Thermodynamic Considerations of Protein Reactions.1,2II. Modified Reactivity of Primary Valence Bonds

1956 ◽  
Vol 78 (22) ◽  
pp. 5793-5798 ◽  
Author(s):  
Michael Laskowski ◽  
Harold A. Scheraga
Keyword(s):  
Primary Valence ◽  
Valence Bonds

Aging of Elastomers. Comparison of Creep with Some Conventional Aging Methods

1949 ◽  
Vol 22 (3) ◽  
pp. 699-711 ◽  
Author(s):  
M. C. Throdahl
Keyword(s):  
High Temperature ◽  
Temperature Region ◽  
Chain Scission ◽  
High Temperature Region ◽  
Secondary Bonds ◽  
Air Atmosphere ◽  
The Stability ◽  
Usual Laboratory ◽  
Primary Valence ◽  
Valence Bonds

Abstract Subjection of elastomers to mechanical stresses results in unusually complicated behavior. Recent theoretical researches have shown that this behavior cannot be described satisfactorily by either of the classical theories of elasticity or viscosity. The general molecular theories which describe the behavior of elastomers have experimental verification manifested by three regions of temperature-stress relationship: (1) a low temperature region in which stiffening is observed, due to the stability of secondary bonds between network chains, (2) an intermediate temperature region in which the secondary bonds are so unstable that complete relaxation occurs before measurements can be obtained; the scission of primary valence bonds is occurring at such a slow rate that no measurable effects are obtained during the course of the usual laboratory experiment; and (3) a high temperature region in which the relaxation of stress with time is associated with a chemical reaction which, through breaking of primary-valence bonds in the network, severs the chains rapidly enough to be measured during the course of usual laboratory experiments. The high temperature region is that in which elastomers soften and (or) harden and finally lose their rubbery characteristics. Oxygen has been shown to be necessary for the chain-scission reaction. Several papers have described this fundamental experimental technique for the stress-relaxation and creep of different elastomers. Well known laboratory methods for artificially aging elastomers in oxygen and air bombs and in circulating air atmosphere have selected conditions somewhat arbitrarily. In exploratory searches for promising compounds to be used as antioxidants in elastomers and in the evaluation of well known antioxidants, it has often been found that the conventional methods of aging do not differentiate among several antioxidants. It is the purpose of this paper to describe an application of the previously described creep technique as a convenient and precise means of studying the relative performance of antioxidants and accelerators in Hevea and GR-S rubbers.

Resonating valence bonds andd-wave superconductivity

1988 ◽  
Vol 37 (7) ◽  
pp. 3664-3666 ◽  
Author(s):  
G. Kotliar
Keyword(s):  
Valence Bonds

Temperature Dependence of the Mechanical and Stress-Optical Behavior of Elastomers

1960 ◽  
Vol 33 (3) ◽  
pp. 763-789
Author(s):  
J. Kruse ◽  
T. Timm
Keyword(s):  
Elastic Modulus ◽  
Natural Rubber ◽  
Optical Constant ◽  
General Rule ◽  
Absolute Temperature ◽  
Double Refraction ◽  
Temperature Function ◽  
Entropy Elasticity ◽  
Optical Behavior ◽  
Valence Bonds

