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The evolution of poly (vinyl alcohol) PVA structure and dichromatic substance induced by uniaxial stretching PVA in the mixed solution with iodine and boric acid (BA) is systematically studied by the combination of in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXD). The experiment is carried out under an orthogonal experimental design for the iodine concentrations of 0.01, 0.2, 1 M, and the BA concentrations of 0.3, 1, and 3 wt%, respectively. We divide the stretch process into three regions, whose boundary is the yield point and strain harden point. The PVA-I 3 - co-crystal generating at high and moderate iodine concentrations is less stable as compared with the PVA crystal, which leads to the earlier decrease of crystallinity at the end of the first region. In the second region, the broken crystal, namely the regular amorphous, is the precursor of the PVA-polyiodide complex. Further increasing the strain, the PVA-I 3 - co-crystals are generated from the one dimension polyiodide surrounded by the PVA chains through a melt-reconstruction process. The adequate BA can make it easy to form the polyiodide at the initial state. Still, the increasing rate of polyiodide is suppressed by further added BA due to the limitation of chain movement by the crosslinking agent. In the final region, the emergence of nanofibril can account for the strain hardening of the film. The evolution of polyiodide and PVA-I 3 - co-crystal revealed by this work gives a better understanding of the polarizer production process, which may guide the resolution of blue leakage of polarizer via an enhancement of the I 3 − in the crystal form during stretching. Image 1 • The structural evolution of dichromatic substance with poly (vinyl alcohol) at different concentrations of boric acid during stretch is investigated by in-situ WAXD and SAXS. • The metastable PVA-I 3 - cocrystal will be easily broken during stretching, which leads to the distinct yield point. • Polyiodide first emerges in the amorphous originated from the broken crystal and then in the highly oriented amorphous during stretching. • The one-dimension polyiodide along with the PVA chain is the precursor of PVA-I 3 - cocrystal. • Dichromatic substance emerges earlier with the increasing of boric acid concentration; however, their final content is less due to the limitation of chain mobility by the crosslinking agent. [ABSTRACT FROM AUTHOR]