Effects of cononsolvency on gelation of poly(vinyl alcohol) in mixed solvents of dimethyl sulfoxide and water

We studied the rates of gelation and phase separation of poly(vinyl alcohol) (PVA) solutions in mixtures of dimethyl sulfoxide (DMSO) and water at 25 °C and found that both the rates show a maximum at a volume fraction of DMSO φ_DMSO=0.60 while gelation was not observed either in pure DMSO or pure water, suggesting that water-DMSO is a cononsolvent system for PVA. On the basis of the data by Cowie [Can J Chem 36 (1961) 2240] we concluded that the 1:2 stable complex between one DMSO molecule and two water molecules is the main cause of this cononsolvency.     

Modification of poly(vinyl alcohol) using maleic acid and its application to the separation of acetic acid-water mixtures by the pervaporation technique

The ‘solution technique’ modification of poly(vinyl alcohol) (PVA) using maleic acid was carried out with the help of triethanolamine/water catalysts. The resulting PVA membranes were characterized by differential scanning calorimetry, infrared spectroscopy, and tensile studies to investigate the reaction between PVA polymer and maleic acid. It was found that the resulting PVA membranes had two portions, branched and crosslinked, and there were no more branched than cross-linked portions. For the pervaporation separation of the acetic acid-water system, two reaction densities (X_cr = mole maleic acid per mole monomeric unit of PVA) of 0.05 and 0.1 were studied for the separation of the entire range of mixture compositions at 25° C. The separation factors of the X_cr = 0.05 modified PVA membranes were higher than those of the X_cr 0.1 modified ones and the highest separation factor of 7.80 was obtained at 70wt% water in the feed with the X_cr = 0.05 modified PVA membrane.     

Expansion of Access Tunnels and Active‐Site Cavities Influence Activity of Haloalkane Dehalogenases in Organic Cosolvents

The use of enzymes for biocatalysis can be significantly enhanced by using organic cosolvents in the reaction mixtures. Selection of the cosolvent type and concentration range for an enzymatic reaction is challenging and requires extensive empirical testing. An understanding of protein–solvent interaction could provide a theoretical framework for rationalising the selection process. Here, the behaviour of three model enzymes (haloalkane dehalogenases) was investigated in the presence of three representative organic cosolvents (acetone, formamide, and isopropanol). Steady‐state kinetics assays, molecular dynamics simulations, and time‐resolved fluorescence spectroscopy were used to elucidate the molecular mechanisms of enzyme–solvent interactions. Cosolvent molecules entered the enzymes' access tunnels and active sites, enlarged their volumes with no change in overall protein structure, but surprisingly did not act as competitive inhibitors. At low concentrations, the cosolvents either enhanced catalysis by lowering K_0.5 and increasing k_cat, or caused enzyme inactivation by promoting substrate inhibition and decreasing k_cat. The induced activation and inhibition of the enzymes correlated with expansion of the active‐site pockets and their occupancy by cosolvent molecules. The study demonstrates that quantitative analysis of the proportions of the access tunnels and active‐sites occupied by organic solvent molecules provides the valuable information for rational selection of appropriate protein–solvent pair and effective cosolvent concentration.     

Adsorption of nile red by poly(N‐isopropylacrylamide) gels in binary water/tetrahydrofuran mixtures

The adsorption of Nile Red by poly(N‐isopropylacrylamide) (PNIPAM) gels in binary water/tetrahydrofuran solutions was investigated using absorption spectrophotometry as a function of the volume fraction of THF, φ. Due to the cononsolvency abilities of such binary solvent mixtures, deswelling of the PNIPAM gels is observed in water‐rich mixtures, 0 ≤ φ ≤ 0.6, while the gels reswell for φ > 0.6. The position of the absorption band of Nile Red before and after equilibration with the PNIPAM gels indicates that the composition of the external solvent mixture is not practically influenced by the swelling process. On the other hand, it is found that the gels can effectively adsorb Nile Red in water‐rich mixtures (φ < 0.6), whereas no significant adsorption was observed in tetrahydrofuran‐rich mixtures. In fact, about 80% of the dye is adsorbed by the PNIPAM gels, at φ = 0.2. Under these conditions, the rather shrunk PNIPAM gel offers a more convenient less polar environment for Nile Red than the water‐rich bulk solvent. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Separation characteristics of pervaporation membrane separation processes using modified poly(vinyl alcohol) membranes

