Preparation and characterization of nanoporous thin films from fully aliphatic polyimides

Preparation of porous polyimide films by selective decomposition of poly(methyl methacrylate) from PI/PMMA-blended films

A series of porous polyimide (PI) films were prepared by decomposing PI/poly(methyl methacrylate) (PMMA)-blended films at appropriate high temperature for a certain period of time in air. The pore-forming agent, PMMA, was blended with poly(amic acid) (PAA) by in situ blending method. As a thermally labile polymer, PMMA was decomposed first to generate substances with low molecular weight, leaving holes in the original location. Fourier transform infrared (FT-IR) spectra were used to characterize the changes in the chemical structure during the process of preparation of porous PI films. The porous morphology was investigated using field emission scanning electron microscopy. Using FT-IR and thermogravimetric analysis, it was confirmed that PMMA disappeared completely after 4 h at 330°C in air. Thus, porous PI films with major differences between surface and bottom morphologies were prepared successfully. The average pore size of the porous films increased with increasing content of PMMA but was not larger than 1 μm.

Effect of unsymmetrical spiro dianhydride structure on properties of fully aliphatic polyimides

Structure-property relationship was studied for fully aliphatic polyimides containing alicyclic dianhydride and diamine units. Rel-(1′R,3S,5′S)-spiro[furan-3(2H),6′-[3]oxabicyclo[3.2.1]octane]-2,2′,4′,5(4H)-tetrone (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. 4,4′-Methylenebis(2-methylcyclohexylamine) (MMCA) and 5-amino-1,3,3-trimethylcyclohexanemethylamine (AMCH) were N-silylated, and reacted with the dianhydrides to prepare the fully alicyclic polyimides. The formation of the polyimides was confirmed by Fourier transform infrared spectroscopy. DAn-based polyimides showed higher solubility than the polyimides derived from CPDA or BOCA. This was explained based on the results of wide-angle X-ray diffraction (WAXD) analysis of the polyimides. The WAXD study showed that DAn-based polyimides have greater full width at half maximum (FWHM) and d-spacing values than the other polyimides. This indicates that DAn-containing polyimides have lower intermolecular order, decreased intermolecular interaction, and less close chain packing compared to the other polyimides. It is considered that the morphology of DAn polyimides is attributable to the unsymmetrical spiro structure of DAn that leads to bulkiness, irregularity, and non-linearity of the polyimide chains. Decomposition temperatures of the polyimides were investigated by thermogravimetric analysis (TGA) and UV-visible spectroscopy was performed to evaluate the optical transparency of the polyimides.