A series of polyimide oligomers endcapped with 4-phenylethynylaniline have been prepared using the chemical compositions as shown in Table 1, in which PI-1 to PI-4 were prepared by the reaction of a diamine mixture of 1,3,4-APB and 3,4'-ODA at mole ratio of 75:25 with four different aromatic anhydrides including ODPA, BPDA, 6FDA, and 9FDA, respectively.
The synthesized polyimide oligomers did not show good solubility in common organic solvents, only the fluorine-containing oligomers, such as PI-2 and PI-4, can be easily dissolved in tetrahydrofuran (THF), demonstrating that the trifluoromethyl groups in the oligomers have remarkable effect on improving solubility.
For instance, PI-2 and PI-4 have the measured [M.sub.n] of 1119 and 1179, respectively, close to the calculated [M.sub.n] (1150).
The crystal phase was also observed in PI-3, PI-5, PI-6, and PI-7, and PI-4, similar to PI-2, was amorphous.
However, PI-2 and PI-4 did not show melting behavior in the DSC curves, due to the trifluoromethyl groups in the polymer backbones.
Figure 4 shows the typical viscosity-temperature curves of the polyimide oligomers, in which the melt viscosities of the polyimide oligomers (PI-2 and PI-4) decreased gradually with increasing temperature beginning at 200[degrees]C, down to a valley stage where the viscosity reach to the lowest point, and then go up gradually with increasing temperature due to the thermal cure of phenylethynyl groups.
The fluorinated polyimide oligomers (PI-2 and PI-4) showed very low viscosities (1.3 and 0.7 Pa s, respectively) at 270[degrees]C, and reached to the minimum viscosities (1.0 and 0.6 Pa s, respectively) at 280-281[degrees]C.
Figure 5 shows the representative curves of melt viscosity on isothermal aging time of PI-4. The complex melt viscosities increased slowly with the isothermal aging time at both 280 and 290[degrees]C.