Induced changes of polymer organization in a gel of poly(vinyl alcohol) crosslinked by Congo red

Different gel microstructures are induced at variable poly(vinyl alcohol) (PVA) and Congo red concentrations, as revealed by ultrarapid freezing and a replica technique for transmission electron microscopy. The polymer microstructures observed include random coils, rigid polymer rods, and long fibers. The development of the different polymer conformations is proposed to be dependent on the degree of intramolecular and intermolecular crosslinking and on the electrostatic interactions of the Congo red ions. The rigid‐rod conformation appears to be the most energetically stable form; it is disrupted by electrostatic effects around the polymer overlap concentration (C*_PVA). We propose that the gel microstructure influences the physical properties of the gel. Gels possessing the rigid‐rod microstructure have increased Young's storage modulus values. Two possible mechanisms of gelation are suggested. The first describes a one‐stage reaction when the polymer concentration approximates C*_PVA, where polymers in an extended random‐coil conformation undergo intermolecular crosslinking without any microstructural changes. The second describes a two‐stage reaction when the polymer concentration is less than or greater than C*, where a disorder–order transition results in the formation of rigid polymer rods and fibers followed by the formation of a macromolecular network. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1471–1483, 2001     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.     

Facile preparation of methylcellulose/poly(vinyl alcohol)physical complex hydrogels with tunable thermosensitivity

Novel complex hydrogels of methylcellulose(MC)and poly(vinyl alcohol)(PVA)with wide-spectrum thermoresponsivity were prepared via physical and mild process.Thermal phase transition of MC/PVA hydrogels exhibited two forms including sol/sol to gel/sol and sol/gel to gel/gel.The phase transition temperature of MC/PVA solution ranged from 38.7 to 60.6℃and was able to be adjusted by simply changing the feeding ratios of two components.The interior morphology of MC/PVA gels was examined with fluorescence analy...     

Structuring in poly(vinyl alcohol)-potassium iodide-iodine systems: Small-angle X-ray-scattering study

The sizes of supramolecular density fluctuations in gels and solid films obtained during formation of iodine inclusion compounds in an aqueous solution of poly(vinyl alcohol) are studied via small-angle X-ray scattering. It is shown that, in the case of iodine-containing poly(vinyl alcohol) samples prepared through drying of iodine-containing gels, several diffraction peaks that are related to the long-range order in the packing of structural fragments manifest themselves. The presence of the peaks indicates periodic density fluctuations in the packing of polymer chains with a period of 1.34 nm and a coherence length of 18.0 × 0.5 nm. The equidistance of peaks in units of scattering-vector modulus makes it possible to suggest the formation of layered systems, that is, systems with a single well-defined repeat period, during gel drying. Wet samples of the iodine-containing gel based on poly(vinyl alcohol) do not reveal any ordering. Heterogeneities in the structure of iodine-free gels may be regarded as voluminous structures with free cavities that survived after the extraction of iodine. These cavities in gels may be again filled with iodine or other complementary molecules.     

Properties of a novel thermal sensitive polymer based on poly(vinyl alcohol) and its layer‐by‐layer assembly

Structurally modified poly(vinyl alcohol) (PVA) was prepared as novel thermally sensitive polymers by partially acetalyzing and/or ionizing the commercially available PVA. Their aqueous solutions experience completely reversible polymer aggregation and dissolution above and below the lower critical solution temperature (LCST), respectively. The LCST of a partially acetalyzed PVA (APVA) can be readily controlled by the degree of acetalysis or the molecular weight of the starting PVA. Introduction of a small amount of cationic group onto the APVA backbone increases the LCST obviously, while the LCST is highly sensitive to NaCl concentration. Then APVA and cationic APVA multilayers are assembled on rayon to make a thermal responsive fiber. The atomic force microscopy (AFM) images of the surface reveal the increment of roughness stimulated by temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

Polycondensation reaction of dimethyl tartrate with hexamethylenediamine in the presence of various matrices

The polycondensation reaction of dimethyl tartrate (DMT) with hexamethylenediamine (HMD) was carried out in dimethyl sulfoxide (DMSO) at 60°C in the presence of various polymer matrices, which were expected to interact with DMT or the resulting polyamide which had pendant hydroxyl groups due to hydrogen bonding. It was found that the rate of polycondensation was enhanced by polymer matrices such as poly(vinyl pyrrolidone) (PVP), Pullulan (polysaccharide) (PF), and poly(vinyl alcohol) (PVA). The rate enhancement became more pronounced with increasing molecular weight of the polymer matrix. When polycondensation in the presence of PVA was carried out in DMSO, a polymer complex was produced. The formation of the polymer complex between the resulting polyamide and PVA during polycondensation was dependent on the concentration of monomers and also on PVA; a gelation of the solution was observed at a concentration of PVA.     

