Study on the Complex Interaction betweetn Poly(vinyl alcohol) and Iodine Complex by Single-molecule Force Spectroscopy

The poly(vinyl alcohol) (PVA)-iodine complex has been widely used in industry due to its unique optical dichroism. The properties of PVA-iodine composites are closely related to their interaction modes and strength. However, due to the complexity of the studied systems, there is still no consensus on the exact structure of the PVA-iodine complex. According to the number of PVA chains in the complex model, the existing models can be divided into the single-chain model and multi-chain model. The single-chain model is proposed based on the helical structure model of the amylose-iodine complex. Thus, to investigate the most probable structure of the PVA-iodine complex, the interactions between PVA-iodine and amylose-iodine have been investigated by single-molecule force spectroscopy in deionized (DI) water and KI/I-2 solution, respectively. When the single amylose chain is stretched in DI water, the sugar ring will undergo a chair-boat conformational transition, showing a shoulder plateau on the force-extension (F-E) curves. When it is stretched in KI/I-2 solution, a long plateau (similar to 40 pN) will appear except the shoulder plateau, which corresponds to the unwinding of the helical structure in the amylose-iodine complex. However, the F-E curves of PVA obtained in the KI/I-2 solution only contain a single peak and do not exhibit a long plateau similar to that of amylose in the KI/I-2 solution. Moreover, the F-E curves of PVA obtained in DI water and KI/I-2 solutions with different concentrations can be superposed well after normalization. The experimental results show that the interaction between PVA and iodine ions is different from that between amylose and iodine ions. In other words, the PVA-iodine complex does not exist in a helical structure, and the single-chain model is not suitable to describe the interaction between PVA and iodine. This study lays the foundation for further study of the multi-chain model of the PVA-iodine complex.