scholarly journals KINETIC STUDIES OF A REDOX POLYMERIZATION

1953 ◽  
Vol 31 (1) ◽  
pp. 13-22
Author(s):  
R. M. Brown ◽  
C. A. Winkler
Keyword(s):  
Free Radicals ◽  
Kinetic Studies ◽  
Activation Energies ◽  
Polymer Chains ◽  
Cross Linking ◽  
Gel Point ◽  
Emulsion System ◽  
Redox Polymerization ◽  
Termination Process

Kinetic studies of a redox-initiated polymerization have indicated activation energies of 9.7 kcal. per mole for the decomposition of the initiating system, approximately 6 kcal. per mole for the propagation reaction, and 9.0 kcal. per mole for the consumption of MTM-4 in the emulsion system. Lower polymerization temperatures raised the gel point without shortening the polymer chains, apparently by decreasing the proportion of cross-linking to propagation reactions. Two types of nonmercaptan termination have been found to occur to appreciable extents: mutual termination of free radicals and a termination process involving the emulsifier.

scholarly journals Enzymatic and Chemical Cross-Linking of Bacterial Cellulose/Fish Collagen Composites—A Comparative Study

2021 ◽  
Vol 22 (7) ◽  
pp. 3346
Author(s):  
Agata Sommer ◽  
Paulina Dederko-Kantowicz ◽  
Hanna Staroszczyk ◽  
Sławomir Sommer ◽  
Marek Michalec
Keyword(s):  
Thermal Stability ◽  
Bacterial Cellulose ◽  
Secondary Alcohol ◽  
Water Vapor Permeability ◽  
Polymer Chains ◽  
Cross Linking ◽  
Vapor Permeability ◽  
Covalent Bonds ◽  
Fish Collagen ◽  
Chemical Cross Linking

This article compares the properties of bacterial cellulose/fish collagen composites (BC/Col) after enzymatic and chemical cross-linking. In our methodology, two transglutaminases are used for enzymatic cross-linking—one recommended for the meat and the other proposed for the fish industry—and pre-oxidated BC (oxBC) is used for chemical cross-linking. The structure of the obtained composites is characterized by scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, and Fourier transform infrared spectroscopy, and their functional properties by mechanical and water barrier tests. While polymer chains in uncross-linked BC/Col are intertwined by H-bonds, new covalent bonds in enzymatically cross-linked ones are formed—resulting in increased thermal stability and crystallinity of the material. The C2–C3 bonds cleavage in D-glucose units, due to BC oxidation, cause secondary alcohol groups to vanish in favor of the carbonyl groups’ formation, thus reducing the number of H-bonded OHs. Thermal stability and crystallinity of oxBC/Col remain lower than those of BC/Col. The BC/Col formation did not affect tensile strength and water vapor permeability of BC, but enzymatic cross-linking with TGGS improved them significantly.

Temperature profiles in endothermic and exothermic reactions and the interpretation of experimental rate data

1970 ◽  
Vol 320 (1540) ◽  
pp. 71-100 ◽  
Keyword(s):  
Steady State ◽  
Correction Factor ◽  
Exothermic Reaction ◽  
Kinetic Studies ◽  
Activation Energies ◽  
Uniform Temperature ◽  
Exothermic Reactions ◽  
Arrhenius Function ◽  
Point To Point ◽  
Reacting Systems