Thermal degradation of copolymers from vinyl acetate and vinyl alcohol

The thermal degradation of poly(vinyl acetate) (PVA), poly(vinyl alcohol) (PVAL), vinyl acetate-vinyl alcohol (VAVAL), vinyl acetate-vinyl-3,5-dinitrobenzoate (VAVDNB) and vinyl alcohol-3,5-dinitrobenzoate (VALVDNB) copolymers have been studied using differential thermal analysis (DTA) and thermogravimetry (TG) under isothermal and dynamic conditions in nitrogen. Thermal analysis indicates that PVA and PVAL are thermally more stable than VAVAL copolymers, being PVAL the most stable polymer. The presence of small amounts of vinyl-3,5-dinitrobenzoate (VDNB) in PVA or PVAL produces a marked decrease in the thermal stability of both homopolymers, being VALVDNB copolymers the less stable materials. The apparent activation energy of the degradative process was determined by the Kissinger and Flynn-Wall methods which agree well.     

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     

Study of the self‐diffusion of poly(ethylene glycol)s in poly(vinyl alcohol) aqueous systems

We have measured the self‐diffusion coefficients of a series of oligo‐ and poly(ethylene glycol)s with molecular weights ranging from 150 to 10,000, in aqueous solutions and gels of poly(vinyl alcohol) (PVA), using the pulsed‐gradient spin‐echo NMR techniques. The PVA concentrations varied from 0 to 0.38 g/mL which ranged from dilute solutions to polymer gels. Effects of the diffusant size and polymer concentration on the self‐diffusion coefficients have been investigated. The temperature dependence of the self‐diffusion coefficients has also been studied for poly(ethylene glycol)s with molecular weights of 600 and 2,000. Several theoretical models based on different physical concepts are used to fit the experimental data. The suitability of these models in the interpretation of the self‐diffusion data is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2396–2403, 1999     

Effect of solvent for vinyl acetate polymerization on microstructure of poly(vinyl alcohol)

The solution polymerization of vinyl acetate was carried out in several solvents at 0 to 100°C, using 2,2′-azobisisobutyronitrile as initiator. For the resulting poly(vinyl alcohol) (PVA), iodinecoloration, 1,2-glycol structure and tacticity were observed. The pentad tacticity of PVA was estimated from its methine carbon spectra by means of ¹³C-FTNMR spectrometer. Iodine-coloration ability of PVA varied markedly with the type of polymerization solvent and decreased in the following order: phenol > aq. phenol > methyl alcohol > ethyl acetate > DMSO, ethylene carbonate. The syndiotactic fraction in PVA also decreased with polymerization solvent in the same order as that of iodine coloration, while 1,2-glycol content of PVA was not almost affected by polymerization solvent except for phenol and aq. phenol. In solution polymerization performed, effect of polymerization temperature on tacticity was less than that of solvent.     

Deliberately designed processes to physically tether the carboxyl groups of poly(pentacosadiynoic acid) to a poly(vinyl alcohol) glassy matrix to make poly(pentacosadiynoic acid) thermochromically reversible in the matrix

In this article, we demonstrate that by tethering carboxyl groups of poly(10,12-pentacosadiynoic acid) (PDA) to a poly(vinyl alcohol) (PVA) matrix, PDA, which is irreversible in its pure form, becomes reversible in the thermochromism. The tethering is realized by simple but deliberately designed processes: (1) Disperse the commercially available monomer 10,12-pentacosadiynoic acid (DA) nanocrystals in a PVA aqueous solution by the "NCCM" method invented in our laboratory. (2) Anneal and dry the mixture solution at a temperature higher than the melting point of pure DA crystal. (3) Polymerize the as-annealed DA/PVA blend films by UV irradiation. After the polymerization, PDA/PVA films with completely reversible thermochromism are obtained. The reversible PDA/PVA films can be easily dissolved in water, leading to water-dispersible nanoaggregates with the reversibility. Blends of PDA with other water-soluble polymers such as poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA) and poly(allyamine) (PAM), were prepared respectively, by the same processes and under the same conditions. It is found that all these nanocomposites are irreversible or partially reversible in the thermochromism; either the relatively low glassy transition temperature of the polymer matrix (in the case of PEO) or the partial ionization nature of the polymer (in the cases of PAA and PAM) is responsible for the irreversibility or the partial reversibility.     

