Dynamic light scattering of dilute PVA-borax aqueous solutions

Poly(vinyl alcohol)-borate (PVA-borate) complexes in dilute aqueous solutions were investigated by dynamic light scattering (DLS). Two relaxation modes obtained, were scattering vector-q² dependent and diffusive. The amplitude of slow mode was independent of borax concentration. In the very dilute PVA concentration regime ( [PVA] < 5 g/L), the PVA di-diol-borate complexation was dominated by intra-molecular reaction, and the fast mode correlation length ς_f which corresponds to the polymer chain dimension increased rapidly with borax concentrations lower than 0.06 M, and reached an asymptote in the higher borax concentration regime. However, for a dilute PVA aqueous solution with higher PVA content (i.e., [PVA] = 9 g/L), both intra-and inter-molecular PVA di-diol-borate complexations might happen. The chain expansion and shrinkage of PVA-borate complex with increasing borax concentration was observed due to the balance between the electrostatic repulsion of the charged diol-borate complexes and the intra-molecular crosslink induced by the intra-molecular di-diol-borate complexation.     

Complexation Equilibrium Constants of Poly(vinyl alcohol)-Borax Dilute Aqueous Solutions – Consideration of Electrostatic Charge Repulsion and Free Ions Charge Shielding Effect

Equilibrium dialysis experiments of dilute poly(vinyl alcohol) (PVA) in borax aqueous solution (PVA concentration was 2 g/L and borax concentration ranged from 0.0 M to 0.05 M) were carried out to determine the concentration of free borate ions in equilibrium with the PVA-borate complex at 25°C, and hence the amount of borate bound to PVA. The binding isotherm of borate ion on PVA molecules was investigated. The concentration of borate ion bound on PVA was found to increase with increasing borax concentration and reached an asymptote at high borax concentrations. The complexation equilibrium constant between borate and PVA diol was found to decrease initially then increases and finally decreases with increasing borax concentration while PVA concentration was fixed in a constant value. The equlibrium constant of free borate ion with borate-PVA diol complex was a balance result of excluded volume of PVA molecular chains, electrostatic charge repulsion between free borate ions and borate-PVA diol complexes, and electro-shielding of free Na⁺ ions on negatively charged PVA-borate complexes. In this study, the electrostatic charge repulsion between free borate ions and borate-PVA diol complex ions and free Na⁺ ions shielding effect are considered in the estimation of equilibrium constants of PVA diol-borate complexation. A fixed value of equilibrium constant is obtained with increasing borax concentration while borax concentraion is below a critical concentration [borax]_crt. As borax cocentration is higher than [borax]_crt, a lots of PVA diols form complexes with borate ions and a high shielding of free Na⁺ ions on negatively charged borate-PVA diol complexes causes the PVA-borate complexes to behave like neutral polymers. Few free PVA diol binding sites are available for reaction with free borate ions while borax concentration is higher than [borax]_crt resulting in a decrease in equilibrium constant with increasing borax concentration.     

Synthesis of boronate-containing copolymers of N,N-dimethylacrylamide, their interaction with poly(vinyl alcohol) and rheological behaviour of the gels

Cross-linking of polyvinylalcohol (PVA) by boronate-containing copolymer of N,N-dimethylacrylamide (DMAA, 1) was studied and compared to cross-linking of PVA by borate buffers in weakly alkaline solutions. The copolymer of M_w=19,000 g mol⁻¹ containing 9 mol% N-acryloyl-m-aminophenylboronic acid (NAAPBA, 2) was prepared by free radical polymerization of the monomers, exhibiting copolymerization constants r₁=0.84 and r₂=2.2. Due to multipoint interaction of the copolymer with PVA via monodiols, the intermolecular cross-linking required for seven-fold and 10-fold lower boron concentrations as compared to borate buffers of pH 8.6 and 7.5, respectively. In rheological measurements, PVA-copolymer gels exhibited storage moduli (G′_max) comparable to those of PVA-borate gels prepared at 7.5-fold higher boron concentration and the same pH 8.6, what testified to the similar concentration of cross-links in the gels. Therefore, DMAA-NAAPBA copolymer is a more effective cross-linker of PVA than borate. The PVA-copolymer gel exhibited much higher relaxation time (97 s) compared to PVA-borate gels (≤20 s) indicating a longer lifetime of junction zones. The ‘shape stability’ of the gel is suggested to originate in the structure of junctions, containing several boronate-diol complexes, between the macromolecules of PVA and the copolymer.     

