PVA-iodine Complex Research Articles

Effect of structural evolution and orientation behavior on optical properties of iodine-dyed PVA film during uniaxial stretching

In the process of preparing polarizing film, based on the research on the influence of the initial condensed structure of Poly (vinyl alcohol) (PVA) film on the dyeing and stretching process, the connection between the structure and performance of polarizing film is established, which is more conducive to understanding the post-processing process of solution casting molding of optical grade PVA film and the optimization of subsequent dyeing and stretching process. In this paper, PVA film with an appropriate swelling degree were selected for dyeing treatment, and the evolution of the condensed structure during stretching was studied in detail. The work also examined the formation and orientation of PVA-iodine complexes and the feedback effect of complex formation on the structural evolution of the film, with the goal of establishing the relationship between stretching ratio, structure, and performance of the polarizing film. The results show that the dyeing process inhibits the recrystallization of the PVA film during uniaxial stretching. However, the PVA-iodine complexes formed act as "anchors" to stabilize the oriented microfiber network, reducing the critical strains (εw and εs) required for the PVA film to enter the strain-hardening zone. Furthermore, as strain increases, the content of polyiodide ions and PVA-iodine complexes in the dyed PVA film also increases, especially making the formation of long iodide chains (I5−) and PVA-I5− complex more sensitive. Additionally, with increasing strain, both the transmittance and degree of polarization of the dyed PVA film improve, reaching optimal values in the strong strain-hardening zone (>εs). This indicates that εs is the minimum critical strain required to ensure optimal polarization performance of the film in the visible light region.

Enhanced Dichroism of Polarizing Composite Films by Embedding Sepiolite.

The present study investigated the use of fibrous nanoparticle-filled polarizing films. Sepiolites were selected as nanoparticles and incorporated into a PVA-iodine complex. The resulting nanocomposite film was elongated and dyed with iodine. Various properties of the nanocomposite polarizing films, including thermal, morphological, optical, and rheological features, were experimentally analyzed. The study demonstrated that an increase in sepiolite loading was accompanied by an enhancement in both the mechanical and viscoelastic properties. In particular, the incorporation of nanoparticles led to an increase in birefringence and the degree of polarization. This was attributed to the alteration of the internal structure of the PVA film caused by the embedded sepiolites. The thermal analysis showed that the composite film with a higher content of sepiolites exhibited higher crystallinity and a higher melting temperature.

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

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 PVA-iodine complex was decomposed by the inverse 13C T1-filter, where 13C 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 (I3-/I5-), since the hydrogen bond(s) (HBs) required a specific direction. Direct evidence for this comes from the similar 13C 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.

Network structure of swollen iodine-doped poly(vinyl alcohol) amorphous domain as characterized by low field NMR

The network structure in the amorphous domain of swollen iodine-doped poly(vinyl alcohol) (PVA) was systematically investigated by low-field (LF) NMR techniques to reveal the PVA-iodine complex formation mechanism. Three PVA-iodine complexes were obtained under different iodine concentrations (ciodine) of KI/I2 solution: (i) ciodine < 0.1 M: PVA-I3-/I5- complex only exists in the non-crystalline region, (ii) 0.1 M < ciodine < 1 M: formation of PVA-I3- complex I, and (iii) ciodine > 1 M: formation of PVA-I3- complex II. It was found that there is no intermediate-magnitude chain motion of PVA under dyeing conditions to induce the substance exchange, as evidenced by the unchanged second moment M2 (∼1.2 × 104 m s-2) at elevated temperature (<380 K). The introduction of iodine ions can affect the chain mobility of the interphase and mobile regions. With increasing ciodine, the chain dynamics become more restricted, as detected by the faster decay of the T2 relaxometry results, which further accelerates the complexation process. The residual dipolar coupling strength, Dres, obtained by the more quantitative double-quantum (DQ) NMR, increases abruptly at ciodine > 1 M. This suggests more constraints form in the amorphous network for the PVA-I3- complex II system. The constant defects fraction further reveals that the complexation prefers to happen along the tie chains. These results supply a possible formation pathway for the PVA-iodine complexes.

Water absorption dependence of the formation of poly(vinyl alcohol)-iodine complexes for poly(vinyl alcohol) films.

