Branched and cross-linked polymers

Make sketches or diagrams showing (a) a linear polymer, (b) a polymer with pendant groups, (c) a polymer with short branches, (d) a polymer with long branches, and cross-linked polymers with (e) low and (f) high cross-linked density. 

All the polyrotaxanes discussed thus far are based on linear backbones. Viewing new properties and applications of branched and cross-linked polymers, three-dimensional polyrotaxanes will surely be interesting in terms of both topology and potential applications. Gong and Gibson extended the hydrogen-bonding theo-... 

The same concept was used in polycondensation systems for the preparation of branched and cross-linked polymers [121]. Copolyurethanes 73 were prepared from tetra(ethylene glycol), bis(5-hydroxymethyl-l,3-phenylene)-32-crown-10 (74)... 

Know the meaning of homopolymer, copolymer, linear, branched, and cross-linked polymer. For copolymers, know the meaning of alternating, random, block, and graft. 

Ham, J. S. Viscoelastic theory of branched and cross-linked polymers. J. Chem. Phys. 26, 625-633 (1957). 

We know two major competing directions of tribodestruction of heterocyclic polymers i) decomposition of a heterocycle (quinoxaline, imide, oxa-diazole or others) as a result of the destruction of weak links of the backbone chain leading to a drop in the polymer molecular mass until the formation of low-molecular products and ii) the reactions resulting in formation of a branched and cross-linked polymer structure [97, 104-106]. Low-molecular products of macromolecular destruction (both gaseous and oligomer) may serve as tribochemical lubricants that reduce the friction coefficient and function as wear inhibitors. 

Figure 1.2 Unear, branched, and cross-linked polymers. 

In terms of their architecture, polymers can be classified as linear, branched and cross-linked polymers (Table 1.2). In the linear polymers, the structural imits are arranged in a linear sequence. Branched polymers may have short and long branches. Branched polymers include comblike and star polymers. Extensive branching may lead to a dendritic structure. 

The molecular weights of the polymers decrease when the ring sizes of the ketone components increase. Excess diisocyanate yields branched and cross-linked polymers. The enamine units in the polymers can be hydrolyzed with formic acid to the corresponding ketones [77]. 

Thermosets are, in essence, one giant molecule since all the polymer chains are connected through the cross-links. Thermosets cannot be melted after they are formed and cannot be dissolved in solvents. Depending on the cross-link density, thermosets can swell in certain solvents. When a cross-linked polymer solidifies or gels, it usually has some linear or uncormected pol)uneT present, which sometimes can be extracted after implanation. Figure 11.2 shows a schematic diagram for linear, branched, and cross-linked polymers. 

Figure 1.3 Linear, Branched and Cross-linked Polymers.

---