Preparation of poly(N-normalpropyl- acrylamide) gel beads

 The gel beads of N-normal-propylacrylamide are prepared by the radical copolymerization of N-normalpropylacrylamide and N,N′-methylene-bis-acrylamide in water. The optimum reaction conditions to obtain the gel beads are revealed from the phase diagram of the reaction system together with the scanning electron microscopy of the reaction products. The scanning electron microscopy of the reaction products also indicates the formation of the spherical gel beads of sub-micron size ranging from 250 to 500 nm in diameter. The viscosity measurements of the suspension of the gel beads indicate that the concentration dependence of the viscosity of the suspension is well described by Einstein’s theory of the viscosity of colloidal particles. The intrinsic viscosity of the suspension of gel beads is then determined. The density of the gel beads, which was obtained from the intrinsic viscosity of the suspension, indicates that the gel beads are in the swollen state at a temperature of 20 ^°C. Received: 12 September 1997 Accepted: 17 December 1997     

Structure of poly(vinylidene fluoride) gel electrolytes

Polymer gel electrolytes have three constituents: polymer, salt and solvent. This paper gives structural information on polymer gel electrolytes made from poly(vinylidene fluoride), lithium triflate and tetraglyme. These electrolytes exhibit a room-temperature ionic conductivity in the region of 10⁻³ S cm⁻¹ while maintaining sufficient mechanical rigidity to form self-supporting films (having elastic moduli in the region of 100 kPa). Differential scanning calorimetry and dynamic mechanical analysis have been used to show that the majority of the network junctions of the gel are crystalline in nature. Wide angle X-ray diffraction has revealed that when no salt is included in the gel, these crystal junctions are almost an order of magnitude larger in their lateral dimensions than when salt is present. The salt is thought to nucleate crystallisation. The modulus is significantly reduced by inclusion of salt; however, DSC suggests that apparent crystallinity is only slightly reduced by the presence of salt. This discrepancy is attributed to either the uncertainty in the heat of fusion of PVDF, or to the formation of small crystalline particles that are not incorporated in the network junctions. Gels with polymer concentrations between 15 and 40% (by weight) maintain their mechanical rigidity up to temperatures around 100 °C. However, once melted, the gel structure only reforms at much lower temperatures. The variation of ionic conductivity of salted gels with temperature shows no such hysteresis, and it is concluded that the ionic conductivity is independent of the mechanical state of the gel.     

Preparation of filled temperature-sensitive poly(N-isopropylacrylamide) gel beads

Temperature-sensitive filled poly(N-isopropylacrylamide) (PNIPAAm) gel beads with diameters in the range of millimeters were prepared using the alginate technique. The polymerization and cross-linking reaction of NIPAAm in the presence of inorganic filling particles was performed in spherical networks of Ca-alginate forming interpenetrating networks (IPN). Thermo-sensitive gel beads could be obtained by washing these IPN with EDTA solution. The PNIPAAm gel beads were analyzed by optical methods to observe there swollen diameter in dependence on the temperature. The diameters of the swollen gel beads were in the range of 0.1 - 2 mm. The influence of the monomer to cross-linker ratio (MCR) and the filling materials (ferrofluid, BaTiO₃, TiO₂, and Ni,) were studied. The phase transition temperature (T_pt) was only weakly influenced by the MCR and the filling material remaining at around 34°C.     

Dynamic adsorption of uranium(VI) and zirconium(IV) on silica gel

Adsorption of uranium(VI) and zirconium(IV) from aqueous nitric acid solution on silica gel has been investigated under dynamic conditions. The influence of temperature, nitric acid concentration (pH), and solution flow rates was studied. If the nitric acid concentration in the solution is higher than 0.05 mol/l, then it is possible to achieve separation of uranium(VI) and zirconium(IV) by passing the solution through a column filled with silicagel.     

Highly concentrated (gel) emulsions,versatile reaction media

Highly concentrated (gel) emulsions are characterised by dispersed phase volume fractions exceeding 0.74, the critical value for the most compact packing of monodispersed undistorted spheres. Their structure consists of polyhedral droplets separated by thin films of continuous phase, a structure resembling gas–liquid foams. Their rheological properties vary from elastic to viscoelastic having a gel appearance. One of the most promising applications is their use as reaction media. The recent advances in the preparation of low-density polymeric materials (solid foams, aerogels) are reviewed and new applications are described. These include the preparation of dual meso/macroporous inorganic oxide materials and the use of gel emulsions as alternative to conventional solvent media in chemical and enzyme-catalysed reactions.     

