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

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 ab initio 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, meso and racemo 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.

Well-dispersed cellulose nanocrystals in hydrophobic polymers by in situ polymerization for synthesizing highly reinforced bio-nanocomposites

“…The band at 932 cm −1 is assigned to the C-O stretching in polyvinyl alcohol, indicating that the degree of hydrolysis of PVAc was enhanced due to the crosslinking reaction. The increased intensity of the band at 807 cm −1 could be assigned to the newly formed borate ring including C-C bonds, as reported by Itou et al, 44 which implies that the crosslinks formed by tetra-coordinated borate ions were generated between the PVAc chains as shown in the chemical structure in Fig. 3a.…”

Crosslinked poly(vinyl alcohol) composite films with cellulose nanocrystals: Mechanical and thermal properties

“…The possible crosslinking structure obtained from PVA and complexes between borate ions and hydroxyl pairs of adjacent PVA chains, as shown in Figure . However, another concept of the crosslinking model between PVA and borax has been introduced . Raman results revealed that borax compounds form crosslinks between with adjacent PVA chains and this crosslinkage is reported to be temperature dependent, the number of hydrogen bonds in the PVA‐borax system decreased at high temperature and can be recovered at a low temperature …”

[Papers] Achromatic Dye-type Polarizer for Paper White Reflective Liquid Crystal Displays

“…Alignment control of dichroic pigments is also important for achieving a highly achromatic polarizer with a uniform dichroic ratio in the visible wavelength range. In general, the optimum conditions for the alignment process are different for each dichroic pigment because of their different chemical structures [22][23][24][25] . The migration of dichroic pigments in the thickness direction of the film also varies depending on the chemical structure; therefore, uniform stretching of the PVA film is important for improving the alignment uniformity of dichroic pigments in the thickness direction of the film.…”