Structure-property correlation of bromine substitution in polyimides

The influence of bromine substitution in polyimides onto physical properties is systematically investigated for the first time. A series of comparable polyimides with and without bromine substituents was synthesized and characterized. Thus, different aromatic or aliphatic diamines containing various flexible linkages were reacted with two analogous dianhydrides, from which one contains tetrabromo substituted phenylene rings. The structure of monomers and polymers was confirmed by infrared spectroscopy and proton nuclear magnetic resonance. The measurements showed that the bromine substitution did not significantly affect the thermal and dielectric properties of the corresponding polymers when compared to related structures. All polyimides exhibited enhanced solubility in polar and in less polar solvents. The introduction of bromine substituents led to the increase of glass transition temperature, being in the range of 147–274 °C for polymers containing only aromatic rings, and in the domain of 72–110 °C for polymers containing aliphatic segments. Also, by the introduction of bromine units thermal stability slightly decreased for the corresponding polymers, with initial decomposition temperature being in the range of 410–510 °C for polymers having only aromatic rings, and in the domain of 355–400 °C for polymers having aliphatic segments. The values obtained for dielectric constant, dielectric loss and activation energy of sub-glass relaxations for the polymer containing bromine units are slightly higher than those of polyimides without substituted phenylene rings.