Polymer chain scission

The role of cavitation in ultrasound degradation has been confirmed repeatably in most experiments where cavitation was prevented, either by applying an external hydrostatic pressure, by degassing the solution, by reducing the sound intensity or the temperature, polymer chain scission was also largely suppressed [117]. 

G(scission) = G(S) = Number of polymer chain scissions per 100 eV absorbed. G(cross-linldng) = G(X) = Number of polymer cross-link sites per 100 eV absorbed. 

The molecular weights of the polymers before and after irradiation were followed by GPC to determine changes in the molecular weights of the polyamides. It was found that photolysis of the polymer resulted in polymer chain scission, leading to the appearance of oligomers containing thymine bases at the end of the molecules. 

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

Ultraviolet Absorbers. These additives are included whenever prolonged outdoor exposure is a prerequisite to performance. They absorb light in the ultraviolet region and prevent polymer chain scission at points of unsaturation created by dehydrochlorination. They are not known to produce adverse effects on the vinyl compound at use concentrations. 

Thus, the strength decreases at rate of about 1.15 gf per micromole of -NH2 per gram of silk. Because of the complex relationship between polymer chain scission, molecular weight, and strength, and the presence of side-chain amino groups in silk, it is difficult to relate these parameters theoretically. 

In addition, mechanistic studies of the photochemical reactions are necessary to determine whether similar processes occur in the solid state. Polymer chain scission is usually the predominant process in the solid state, although cross-linking reactions become more important in the presence of pendant unsaturation. However, little is known about the nature of the intermediates produced in the solid state. Information of this type is important, because most of the applications of polysilane derivatives require the materials as solid films. 

Fig. 6.1. Pol5

The data from Table 3.7.2 show that during polymer chain scission the initiation reaction is more likely to take place by p scission (lower dissociation energy). Methyl elimination and a scission probably play a less important role in the initiation reaction. The values calculated for 25° C and 500° C did not show differences between them. 

A simple, rapid, and reliable viscometric technique for evaluating the protective capacity of commercially available materials as inhibitors of ozone-induced polymer chain scission has been described (1). This work included the results of an evaluation of several chemicals such as A,A -di-sec-butyl-p-phenylenediamine, nickel dibutyl di-thiocarbamate, l-(m-aminophenyl)-2,5-dimethylpyrrole, and 2,6-di-tert-butyl-4-methyl-phenol as antiozonants A,A -di-sec-butyl-p-phenylenediamine exhibited superior inhibiting characteristics. However, because of the potential toxic effects and relatively high vapor pressure of this chemical, its use is considered impractical. 

As further shown, polymer chain scission or side scission of many linear polymers takes place through this mechanism. At higher temperatures (600-900° C) this type of reaction is also common for small molecules, which explains in some cases the formation of unsaturated or aromatic hydrocarbons from larger aliphatic ones. 

Not only small molecules are eliminated in this type of reaction. Polymer chain scissions also may occur with p-elimination. 

Once the transesterification step is complete, the metal salt is always deactivated by complexation with a phosphate or phosphite compound [18], preventing unwanted side reactions as the reaction temperature is increased during polycondensation. Such side reactions lead to polymer chain scission and loss of molecular weight, as well as development of unwanted discoloration of the polymer. Care has to be taken that excess phosphorous compounds are not added to the reaction, as these compounds can significantly reduce the effectiveness of the polycondensation catalyst. 

In most cases direct evidence for actual devulcanization, i.e., breaking sulfur-sulfur and carbon-sulfur bonds without polymer chain scission, is lacking. However, in many instances the so-called devulcanization process increases the suitability for reuse (Fig. 2). 

Photooxidation reactions only take place in the presence of oxygen (15, 17). These reactions are the primary source of most of the sunlight damage to textiles. Photooxidation of nylon and model compounds have shown that oxidative attack usually produces free radicals, peroxides, and ultimately polymer chain scission. This results in lower tensile strength and ultimately a shorter useful lifetime of the textile product. 

In an effort to determine the effect of aqueous acid on nylon in the presence of light, Zeronion et al (12) submerged nylon fabric in 20% sulfuric acid at 50 C in a flint glass jar and exposed it to irradiation from a 275 watt sunlamp at a distance of 6 inches from the fabric. The nylon showed more polymer chain scission and greater [NH ] than nylon degraded by light and SO gas both with and without a water spray. From these experiments, it was concluded that if sulfuric acid was present in the atmosphere, its attack on nylon was accelerated by the presence of light. 

Fig. 2. Multi-ionization spur in C,H polymers chain scission, and if spur energy > 100 eV, also debris present, e.g. acetylene, methane and increased yield of hydrogen.

Table IV. Efficiency of Polymer Chain Scission Relative to Photodecomposition of Hydroperoxide at 313 nm...

The external effects of the environment on polymer blends are chemical in nature, and normally lead to degradation of the polymers. Chain scission, depolymerization and reactions on the side-chain substituents all contribute to overall deterioration of blend properties. These are described for blends containing polyvinyl chloride, polystyrene, acrylics and polyolefins mixed with a variety of other polymers. The general feamres of radiation damage and the detrimental effects of processing are reviewed. 

In principle, the rupture of a fibril may occur by at least two or three mechanisms, which are not always clearly separable. One is the end point of the drawing (creep) mechanism at constant mass of polymer in the fibril. A condition of local necking down could develop, leading to failure (22). See Fig. 1. This would be more likely to exist with low than with high MW polymers. Alternatively, and particularly with high MW polymers, chain scission might occur. 

Paturej J, Milchev A, Rostiashvili VG, Vilgis TA (2011) Polymer chain scission at constant tension - an example of force-induced collective behaviour. Europhys Lett 94 48003... 

Islam MT, Vanapalli SA, Solmnon MJ (2004) Inertial effects on polymer chain scission in planar elongational cross-slot flow. Macrmnolecules 37 1023-1030... 

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