Electrosynthesis and protection role of polyaniline–polvinylalcohol composite on stainless steel

Polyaniline–polyvinyl alcohol (PANI–PVA) composite has been electrodeposited on stainless steel surface from aqueous sulfuric acid solution of aniline monomer in presence of soluble PVA at different concentrations. The PVA increased the rate of electropolymerization where 4 g/L PVA formed a composite of 37 wt% PANI and 63 wt% PVA composition. The composite layer exhibited more adhesion to the steel surface in comparison with PANI layer but with less thermal stability. It has higher protection role for the stainless steel (SS) against general and pitting corrosion. It enhanced the passivation of the SS surface by increasing the thickness of oxide film and improving the composition.     

Synthesis of star polymer by means of the living polymerization of [(o-trifluoromethyl)phenyl]acetylene using a MoOCl4-based catalyst and the linking method

A star polymer was synthesized by addition of 1,4-diethynyl-2,5-dimethylbenzene as linking agent (30 °C, 24 h) after living polymerization of [(o-trifluoromethyl)phenyl]acetylene (o-CF₃PA) with MoOCl₄-n-Bu₄Sn-EtOH catalyst (in anisole, 30 °C, 20 min; [Mo]=10 mM, [P^∗]/[Mo]=40%, [o-CF₃PA]₀=200 mM). The M_n values of the living and star polymers were 8.1×10³ and 5.3×10⁴, respectively, according to gel permeation chromatography, while these values determined by multi-angle laser light scattering (MALLS) were 7.8×10³ and 2.5×10⁵. The M_w/M_n and arm number of the star polymer were 1.04 and 29, respectively, according to MALLS. The molecular weight and arm number of star polymer increased with increasing linking agent concentration and polymerization temperature.     

Transition of hydration states of poly(vinyl alcohol) in aqueous solution

A straightforward method for determination of the hydration number of polymer in aqueous solution based on ice-melting technique of DSC is proposed. The simple yet precise method has been applied to determine the hydration number of poly(vinyl alcohol) (PVA) in aqueous solution covering a wide range of concentrations, from 0.005 to 0.3 g(solute)/g(solution), for three samples with different molar masses. The hydration number of PVA maintains a constant lower value of 2.5 when the concentration exceeds 0.2 g(solute)/g(solution). It increases to a value of 7 when the concentration decreases to the overlap concentration C^∗ of the polymer, where C^∗ was estimated as the reciprocal of its intrinsic viscosity. For solutions of C < C^∗, the hydration number keeps constant again at the value of 7. This behavior evidently demonstrates that PVA has two hydration states, one occurs at the dilute regime and the other occurs at concentrated regime. The concentration dependent transition from one state to another is treated mathematically by a quantitative formula which involves two parameters: one denotes the transition concentration and the other denotes the width of the transition region. The transition concentration decreases linearly with increasing molar mass resembling the behavior of molar mass dependence of overlap concentration. The structural features for the two states of hydrated PVA are briefly discussed.     

Effect of 1-pentanol on the styrene emulsion polymerization

The influence of 1-pentanol (C₅OH) on the ST emulsion polymerization mechanisms and kinetics is investigated. The CMC of the ST emulsions first decreases rapidly and then levels off when the C₅OH concentration ([C₅OH]) increases from 0 to 72 mM. The effect of C₅OH increases to a maximum and then decreases when the SDS concentration ([SDS]) increases from 2 to 18 mM. At [SDS]=2 mM, homogeneous nucleation controls the polymerization kinetics regardless of [C₅OH]. At [SDS]=4 mM, the effect of [C₅OH] appears due to the transition from homogeneous nucleation to a mixed mode of particle nucleation (homogeneous nucleation and micellar nucleation) occurs when [C₅OH] increases from 0 to 72 mM. The effect of [C₅OH] is the strongest at [SDS]=6 mM since the particle nucleation mechanisms span homogeneous nucleation (low [C₅OH]), a mixed mode of particle nucleation (homogeneous nucleation and micellar nucleation) (medium [C₅OH]) and micellar nucleation (high [C₅OH]). At [SDS] >6 mM, in which micellar nucleation controls the polymerization kinetics, the effect of [C₅OH] decreases rapidly with increasing [SDS].     

