Investigation into the gelation and crystallization of polyacrylonitrile

Polyacrylonitrile (PAN) is soluble in dimethyl sulfoxide (DMSO) and the resulting solution can gel by various mechanisms. The effects of temperature and water on the thermoreversible gelation of PAN have attracted much attention because of their importance in the fiber formation and film casting. Rheological tests were employed in this study to examine the gelation behavior and determine the gel point temperature of PAN-DMSO solution. Calorimetric studies of PAN-DMSO gels were conducted through Differential Scanning Calorimetry and no crystallization was discovered in these gels prepared at low temperature. X-ray diffraction of different PAN gels indicated that in the absence of water, gels resulted from PAN-DMSO solution by decreasing temperature were not crystallizable. In contrast, water-induced gelation led to crystallization of PAN gel. The water content in the formed gel is responsible for its crystallinity and average crystallite size.     

Viscoelastic behavior of polyacrylonitrile/dimethyl sulfoxide concentrated solution with water

The spinnability and polydispersity of polyacrylonitrile/dimethyl sulfoxide (PAN/DMSO)/H₂O spinning solutions with conventional PAN molecular weight and comparative high PAN concentration have been investigated using a cone‐plate rheometer. It is observed from the measurements that, the viscosities of the solutions decreased with the rising of shear rate, and then stabilized to almost the same value, regardless of the PAN concentration. The chain orientation in the fiber formed under constant shear rate cannot be changed considerably even after long relaxation of more than 900s. For dynamic experiments, a steady increase of both G′ and G″ with escalating oscillation frequency was seen for all samples. Higher viscous‐elastic modulus at higher H₂O content was found, too. It is also concluded from the log G′ ? log G″ plot and the gel point that the PAN/DMSO/H₂O system with regular PAN molecular weight behaves very close to a mono‐disperse system, thus very suitable for gel spinning and for preparation of high performance PAN precursor fiber. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1437–1442, 2009     

Rheological properties of cellulose solution in paraformaldehyde/dimethyl sulfoxide system (1)

The rheological properties of three cellulose samples are investigated, including the dependence of the non‐Newtonian Index, structural viscosity and zero shear viscosity on temperature and the concentration of their paraformaldehyde/dimethyl sulfoxide solutions; the values of viscous flow activation energy of them are higher than that of the viscose solution. With the increase of molecular weight, solution concentration and the decrease of temperature, the rheological properties become worse. The rheological properties of cotton linters Cotton 1 are better than those of wood pulp Wood 2 despite a similar degree of polymerization. Copyright © 1999 John Wiley & Sons, Ltd.     

Enthalpic effect of polyacrylonitrile on the concentrated solutions in dimethyl sulfoxide: Strong thixotropic behavior and formation of bound solvents

The effect of dipole–dipole interaction by nitrile groups of PAN on the bound state of solvent molecules and the concentrated solution properties in DMSO was investigated. Variation of a solution viscosity exhibited three overlap concentrations, C1*, C2*, and C3*, at 2.7, 8.6, and 16.3 wt%, respectively, representing the transition of concentration regions in the order of dilute, unentangled‐semi dilute, entangled‐semi dilute, and concentrated regions. The two‐dimensional mapping of FT‐IR analysis and dielectric measurement confirmed that the intermolecular interaction of PAN was suddenly enhanced at the C*s, inducing polarization to DMSO. In the ice‐melting process of PAN solutions, two different melting peaks (T_m2 and T_m3) of DMSO newly appeared at each C2* and C3*, suggesting the different types of bound solvents. In the concentrated solutions, the saturated dielectric constant and the strongly delayed evaporation of the solvent even at the boiling point of DMSO along with strong thixotropic behavior were indicative of the stronger confinement state of bound DMSO than in the semidilute solutions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1080–1089     

Geometry and electronic structure of the dimethyl sulfoxide dimer. Calculation by the MNDO method

