Chemistry Reference
In-Depth Information
3.1.2. Characterisation of bisitaconimides
Bisitaconimides (60-70% yield) obtained by cyclodehydration of bisitaconamic
acids were yellow in colour and were soluble in low boling solvents like acetone
and chloroform.
In the FTIR spectra of bisitaconimides, the characteristic absorption peaks due
to imide groups were observed at 1775
±
8 cm
-1
and 1710
±
10 cm
-1
. The >C=C<
stretch of bisitaconimides was observed at 1660
±
5 cm
-1
. Isomerisation of ita-
conimide to citraconimide was indicated in sample IET where an additional ab-
sorption peak was observed at 1643 cm
-1
due to the >C=C< stretch in a 5 mem-
bered ring (Fig. 3).
In the
1
H-NMR spectra of bisitaconimides (Fig. 4) vinylidene protons were ob-
served as singlets at 5.6
±
0.1 ppm and 6.4
±
0.1 ppm. The methylene protons ap-
peared at 3.4
±
0.1 ppm. Aromatic proton resonance signals were observed at 7-8
ppm. An additional proton resonance signal of varying intensity was present at
2.06
±
0.05 ppm. This is due to -CH
3
protons of citraconimide, thereby indicating
isomerisation of bisitaconimide to biscitraconimide.
The intensity of the 6.4 ppm signal was also higher compared to 5.6 ppm due to
the presence of vinylidene proton in citraconimide. The content of biscitraconim-
ide in these products was estimated from the intensity ratio of proton resonance
signals at 2.06 and 5.6 ppm, and was found to be highest in IET (77%) and lowest
in IEF (7.2%). The extent of isomerisation was comparable in IEA (29.5%), IEC
(24%) and IEN (24%).
The DSC scans (Fig. 5) of IET, IEC and IEF showed sharp melting endotherms
at 191°C, 201°C and 208°C, respectively and the curing exotherms were immedi-
ately after melting. The melting endotherms in IEA and IEN were at 196 and
201°C, respectively. It may be concluded that as the % biscitraconimide increases
the melting temperature decreases. The exothermic peak temperature was lowest
in IET and increased as the % biscitraconimide decreased. This could be due to
presence of electron donating methyl group in citraconimide that increases the re-
activity of the double bond. Due to partial overlap of melting endotherm and cur-
ing exotherm it was difficult to calculate the heat of curing (
H). The extent of
isomerisation was maximum in bisitaconimide prepared by refluxing in toluene
and lowest when tetrahydrofuran was used as solvent.
The TG traces of bisitaconimide resins cured at 200°C showed one- or two-step
decomposition (Fig. 6). The characteristic decomposition temperatures (T
i
, T
max
or
T
f
) and char yield at 800°C of bisitaconimides are summarized in Table 3. In case
of IEC, IEN and IET a weight loss of 4-6% was observed in the temperature
range 240-400°C. T
i
value for the first step was lowest for IET, while for IEC and
IEN the T
i
values were 290°C and 272°C, respectively.
The citraconimide content was highest in IET; therefore, the poor thermal sta-
bility of this resin compared to other samples may be attributed to the methyl
group. In all the imide resins, major weight loss due to scission of the main chain
was observed at temperatures >440°C. The T
max
values for all the imides were ob-
∆
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