Proton conducting polyimides from novel sulfonated diamines - Polyimides and Other High Temperature Polymers - page 139

Differential scanning calorimetry (DSC) was performed with a Seiko DSC-5200
at a heating rate of 10°C/min. Thermogravimetry-mass spectrometry (TG-MS)
was conducted with a JEOL MS-TG/DTA220 instrument in helium (flow rate:
100 cm 3 /min) at a heating rate of 5°C/min. Gel permeation chromatography
(GPC) was performed with a HLC-8020 apparatus (column: Shodex KD-80M).
NMP was used as the eluent at a flow rate of 1.0 mL/min. Polymer solutions con-
taining 0.05 M LiCl and 0.05 M phosphoric acid were filtered through a 0.5 µm
PTFE filter prior to injecting into the column. Molecular weights were calculated
against poly(ethylene oxide) standards.
Proton conductivity was measured by an ac impedance method with two black
platinum electrodes using Hioki 3552 Hitester instrument over the frequency
range from 100 Hz to 100 kHz.
Water sorption experiments were carried out by immersing three sheets of film
(20-30 mg per sheet) of the polyimide into water at 80°C for 5 h. Then the films
were taken out, wiped with tissue paper, and quickly weighted on a microbalance.
Water uptake of the films, S, was calculated from
S = (W s - W d )/W d ×100 (%) (1)
where W d and W s are the weights of the dry and corresponding water-swollen
film sheets, respectively. The average value of three sheets of film was used as
the water uptake of the polyimide film.
3. RESULTS AND DISCUSSION
3.1. Monomer synthesis and characterization
All the sulfonated diamines, ODADS, BAPFDS, and BAPBDS, were prepared by
direct sulfonation of the corresponding parent diamines, ODA, BAPF, and BAPB,
respectively. Primarily the parent diamines reacted with concentrated sulfuric acid
to form the sulfuric acid salt of the diamines. SO 3 in fuming sulfuric acid was the
real sulfonating agent, and the sulfonation reaction was carried out at different
temperatures depending on the starting diamine. In the case of ODA, the appro-
priate sulfonation temperature was 80°C, and the sulfonation reaction could not
occur below this temperature. This is because the protonated amino group is a
strong electron-withdrawing group which deactivates the phenyl rings. For the
same reason, the sulfonation reaction mainly occurred in the meta-position of the
amino groups of ODA. The ether bond in the para-position of amino group also
supported such a meta-position substitution.
For BAPF, the sulfonation reaction could readily be carried out at 60°C. The
two phenyl-rings to which the two amino groups are attached are deactivated due
to the strong electron withdrawing effect of the protonated amino groups, whereas
the central fluorenylidene ring is highly reactive. As a result, the sulfonation reac-
tion occurred in 2,7-positions of the fluorenylidene ring (these two positions were