Poly(vinyl alcohol) (PVA) films annealed at different temperatures are used to explore the effects of the water absorption on the formation of PVA–iodine complexes. It's found that the higher the annealing temperature, the stronger the interaction force between PVA segments, and the smaller the free volume of the PVA films. These mainly lead to the reduction of the amount of PVA segments with a moderate degree of hydration (i.e., PVA segments with moderate mobility), which are the major segments participating in the formation of PVA–iodine complexes. Therefore, PVA films with higher water absorption not only possess faster complexation speed and form more PVA–iodine complexes, but also increase the proportion of polyiodide ions with a longer length. Moreover, the complexation restricts the PVA segments with high mobility, resulting in the formation of the intermolecular ordered structure. The water absorption dependence may guide the dyeing process to obtain PVA polarizers with excellent optical performance.

Anomalous dichroism of cellulose nanowhisker embedded composite film

While natural nanoparticles are currently being investigated in a wide range of applications due to their compelling strong potential, they have yet to be employed for practical engineering material systems. Here, we demonstrate a dichroic nanocomposite film that shows outstanding optical properties by introducing cellulose nanowhiskers (CNWs) into PVA iodine complex. CNWs were isolated via acid hydrolysis and the nanocomposite film was elongated and dyed with iodine. The significantly reduced extension and treatment time of iodine were applied for the alignment of iodine and PVA molecules in the film. The results revealed that the degree of polarization and transmittance were enhanced as the draw ratio and the content of the CNWs increased. We expect that this material system can offer a new pathway for designing and processing more advanced engineering materials with high performances by using various natural biomaterials.

Details of the intermolecular interactions in poly(vinyl alcohol)-iodine complexes as studied by quantum chemical calculations

Abstract The electronic state of poly(vinyl alcohol) (PVA)-I 3 − ion complex has been successfully described for the first time on the basis of the density functional theoretical (DFT) calculations. (1) As the first trial, complex structure models between a short PVA chain (5-mer) and I 3 − ion were constructed, which were geometrically optimized and the energy change during the complexation was analyzed. Mainly the interaction between OH groups of PVA and iodine has been found to determine the stability of the complex. The interaction strength depends on the stereoregularity of the chain: the syndiotactic PVA sequence gave the strongest interactions between I 3 − and four or five OH sequence along the chain axis, while the atactic and isotactic chain models are less stable than the former case. The calculated interaction energy between PVA and I 3 − species was 36.0, 31.2 and 25.6 kcal/mol for syndiotactic , atactic and isotactic PVA, respectively. The calculation of orbital interactions revealed that the highest occupied molecular orbital (HOMO) of I 3 − is relatively close to the lowest unoccupied MO (LUMO) of PVA, causing an electron transfer from I 3 − to PVA. (2) In the second-stage calculation, the crystal structure model of PVA-iodine complex including counter ions (K + ) was built up by referring to the X-ray analyzed structure. The K + ions were found to be localized in the highly-negatively-charged space surrounded by OH groups and iodine atoms, corresponding well to the electrostatic potential minimum created by the PVA chains and iodine ions. The local molecular orbitals (LMO) calculated for these two models [(1) and (2)] revealed the existence of H-bonding interaction between the OH group and the iodine ion.

The effect of counter cation species on the formation of various crystal forms and their phase transition behavior of poly(vinyl alcohol)-iodine complex

Poly(vinyl alcohol) [PVA, -(CH2CH(OH))n-] forms the crystalline complexes I and II with iodine ions (I3−) when PVA is immersed in the KI/I2 aqueous solutions of 0.1–0.5 and 1–3 mol/l concentration, respectively. On the basis of the X-ray diffraction and the ultraviolet-visible, IR and Raman spectral data, we have found that the immersion of PVA films into the iodine aqueous solutions containing the different type of counter ions (K+, Na+, Li+, H+, or Cs+) creates the various types of the crystal modification. When PVA sample was immersed in the solution of relatively low concentration, all the types of counter ion gave the complex I structure. When the concentration of the solution was increased to 1–3 mol/l, the sample immersed for several hours showed the formation of the complex II. In the case of H+ ion, the longer immersion was found to give a new crystal form or complex III with the different packing mode of PVA and I3− ions as determined by the X-ray structure analysis. In the cases of Na+ and Li+ ions, the complex II was disordered by the longer immersion in such a highly-concentrated solution. As clarified by the infrared spectral data, the hydrogen bonds of O–H ⋯ I type are formed in the iodine complex, the strength of which was found also to be different depending on the type of counter ion species. In this way, the present paper has revealed the important role of the counter cations on the formation and stability of the PVA-iodine complexes for the first time.