Boroxine ring compounds as dissociation enhancers in gel polyelectrolytes

In order to combine the advantages of both traditional gel electrolytes and polyelectrolytes, a novel polyelectrolyte which incorporates a boroxine ring-containing anion-trapping agent has been explored. Poly(lithium 2-acrylamido-2-methyl-1-propanesulfonate) (PAMPSLi), ethylene carbonate (EC) and tri(methoxyethoxyethoxyethoxy)boroxine (TME₃Bx) were combined to prepare various gel systems. The thermal properties and conductivities of these gels have been investigated. A conductivity of 10^−3.6 S cm⁻¹ at 20 °C has been achieved in a gel polyelectrolyte system with a molar ratio of [EC]:[TME₃Bx]:[Li⁺]=24:1.7:1. Temperature-dependent NMR measurements indicated that a significant interaction exists between the boroxine ring and the polyelectrolyte.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.     

Interaction of nanogel with cyclodextrin or protein: Study by dynamic light scattering and small-angle neutron scattering

The structure of hydrogel nanoparticles (CHP nanogels), formed by self-aggregation of cholesterol-bearing pullulan (CHP) was studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The interactions between the CHP nanogel and methyl-β-cyclodextrin (CD) or protein (hen egg white) were also investigated. It was revealed by SANS that the nanogels were spherical in shape with a radius of 6.7 nm. The following two functions were disclosed. (1) CHP nanogels were dissociated by the addition of CD and formed inclusion complexes with cholesteryl groups, leading to suppression of hydrophobic interaction between the cholesteryl groups. (2) The nanogel behaved as a molecular chaperone (heat shock protein-like activity) when CHP nanogel was mixed with hen egg white and heated up to 75 °C. The egg white aqueous solutions with CHP nanogel remained transparent while the egg white without CHP nanogel became opaque.     

Electrostatic self-assembly of thermally responsive zwitterionic poly(N-isopropylacrylamide) and poly(ethylene oxide) modified with ionic groups

The electrostatic self-assembly of thermally responsive copolymers of N-isopropylacrylamide (NIPAM) containing up to 10 mol% of the sulfobetaine monomer 3-[N-(3-methacrylamidopropyl)-N,N-dimethyl]ammoniopropane sulfonate) (SPP) and poly(ethylene oxide) modified with terminal cationic or anionic groups (IMPEO) was studied in methanol and aqueous solutions by static light scattering, turbidimetry, viscometry, and rheological measurements. The formation of graft-like complexes at stoichiometric dipole-ion ratio and their self-association was detected in the dilute and semidilute regime at temperatures below and above the lower critical solution temperature (LCST). The ability of the graft-like complexes to associate below the LCST depended on the sulfobetaine content of the copolymers, the functionality of IMPEO, and the polymer concentration. The effect of the IMPEO terminal group on the solution behavior of the graft-like complexes was less pronounced. With increasing temperature their semidilute aqueous solutions form gels, stable over a wide temperature range.     

TEMPO-mediated radical polymerization of styrene in aqueous miniemulsion: Macroinitiator concentration effects

The control/livingness in nitroxide-mediated polymerization of styrene (S) in aqueous miniemulsion at 125 °C employing a poly(S)-2,2,6,6-tetramethylpiperidinyl-1-oxy (PS-TEMPO) macroinitiator and the surfactant sodium dodecylbenzenesulfonate has been shown to depend strongly on the macroinitiator concentration for particles of approximate number-average diameter 65 nm. The control/livingness was relatively poor at [PS-TEMPO]₀ ≤ 0.02 M due to the combined effect of enhanced spontaneous initiation and the interface effect (whereby deactivation is suppressed due to interfacial activity of TEMPO). Satisfactory control/livingness was obtained at higher [PS-TEMPO]₀ as a result of the interface effect and enhanced spontaneous initiation exerting less pronounced influence per chain than at lower [PS-TEMPO]₀. Polymerizations using the sulfonate surfactant DOWFAX 8390 gave similar results, indicating that the present macroinitiator concentration effects are not specific to SDBS-based systems. The results also demonstrate that TEMPO-mediated polymerization of S in miniemulsion can proceed at a higher rate than in bulk with good control/livingness.     

