Poly(vinyl alcohol)-poly(acrylic acid) interpenetrating networks. Study on phase separation and molecular motions

We report on the preparation and characterization of interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) formed by a sequential method. Interpolymer interactions were examined using ¹³C CP/MAS NMR and DSC methods. Evidence on the formation of a PVA-PAAc complex through hydrogen bonds between the hydroxyl groups of the PVA chains and the carbonyl group of the PAAc chains was obtained. The existence of polymer interactions between PVA and PAAc and its effect on the molecular motion of polymer chains, was further investigated by means of the analysis of the ¹³C spin-lattice relaxation times in the rotating frame, . To elucidate the scale of the mixing in the PVA/PAAc IPNs, proton spin-lattice relaxation times in the rotating frame, , were also investigated. The analysis of the results reveals the compatibility between the PVA and the PAAc polymer networks at low PVA concentrations and the occurrence of phase separation at relatively high PVA concentrations.     

Glass transition and molecular dynamics in poly(dimethylsiloxane)/silica nanocomposites

The molecular dynamics of a series of poly(dimethylsiloxane) networks filled with silica nanoparticles synthesized in situ was investigated using thermally stimulated depolarization currents, broadband dielectric relaxation spectroscopy and differential scanning calorimetry. The techniques used cover together a broad frequency range (10⁻³-10⁹ Hz), thus allowing to gain a more complete understanding of the effects of the nanoparticles on the chain dynamics. In addition to the α relaxation associated with the glass transition of the polymer matrix, we observe in dielectric measurements a slower α relaxation which is assigned to polymer chains close to the polymer/filler interface whose mobility is restricted due to interactions with the filler surface. The thickness of the interfacial layer is estimated to be about 2.1-2.4 nm. Differential scanning calorimetry shows a change in the shape of the glass transition step, as well as a decrease in both the degree of crystallinity and the crystallization rate by the addition of silica.     

Characterization of sodium alginate and poly(vinyl alcohol) blend membranes in pervaporation separation

By blending a rigid polymer, sodium alginate (SA), and a flexible polymer, poly(vinyl alcohol) (PVA), SA/PVA blend membranes were prepared for the pervaporation separation of ethanol–water mixtures. The rigid SA membrane showed a serious decline in flux and a increase in separation factor due to the relaxation of polymeric chains, whereas the flexible PVA membrane kept consistent membrane performance during pervaporation. Compared with the nascent SA membrane, all of the blend membranes prepared could have an enhanced membrane mobility by which the relaxation during pervaporation operation could be reduced. From the pervaporation separation of the ethanol–water mixtures along with the temperature range of 50–80°C, the effects of operating temperature and PVA content in membrane were investigated on membrane performance, as well as the extent of the relaxation. The morphology of the blend membrane was observed with PVA content by a scanning electron microscopy. The relaxational phenomena during pervaporation were also elucidated through an analysis on experimental data of membrane performance measured by repeating the operation in the given temperature range. SA/PVA blend membrane with 10 wt % of PVA content was crosslinked with glutaraldehyde to enhance membrane stability in water, and the result of pervaporation separation of an ethanol–water mixture through the membrane was discussed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:949–959, 1998     

Effects of mixed solvent on gelation of poly(vinyl alcohol) solutions

The relationship between the polymer–solvent interaction and gelation behavior of poly(vinyl alcohol) (PVA) solutions prepared from ethylene glycol/water (EG/water) mixed solvents was investigated using a viscometer, light scattering, FTIR, X‐ray, and pulsed NMR analyses. The viscometric result showed that the affinity to PVA for water is higher than that for EG. The light scattering result showed that the spinodal decomposition rate of the PVA solution decreases rapidly as the water content in the EG/water mixed solvent is increased. On the other hand, the FTIR and X‐ray results both indicated that the crystallinity of the PVA gel decreases with water content. These results imply that the water molecules must improve the affinity of the solvent to PVA to inhibit the aggregation or crystallization of PVA chains. The pulsed NMR measurement results showed that the spin–spin relaxation times related to the polymer‐rich and polymer‐poor phases of the PVA gel increase, and the fractional amount of the polymer‐poor phase increases while that of the polymer‐rich phase decreases with increasing water content. These facts indicated that the increase in the mobility of PVA chains must give rise to the difficulty in chain aggregation of PVA solutions with increasing water content. Two transition temperatures were found in the phase transition of the polymer‐rich phase. The lower transition temperature was attributed to the destruction of the denser chain entanglements in the polymer‐rich phase and the higher transition temperature was mainly concerned with the melting of the crystallites. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1113–1120, 2001     