Modified poly(vinyl alcohol) (PVA) membranes prepared by the ‘solution technique’ were tested for ethanol-water mixtures by varying the reaction density (X_cr = 0.05, 0.1) at various temperatures. The results are compared with those of PVA membranes (X_cr = 0.05) prepared by the technique of the GFT Company, Germany.     

Dehydration of D-xylose into furfural over H-zeolites

The liquid-phase dehydration of D-xylose into furfural was carried out over various H-zeolites-H-ferrierite, H-β, H-ZSM-5, H-Y and H-mordenite-with various SiO₂/Al₂O₃ molar ratios in different solvent systems: water, dimethyl sulfoxide (DMSO) and a mixture of water and toluene (water/toluene). For comparison, γ-Al₂O₃ and silica-alumina were also examined. FT-IR spectroscopy after pyridine adsorption was conducted to probe the acidity of the H-zeolites. The D-xylose conversion and furfural yield generally decreased with increasing SiO₂/Al₂O₃ molar ratio over the H-zeolites having the same crystal structure irrespective of the kind of solvent system. This is closely related to the accessible acid sites. In a comparison study using the three different solvent systems, the D-xylose conversion and furfural selectivity generally decreased in the following order: water/toluene>DMSO>water. In water and water/toluene, H-β (25) showed the highest furfural selectivity at a similar D-xylose conversion among the tested zeolites. On the other hand, H-mordenite (20) showed the highest furfural selectivity at a similar D-xylose conversion in DMSO.     

Effects of cononsolvency on preferential adsorption phenomenon in poly(N‐isopropylacrylamide) ternary solutions

In this work, the effect of cononsolvency on the phase transition and preferential adsorption phenomenon behaviors of poly(N‐isopropylacrylamide)/methanol/water ternary solutions was studied. In this cononsolvent system, the , , and χ₁₂ values show a nonlinear behavior and the minimum values of and , while the maximum value of χ₁₂ at ϕ₂ is around 0.7. These facts indicated that one water molecule could directly bond with one methanol molecule to form the H₂O MeOH complex. The H₂O MeOH complex structure was found to remarkably affect the phase transition of poly(N‐isopropylacrylamide) (PNIPAM) in ternary solution. However, at the composition of mixed cononsolvent, ϕ₂ < 0.2, the PNIPAM molecules may preferentially adsorbed pure water molecules; therefore, the LCST decreases slightly with composition of mixed cosolvent and this may be because of the small amount of H₂O MeOH complexes in the mixed cononsolvent. While, at ϕ₂ > 0.7, the PNIPAM molecules may preferentially adsorbed pure methanol molecules. PNIPAM ternary solutions were transparent and no transition occurred in this region. This indicates that the PNIPAM coils exhibited a much‐extended conformation in solutions. In contrast, at 0.2 < ϕ₂ < 0.4 and 0.4 < ϕ₂ < 0.7, PNIPAM molecules preferentially adsorbed water and methanol molecules, respectively, and also adsorbed large amount of H₂O MeOH complexes. In these regions, the clathrate‐like structure around the side chain of PNIPAM molecule became more defected with adsorbing H₂O MeOH complex. Therefore, we considered that the various thermodynamic behaviors between PNIPAM and mixture solvents must be related different preferential adsorption phenomena, which were mainly related to different degrees of polymer–solvent interaction and structures of solvent used. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

