Network structure of swollen iodine-doped poly(vinyl alcohol) amorphous domain as characterized by low field NMR

Abstract

The network structure in the amorphous domain of swollen iodine-doped poly(vinyl alcohol) (PVA) was systematically investigated by low-field (LF) NMR techniques to reveal the PVA–iodine complex formation mechanism. Three PVA–iodine complexes were obtained under different iodine concentrations (c_iodine) of KI/I₂ solution: (i) c_iodine < 0.1 M: PVA–I₃⁻/I₅⁻ complex only exists in the non-crystalline region, (ii) 0.1 M < c_iodine < 1 M: formation of PVA–I₃⁻ complex I, and (iii) c_iodine > 1 M: formation of PVA–I₃⁻ complex II. It was found that there is no intermediate-magnitude chain motion of PVA under dyeing conditions to induce the substance exchange, as evidenced by the unchanged second moment M₂ (∼1.2 × 10⁴ m s⁻²) at elevated temperature (<380 K). The introduction of iodine ions can affect the chain mobility of the interphase and mobile regions. With increasing c_iodine, the chain dynamics become more restricted, as detected by the faster decay of the T₂ relaxometry results, which further accelerates the complexation process. The residual dipolar coupling strength, D_res, obtained by the more quantitative double-quantum (DQ) NMR, increases abruptly at c_iodine > 1 M. This suggests more constraints form in the amorphous network for the PVA–I₃⁻ complex II system. The constant defects fraction further reveals that the complexation prefers to happen along the tie chains. These results supply a possible formation pathway for the PVA–iodine complexes.