The geometric and electronic structure of the complex formed by dipole-dipole interaction between two molecules of DMSO in the "head-to-tail" orientation were calculated by the MNDO quantum-chemical method. The minimum total energy corresponds to a distance of 5.5 Å between the sulfur atoms, and the angle between the axis of the molecular dipoles is 16.4°. This agrees with calculations for liquid DMSO by molecular dynamics. The large equilibrium distance between the DMSO molecules explains its low density in the liquid phase and the high intensity of microwave absorption due to free volume sufficient for rotation of one molecule in the complex in relation to the dipole axis of the other.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117334 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1340–1344, June, 1992.     

Measurement and Prediction on the Surface Properties of Dimethyl Sulfoxide/Water Mixtures

Pendent drop method was adopted to measure the surface tension of dimethyl sulfoxide(DMSO)/water mixtures. A new pendent drop apparatus was built up and checked with water, and a good agreement of our data with literature could be found. With the new apparatus, the surface tensions of nine DMSO/water mixtures with mass fractions of water from 0.1 to 0.9 were investigated in a temperature range of 298-338 K. The expanded uncertainty for surface tension measurement was estimated to be 0.5% at a confidence level of 95%(k=2) in the whole temperature range. A thermodynamic-based relation was used to predict the surface properties of DMSO/water mixtures. Based on the relation and Gibbs adsorption theory, a prediction model was proposed for the calculation of surface relative excess and the thickness of the surface molecule layer.     

Water effect on the rheologic behavior of PAN solution during thermal‐induced gelation process

The thermal‐induced gelation process in a polyacrylonitrile (PAN) solution containing different amounts of water was investigated through both dynamic and steady‐state rheologic measurements. During the cooling process, the storage modulus G′ was found to intersect with the loss modulus G″ at a temperature which can be considered as the incipient gel point T_ig. This crossover point increased with the water content in the PAN solution. On the basis of the power law $G'\tilde{G}\tilde{\omega} {}^{n} $ and the relation ${\rm tan}\ \delta = G(\omega )/G^{'}(\omega ) = {\rm tan}(n\pi /2) = {\rm const}$ , the loss tangent tan δ was traced with decreased temperature at various oscillation frequencies, and a crossover point was observed at the temperature a little higher than the T_ig above for all PAN solutions. The water content might affect the value of the relaxation exponent n via two different mechanisms, that is, hydration decoupling of the pendant nitrile groups in PAN at low water content (2 wt% water) and molecule aggregation at high water content (4 and 6 wt%). Nevertheless, the slight difference in the n values reflects the unique structure of the critical gels irrespective of the water level. The apparent viscosity η_a of either the PAN solution or the PAN gel was found to rise with increased water content and both showed pronounced shear‐thinning behavior. Compared with temperature, the water effect on the apparent viscosity η_a is more significant due to the water‐induced aggregation of the molecular chains. Copyright © 2010 John Wiley & Sons, Ltd.     

Application of the solvent dimethyl sulfoxide/tetrabutyl-ammonium fluoride trihydrate as reaction medium for the homogeneous acylation of Sisal cellulose

Two types of Sisal cellulose were studied as starting material for homogeneous acylation in the solvent dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride trihydrate (TBAF). The native Sisal cellulose investigated contains 14% hemicellulose (mainly composed of xylose) as confirmed by ¹³C-NMR spectroscopy in DMSO-d₆/TBAF and HPLC analysis after complete polymer degradation. Alkali treatment of Sisal cellulose decreases the amount of hemicellulose, the degree of polymerization and the crystallinity. Both Sisal cellulose samples can be dissolved in DMSO/TBAF after treatment at elevated temperature. GPC measurements showed high aggregation in the solution. Different homogeneous acylation reactions using carboxylic acid anhydrides and vinyl esters were carried out, showing a pronounced tendency of the anhydride towards hydrolysis in the solvent. This disadvantage can be diminished by decreasing the amount of the salt hydrate (TBAF trihydrate) or by a distillative removal of the majority of water. A strong interaction of the polymer with the water in the solvent was observed.     

