Elastomers chain scissions

The classic model that describes chain scission in elastomers was proposed many years ago by Lake and Thomas [26J. The aim of the model is to calculate the energy dissipated in breaking all the polymer strands that have adjacent cross-links on either side of the crack plane. The basic assumption of this model is that all the main chain bonds in any strand that breaks must be strained to the dissociation... 

Obviously, there are many subtle differences in the structure, morphology, or network topology between radiation cured and sulfur cured elastomers, but their physical properties may be nearly equal, provided that precautions are taken to avoid the occurrence of chain scissions. A comparison of radiation cross-linked and sulfur cured natural rubber (gum and carbon-black-reinforced compounds) is in Table 5.4. ... 

Since fluorocarbon elastomers being discussed here contain hydrogen in their molecules, they have the tendency to cross-link in addition to scission, common in fluoropolymers when exposed to radiation. The cross-linking predominates, but there is still a significant degree of chain scission. ... 

Mixing and extrusion techniques are usually applied to elastomers for processing and improvement of physical properties. These mechanical processes normally lead to the occurrence of radical reactions such as chain scission, chain coupling, and crosslinking. The 13C-NMR analysis of EP showed that the shearing of polymer resulted in hydrogen abstraction. This was followed by disproportionation reaction to form olefins [75]. 

The main problem is related to exposure of these materials to high temperatures in the presence of oxygen. These conditions favor the oxidative degradation of the elastomer, which leads to opposing changes in its structure. On the one hand, chain scission can occur, and on the other hand, cross-links can be generated. If chain scission dominates, the hardness and modulus of the elastomer will decrease. If, however, cross-links are generated, both the hardness and the modulus increase and the strain at fracture will decrease. Other mechanical properties are altered by an increase in... 

As can be imagined the radiation produced crosslinks and chain scissions act in opposite directions with respect to changing the physical properties of plastics. Crosslinks transform molten polyethylene from a liquid into an elastomer. The elastomeric properties thus produced make possible the so-called "memory effect" in which the irradiated PE, for example, when stretched at temperatures above its melting point and then cooled... 

The title block copolymers with characteristics ranging from thermoplastic elastomers to polyethylene-like thermoplastics are obtained from ring opening polymerization of hexamethylcyclotrisiloxane with living a,(D-dilithiopoly-styrene. Chain scissions and oligomerizations which usually complicate siloxane polymerization are avoided, and molecular parameters regulating physical and mechanical properties are conveniently controlled to provide a unique family of thermoplastic materials. 

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