Network chains chemical scission

Note The aging of a gel can involve polymerization, crystallization, aggregation, syneresis, phase changes, formation of branch points and junction points as well as scission and chemical changes to constitutional units of network chains. 

The resolution capability of a resist is directly related to resist contrast (7) which, for a negative resist, is related to the rate of crosslinked network formation at a constant input dose. It is somewhat more complicated for a positive resist being related to the rate of chain scission and the rate of change of solubility with molecular weight with the latter being markedly solvent dependent. Contrast, like sensitivity, is governed by the type of chemical reactions that occur in the polymeric resist and is affected by molecular parameters such as molecular weight distribution and chemical composition. 

THe network structure of these block polymers can also be altered by actually crosslinking the elastomeric polydiene chains, thus introducing a chemical network. In this approach it was necessary to use a crosslinking method which would not result in any measurable chain scission. Dicumyl peroxide (Dicup) was chosen for crosslinking an SIS polymer since this peroxide is known to accomplish exclusive crosslinking without any observable chain scission (3). The Dicup was dissolved in a THF solution of the polymer, and a cast film was prepared which was then vulcanized in a press at 155 °C for 35 min. A control sample, without Dicup was treated in the same way. 

Antidegmdents. This group of chemicals is added to prevent undesirable chemical reactions with the polymer network. The most important are the antioxidants, which trap free radicals and prevent chain scission and cross-linking. Antiozonants are added to prevent ozone attack on the rubber, which can lead to the formation and growth of cracks. Antiozonants function by diffusion of the material to the surface of the rubber, thereby providing a protective film. Certain antioxidants have this characteristic, and waxes also are used for this purpose. 

It follows from this Fig. that the amount of chemical junctions in silicon rubber increases with increasing fractions of Aerosil. The chemical junctions are apparently formed by scission of PDMS chains under the mechanical forces during milling. However, the fraction of these junctions is the lowest. The fraction of adsorption junctions increases proportionally to the filler content as shown in Fig. 11. The major contribution to the network structure is provided by topological hindrances near the filler surface as shown in Fig. 11. 

A well defined way of obtaining kinetic data for chemical reactions which lead to chain scission in bulk network polymers is by stress relaxation. The formalism of this method has been developed by Tobolsky and others (24-26), and can be summarized as follows. 

Chain scission, a form of chemical degradation, which is often important in crosslinked elastomeric networks, especially at elevated temperatures. 

The chemical stability against chain scission and oxygen attack also plays a role in the application of plastics at still higher temperatures. The statistical probability of chain scission is indeed practically the same for linear chains, cross-linked network polymers, and ladder polymers. But, whereas, a chain scission in linear polymers leads to lower degrees of polymerization because of chain degradation, with consequent diminished mechanical properties, chain scission with, for example, a ladder polymer, still leaves one chain of the ladder intact since it is improbable that both chains of the ladder should break at exactly the same distance from each end (see also Chapter 23). 

Under the conditions of mechanical solicitation, firstly are split the most tensioned bond between particles that behave as authentic centres of crosslinking, which should be assimilated to the knots of chemical bonds subsequently these ones do not contribute to the material resistance,when exposed to the subsequent solicitation. In this case, the chains distribution between the particles is not statistic one, since the weakened bonds have been split. At high temperatures, the scissions of and remakes of the chemical bond occur in the elastomer network. Consequently, after a certain period the chains distribution will regain its statistic character and the split chains during the first stressing period are replaced with other ones identical. Therefore, a recovering of the modulus to the initial values is expected. However, at small deformations and especially in the case of a poor dispersion of the particles, a decay of modulus is recorded, which was not recuperated during the elastomer thermal treatment. 

Verbruggen and co-workers [13] prepared a model NR gumstock (i.e., cure system only, no fillers or other additives) compound and subjected a thin film (300 pm thick) of it to DPDS at 200 "C. Solvent extraction (with acetone and tetrahydrofuran), swelling experiments (in toluene) and chemical analysis by GPC and nuclear magnetic resonance (NMR) spectroscopy were used to assess the degree of devulcanisation and to characterise the samples produced. It was found that complete network breakdown was obtained with 2.4% w/w of DPDS after 2 h of heating, but that both crosslink scission and main-chain scission had occurred. 

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