Single-molecule Force Spectroscopy Studies On The Structure-property Relationship Of Polyvinyl Alcohol And Its Iodine Complexes

Polyvinyl alcohol(PVA)is a macromolecule with carbon-carbon main chain and a large number of hydroxyl groups.PVA itself has many good mechanical properties and physical and chemical properties,so it is widely used in all walks of life.When people use PVA,it is often necessary to dissolve it in water before subsequent processing.However,the solubility of PVA is greatly affected by the degree of alcoholysis and polymerization.When the degree of alcoholysis is 99%,it is not easy to dissolve at room temperature.The researchers propose that the solubility of PVA can be improved by interfering with the continuous intramolecular hydrogen bonds between adjacent hydroxyl groups.Since the discovery of PVA-iodine complex,it has been widely used in people’s daily life.Since its properties are closely related to the interaction between them,the structure of PVA-iodine complexs has received extensive attention.Researchers have proposed many structural models,such as helical model,aggregation model,etc.Although many scholars have studied the PVA-iodine complexs by various clustering methods,the objects studied by the clustering method are macroscopic systems containing a large number of PVA-iodine complexs.Due to the complexity of the system,there are still many uncertainties in the proposed structural model,and the interaction mechanism of PVA and iodine is still unclear.In this thesis,the intramolecular hydrogen bonding of PVA with different alcoholysis degrees and the interaction between PVA and iodine were studied at the single molecular level by single molecular atomic force microscopy.Firstly,the single chain elasticity of PVA with different alcoholysis degrees in dimethyl sulfoxide,vacuum and nonane was studied,and the possible reason for the low water solubility of PVA-99 at room temperature was proposed.Secondly,the rationality of single chain helical structure model of PVAiodine complex was studied.Finally,the effect of iodide ion on the single chain behavior of PVA was studied in potassium iodide solution.Based on the above research,this thesis mainly draws the following conclusions:(1)The intramolecular hydrogen bond strength of PVA increases with the increase of its alcoholysis degree,and there is a synergistic enhancement effect between intramolecular hydrogen bonds.The strong synergy of intramolecular hydrogen bonding of PVA-99 may be a key factor for its poor solubility at room temperature.(2)In potassium iodide/iodine solution(polyiodide ion),the interaction between Amylose single chain and iodine,PVA single chain and iodine was studied.the results showe that Amylose can form the helical structure mentioned in the literature.However,the force-extension curve of PVA does not appear as a platform representing the helical structure,which indicates that the single chain helical structure proposed by the predecessors should not be the binding mode of the PVA-iodine complex.(3)The single chain mechanical behavior of PVA was studied in potassium iodide solution(iodine ion),and the results showed that there was an interaction between the PVA single chain and iodide ions,which was reflected as a long platform on the force curve,and extra energy was consumed during the stretching process,resulting in the PVA single chain become more flexible.