to form in 2 h monitored by
1 H NMR technique. The SEM image in Fig. 5.21c
demonstrates the highly cross-linked morphology of the polymerized MUC gel.
Compared with the molecular network prior to polymerization, a smaller diameter of
approximately 25 nm could be achieved for the thinnest fibres within the
three-dimensional network (Fig. 5.21c). The increased degree of cross-linking arising
from the polymerization was explained as the primary reason for the enhanced
macroscopic stability of the MUC gel with covalent linkages.
Further to the radical polymerization between methyl methacrylate moieties,
gelators functionalized with other reactive groups have been designed and implemented. Examples of reaction sites include acrylate [77], conjugated diacetylene units
[78, 79], alkene [80], alkyne and azide [81] and triethoxysilane end groups [82].
Figure 5.22a shows a diacetylene derivative designed and synthesized by Weiss
and co-workers [78]. When the diacetylene units are properly aligned, the derivatives
can undergo 1,4-addition reactions forming a polymerized network (Fig. 5.22b).
Prior to UV irradiation, the gelation performance of the diacetylene derivative was
examined in silicone oil. A milky white gel was observed to form (Sample 1 shown in
Fig. 5.22c). The gel was stabilized by both the hydrogen bonding between the amide
hydrogen and carbonyl groups and the hydrophobic interactions of long alkyl chains.
Upon irradiation in silicone oil at room temperature for 10 min, the colour of the gel
turned from milky white to blue (Sample 2 shown in Fig. 5.22c). The realization
Fig. 5.21 a Chemical structure of a methacrylate derivatives of trans-1,2-bis
(3-methylureido)cyclohexane (MUC), b TEM image of a butyl acetate gel of the compound
before polymerization (scale bar = 1000 nm), c SEM image of a benzene gel of the compound
after irradiation for 1 h (scale bar = 400 nm). Adapted with permission from Ref. [76]. Copyright
1997 American Chemical Society
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
179
1 H NMR technique. The SEM image in Fig. 5.21c
demonstrates the highly cross-linked morphology of the polymerized MUC gel.
Compared with the molecular network prior to polymerization, a smaller diameter of
approximately 25 nm could be achieved for the thinnest fibres within the
three-dimensional network (Fig. 5.21c). The increased degree of cross-linking arising
from the polymerization was explained as the primary reason for the enhanced
macroscopic stability of the MUC gel with covalent linkages.
Further to the radical polymerization between methyl methacrylate moieties,
gelators functionalized with other reactive groups have been designed and implemented. Examples of reaction sites include acrylate [77], conjugated diacetylene units
[78, 79], alkene [80], alkyne and azide [81] and triethoxysilane end groups [82].
Figure 5.22a shows a diacetylene derivative designed and synthesized by Weiss
and co-workers [78]. When the diacetylene units are properly aligned, the derivatives
can undergo 1,4-addition reactions forming a polymerized network (Fig. 5.22b).
Prior to UV irradiation, the gelation performance of the diacetylene derivative was
examined in silicone oil. A milky white gel was observed to form (Sample 1 shown in
Fig. 5.22c). The gel was stabilized by both the hydrogen bonding between the amide
hydrogen and carbonyl groups and the hydrophobic interactions of long alkyl chains.
Upon irradiation in silicone oil at room temperature for 10 min, the colour of the gel
turned from milky white to blue (Sample 2 shown in Fig. 5.22c). The realization
Fig. 5.21 a Chemical structure of a methacrylate derivatives of trans-1,2-bis
(3-methylureido)cyclohexane (MUC), b TEM image of a butyl acetate gel of the compound
before polymerization (scale bar = 1000 nm), c SEM image of a benzene gel of the compound
after irradiation for 1 h (scale bar = 400 nm). Adapted with permission from Ref. [76]. Copyright
1997 American Chemical Society
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
179