Short branching in poly(vinyl alcohol). III. Some properties of model polymers of short-branched poly(vinyl alcohol)

Melting point, the iodine color reaction, and foam fractionation were studied on model poly(vinyl alcohol) (PVA) having short branches of one or two monomer units in length. An increase in the amount of short branching units caused a marked decrease in color intensity of the PVA–iodine reaction and in the melting point. These tendencies were more remarkable when the short branching was two monomer units in length than when it was one monomer unit. It was also found that foam fractionation of an aqueous PVA solution produced PVA fractions with different degree of short branching, the degree increasing with increase in the fraction number. The color intensity of the PVA–iodine reaction has been confirmed to decrease with increase in the fraction number, but this result cannot be explained solely in terms of the short branching. It is concluded that the phenomenon of foam fractionation of PVA and the iodine color reaction of the fraction appear to be governed by many factors such as molecular weight, stereoregularity, and short branching.     

Pyrene functional poly(vinyl alcohol) by “click” chemistry

Side‐chain pyrene functional poly(vinyl alcohol) (PVA) was synthesized by using “click chemistry” strategy. First, partial tosylation of PVA with p‐toluene sulfonyl chloride were performed. The resulting PVA‐Ts polymer was then quantitatively converted into poly(vinyl alcohol)‐azide (PVA‐N₃) in the presence of NaN₃/DMF at 60 °C. Propargyl pyrene was prepared independently as a photoactive click component. Finally, azido functionalized PVA was coupled to propargyl pyrene with high efficiency by click chemistry. Incorporation of pyrene functionality in the resulting polymer was confirmed by spectral analysis. It is also shown that pyrene functionalized PVA (PVA‐Py) exhibited characteristic fluorescence properties and improved solubility in highly polar solvents such as water, DMSO, and DMF as well as less polar solvent such as THF compared with pristine PVA. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1317–1326, 2009     

Study of cryostructuring of polymer systems: 27. Physicochemical properties of poly(vinyl alcohol) cryogels and specific features of their macroporous morphology

Based on aqueous poly(vinyl alcohol) (PVA) solutions with different content of polymer having different molecular masses and chain tacticity, macroporous viscoelastic gels (PVA cryogels) are prepared in various regimes of freezing-storage in a frozen state-thawing. Shear modulus and fusion temperature of corresponding samples are measured; the structure of thin sections is studied by optical microscopy and the images are processed and analyzed. It is shown that the rigidity and heat endurance of cryogels rise with an increase in the concentration of initial PVA solution and a decrease in the rate of thawing. The influence of the temperature of cryogenic treatment and the PVA molecular mass has an extreme character. At the same time, the effect of the main parameters of cryotropic gelation on the macroporous morphology of PVA cryogels is manifested in the form of more complex dependences because of its multiple-factor character. Therefore, distinct structure-property correlations are not observed in many cases. Cluster analysis of the morphometric characteristics of cryogels in comparison with data on their rigidity makes it possible to classify these systems.     

A novel type of optically active helical polymers: Synthesis and characterization of poly(N‐propargylureas)

This article presents two novel artificial helical polymers, substituted polyacetylenes with urea groups in side chains. Poly( 4 ) and poly( 5 ) can be obtained in high yields (≥97%) and with moderate molecular weights (11,000–14,000). Poly( 4 ) contains chiral centers in side chains, and poly( 5 ) is an achiral polymer. Both of the two polymers adopted helical structures under certain conditions. More interestingly, poly( 4 ) exhibited large specific optical rotations, resulting from the predominant one‐handed screw sense. The helical conformation in poly( 5 ) was stable against heat, while poly( 4 ) underwent conformational transition from helix to random coil upon increasing temperature from 0 to 55 °C. Solvents had considerable influence on the stability of the helical conformation in poly( 4 ). The screw sense adopted by the helices was also largely affected by the nature of the solvent. Poly( 4 ‐co‐ 5 )s formed helical conformation and showed large optical rotations, following the Sergeants and Soldiers rule. By comparing the present two polymers (with one ? N? H groups) with the three polymers previously reported (with two ? N? H groups in side chains), the nature of the hydrogen bonds formed between the neighboring urea groups played big roles in the formation of stable helical conformation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4112–4121, 2008     

