Elsevier

Polymer

Volume 233, 26 October 2021, 124180
Polymer

X-ray study of Poly(vinyl Alcohol)-Iodine complex prepared from the dilute iodine solution as a hint to know the inner structure of polarizer

https://doi.org/10.1016/j.polymer.2021.124180Get rights and content

Highlights

  • A new crystal form IV was discovered for poly (vinyl alcohol)-iodine complex.

  • Crystal structure of form IV has been analyzed by X-ray, Raman and DFT method.

  • The inner structure of a commercial optical polarizer was derived concretely.

Abstract

As well known, poly (vinyl alcohol) (PVA) forms the various kinds of iodine complexes (forms I, II and III) when the sample is immersed in the KI/I2 aqueous solution of various iodine concentrations of 0.1–3 M. By immersing the PVA sample in more dilute solution of 0.001–0.05 M, a new X-ray diffraction pattern has been found (form IV). The quantitative analysis of the X-ray diffraction pattern combined with the resonance Raman data and the density functional theoretical calculation allowed us to derive the concrete structure model of form IV. The vertically-arrayed linear I5 ions are surrounded by the 6 PVA chains of planar-zigzag conformation to form the cylindrical structure. These cylinders are gathered together to form the hexagonal lattice. On the basis of the thus-determined structural information of a series of complexes, the transformation mechanism has been proposed for forms IV, I, II and III. Besides, the X-ray diffraction pattern of form IV was found to be similar to that measured for a commercial polarizer. The inner structure of the polarizer has been discussed concretely using the thus-revealed aggregation structure of form IV.

Introduction

When a highly-oriented poly (vinyl alcohol) (PVA, -[CH2CH(OH)]n-) film is immersed into an aqueous KI/I2 solution of an iodine concentration of 0.001–0.01 M, PVA-iodine complex is formed, which is industrially utilized as an optical polarizer [[1], [2], [3], [4], [5], [6]]. The complex is said to be created in the amorphous region of PVA [[4], [5], [6], [7], [8], [9], [10], [11], [12]]. Because of the unclear X-ray diffraction pattern, however, the inner structure of the polarizer has not yet been established enough well beyond the speculative images, in spite of the increasing usage of the polarizer in these several decades. When the iodine concentration is increased in 1–2 order (ca. 0.1–3 M), the iodine complex is formed even in the crystalline region of PVA [[13], [14], [15], [16], [17], [18], [19], [20]]. In a series of the papers [[16], [17], [18], [19], [20]], we found the various types of crystalline PVA-iodine complexes, the formation of which depends on the iodine concentration, the type of counter cation species (K+, Na+, Li+, H+, etc.) and so on. These complexes are called the forms I, II and III. Based on the quantitative analyses of the 2D X-ray diffraction and IR/Raman vibrational spectroscopic data in combination with the computer simulation technique, we proposed the aggregation structures of PVA chains and iodine ion species in the crystalline lattices of these complex forms. The detailed structure information of these crystalline complexes should be a good hint for the study of inner structure of the polarizer.

However, the iodine concentration used in our previous studies is appreciably higher than that required for the industrial production of an optical polarizer [[1], [2], [3], [4], [5], [6],21,22]. In the present study, we have changed our target on the PVA-iodine complex formed from a dilute iodine solution of 0.001–0.05 M concentration. The 2-dimensional X-ray diffraction pattern detected for the thus-prepared sample was found to be different from those of forms I, II and III. The crystal structure analysis was performed successfully for the thus newly-found crystalline form by combining the X-ray diffraction data, the Raman data, and the quantum mechanical calculation. This new crystal form was found to be similar to that of a commercial polarizer itself. By referring to all the experimental data and the structure information revealed for the various forms of PVA-iodine complexes, the inner structure of the polarizer was investigated concretely.

Before the detailed description of the analyzed results, it is useful to remind about the X-ray diffraction patterns and the corresponding crystal structures characteristic of the previously-revealed crystalline forms of PVA-iodine complexes. As mentioned above, depending on the iodine concentration, the crystalline PVA-iodine complexes are classified into 3 types; I (ca. 0.1 ∼ ca. 0.8 M), II (ca. 0.8 ∼ ca. 3 M) and III (ca. 3 M, only for H+ counter ion) [[16], [17], [18], [19], [20]]. The observed 2D X-ray diffraction patterns of these complexes are shown in Fig. 1. The corresponding crystal structures are shown in Fig. 2 [20].