State of water in chitosan–PVA hydrogel

The bound water fraction (X_BW) of a newly‐ developed pH‐sensitive, biodegradable chitosan‐polyvinyl alcohol (PVA) hydrogel crosslinked with glutaraldehyde (GA) was investigated as a function of the chitosan/PVA molar ratio, GA concentration (C_GA), and ionization state. Differential scanning calorimetry (DSC) was used to determine the X_BW of the initial hydrogel, and of the hydrogel equilibrated in pH 3 and pH 7 buffers. Changes in X_BW during swelling and shrinking of hydrogel were also investigated. In the initial state of hydrogel, X_BW increased with increasing PVA concentration (C_PVA), without being significantly affected by C_GA. In the buffer‐equilibrated hydrogels, X_BW decreased with increasing C_PVA and decreasing C_GA. The amount of bound water based on dry mass (C_BW) was substantially higher when the hydrogel was in the ionized (swollen) state compared to its unionized counterpart. This may be due to the association of a large quantity of water molecules with ? NH₃⁺ groups of chitosan when the gel swelled in the acidic environment. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3227–3232, 2006     

Solvent‐specific swelling behavior of poly(allylamine) gel

Poly(allylamine) (PAlAm) gel was prepared by γ‐ray irradiation. Swelling behavior of the charged PAlAm gels having Cl^? and NO₃^? counter‐ions (PAlAmHCl and PAlAmHNO₃, respectively) was investigated in aqueous organic solvent mixtures as functions of solvent species (dimethylsulfoxide (DMSO), methanol (MeOH), ethanol (EtOH), 2‐propanol (2PrOH), tert‐butanol (tert‐BuOH), acetone, tetrahydrofuran (THF) and dioxane) and the concentration. With increasing solvent concentration, the gels (except for the DMSO system) showed a sharp deswelling comparable to the volume phase transition. The solvent specificity for the PAlAmHNO₃ gel transition was correlated with the dielectric constant of the organic solvents, while that for the PAlAmHCl gel suggested that solvency for the counter anion is another important factor. Copyright © 2004 Society of Chemical Industry     

Polymer-cosolvent systems: 7. The application of free volume theory to the classical cosolvent systems formed from polystyrene in acetone and n-alkanes

The Prigogine-Patterson theory of polymer solutions has been applied to the cosolvent systems formed from mixtures of acetone (1) with n-alkanes (2). The necessary reduction parameters were calculated taking due account of acetone/alkane interactions as characterized by the Flory X₁₂ parameter. The effect of relatively increased free volume differences in the solvent mixtures is counteracted by a marked decrease in the enthalpy term which brings about an overall decrease in the polymer/solvent interaction parameter (χ). It is suggested that cosolvency is predominantly enthalpic in origin and a possible mechanism is postulated.     

Polyacrylonitrile‐g‐poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures

Poly(vinyl alcohol) as well as its grafted copolymer membranes with polyacrylonitrile (PAN‐g‐PVA) were prepared and used to separate water and dimethyl formamide mixtures by the pervaporation technique. The three following membranes were prepared: (1) pure PVA; (2) 46% grafted PAN‐g‐PVA; and (3) 93% grafted PAN‐g‐PVA. Pervaporation separation experiments were carried out at 25°C for the feed mixture containing 10 to 90% water. By use of the transport data, permeation flux, separation selectivity, swelling index, and diffusion coefficients have been calculated. By increasing the grafting of the membrane, flux decreased, whereas separation selectivity increased slightly over that of pure PVA membrane. Arrhenius activation parameters for transport processes were calculated for 10 mass % water containing feed mixture by using flux and diffusion data obtained at 25, 35, and 45°C. Transport parameters were discussed in terms of sorption‐diffusion principles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 4091–4097, 2004     

Preferential adsorption in polymer/solvent-1 /solvent-2 solutions by infrared spectroscopy

IR spectroscopy is shown to be a suitable technique for preferential adsorption studies in the following solvent-1/solvent-2/polymer systems: poly(N-vinylcarbazole) and polyacenaphthene in nitrobenzene/dioxane, nitrobenzene/ tetrahydrofuran, and nitrobenzene/cyclohexanone. Values of the preferential adsorption parameters derived from the Schultz-Flory theory agree with those from IR when both solvents in the ternary system are considered to be good solvents for the polymer. In the systems studied, the relative adsorptions of solvent-1 and solvent-2 by the polymer depend on the solvent mixture composition. The number of adsorbed molecules and their performance are correlated with the ternary interaction parameters, X_m3, and the Mark-Houwink-Sakurada exponent, a′. The adsorption changes observed are greater in those systems in which tetrahydrofuran was present.     