Block copolymers of poly(ethylene oxide) and poly(vinyl alcohol) synthesized by the RAFT methodology

A methodology for the synthesis of well‐defined poly(ethylene oxide)‐block‐poly(vinyl alcohol) (PEO‐b‐PVA) and PVA‐b‐PEO‐b‐PVA polymers was reported. Novel xanthate end‐functionalized PEOs were synthesized by a series of end‐group transformations. They were then used to mediate the reversible addition–fragmentation chain transfer polymerization of vinyl acetate to obtain well‐defined poly(ethylene oxide)‐b‐poly(vinyl acetate) (PEO‐b‐PVAc) and PVAc‐b‐PEO‐b‐PVAc. When these block copolymers were directly hydrolyzed in methanol solution of sodium hydroxide, polymers with brown color were obtained, which was due to the formation of conjugated unsaturated aldehyde structures. To circumvent these side reactions, the xanthate groups were removed by adding a primary amine before hydrolysis and the products thus obtained were white powders. The polymers were characterized by gel permeation chromatography, ¹H NMR spectroscopy and FT‐IR. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1901–1910, 2009     

A study on optical limiting properties of Eosin-Y and Eriochrome Black-T dye-doped poly (vinyl alcohol) composite film

A poly (vinyl alcohol) PVA/Eriochrome Black-T (EBT) dye, and PVA/Eosin-Y (EY) dye composite film was prepared using a solution casting process. The dye-doped composite polymer films were characterized by UV–vis spectroscopy. An optical band gap (E_g) of pure PVA reduced from 4.22?eV to 2.80?eV for PVA/EBT film and 2.14?eV for PVA/EY film respectively. This result indicates the occurrence of inter-molecular hydrogen bonding between the –OH functional group in PVA chains and sulfonate (EBT) and carboxyl group (EY) in dye molecules, respectively. Moreover, the experimental result of PVA/EBT and PVA/EY composite film showed the excellent properties of a large scale cut-off filter in the ultraviolet and visible range region.     

Poisson's ratio of polyacrylamide (PAAm) gels

Poisson's ratio (μ₀) of polyacrylamide (PAAm) gels was estimated. The value of μ₀ for PAAm gels was found to be 0.457, which is close to that for poly (vinyl alcohol) (PVA) gels swollen in the mixture of dimethylsulfoxide (DMSO) and water, but is higher than the value for PVA hydrogels.     

Complex formation of boric acids with DI- and TRI- carboxylic acids and poly(vinyl alcohol) in aqueous solutions

Complexation of dihydroxyboryl compounds such as dihydroxyboryl phenylalanine and boric acid with polycarboxylic acids as well as polyols was studied by infrared spectroscopy and zone electrophoresis. By consulting the results obtained, gel formation of poly(vinyl alcohol) (PVA) in aqueous solutions of boric acids (BA) and borax was studied. The IR spectra of aqueous PVA-BA gels indicate that BA mainly assumes a planar triangle form, while the local conformation of PVA in the gel is different from that in aqueous solutions.     

Novel approach to the chemical modification of poly(vinyl alcohol): Phosphorylation

The chemical modification of poly(vinyl alcohol) (PVA) was performed through oxidation followed by nucleophilic addition. PVA was oxidized by KMnO₄ to form vinyl ketone units along the polymer backbone. The chemical modification of PVA was then conducted through the reaction of the carbonyl group of the vinyl ketone unit with 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) as a nucleophile. Through this approach, the phosphorous DOPO group was attached onto the carbon atom of the polymer main chain rather than onto the pendent hydroxyl groups of PVA. The formed DOPO‐containing PVA showed improved thermal stability, organosolubility, and flame retardance. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1107–1113, 2003