small-angle-X-ray scattering (SAXS), NMR, SEM, TEM, confocal laser scanning
microscopy, circular dichroism (CD) and absorption and fluorescent spectroscopy,
theoretical calculation, the designing and studying new molecular systems get more
guidance that makes gelation more predicted, which are associated with the intrinsic
nature of gelation and the corresponding response processes.
2.4 Chemical Responsive Gels
Compared to the serendipity of physical stimuli, the structures of the gels controlled
by chemical stimuli are more predictable. Through introducing the molecular
design functional groups that interact either by covalent or non-covalent interactions with other molecules present in the medium into the systems, the properties of
the gels may be altered. Generally, on the one hand, the pre-gelators might be
activated by the chemical stimuli to form gels. On the other hand, the gels would be
disassembled by solubilization or precipitation processes after stimuli treatment.
And the stimuli might product irreversible properties or it may be regenerated by
the addition of an antagonist stimuli. According to the type of gelator-stimuli
interaction systems, we mainly focus on the acid–base reaction, ion-binding, redox
reactions, neutral species.
2.4.1 Acid and Base
pH-responsive gelators functionalized with ionizable groups allow the control of
gelation processes by acid–base behaviour. Basically, the sol-to-gel transition can
be tuned by pH changes. Escuder and co-workers reported a hydrogelator 40
derived from isophthalic acid (Scheme 2.7) [123]. The gelator can be solubilized in
basic media and can be formed into a hydrogel at pH = 1 with an addition of HCl
solution (Fig. 2.27). As for basic ionizable gelator, it is soluble in acidic media, and
can be converted into hydrogels exposure to basic conditions like NaOH solution or
ammonia vapours [124]. And the sol–gel transitions are both reversible by the
addition of appropriate antagonist stimulus as for as the concentration of gelator
remains above CGC. In fact, the reversibility of the sol–gel transformation processes also depends on the present of side products like salts in the medium.
Generally, the physical properties can be sensitive to the rate of addition of the
stimulus. Sometimes, the high reproductivity of gel properties is hard to realize,
because the heterogeneous gels particles even precipitates might form if the acidic
stimulus is added too fast due to the kinetics of mixing being slower than the initial
kinetic of gelation. In order to avoid this effect, Adam and co-workers introduce an
approach to control the rate of pH changes by using hydrolysation characteristic of
d-gluconolactone to produce homogeneous hydrogels [125].
2.3 Sonication and Mechanical Stress Responsive Gels
37
microscopy, circular dichroism (CD) and absorption and fluorescent spectroscopy,
theoretical calculation, the designing and studying new molecular systems get more
guidance that makes gelation more predicted, which are associated with the intrinsic
nature of gelation and the corresponding response processes.
2.4 Chemical Responsive Gels
Compared to the serendipity of physical stimuli, the structures of the gels controlled
by chemical stimuli are more predictable. Through introducing the molecular
design functional groups that interact either by covalent or non-covalent interactions with other molecules present in the medium into the systems, the properties of
the gels may be altered. Generally, on the one hand, the pre-gelators might be
activated by the chemical stimuli to form gels. On the other hand, the gels would be
disassembled by solubilization or precipitation processes after stimuli treatment.
And the stimuli might product irreversible properties or it may be regenerated by
the addition of an antagonist stimuli. According to the type of gelator-stimuli
interaction systems, we mainly focus on the acid–base reaction, ion-binding, redox
reactions, neutral species.
2.4.1 Acid and Base
pH-responsive gelators functionalized with ionizable groups allow the control of
gelation processes by acid–base behaviour. Basically, the sol-to-gel transition can
be tuned by pH changes. Escuder and co-workers reported a hydrogelator 40
derived from isophthalic acid (Scheme 2.7) [123]. The gelator can be solubilized in
basic media and can be formed into a hydrogel at pH = 1 with an addition of HCl
solution (Fig. 2.27). As for basic ionizable gelator, it is soluble in acidic media, and
can be converted into hydrogels exposure to basic conditions like NaOH solution or
ammonia vapours [124]. And the sol–gel transitions are both reversible by the
addition of appropriate antagonist stimulus as for as the concentration of gelator
remains above CGC. In fact, the reversibility of the sol–gel transformation processes also depends on the present of side products like salts in the medium.
Generally, the physical properties can be sensitive to the rate of addition of the
stimulus. Sometimes, the high reproductivity of gel properties is hard to realize,
because the heterogeneous gels particles even precipitates might form if the acidic
stimulus is added too fast due to the kinetics of mixing being slower than the initial
kinetic of gelation. In order to avoid this effect, Adam and co-workers introduce an
approach to control the rate of pH changes by using hydrolysation characteristic of
d-gluconolactone to produce homogeneous hydrogels [125].
2.3 Sonication and Mechanical Stress Responsive Gels
37