Topological scission

Finally, in Figure 9.11 we show MWDs for two different scission models. The linear scission case assumes scission of unbranched chains. The topological scission case employs the fragment length function of Eq. (79). A marked difference is observed. 

Fig. 9.11. Radical polymerization of ethylene in a CSTR. Linear and topological scission with the same polydispersity of 26 as experimental MWD from SEC-MALLS [35], Solid line, experimental MWD dash-dot line, topological scission result dash-dot-dot line, linear scission result.

An example of the possibility that network scission experiments may be subject to topological interpretation is suggested by the results reported by Hookway and Shelton (2). Of particular interest is the degelation point where the network dissolves. (Degelation implies transition through a gel point that may or may not be related structurally to the usual non-gel to gel transition observed in the corresponding network synthesis. The data (ref. 2, Fig. 3) show that hydrogen peroxide causes the release of about 0.5 mole... 

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. ... 

Iedema, P.D. and Hoefsloot, H.C.J. (2001) Synthesis of branched polymer architectures from molecular weight and branching distributions for radical polymerisation with long-chain branching, accounting for topology-controlled random scission. Macromol. Theor. Simul., 10, 855. 

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. 

From Sects. 4 to 8 we have summarized the mechanochemistry of polymers of complex topologies. These topological structures are stable and can only be altered by the scission of covalent bonds. In some other cases, the topology of the molecules (or their assembly) is dynamic and flucmates in a stress-free state. In this section, we explore the mechanochemistry of two paradigmatic systems. 

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