  • (i)

    PVA Fig. 1 (a) shows the 2-dimensional wide-angle X-ray diffraction (WAXD) pattern of a uniaxially-oriented PVA sample measured using a Mo-Kα X-ray beam. As seen in Fig. 2 (a), the 2 planar-zigzag PVA chains are contained in the monoclinic unit cell, and they are combined together by the intermolecular hydrogen bonds of OH⋯O type [20,[23], [24], [25]]. But, this structural model is only an approximation, and could not satisfy the independently-measured neutron diffraction data enough well [20]. The introduction of the chain packing disorder was found to reproduce both of the X-ray and neutron diffraction data consistently. The individual domains are composed of the above-mentioned pairs of the hydrogen-bonded PVA chains. But, these domains are aggregated together with the 1/2 slippages along the 110 planes [see Fig. 2 (a)]. In this way, the crystalline phase of PVA consists of the disordered arrays of the domains composed of the hydrogen-bonded chains [20].

  • (ii)

    PVA-iodine complex form I By the immersion of an oriented PVA film into a dilute iodine solution of ca. 0.1 M ∼ ca. 0.8 M concentration, the crystal form I is created [[16], [17], [18], [19]]. As seen in Fig. 1 (b), the several sharp diffraction peaks are observed along the equatorial line in a low diffraction angle range. The strong streak lines are detected along the layer lines, which are originated from the 1-dimensionl columnar structure of I5 and I3 ions with the relative height disordering [16,17]. These iodine columns invade into the spaces between the originally-hydrogen-bonded PVA chains, and the OH⋯I hydrogen bonds are formed between PVA chains and iodine ions. However, the crystalline phase is not totally occupied by the iodine complex structure, but the PVA chain pairs remain coexistent partly in the crystalline lattice [Fig. 2 (b)]. The occupancy of iodine ions is not very high.

  • (iii)

    PVA-iodine complex form II As the iodine concentration is increased to ca. 1–3 M, the form II complex is obtained. The equatorial diffraction peaks are sharp. The strong streak lines characteristic of the I3 columnar structure are detected along the layer lines. In the cases of the original PVA crystal and form I, the zigzag planes of the PVA chains are oriented along the rolled plane (or the film plane). In form II, these PVA chains are rotated by 38° around the chain axis so as to form the more effective packing structure of PVA and I3 ion columns. The O–H⋯I hydrogen bonds are formed in higher population, as known from the IR and Raman spectral data and the detailed calculation by the density functional theory (DFT) [19]. The packing disorder of the finite domains, as observed in the original PVA crystal, is still detected in forms I and II, as illustrated in Fig. 2 (b) and (c), respectively [20].

  • (iv)

    PVA-iodine complex form III This complex is formed by immersing the PVA film in the HI/I2 solution of 3 M concentration for a long time [18]. The counter cation is H+. Such counter cation as Li+, Na+ and K+ does not create the form III even when the immersion time is longer. The packing structure of PVA chains and I3 ion columns becomes more regular [see Fig. 2 (d)].

Purpose of the present study In this way, the various types of PVA-iodine complexes are detected. However, these complexes are formed for the solutions of iodine concentration ca. 0.1 ∼ ca. 3 M. How about the case of highly-diluted iodine solution of 0.001–0.05 M, as used for the industrial production of a polarizer ? The several models were proposed for the aggregation state of PVA and iodine ion species in the dilute solution or in the amorphous region. For example, Zwick proposed a helical structure model [4], where the long iodine rod is trapped inside the helical PVA chain as seen in the iodine complex of amylose [26] (see Fig. 3 (a)). Choi and Miyasaka proposed the PVA-iodine complex model in the amorphous region, where an I5 ion is enclosed by the 4 PVA chain segments as illustrated in Fig. 3 (b) [7]. Unfortunately, however, any clear experimental evidence, for example, X-ray diffraction data, was not presented for deriving the model, which is only speculative. Matsunaga et al. drew a model of the I3 ion rods adsorbed on the PVA crystallite surfaces [Fig. 3 (c)] [8]. By analyzing the small-angle neutron scattering data collected for the solution containing the PVA oligomers of about 15 monomeric units, iodine ions, and CuCl2 as a color inducing agent, Yajima [9] proposed a double-cylindrical model in which the I3 ion column of 66 Å length is surrounded by the 4 PVA chains and then by the 8 PVA chains as the second cylindrical tube. The thus-created cylinders were speculated to aggregate together in the solution. However, their inner structure of the cylinders was that transferred from the speculative model proposed by Choi and Miyasaka [7].

