The dependence of the retardation spectrum width on the network chain length and on its distribution

2007 ◽  
pp. 21-22
Author(s):  
A. Havránek
Keyword(s):  
Chain Length ◽  
Network Chain ◽  
Retardation Spectrum ◽  
Spectrum Width

Networks Having Multimodal Chain-Length Distributions

1997 ◽  
Author(s):  
Burak Erman ◽  
James E. Mark
Keyword(s):  
Free Radical ◽  
Chain Length ◽  
Length Distribution ◽  
Bimodal Distribution ◽  
High Energy ◽  
Chain Length Distribution ◽  
Network Chain ◽  
Ultimate Properties ◽  
Non Gaussian ◽  
Chain Lengths

As was mentioned in chapter 10, end-linking reactions can be used to make networks of known structures, including those having unusual chain-length distributions. One of the uses of networks having a bimodal distribution is to clarify the dependence of ultimate properties on non-Gaussian effects arising from limited-chain extensibility, as was already pointed out. The following chapter provides more detail on this application, and others. In fact, the effect of network chain-length distribution, is one aspect of rubberlike elasticity that has not been studied very much until recently, because of two primary reasons. On the experimental side, the cross-linking techniques traditionally used to prepare the network structures required for rubberlike elasticity have been random, uncontrolled processes, as was mentioned in chapter 10. Examples are vulcanization (addition of sulfur), peroxide thermolysis (free-radical couplings), and high-energy radiation (free-radical and ionic reactions). All of these techniques are random in the sense that the number of cross-links thus introduced is not known directly, and two units close together in space are joined irrespective of their locations along the chain trajectories. The resulting network chain-length distribution is unimodal and probably very broad. On the theoretical side, it has turned out to be convenient, and even necessary, to assume a distribution of chain lengths that is not only unimodal, but monodisperse! There are a number of reasons for developing techniques to determine or, even better, control network chain-length distributions. One is to check the “weakest link” theory for elastomer rupture, which states that a typical elastomeric network consists of chains with a broad distribution of lengths, and that the shortest of these chains are the “culprits” in causing rupture. This is attributed to the very limited extensibility associated with their shortness that is thought to cause them to break at relatively small deformations and then act as rupture nuclei. Another reason is to determine whether control of chain-length distribution can be used to maximize the ultimate properties of an elastomer. As was described in chapter 10, a variety of model networks can be prepared using the new synthetic techniques that closely control the placements of crosslinks in a network structure.

Network Chain Distribution and Strength of Vulcanizates

1969 ◽  
Vol 42 (3) ◽  
pp. 659-665 ◽  
Author(s):  
S. D. Gehman
Keyword(s):  
Physical Properties ◽  
Chain Length ◽  
Random Distribution ◽  
Length Distribution ◽  
Chain Scission ◽  
Point Of View ◽  
Average Chain Length ◽  
Chain Length Distribution ◽  
Network Chain ◽  
Chain Molecules

Abstract Physical characteristics of rubber network structures usually enumerated and discussed are network chain density, crosslink functionality, average chain length between crosslinks, entanglements which act somewhat like crosslinks, and free chain ends which are network defects. Chemical factors include structure of the chain molecules, type of crosslinks, whether monosulfide, disulfide or polysulfide, or direct carbon-to-carbon bonds. Side effects of vulcanization reactions such as chain scission or combination of minor quantities of chemical fragments from the vulcanizing system are also recognized. One might think that these variables would be adequate to account for physical properties of elastomers but explanations of strength aspects of vulcanizates are still unsatisfactory. Something is missing in these considerations, that is, the distribution of crosslinks along a main chain or the length sequences of monomer units in network chains. Usually a random distribution is implicitly assumed. If the distribution is always random and nothing can be done about it and it cannot be measured anyway, there may seem to be little point in writing about it. However, an ideally random distribution for all crosslinking systems and polymers seems very improbable. The importance of network chain length distribution for physical properties has been, of course, well recognized in theory. Bueche's calculations showed that viscoelastic resistance to deformation increased markedly with increased crosslink functionality, that is, as more chains are involved in the displacement of a crosslink. His molecular theory of tensile strength was based on the concept of short, overloaded network chains which snapped and transferred their loads to neighboring chains. An alternate point of view is that short chains are detrimental because they do not stress orient as well as longer chains.

Improved Elastomers through Control of Network Chain-Length Distributions

1999 ◽  
Vol 72 (3) ◽  
pp. 465-483 ◽  
Author(s):  
J. E. Mark
Keyword(s):  
Chain Length ◽  
Addition Reaction ◽  
Condensation Reaction ◽  
Bimodal Distribution ◽  
Polymer Chains ◽  
Network Chain ◽  
End Linking ◽  
Chain Lengths ◽  
Bimodal Elastomers

Abstract Methods are described for obtaining elastomers of controlled network chain-length distributions by restricting the reactivity of the polymer chains to their ends, and then end linking these chains with a multi-functional reactant. The networks of this type that have proved to be of greatest interest consist of short chains end linked with long chains to yield a bimodal distribution of network chain lengths. These bimodal elastomers have unusually high extensibility for their values of the modulus and ultimate strength, and thus considerable toughness, even in the unfilled state. Most such elastomers have been prepared from chains of poly(dimethylsiloxane), by carrying out either a condensation reaction between hydroxyl-terminated chains and tetraethoxysilane, or an addition reaction between vinyl-terminated chains and a poly(methylhydrogen siloxane) oligomer. The material presented in this review discusses the preparation of such elastomers, the characterization of some of their properties, and the interpretation of some of these properties in terms of the molecular theories of rubber-like elasticity.

Satiation in humans is modulated by fatty acid chain length

2001 ◽  
Vol 120 (5) ◽  
pp. A710-A710
Author(s):  
S LAL ◽  
J MCLAUGHLIN ◽  
O NIAZ ◽  
G DOCKRAY ◽  
A VARRO ◽  
...  
Keyword(s):  
Fatty Acid ◽  
Chain Length ◽  
Fatty Acid Chain Length ◽  
Fatty Acid Chain
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