Highly transparent polyimides derived from 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene: preparation, characterization, and optical properties

Abstract

To investigate the structure–property relationships of polyimides containing pyrimidine or pyridine moieties, two diamine monomers, 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene, were designed and synthesized. The diamines reacted with different commercially available aromatic dianhydrides to yield a series of heterocyclic polyimides via a classical two-step polymerization method. The polyimides, PI-1 and PI-6, exhibited excellent solubility in strong polar solvents, such as N,N-dimethylacetamide, N,N-dimethylformamide and N-methyl-2-pyrrolidone. The flexible, strong and transparent polyimide films were formed with UV-visible absorption cut-off wavelengths at 344–399 nm. Glass transition temperatures (T_g) of the polyimides PI-(1–6), derived from PBAPD and BADB with dianhydrides, in the range of 212–260 °C were obtained by DSC, and the temperatures for 5% wt loss of the polyimides in nitrogen and air atmospheres were obtained from TGA in the range of 430–503 °C and 422–465 °C, respectively. Moreover, the polyimide films showed a low moisture absorption of 0.18–1.38% and outstanding mechanical properties with tensile strengths of 81–111 MPa, an elongation at break of 4.3–8.7%, and a tensile modulus of 1.3–2.4 GPa. The coefficients of thermal expansion (CTEs) of the polyimides ranged from 28 to 62 ppm °C⁻¹.