Evidence for complexation-induced micro-extension of poly(vinyl alcohol) chains in interphase and amorphous domains from solid-state NMR

• Published in: Soft matter 2022-12, Vol.18 (47), p.8974-8982
• Main Authors: Li, Yahui, Tao, Wei, Chen, Wei
• Format: Article
• Language: English
• Subjects: Chains; Conformation; Domains; Hydrogen bonding; Hydrogen bonds; Interphase; Iodine; NMR; Nuclear magnetic resonance; Polyvinyl alcohol; Resonance lines; Solid phases; Solid state
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d2sm01136k

The three-phase structure of poly(vinyl alcohol) (PVA)iodine complexes was elucidated by solid-state NMR (SSNMR), of which the micro-extension of the PVA segment in the interphase and amorphous domains was directly confirmed. The three-phase structure of the PVAiodine complex was decomposed by the inverse 13 C T 1 -filter, where 13 C NMR resonance lines of a C(H) triplet were observed in all three phases. The chain axis of 26% extended chains in the interphase deviates 35°-50° relative to the stretching direction, while there is only a 1° deviation for the extended chains in the crystalline domain. The increasing iodine concentration results in the increment of hydrogen-bonded C(H) fractions in both the amorphous and interphase domains, while the distribution of different C(H) fractions remains almost constant in the crystalline domain. Such an increment results from the locally extended PVA chains induced by polyiodine species (I 3 − /I 5 − ), since the hydrogen bond(s) (HBs) required a specific direction. Direct evidence for this comes from the similar 13 C CP/MAS spectra of C(H) in the three phases between unoriented iodine-doped PVA and highly oriented pure PVA. This supports the aggregation model for the formation mechanism of PVA-iodine complexes, where the PVA chain takes an extended zig-zag conformation. The effect of stretch-induced macro-extension vs. complexation-induced micro-extension on PVA films as revealed by the separation of a three-phase structure via Torchia 13 C CP T 1 experiments.