From Molecular Defect to Macroscopic Performance: Manipulating Dichroism and Mechanics in Iodine-doped Poly(vinyl alcohol) via Ethylene Incorporation

Abstract

Molecular chain defect engineering is a pivotal strategy for tailoring the macroscopic properties of polymeric materials. However, establishing a definitive relationship between molecular-level defects and bulk material performance remains challenging, due to the difficulty in precisely controlling not only the molecular architecture but also the subsequent chain packing, which critically governs material performance. In this work, we demonstrate that introducing ethylene units as chain defects into poly(vinyl alcohol) (PVA) enables precise modulation of the dichroism in iodine-doped PVA polarizers. This approach allows for concurrent control over the iodine-PVA complexation, the crystalline network and the mechanical properties. The dichroic behavior originates from the formation of oriented I₃⁻ and I₅⁻ species, with I₃⁻ absorbing between 400–520 nm and I₅⁻ between 520–780 nm. Therefore, optimizing optical performance—namely transmittance and polarization efficiency—requires careful tuning of the I₃⁻/I₅⁻ ratio and their overall concentration. The incorporation of ethylene defects addresses this need through three synergistic effects: (1) It increases the I₃⁻/I₅⁻ ratio by shortening the vinyl-alcohol sequence length in the amorphous regions, given that ethylene unit does not complex with iodine; (2) It reduces the overall crystallinity, as ethylene units are excluded from the crystalline domains; (3) It weakens intermolecular interactions (reflected by an increased Flory-Huggins parameter), which lowers the modulus but enhances drawability. This study illustrates how molecular-level defect engineering can be effectively translated into precise control over both chemical complexation and physical networks, thereby enabling the simultaneous manipulation of optical and mechanical properties in polymer materials.