Clarification of Cross-Linkage Structure in Boric
Acid Doped Poly(vinyl alcohol) and Its Model Compound As Studied by
an Organized Combination of X‑ray Single-Crystal Structure
Analysis, Raman Spectroscopy, and Density Functional Theoretical Calculation
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Abstract
When
boric acid (BA) is added to poly(vinyl alcohol) (PVA), a chemical
reaction occurs to form the cross-linkages between the amorphous PVA
chains. The local structural change caused by this reaction has been
clarified concretely from the microscopic level on the basis of the
X-ray-analyzed crystal structure, Raman spectra, and <i>ab initio</i> density functional theory using a model compound produced by the
reaction between pentanediol (PENT) and boric acid (PENT–BA).
The PENT–BA compound was found to take the TT and TG conformations
in the methylene segmental parts depending on the stereoregularity
of the PENT molecule itself, <i>meso</i> and <i>racemo</i> configurations, respectively. These two conformations give the Raman
bands at the different positions. By comparison of the Raman spectra
between the PVA–BA and PENT–BA model compounds, the
local structures of PVA chains connected to BA molecules have been
derived concretely: the syndiotactic PVA parts in the amorphous region
form the TG-type ring structure with the 3-coordinate boron atom,
where T and G are trans and gauche conformers, respectively. On the
other hand, the isotactic PVA part takes the TT conformation when
it forms a ring with boron atom. The thus-created rings are hydrogen-bonded
to form a dimer, which plays a role as cross-linkage between the neighboring
PVA chain segments in the amorphous region