New highly phenylated bis(phthalic) and bis(naphthalic) anhydrides and polyimides therefrom - Polyimides and Other High Temperature Polymers - page 107

X-ray diffractograms showed that the phenylated PNIs were fully amorphous.
This morphology probably contributes to their improved solubility. All the poly-
mers were soluble in aprotic solvents - NMP, DMF, DMAA, DMSO, as well as
in m-cresol and THF. When heated, the polymers were even soluble in hot chloro-
form and toluene.
The solubility of these polymers depended on the structure of the aromatic
diamines used. Polymers containing ether, methylene, hexafluoro-

isopropylidene, and bulky trifluoromethyl groups rapidly dissolved in organic sol-
vents and displayed better solubility compared to the phenylated PNIs

VIC , VIE-VIG

VIA , VIB

with more rigid chains. Polymer was insoluble in chloroform and toluene

possibly due to the presence of rigid and polar benzimidazole groups, which
strongly enhance intermolecular dipole-dipole interaction. Finally, the solubility
of the phenylated PNIs

VID

was appreciably higher compared to the PNIs
based on 1,4,5,8-naphthalenetetracarboxylic dianhydride [27-30]. The significant
increase in solubility can be explained by the presence of various isomeric struc-
tures and to the large amount of phenyl substituents.
Polymers

VIA-VIG

displayed glass transition temperatures ranging from 340
to 400°C. These values decreased in the sequence

VIA-VIG

VIB>VIDV>IC>VIG>

VIA>VIE>VIF As might be expected, a higher glass transition temperature was

observed for the rigid-chain phenylated PNI

.

VIB containing p-diphenylene moie-

ties. For polymer , the high glass transition temperature was apparently re-

lated to the polarity and rigidity of the benzimidazole moiety. Contrary to expec-
tations, polymer

VID

VIA containing p-phenylene moieties exhibited a lower T g than

polymer This may be due to the high percentage of side phenylene rings

(per phenylene group of the backbone) in the repeat unit. In fact, polymers

VIB .

VIA-

VIG have lower glass transition temperatures than those of nonphenylated PNIs

[7, 31].
Phenylated PNIs displayed excellent thermal stability (Table 5). Dynamic TGA
showed that the temperatures corresponding to 10% weight loss in air and argon
were between 580 and 615, and 625 and 680°C, respectively. The wide ranges be-
tween the decomposition temperatures and the glass transition temperatures (T dec -
T g ) of the phenylated PNIs indicate that these polymers should be capable of be-
ing processed by compression molding.
Table 6 lists the mechanical properties of films based on phenylated PNIs. The
tensile strength of these films ranged from 66.1 to 83.0 MPa, the elastic modulus
was between 1.55 and 2.55 GPa, and the elongation at break ranged from 3 to 9%.
It should be pointed out that the film mechanical properties are comparable to
those based on the PNIs described in [32].
The dielectric constants,

', of the phenylated PNIs were measured at relative
humidies of 0 and 50% and an electric field frequency of 1 kHz (Table 6). When
the relative humidity was 0%, the values of

ε

polymers varied
from 2.85 to 3.00. These results are rather close to the calculated values (Table 6).
The low values of

' for the VIA-VIG

ε

ε ' for the phenylated PNIs compared to the values reported for