38
boiling water. After this, these hydrogels were immersed in different solvents in
order to fix the temporary shape during the cooling process, thus obtaining thermosensitive hydrogels. Skrzeszewska et al. (2011) also found that the chemically
crosslinked telechelic peptide hydrogel showed excellent thermo-sensitive shape
memory performance, as a result, the hydrogel could be stretched 2 times its initial
length by increasing temperature. After cooling, the collagen-like segments at different molecular chain ends were combined by winding to form a triple screwrotating node. Thus, the hydrogel sample could fix the shape temporarily, which
could be maintained for a long time at room temperature, as well as the initial shape
could be recovered in a few minutes at a temperature of 50 °C or more. However,
the mechanical properties of this material were so poor that the E values varied
between 10
2
–10
4
Pa, which greatly limited its potential application. In order to overcome this disadvantage, the Osada’s group. prepared a chemically crosslinked
copolymer of acrylic acid (AAc) and octadecyl acrylate (OA) by solution polymerization using ethanol as solvent (Osada 1995; Matsuda et al. 1994; Miyazaki et al.
2002). These authors reported that the copolymer gel showed excellent thermosensitive shape memory performance, due to the ordered-disorder transition of the
microdomain formed by the hydrophobic octadecyl side chain of the copolymer,
resulting in a E value of 1.7 × l0
7
Pa at 30 °C, and dropped to 2.2 × 10
5
Pa when
heated above 49 °C.
Bilici and Okay (2013) took advantage of sodium dodecyl sulfate (SDS) as a
surfactant to be used in the development of micelles in aqueous solution to build a
chemically crosslinked hydrogel from a OA-co-AA copolymer through polymerization. Thereby, the resulting thermo-sensitive hydrogel was positively affected in
terms of their E values.
According to Candau and Selb (1999), the water solubility of the OA monomer
is so poor, that the direct obtaining of the copolymer SA-co-AA in aqueous solution
is difficult. For this reason, a certain amount of sodium chloride (NaCl) plus the
surfactant could dissolve the hydrophobic monomer (OA) as much as possible.
These authors reported that spherical micelles were formed once the NaCl concentration in aqueous solution exceeded the critical micelle concentration (CMC). As a
result, interactions between the Na
+
cations and the negatively charged external sulfate groups of the micelles were given, thus allowing to obtain worm-like capsules,
which could tailor the mechanical strength and thermosensitivity of the hydrogel to
a certain extent.
3.4.1.2 Light-Induced SMGs
As a clean and controllable environmental stimulus, illumination is indispensable for
the development and application of new smart polymers. In recent years, scientists’
attention has focused on light-induced SMPs. This type of polymer has mainly phoZ. Gao and G. Gao
boiling water. After this, these hydrogels were immersed in different solvents in
order to fix the temporary shape during the cooling process, thus obtaining thermosensitive hydrogels. Skrzeszewska et al. (2011) also found that the chemically
crosslinked telechelic peptide hydrogel showed excellent thermo-sensitive shape
memory performance, as a result, the hydrogel could be stretched 2 times its initial
length by increasing temperature. After cooling, the collagen-like segments at different molecular chain ends were combined by winding to form a triple screwrotating node. Thus, the hydrogel sample could fix the shape temporarily, which
could be maintained for a long time at room temperature, as well as the initial shape
could be recovered in a few minutes at a temperature of 50 °C or more. However,
the mechanical properties of this material were so poor that the E values varied
between 10
2
–10
4
Pa, which greatly limited its potential application. In order to overcome this disadvantage, the Osada’s group. prepared a chemically crosslinked
copolymer of acrylic acid (AAc) and octadecyl acrylate (OA) by solution polymerization using ethanol as solvent (Osada 1995; Matsuda et al. 1994; Miyazaki et al.
2002). These authors reported that the copolymer gel showed excellent thermosensitive shape memory performance, due to the ordered-disorder transition of the
microdomain formed by the hydrophobic octadecyl side chain of the copolymer,
resulting in a E value of 1.7 × l0
7
Pa at 30 °C, and dropped to 2.2 × 10
5
Pa when
heated above 49 °C.
Bilici and Okay (2013) took advantage of sodium dodecyl sulfate (SDS) as a
surfactant to be used in the development of micelles in aqueous solution to build a
chemically crosslinked hydrogel from a OA-co-AA copolymer through polymerization. Thereby, the resulting thermo-sensitive hydrogel was positively affected in
terms of their E values.
According to Candau and Selb (1999), the water solubility of the OA monomer
is so poor, that the direct obtaining of the copolymer SA-co-AA in aqueous solution
is difficult. For this reason, a certain amount of sodium chloride (NaCl) plus the
surfactant could dissolve the hydrophobic monomer (OA) as much as possible.
These authors reported that spherical micelles were formed once the NaCl concentration in aqueous solution exceeded the critical micelle concentration (CMC). As a
result, interactions between the Na
+
cations and the negatively charged external sulfate groups of the micelles were given, thus allowing to obtain worm-like capsules,
which could tailor the mechanical strength and thermosensitivity of the hydrogel to
a certain extent.
3.4.1.2 Light-Induced SMGs
As a clean and controllable environmental stimulus, illumination is indispensable for
the development and application of new smart polymers. In recent years, scientists’
attention has focused on light-induced SMPs. This type of polymer has mainly phoZ. Gao and G. Gao