Modulation-Excitation Spectroscopic Analysis of Dynamic Behavior of Iodine in Poly(Vinyl Alcohol) Film

The development of highly durable polarizing films necessitates a comprehensive understanding of the molecular dynamics occurring within the material. In this study, we employed modulation-excitation infrared spectroscopy (ME-IR) and modulation-excitation X-ray absorption spectroscopy (ME-XAS) under simultaneous control of temperature and humidity to investigate the dynamic behavior of iodine-containing poly(vinyl alcohol) (I-PVA). ME-IR provided spectroscopic evidence of a sequence of events upon H2O exposure: initial adsorption of H2O onto I-PVA, followed by the formation of bulk H2O through interactions with boronic acid, and subsequent decomposition of the iodine complex I5– into I–. Differentiated ME-IR spectra under H2O-concentration modulation (wet–dry alternation) revealed distinct signals corresponding to adsorbed H2O, bulk H2O, and boronic acid, indicating their dynamic behavior in response to humidity changes. ME-XAS detected reversible transformations among mono- and polyiodide species (I–, I3– and I5–), and phase-sensitive detection analysis elucidated the sequential nature of these processes. These findings provide critical insights into the molecular-level mechanisms governing the stability and performance of I-PVA-based polarizing films under varying environmental conditions.