Reversible and irreversible biopolymer gels - Structure and mechanical properties

The majority of synthetic polymer gels are formed by the covalent cross-lin king of linear or branched macromolecules using multi-functional cross-link ing agents. Such gels are networks, or true macromolecules with (nominally) infinite molecular weight and consequently they swell rather than dissolve if immersed in a good solvent. However, there are also a whole class of ma terials called physical gels where non-covalent crosslinks occur. These sho w similarities to covalent networks, but because the cross-links are not pe rmanent, they will show creep behaviour at very long times. Some of these f orm on cooling a heated solution, such as gelatin gels, while others form o nly on heating. Also, some gels are thermoreversible, while others are not. Physical gels can be formed from synthetic or biopolymers. In the latter ca se non-covalent cross-links often comprise more specific and complex mechan isms involving, rather than point-like cross-links, "junction zones" of kno wn, ordered secondary structure such as multiple helices. Typically there i s a specific, and often intricate, hierarchy of arrangements, which are mor e Familiar to molecular biologists than to polymer physical chemists. In th is paper we introduce viscoelastic techniques for characteristing physical gels, and then relate the properties to the underlying structure at the mac romolecular and junction zone level. The parallels between synthetic and bi opolymer gels will also be illustrated. Finally we describe recent work on the heat-set gelation of globular proteins, and an attempt to relate the im portant parameter of the gelation time, t(c). to both polymer concentration and temperature.