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in gel-film to polyimide was detected after stretching. The drawn film was
washed with isopropanol to remove NMP.
3.4. Heat treatment
The drawn gel-film was dried in dehumidified air by fixing the ends of the film to
a metal frame to prevent shrinkage. The dried film was subjected to high tempera-
ture heat treatment to convert polyisoimide to polyimide under the prescribed
heating condition of 200°C/10 min, 250°C/8 min, 350°C/5 min and 350 to
450°C/5 min. In this report the heat treatment temperature (HTT) represents the
final treatment temperature. Imidization gradually occurred above 200°C and
completed at around 300°C. The conversion to polyimide was confirmed by FT-
IR spectroscopy as shown in Figure 3(c). The final thickness of studied PPPI film
was 8.0
µ
m.
4. HIGH-MODULUS PPPI FILMS
Figure 4 shows the dependence of tensile properties of heat-treated PPPI films on
gel-draw ratio, where HTT was 450°C. Undrawn films are too brittle to carry out
tensile test after heat treatment. Both tensile strength and modulus dramatically
increase with increasing draw ratio; especially the improvement in tensile strength
is quite dramatic. One can see that a tensile strength of 500 MPa and a Young's
Modulus of 20 GPa are simultaneously realized at a draw ratio of 5.0. We can
conclude that the gel-drawing technique overcomes the problem of intrinsic brit-
tleness of PPPI polymer. Figure 5 shows dependence of Young's modulus on the
heat treatment temperature. Young's modulus of PPPI almost saturated at 350°C,
which corresponds to completion of conversion from polyisoimide to polyimide.
Figure 4. Dependence of (a) tensile strength and (b) Young's modulus for gel-drawn (
λ
=4.8) PPPI
film on gel-draw ratio.
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