Chemistry Reference
In-Depth Information
7.5 Polyimides
It is interesting that formation of a linear aromatic polyimide was observed as early as 1908 when a
polyimide was formed by heating 4-aminophthalic anhydride:
O
O
O
N
n
H
2
N
n
O
O
Formation of polymers, however, was at the time considered undesirable, so the material was not
pursued [
87
]. It was learned since that an imide link is more thermally stable than an amide one and
that polyimides can be very useful materials. Many polyimides were developed since. Aromatic
structures in the polymeric backbone raise the melting temperatures and yield stiffer and tougher
materials. Most sought-after polyimides, therefore, are products from aromatic tetra acids (or
dianhydrides) and aromatic diamines.
Many commercial preparations of aromatic polyimides include a preliminary step of forming
polyamic acids first [
88
]:
OO
OO
H
HO
OH
+
R'
n
O
R
O
R
N
N
H
2
N
NH
2
R'
n
H
OO
O
O
This is followed by imidation, often after the polymer has been applied to a substrate as a coating
or was cast as a film:
O
O
OO
HO
OH
H
R
N
R'
N
NRN '
n
n
H
O
O
OO
The polyamic acids are usually prepared in solution. Suitable solvents are
N
,
N
-dimethyl-
formamide, dimethyl sulfoxide, and
-methyl-2-pyrrolidone. The reactions require anhydrous
conditions at relatively low temperatures, like 50
C (or lower). Some, however, need high
temperatures, as high as 175
C[
88
]. The two reagents are combined in solution. The order of
addition and reagent purity can influence the molecular weight of the products that may range from
13,000 to 55,000 [
88
]. Some imidation accompanies the first step. It is desirable that during polyamic
acid formation, the degree of imidation does not exceed 50%.
The step of conversion of polyamic acids to polyimides can take place at about 300
C in thin films.
With cyclizing agents, however, it can take place at much lower temperatures [
88
].
Lozano et al. reported on an improved method for forming high molecular weight polyimides [
88
].
They used in situ sililation of the diamines to carry out the reaction. Trimethylchlorosilane is added to
the diamine solution prior to the addition of the electrophilic monomer.
Commercially, the most commonly used aromatic dianhydrides are pyromellitic dianhydride
and benzophenone tetracarboxylic dianhydride. The common amines in industrial practices are
n
