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**Important Content:** The research group of Cui Shuxun at Southwest Jiaotong University conducted a comparative study on the interactions between polyvinyl alcohol (PVA) and iodine, and between amylose and iodine. They ruled out the single-chain PVA-iodine helical structure from several possible structures of the PVA-iodine composite, laying the foundation for further research on the multi-chain model of the PVA-iodine composite.
**Background:** Due to its unique optical dichroism, the PVA-iodine composite is widely used as the core substrate film for optical thin films and is an indispensable key component in optical devices such as liquid crystal displays (LCDs) and complementary metal-oxide-semiconductor image sensors (CMOS). The performance of PVA-iodine composites is closely related to the interaction between the two and the composite structure. However, due to the complexity of the system, a reasonable structure for the PVA-iodine composite has remained undetermined. Possible structural models for the PVA-iodine composite include the helical model, the aggregate model, and the four-chain structure model. If we use the number of PVA molecular chains involved in the interaction in the model as a standard, they can be divided into two categories: single-chain models where a single PVA chain interacts with polyiodide ions, and multi-chain models where multiple PVA chains interact with polyiodide ions. Determining the exact structure of the PVA-iodine complex helps to establish the relationship between the molecular structure of the complex and its mechanical and optical properties, and is expected to provide guidance for the rational design of materials. Single-molecule force spectroscopy (SMFS) based on atomic force microscopy is a high-precision experimental technique developed in the 1990s and has been widely used to study the elasticity and conformation of polymer single chains and the interactions between polymers and other molecules. SMFS technology can operate in different liquid environments and is suitable for studying the interaction between PVA single chains and iodine.
Article Overview: The research group of Cui Shuxun at Southwest Jiaotong University first determined the single-chain elasticity of amylose in pure water using SMFS experiments, and used this as a benchmark for studying the interaction between amylose and iodine. The typical normalized force-distance (F-E) curve of amylose in deionized water initially shows a horizontal line; followed by a slowly rising shoulder plateau between 200 and 400 pN, representing the transition of amylose from the chair conformation to the boat conformation; finally, a single peak appears at F > 400 pN, representing the enthalpy elasticity of amylose.
Subsequently, the single-chain elasticity of amylose in KI/I₂ solution (a mixed solution of 1 × 10⁻⁴ mol/L I₂ and 3 × 10⁻⁴ mol/L KI) was investigated. The F-E curve of amylose in this environment initially shows a horizontal line, representing the stretching of the complex from a free helical state to a straightened helical state; then a long plateau of approximately 40 pN appears, representing the gradual unwinding process of the amylose-iodine complex helical structure under external force; the shoulder plateau and the stretching peak representing the enthalpy elasticity of amylose are also observed. It should be noted that the different heights of the shoulder plateaus of amylose in deionized water and KI/I₂ solution are related to the different bound water contents in the two environments.
The single-chain elasticity of PVA in deionized water was studied using the same method, and the resulting F-E curve contained only a single peak representing the enthalpy elasticity of PVA. The F-E curve of PVA in KI/I₂ solution also contained only a single peak. The F-E curves obtained in these two experimental environments overlapped well, indicating that the single-chain mechanical behavior of PVA in KI/I₂ solution was basically consistent with its behavior in deionized water.
The F-E curve of amylose in KI/I₂ solution shows a long plateau representing the unwinding of the helical structure of the amylose-iodine complex. If PVA and iodine form a similar helical structure, then an F-E curve with a similar long plateau should also be obtained when PVA is stretched in KI/I₂ solution, corresponding to the unwinding of the helical structure. However, no similar plateau was observed in the F-E curve of PVA in KI/I₂ solution, indicating that single-chain PVA does not form a helical structure with polyiodide ions in KI/I₂ solution.
Through this study, the single-chain PVA-iodine helical structure was ruled out among several possible structures of the PVA-iodine complex, which also means that the multi-chain model is likely the main mode of interaction between PVA and iodine. The specific mechanism of interaction between multi-chain PVA and iodine will be investigated using SMFS technology in future studies.
This work will be published in print in the September 2023 issue of *Acta Polymerica Sinica*. The first author is Xinxin Bao, a master's student from Southwest Jiaotong University, and the corresponding authors are Dr. Yu Bao and Professor Shuxun Cui from the School of Chemistry, Southwest Jiaotong University.
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