Probing structural evolution of the poly(amic acid) containing benzoxazole moieties in backbone during thermal imidization

Structural evolution of a poly(amic acid) based on 5-amino-2-(4-aminophenyl) benzoxazole (BOA) and 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) in solid film during thermal imidization was investigated using Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), ultraviolet-visible spectroscopy and computation techniques. The results of FTIR and geometries of the model compounds optimized using density functional theory (DFT) method revealed the changes of the chemical functionalities and electrical effect accompanying the thermal imidization reaction. The electronic charges of both the para-disubstituted phenyl and oxazole rings in BOA residues would be simultaneously withdrawn towards the BPDA residues as the imidization reaction proceeded. The WAXD results suggested that the molecular aggregation structures evolved from an overall amorphous morphology to a mixture of a highly ordered crystalline domain and an amorphous matrix. The highly ordered structures evolved via a three-step process: initial formation of a face-to-face pi-stacking order in the short range, subsequent formation of an interchain ordered packing and final formation of crystalline structure by simultaneous improvement of the two orders. In particular, the formation of the interchain ordered packing required a very high imidization degree. Meanwhile, the average interchain distances in the amorphous matrix decreased with the evolution of the ordered structures in the ordered domain. Additionally, the cut-off wavelength and the transmittance (%) at 500 nm of the films changed obviously due to the structural evolution during the thermal imidization.