Bending of poly(vinyl alcohol)–poly(sodium acrylate) composite hydrogel in electric fields

Bending of poly(vinyl alcohol) hydrogel mixed with poly(sodium acrylate) chains, PVA–PAA gel, under the influence of dc electric fields was studied. The PVA–PAA gel was prepared by repeatedly freezing and thawing a mixture of PVA and polyacrylic acid aqueous solutions. The PVA–PAA gel was a hydrogel with the PAA chains, which were entangled with the PVA polymer network and were fixed in the gel. The PVA–PAA gel bent toward the negative electrode in electrolyte solutions under dc electric fields as did the polyelectrolyte gel with negatively charged polyions. The PVA gel, free of PAA, was insensitive to dc electric fields. The deflection of the bending and the bending speed were influenced by the filed intensity, the concentration of the polyion in the gel, and the thickness of the gel. The bending of the PVA–PAA gel was qualitatively explained by a bending theory of polyelectrolyte gel, based upon the change of the osmotic pressure due to the ion concentration difference between the inside and the outside of the gel.     

Dye-labelled polymer chains at specific sites: Synthesis by living/controlled polymerization

Dye-labelled polymer chains are extremely useful in many fields, such as optical imaging, signal amplification in biological diagnostics, light-harvesting and photochromic materials as well as in fluorescence studies about intra- and inter-molecular polymer chain associations, conformations and dynamics of polymer chains. However, in many cases, it is particularly useful that the dye is localized at a specific site, such as the chain-end or the junction between blocks. With the development of living/controlled polymerization techniques, end- and junction-functionalized polymers can be prepared with controlled molecular weights from a huge variety of monomers. This review highlights the state of the art in the strategies leading to one and only one precisely localized dye per polymer chain. Such dye can be introduced at three different steps of the polymerization: i) at the very beginning via the initiator or a chain transfer agent, ii) during polymerization via a functional monomer or a quencher, or iii) after polymerization via covalent binding of a dye-derivative.     

Studies on the Hydration Behavior of Polyvinyl Alcohol in Aqueous Solution: A Kinetic Approach through the Hydrolysis of Benzocaine

The present work investigates the structural and dynamical features of Polyvinyl alcohol (PVA) and surfactant–PVA system through the change in the rate of hydrolysis of benzocaine. The UV–Vis spectra of benzocaine showed that the absorbance values at λ_max (284 nm) increased with the increase in PVA. The rate of hydrolysis decreased in the dilute- and high-concentration regions of PVA, whereas it remains unchanged in the mid-concentration range of PVA, known as the polymer–polymer interaction region. These observations correspond to the structural changes in PVA with the increase in PVA concentration in aqueous solution. The aggregation behavior of PVA was further studied using transmission electron microscopy micrographs. The rate constant for the hydrolysis of benzocaine showed peaked behavior with the increase in [CTABr] in the CTABr-PVA mixed system. The sodium dodecyl sulphate (SDS) molecules modify the PVA chains into a more compact form, and so, the hydration number around PVA is decreased. Consequently, the less hydrophilic benzocaine entangles with SDS-PVA, whereas OH^– ions aggregate into the aqueous pseudophase, resulting in a decrease in the rate of hydrolysis in the SDS–PVA mixed medium. The calculated value of binding constant increased with the increase in PVA in both CTABr-PVA and SDS-PVA mixed systems.     

