Because the sulfonic acid group is a strong electron-
withdrawing group, the electron density of the phenyl rings to which the amino
groups are attached should be larger for BAPFDS than for ODADS or BDSA, i.e.,
BAPFDS is more basic than ODADS or BDSA. It is well known that aromatic
diamines with higher basicity are generally more reactive with dianhydrides than
those with lower basicity [14]. Since hydrolysis is the reverse reaction of polym-
erization, polyimides derived from more basic diamines should have higher hy-
drolysis stability. As a result, the high basicity of BAPFDS is favorable for main-
taining the stability of the imido rings, which offsets the unfavorable effect on
water stability due to its rigid structure, and this might be the reason that
BAPFDS-based polyimide membranes had much better water stability than
BDSA-based ones with similar IEC. BAPBDS is a “type 2” and highly flexible
sulfonated diamine, and therefore NTDA-BAPBDS polyimide membrane dis-
played very high water stability in comparison to others.
It should be noted that in the literature [6, 15] the criterion for the judgment of
the loss or maintaining mechanical properties of membranes is described by the
statements, “the membranes become highly brittle” or “keep their form”, which
are not so strict as that proposed in this paper. Using the literature description, the
time for characterization of the stability of the copolyimide membranes should be
much longer than these listed in Table 3. NTDA-ODADS/ODA(1/1) membrane,
for example, did not break into pieces (i.e., kept its form) after being soaked in
water at 80°C for more than 200 h even by vigorously shaking the bottle where
the membrane and distilled water were charged.
Membrane stability to oxidation was also investigated. The copolyimide mem-
branes (the size of each sheet: 0.5
×
1.0 cm
2
) were soaked in Fenton's reagent (30
ppm FeSO
4
in 30% H
2
O
2
, see ref. 15) at room temperature. Oxidative stability of
the membranes was characterized by the time elapsed when the membranes
started to become slightly brittle (the membranes broke when bent) or started to
dissolve in the solution. As shown in Table 4, the sulfonated copolyimide mem-
branes displayed fairly good stability to oxidation, which is much better than that
of the sulfonated arylene ether/fluorinated alkane copolymers reported in the lit-
erature [15]. In addition, the oxidative stability of NTDA-BAPBDS polyimide
membrane is relatively poorer than that of others.
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