spectroscopy (FT-IR) technique. By varying the molecular ratio between PEI and
PEA-(COOH) 2 , homogeneous hybrid gels could be formed. The driving force
consists of two competitive interactions including attractive hydrogen bonding and
repulsive interactions induced by phase separation. The gel preparation procedure
was further extended to other similar molecular systems. When replacing PEA(COOH) 2 with commercial carboxyl-terminated telechelic polydimethylsiloxane
(PDMS-(COOH) 2 ), polymer gels could also be formed at a high molecular ratio of
PEI.
5.3.2 Metal–Organic Coordination Polymer Gels
Based on metal–organic coordination interactions, multidimensional polymer networks may be formed to yield gel phases. The binding strength of metal–organic
coordinated linkages ranges from strong bonds to weak interactions. The polymer
gels coordinated by weak metal–organic interactions can exhibit reversible physical
properties and demonstrate an assembly–disassembly equilibrium in response to
environmental stimuli including heat, shaking and sonication [45]. The dynamic
feature of weak coordination gel systems enables these materials to have potential
applications in drug delivery, chemical catalysis, fluorescence and sensing. The
structures and functionalities of polymer metallogels can be finely adjusted by
judicious choice of the metal ions, rational design of the binding sites and conformation of polymeric organic ligands [6].
Metal–ligand coordination has been found to play a crucial role in controlling
the adhesion, self-assembly, toughness and hardness of biological materials [46].
Inspired by the structure and property of the natural products, biomimetic polymer
metallogel systems have been explored by materials scientists. Harrington and
co-workers elucidated the peculiar catecholato-iron metallopolymeric structures as
underlying mechanical supports for the load-bearing network in byssus cuticles
[47]. On this basis of this work, Holten-Andersen and co-workers developed a
pH-responsive polymer gel cross-linked by the coordination between catechol
derivatives and Fe
3+ ions [48]. Figure 5.7a shows the pH-dependent formation of
catechol-Fe
3+ coordinated complexes with varying stoichiometry. At basic pH
(pH ˃ 7), the deprotonation of the catechol hydroxyl is favoured; however, the
solubility of Fe
3+ is decreased. In order to solve this problem, mussels offer a good
solution to prevent Fe
3+ from precipitating under basic conditions. As presented in
Fig. 5.7b, Fe
3+ was first bounded to mono-PEG-dopa 4 under acidic conditions
(pH
5). When increasing pH to approximately 8 by releasing in seawater, the
cuticle materials cross-linked to multi-Fe
3+ -dopa coordinated networks. In experiments, a dopa-functionalized polyethylene glycol polymer (PEG-dopa 4 ) was used
as a gelator (Fig. 5.7c). By adopting the mussel-inspired preparation scheme, the
cross-linking between PEG-dopa 4 and Fe
3+ could be realized by increasing pH
while maintaining Fe
3+ solubility (Fig. 5.7d). The formation of gel phases started
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
165
PEA-(COOH) 2 , homogeneous hybrid gels could be formed. The driving force
consists of two competitive interactions including attractive hydrogen bonding and
repulsive interactions induced by phase separation. The gel preparation procedure
was further extended to other similar molecular systems. When replacing PEA(COOH) 2 with commercial carboxyl-terminated telechelic polydimethylsiloxane
(PDMS-(COOH) 2 ), polymer gels could also be formed at a high molecular ratio of
PEI.
5.3.2 Metal–Organic Coordination Polymer Gels
Based on metal–organic coordination interactions, multidimensional polymer networks may be formed to yield gel phases. The binding strength of metal–organic
coordinated linkages ranges from strong bonds to weak interactions. The polymer
gels coordinated by weak metal–organic interactions can exhibit reversible physical
properties and demonstrate an assembly–disassembly equilibrium in response to
environmental stimuli including heat, shaking and sonication [45]. The dynamic
feature of weak coordination gel systems enables these materials to have potential
applications in drug delivery, chemical catalysis, fluorescence and sensing. The
structures and functionalities of polymer metallogels can be finely adjusted by
judicious choice of the metal ions, rational design of the binding sites and conformation of polymeric organic ligands [6].
Metal–ligand coordination has been found to play a crucial role in controlling
the adhesion, self-assembly, toughness and hardness of biological materials [46].
Inspired by the structure and property of the natural products, biomimetic polymer
metallogel systems have been explored by materials scientists. Harrington and
co-workers elucidated the peculiar catecholato-iron metallopolymeric structures as
underlying mechanical supports for the load-bearing network in byssus cuticles
[47]. On this basis of this work, Holten-Andersen and co-workers developed a
pH-responsive polymer gel cross-linked by the coordination between catechol
derivatives and Fe
3+ ions [48]. Figure 5.7a shows the pH-dependent formation of
catechol-Fe
3+ coordinated complexes with varying stoichiometry. At basic pH
(pH ˃ 7), the deprotonation of the catechol hydroxyl is favoured; however, the
solubility of Fe
3+ is decreased. In order to solve this problem, mussels offer a good
solution to prevent Fe
3+ from precipitating under basic conditions. As presented in
Fig. 5.7b, Fe
3+ was first bounded to mono-PEG-dopa 4 under acidic conditions
(pH
5). When increasing pH to approximately 8 by releasing in seawater, the
cuticle materials cross-linked to multi-Fe
3+ -dopa coordinated networks. In experiments, a dopa-functionalized polyethylene glycol polymer (PEG-dopa 4 ) was used
as a gelator (Fig. 5.7c). By adopting the mussel-inspired preparation scheme, the
cross-linking between PEG-dopa 4 and Fe
3+ could be realized by increasing pH
while maintaining Fe
3+ solubility (Fig. 5.7d). The formation of gel phases started
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
165