Viscosity studies of some polyamides dissolved in mixed solvents (cosolvents)

The solubility of poly (octamethylene sebacamide) (POS), poly (octamethylene terephthalamide) (POT) and poly (octamethylene tetrachloroterephthalamide) (POTC1), in $\text{m-}\text{cresol}\text{cyclohexane}$ mixtures, a cosolvent system, has been studied by measuring the intrinsic viscosity [η], at 25°C. The intrinsic viscosities of POT, POS and POTC1 in various solvent/non-solvent mixtures are higher than the corresponding intrinsic viscosities in the pure solvent m-cresol. The POT and POTC1 polyamides exhibit a sharp increase in [η] values compared with the corresponding small increase in the POS polyamide. The POTC1 polyamide presents a true cosolvency effect in $\text{m-}\text{cresol}\text{cyclohexane}$ mixtures; m-cresol and cyclohexane are non-solvents for this polyamide. The cosolvent behaviour is discussed in terms of the most energetically favourable interactions. The aromatic ring in POT and POTC1 would expand the chain, would favour a better packing between neighbouring solvent molecules and would favour the selective adsorption in the cosolvent system.     

Characteristic rheological features of PVA solutions in water-containing solvents with different hydration states

Two organic solvent systems containing equimolar ratio of water but different association strenght, 8/2 by wt mixture of dimethyl sulfoxide (DMSO) and water and 86.7/13.3 by wt mixture of N-methyl morpholine N-oxide and water (NMMO monohydrate), were chosen and the rheological properties of polyvinyl alcohol (PVA) solutions in the solvents were investigated and compared. The associated state of water in the solvent systems had a significant effect on the rheological properties of the solutions. In dilute concentration regime, NMMO monohydrate gave higher intrinsic viscosity than DMSO/water although two solvents gave similar values of the Mark-Houwink exponent. More noticeable difference was observed on the viscosity curve in high concentration regime. DMSO/water exhibited near a Bingham behavior and gave much lower power-law index than NMMO monohydrate. On the logarithmic plot of storage modulus (G′) against loss modulus (G″), both solutions exhibited slight decrease in slope with increasing concentration. Of two solvent systems DMSO/water gave lower values of slope than NMMO monohydrate. In addition, NMMO monohydrate gave greater than one at low frequency whereas DMSO/water gave much less than one over the entire frequency range examined. Further, the frequency to cause gelation was decreased with increasing concentration in the case of NMMO monohydrate. This indicates that PVA/DMSO/water system is more heterogeneous because weakly bonded water molecules produce hydrogel structure.     

Microwave dielectric relaxation and molecular dynamics in binary mixtures of poly(vinyl pyrrolidone)–poly(ethylene glycol)s in non‐polar solvent

