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R. G. Weiss
within zones, despite their playing a crucial role in determining the strength, elastic
properties and longevity of the gels. Unfortunately, there does not appear to be a
general means to assess the molecular packing within junction zones at this time,
and it will be difficult to do so in the future because of their low concentrations and
(presumably) greater disorder than within the gelator objects.
Their gel networks are comprised of molecules in which the networks responsible
for providing the viscoelastic properties are linked physically through non-covalent
interactions (usually, ionic, H-bonding and/or molecular dispersion forces). Many of
them can be cycled with their corresponding sol/solution phases by heating above and
cooling below their characteristic gelation temperatures; they are reversible thermally
even if the non-covalent bonds are broken. Of course, there is a critical gelator
concentration, usually defined at ambient temperqature, below which percolation of
the gelator molecules into fibrillar or other objects does not lead to a 3D network and
gelation of the liquid component. Polymer gels, in which the networks responsible for
providing the viscoelastic properties are from monomers held together by covalent
bonds (i.e., linked chemically), will not be discussed except as needed to provide
context for molecular gels. As opposed to many molecular gels (i.e., physical gels),
polymer gels are not reversible thermally with their corresponding sol/solution phases
if the covalent bonds of the polymer network are broken. In fact, Flory included
molecular gels almost as an afterthought, presumably in his fourth class of gels [41]:
(1) well-ordered lamellar structures; (2) cross-linked polymeric networks swollen
with solvent; disordered polymer chains; (3) polymer networks in which the chainchain interactions are physical; (4) particulate disordered structures.
7.4 Making Molecular Gels
As mentioned above, the most common method to make molecular gels is by cooling
their sols/solutions and by heating molecular gels to make sols/solutions. However, in
each case, the shapes and sizes of the objects constituting the gel network depend on
the rate of cooling of the sol phase (Fig. 7.2) [42] and critically on the detailed nature
of solvent-gelator molecular interactions during the aggregation process leading to D
in Fig. 7.3 [43]. The potential sensitivity of the gelator aggregation mode to cooling
rate and liquid composition is clearly displayed by 1.5 wt% 3β-cholesteryl 4-(2anthryloxy)butanoate (CAB) gels (Fig. 7.4) [44, 45]: they exhibit T g values and
fluorescence maxima at ~39 °C and 421 nm, respectively, in hexadecane and ~60 °C
and 427 nm, respectively, in 1-octanol; the hexadecane-like or the 1-octanol-like
gel network can be formed repeatedly and reproducibly when the sol phases are
fast–cooled or slow-cooled and then reheated to the sol phase and re-cooled within
a specific, intermediate range of liquid compositions. Neutron diffraction and X-ray
scattering studies demonstrate that the packing arrangement of the CAB molecules
and the shapes of the constituent fibers are different in the networks prepared by the
different cooling protocols [46].
R. G. Weiss
within zones, despite their playing a crucial role in determining the strength, elastic
properties and longevity of the gels. Unfortunately, there does not appear to be a
general means to assess the molecular packing within junction zones at this time,
and it will be difficult to do so in the future because of their low concentrations and
(presumably) greater disorder than within the gelator objects.
Their gel networks are comprised of molecules in which the networks responsible
for providing the viscoelastic properties are linked physically through non-covalent
interactions (usually, ionic, H-bonding and/or molecular dispersion forces). Many of
them can be cycled with their corresponding sol/solution phases by heating above and
cooling below their characteristic gelation temperatures; they are reversible thermally
even if the non-covalent bonds are broken. Of course, there is a critical gelator
concentration, usually defined at ambient temperqature, below which percolation of
the gelator molecules into fibrillar or other objects does not lead to a 3D network and
gelation of the liquid component. Polymer gels, in which the networks responsible for
providing the viscoelastic properties are from monomers held together by covalent
bonds (i.e., linked chemically), will not be discussed except as needed to provide
context for molecular gels. As opposed to many molecular gels (i.e., physical gels),
polymer gels are not reversible thermally with their corresponding sol/solution phases
if the covalent bonds of the polymer network are broken. In fact, Flory included
molecular gels almost as an afterthought, presumably in his fourth class of gels [41]:
(1) well-ordered lamellar structures; (2) cross-linked polymeric networks swollen
with solvent; disordered polymer chains; (3) polymer networks in which the chainchain interactions are physical; (4) particulate disordered structures.
7.4 Making Molecular Gels
As mentioned above, the most common method to make molecular gels is by cooling
their sols/solutions and by heating molecular gels to make sols/solutions. However, in
each case, the shapes and sizes of the objects constituting the gel network depend on
the rate of cooling of the sol phase (Fig. 7.2) [42] and critically on the detailed nature
of solvent-gelator molecular interactions during the aggregation process leading to D
in Fig. 7.3 [43]. The potential sensitivity of the gelator aggregation mode to cooling
rate and liquid composition is clearly displayed by 1.5 wt% 3β-cholesteryl 4-(2anthryloxy)butanoate (CAB) gels (Fig. 7.4) [44, 45]: they exhibit T g values and
fluorescence maxima at ~39 °C and 421 nm, respectively, in hexadecane and ~60 °C
and 427 nm, respectively, in 1-octanol; the hexadecane-like or the 1-octanol-like
gel network can be formed repeatedly and reproducibly when the sol phases are
fast–cooled or slow-cooled and then reheated to the sol phase and re-cooled within
a specific, intermediate range of liquid compositions. Neutron diffraction and X-ray
scattering studies demonstrate that the packing arrangement of the CAB molecules
and the shapes of the constituent fibers are different in the networks prepared by the
different cooling protocols [46].