Abstract The temperature functions of the elastic modulus K2 and of the stress-optical constant K1 or its reciprocal 1/K1 were investigated for several elastomers. In the case of a hypothetical rubber which we have called “ideal” rubber—in analogy to gases—theory requires a direct proportionality between K2 or 1/K1 and the absolute temperature. The temperature functions of K2 and 1/K1 which we found by experiments with “real” elastomers show characteristic negative and positive deviations Δa2 and Δa1 from “ideal” values. When we put these values of Δa2 and Δa1 into a coordinate system, we find a certain orderly arrangement of the different elastomers, which allows us to picture a relationship between molecular structure and the values of Δa2 and Δa1. This brings up the possibility of explaining the experiments with the help of already known molecular-physical concepts. Although other explanations are conceivable the attempt is made to develop the simplest and most obvious ideas. It is conjectured that negative values of Δa2 and Δa1 come about from a loosening of secondary valence bonds—in certain ways, like crystal bonds— between neighboring molecules. Negative Δa1 values were found only in the crystallizable elastomers. It is further conjectured that positive values of Δa2 and Δa1 may result from the liberation by heat, of blocked, bulky molecular segments. These molecular segments can then contribute to the entropy elasticity only at higher temperatures. Positive Δa2 and Δa1 values are found chiefly in strongly crosslinked elastomers. Brief attention is given to the physical processes which are responsible for the elongation—double refraction and the entropy-elasticity. From this, it seems that the stress-optical constant and its temperature function are connected with properties of the molecular chains and on their orientability and crystallizability. The elastic modulus and its temperature function are strongly affected by the structure of the network and the molecular cohesive forces. Worthwhile hints about crystallization tendency, polarity and degree of symmetry of the different systems are given by the Δa1 and Δa2 values in the above mentioned coordinate systems. Natural rubber was tested in different recipes. The results of milling, of sulfur and accelerator additions, of time and temperature of vulcanization, on the values of K2, 1/K1, Δa2 and Δa1 were all investigated. The values of 1/K1 are at their highest level for dried latex films (unvulcanized). Milling and vulcanization, particularly the use of rather long periods and high temperatures, lower the value of 1/K1. A drop in the value of 1/K1, which regularly appears with a reduction of the negative Δa1 value, is explained as a loosening of secondary valence molecular couplings. According to this, natural rubber in the latex state is most strongly associated. According to this explanation, stretching in the unvulcanized condition is sufficient to loosen the secondary valence molecular bonds. Milling and vulcanization also act to loosen the linkages. Secondary valence bonds which are loosened by warming, as a general rule, are reestablished by prolonged cooling. It is to be supposed that the secondary valence molecular bonds under consideration are limited to small regions, somewhat comparable to the ordering in liquids. With an increasing degree of vulcanization, the Δa2 values go through a maximum which perhaps coincides with the condition of optimum vulcanization. This is explained as a maximum of the entropy-elasticity. In the case of slightly milled natural rubber which is appropriately vulcanized, the value of Δa2 can become practically zero. The change of the elastic modulus with temperature then is “ideal.” Nevertheless, no “ideal” rubber exists here, for Δa1 is less than zero.

scholarly journals Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4

2018 ◽  
Vol 8 (3) ◽  
Author(s):  
Itamar Kimchi ◽  
Adam Nahum ◽  
T. Senthil
Keyword(s):  
Quantum Magnets ◽  
Valence Bonds

Chains of Metallic Clusters With Ligands

2021 ◽  
pp. 96-119
Keyword(s):  
Geometric Approach ◽  
Higher Dimension ◽  
Metallic Clusters ◽  
The Core ◽  
Metal Atoms ◽  
Dimension Space ◽  
Metal Chain ◽  
Valence Bonds

This chapter geometrically investigated the structure of clusters, the core of which represent the metal chains (linear or curved) of both identical and different elements. It was shown that the dimension of the structures of these clusters is more than three. To create a model of these chains in a higher dimension space, a new geometric approach has been developed that allows us to construct convex, closed polytopes of these chains. It consists of removing part of the octahedron edges necessary for constructing the octahedron and adding the same number of new edges necessary to build a closed polytope chain while maintaining the number of metal atoms and ligands and their valence bonds. As a result, it was found that metal chain polytopes consist of polytopes of higher dimension, adjacent to each other along flat sections.

scholarly journals Valence bonds in planar and quasi-planar boron disks

2019 ◽  
Vol 21 (2) ◽  
pp. 729-735 ◽  
Author(s):  
Issofa Patouossa ◽  
Athanasios G. Arvanitidis ◽  
Jules Tshishimbi Muya ◽  
Minh Tho Nguyen ◽  
Arnout Ceulemans
Keyword(s):  
Boron Clusters ◽  
Electron Counting ◽  
Counting Rules ◽  
Valence Bonds

Valence bonds within the perimeter of disk-like boron clusters with a concentric topology follow simple 4n and 8n electron counting rules.

scholarly journals Resonating valence bonds and mean-fieldd-wave superconductivity in graphite

2007 ◽  
Vol 75 (13) ◽  
Author(s):  
Annica M. Black-Schaffer ◽  
Sebastian Doniach
Keyword(s):  
Valence Bonds

Relation of Volume Change to the Mechanism of Rubber Vulcanization

1934 ◽  
Vol 7 (4) ◽  
pp. 693-699
Author(s):  
Ira Williams
Keyword(s):  
Physical Properties ◽  
Volume Change ◽  
Physical State ◽  
Direct Relation ◽  
Polymeric State ◽  
Primary Valence

Abstract The present investigation shows that no direct relation exists between either combined sulfur or density and the degree of vulcanization. It is not probable that the various changes in physical properties of the rubber during vulcanization are due to changes in the polymeric state involving a change in primary valence forces. Neither is it apparent in what manner combination of sulfur has contributed directly to the change in physical properties of the rubber. It is probable that the change in physical state is due to a change in manner or degree of aggregation of the rubber molecules.

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