Aromatic poly(amic acid) precursors always form complexes
with dipolar aprotic solvents
via strong acid/base interaction and are expected to have relatively
high glass transition temperatures
(T
g's) which are overlapped with or higher than
the imidization temperatures, so that their
T
g's could
not be determined in spite of their wide usages. In the present
study, the measurement of T
g was
attempted for poly(amic acid) precursors of three different
aromatic polyimides synthesized in N-methyl-2-pyrrolidone (NMP) from the respective dianhydrides and diamines:
poly(4,4‘-oxydiphenylene pyromellitamic acid) (PMDA−ODA), poly(p-phenylene
3,3‘,4,4‘-oxydiphthalamic acid) (ODPA−PDA), and
poly(p-phenylene benzophenonetetracarboxamic acid) (BTDA−PDA). Phase
transitions, as well as imidization
reactions in the precursor/NMP mixtures, were measured with varying
compositions by a newly developed
oscillating diffferential scanning calorimetry. Compositions in
the mixtures were determined by proton
nuclear magnetic resonance spectroscopy. For solvent-rich
mixtures, a melting point depression of the
NMP solvent was observed, whereas for precursor rich mixtures,
T
g depression was detected. In
particular,
T
g's measured for the precursor rich mixtures
were best fitted by a modified Gordon−Taylor equation as
a function of composition, in order to estimate
T
g's of the poly(amic acid)s in solvent
free, that is, true
T
g's of the precursor polymers: 207.4 °C
for PMDA−ODA, 166.3 °C for ODPA−PDA, and 213.2 °C
for
BTDA−PDA precursor. The Kuhn segment length, which is a measure
of chain flexibility, was estimated
to be 43.3 Å for PMDA−ODA, 34.6 Å for ODPA−PDA, and 34.6 Å for
BTDA−PDA. In addition, a phase
diagram was constructed for the PMDA−ODA precursor/NMP mixture.
For the highly dried precursor
samples, the chemical repeat unit was also determined to complex with
1.4−1.7 NMP molecules, depending
on the precursors.
