PVA-Iodine complexes: Formation, structure, and properties

Hybridization of Wide-Angle X-ray and Neutron Diffraction Techniques in the Crystal Structure Analyses of Synthetic Polymers

The development in the crystal structure analysis of synthetic polymers using the hybridized combination of wide-angle X-ray and neutron diffraction (WAXD and WAND, respectively) techniques has been reviewed with many case studies performed by the authors. At first, the technical development was reviewed, in which the usage of high-energy synchrotron X-ray source was emphasized for increasing the total number of the observable diffraction peaks, and several examples were introduced. Secondly, the usage of the WAND method was introduced, in which the successful extraction of hydrogen atomic positions was described. The third example is to show the importance for the hybrid combination of these two diffraction methods. The quantitative WAXD data analysis gave the crystal structures of at-poly(vinyl alcohol) (at-PVA) and at-PVA-iodine complex. However, the thus-proposed structure models were found not to reproduce the observed WAND data very much. The reason came from the remarkable difference in the atomic scattering powers of the constituting atomic species between WAXD and WAND phenomena. The introduction of statistical disorder solved this serious problem, which reproduced both of the observed WAXD and WAND data consistently. The more systematic combination of WAXD and WAND methods, or the so-called X-N method, was applied also to the quantitative evaluation of the bonded electron density distribution along the skeletal chains, where the results about polydiacetylene single crystals were presented as the first successful study. Finally, the application of WAND technique in the trace of structural changes induced under the application of external stress or temperature was described. The future perspective is described for the development of structural science of synthetic polymers on the basis of the combined WAXD/WAND techniques.

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 (c_iodine) of KI/I₂ solution: (i) c_iodine < 0.1 M: PVA-I₃^-/I₅^- complex only exists in the non-crystalline region, (ii) 0.1 M < c_iodine < 1 M: formation of PVA-I₃^- complex I, and (iii) c_iodine > 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 c_iodine, 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, D_res, obtained by the more quantitative double-quantum (DQ) NMR, increases abruptly at c_iodine > 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.

Swelling process improves the mobility of PVA segments, while dyeing process restricts that. And there is a large water absorption dependence on the formation of PVA–iodine complexes.

Molecular iodine/polymer complexes

A unique feature of molecular iodine by far, is its ability to bind to polymeric materials. A plethora of natural and synthetic polymers develop complexes when treated with molecular iodine, or with a mixture of molecular iodine and potassium iodide. Many unexpected findings have been encountered upon complexation of iodine and the polymer skeleton, including the color formation, the polymer morphology changes, the complexation sites or regions, the biological activity, and the electrical conductivity enhancement of the complexes, with polyiodides (In¯), mainly I3¯ and I5¯, as the actual binding species. Natural polymers that afford such complexes with iodine species are starch (amylose and amylopectin), chitosan, glycogen, silk, wool, albumin, cellulose, xylan, and natural rubber; iodine-starch being the oldest iodine-natural polymer complex. By contrast, numerous synthetic polymers are prone to make complexes, including poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), nylons, poly(Schiff base)s, polyaniline, unsaturated polyhydrocarbons (carbon nanotubes, fullerenes C60/C70, polyacetylene; iodine-PVA being the oldest iodine-synthetic polymer complex.

Multilayer PVA adsorption onto hydrophobic drug substrates to engineer drug-rich microparticles

Despite the availability of numerous crystal engineering techniques, generating drug-rich microparticles with a predetermined size, morphology and crystallinity still represents a significant challenge. A microparticle manufacturing method has recently been developed that attempts to 'shield' the physicochemical properties of micronised drugs by the application of a microfine polymer coating. The aims of this study were to investigate the nature of the drug-polymer interactions and determine the effects of this manufacturing strategy upon release of the drug from the microparticles. The adsorption of poly(vinyl alcohol) (PVA) on the micronised hydrophobic drug surface was found to reach equilibrium between 23 and 27 h. The Freundlich isotherm model was shown to give the most accurate fit to the experimental data and thus multilayer adsorption was assumed. The adsorptive capacity (1/n) was specific to the substrate and PVA grade. An increase in the PVA (%) hydrolysis value caused 1/n to increase from 0.76 to 1.05 using budesonide and from 0.31 to 0.79 when betamethasone valerate (BMV) was used. Increasing the molecular weight of the adsorbing polymer caused a reduction in the strength of PVA-adsorbate interaction when budesonide was used as the substrate (from 0.76 to 0.59), whereas a three-fold increase (from 0.31 to 0.86) was achieved when the BMV substrate was employed. A proportion of the adsorbed polymer was shown to remain associated with the substrate during the spray-drying process and the polymer coating resulted in a significantly higher (p<0.05, ANOVA) amount of drug release in 60 min (ca. 100%) compared to budesonide alone.