磺酸多相催化剂用于二甲基亚砜中C6-单糖脱水制5-羟甲基糠醛（英）

Sulfonic acid-functionalized heterogeneous catalysts have been evaluated in the catalytic dehydration of C(,monosaccharides into 5-hydroxymethylfurfural(HMF) using dimethyl sulfoxide(DMSO)as solvent.Sulfonic commercial resin Amberlyst-70 was the most active catalyst,which was ascribed to its higher concentration of sulfonic acid sites as compared with the other catalysts,and it gave 93 mol%yield of HMF from fructose in 1 h.With glucose as the starting material,which is a much more difficult reaction,the reaction conditions(time,temperature,and catalyst loading) were optimized for Amberlyst-70 by a response surface methodology,which gave a maximum HMF yield of 33 mol%at 147 °C with 23 wt%catalyst loading based on glucose and 24 h reaction time.DMSO promotes the dehydration of glucose into anhydroglucose,which acts as a reservoir of the substrate to facilitate the production of HMF by reducing side reactions.Catalyst reuse without a regeneration treatment showed a gradual but not very significant decay in catalytic activity.     

水/二甲基亚砜体系中无机盐高效催化木糖脱水制备糠醛(英)

提出了一种高效的均相催化木糖制备糠醛的方法,探讨了水/二甲基亚砜（DMSO）均相体系中不同类型的无机盐对催化木糖制备糠醛的影响.结果表明,氯盐较硫酸盐和硝酸盐表现出较高的催化活性,其中以SnCl₄的催化效果最高.且它与LiCl复配时催化性能更高.当SnCl₄的摩尔分数为0.8时,催化效果最好,糠醛收率达56.9%.反应条件优化实验发现,当催化剂与木糖的摩尔比为0.5:1,固液比为1:20,水与DMSO体积比为5:5时,于130℃下反应6h,糠醛收率达63%.     

Effect of mixed solvent on solution properties and gelation behavior of poly(vinyl alcohol)

In this work, the static and dynamic light scattering measurements were used to investigate the solution properties and the aging effects on PVA/DMSO/water ternary system in dilute region at 25 °C. It was found that the phase separation and aggregate behavior occurs rapidly and obviously when DMSO mole fraction (X₁) in the solvent mixture is between 0.2 and 0.33, especially at 0.25. In this solvent composition range, a broad peak which indicates phase separation and chain aggregation can be observed from static light scattering measurement. However, when DMSO mole fraction is increased to 0.37, no such peak is present. For this ternary system, the gelation mechanism and the relationship between the phase separation behavior and the gelation of the formed physical gels were also investigated through the gelation kinetic analyses in the dilute and semi-dilute region. It is concluded that the cononsolvency effect in the dilute solution is not the sole origin that affects the phase separation, aggregation, and gelation behavior for the ternary system in a higher polymer concentration range. The hydrodynamic factors such as the higher viscosity and slower polymer chain diffusion that are resulted from higher polymer concentration should be also considered.     

Mechanism of the reaction of radicals with peroxides and dimethyl sulfoxide in aqueous solution

The reactions of methyl and methylperoxyl radicals derived from dimethyl sulfoxide (DMSO) with hydrogen peroxide, peroxymonocarbonate (HCO4 (-)), and persulfate were studied. The major reaction observed for the hydroperoxides was the abstraction of the hydrogen atom by the radicals. The radicals interact with a lone pair of electrons on the peroxide to produce methanol and formaldehyde. Furthermore, the results indicate that in RO2H and RO2R', electron-withdrawing groups cause a considerable increase in the reactivity of the peroxides towards the radicals and not only towards nucleophiles. The HO2 (.)/O2 (.-) and CO3 (.-) radicals react with DMSO to produce methyl radicals. Thus, the formation of the (.)CH3 radicals in the presence of DMSO is not proof of the formation of the (.)OH radicals in the system. These reactions must be considered when radical processes, such as in biological and catalytic systems, are studied. Especially, the plausible role of HCO4 (-) ions in biological systems as a source of oxidative stress cannot be overlooked.     

