Synthesis of poly(propylene glycol)-grafted polyimide precursors and preparation of nanoporous polyimides

A novel approach to prepare a polyimide nanofoam was explored by using a polyimide precursor grafted with a labile poly(propylene glycol) (PPG) oligomer. The PPG-grafted polyimide precursor, poly((amic acid)-co-(amic ester)), was synthesized via partial esterification of poly(amic acid) derived from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) with bromo-terminated poly(propylene glycol) in the presence of K₂CO₃ in hexamethylphosphoramide and N-methylpyrrolidone. The precursor polymer film was spin-coated onto a glass substrate, then imidized at 200°C under nitrogen, and subsequently the PPG graft was decomposed by heating the film at 300°C for 9 h in air, resulting in the PMDA/ODA polyimide nanofoam. The precursor polymers, polyimides and foamed polyimides were characterized by a variety of techniques including ¹H-NMR spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The homogeneously distributed nano-sized pores of 20-40 nm were observed by transmission electron microscopy (TEM) of the foamed polyimide.