Effect of boric acid and heat treatment for the formation of poly(vinyl alcohol)/iodine complex films iodinated at solution before casting

Abstract

This study examined the role of boric acid and the effect of heat treatment on PVA-iodine polarizing films prepared in the solution state before casting (IBC) of PVA/iodine/boric acid films. The films were prepared by casting aqueous solutions of 10 wt % poly(vinyl alcohol) (PVA) containing boric acid with 0, 0.1, 0.3, and 0.5 mol/l of I 2 /KI aqueous solution, and I 2 /KI(1 : 2) with 5 wt % of PVA. The effect of boric acid and heat treatment on the durability of the IBC PVA polarizing sheet films was investigated by UV-vis absorption spectroscopy. Boric acid was found to be essential for the complex formation in PVA/iodine solutions at relatively low I 2 /KI concentrations and high temperatures. The strength of the complex peak at $ 600 nm in UV-vis absorption spectra increased with increasing boric acid concentration. With increasing heating temperature over 90 C the intensity of the peak at 600 nm corresponding to the complex decreased due to the evaporation of I 2 decomposed from I 5 À , but the peak at 355 nm corresponding to free I 2 ÁI 3 À was remained unchanged. From heat treatment at 150 C, the intensity of the peak at 600 nm decreased but the intensity of the complex peak (600 nm) of the sample with 0.5 mol/l boric acid was unaffected. The transmittance and degree of polarization for the films increased and decreased with increasing heat treatment time under heat and a humid atmosphere, respectively. However, this tendency decreased with increasing boric acid concentration and heat treatment.

Molecular iodine/polymer complexes

“…High amounts of solvents such as acetone, DMF, methanol, ethanol, pyridine, or formic acid, will destroy the complex and the color will fade. Shin et al [173,174] reported the use of boric acid treatment to form I 2 PVA polarizing film characterized by its transmittance and polarization degree. The acid promoted the formation of iodide species I 3¯ and I 5¯, and improved the durability of the film under heat and humidity.…”

“…The acid promoted the formation of iodide species I 3¯ and I 5¯, and improved the durability of the film under heat and humidity. Boric acid seemed to disfavor the crystallization, inducing the formation of an amorphous region [173]. In fact, Woo et al [172] worked on the effect of boric acid on the crystallinity and the drawability of the I 2 PVA films made by iodinating the PVA solution before and after casting its solution, BC and AC, respectively.…”

Effect of boric acid treatment method on the characteristics of poly(vinyl alcohol)/iodine polarizing film

“…The five absorption peaks that appeared at ∼ 200, 310, 370, 480, and 650 nm were assigned to I − , I 3 − , I 2 ·I 3 − , I 3 − , and I 5 − ions, respectively. The first three peaks were confirmed by the experiment for PVA dissolved in the I 2 /KI solution, and the last two peaks were attributed to the formation of the PVA/iodine complexes 6, 13. The strength of the first three peaks was greater than the last two peaks, indicating that mostly the iodine ions (I − , I 3 − , and I 2 ·I 3 − ) existed in the PVA films, but a few PVA/iodine complex ions (PVA‐I 3 − and PVA‐I 5 − ) were also present.…”

“…In our previous studies, we have estimated the effect of boric acid on durability of PVA–iodine polarizing sheet film12, 13 in the iodination at solution before casting (IBC); boric acid combined with the PVA chain to form intramolecular crosslinking segments that were regarded as a defect in the crystal, which decreased the melting temperature of PVA. The formation of I 3 − and I 5 − increased with increasing boric acid content in IBC state.…”

Formation and dichroism of poly(vinyl alcohol)–iodine complex in photocurable film

“…Boric acid forms crosslinked bonding in PVA, and it stabilizes PVA–polyiodine complexes in the polymer. The effects of boric acid on PVA–polyiodine complexes and the reactions have been studied, and a mechanism has been proposed …”