In this way, the PVA–iodine complex produced from the dilute iodine solution has not yet been investigated enough well. Besides, no experimentally-confirmed structure model was reported. As will be described here, we have found out a new X-ray diffraction pattern intrinsic to the PVA sample immersed in a dilute iodine solution. This new complex is called form IV here. By using mainly the wide-angle X-ray diffraction method, the various factors (iodine concentration, immersion time, effect of boric acid etc.) have been investigated to establish the formation condition of form IV. The quantitative analysis was performed for the observed X-ray diffraction data, and the concrete structure model has been derived successfully by combining with the Raman spectral data and the DFT calculation. As a result, we have obtained the information of the aggregation structures for 4 types of PVA-iodine complexes in total. On the basis of these structural information, the formation mechanism of a series of complexes has been discussed. Finally, the X-ray diffraction pattern obtained for the commercial polarizer was found similar to that of form IV, although the details of the preparation conditions of the polarizer are not necessarily the same as those reported here. The concrete information about the chain aggregation structures in forms I–IV can help us to speculate the aggregation state of iodine complexes in a commercial polarizer.

In order to avoid the misunderstanding of the readers, we need to point out here the significance of the present report. As mentioned above, the several models were reported about the PVA-iodine complex in the oriented amorphous region or in the solution [4,[7], [8], [9]], which were believed to be the real model of the polarizer. The SANS analysis derived the cylindrical aggregation model experimentally [9]. However, the inner structures proposed in Refs. [4,7,9] were only speculative and were not supported confirmatively by the experimental data. Especially, no X-ray diffraction pattern had been reported about the oriented-amorphous PVA-iodine complex at all. The discovery of form IV complex has allowed us to imagine the concrete structural image of the polarizer.

Section snippets

Samples and preparation conditions

Samples PVA samples were supplied by Kuraray Co. Ltd., Japan. The degree of polymerization was 1700, and the degree of saponification was 99.9%. The sample was dissolved into hot water and cast to the films at room temperature. The films were stretched about 5 times the original length on the hot plate at 90 °C. The film thickness was about 80 μm. Annealing of the film was performed at 160 °C for 60 min in a vacuum oven. The as-stretched or heat-treated films of the same thickness were immersed

Effect of iodine concentration on X-ray diffraction pattern

The 2-dimensional X–ray diffraction patterns were measured for the as-stretched and un-annealed PVA samples immersed in the KI/I2 solutions of the various iodine concentrations. Fig. 4 (a) shows the typical 2D WAXD patterns and Fig. 4 (b) shows the corresponding 1D diffraction profiles along the equatorial line. The diffraction patterns of forms I (0.5 M concentration) and II (1.0 M concentration) are the same as those shown in Fig. 1. The newly-found diffraction pattern is that of form IV

Summary and conclusion

In the present report, the creation of a new form IV was detected for the PVA sample immersed in an iodine solution of an extremely low concentration of 0.001–0.05 M. The structural characteristics of form IV were extracted successfully by the detailed quantitative analysis of X-ray diffraction and Raman data as well as the DFT calculation. The crystal structure of form IV was proposed, which is the hexagonal-type aggregation of the cylinders composed of I5 ion rod enclosed by 6 PVA zigzag

CRediT authorship contribution statement

Kohji Tashiro: Conceptualization, Methodology, Data curation, Formal analysis, Data analysis, Writing – original draft. Tomohiko Takahama: Formal analysis, Simulation, Analysis. Meng Fan Wang: Data curation, Formal analysis, Data collection, Analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors wish to thank the Nitto Denko Cooperation, Japan, for supplying a commercial polarizer. They thank also the Kuraray Co. Ltd., Japan, for supplying the PVA sample.

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