Structure–property relationships for partially aliphatic polyimides

The structure–property relationship was studied for partially aliphatic polyimides containing alicyclic dianhydride and aromatic diamine unit. Rel-[1S,5R,6R]-3-oxabicyclo[3,2,1]octane-2,4-dione-6-spiro-3′-(tetrahydrofuran-2′,5′-dione) (DAn) was used as an unsymmetrical spiro dianhydride, and 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BOCA) were used as symmetrical non-spiro dianhydrides. The dianhydrides were polymerized with two aromatic diamines, 4,4′-oxydianiline (ODA) and 4,4′-(hexafluoroisopropylidene)dianiline (FDA), using a conventional two-step chemical imidization method. Structures of the PAl-PIs prepared were confirmed by ¹H-NMR and FT-IR spectroscopy. Solubility of the polyimides was tested in various organic solvents. Thermal properties of the PAl-PIs were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). UV-visible spectroscopy was performed to evaluate the optical transparency of the polyimides. The effect of monomer structure on the properties was studied. The PAl-PIs prepared from DAn showed improved solubility, thermal properties, and transparency when compared with PAl-PIs derived from CPDA and BOCA. It is considered that the rigid, unsymmetrical spiro structure of DAn leads to rigidity, bulkiness, irregularity, and non-linearity of the polyimide chains, resulting in the enhanced properties of DAn polyimides. The molecular arrangement in the polyimides has also been studied by wide-angle X-ray diffraction (WAXD) and was correlated with the properties.