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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]

Mechanical and structural evolution of poly (vinyl alcohol) (PVA) induced by uniaxial deformation at different concentrations of boric acid is systematically studied with in-situ synchrotron radiation small- and wide-angle X-ray scattering (SAXS and WAXS). The PVA films were stretched uniaxially in the aqueous solution at room temperature with concentrations of boric acid at 0.3 wt%, 1 wt% and 3 wt%, respectively. The stretching process can be divided into three stages through WAXS and SAXS results, whose boundary is the end of the crystallinity platform and the onset of the formation of nanofibrils. In the first stage, the films show elastic stretch with the crystallinity keeping constant and the d -spacing of (101) plane (d 101) decreasing. The unstable crystal breaks up and transfers into amorphous during the second stage so that the force is gradually removed from the unit cell. The connection between the crystal network breaks up in the third stage when the content of boric acid is not sufficient enough. Moreover, increasing the concentration of boric acid leads to the earlier emergence of nanofibrils due to the lower energy barrier induced by entropy decrease, which also results in a reduction of nanofibrils content by limiting the mobility of molecular chain. The results have been applied to establish relationships between the microstructure and the physical characteristics of PVA to help us tune the processing parameters during the practical production. •The structural evolution of poly (vinyl alcohol) at different concentrations of boric acid during stretch were investigated with in-suit WAXS and SAXS. •The addition of adequate boric acid can maintain the connection between the crystal. •Increasing the concentration of boric acid leads to the earlier emergence of nanofibrils. •The boric acid works as the crosslinking agent which limits the mobility of the molecular chain to form more nanofibrils. [ABSTRACT FROM AUTHOR]