Thallium-205 NMR spectroscopy. Solvation of the Tl(l) ion in the binary solvent systems water/pyridine,water/dimethyl sulfoxide,and pyridine/dimethyl sulfoxide

Solvation of the Tl⁺ ion in 0.005M solutions of water/pyridine, water/dimethyl sulfoxide, and pyridine/dimethyl sulfoxide was studied with ²⁰⁵ Tl NMR spectroscopy as a function of solvent composition and anion (NO ₃ ^– and ClO ₄ ^t- ). Dimethyl sulfoxide solvated the Tl⁺ ion more strongly than did pyridine, despite the latter's greater electron-donating ability. This was explained in terms of structural effects, which were found to be large for all three binary solvent systems. Ion pairing was evident in the DMSO/pyridine and water/pyridine solvent systems in which the pyridine mole fraction was greater than 0.8.     

Morphology and mechanical properties of poly(vinyl alcohol) and starch blends prepared by gelation/crystallization from solutions

 In an attempt to produce biodegradation materials, poly(vinyl alcohol) (PVA)–starch (ST) blends were prepared by gelation/crystallization from semidilute solutions in dimethyl sulfoxide (Me₂SO) and water mixtures and elongated up to 8 times. The content of mixed solvent represented as Me₂SO/H₂O (volume percent) was set to be 60/40 assuring the greatest drawability of PVA homopolymer films. The PVA/ST compositions chosen were 1/1, 1/3, and 1/5. The elongation up to 8 times could be done for the 1/1 blend but any elongation was impossible for blends whose ST content was beyond 50%. When the blends were immersed in water at 20 or 83 °C, the solubility became considerable for an undrawn blend with 1/5 composition and a drawn 1/1 blend with λ=8. To avoid this phenomenon, cross-linking of PVA chains was carried out by formalization under formaldehyde vapor. Significant improvement could be established by the cross-linking of PVA chains. For the 1/1 blend, the amount of ST dissolved in water at 23 °C was less than 3% for the undrawn state and 25% for the drawn film. The decrease in the ST content was enough for use as biodegradation materials. Namely, the water content relating to the biodegradation in soil is obviously different from such a serious experimental condition that a piece of blend film was immersed in a water bath. At temperatures above 0 °C, the storage modulus of the formalization blends became slightly higher than those of the nonformalization blends. The Young's modulus of the drawn films with a draw ratio of 8 times was 2 GPa at 20 °C. Received: 23 June 2000 Accepted: 30 October 2000     

Carbon-bromine cleavage by dimethyl sulfoxide: the key step of C(5) functionalization of push-pull 2-alkylidene-4-oxothiazolidine vinyl bromides

Push-pull 2-alkylidene-4-oxothiazolidine vinyl bromides undergo efficient C(5) functionalization through DMSO-assisted carbon-bromine cleavage, followed by a bromine transfer-substitution (or elimination) sequence. A mechanism for this novel transformation is proposed.     

Rheological properties of solutions of general‐purpose poly(vinyl alcohol) in dimethyl sulfoxide

