Cation/crosslink interactions

The mobility of ions in polymeric systems, particularly those that contain multivalent cations, is mainly based on breaking and re-forming ion-chain bonds. For example, in PEO, as in other aprotic solvents, cations are more strongly solvated than anions. In the transitions between different conformational states coupled with chain mobility, ether oxygen-cation bonds will be broken and restored again to produce a new chain conformation. Cation motion and segmental polymer motion are closely coupled in this manner. Cation-chain interaction can involve single polymer chains and also create pseudo-crosslinks between chains. 

Interaction with plurivalent cations via ligand exchange mechanism is one more rather widely applied crosslinking technique. The network bonds of ionic or donor-acceptor nature are located, with respect to lifetime, between the truly covalent crosslinks and physical entanglements. Generally speaking, gelation in these systems is reversible. 

In 1956 Brown, in a series of patents(68-75), disclosed that clays could be treated with di-, tri-, or tetra-substituted ammonia derivatives. Later, McLaughlin, et al.(76,77), introduced cationic polymers as permanent clay protective chemicals. A series of published results describing laboratory and field applications soon became available(78-81). Structural details of the cationic polymers appeared in patents(82-85). In general the polymers are polyamine derivatives, mostly quaternary in nature. Theng(86,87) has discussed how the multiple cationic centers in these polymers can interact and permanently protect clays. Callaway(88) et al. has noted that cationic polymers may interfere with the performance of crosslinked fracturing fluids. 

Figure 28.4 Formaldehyde can be used to capture protein interactions if it is used at low concentrations. The reaction proceeds through modification of a protein to create an intermediate immonium cation, which then goes on to react with a neighboring protein to form the crosslinked product via secondary amine bonds.

Hofmann et al.361 have obtained evidence that glycolaldehyde can interact with e-amino groups to crosslink proteins with generation of colour due to pyrazinium radical cations (cf. CROSSPY 84) under physiological conditions. This has direct bearing on, for example, cataract formation. 

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