Semicrystalline polyimides for advanced composites - Polyimides and Other High Temperature Polymers - page 524

3. RESULTS AND DISCUSSION

3.1. Thermomechanical characteristics of PI films
Thermomechanical characteristics of PI films and temperatures of main transi-
tions - glass transition and melting (according to DSC data) of the above polymers
are given in Table 1.
The (BPDA-TPER)TPER polymer which is actually a copolymer based on two
different dianhydrides of DF and R types was amorphous. It also follows from refs.
[9, 10] that PI copolymerization using two different diamines or dianhydrides usu-
ally leads to material amorphization. Other polymers listed in Table 1 are semicrys-
talline and their melting temperature can be determined using DSC. The melting
temperature of PM-BAPB is relatively high and is probably close to its thermal
degradation temperature. Therefore, it is not possible to determine it precisely using
DSC. Nevertheless, according to XRD, this polymer is also semicrystalline. The
melting temperature of R-BAPB and ODPA-BAPB can increase by 15-20 ° C after

annealing at a temperature 5-10 C lower than that of the main melting transition.

This is probably caused by a certain ordering of the size and shape of crystalline re-
gions in the initial polymer obtained from PAA. Hence, the melting temperature
range is given in Table 1 for these polymers. Its lower limit corresponds to the melt-
ing temperature of the unannealed polymer, whereas its upper limit corresponds to
that of the annealed sample. It should also be noted that according to DSC data, af-
ter the first temperature scanning, provided the temperature exceeds the melting
temperature of the polymer by 20-30

°

C, the polymers of the ODPA-BAPB and R-
BAPB types are amorphized and are not crystallized during cooling. Thus, they be-
come amorphous, which is confirmed by the results of the second scanning. The
BPDA-TPER polymer retains its crystallinity after the first scanning and cooling,
but to a much lesser extent, i.e. its melting enthalpy decreases from 22.6 J/g during
the first scanning to 2.0 J/g during the second scanning.

°

Table 1.
Thermomechanical characteristics at T=25 ° C and main transitions temperatures of semicrystalline
PI films obtained from NMP solutions

Polymer E [GPa]

σ [MPa] ε [%] T g [ ° C] T m [ ° C] ∆ H [J/g]

PM-BAPB 2.9-3.15 130-135 10-15 300 >450 ND

ODPA-BAPB 3-3.1 90 3-4 245 380-412 28

R-BAPB 2.7-3.3 110 10-15 198-213 317-325 32-45

BPDA-TPER 3.6-3.7 100-110 5 240 387 23

(BPDA-TPER)TPER 3.2 100-110 9-10 202 No No

E = Young's modulus; σ = tensile strength; ε = elongation at break; T g = glass transition
temperature; T m = melting temperature; ∆ H = melting enthalpy; ND - not detectable.