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

Yahui Li; Junsheng Yang; Hong Cheng; Linkun Cai; Ke Ye; Zhijie Xia; Qianlei Zhang; Daoliang Wang; Wei Chen

doi:10.1039/d1sm00988e

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

Soft Matter. 2021 Oct 13;17(39):8973-8981. doi: 10.1039/d1sm00988e.

Authors

Yahui Li¹, Junsheng Yang^{1

2}, Hong Cheng¹, Linkun Cai², Ke Ye¹, Zhijie Xia¹, Qianlei Zhang¹, Daoliang Wang¹, Wei Chen¹

Affiliations

¹ National Synchrotron Radiation Laboratory, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China. wc003@ustc.edu.cn.
² South China Advanced Institute for Soft Matter Science and Technology, School of Molecular Science and Engineering, Guangdong Provincial Key Laboratory of Functional and Intelligent Hybrid Materials and Devices, and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China.

PMID: 34558595
DOI: 10.1039/d1sm00988e

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.