5-fluro-2-formylpyridine, 4,4′-diaminobiphenyl-2,2′-disulfonic acid, tetramethylammonium hydroxide and an aqueous solution of iron(II) sulphate heptahydrate.
The polymer network cross-linked by tetrahedral-shaped MOCs (Fig. 5.10b) has
the ability to absorb water giving rise to the formation of a purple gel (Fig. 5.10c).
Following the gel preparation, guest encapsulation and release studies were conducted to show the different behaviours of molecules within cage cavities and gel
pores. For these investigations, furan was chosen as a competing guest against
benzene and anisole. Results from adsorption experiments monitored by UV-Vis
spectroscopy showed that the cages are only occupied by benzene in the absence of
furan. While in the presence of furan, the benzene within the cages was released and
subsequently replaced by furan partially. Therefore, the cage cavities and gel pores
could be accessed selectively upon exposure to different external stimuli. This study
has paved the way for further investigations on the design of cage-containing
polymer gels for the controllable encapsulation of technically important guest
molecules such as drugs, pesticides and fragrances.
Johnson and co-workers have increased the average number of bridges connecting network junctions by developing highly branched polymer gels linked with
stable MOCs [53]. As shown in Fig. 5.11a, the polyMOCs exhibit tunable
cavity-containing junctions and an enhanced network branch functionality compared
with conventional metal–organic gels where point junctions are generated. Due to
the increase in the network branch functionality, the resulting polymer gels connected with MOCs are expected to show an increase in elastic modulus. Two
polyethylene glycol (PEG) derivatives terminated with bis-para-pyridyl (PL1) and
bis-meta-pyridyl (PL2) groups were designed for the formation of MOC junctions,
respectively (Fig. 5.11b, c). Upon mixing with Pd
2+ ions, the formation of coordinated M 12 L 24 cages and M 2 L 4 paddlewheels could be activated by thermal
annealing. Resulting from the formation of cage junctions, the gelation of two
polyMOC gel species (gel-1 and gel-2 in Fig. 5.11a) could be initiated as illustrated
by
1 H magic-angle spinning (MAS) nuclear magnetic resonance (NMR),
small-angle neutron scattering (SANS) and molecular dynamic simulation techniques. Using oscillatory rheometry, the mechanical properties of gel-1 and gels-2
show close correlation with their network structures. As compared to gel-2, gel-1
based on M 12 L 24 cages has a higher average number of bridges connecting network
junctions and exhibits large shear moduli. The large number of elastically inactive
loop defects within gel-1 could be replaced with functional ligands without altering
the shear modulus of the gels. While not applicable to such ligand substitution, gel-2
demonstrates a self-healing behaviour possibly related with the dynamic feature of
M 2 L 4 junctions.
Thereafter, Johnson and co-workers further extended their efforts to apply rigid
MOCs as junctions for block copolymer (BCP) self-assembly [55]. The integration
of MOCs with BCP self-assembly produces a new class of hybrid materials, namely
block co-polyMOCs (BCPMOCs). Figure 5.12a shows the chemical structure of
the molecular ligand synthesized for the preparation of BCPMOCs, poly(methylmethacrylate)-block-poly(n-butyl acrylate) (PMMA-PBA). As each PMMA-PBA
contains two pyridyl groups at its chain end, these ligands form star polymers
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
169
The polymer network cross-linked by tetrahedral-shaped MOCs (Fig. 5.10b) has
the ability to absorb water giving rise to the formation of a purple gel (Fig. 5.10c).
Following the gel preparation, guest encapsulation and release studies were conducted to show the different behaviours of molecules within cage cavities and gel
pores. For these investigations, furan was chosen as a competing guest against
benzene and anisole. Results from adsorption experiments monitored by UV-Vis
spectroscopy showed that the cages are only occupied by benzene in the absence of
furan. While in the presence of furan, the benzene within the cages was released and
subsequently replaced by furan partially. Therefore, the cage cavities and gel pores
could be accessed selectively upon exposure to different external stimuli. This study
has paved the way for further investigations on the design of cage-containing
polymer gels for the controllable encapsulation of technically important guest
molecules such as drugs, pesticides and fragrances.
Johnson and co-workers have increased the average number of bridges connecting network junctions by developing highly branched polymer gels linked with
stable MOCs [53]. As shown in Fig. 5.11a, the polyMOCs exhibit tunable
cavity-containing junctions and an enhanced network branch functionality compared
with conventional metal–organic gels where point junctions are generated. Due to
the increase in the network branch functionality, the resulting polymer gels connected with MOCs are expected to show an increase in elastic modulus. Two
polyethylene glycol (PEG) derivatives terminated with bis-para-pyridyl (PL1) and
bis-meta-pyridyl (PL2) groups were designed for the formation of MOC junctions,
respectively (Fig. 5.11b, c). Upon mixing with Pd
2+ ions, the formation of coordinated M 12 L 24 cages and M 2 L 4 paddlewheels could be activated by thermal
annealing. Resulting from the formation of cage junctions, the gelation of two
polyMOC gel species (gel-1 and gel-2 in Fig. 5.11a) could be initiated as illustrated
by
1 H magic-angle spinning (MAS) nuclear magnetic resonance (NMR),
small-angle neutron scattering (SANS) and molecular dynamic simulation techniques. Using oscillatory rheometry, the mechanical properties of gel-1 and gels-2
show close correlation with their network structures. As compared to gel-2, gel-1
based on M 12 L 24 cages has a higher average number of bridges connecting network
junctions and exhibits large shear moduli. The large number of elastically inactive
loop defects within gel-1 could be replaced with functional ligands without altering
the shear modulus of the gels. While not applicable to such ligand substitution, gel-2
demonstrates a self-healing behaviour possibly related with the dynamic feature of
M 2 L 4 junctions.
Thereafter, Johnson and co-workers further extended their efforts to apply rigid
MOCs as junctions for block copolymer (BCP) self-assembly [55]. The integration
of MOCs with BCP self-assembly produces a new class of hybrid materials, namely
block co-polyMOCs (BCPMOCs). Figure 5.12a shows the chemical structure of
the molecular ligand synthesized for the preparation of BCPMOCs, poly(methylmethacrylate)-block-poly(n-butyl acrylate) (PMMA-PBA). As each PMMA-PBA
contains two pyridyl groups at its chain end, these ligands form star polymers
5.3 Nature of Cross-Linking Leading to the Formation of Polymer Gels
169