Any endothermic or exothermic reaction is accompanied by self-cooling or self-heating. In reacting systems in which heat transfer is controlled by conduction, non-uniform temperature-position profiles are established. Examples of this situation are the exothermic decomposition of gaseous diethyl peroxide and the endothermic decomposition of nitrosyl chloride at low pressures (when convection is unimportant). In kinetic studies, allowance must be made for the non-uniform temperature to derive accurate isothermal velocity constants and Arrhenius parameters. In the present paper, the necessary corrections have been derived for a reactant in the steady state whose reaction rate varies exponentially with temperature and in which the temperature excess varies from point to point, being zero at the boundary (Frank-Kamenetskii’s conditions). The geometries considered are the slab, cylinder and sphere. The temperature gradient at the surface in the steady state ( Г ) occupies a key position, and this is exploited to find the correction factor required to convert 'observed’ rate constants to isothermal conditions, and thence to correct ‘observed’ activation energies and pre-exponential factors. The correction factor is found to be simply related to Frank- Kamenetskii’s δ (a dimensionless measure of heat-release rate). A similar analysis is given for systems hotter or cooler than their surroundings but uniform in temperature—such as well stirred fluid systems or small solid crystals (Semenov’s conditions). In these circumstances, systems of arbitrary geometry may be studied, and no approximation need be made to the Arrhenius function. For either type of boundary condition, uncorrected activation energies are overestimates in exothermic reactions and underestimates in endothermic reactions. Explicit relations are derived for making corrections. Boundary conditions intermediate between the two extremes investigated can also be treated though the resulting expressions are more cumbersome. In an appendix, an alternative ‘experimental’ approach is made to the elimination of errors from measured reaction velocities. This approach identifies the measured velocities with a temperature intermediate between those at centre and surface. The optimum choice, which weights the central and surface temperatures in the ratios 2:1 (slab), 1:1 (cylinder) and 2:3 (sphere), gives exactly correct results for the cylinder and acceptable precision for the slab and sphere even to within 5 K of the explosion limit. Other correction methods are also discussed.

scholarly journals Printable Alginate Hydrogels with Embedded Network of Halloysite Nanotubes: Effect of Polymer Cross-Linking on Rheological Properties and Microstructure

Polymers ◽  
2021 ◽  
Vol 13 (23) ◽  
pp. 4130
Author(s):  
Svetlana A. Glukhova ◽  
Vyacheslav S. Molchanov ◽  
Boris V. Lokshin ◽  
Andrei V. Rogachev ◽  
Alexey A. Tsarenko ◽  
...  
Keyword(s):  
3D Printing ◽  
Rheological Properties ◽  
Shear Thinning ◽  
Halloysite Nanotubes ◽  
Polymer Chains ◽  
Cross Linking ◽  
Saxs Data ◽  
Nanocomposite Hydrogels ◽  
Embedded Network ◽  
The Cross

Rapidly growing 3D printing of hydrogels requires network materials which combine enhanced mechanical properties and printability. One of the most promising approaches to strengthen the hydrogels consists of the incorporation of inorganic fillers. In this paper, the rheological properties important for 3D printability were studied for nanocomposite hydrogels based on a rigid network of percolating halloysite nanotubes embedded in a soft alginate network cross-linked by calcium ions. Particular attention was paid to the effect of polymer cross-linking on these properties. It was revealed that the system possessed a pronounced shear-thinning behavior accompanied by a viscosity drop of 4–5 orders of magnitude. The polymer cross-links enhanced the shear-thinning properties and accelerated the viscosity recovery at rest so that the system could regain 96% of viscosity in only 18 s. Increasing the cross-linking of the soft network also enhanced the storage modulus of the nanocomposite system by up to 2 kPa. Through SAXS data, it was shown that at cross-linking, the junction zones consisting of fragments of two laterally aligned polymer chains were formed, which should have provided additional strength to the hydrogel. At the same time, the cross-linking of the soft network only slightly affected the yield stress, which seemed to be mainly determined by the rigid percolation network of nanotubes and reached 327 Pa. These properties make the alginate/halloysite hydrogels very promising for 3D printing, in particular, for biomedical purposes taking into account the natural origin, low toxicity, and good biocompatibility of both components.

