Semicrystalline polyimides for advanced composites - Polyimides and Other High Temperature Polymers - page 525

3.2. Study of the possibility of increasing the degree of crystallinity of the PI
matrix in the composite
Preliminary DSC investigations showed that the possibility of crystallinity reten-
tion in the matrix was rather small. However, to obtain a homogeneous composite
with a uniform matrix distribution throughout the fiber bundle, it is necessary to
carry out the binder melting stage in the mold. Moreover, it is desirable to have
the mold temperature much higher than the binder melting temperature in order to
attain maximum flowability of the polymer melt. Special experiments were car-
ried out to study the possibility of polymer matrix recrystallization after its first
melting. The R-BAPB polymer was selected for these experiments because its
melting temperature was lower than that of other polymers (Table 1). Therefore, it
can be more easily processed into the composite matrix without any danger of
thermal degradation which, according to TGA data, starts at 470 C.

The experiment was performed as follows. The PI R-BAPB film obtained from
PAA by imidization on glass at T = 300

°

C and, therefore, being highly crystalline
(Table 1), was subjected to melting in a DSC unit for 5 min at a temperature
higher than that of polymer melting and subsequently cooled. T x (see Table 2) is
the maximum temperature to which the sample was heated during thermal treat-
ment and at which it started to flow. Subsequently, the sample was cooled to
300

°

C and was maintained at this temperature for 45 min after which it was
cooled to room temperature. After this treatment the sample was heated for the
second time and during this second scanning its melting temperature T m2 and
melting enthalpy

°

H 2 were determined. The results are listed in Table 2.
The DSC results (Table 2) for the R-BAPB polymer show that the maximum
temperature to which the polymer can be heated so as to retain crystallinity after
cooling must not exceed 320

∆

C, which is quite insufficient for uniform impregna-
tion of fibers. Under the actual molding conditions it is rather difficult to maintain
the temperature 5-10

°

C higher than the melting temperature of the matrix to
achieve its flowability state on the one hand and not to exceed too much the upper
temperature limit for retaining polymer crystallinity after cooling, on the other.
Hence, using a pure polymer of the R-BAPB type as the matrix for composite, it
is rather difficult to obtain as a result of melting a composite with a semicrystal-

°

Table 2.
Melting temperature and melting enthalpy of the R-BAPB film according to DSC data during the
second scan

T x [ ° C] T m2 [ ° C] ∆ H 2 [J/g] T g [ ° C]

310 324 24.5 ND

320 314 7.8 ND

330 no ND ND

350 no ND 198