Random scission and cross-linking

Molecular Weight Distribution. A given polymer sample is composed of many polymer chains, which in most cases are not of the same length. This variability can be a result of the synthesis process or of possible random scission and cross-linking that can occur upon processing. For economic reasons, it is not possible to separate the various polymer chains by length prior to use so it is important to characterize this distribution in order to describe the polymer and understand its performance. As with any distribution, no single number is a totally satisfactory descriptor. 

Figure 2. (a) Random scission and cross-linking of narrow molecular... 

An important radiation chemical effect in polymers is the production of cross-links (cl) and main-chain scissions (sc), since these have a profound effect on various macroscopic properties of the polymer material. Scission decreases the molecular weight of the polymer and cross-linking increases it. It may be shown that under conditions of random scission and cross-linking and for an initially random distribution of molecular weights, one can write for the weight-average molecular weight as a function of dose i ... 

The constant 2 is equal to 4.82 x 10 if the dose is expressed in kGy. Equation 9 assumes a post-probable initial molecular weight distribution, random scission and cross-linking reactions, and cross-linking by an H-linking mechanism. If cross-linking occurs by a Y-linking mechanism, the relationship between solnble fraction and dose is given by (96)... 

Shyichuk has developed a Monte Carlo method of computer-aided simulation of the MWDs of degraded polymer derived from the MWD of the unexposed polymer and assuming scission and cross-linking are random events. The results of trial scission cross-linking concentrations are compared with measured MWDs for the exposed samples using the sum of the squares of the... 

Simultaneous random chain scissions and cross-linking... 

The thermotropic polymers used in these studies are all random copolyesters. As has previously been documented for other types of polymers, the presence of aromatic rings and/or conjugated bonds, and the absence of CH2—CH2 and ether linkages yield materials most resistant to beam damage (by processes such as chain scission and cross-linking). The family of thermotropic copolyesters most extensively studied here comprises the following randomly sequenced segments ... 

Non-sequence-specific or global DNA damage is generally caused by random DNA alkylation, cross-linking or strand scission, the mechanisms and consequences of which have been previously mentioned. 

Several theoretical models have been published which predict the change in MWD during degradation especially for a random mechanism. For detail, the reader is referred to specialized books (see, for example, Conley [2]). Kotliar, using Monte Carlo techniques, calculated the changes in distribution for pol)miers with Schulz-Zimm distributions of varying initial breadth. He considered random scission [3-6] and scission plus cross-linking. He concluded that an increase in amount of chain scission for a broad MWD, M jMn > 2, results in a decrease in the MWD, for the case of random... 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

Equation (7.20) is general, although the expression for Te itself depends on the primary distribution, the gel fraction, and the relative rates of random cross-linking and chain scission. 

High energy radiation splits covalent bonds into unpaired radicals, which can then recombine randomly. Depending upon the relative rates of recombination and scission, an irradiated polymer can be cross-linked or it degrades into low molecular weight fragments. The influence of irradiation on polymers depends on their chemical structure ... 

Ladder polymers are double-strand linear polymers. Their permanenee properties are superior even to those of conventional network polymers. The latter are randomly cross-linked, and their molecular weight ean be redueed by random scission events. When a chemical bond is broken in a ladder polymer, however, the second strand maintains the overall integrity of the molecule and the fragments of the broken bond are held in such close proximity that the likelihood of their recombination is enhanced. 

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