Dielectric relaxation study of binary mixtures of poly(vinyl pyrrolidone) (PVP) (M_w = 40 000 g mol^?1) and poly(ethylene glycol)s (PEGs) (M_n = 200, 400 and 600 g mol^?1) with concentration variation was carried out in dilute solutions of benzene at 10.1 GHz and 35 °C. The average relaxation time τ_o, corresponding to segmental motion τ₁ and group rotations τ₂ was determined for PVP–PEGs mixtures. A comparison of these mixtures relaxation times was made with the relaxation times of PEGs in benzene solvent. The evaluated τ_o values of PVP–PEGs mixtures in benzene solution are assigned to the reorientation of PEG molecules. It has been observed that the τ_o value of PVP–PEG200 mixtures increases with increasing concentration of PVP but their values are small in comparison with the τ_o value of PEG200 molecules. In the case of PVP–PEG400 and PVP–PEG600 mixtures, the evaluated values of τ_o are greater than the corresponding τ_o values of PEG400 and PEG600 molecules in benzene solvent. The variation in τ_o values in these systems has been discussed by considering the stretching effect in the PEGs molecular chains in PVP–PEGs mixtures in benzene solutions. The high value of distribution parameter α (≈0.4 to 0.7) suggests that in these mixtures there is a large contribution of segmental motion and group rotations to the relaxation processes. The nature of the formation of hydrogen‐bonded PVP–PEG complex heterogeneous network due to hydrogen bonding between carbonyl groups of PVP monomer units and terminal hydroxyl groups of PEGs is discussed. Furthermore, the elongation behaviour of PVP–PEG complex networks in benzene solvent and the molecular dynamics in the mixture due to breaking and reforming of hydrogen bonds has been explored by comparing the evaluated relaxation times and the Kirkwood correlation factor of pure PEG molecules for their possible use in drug control release systems. The relaxation times of these mixtures are independent of their viscosity, but the elongation of the mixture network is influenced by the PEG chain length and the number of hydroxyl groups in comparison with the number of carbonyl groups in the mixtures. Copyright © 2003 Society of Chemical Industry     

Synthesis,characterization, and drug release activity of structurally modified poly(vinyl alcohol)

The main aim of the present investigation is synthesis of drug‐grafted poly(vinyl alcohol) (PVA) for sustainable drug release in order to avoid bulk release and unwanted side effects. Here, the PVA was structurally modified with five different drug molecules in DMSO medium at 85 °C under N₂ atmosphere for 2 h. The structure of modified PVA was confirmed by FTIR and ¹H NMR spectra and further it was characterized by TGA, DSC, and SEM. The tensile strength and % elongation for the structurally modified PVA were determined. The FTIR spectrum showed peaks corresponding to the C?O and C? S stretching due to the grafted drug molecules. The ¹H NMR spectrum showed the acrylic CH₂ proton signal of PVA around 1.6 ppm. The SEM showed different surface morphology for the structurally modified PVA. The mechanical properties of the structurally modified PVA was found to be reduced due to the presence of traces of solvent molecules and the breaking of inter‐ and intramolecular hydrogen bonding. The sustainable drug release through hydrolysis mechanism was tested at the pH of 7.3. Generally, the drug release followed the Korsmeyer–Peppas model with Fickian drug transportation mechanism except Furosemide (Fur)‐grafted PVA system at the pH of 7.3. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46620.     

Synthesis of poly(vinyl alcohol)‐graft‐poly(ε‐caprolactone) and poly(vinyl alcohol)‐graft‐poly(lactide) in melt with magnesium hydride as catalyst

Grafting of poly(ε‐caprolactone) (PCL) and poly(lactide) (PLA) chains on poly(vinyl alcohol) backbone (PVA degree of hydrolysis 99%) was investigated using MgH₂ environmental catalyst and melt‐grown ring‐opening polymerization (ROP) of ε‐caprolactone (CL) and L ‐lactide (LA), that avoiding undesirable toxic catalyst and solvent. The ability of MgH₂ as catalyst as well as yield of reaction were discussed according to various PVA/CL/MgH₂ and PVA/LA/MgH₂ ratio. PVA‐g‐PCL and PVA‐g‐PLA were characterized by ¹H‐ and ¹³C‐NMR, DSC, SEC, IR. For graft copolymers easily soluble in tetrahydrofuran (THF) or chloroform, wettability and surface energy of cast film varied in relation with the length and number of hydrophobic chains. Aqueous solution of micelle‐like particles was realized by dissolution in THF then addition of water. Critical micelle concentration (CMC) decreased with hydrophobic chains. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Comparative analysis of PVA hydrogels incorporating two natural antimicrobials: Punica granatum and Arnica montana tinctures

