To program the properties of polymer gels, several factors can be tuned including
polymer backbones, liquid species, polymer–liquid interactions, addition of extra
additives and liquid loading.
Most of the gels we come into contact in everyday life or apply in high-tech
areas are made of polymers, such as materials for uses in contact lenses [1], robotics
[2], tissue surrogates [3], sensing [4] and battlefield health care [5]. According to
the nature of swelling agent in liquid phases, polymer gels can also be broadly
categorized into two sub-classes: hydrogels and organogels [6]. Hydrogels are made
of three-dimensional cross-linked polymer chains, which absorb water rather than
dispersing or dissolving. For organogels, the swelling agents for polymer networks
are organic solutions. Commonly, the liquid phases of gels are made of more than
95% (by weight) of water content or organic solutions [7–9]. In the following part
of this section, several key aspects of the formation of polymer gels are discussed in
detail.
5.1.1 Cross-Linking of Polymer Chains
Cross-linking between polymer chains or between segments of the same chain plays
a pivotal role in determining the formation of polymer gels. Figure 5.1 is a schematic illustration showing the interchain cross-linking and entanglements between
polymer chains. In Fig. 5.1a, the entanglement of uncross-linked polymer chains is
presented. The good affinity of the polymer chains with organic solvents or water
often results in the formation of a uniform solution. Microscopically, the solution is
composed of polymer chains, each one of which is surrounded by organic solvent
or water molecules. The viscosity of the polymer solution is usually significantly
larger than that of the solvent. If the polymer chains or different segments within the
same chain are cross-linked via varying interactions, a strongly linked molecular
assembly is established (Fig. 5.1b). The bonding strength of the cross-linkers
ranges from strong covalent bonds to weak non-covalent interactions such as
Fig. 5.1 Schematic illustrations of a uncross-linked polymer chains, b cross-linked polymer
networks. The blue knots correspond to cross-linkers between polymer gelators
154
5 Polymer Gels
polymer backbones, liquid species, polymer–liquid interactions, addition of extra
additives and liquid loading.
Most of the gels we come into contact in everyday life or apply in high-tech
areas are made of polymers, such as materials for uses in contact lenses [1], robotics
[2], tissue surrogates [3], sensing [4] and battlefield health care [5]. According to
the nature of swelling agent in liquid phases, polymer gels can also be broadly
categorized into two sub-classes: hydrogels and organogels [6]. Hydrogels are made
of three-dimensional cross-linked polymer chains, which absorb water rather than
dispersing or dissolving. For organogels, the swelling agents for polymer networks
are organic solutions. Commonly, the liquid phases of gels are made of more than
95% (by weight) of water content or organic solutions [7–9]. In the following part
of this section, several key aspects of the formation of polymer gels are discussed in
detail.
5.1.1 Cross-Linking of Polymer Chains
Cross-linking between polymer chains or between segments of the same chain plays
a pivotal role in determining the formation of polymer gels. Figure 5.1 is a schematic illustration showing the interchain cross-linking and entanglements between
polymer chains. In Fig. 5.1a, the entanglement of uncross-linked polymer chains is
presented. The good affinity of the polymer chains with organic solvents or water
often results in the formation of a uniform solution. Microscopically, the solution is
composed of polymer chains, each one of which is surrounded by organic solvent
or water molecules. The viscosity of the polymer solution is usually significantly
larger than that of the solvent. If the polymer chains or different segments within the
same chain are cross-linked via varying interactions, a strongly linked molecular
assembly is established (Fig. 5.1b). The bonding strength of the cross-linkers
ranges from strong covalent bonds to weak non-covalent interactions such as
Fig. 5.1 Schematic illustrations of a uncross-linked polymer chains, b cross-linked polymer
networks. The blue knots correspond to cross-linkers between polymer gelators
154
5 Polymer Gels