Effect of Metal Ion Loading on Thermal Decomposition Temperature of Surface Modified Polyimide Films

The surface of the pyromellitic anhydride/4,4'-diaminodiphenyl ether (PMDA/ODA)-based polyimide (PI) film was subjected to alkali-cleaving and ring-opening treatment with potassium hydroxide (KOH) solution. After acidification with acetic acid, a surface-modificated layer of polyamic acid (PAA) was obtained on the surface of PI. Six different metal ions (K⁺, Na⁺, Ca²⁺, Mg²⁺, Ag⁺ and Al³⁺) were introduced into the PAA layer via ion exchange reaction. The ion exchange processes of different metal ions were investigated by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and inductively coupled plasma emission spectroscopy (ICP). As a result, the load amount of metal ion showed a completely inverse relationship with its valence, indicating that the ion exchange reaction was carried out exactly according to the ionic charge ratio of 1∶1. Meanwhile, the thermal gravimetric (TG) analyzer under a nitrogen atmosphere was performed to investigate the effect of different metal ions on the thermal decomposition temperature of PI film. It was found that the strong alkali metal ions, K⁺ and Na⁺, provided obvious degradation effects on the PI film, which caused a significant decrease in the thermal decomposition temperature of the PI film. However, the weaker alkali metal ions such as Ca²⁺, Mg²⁺, Ag⁺ and Al³⁺ exhibited a rather weak influence on the thermal decomposition temperature of the PI film. The effect of metal ions on the thermal decomposition temperature of the film was positively correlated with the alkalinity（pK_b） of its corresponding hydioxide, ie K⁺> Na⁺ >> Ca²⁺ ≈ Mg²⁺ > Ag⁺ ≈ Al³⁺.