Effects of the composition ratio,eosin addition,and γ irradiation on the dielectric properties of poly(vinyl alcohol)/glycogen blends

The dielectric relaxation spectra of various poly(vinyl alcohol) (PVA)/glycogen blends and irradiated blend samples with 70 wt % PVA content that were undoped and doped with eosin were measured in extended temperature (30–160°C) and frequency (1 kHz to 1 MHz) ranges. Dielectric relaxation spectroscopy separates different molecular groups of a repeating unit of a polymer with respect to the rate of its orientation dynamics. In the high‐temperature range (>100°C), the σ relaxation, which is associated with the hopping motion of ions in the disordered structure of the biopolymeric material, can be measured. The electric dipole moment and the activation energy of the glass‐transition temperature relaxation process were calculated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The interactions of ionic dye permeants with poly(vinyl alcohol) membranes

The well-known interaction between Congo Red and poly(vinyl alcohol), PVA, was studied by equilibrium dialysis. The diffusion of Congo Red into PVA membranes was much more rapid in 0.1N NaCl solution than in water. The dye appeared to be practically immobilized by the membranes in both solvents. A short survey of the dialytic behavior of various classes of ionic dyes through PVA membranes with water as solvent was undertaken. Anionic dyes permeated the membranes only very slowly, whereas cationic dyes permeated the membranes relatively rapidly and dyed them considerably. The existence of negative charges on the PVA membranes was demonstrated by performing dialysis experiments with the anionic dye Orange II and the cationic dye Acridine Orange in water and in excess electrolyte (1N NaCl).     

Revisal of the chain equilibration process in strained polymer melts

Summary The retraction of stretched molecules explains the intermediate step of relaxation observed at short times with strained polymer melts instead of a single rubberlike plateau.The stress relaxation following a rapid deformation is derived in this zone of the spectrum from the non linear, quasi elastic response of the transient network formed by the system of entangled chains. This method corrects an approximate calculation of the modulus decline in DOI's original treatment of the molecular equilibration process.     

Binding energetics of ferredoxin-NADP+ reductase with ferredoxin and its relation to function

To obtain insight into the motional features of proteins for enzymatic function, we studied binding reactions between ferredoxin-NADP(+) reductase (FNR) and ferredoxin (Fd) using isothermal titration calorimetry and NMR-based magnetic relaxation and hydrogen/deuterium exchange (HD(ex)). Fd-FNR binding was accompanied by endothermic reactions and driven by the entropy gain. Component-wise analysis of the net entropy change revealed that increases in the conformational entropy of the Fd-FNR complex contributed largely to stabilizing the complex. Intriguingly, analyses of magnetic relaxation and HD(ex) rates with X-ray B factor implied that Fd binding led to both structural stiffening and softening of FNR. Enhanced FNR backbone fluctuations suggest favorable contributions to the net conformational entropy. Fd-bound FNR further showed that relatively large-scale motions of the C terminus, a gatekeeper for interactions of NADP(+) (H), were quenched in the closed form, thereby facilitating exit of NADP(+) (H). This can provide a first dynamic structure-based explanation for the negative cooperativity between Fd and NADP(+) (H) via FNR.     

Characterization of grafting in the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as stabilizer

Emulsion polymerizations of vinyl acetate (VAc) with polyvinyl alcohol (PVA) as emulsifier were carried out by both batch and semicontinuous processes. The extent of grafting of vinyl acetate onto the PVA chains was investigated by a new method for separating the various polymer fractions in high solids content latexes. The quantification was carried out by a three‐step separation and selective solubilization of the PVAc latexes. After the separation, the water‐soluble PVA and the solvent‐soluble PVAc components were characterized by gel permeation chromatography and ¹³C–NMR, from which the accuracy of this method was verified. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1739–1747, 2001     

Synthetic absorbent for dyestuffs based on gamma crosslinked poly(vinyl alcohol) (PVA)

This work reports on the characterization and uses of gamma radiation‐crosslinked poly(vinyl alcohol) (PVA) films as absorbents for different classes of dyestuffs normally released from textile factors. Dyestuffs were selected from different producers. These dyestuffs were Remacryl Blue (basic dye), Remazol Golden Orange (reactive dye), Solar Orange (direct dye), and Sandolan (acid dye). The percentage dye sorption by PVA was determined by spectroscopic analysis and color‐strength measurements of the PVA films after absorbing the different dyestuffs. The results showed that the concentration of dimethylbisacrylamide as a crosslinking agent has a great effect on the gel fraction and percentage swelling in water of the PVA polymer. In general, crosslinked PVA showed a low tendency to absorb the different dyestuffs under investigation at room temperature. This tendency was shown to increase with an increasing temperature of the dye solution from room temperature to 70°C. Meanwhile, the highest percentage of dye sorption was found in the case of the basic dye and the lowest one in the case of the reactive dye depending on the active groups on the dyes. Moreover, the percentage dye sorption by PVA was slightly higher in the acid medium rather than in the alkaline one. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 349–354, 2003     