Effect of syndiotacticity on the aqueous poly(vinyl alcohol) gel 2. Syneresis of gel

Summary The mechanism of syneresis of an aqueous gel of syndiotacticity-rich poly(vinyl alcohol) (PVA) with the rise of temperature was studied using the PVA having the content of syndiotacticity in diad from 58.4 to 66.2%. The extent of syneresis was estimated by gravimetry. Syneresis of gel with the rise of temperature depends upon the syndiotacticity and gelling temperature. The gel chilled at lower temperature of PVA having the content of syndiotacticity in diad above 60.9% showed remarkable syneresis with the rise of temperature. The maximum of the extent of syneresis appeared between the content of syndiotacticity in diad of 60.9% and that of 64.2%. The phenomena of syneresis was not thermally reversible. Syneresis occurs accompanying the breakdown of the unstable junction and further growth of the crystallite.

---

Zusammenfassung Der Mechanismus der Synärese an wäßrigen Gelen von Polyvinylalkohol, reich an syndiotaktischem Anteil, wurde mit steigender Temperatur untersucht, wobei der Polyvinylalkohol in Bezug auf seinen Gehalt an syndiotaktischen Diaden zwischen 58,4 und 66,2% variiert. Die Synärese wurde gewichtsmäßig verfolgt. Die Synärese hängt von der Syndiotaktizität und der Gelierungstemperatur ab. Gele, erstarrt bei tieferen Temperaturen mit einer Syndiotaktizität von 60,9%, zeigten beträchtliche Synärese mit steigender Temperatur. Das Maximum an Synärese wurde zwischen 60,9 und 64,2% Syndiotaktizität beobachtet. Das Phänomen der Synärese ist nicht thermisch reversibel. Die Synärese läuft ab unter Zusammenbruch von unstabilen Bindungen und weiterem Wachstum der Kristallite.

---

---

Part 1. cf. reference 4.     

Preparation of single-walled carbon nanotube (SWNT) gel composites using poly(ionic liquids)

This paper reports a new and practical route for synthesizing nanotube-polymeric ionic liquids gel by non-covalent functionalization of oxidized single-walled carbon nanotube (SWNT) surfaces with imidazolium-based poly(ionic liquids) (PILs), using in situ radical polymerization method. A black and homogeneous precipitate SWNTs was obtained as a gel form, which is well dispersed in aqueous solution without any aggregation. The formation of SWNT gels is explained by the electrostatic attractions or π-bonds between the SWNT surface and the PIL matrix. By anion-exchange reaction of PIL bound to SWNTs, hydrophilic anions in PIL were substituted with hydrophobic anions, resulting in an effective transfer of SWNT-PIL hydrogels to organogels. The result also showed that SWNTs can effectively improve the conductivity along with the thermal stability of nanocomposite gels.     

Ion-specific swelling behavior of uncharged poly(acrylic acid) gel

The ion-specific swelling behavior of poly(acrylic acid) (PAA) gel prepared by -ray irradiation was investigated as a function of salt concentration in the presence of 0.01 M HCl. The anion specificity for the swelling ratio was similar to that for many kinds of hydrogels, i.e., Cl^–<Br^–<NO₃ ^–<I^–, while the cation specificity proved to be rather unusual, i.e., Mg²⁺<Ca²⁺<Li⁺<Na⁺<K⁺<Cs⁺. In order to find any differences in the hydration of uncharged PAA from that of other polymers having typical polar groups, the hydrogen-bonding hydrations on the relevant polar groups were compared for small molecule analogues with an ab initio molecular orbital calculation. According to the results, the marked deswelling of PAA gel in the presence of strongly hydrated cations was ascribed to the unfavorable hydration to the acidic proton of PAA due to the reduced availability of water oxygen as well as to the destabilization of hydrophobic hydration developing around the uncharged PAA.     

Effect of syndiotacticity on the aqueous poly(vinyl alcohol) gel 3. Mechanical property of gel

Summary The mechanical property of aqueous gel was studied using the PVA having the content of syndiotacticity in diad from 58.4 and 66.2%. The elastic modulus of gels was 105–106 dyne/cm2 and the variation of the elastic modulus of gel with the rise of temperature was corresponded with the variation of the concentration of PVA in gel accompanying syneresis. The elastic modulus of gel having the same content of syndiotacticity and concentration of PVA increased in the order as follows, the first is the gel chilled at room temperature, the second is the gel chilled at 60 °C and last is the gel chilled at 0 °C. The variation of the elastic modulus of gel with the rise of temperature became smaller with increasing content of syndiotacticity. The increase of elastic modulus is considered to be attributed to the further growth of stronger junctions through the recrystallization followed by syneresis rather than the entropic effect of polymer networks with the rise of temperature.