Color change of an iodinated poly(vinyl alcohol) film by physical deformation

ABSTRACTThe color change of an iodinated poly(vinyl alcohol) (PVA) film caused by physical deformation was investigated in this study. The color of a PVA film soaked in an aqueous potassium iodide (KI)/I2 solution was light yellow, but it turned light blue when the film was physically deformed. The ultraviolet–visible absorption spectrum of the iodinated PVA film extended uniaxially in air was measured at various extension levels. Without deformation, the film showed UV absorption bands at 210, 290, and 360 nm. However, under deformation, the film showed new visible light absorption bands at 440 and 620 nm. From the UV–vis absorption spectra of several iodinated solutions, we found that the absorption wavelength of iodine was affected by the cohesive energy of the solvents. The KI/I2 diethyl ether solution showed an absorption band at 460 nm, and this provided a clue to understanding the color change of the PVA–iodine complexes caused by physical deformation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43036.

Effect of boric acid treatment on the crystallinity and drawability of poly(vinyl alcohol)–iodine complex films

A film iodinated at solution state before casting (BIBC film) and a film iodinated after casting (BIAC film) were prepared by casting an aqueous solution of poly(vinyl alcohol) (PVA) including I2/KI and boric acid, and by successively soaking the PVA film in aqueous solutions of boric acid and I2/KI, respectively. The boric acid-induced and I2/KI-induced weight gains relative to the PVA were 3, 5, 7, and 10%, and 3, 5, 10, and 20%, respectively. The effects of boric acid and iodine on the crystallinity and drawability of the films were investigated. Although the crystalline structure of the BIAC films was not affected by boric acid, the boric acids in the PVA solution containing I2/KI may have formed intra-molecular cross-links on the PVA chain to accelerate the formation of the PVA–iodine complex evenly, and subsequently interrupt the PVA crystallization through the BIBC film formation to render the resultant film slightly crystalline or practically amorphous. This occurred even at a much lower I2/KI-induced weight gain (20%) than the minimum weight gain (125%) at which the iodinated at solution state before casting film without boric acid indicated a practically amorphous state. The maximum draw ratio of the films generally decreased with increasing boric acid content, which was mainly attributed to the increase of the extended segments of the PVA chains in the amorphous region due to the cross-links formed with the boric acids. The maximum draw ratios of the BIBC films tended to decrease more severely than those of the BIAC films.

Role of boric acid in the formation of poly(vinyl alcohol)–iodine complexes in undrawn films

The role of boric acid in the formation of poly(vinyl alcohol) (PVA)–iodine complexes in undrawn films has been investigated by using wide-angle X-ray diffraction (WAXD) and high-resolution solid-state 13C NMR spectroscopy. From UV–vis absorption spectroscopy, it is confirmed that boric acid is necessary for the formation of the complexes in films that are treated with I 2/KI aqueous solutions at relatively low I 2 concentrations. The WAXD profiles indicate that, irrespective of the presence of iodine, crystallite sizes perpendicular to the chain axis become smaller by the addition of boric acid in the swelling media. Moreover, small crystallites and surficial parts of larger crystallites may be partially dissolved in the swelling process with water and boric acid suppresses the re-crystallization in the drying process with or without iodine. The 13C spin–lattice relaxation time analysis reveals that there exist two components called the mobile and the less mobile components in the films and the latter component, which contains the complexes and the crystalline component, is increased in the fraction by the presence of boric acid. The evaluation of the CH resonance line shows that some of the intermolecular hydrogen bonds are broken by boric acid, which increases the intramolecular hydrogen bonds. The CH 2 lineshape analysis also reveals that the gauche fraction is appreciably increased in the less mobile component by the addition of boric acid. These facts suggest that boric acid may promote the formation of PVA–iodine complexes particularly in the surficial areas of the crystallites probably by reducing the molecular mobility of the PVA chains by causing cross-linking among them.

