interactions with solvent. DMSO is less compatible with PAAm chains and more
compatible with the PPO blocks. Therefore, with increasing DMSO content in the
gels, the F127DA micelles are swollen, whereas the PAAm chains adopt a collapsed
conformation. As a result, the hydrophobic association in the micelles and network is
weakened, resulting in lower tensile strength but larger stretchability. Since the
overall F127DA content remains constant, the global hydrophobic association in
the gels is not significantly changed. Therefore, the total energy dissipation capability remains constant, despite changes in solvent quality.
3 Multi-responsive Hydrogels
Responsive hydrogels that change their volume or shape upon external stimuli have
potential applications in soft actuators [29, 30], artificial muscles [31], drug delivery
[32, 33], shape morphing [6, 34], and shape memory [35] devices. It is convenient to
incorporate functional moieties, including ionic, chargeable [36], host-guest [31],
magnetic [37], thermo-sensitive [38], and light-sensitive groups [39], into the polymer networks, to synthesize multi-responsive hydrogels. In history, however, it
remains a challenge for most conventional responsive hydrogels for practical applications, primarily due to the poor mechanical properties. Some hydrogels even could
hardly survive the volume changes induced by external stimuli.
Recent developments in tough hydrogels have enabled the preparation of a series
of responsive hydrogels with outstanding mechanical properties. Double network
hydrogels [40] and nanocomposite hydrogels [41, 42] containing thermo-responsive
poly(N-isopropylacrylamide) (PNIPAM) chains or segments experience reversible
volume or shape changes, which have been employed for the fabrication of soft
actuators and walkers. Hydrogels with intrinsic supramolecular recognition are
sensitive to redox [43], small molecules [44], or radiation [31] that alters the
supramolecular crosslinks and thus changes the volume or shape of hydrogels.
Polyelectrolyte hydrogels with ionic or chargeable moieties are responsive to external stimuli that change the charge density inside the network or alter the osmotic
pressure balance. To date, it is desired to create novel strategies to synthesize multiresponsive hydrogels with outstanding mechanical properties by using a versatile
method that is applicable for many functional groups.
The self-assembled F127DA micelles are compatible with most hydrophilic
monomers, including ionic monomers. Since F127DA triblock copolymer is a
nonionic macromolecular surfactant, its micelle solution is stable with the presence
of ionic monomers [45]. The F127DA micelles remain their size and shape
when mixed with ionic monomers, initiator, and catalyst. Polyelectrolyte
hydrogels are easily obtained by free radical polymerization. Here, 2-acrylamide2-methylpropanesulfonic acid (AMPS) [46] and (2-(acryloyloxy)ethyl)
trimethylammonium chloride (DMAEA-Q) [45] are separately copolymerized with
AAm and F127DA micelles to produce anionic and cationic hydrogels.
Triblock Copolymer Micelle-Crosslinked Hydrogels
223
compatible with the PPO blocks. Therefore, with increasing DMSO content in the
gels, the F127DA micelles are swollen, whereas the PAAm chains adopt a collapsed
conformation. As a result, the hydrophobic association in the micelles and network is
weakened, resulting in lower tensile strength but larger stretchability. Since the
overall F127DA content remains constant, the global hydrophobic association in
the gels is not significantly changed. Therefore, the total energy dissipation capability remains constant, despite changes in solvent quality.
3 Multi-responsive Hydrogels
Responsive hydrogels that change their volume or shape upon external stimuli have
potential applications in soft actuators [29, 30], artificial muscles [31], drug delivery
[32, 33], shape morphing [6, 34], and shape memory [35] devices. It is convenient to
incorporate functional moieties, including ionic, chargeable [36], host-guest [31],
magnetic [37], thermo-sensitive [38], and light-sensitive groups [39], into the polymer networks, to synthesize multi-responsive hydrogels. In history, however, it
remains a challenge for most conventional responsive hydrogels for practical applications, primarily due to the poor mechanical properties. Some hydrogels even could
hardly survive the volume changes induced by external stimuli.
Recent developments in tough hydrogels have enabled the preparation of a series
of responsive hydrogels with outstanding mechanical properties. Double network
hydrogels [40] and nanocomposite hydrogels [41, 42] containing thermo-responsive
poly(N-isopropylacrylamide) (PNIPAM) chains or segments experience reversible
volume or shape changes, which have been employed for the fabrication of soft
actuators and walkers. Hydrogels with intrinsic supramolecular recognition are
sensitive to redox [43], small molecules [44], or radiation [31] that alters the
supramolecular crosslinks and thus changes the volume or shape of hydrogels.
Polyelectrolyte hydrogels with ionic or chargeable moieties are responsive to external stimuli that change the charge density inside the network or alter the osmotic
pressure balance. To date, it is desired to create novel strategies to synthesize multiresponsive hydrogels with outstanding mechanical properties by using a versatile
method that is applicable for many functional groups.
The self-assembled F127DA micelles are compatible with most hydrophilic
monomers, including ionic monomers. Since F127DA triblock copolymer is a
nonionic macromolecular surfactant, its micelle solution is stable with the presence
of ionic monomers [45]. The F127DA micelles remain their size and shape
when mixed with ionic monomers, initiator, and catalyst. Polyelectrolyte
hydrogels are easily obtained by free radical polymerization. Here, 2-acrylamide2-methylpropanesulfonic acid (AMPS) [46] and (2-(acryloyloxy)ethyl)
trimethylammonium chloride (DMAEA-Q) [45] are separately copolymerized with
AAm and F127DA micelles to produce anionic and cationic hydrogels.
Triblock Copolymer Micelle-Crosslinked Hydrogels
223