---

Zusammenfassung Es wurden die mechanischen Eigenschaften wßriger Gele aus Polyvinylalkohol untersucht, mit Gehalten an Syndioaktizität in Diaden zwischen 58,4 und 66,2%. Die elastischen Moduln der Gele lagen bei 105 bis 106 dyn/cm2 und die Variation der elastischen Moduln der Gele mit Steigerung der Temperatur entsprach der Konzentrationsänderung von Polyvinylalkohol im Gele gemäß der stattgefundenen Synärese. Der elastische Modul des Gels mit konstantem Gehalt an Syndiotaktizität und Konzentration des PVA steigen in folgender Weise: Zuerst das Gel abgekühlt bei Raumtemperatur, dann das Gel abgekühlt bei 60 °C und zuletzt das Gel erstarrt bei 0 °C. Die Variation der elastischen Moduln mit steigender Temperatur wird kleiner mit steigendem Gehalt an Syndiotaktizität. Das Anwachsen der Moduln sollte vermutlich dem weiteren Wachstum strengerer Kopp lungen durch Rekristallisation infolge der Synärese zuzuschreiben sein mehr als einem Entropieeffekt der polymeren Netzwerke mit steigender Temperatur.

---

---

With 6 figures and 2 tables

---

Part 2. cf. reference 2.     

Characterization of polyethylene terephthalate by gel permeation chromatography (GPC)

The use of the solvent mixture chloroform/hexafluoroisopropanol (98 : 2 vol %) in the GPC-analysis of polyethylene terephthalate (PET) is described. PET-samples of different molecular weight have been prepared and characterized by light scattering, viscometry, and improved techniques of osmometry and end group titration. These well characterized samples were used in calibrating the GPC. The calibration with polydisperse standards was found superior to the universal calibration procedure in the solvent system chosen.     

Production of poly(chlorosulfostyrene) by synthesis in a gel

Synthesis in a gel as a method for the preparation of poly(chlorosulfostyrene) by treating polystyrene with chlorosulfonic acid was proposed, which makes it possible to obtain a polymer with a total degree of functionalization of sulfo and chlorosulfo groups up to 1.0. The possibility of regulating the degree of chlorosulfonation in the range from 50 to 100% was shown. The composition and structure of the products of polymer-analogous transformations were characterized by elemental analysis, FTIR spectroscopy, and thermogravimetry with IR identification of the decomposition products. Chlorosulfonation in the gel makes it possible to achieve a high degree of functionalization even at lower temperatures, at room temperature, and at less time than compared with known methods.     

Thermoxidative degradation leading to gel in poly(ethylene terephthalate)

Poly(ethylene terephthalate) oxidizes on heating at 300°C in an air stream. Various literature has cited crosslinking under such conditions through a butanetriol or a biphenyltricarboxylic acid. In this study gelled polymer was found to contain the latter trifunctional compound, but not the former. In addition, 4,4′-biphenyldicarboxylic acid was found in the degraded gel. There is further indication for the presence of other multifunctional species. All components are consistent with general free-radical arylation mechanisms.     

Salt resistivity of poly (4-vinyl benzoic acid) gel

Salt resistivity of poly (4-vinyl benzoic acid) (P4VBA) gel was investigated to compare with the super salt-resistivity that was found for poly (4-vinyl phenol)(P4VPh) gel containing an acidic proton and π-electron system. Poly(acrylic acid) (PAA) gel was also prepared and used as a reference gel containing only an acidic proton. P4VBA gel showed a moderate salt resistivity, which was less significant than that for P4VPh gel, in many kinds of inorganic salt solutions (MgCl₂, LiCl, NaCl, KCl, CsCl, KI, KSCN, Na₂SO₄). On the other hand, PAA gel showed a drastic deswelling in the presence of concentrated MgCl₂, LiCl, Na₂SO₄, and (NH₄)₂SO₄ solutions, and a significant swelling for KSCN solution. These contrastive behaviors between P4VBA and PAA gels strongly suggest that the combination of acidic proton and π-electron system is essential and necessary for polymer gels to be endowed with the salt resistivity.     

Photoactive proton conductor: poly(4-vinyl pyridine) gel

We describe a hydrogen-bonded poly(4-vinyl pyridine)-based dielectric material, in which conductivity can be induced due to the presence of side-chain protonated species that form spontaneously when the polymer is dissolved in pyridine. The conductivity of the proton conductive gel can be controlled by direct irradiation at the proton-transfer center: a reversible change of conductivity was observed in response to the on/off switching of 385 nm wavelength radiation. Over most of the range of intensities used, the proton conductivity exhibited a bimolecular character. We present a model of the protonated pyridine side-chain unit in the ground and excited states (DFT level). In the ground state, the protonated pyridine moiety has a cyclic, conjugated structure.     

Adsorption thermodynamic and desorption studies of U(VI) on modified silica gel (SiAPMS-HL)

In this study, the adsorption process was examined by various isotherm models Langmuir, Freundlich and Dubinin–Radushkevich and equilibrium data were successfully described by Langmuir model. Adsorption thermodynamics of uranium (VI) on modified silica gel (SiAPMS-HL) has been studied within a temperature range from 293 to 333 K and the thermodynamic parameters, such as equilibrium constant (K _D), standard free energy changes (ΔG°), standard enthalpy change (ΔH°) and standard entropy change (ΔS°), have been obtained. The desorption studies were conducted in batch system to investigate the kind, concentration and volume of the eluent.     

