Cross-linking naturally occurring

Intracatenary cross-linked macromolecules occur very often in nature. With the enzyme, ribonuclease, for example, the single peptide chain is cross-linked to itself via four disulfide bridges (Figure 2-5). Another protein, insulin, has, on the other hand, two chains, A and B, of differing composition and peptide residue sequence, joined together by a total of two disulfide bridges. 

This chapter has considered the application of several well-known reactions of alkenes to diene polymers. Whilst the basic reactions are generally predictable from a knowledge of alkene reactivity they are influenced by the fact that the double bond is part of a very long chain molecule consisting of double bond-containing repeat units. In particular the tendency to cyclize has been noted, for example in the case of chlorinated natural rubber and with rubber hydrochloride whilst in other cases degradation or cross-linking has occurred. It may be noted here that the ability to produce cyclized natural rubber is a direct consequence of the affinity of a carbonium ion for a double bond when activated in a polymeric environment. 

As the temperature is decreased, free-volume is lost. If the molecular shape or cross-linking prevent crystallisation, then the liquid structure is retained, and free-volume is not all lost immediately (Fig. 22.8c). As with the melt, flow can still occur, though naturally it is more difficult, so the viscosity increases. As the polymer is cooled further, more free volume is lost. There comes a point at which the volume, though sufficient to contain the molecules, is too small to allow them to move and rearrange. All the free volume is gone, and the curve of specific volume flattens out (Fig. 22.8c). This is the glass transition temperature, T . Below this temperature the polymer is a glass. 

The overall objective of this chapter is to review the fundamental issues involved in the transport of macromolecules in hydrophilic media made of synthetic or naturally occurring uncharged polymers with nanometer-scale pore structure when an electric field is applied. The physical and chemical properties and structural features of hydrophilic polymeric materials will be considered first. Although the emphasis will be on classical polymeric gels, discussion of polymeric solutions and nonclassical gels made of, for example, un-cross-linked macromolecular units such as linear polymers and micelles will also be considered in light of recent interest in these materials for a number of applications... 

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