Investigation of the structure of poly(vinyl alcohol)–iodine complex hydrogels prepared from the concentrated polymer solutions

The structure and dynamics of poly(vinyl alcohol) (PVA)–iodine complex hydrogels that were prepared from concentrated PVA solutions have been characterized by high-resolution solid-state 13C NMR spectroscopy. The fully relaxed dipolar decoupling (DD)/MAS 13C NMR spectrum indicates that the hydrogels contain at least two components, a highly mobile and broader components. The former is assigned to the soluble or well water-swollen PVA chains that are not closely associated with the PVA–iodine complexes, whereas the latter may be mainly ascribed to the aggregated PVA chains that are produced by the formation of the PVA–iodine complexes because no diffraction peaks due to the conventional PVA crystallites are observed by wide-angle X-ray diffractometry. Furthermore, 13C spin-lattice relaxation time (T1C) analyses reveal that the broader component is composed of the highly restricted component probably assignable to PVA molecular chain aggregates containing the PVA–iodine complexes and the less mobile component. As for the former component, their CH resonance line measured by the T1C-filtering method is successfully resolved into 7 constituent lines by the least-squares curve fitting. The statistical analysis of the integrated intensities of the constituent lines thus obtained also reveals that the probability fa for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability ft of the trans conformation are, respectively, as high as 0.86 and 0.88. This fact indicates that the PVA molecular chain aggregates containing the PVA–iodine complexes should be composed of PVA segments with the trans-rich conformation and the PVA–iodine complexes therein may also be formed with these several trans-rich segments surrounding the rod-like polyiodine cores in agreement with the so-called aggregation model. Moreover, several new diffraction peaks that should be interpreted in terms of the hexagonal structure are observed for the PVA–iodine complex hydrogels in the low 2θ region in the wide-angle X-ray diffraction (WAXD) profile measured by a strong X-ray source at SPring-8. This suggests the necessity of more detailed WAXD characterization to propose a new structure model, which should be referred to as the hexagonal aggregation model, for the PVA–iodine complexes.

Effect of an Anionic Surfactant on the Complexation of Some Nonionic Polymers with Iodine in Aqueous Media

Complexation of some water soluble nonionic polymers, namely, polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), and hydroxypropyl cellulose (HPC), with iodine has been studied in aqueous and aqueous sodiumdodecylsulfate (SDS) solution. While the complexation was indicated by a red shift of the tri-iodide band in case of PVP or HPC, the PVA-iodine complex showed its characteristic band around 500 nm. It was observed for the first time that presence of SDS led to complete break down of the PVA-iodine complex and its characteristic blue color. The presence of monomers of SDS, however, appeared to favor the formation of the iodine complex with PVP or HPC. Addition of n-propanol, which is known to prevent the formation of gels or microgels in polymer solutions, was found to enhance the polymer-iodine complex. Gels of pure HPC and HPC with iodine both in presence and absence of SDS have been prepared and studied.

Conductivity studies of poly (vinyl alcohol)-iodine complex membrane

The conductivities of polymers like poly (vinyl alcohol) (PVA) and its iodine complex membranes are reported here. PVA-iodine complex membrane was prepared by dipping PVA film into an I2–KI solution. The formation of the complex membrane was confirmed by IR spectra. Conductivities were determined from 30 to 300 °C with a frequency ranges from 42 Hz to 500 KHz in solid state. It was observed that iodine is known to act as a catalyst for dehydration of PVA. A possible mechanism for the dehydration of PVA catalyzed by iodine is also explained.

Role of adsorbed iodine into poly(vinyl alcohol) films drawn in KI/I 2 solution

We have investigated the microstructure of the poly(vinyl alcohol) (PVA) films using small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) techniques. The samples were uniaxially drawn in water or KI/I 2 aqueous solution and then dried in an air-oven at 333 K for 1 h prior to SAXS and WAXS measurements. It was found that for the films drawn in KI/I 2 solution PVA chains in the microfibrillar structure are more extended upon the film drawing compared to the case of the films drawn in pure water, which is resulted from the correlation function analysis on the SAXS data. Adsorbed iodines into the film were anticipated to act as junction points between the microfibrils via the formation of the PVA-iodine complexes.