Studying the thermodynamic parameters of disperse dyeing of modified polyethylene terephthalate sheets using hyperbranched polymeric additive as a nanomaterial

Thermodynamic parameters of disperse dyeing of modified PET sheets by various loads of a polyesteramide hyperbranched polymer are investigated in terms of standard affinity (?Δμ°), enthalpy change (ΔH°) and entropy change (ΔS°). The results show that the standard affinity of dye to the modified PET is higher than that to the virgin PET. The bath containing virgin PET displays the highest negative values of the ΔH° and ΔS°, while the bath containing the modified PET with 2% hyperbranched polymer shows the lowest negative values of them. Surface morphology and thermal properties of the samples are analyzed by AFM and DSC.     

Structure and properties of hydrophobic cationic poly(vinyl alcohol)

A new class of derivatives of poly(vinyl alcohol) (PVA) was prepared through hydrophobic cationic modification. The structure and composition of PVA grafted with glycidyl‐N‐alkyl‐N,N‐dimethyl‐ammonium chloride (DA) (PVA‐ graft ‐DA) was confirmed with Fourier transform infrared spectral analysis and ¹H NMR spectral analysis. The stress‐strain curves of PVA‐ graft ‐DA samples all exhibited an elastic deformation stress plateau, and strain hardening behavior can be observed, indicating the transition of PVA from brittle fracture to ductile fracture. Compared with virgin PVA, the relaxation peak (T_g) of PVA‐ graft ‐DA shifted to a lower temperature. With increasing alkyl chain length and grafting ratio of DA, T_g decreased, and PVA‐ graft ‐DA exhibited a gradually decreasing storage modulus over the whole temperature range of testing due to the relatively weak intermolecular hydrogen bonding and increasing flexibility of molecular chains by introduction of long alkyl chains. PVA crystallites were not affected by grafting with DA, while the crystallization temperature and crystallinity of PVA were improved and the grain size decreased. On grafting with DA, the fracture surface of PVA changed from a smooth surface to regularly distanced striations, displaying much obvious character of tough fracture, indicating that appropriate intermolecular association of the hydrophobic groups facilitated the formation of physical entanglement of molecular chains to strengthen and toughen the PVA matrix. PVA‐ graft ‐DA showed a significant decreasing surface tension with polymer concentration, while the surface tension of PVA‐ graft ‐DA12 dropped most dramatically and declined with increasing grafting ratio of DA12, indicating improvement of the surface activity of PVA by introduction of hydrophobic alkyl chains and hydrophilic cationic groups. Copyright © 2011 Society of Chemical Industry     

PH-induced structure change of poly(vinyl alcohol) hydrogel crosslinked with poly(acrylic acid)

The structure of the hydrogel of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was investigated by small angle X-ray scattering (SAXS) of synchrotron radiation. A physically crosslinked blend gel, which was prepared by repetitive freezing and thawing of an aqueous solution of PVA and PAA, could be chemically crosslinked by esterfication of PVA with PAA even in the hydrogel state. The chemical crosslinking induced the destruction of physical crosslinks into a folded structure, indicating that the chemical crosslinking proceeds at the sites around the physical crosslinks that contain PVA and PAA in much higher concentration than other portion of the gel. The pH-induced structure changes of the PVA hydrogels, chemically crosslinked with poly(acrylic acid) (PAA) were investigated by SAXS on the samples of various chemical crosslinking time. The gels were shrunk at pH4, and swollen at pH8. The results of SAXS showed, that the Porod slope changed with chemical crosslinking time from -3.5 to ?2.9 at pH4, and from ?2.9 to ?2.4 at pH8. The results suggest that a folded structure as a structural domain, which is characterized by fractally rough interface, tends to change into the structure that corresponds to percolation cluster, particularly at pH8. The gels immersed in pH8 showed a remarkable structure change accompanying swelling. The results revealed that a conformational change of PAA chains, induced by the pH change, can be explained by the presence of a structural domain in the gel network, where both PVA chains and PAA chains get entangled and partially form a interpenetrating polymer network(IPN).     