Amphoteric poly(vinyl alcohol) gel as a force generator: Observation of microporous gel formation with cryo-SEM

Amphoteric PVA hydrogels were made from mixed aqueous solutions of poly(acrylic acid), poly(allylamine) hydrochloride and poly(vinyl alcohol) by repetitive freezing and thawing. The hydrogen bond induced gelation produced an elastic and tough microporous hydrogel which generated tension as high as 0.3MPa corresponding to the solvent exchange from water to acetone. In this paper we show the microscopic structure of hydrogels determined by means of cryo-SEM technique and X-ray analysis.     

Effect of syndiotacticity on the aqueous poly(vinyl alcohol) gel 4. X-ray diffraction analysis of gel

Summary The structure of syndiotacticity-rich poly(vinyl alcohol) (PVA)-water gel was investigated by the X-ray diffraction analysis. Diffuse haloes appeared in the gel prepared from lower aqueous solution of PVA (5 wt%) but diffraction pattern became sharper with increasing concentration of PVA. Thed-spacings of gels and films are agreed with each other. The half height width of value of peak position of (101) obtained by photodensitometer decreased with increasing syndiotacticity of PVA. The radius of gyration of gel obtained from theGuinier's plots by the small angle X-ray diffraction increases with increasing syndiotacticity. These results suggest that the syndiotactic sequence length increases with increasing content of syndiotacticity and the crystallites in gel networks become also larger with increasing syndiotacticity. The average size of gel junction is 73–89 Å (s-(diad)% = 49.₄-66.₂).

---

Zusammenfassung Es wurden mit Röntgeninterferenzen die Strukturen von einem Polyvinylalkohol-Wasser-Gel mit syndiotaktisch angereichertem Polyvinylalkohol untersucht. Es erscheinen diffuse Halos in dem Gel, das aus niedrigeren wäßrigen Lösungen (5% PVA) präpariert wurde. Doch wurden die Beugungsinterferenzen schärfer mit steigender Konzentration an PVA. Died-Abstände in Gelen und Filmen stimmten miteinander überein. Die Halbwertsbreite der 101-Interferenzen, gemessen mit einem Photodensitometer, nahm mit steigender Syndiotaktizitat des PVA ab. Der Trägheitsradius des Gels aus Guinier-Auftragungen für die Kleinwinkelstreuung nimmt mit steigender Syndiotaktizitat zu. Diese Ergebnisse legen nahe, daß die syndiotaktische Segmentläange mit steigendem Gehalt an Syndiotaktizitat zunimmt und daß die Kristallite in den Gelnetzwerken mit zunehmender Syndiotaktizität gröBer wurden. Die mittlere Größe des Netzwerkabstandes ist 73–89 Å (s-(diad)% = 49.₄-66.₂).

---

---

With 7 figures in 15 details and 3 tables     

Photo-quenched luminescence in Co(II)-doped sol–gel zirconia

The quenching process of the laser-induced luminescence of Co(II)-doped zirconia samples prepared by the sol–gel method is reported. Zirconia monoliths doped with Co(II) at different concentrations, were laser irradiated producing fluorescence; its intensity, measured by the slope at the low energy side of the Raman spectra, is reduced with the irradiation time. The rate the fluorescence decays can be modeled as a double exponential function of the irradiation time; the characteristic times involved in this quenching process are in the range of seconds. The suppression of the luminescence has been associated to the local heating produced when the laser beam is focused in a small area (≈2 microns in diameter) on the sample. This heating process reduces physical (such as grain boundaries and surface states) and chemical (oxygen vacancies) defects; however, some residual fluorescence still remains after long periods of illumination. R. Rodríguez is on sabbatical leave at Cinvestav, Unidad Queretaro.     

Formation of gold and silver nanostructures within polyvinylpyrollidone (PVP) gel

Study on reduction of Au(III) and Ag(I) and the formation of Au and Ag nanostructures was performed on the gels of metal precursor and PVP polymer mixture. Some comparing samples were prepared for better understanding the role of reactants on the reduction of metal ions and further growth of nanocrystals. The results suggest that, in addition to its function of generating stable colloids, PVP not only has a reducing effect on metal ions, but also acts as a crystal growth modifier. At low temperatures, the reducing effect of PVP is strong on Ag(I) ions in AgNO₃, while the reduction of complex Au(III) ions in HAuCl₄ is slow, involving two steps of Au(III)→Au(I)→Au. In the study of temperature disturbance on crystal growth, Au nanoplates of new and well-defined star shape were observed. The differences in the size and shape of nanoparticles are discussed from the colloid chemistry.