Poly(vinyl alcohol) (PVA) hydrogels are biocompatible materials that retain relatively high amounts of aqueous fluids in their network. The incorporation of antimicrobial natural products, such as pomegranate and arnica, is a strategy for coupling their bactericidal effects with the favorable properties of the gels for wound healing treatment. The goal of this work was to investigate the microstructural, mechanical, and release properties of PVA–pomegranate and PVA‐arnica samples. Physical interactions were observed by Fourier transform infrared spectra in PVA–pomegranate samples. The addition of pomegranate to PVA led to high T_g, probably related to the physical interactions, and low T_m and X_c, possibly due to the pomegranate location between PVA chains. Increased levels of arnica were associated with a slight decrease in samples' X_c, probably due to a physical barrier imposed by arnica to the interaction of PVA chains. PVA samples presented higher swelling than the samples containing natural products. High amounts of pomegranate in the samples led to high weight loss and high phenols and flavonoids delivery. Arnica release seemed to be more difficult than the pomegranate release. All samples presented relative high mechanical properties. In addition, pomegranate samples present higher potential to deliver active compounds than arnica samples. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45392.     

Blends of aliphatic polyesters. IV. Morphology,swelling behavior,and surface and bulk properties of blends from hydrophobic poly(L-lactide) and hydrophilic poly(vinyl alcohol)

Blend films were prepared from hydrophobic poly(L -lactide) (PLLA) and hydrophilic poly(vinyl alcohol) (PVA) with different PLLA contents [X_PLLA (w/w) = PLLA/(PVA + PLLA)] by solution casting and melt quenching. Their morphology, swelling behavior, and surface and bulk properties were investigated. Polarizing optical microscopy, scanning electron microscopy, differential scanning calorimetry, X-ray diffractometry, and tensile testing revealed that PLLA and PVA were phase separated in these blend films and the PLLA-rich and PVA-rich phases both formed a continuous domain in the blend film of X_PLLA = 0.5. The water absorption of the blend films was higher for the blend films of low X_PLLA values when compared at the same immersion time, and it was larger than expected from those of nonblended PLLA and PVA films. The dynamic contact angles of the blend films were linearly increased with an increase in X_PLLA. The tensile strength and Young's modulus of the dry blend films decreased with a rise in X_PLLA, but this dependence was reversed because of the large decreases in tensile strength and Young's modulus for the blend films having high X_PLLA values after immersion in water. The elongation at break was higher for the wet blend film than for the dry blend film when compared at the same X_PLLA and that of the dry and wet blend films decreased with an increase in X_PLLA. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2151–2160, 2001     

Permselectivity of polyamide composite membrane modified by solvent

Asymmetric reverse osmosis (RO) polyamide (PA) composite membrane may be modified by means of a solvent‐processing technique into pervaporation (PV) membrane for separating organic aqueous solution. Formic acid, acetic acid, phosphoric acid, hydrochloric acid, phenol, and so forth, better solvents for PA, were selected as the modifying agents in this study. The effect of the modifying conditions (type and concentration of the modifying agent, processing time of membrane in liquid medium) on the sorption of modified membranes was investigated. After the PA composite membrane was treated with 8 wt % acetic acid for 1 h, the swelling rate for water (S_w) increased, whereas that for isopropanol (S_IPA) decreased, compared with that of the nonmodified membrane. The difference between S_w and S_IPA of the membrane treated with acetic acid was greater than that with other modifying agents. The separation factor and flux of the modified membrane were correspondingly maximal when it was used in a PV separating isopropanol aqueous solution. The contact of solvent molecules in liquid medium with polymer chains in the PA membrane and the intranodular chains in the PA membrane tending to diffuse by reptation toward the solvent molecules were the principal causes leading to a change of structure of asymmetric PA composite membrane. The mechanism of changing membrane structure by means of the solvent‐processing technique may better interpret the modification of PA composite RO membrane. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1005–1010, 2004