Intramolecular Cyclization Reaction Promoted Self-Assembly of a Poly(amic Acid) in DMF/Water

The chemical composition and chain structure of polymers have a critical influence on their self-assembly behavior and corresponding morphology of the assemblies. Herein, an alternative strategy based on the intramolecular cyclization reaction is proposed to precisely regulate the main chain structure and self-assembly behavior of poly(amic acid) (PAA). The PAA is synthesized via stepwise polymerization of pyromellitic dianhydride and 3,5-diamino-1,2,4-triazole under mild conditions, which undergoes intramolecular cyclization reaction to generate polyimide (PI) segments in the backbone upon thermal annealing (named as P(AA-stat-I)). The generation of PI segments enhances the hydrophobicity, chain rigidity, crystallinity, and conjugation of the polymer, promoting its self-assembly to form crystalline nanodiamonds, needles up to millimeter, and nanowires and their aggregates, in N,N-dimethylformamide (DMF), water, and DMF/water, respectively. Contact angles of PAA and P(AA-stat-I) with different degrees of imidization confirm the enhancement of hydrophobicity with the generation of PI segments. UV–vis absorption and fluorescence spectra reveal the expansion of main chain conjugation of the polymer after annealing, leading to the red shift of maximal absorption and emission peaks, as well as the enhancement of the fluorescence intensity. Furthermore, the calculated band-gap energies of different polymers exhibit a pronounced decrease with the increase of degree of imidization. This observation further verifies that π-conjugation extends the delocalized electronic structure and narrows the band gap, in agreement with the red shift of fluorescence emission.