Synthesis and characterization of 5-5 ring-fused polyimides based on imidazole.

Abstract

AAB-type monomers based on imidazole for the preparation of 5-5 ring-fused polyimides were synthesized by carrying out a substitution at the 1-position of 2-amino-4,5-dicyanoimidazole, followed by hydrolysis. Thus, various pendant groups such as methyl, hexyl and the 2,4-dinitrophenyl group were introduced at the 1-position by electrophilic substitution. A solid-state polymerization was employed to prepare the poly (imidazoleamic acid chlorides) which can be directly imidized to the corresponding polyimides, used to obtain poly(amic acids) or poly(amic esters). As an another approach to synthesizing an active precursor, 2-amino-1-methyl-4,5-imidazoledicarbonyl chloride hydrochloride was prepared using thionyl chloride. Thermal and chemical curing behavior were investigated using FT-IR spectroscopy. Finally, the poly(imidazole imides) were prepared in the form of a film from poly(imidazoleamic acid chlorides) by heating up to 180$\sp\circ$-200$\sp\circ$C. The carbonyl stretching peaks of the imide bond appear at 1808(sym) and 1756(antisym) cm$\sp{-1}$. The imidization process and the cleavage reaction of the imide bond were monitored by FT-IR. The difference in the hydrolysis rate between polyimides having short or long aliphatic pendant groups at the 1-position was observed using FT-IR. The highest value of inherent viscosity of the poly(amic acid), 1.52 dl/g in methanesulfonic acid, was obtained after hydrolysis of the N-methyl substituted poly (imidazole imide) while that of N-hexyl polyimide was 1.26 dl/g in N-methyl pyrrolidinone (NMP) and 0.22 dl/g in the case of N-2,4-dinitrophenyl poly(amic acid) in methanesulfonic acid. The structural, physical and material properties of the polyimides and the poly(amic acids) were characterized by infrared, nuclear magnetic resonance, UV-visible, luminescence, viscosimetric methods, differential scanning calorimetry, thermogravimetric analysis, optical microscopy and wide angle x-ray scattering. Solution properties were also investigated by monitoring the viscosity as a function of time at 30$\sp\circ$C. Luminescence spectroscopy of the poly(1-methyl imidazole imide) and poly(1-methyl imidazoleamic acid) films shows an emission band centered at 535 nm and 505 nm, respectively. The poly(1-hexylimidazoleimide) decomposes endothermally at a temperature in excess of 330$\sp\circ$C. (Abstract shortened with permission of author.).