Crosslinking physical process

Certain block copolymers comprise a category of elastomer in which crosslinking occurs by a physical process. The two main types of block copolymers which have been commercialized are the ABA and -(AB n block copolymers (X and XI) where A and B represent... 

The neutral polymer radicals which are produced also often undergo further reactions, which can result in chemical changes in the polymer. These reactions may include crosslinking or scission of polymer chains, formation of small molecule products, changes in the stereochemistry of the polymer chains, changes in the crystallinity of the polymer or a variety of other chemical and physical processes. 

By far the most interesting application to date is the ability to crosslink polymers in the solid state, and much research has been devoted to studying the reactions involved. The initial physical process of energy absorption and the final chemical change—formation of crosslinks—can be readily determined. However, there is still considerable doubt as to the intermediate reactions, and this problem offers an appropriate start of this review. 

Figure 3 shows that the delta Mooney indexes show an abrupt change from negative to positive values in the same region. Thus, there is a clear relationship between the onset of dominant crosslinking and processability. The deterioration in physical characteristics is not coincidental with the formation of gel since, in both of these examples, the gel point occurs at conversions greater than 60%. 

The detailed physical process of an increase in viscosity followed by gelation and vitrification of network polymers that results from the following crosslinking chemistry is discussed in Section 2.4. 

In the network structure of Fig. V. 1 the crosslinks need not be produced by chemical reaction. Any physical process which favors association between certain (but not all) points on different chains may also lead to gels. Many examples of this are found with biological molecules, such as gelatin" or certain polysaccharides. In many of these sterns the association process is still disputed. There are three main posabilities ... 

In this review article, after a description of the historical background of the present subject, some general features of reactions in amorphous polymer solids are summarized and then specific features of photo-physical processes, photoisomerization, photodimerization, chain scission, and crosslinking reactions are discussed separately. Typical theoretical treatments of solid-state reactions are reviewed in relation to the interpretation of experimental results. 

Crosslinking occurs by a physical process in block copolymers which possess a microheterogeneous, two-phase network morphology. The two important types of block copolymers are the triblock, ABA, and multiblock (AB) copolymers (III and IV, respectively) where A and B... 

In the present monograph these problems were studied from the principally new point of view, when object structure, either macromolecular coil (microgel) or condensed state structure, was considered in terms of chemical or physical processes. Such an approach is possible with the availability of appropriate structural models, of which fractal (multifractal) analysis and the cluster model of the amorphous state structure of polymers were used. The first of the indicated approaches is a general mathematical calculus, whereas the second represents itself as a purely polymeric concept. This circumstance defines the excellent combination and addition to one another of crosslinked polymers. 

Thermoset plastics are chemically setting synthetic resins. Thermoset plastics set in the hot mould by crosslinking (chemical process), as opposed to thermoplastic fieezing in a sufficiently cold mould (physical process). Synthetic resins tend to form shrinkage cracks, since a plastic deformation is practically impossible. The various resin types are shown in Table 7.1. Phenohc resins may also be referred to as Bakelite (after the chemist Baekeland who invented phenolic resin). 

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