1
506
SIDOROVICH et al.
In the compound with the bischalcone (C1) frag-
(
(
a)
b)
ment, replacement of the norbornenedicarboxylic acid
imide group (Fig. 3b, curve 1) by the maleimide
group (Fig. 3b, curve 5) affects the melting range of
the mesophase insignificantly, but considerably ex-
tends the curing range: Curing is not complete even
at 400 C, whereas in the case of 1 (Fig. 3b, curve 1)
it is virtually complete at 400 C. In oligomers with
the chalcone (C2) fragment, the norbornenedicarbox-
ylic acid imide group (Fig. 3c, curve 2), compared to
the maleimide group (Fig. 3c, curve 6), does not alter
the range of transition from the amorphous to softened
state but noticeably (by 50 60 C) shifts the exother-
mic effect to higher temperatures. Compound 6 with
the maleimide group (Fig. 3c, curve 6) exhibits the
narrowest curing range and the lowest temperature in
the maximum of the exothermic effect, 270 C.
Exo
Endo
(
c)
T, C
Fig. 3. Thermograms of oligomers (a) 3 and 4, (b) 1 and 5,
and (c) 2 and 6.
phous oligomers 2 and 6. The maximum of the melt-
ing peak in the DSC curve of oligomer 3 (Fig. 2) is
lower, compared to the corresponding monomer 3 , by
The thermograms in Fig. 3 show that the tempera-
ture ranges of the exothermic effects of the oligomers
with the norbornenedicarboxylic acid imide terminal
groups (curves 1 3) differ to a lesser extent compared
to the oligomers with the maleimide terminal groups
4
0 C, whereas in the case of 4 it is 25 C higher.
Crystalline oligomer 3 has the lattice similar to that
(curves 4 6). For example, in the case of 4 the exo-
of the monomer, as suggested by coincidence of sev-
eral reflections, but more defective, as indicated by
lower intensity of reflections. The lattice of 4 differs
from that of 4 .
thermic effect is complete at 400 C; in the case of 5,
it is not complete at this temperature; and in the case
of 6, it is complete even at 370 C, which is more
favorable for using the oligomer as a binder. There-
fore, we chose 6 for further experiments aimed to
determine the activation energy of curing. For this
purpose, we heated samples of 6 to 523, 533, 543, and
Figure 3 shows the thermograms of the oligomers
grouped with respect to the phase state: crystalline
(
3, 4, Fig. 3a), mesomorphic (1, 5, Fig. 3b), and
amorphous (2, 6, Fig. 3c). It is clearly seen that the
energy characteristics of the curing transitions corre-
late with the phase state. The heats of endothermic
transitions regularly decrease in going from crystalline
5
73 K at a rate of 32 deg min 1 and obtained the iso-
therms of heat release capacity Qt. The isotherms
plotted in the coordinates Qt t (Fig. 4a) and ln(Qt) t
(Fig. 4b) show that curing at 523, 533, 543, and
573 K is a first-order reaction; its rate constant is k =
d[ln(Qt)]/dt. Fig. 4c shows a plot of lnk vs. recip-
rocal temperature 1/T. From this plot, we determined
1
1
(3, 4; 42 kJ kg ) to mesomorphic (1, 5; 26 kJ kg )
1
and amorphous (2, 6; 21 kJ kg ) oligomers. The re-
spective heats of exothermic transitions are 78, 161,
1
and 51 kJ kg . The considerably larger heat of the
the activation energy Ea = 8.314d(lnk)/d(1/T) =
exothermic transition (curing) in the case of 1 and 5,
with the transition extending to 400 C, is due to the
more pronounced conformational isomerism, com-
pared to 2 and 6.
1
7
9 kJ mol . For cross-linking of bisbornenimide, Liu
1
et al. report a somewhat higher value, 94 kJ mol .
This fact is consistent with our DSC data that oligo-
mers 1 and 3 with the norbornenedicarboxylic acid
imide terminal groups exhibit higher temperatures of
the onset and end of the exothermic effect and higher
heat of curing, compared to 6.
Within each pair in Fig. 3, the oligomers differ
only in the structure of the terminal group. Compari-
son of the thermograms of 3 and 4, 1 and 5, or 2 and
6
reveals the effect of the terminal group on curing.
In the case of oligomers 3 and 4 containing the di-
phenyl oxide unit and no chalcone units, replacement
of the terminal norbornenedicarboxylic acid imide
group (Fig. 3a, curve 3) by maleimide group (Fig. 3a,
curve 4) shifts the melting range, curing range, and
maximum of the exothermic effect to lower tempera-
tures by 50, 25, and 30 C, respectively.
We found that the course of cross-linking is influ-
enced by preliminary pressing of oligomer powders:
The exothermic effect starts at lower temperatures.
We performed experiments with samples of oligomer
powders compacted before curing at a pressure of
50 atm in a mold with a punch diameter of 10 mm.
Upon pressing, the initially weakly colored powders
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 76 No. 9 2003
Sidorovich