Electrodeposition of selenium, indium and copper in an air- and water-stable ionic liquid at variable temperatures

The electrochemical behaviour of Au(1 1 1) and highly oriented pyrolytic graphite (HOPG) substrates in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]Tf₂N) was investigated using in situ scanning tunneling microscopy (STM). Furthermore, the electrodeposition of Se, In and Cu in the same ionic liquid was investigated. The high thermal stability as well as the large electrochemical window of this ionic liquid compared with aqueous electrolytes allow the direct electrodeposition of grey selenium, indium and copper at variable temperatures, as the first step in making CIS solar cells electrochemically, in a one pot reaction. The results show that grey selenium can be obtained at temperatures ≥100 °C. XRD patterns of the electrodeposit obtained at 100 °C show the characteristic peaks of crystalline grey selenium. Nanocrystalline indium with grain sizes between 100 and 200 nm was formed in the employed ionic liquid, containing 0.1 M InCl₃, at room temperature. It was also found that copper(I) species can be introduced into the ionic liquid [BMP]Tf₂N by anodic dissolution of a copper electrode and nanocrystalline copper with an average crystallite size of about 50 nm was obtained without additives in the resulting electrolyte.     

Swelling behavior of amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate

The amphiphilic gels based on hydrophobically modified dimethylaminoethyl methacrylate with different 1-bromoalkanes (1-C_nH_2n+1Br, n = 2, 4, 6, 8, 12) were synthesized by radiation-induced polymerization and crosslinking. The length of alkyl side chains had significant influence on the swelling behavior of the resulting gels. The swelling degree of the gels decreased with the increase of side chain length, and the gel hardly swelled when n = 12. The effect of temperature and ionic strength on the swelling behavior of the resulting gels revealed that (1) the gels with longer side chains (n ≥ 8) had upper critical solution temperature, while other gels were not thermo-sensitive. (2) Antipolyelectrolyte effect was observed when immersing the gels (n ≥ 8) in NaCl solutions in certain concentration range. The dramatic difference in swelling behavior was attributed to the different gel structures. The gels with short side chains (n ≤ 6) had cellular structure of normal polyelectrolyte gels. The gels (n ≥ 8) had an aggregation gel structure caused by the hydrophobic interaction among alkyl groups and the formation of ion-cluster between tetra-alkyl ammonium cation and Br⁻, which had been analyzed with the aid of SEM, Br⁻-selective electrode and fluorescence molecular probe.     

Effects and optimization of initial pH and sewage sludge compost content on leaching of lead and zinc in contaminated soil

We investigated the effects of initial pH (2≤pH₀≤6) and sewage sludge compost content (5≤[SSC]≤25 g/kg) on leaching characteristics of lead (Pb) and zinc (Zn) in contaminated field soil. pH₀ and [SSC] significantly affected the leaching of Pb and Zn in soils contaminated with them. The pH in the solution increased as reaction time and [SSC] increased. The leached amounts of Pb and Zn were highest at pH₀=2 and increased with reaction time. As [SSC] increased, the leached amount of Pb decreased (50.4 mg/kg at control condition ([SSC]=0 g/kg); 22.9 mg/kg at [SSC]=25 g/kg at pH₀=2) and the leached amount of Zn increased (20.1 mg/kg at [SSC]=0 g/kg; 31.7 mg/kg at [SSC]=25 g/kg at pH₀=2). The change increased as pH₀ decreased. Within the design boundaries, minimum leaching of Pb (14.7 mg/kg) occurred at pH₀=5.1 and [SSC]=25 g/kg, and minimum leaching of Zn (5.0 mg/kg) occurred at pH₀=5.1 and [SSC]=5 g/kg.     

Dimensions and viscosity behavior of polyelectrolyte brushes in aqueous sodium chloride. A polymacromonomer consisting of sodium poly(styrene sulfonate)

Nine samples of a polymacromonomer consisting of sodium poly(styrene sulfonate) and having a side-chain polymerization degree of 15 are investigated by light scattering, small-angle X-ray scattering, and viscometry with 0.05 M aqueous NaCl at 25 °C as the solvent. The (total) weight-average molecular weight M_w ranges from 2.2 × 10⁴ to 7.1 × 10⁶. The radii of gyration, scattering functions, and intrinsic viscosities determined as functions of M_w are analyzed in terms of the cylindrical wormlike chain model. The estimated Kuhn segment length of about 120 nm is much larger than that (16 nm) for the polystyrene polymacromonomer with the equivalent side-chain length in toluene, a good solvent, while the chain thickness (5.5 nm) is comparable to or only slightly larger than that (5 nm) of the nonionic polymer. It is thus concluded that the electrostatic repulsion between side chains significantly stiffens the main chain of the polyelectrolyte brush.     

