Structural Evolution Of Poly(vinyl Alcohol) Film During Polarizer Processing By Synchrotron Radiation And Nuclear Magnetic Resonance

New displays are becoming increasingly important as interfaces for humanmachine interaction.With the demand for diverse application scenarios,portable production,and intelligent engineering technology,new displays are gradually evolving towards lightweight,flexible,high-contrast,and high-durability.Polarizing films are one of the core components of new displays and play a crucial role in determining the display quality.The most commonly used polarizing film currently is poly(vinyl alcohol)(PVA)-iodine polarizing film.PVA possesses a multi-hydroxyl structure that can form dichroic PVA-iodine complexes through the coordination of polyiodide ions.The formation of this complex is the source of dichroism of the PVA polarizing film.The I5and I3-ions present in the complex cover the long and short wavelengths of ultravioletvisible(UV)light,respectively,allowing stretched PVA-iodine complexes to exhibit polarized absorption of UV light across the entire wavelength range.Therefore,regulating the structure of the PVA-iodine polarizing film and the internal PVA-iodine complex is critical for enhancing the polarization performance and service performance of the polarizing film.Currently,the knowledge regarding the processing and manufacturing of PVA-iodine polarizing films is mostly concentrated in Japan.In China,a complete theoretical system has not been established for the processing and manufacturing of PVA polarizing films,and there is a lack of reliable indicators for key parameters and technical control.This lack of information has constrained the development of PVA-iodine polarizing films towards high polarization,high transmittance,and high durability,limiting their potential applications in high-end display products.In industrial processes,the fabrication of PVA-iodine polarizing films involves a highly intricate and multi-step manufacturing procedure that includes processes such as iodine doping,boric acid crosslinking,and stretching.These processes are governed by various intermolecular interactions and the evolution of the internal multiscale structure of the film,which face significant challenges in achieving precise control over the final oriented PVA-iodine complex structure.By distilling the scientific issues involved in each processing step and investigating the underlying physical mechanisms,not only can a robust theoretical framework be established,but it can also play an important role in constructing the dependence relationship between the microscopic structure and macroscopic properties,as well as in the innovation and creation of controllable highperformance products.This work systematically investigates the multi-scale structure and dynamics of PVA-iodine polarizing film,as well as the microstructure evolution mechanism involved in various processing steps,using synchrotron radiation X-ray scattering and solid-state nuclear magnetic resonance(NMR)techniques.The main works and findings are summarized as follows:(1)This study proposes a mechanism of selective spatial formation of the PVAiodine complexes.Through low-field NMR and synchrotron radiation wide/small-angle X-ray scattering experiments,the study investigated the effect of iodine concentration on the crystal structure and amorphous structure of PVA films and connected the PVAiodine complexes in amorphous and crystal regions.Based on crystal types,iodine concentration was divided into three ranges:ⅰ)0～0.1 M for pure PVA crystal,ⅱ)0.1～1 M for PVA-I3-complex Ⅰ crystal,and ⅱ)1-5 M for PVA-I3-complex Ⅱ crystal.it is observed that the introduction of iodine reduces the chain mobility of the entire system,but does not affect the long period,lamellar thickness,and entanglement density of the amorphous domain within the first two concentration ranges.Furthermore,the molecular chains in the PVA crystalline domain do not exhibit any chain motion on the time scale of～100 μs at all iodine concentrations.These findings indicate that the polyiodide ions preferentially complex with tie chains located near the interphase of PVA.(2)This study utilizes 13C T1-filtered NMR spectra to selectively detect the threephase structure of the PVA-iodine complex system.The consistency between complexation-induced micro-orientation and stretch-induced macro-orientation is proposed.The findings include the following:ⅰ)the three phases contain three different types of hydrogen bonds,and complexation can cause changes in their relative fraction;ⅱ)segmental orientation differs among the three phases,with chains in the crystalline domain oriented basically along the stretching direction,those in the interphase exhibiting a disordered all-trans conformation with the deviation of 35°～50° to the stretching direction,and chains in the amorphous domain being more random;ⅲ)linear polyiodide complexation and stretching have consistent effects on the local orientation of PVA segments;ⅳ)PVA segments with a zig-zag conformation are more likely to form PVA-iodine complexes.Additionally,stretching reduces the entropy of PVA chain conformation,allowing complexation to occur in the amorphous domain at low iodine concentrations and promoting the formation of hydrogen-bonded structures in this domain.These results provide further evidence for the proposed complexation mechanism in(1).(3)The relationship between the performance,microstructure,and dynamics of iodine-doped PVA films is elucidated.Based on the conclusions from the previous work,four iodine-doped PVA films with typical PVA-iodine complexes are selected.Onedimensional and two-dimensional spectra from solid-state NMR are used to analyze the stability of hydrogen bonding and molecular geometry in the iodine-doped PVA films.It is revealed that the thermal stability of the iodine-doped PVA films is not related to the stability of molecular geometry but rather to the strength of the 1H-13C dipolar interaction.Based on the results of stability studies,appropriate iodine concentration is chosen to investigate the relationship between orientation and polarization performance.Based on the results of synchrotron radiation wide-angle X-ray scattering and polarization performance,it is found that the formation of PVA-iodine crystals is not the key to achieving high polarization properties,but rather the precise control of macroscopic polarization performance is dependent on the qualitative and quantitative analysis of the orientation of PVA molecular chains,oriented polyiodide ions,and iodine ion species.(4)This work reveals the enhanced recoverability of the amorphous network of PVA due to boric acid crosslinking and elucidates the deformation mechanisms of the multi-network structure of PVA.Synchrotron radiation X-ray experiments track the evolution of the crystal skeleton with boric acid concentration.It is revealed that crosslinking mainly occurs on the lateral side of the crystal planes at low to moderate concentrations(below 2 wt%).Crosslinking increases the stiffness of the crystal network and reduces the deformation of the amorphous network,resulting in enhanced recoverability.At high boric acid concentrations,the onset of damage to the surface of PVA crystal is observed.Meanwhile,the density of restricted points within the amorphous network,as determined by low-field NMR,exhibits a linear dependence with the elastic modulus obtained from the linear region of the mechanical stress-strain curves.A deformation model based on the Maxwell model is employed to explain the enhancement in the elasticity of the deformed network resulting from boric acid crosslinking.