Structural evolution from poly(amic acid) to polyimide fibers during thermal imidization process

Abstract

In this study, poly(amic acid) (PAA) precursor fiber was prepared via a two-step wet spinning method and subsequently heat-treated to obtain the polyimide fiber by thermal imidization. The structural evolution of the PAA precursor during the imidization was traced using various measurements. The imidization degree of the PAA fibers treated at different temperatures was calculated by Fourier transform infrared analysis, and the cyclization reaction occurred accompanied by the decomplexation of the H-bonded N,N′-dimethylacetamide (DMAc). The thermogravimetry test illustrated that the residual solvent evaporation took place prior to imidization. Moreover, in situ wide-angle X-ray diffraction and small-angle X-ray scattering measurements were employed to investigate the development of the aggregation structure and microvoids in PAA fibers. The results indicated that molecular chains were thermally extended during the thermal imidization, resulting in the increase of Hermans’ orientation factor and the increased strain. The thermal imidization process also led to the orientation and elongation of microvoids along the molecular chains.