case of hydrophilic gels, called hydrogels, an ionic comonomer is also included in
the monomer mixture to increase the swelling capacity in aqueous environment.
The desired property of gels, such as the swelling capacity, the modulus of
elasticity, and the degree of heterogeneity, is obtained by adjusting the concentration as well as the composition of the reaction constituents [1]. Ionic hydrogels
swell up to 1,000 times their dry volume when in contact with water. They also
exhibit drastic volume changes in response to specific external stimuli such as
temperature, solvent quality, pH, electric field, etc [2]. Depending on the design of
the hydrogel matrices, this volume change may occur continuously over a range of
stimulus level, or discontinuously at a critical stimulus level. These properties of
hydrogels and their similarities to biological systems have received considerable
interest over the last three decades. Today, these soft and smart materials belong to
the most important class of functional polymers in modern biotechnology [3]. They
are useful materials for drug delivery systems, artificial organs, separation operations in biotechnology, processing of agricultural products, on–off switches, sensors, and actuators [1, 4].
Despite this fact and considerable research in this field, the design and control of
gel-based devices still present some problems because a number of network properties are inversely coupled. For example, decreasing the degree of crosslinking of
gels in order to increase their mesh size results in their accelerated degradation.
Further, loosely crosslinked gels are fragile materials when handled in the swollen
state; typically, they rupture at very low strains due to the lack of an efficient energy
dissipation mechanism in the gel network. Moreover, the response rate of gels
to external stimuli is not as fast as required in many application areas. This is due to
the fact that the kinetics of the gel volume change involves absorbing or desorbing
solvent by the polymer network, which is a diffusive process. This process is slow,
and even slower near the critical point [5]. Hence, design of gels with a good
mechanical performance together with a fast response rate is crucially important in
many existing and potential application areas of soft materials.
A number of techniques for toughening of gels have recently been proposed,
including double network gels [6], topological gels [7], gels formed by hydrophobic
associations [8], gels made by mobile crosslinkers such as clay nanoparticles
(nanocomposite hydrogels) [9], and microsphere composite hydrogels
[10]. Although these techniques create energy dissipation mechanisms to slow
crack propagation and, thus, improve the mechanical properties of gels, they exhibit
a slow response rate to external stimuli. In order to achieve rapid changes in the gel
volume, a common strategy is to create an interconnected pore structure inside the
gel network [11]. For a polymer network having an interconnected pore structure,
absorption or desorption of solvent occurs through the pores by convection, which
is much faster than the diffusion process that dominates the nonporous polymer
networks.
1 The basic technique for obtaining polymer gels with a macroporous
1 In the early works, the space between the network chains in a swollen homogeneous gel was
defined as “porosity” or “molecular porosity.” However, this term is clearly misleading because
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O. Okay and V.I. Lozinsky
the monomer mixture to increase the swelling capacity in aqueous environment.
The desired property of gels, such as the swelling capacity, the modulus of
elasticity, and the degree of heterogeneity, is obtained by adjusting the concentration as well as the composition of the reaction constituents [1]. Ionic hydrogels
swell up to 1,000 times their dry volume when in contact with water. They also
exhibit drastic volume changes in response to specific external stimuli such as
temperature, solvent quality, pH, electric field, etc [2]. Depending on the design of
the hydrogel matrices, this volume change may occur continuously over a range of
stimulus level, or discontinuously at a critical stimulus level. These properties of
hydrogels and their similarities to biological systems have received considerable
interest over the last three decades. Today, these soft and smart materials belong to
the most important class of functional polymers in modern biotechnology [3]. They
are useful materials for drug delivery systems, artificial organs, separation operations in biotechnology, processing of agricultural products, on–off switches, sensors, and actuators [1, 4].
Despite this fact and considerable research in this field, the design and control of
gel-based devices still present some problems because a number of network properties are inversely coupled. For example, decreasing the degree of crosslinking of
gels in order to increase their mesh size results in their accelerated degradation.
Further, loosely crosslinked gels are fragile materials when handled in the swollen
state; typically, they rupture at very low strains due to the lack of an efficient energy
dissipation mechanism in the gel network. Moreover, the response rate of gels
to external stimuli is not as fast as required in many application areas. This is due to
the fact that the kinetics of the gel volume change involves absorbing or desorbing
solvent by the polymer network, which is a diffusive process. This process is slow,
and even slower near the critical point [5]. Hence, design of gels with a good
mechanical performance together with a fast response rate is crucially important in
many existing and potential application areas of soft materials.
A number of techniques for toughening of gels have recently been proposed,
including double network gels [6], topological gels [7], gels formed by hydrophobic
associations [8], gels made by mobile crosslinkers such as clay nanoparticles
(nanocomposite hydrogels) [9], and microsphere composite hydrogels
[10]. Although these techniques create energy dissipation mechanisms to slow
crack propagation and, thus, improve the mechanical properties of gels, they exhibit
a slow response rate to external stimuli. In order to achieve rapid changes in the gel
volume, a common strategy is to create an interconnected pore structure inside the
gel network [11]. For a polymer network having an interconnected pore structure,
absorption or desorption of solvent occurs through the pores by convection, which
is much faster than the diffusion process that dominates the nonporous polymer
networks.
1 The basic technique for obtaining polymer gels with a macroporous
1 In the early works, the space between the network chains in a swollen homogeneous gel was
defined as “porosity” or “molecular porosity.” However, this term is clearly misleading because
106
O. Okay and V.I. Lozinsky