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 (<i>c</i>_iodine) of KI/I₂ solution: (i) <i>c</i>_iodine < 0.1 M: PVA-I₃^-/I₅^- complex only exists in the non-crystalline region, (ii) 0.1 M < <i>c</i>_iodine < 1 M: formation of PVA-I₃^- complex I, and (iii) <i>c</i>_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 <i>M</i>₂ (∼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 <i>c</i>_iodine, the chain dynamics become more restricted, as detected by the faster decay of the <i>T</i>₂ relaxometry results, which further accelerates the complexation process. The residual dipolar coupling strength, <i>D</i>_res, obtained by the more quantitative double-quantum (DQ) NMR, increases abruptly at <i>c</i>_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.