Proton conducting polyimides from novel sulfonated diamines - Polyimides and Other High Temperature Polymers - page 140

more reactive than the others), and the reaction temperature was lower than that
for ODA. Just like the case of BAPF, BAPB could also readily undergo sulfona-
tion reaction at 60°C. At this temperature the sulfonation reaction selectively oc-
curred at the two central phenyl rings due to their high reactivity.
The chemical structures of all the resulting sulfonated diamines were character-
ized by IR and 1 H NMR spectroscopies.
3.2. Preparation and properties of sulfonated polyimides
As shown in Scheme 1, polymerization of NTDA and the sulfonated diamines
(ODADS, BAPFDS, BAPBDS, and BDSA) was carried out by “one-step”
method in m-cresol in the presence of triethylamine (Et 3 N) and benzoic acid. Et 3 N
was used to liberate the protonated amino groups for polymerization with NTDA,
and benzoic acid functioned as catalyst. This is a literature method which has
been employed for preparation of a series of BDSA-based polyimides [6]. Ran-
dom copolymerization of NTDA, the sulfonated diamines, and non-sulfonated
diamines was also carried out using this method. For comparison purpose, BDSA-
based polyimides were also prepared. The as-synthesized polyimides were in the
triethylammonium sulfonate form and were converted to the proton form by treat-
ing with 1.0 N hydrochloric acid at room temperature. The completion of proton
exchange was confirmed by the disappearance of the peaks corresponding to the
protons of triethylamine in the 1 H NMR spectra of the polyimides (for NTDA-
BAPBDS, IR spectroscopy was used instead of 1 H NMR spectroscopy because it
was insoluble in DMSO-d 6 or other solvents). The IR spectra of the sulfonated
polyimides were recorded. For NTDA-ODADS, the strong absorption bands
around 1717 cm -1 and 1671 cm -1 are assigned to the stretching vibration of car-
bonyl groups of the imido rings. The broad band around 1255 cm -1 and the band
around 1088 cm -1 correspond to the stretching vibration of sulfonic acid groups.
Solubility behaviors of the sulfonated polyimides (in triethylammonium salt
form) are shown in Table 1. All the polyimides except NTDA-BDSA/BAPB(1/1)
are soluble in m-cresol but insoluble in common dipolar aprotic solvents such as
1-methyl-2-pyrrolidinone (NMP) and N,N-dimethylacetamide (DMAc) which are
good solvents for many five-membered ring (non-sulfonated) polyimides.
BAPFDS-based polyimides are still soluble in DMSO besides in m-cresol;
whereas some ODADS- or BDSA-based polyimides are insoluble in DMSO, in-
dicating a better solubility of the former type of polyimides. Proton exchange
generally led to significant improvement in solubility of the polyimides. For ex-
ample, most of the sulfonated polyimides in proton form could be dissolved in
NMP by slight heating. NTDA-BAPBDS is an exception, as no large difference
in solubility of this polyimide was observed before and after proton exchange.
Thermal stability of the sulfonated polyimides (in proton form) was investi-
gated by TG-MS measurements. For NTDA-ODADS, the weight loss starting
from 275°C is due to the decomposition of sulfonic acid groups judging from the
evolution of sulfur monoxide and sulfur dioxide, indicating fairly good thermal