Synthesis and characterization of soluble polyimides containing trifluoromethyl groups in their backbone

A series of polyimides were synthesized from 2,2-Bis(3,4-dicarboxyphenyl)-hexafluoropropane, 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane, and 4,4′-oxydianiline by chemical imidization. The effects of the diamine ratios on the properties of the films were evaluated through the study of their thermal, electrical, and morphological properties. All the polymers exhibited better solubility in most of the organic solvents and hence were easily processable. Polyimides with more 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane exhibited better solubility and a low refractive index, which is highly desired for microelectronic applications. The dielectric constant and birefringence were strongly dependent on the fluorine content. With an increase in the fluorine substitution, both the dielectric constant and birefringence decreased. All the polymers exhibited high thermal stability (>400°C). The absence of crystalline melting in differential scanning calorimetry and broad wide-angle X-ray diffraction patterns revealed the amorphous nature of the polymers, which was due to the presence of bulky CF₃ groups and hinged ether linkages of the diamine component. The residual stress values decreased with an increase in the 4,4′-oxydianiline content, and the results were in agreement with the dielectric constant.