Structure cross-link density

Because thermosetting plastics have an irregular form they are amorphous and because of the network structure are invariably rigid. They do not dissolve without decomposition but may swell in appropriate solvents, the amount of swelling decreasing with increased cross-link density. 

Cross-link density, 10 415-416, 417-418 direct measurement of, 10 426 427 Cross-linked copolymers, 7 6 lOt Cross-linked high amylose starch, 13 742 Cross-linked hydrogels, 13 729-730 Cross-linked polymers, internal stresses and, 10 423 424 Cross-linked starches, 4 721 Cross-linked thermoset polymer structure, 10 418... 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

FIGURE 2.3 Skeletal structural formulas of a linear polymer (a) and a network (cross-linked) polymer with low cross-linking density (b) and high cross-linking density (c). 

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

Poly(ethylene oxide). The synthesis and subsequent hydrolysis and condensation of alkoxysilane-terminated macromonomers have been studied (39,40). Using 29Si-nmr and size-exdusion chromatography (sec) the evolution of the silicate structures on the alkoxysilane-terminated polyethylene oxide) (PEO) macromonomers of controlled functionality was observed. Also, the effect of vitrification upon the network cross-link density of the developing inorganic—organic hybrid using percolation and mean-fidd theory was considered. 

Since the cross linking density of the gel is low enough, there are many contact points between polymer chains. The structure of the gel, therefore, is analogous to that of the semi-dilute polymer solution suggesting that the concentration dependence of the correlation length is explained by the following power law relationship [6]... 

When reacting either ethylene carbonate or propylene carbonate with an aliphatic diamine, a polyurethane can be produced (Figure 2.14). Polyethylene ether carbonate) diols (Harris et al., 1990), when fabricated into polyurethanes using MDI and BDO, produce elastomers that have polyester polyol features. This was shown using 13C NMR. The structure gives rise to potential for a very high virtual cross-linking density. These carbonate-derived polyesters have superior hydrolysis resistance compared to the traditional materials. 

In addition to forming the chains of polyacrylate, the chains are cross-linked. This is a process in which two or more chains are held together by other compounds in a network. Typical cross-linkers for this polymer include di- and tri-acrylate esters. The swelling and elasticity of the polyacrylate polymer depends on the structure of this network and the number of cross-links. The swelling capacity of the polymer decreases with increased cross-link density. After formation, the polyacrylate is dried and formed into microparticles of irregular shape that can be stored for a long time. 

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