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high melting point of curing agent results in high curing temperature [12-14]. As ‘solvent or catalyst’, ionic liquids (ILs) are known as
versatile chemicals in diverse fields of the synthetic chemistry [15]. They have unique properties such as low vapor pressure, good
thermal stability, high ion conductivity and simple functionality [16], and have been employed in a number of chemical processes, such
as separations [17], homogeneous two-phase catalysis [18] and polymerization [19]. Here, we employed multiple-SO3H functioned
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ionic liquid (MIL) as curing agent for phthalonitrile resins. The MIL shows low melting point (below 100 C), high decomposition
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temperature (326 C), and strong acidity (Brønsted acid) [20], which is active for curing phthalonitrile resins. Combined with these
advantages, the new curing agent MIL may overcome the shortcoming of the traditional solid curing agent.
Our laboratory has synthesized a series of phthalonitrile-terminated poly(phthalazinone ether nitrile) (PPEN-Ph) [21-23], as
organosoluble PPENs show high mechanical strengths and good thermal properties, and they have been applied in many fields [24,25].
In order to improve the curing process of PPEN-Ph, as well as the properties of resulting PPEN-Ph resins, here MIL is employed as
curing agent for PPEN-Ph for the first time. In fact, the performance of our MIL for curing PPEN-Ph is better than the common curing
agent ZnCl2.
2. Results and discussion
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The MIL was synthesized via three steps as shown in Scheme 1. The H NMR and FT-IR spectra of resulting MIL were shown in
Fig. 1, and the assignment of the corresponding spectra were listed in experimental part. The results confirmed that multiple-SO3H
functioned ionic liquid has been prepared successfully [20]. To check the feasibility of employing MIL as curing agent for
phthalonitrile-terminated oligomers, herein, trimerization of phthalonitrile was chosen as the model reaction to compare the activity of
formation of 1,3,5-triazine (TCPT) over various catalysts (Scheme 2). Under the identical reaction conditions, the total yield of TCPT
over MIL was 75.9%, whereas this value just was 43.8% over the common curing agent ZnCl2 (Fig. 2a). Moreover, the FT-IR spectra
(Fig. 2b) indicated that the main products over these two catalysts both were TCPT, the peaks around 1525 cm-1 and 1363 cm-1 were
assigned to the vibration of C=N and C-N in TCPT, respectively. And the intensities of peaks around 2222 cm-1 attribute to C≡N were
declined compared with that in the pristine phthalonitrile (the spectrum was not shown here). Based on these results, it can be
concluded that the pre-prepared MIL was an efficient catalyst for trimerization of phthalonitrile.
Trifluoromethanesulfonic acid and chlorosulfonic acid both are strong acid, liquid and can be soluble in polar solvents, and they
have been employed as efficient curing agents for trimerization of aromatic nitriles to afford aromatic s-triazine polymers [26, 27].
However, these two acids show strong erosion and low thermal stability, which limit their application as curing agent for phthalonitrile
resins. Here, the ionic liquid functionalized with –SO3H group is introduced to catalyze the trimerization of phthalonitrile for the first
time, and the catalytic performance is better than that of ZnCl2 as its strong acidity. To further study the performance of curing PPEN-
Ph (the structure was shown in Scheme 3) over MIL, curing kinetics was determined by DSC. PPEN-Ph was blended with different
curing agents with a mass ratio of 20:1, then the mixtures were successively scanned at different heating rates (5, 10, 15 and 20 oC/min)
from 25 oC to 400 oC on DSC. From the DSC curves shown in Figs. S1 and S2 (Supplementary information), it is clear that two distinct
curing peaks appear over the curing agents, which indicated that higher post-cure temperature was necessary to complete the curing
reaction in these systems. However, in order to discuss conveniently, we just took the first curing peak (Tp’) as an example to calculate
the curing parameters (Supplementary Information), and the details about the calculation also can be seen in our previous report [28].
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Based on the DSC curves, the curing temperature under isothermal conditions T’p0 over MIL and ZnCl2 were 207.9 C and 268.5 C
(Fig. 3), respectively, it seems that the curing temperature for PPEN-Ph can be reduced effectively with MIL as curing agent.
Furthermore, the apparent activation energy Ea’ was calculated via the Kissinger equation, and the value for MIL (101.5 kJ/mol) was
nearly half of that over ZnCl2 (201.5 kJ/mol). Thus, the curing reaction with MIL as curing agents can be much more easily proceeded
under mild reaction conditions compared with ZnCl2. And this phenomenon maybe was caused by the lower melting point and stronger
acidity on MIL.
Thermal stability of the resulting thermosetting resin was also tested to evaluate the performance of curing agents, and the TGA
curves of the cured PPEN-Ph were shown in Fig.4, the main data including the temperature at weight loss 5% (Td5%), 10% (Td10%) and
the char yields at 800 oC (Cy800) were summarized in Table 1. After curing over various curing agents, the thermal stability of PPEN-Ph
can be improved significantly. Moreover, it is noted that the values of Td5% and Td10% over MIL are both higher around 14 oC than those
over ZnCl2. With these results in hands, we can conclude that as a curing agent for phthalonitrile resin, the performance of MIL is
better than ZnCl2.
3. Conclusion
Based on above results, we can conclude that employing ionic liquid to promote the curing rate and reduce the curing temperature
of phthalonitrile resin was feasible. Introduction of multiple-SO3H functioned ionic liquid (MIL) as curing agent into phthalonitrile-
terminated poly(phthalazinone ether nitrile) (PPEN-Ph) oligomers, resulted in considerable reduction of curing temperature and
apparent activation energy without compromise in their thermal stability, compared with present common curing agent ZnCl2.
Although the curing pathway of PPEN-Ph over MIL and other properties of resulting thermosetting resins are studied currently under