Double helix of Agrobacterium tumefaciens succinoglycan in dilute solution

Succinoglycan samples ranging in weight-average molecular weight from 1.4×10⁵ to 10.5×10⁵ (in 0.1 M aqueous NaCl at 25°C), prepared from native succinoglycan by sonication, were investigated by static light scattering and viscometory in 0.1 M aqueous NaCl. It is well known that the conformation of the polysaccharide changes at around 65°C. In order to confirm the conformation of the polysaccharide, the property of the polysaccharide in dilute solution was studied at both 25 and 75°C. The molecular weight of a sample at 25°C is the double of the value at 75°C. The molecular weight dependence of 〈s²〉_Z and [η] for succinoglycan shows that the polysaccharide is a rod-like polymer at 25°C and behaves like a semi-flexible polymer at 75°C in 0.1 M aqueous NaCl. Moreover, the linear mass density of the polysaccharide was almost twice that expected for single succinoglycan molecules. These results indicate that the conformation of the polysaccharide would be a double helix at 25°C and the helix melts to a single strand behaving as a semi-flexible chain above 65°C.     

Creating new materials with melamine resins

Acid catalyzed condensation of hexa-methoxy methyl melamine (HMMM) in aqueous phase leads to new functional particles and up to now unknown lamellar mesoscopic gels. The investigation with transmission electron microscopy (TEM) shows that the polymer formation starts with nonspherical nanoparticles. Dynamic light scattering experiments reveal a particle size of about 60–100 nm. Atomic force microscopy (AFM) measurements disclose nonuniform flat particles with an aspect ratio of about 0.3. These nanoparticle dispersions form thermoreversible gels. Molecular modeling investigations indicate energy minimized layer-by-layer condensation of the melamine resin molecules. The next step in growth is the nucleation of the nanoparticles via the narrow sides. This forms nonperfect lamellar layers. This time, we get a thermoreversible gel which is fluid at 80 °C and gets fixed at 20 °C. Out of these platelet structures as precursors, a mesoporous, nonthermoreversible gel with essentially lamellar sides and pore sizes about 10 μm is formed. Scanning electron microscopy (SEM) studies show very uniform wall and plate sizes with a directed three-dimensional structure.     

Morphology of nanoholes formed in silicon by wet etching in solutions containing HF and H2O2 at different concentrations using silver nanoparticles as catalysts

Holes with diameters of tens of nanometers were bored in Si(1 0 0) in aqueous solutions containing hydrofluoric acid and hydrogen peroxide utilizing silver nanoparticles as catalysts. The holes grew deeply in the [1 0 0] direction when the concentration of hydrogen peroxide was about 0.18 M. In addition to these vertical holes, holes were generated horizontal to the surface in the 〈1 0 0〉 directions near the surface of the sample. We found that the silver particles making the deep holes in the [1 0 0] direction were more spherical than those making the horizontal holes near the surface. These results indicate that the shape of the silver particles is an important factor controlling the direction of the holes. When the hydrogen peroxide concentration was increased to 1.8 M, as well as the vertical and horizontal holes, a microporous layer was formed on the top surface region. On the other hand, when the hydrogen peroxide concentration was lowered to 0.0018 M, holes did not show any preference for growing direction and were crooked. Even in HF solutions that did not contain hydrogen peroxide, holes were generated at a very slow rate in random directions if the solutions contained oxygen.     

Effect of saccharin addition on the corrosion resistance of polypyrrole coatings

Electropolymerization baths for polypyrrole coating typically consist of two main constituents: monomer and counter-ion. By adding saccharin as the third constituent, a new branch emerges in electropolymerization that will yield coatings with modified properties. In the present study, polypyrrole is galvanostatically deposited onto carbon steel in an aqueous solution containing 0.2 M pyrrole and 0.1 M oxalic acid as the supporting electrolyte with (0.25, 0.5, 2.5 g/l) and without saccharin addition. Determination of the corrosion resistance of coatings in 0.1 M NaCl solution shows that in the presence of 0.5 g/l saccharin, a more noble corrosion potential and a greater corrosion resistance are obtained. Electrochemical impedance spectroscopy (EIS) revealed that the corrosion process was under infinite diffusion control of ions during prolonged immersion. The results indicate that the lower diffusion rate of ions in the coating produced in the presence of 0.5 g/l saccharin is responsible for its better corrosion resistance. Atomic force microscopy (AFM) shows that greater compaction of the coating produced in the presence of 0.5 g/l saccharin could be the reason for its higher corrosion resistance.