Feasibility study on the recovery of chromium (III) by polymer enhanced ultrafiltration

The viability of recovering chromium (III) was studied by polymer enhanced ultrafiltration (PEUF) and its application to tannery wastewater treatment. Four polymers with different functional group, molecular weight and acid-basic equilibrium were tested at 40 and 60 °C in order to investigate the complexation and decomplexation of chromium (III) ions on the polymer chains: polyvinyl alcohol (PVA), polyacrylic acid-co-maleic acid (PACM), polyethylenimine (PEI) and ethoxylated polyethylenimine (EPEI). The reversibility of the polymer metal binding was studied by operating in a dead-end PEUF system. The best chromium (III) rejections for both reactions were obtained by EPEI. The feasibility of recovering chromium (III) with EPEI was also studied in a cross-flow PEUF system. Chromium ions were complexed by EPEI chains at pH = 5 and decomplexed at pH = 1. Membrane fouling due to conformational variations of EPEI was observed at high ionic strengths.     

One-step fabrication of branched poly(vinyl alcohol) nanofibers by magnetic coaxial electrospinning

Branching has been emerging in 3-D interconnecting building blocks. Branched and hyperbranched poly(vinyl alcohol) (PVA) nanofibers were fabricated by coaxially electrospinning two-liquids under an alternating magnetic field in a facile manner. Both the PVA nanofiber trunks with diameter of 100–200 nm and the PVA nanofiber branches with diameter of 10–30 nm were formed in a single step. The length and the morphology of the branched PVA nanofibers could be controlled through a rational design of the magnetic field. The facile technique may readily be extended to prepare 3-D branched nanofibers from other materials such as various polymers and polymer–ceramic materials. Moreover, the multifunctional and multicomponent materials with branched nanostructure could be expected by using the magnetic coaxial electrospinning technique. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Interaction of dimethyl methylphosphonate with Nafion in acid and cation modifications

Sorption kinetics and sorption isotherms have been determined for dimethyl methylphosphonate, DMMP, in a perfluoroionomer, Nafion™, in the acid form and as calcium and ferric salts. Nafion is a phase segregated polymer with a fluorocarbon main chain and sulfonic acid terminated fluoroether side chain comprising 33 wt% of the polymer. DMMP swells only the fluoroether region. Under immersion conditions, equilibrium solubilities for all three forms are essentially equal, averaging about 130 wt%. Kinetics are non-Fickian, dominated by relaxation effects, with calcium much slower than the acid form of Nafion, and ferric slower still. Analysis with the Joshi-Astarita model of relaxation coupled diffusion indicates that the effect of cation modification is a more than 200-fold decrease in both diffusion coefficient and relaxation rate. Vapor sorption kinetics is also non-Fickian and dominated by relaxation, frequently involving a series of steps or several maxima at high vapor activity. Additionally, kinetics and, to a lesser extent equilibrium solubilities, are affected by sample history and conditions of sample exposure. The sorption isotherm, for acid Nafion exhibits a rapid concentration increase at vapor activities below 0.2, a low slope at intermediate activities and a rapid increase above activity 0.8. Isotherms for salt modifications are similar but with even lower slope at intermediate activities. Effective diffusion coefficients for acid Nafion, calculated from boundary layer corrected steady state permeabilities combined with the sorption isotherm, exhibit a pronounced rise at a volume fraction of 0.6. A transition in NMR solvent self-diffusion coefficients attributed to the emergence of a new phase segregated structure is reported at the same concentration. When effective diffusion coefficients are converted to solvent self-diffusion coefficients, the values are 30-fold lower than NMR values at low concentrations, taken as a measure of tortuosity, but closely approach NMR values above the transition. The prominence of relaxation controlled sorption kinetics and high degree of tortuosity below the transition suggest that Nafion is a disordered structure of small interspersed fluoroether and fluorocarbon regions, whereas the occurrence of discrete relaxation events, mainly at high activities, is probably associated with larger scale reorganization of the partially coalesced phases above the transition.     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.