physical gels are formed via cooperative intermolecular interactions including hydrogen bonding, metal–organic coordination, van der Waals interactions and
electrostatic interactions. In some cases, the entanglement of long polymer chains at
high concentrations also contributes to the structural stabilization of physical gels.
In addition to the nature of cross-linking, polymer gels can also be classified
according to whether the polymer chains are biological or synthetic molecules.
Biology-oriented polymer gels, also referred to as biogels, are obtained from natural
polymers with animal, plant and algal origins such as protein and polysaccharide.
Examples of these natural polymers include collagen, agarose, carrageenan, starch,
cellulose, pectin, chitosan, cellulose, alginic acid and hyaluronic acid derivatives.
The nature-inspired biogels exhibit advantages in particular, the ability to mimic
biological transport processes [26, 27]. Physically cross-linked biogels can often
undergo reversible sol–gel transitions in response to thermal or chemical factors.
Synthetic hydrophilic polymers are common components for artificial hydrogels.
Acrylics, including acrylic acid, acrylamide and maleic anhydride polymers and
copolymers, are typical hydrophilic polymers for synthetic hydrogels. Other
examples include amine-functional polymers (such as allylamine, ethyleneimine
and oxazoline) and other polymers containing amine groups in their main chains or
side chains.
5.2.2 Mechanical Properties
Polymer gels are soft materials with a hybrid structure consisting of
three-dimensional polymer networks and a large amount of liquids. As a result of
this unique bi-phase structure, the gels can be treated as solids macroscopically
while as polymer solutions microscopically. Therefore, the chemical reactivity, the
material permeability of liquids and the self-standing ability of solids also exhibit
duality. The mechanical properties of polymer gels are an important aspect to
consider when designing and synthesizing functional gel materials. In order to be
used in real-world applications, polymer gels are required to be mechanically
robust. The commonly encountered parameters used to characterize the mechanical
robustness of polymer gels include Young’s modulus [28], tensile or compressive
fracture [29], fracture energy [30] and viscoelastic properties [31].
5.2.3 Swelling and Shrinking Properties
Swelling is a concept describing the dynamic volume change of polymer gels on
immersion in liquids. Upon exposure to a solvent, a cross-linked polymer network
is commonly observed to swell rather than dissolving completely. Due to the
absorption of the solvent molecules by the network, the total volume of the polymer
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5 Polymer Gels
electrostatic interactions. In some cases, the entanglement of long polymer chains at
high concentrations also contributes to the structural stabilization of physical gels.
In addition to the nature of cross-linking, polymer gels can also be classified
according to whether the polymer chains are biological or synthetic molecules.
Biology-oriented polymer gels, also referred to as biogels, are obtained from natural
polymers with animal, plant and algal origins such as protein and polysaccharide.
Examples of these natural polymers include collagen, agarose, carrageenan, starch,
cellulose, pectin, chitosan, cellulose, alginic acid and hyaluronic acid derivatives.
The nature-inspired biogels exhibit advantages in particular, the ability to mimic
biological transport processes [26, 27]. Physically cross-linked biogels can often
undergo reversible sol–gel transitions in response to thermal or chemical factors.
Synthetic hydrophilic polymers are common components for artificial hydrogels.
Acrylics, including acrylic acid, acrylamide and maleic anhydride polymers and
copolymers, are typical hydrophilic polymers for synthetic hydrogels. Other
examples include amine-functional polymers (such as allylamine, ethyleneimine
and oxazoline) and other polymers containing amine groups in their main chains or
side chains.
5.2.2 Mechanical Properties
Polymer gels are soft materials with a hybrid structure consisting of
three-dimensional polymer networks and a large amount of liquids. As a result of
this unique bi-phase structure, the gels can be treated as solids macroscopically
while as polymer solutions microscopically. Therefore, the chemical reactivity, the
material permeability of liquids and the self-standing ability of solids also exhibit
duality. The mechanical properties of polymer gels are an important aspect to
consider when designing and synthesizing functional gel materials. In order to be
used in real-world applications, polymer gels are required to be mechanically
robust. The commonly encountered parameters used to characterize the mechanical
robustness of polymer gels include Young’s modulus [28], tensile or compressive
fracture [29], fracture energy [30] and viscoelastic properties [31].
5.2.3 Swelling and Shrinking Properties
Swelling is a concept describing the dynamic volume change of polymer gels on
immersion in liquids. Upon exposure to a solvent, a cross-linked polymer network
is commonly observed to swell rather than dissolving completely. Due to the
absorption of the solvent molecules by the network, the total volume of the polymer
158
5 Polymer Gels