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and OI obtained by chemical imidization are characterized by X-ray diffraction
patterns with several reflections of low intensity. A comparison of these data sug-
gests that the chemical imidization of PI and OI yields inferior crystalline struc-
tures, i.e. more defective structures with smaller crystals.
3.4. Thermomechanical properties of the composites based on the PI matrix alone
and on PI with the addition of the oligoimide
CFRC's based on an ELUR carbon fiber were obtained from a pure R-BAPB PI
matrix and from those with the addition of the oligoimide OI in the ratio of PI:OI
= 60:40 and PI:OI = 90:10. The aim of the experiment was to evaluate the effect
of crystallinity on the thermomechanical properties of the polymer. The prepreg
was impregnated using PAA solution (with or without OI). Prepregs were dried to
remove the solvent and imidized at 280-300
C. Subsequently, they were packed
into a pile consisting of 24-25 layers, placed in a mold and molded at 330
°
C for 5
min. To increase the degree of matrix crystallinity in the CFRC, an additional
thermal treatment was also carried out during cooling. It consisted of isothermal
maintenance of the CFRC in the mold at T = 250
°
°
C for 0.5 hr, i.e. at a tempera-
ture close to that of matrix crystallization.
Fig. 2 shows thermomechanical analysis data for CFRC using a torsion pendu-
lum. It can be seen that in the CFRC with a semicrystalline matrix the shear stor-
age modulus G' (Fig. 2, curves 1' and 2') retains a high value at a level of 0.5-1
GPa up to the melting temperature of about 300
C. For the CFRC with an amor-
phous matrix (pure PI without the addition of OI), a sharp drop in the G' modulus
(Fig. 2, curve 3') takes place in the range of glass transition temperature, i.e. at
200
°
C. Hence the crystallization of PI/OI matrix raises the limit of CFRC per-
formance by almost 100
°
up to the melting temperature of the matrix.
For additional confirmation of the presence or absence of crystallinity in the PI
matrix of the composite, a DSC analysis on the CFRC obtained was performed. In
fact, semicrystalline PI matrix with the addition of OI (60:40) in CFRC has a
melting temperature of 305
°
C, i.e. approximately the same as in pure polymer
without fibers (Table 3, No. 4). Moreover the melting enthalpy
°
H of CFRC is
8.3 J/g, which, based on the weight of pure matrix in CFRC, is about 25 J/g (vol-
ume fraction of the fiber - 0.6, fiber density - 1.75 g/cm
3
and matrix density - 1.4
g/cm
3
). This value corresponds to the
∆
H value for the matrix in the form of a
film (Table 3, No. 4). Consequently, the degree of matrix crystallinity in CFRP
approximately corresponds to that of a pure matrix obtained in the form of a film
and crystallized under the same conditions. The T
g
values for CFRC's with an
amorphous matrix also approximately correspond to that of pure PI matrix in the
form of films (Table 1) and is 204
∆
C for the amorphous matrix. Slightly higher T
g
values for matrices in CFRP can be caused by partial limiting of segmental mobil-
ity of the macromolecules in the polymer in the vicinity of carbon fibers or due to
possible cross-linking of PI at high temperatures.
°
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