Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.4 Effect of polymerization solvent 845
tent of cyclic imide bond and all imide bonds of PAI-1 and PAI-2 further increased by heating
of the corresponding PAAAs at 180°C for 2 h followed by heat treatment at 260°C for 2
h. The content of amide bond decreased by heating of PAAA at 180°C for 2 h, but increased
by post-heating at 260°C for 2 h.
Table 13.4.4. IR absorbance ratio and insoluble part of samples after heat treatment in
air [Data from reference 7]
Sample
No.
Heat treatment Absorbance ratio
Insoluble
part
%
Temperature Time Cyclic imide Amide All imide
o
C
h D1780/D1510 D1530/D1510 D1380/D1510 PAAA-1 0.05 0.68 0.37 0
PAI-1 180 2 0.24 0.43 0.57 0
PAI-1 260 2 0.33 0.56 0.71 76.7
PAAA-2 0.10 0.59 0.36 0
PAI-2 180 2 0.26 0.45 0.58 0
PAI-2 260 2 0.31 0.50 0.64 16.8
13.4.3.2 Curing PAI by post-heating
The formation of the insoluble part in NMP was observed and the amount of insoluble part
formed by heating of PAI-1 (obtained in MEK/water mixed solvent) was larger than that by
heating of PAI-2 (obtained in DMAc).
The increase of imide and amide bond content by post-heating at 260°C is considered
as follows: the imidation of amic acid structure may be proceeded by both intermolecular
and intramolecular imidation. The latter produces cyclic imide bond, but the former produces
acyclic imide bond to give crosslinking material as shown in Eq. [13.4.3].
Since PAIs obtained by heating of PAAAs at 180°C are completely soluble in NMP,
the intramolecular imidation preferentially proceeds at 180°C, but intermolecular imidation
scarcely proceeds.
On the other hand, by post-heating at 260°C, the crosslinking reaction proceeds by the
intermolecular imidation of terminal carboxyl groups of PAI with remaining amic-acid
structure (Eq. [13.4.4]) and the amide bond in main chain of PAI (Eq. [13.4.5]) to give the
insoluble part in NMP. The reaction induced the increase of imide bond content of PAI after
post-heating. The increase of amide bond after post-heating may be due to the formation of
crosslinking structure by the amidation of terminal amino groups of PAI with carboxyl
groups of PAI in main chain (Eq. [13.4.5]-[13.4.8]).
Therefore, PAI obtained from the heating of PAAA obtained in MEK/water mixed
solvent, which has many carboxyl groups, produces more insoluble part by post-heating.
Figure 13.4.2 shows the effect of heating temperature on the curing of PAI obtained by
heating of PAAA at 180°C in MEK/water mixed solvent and DMAc. As shown in Figure
13.4.2, by post-heating at 280°C for2hinair, insoluble part in NMP reached 100%, indicating
the almost complete curing of PAIs. It is interesting to note that PAIs formed by heating
of PAAA obtained in MEK/water mixed solvent (PAI-1 and PAI-3) were found to be cured