Cross Linkages:

The second variation which can be introduced into the copolymer bead is that of cross linkage. As mentioned earlier, the cross linkage in a styrene- divinylbenzene polymer refers to the fraction of divinylbenzene content. Thus, a resin of 8% crosslinkage is made with beads containing 8% divinylbenzene and 2% styrene and other monovinyl monomers.

The cross linkage affects the resin in two ways. As the amount of cross linkage increases, the dry weight capacity decreases. That decreased capacity give output from the greater difficulty of substituting active groups on the copolymers probably due to steric factors. Therefore, as compared to this, the change in water content is more pronounced. Thus, as the cross linkage increases, the resin has a - swollen volume for essentially the same number of sites and the wet volume capacity increases.

There are other properties which are affected by the degree of cross linkage. With the decrease in the cross linkage, a resin swells more and therefore, the diffusion of ions within the resin becomes faster. That, in turn, provides faster equilibrium rate particularly, for large ions. Instead, if the cross linkage is increased; the diffusion paths may become small enough for the entrance of large ions. This offers a possibility of separation of ions based on ionic sizes. A classical example is the separation of sulphate from high molecular weight sulphonic acid by using highly cross linked anion exchange resin. In the similar light, we could say that if the cross linking is decreased; the permeable selectivity difference is also decreased.

Cross linkage affects the physical properties also. Highly crosslinked resin is brittle. On the other hand, low cross linked resins are highly swollen; therefore, soft and easily deformed.