Pulses of Fast Electrons as a Tool To Synthesize Poly(acrylic acid) Nanogels. Intramolecular Cross-Linking of Linear Polymer Chains in Additive-Free Aqueous Solution

2003 ◽  
Vol 36 (7) ◽  
pp. 2484-2492 ◽  
Author(s):  
Slawomir Kadlubowski ◽  
Jaroslaw Grobelny ◽  
Wielislaw Olejniczak ◽  
Michal Cichomski ◽  
Piotr Ulanski
Keyword(s):  
Aqueous Solution ◽  
Acrylic Acid ◽  
Polymer Chains ◽  
Linear Polymer ◽  
Cross Linking ◽  
Fast Electrons ◽  
Poly Acrylic Acid

Effects of low-energy electron bombardment on the surface chemical structure and adhesive properties of polytetrafluoroethylene (PTFE)

1986 ◽  
Vol 1 (5) ◽  
pp. 717-723 ◽  
Author(s):  
J.A. Kelber ◽  
J.W. Rogers ◽  
S.J. Ward
Keyword(s):  
High Vacuum ◽  
Nitrogen Adsorption ◽  
Electron Bombardment ◽  
Polymer Chains ◽  
Ultra High Vacuum ◽  
Cross Linking ◽  
Cohesive Failure ◽  
Adhesive Properties ◽  
Rate Determining Step ◽  
Adsorption Of Nitrogen

The x-ray photoemission studies of polytetrafluoroethylene (PTFE) bombarded by lowenergy electrons in ultra-high vacuum conditions indicate that the major chemical changes induced by electron bombardment are defluorination of the surface and cross-linking of the polymer chains. The same electron bombardment process, when performed in the presence of 1×10−6 Torr ND3, also results in the adsorption of nitrogen-containing groups at the surface. The rate of nitrogen adsorption is linear for short electron bombardment times while the rates of defluorination and cross-linking are roughly exponential. However, at long bombardment times, the rates of nitrogen uptake, defluorination, and cross-linking become zero at the same time, indicating that defluorination of the surface is the rate-determining step in electron beam-induced adsorption of nitrogen-containing species. Regardless of whether the bombardment is carried out in ultra-high vacuum or in the presence of ND3, the maximum modification depth is less than 30 Å. Pull tests performed on PTFE samples bombarded by electrons in ultra-high vacuum, then removed into air and bonded to epoxy show epoxy-PTFE joint strengths of 280–360 1b/in.2 (psi), are compared to zero psi for untreated PTFE and ≃2000 psi for cohesive failure within the PTFE layer.

Proton-Conducting Cross-Linked Sulfonated Aromatic Polymers for Fuel Cells Application

2012 ◽  
Vol 1384 ◽  
Author(s):  
B. Maranesi ◽  
L. Pasquini ◽  
M. Khadhraoui ◽  
P. Knauth ◽  
M.L. Di Vona
Keyword(s):  
Mechanical Properties ◽  
Thermal Stability ◽  
Hydrolytic Degradation ◽  
Exchange Capacity ◽  
Polymer Chains ◽  
Cross Linking ◽  
Ionic Exchange ◽  
Acid Base ◽  
Acid Base Titration ◽  
Aromatic Polymers

AbstractThermal stability, hydration and mechanical properties of thermally cross-linked Sulfonated Aromatic Polymers (SAP) with high ionic exchange capacity (IEC) were measured and compared to untreated samples. The formation of cross-linking greatly stabilizes SAP in terms of thermal, mechanical, and hydrolytic degradation: they can resist in water even at a temperature of 145 °C with improved mechanical properties. Acid-base titration and FTIR spectra consistently indicate that SAP microstructure stabilization is related to cross-linking of the polymer chains by SO2 bridges, which is promoted by temperature.

scholarly journals Kinetic studies of metalloporphyrin formation. Part I. Mn(II) with hematoporphyrin

1968 ◽  
Vol 46 (22) ◽  
pp. 3431-3436 ◽  
Author(s):  
Doreen A. Brisbin ◽  
Robert J. Balahura
Keyword(s):  
Acetic Acid ◽  
Glacial Acetic Acid ◽  
Kinetic Studies ◽  
Activation Energies ◽  
Kinetics Of

The kinetics of the formation of Mn(III) hematoporphyrin in glacial acetic acid has been studied spectrophotometrically.On addition of Mn(II) to porphyrin in glacial acetic acid a species which appears to be the monocation immediately is formed and is then converted to Mn(III) hematoporphyrin at a rate suitable for kinetic studies.The rates of reaction were measured at 45, 50, and 55 °C and the activation energies were calculated.

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