The characteristic rheological responses of solutions of atactic poly(vinyl alcohol) (PVA) in dimethyl sulfoxide were investigated in terms of the concentration and molecular weight. The syndiotactic dyad content and weight‐average molecular weight of PVA were 52% and 85,000–186,000, respectively. On a modified Cole–Cole plot, the solutions did not give a single master curve with a slope of 2 but gave various curves with slopes of less than 2. Furthermore, the slope slightly decreased with increasing concentration. The deviation from the master curve indicated that the solutions were rheologically heterogeneous, despite optical transparency. Among the PVA solutions, the 14 wt % solutions exhibited very unusual rheological behavior. The loss tangent changed with the shear rate and produced three distinct regions, which indicated a shear‐induced phase transition. With respect to Winter's view on gelation, the 14 wt % solutions underwent a double sol–gel transition as the shear rate increased over the frequency range of 0.05–500 rad/s. However, the molecular weight did not have such noticeable effects on the rheological behavior over the concentration range of 10–14 wt %. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1451–1456, 2004     

Effect of Dimethyl Sulfoxide on the Lamellar Phase of a Zwitterionic Surfactant

We study the effect of dimethyl sulfoxide, a solvent used in biological applications, on the lamellar phases of a zwitterionic surfactant. We used solutions with different proportions of dimethyl sulfoxide/water as solvents in order to analyze the effect on the lamellar phases (regular stacks of surfactant bilayers). The samples were analyzed by macroscopic visual observation, polarized light microscopy, linear rheology, and electron microscopy experiments. The results show that the variation of the dimethyl sulfoxide proportion in the solvent increases the curvature of the bilayers which leads to a transformation from a lamellar phase to an isotropic phase.     

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E₁₀₂P₃₇, E₁₀₄P₅₂, E₉₂P₅₅, E₁₀₄P₆₀ and E₉₈P₇₃ where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock E_mP_nE_m copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of E_mB_n copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E₉₈P₇₃ were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of E_mP_n copolymers in the solubilisation and controlled release of drugs.     

Effect of solution concentration on the gelation of aqueous polyvinyl alcohol solution

The concentration dependence of cryogenic gelation for aqueous solution of poly(vinyl alcohol) was studied by measuring the apparent gel fraction G and the swelling ratio Q of the gel formed by freezing and thawing. It was found that for the gelation process there were three distinct regions of solution concentration bounded by two concentrations C_gel and C. The gel started to form at C = C_gel, while no visible gel could be detected even upon repeated freezing and thawing of the extremely dilute solutions of C < C_gel. The entire solution was gelatinized as a whole in the high concentration region of C > C. In the intermediate concentration region, C_gel < C < C, which covers three orders of magnitude in concentration, gel and sol phases coexist. Both concentration dependencies of G and Q show two branches jointed at a concentration very close to the overlap concentration C^*. The curve of G?Q versus C shows a sharp cusp. In case the sharp cusp concentration is really the value of C^*, gelation offers a precise method to determine the overlap concentration. ©1995 John Wiley & Sons, Inc.     

Investigation on the gelation behavior of biodegradable poly(butylene succinate) during isothermal crystallization process

The early stage of polymer crystallization may be viewed as physical gelation process,i.e.,the phase transition of polymer from liquid to solid.Determination of the gel point is of significance in polymer processing.In this work,the gelation behavior of poly(butylene succinate)(PBS) at different temperatures has been investigated by rheological method.It was found that during the isothermal crystallization process of PBS,both the storage modulus(G′) and the loss modulus(G″) increase with time,and the rheological response of the system varies from viscous-dominated(G′G″),meaning the phase transition from liquid to solid.The physical gel point was determined by the intersection point of loss tangent curves measured under different frequencies.The gel time(t_c) for PBS was found to increase with increasing crystallization temperature.The relative crystallinity of PBS at the gel point is very low(2.5%-8.5%) and increases with increasing the crystallization temperature.The low crystallinity of PBS at the gel point suggests that only a few junctions are necessary to form a spanning network,indicating that the network is"loosely"connected,in another word,the critical gel is soft.Due to the elevated crystallinity at gel point under higher crystallization temperature,the gel strength S_g increases, while the relaxation exponent n decreases with increasing the crystallization temperature.These experimental results suggest that rheological method is an effective tool for verifying the gel point of biodegradable semi-crystalline polymers.