Formation of poly(vinyl alcohol)-iodine complex in aqueous solution: a SEM study of the freeze-dried substances

The freeze-dried samples prepared from dilute PVA aqueous solutions and PVA–iodine complex aqueous solutions have been examined using a scanning electron microscope. The samples prepared from syndiotacticity-rich PVA (S-PVA) solutions were found to have a network structure due to the formation of intermolecular hydrogen bonds, whereas in the case of atactic PVA (A-PVA) a network structure was not found. The network structure became more finely structured with increasing syndiotacticity. The structure of the freeze-dried sample of the complex solution prepared from S-PVA having a syndiotactic diad content of 63·8%, and iodine, was coarse in comparison with that of the freezedried sample of the S-PVA solution. In addition, the formation of spherical bulges, which are considered to correspond to microgels in the aqueous solution, were observed in several places. In the S-PVA having a syndiotactic diad content of 57·8%, the spherical bulges were not observed, whereas the absorbance of the aqueous solution was the highest. Although A-PVA did not form a PVA– iodine complex at 30°C in solution, the frozen solution turned blue due to the formation of aggregates. These phenomena were confirmed by the degree of crystallinity estimated from IR spectra, and the amount of iodine estimated from X-ray microanalysis of the freeze-dried samples. The PVA–iodine complexes are formed by the interaction of the aggregates of PVA molecules with iodine molecules. However, the PVA microgels do not interact with iodine. © 1998 Society of Chemical Industry

Formation of poly(vinyl alcohol)-iodine complexes in solution

The effect of the dissolved state of poly(vinyl alcohol) (PVA) molecules in water on the color development due to PVA–iodine complexes was investigated at each given PVA and iodine concentration using two kinds of syndiotactic-rich PVA (S-PVA) which are unstable in water because of the formation of intermolecular hydrogen bonds and form the complex easily. In the reaction mixtures prepared by mixing PVA solutions and an iodine solution, the color development was constant and independent of standing time of the PVA solution before the addition of iodine up to a certain time, after which it decreased with the standing time. The color development obtained with use of the PVA solution allowed to stand for a fixed time was higher for S-PVA with a lower s-(diad)%. In the case of the reaction mixture prepared by dissolving PVA in an iodine solution, the color development was higher for S-PVA with a higher s-(diad)%. The initial ratio of the I5−/I3− and the rate of decrease in the ratio of I5−/I3− were larger than those in the preceding case. The color development decreased for the PVA with an s-(diad) % of 58, whereas it increased for the PVA an s-(diad) % of 61.3 with increasing propanol content, an inhibitor of gelation. From these results, the aggregates of PVA molecules have been assumed to play an important role in forming the complexes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1701–1709, 1997

Temperature-dependent resonance Raman study of iodine-doped poly(vinyl alcohol) films

The resonance Raman spectrum of iodine-doped poly(vinyl alcohol) films was studied as a function temperature from 77 t0 290 K. The main bands in the spectrum are assigned to fundamental and overtone bands of linear I 3 − and I 5 −. The presence of combination bands in the spectrum is attributed to exciton coupling between these species in polyiodide chains. The low-temperature measurements allowed us to resolve new spectral features. At 77 K, the spectrum showed evidence for minor amounts of I 2 and isomers of the main polyiodide components. The optical properties of the PVA-iodine complex are not simple and cannot be assigned to any single polyiodide species.

Properties of Branched and NetworkPoly(vinyl alcohols) Prepared withPoly(vinyl alcohol) Having AldehydeGroups at the Chain Ends

Branched and network poly(vinyl alcohols) (PVAs) were prepared with inter-acetalization of the PVA with aldehyde groups at the chain ends which was prepared by the cleavage of 1,2 glycol bonds in commercial PVA. The numbers of branches estimated from molecular weights were compared with those estimated by theory. Huggins' constant and crystallinity decreased with increasing branch number. Dissolution of branched PVAs into dimethylsulphoxide was not so easy compared with commercial PVA. The colour of branched PVA–iodine complex decreased rapidly with standing while that of commercial PVA decreased gradually. Network PVAs with Young's modulus of 1–8 MPa were prepared.

特集 中性子散乱による最近の研究 中性子小角散乱コントラスト変調法によるポリビニルアルコール・ヨウ素錯体の構造研究

The structural characteristics of the blue-colored poly (vinyl alcohol) (PVA) -iodine complex (PIC) in aqueous solution were studied by means of small-angle neutron scattering with contrast variation method, using various D2O/H2O. The concentrated PIC solutions were prepared by using PVA with the degree of polymerization of 15, containing excess KI and the color inducing agent CuCl2. The results from the computer simulation for the measured Stuhrmann plot with theoretical scattering functions suggested that the PIC forms a cylindrical aggregate compposed of the complexed units of more than 4 (insert space) linearchains of PVA surrounding triiodide dimer.