356
sample is mostly amorphous, and cross-linking is not homogeneous. The amorphous character is caused by the loss of cyclodextrin crystallinity during the crosslinking reaction. The structure is heterogeneous and presents different regions with
different mobility properties. For the first time, NMR studies have shown cyclodextrin gels are composed of a relatively dense, rigid, and hydrophobic cross-linked
core and a more hydrophilic surface, less cross-linked containing long and highly
mobile hydroxyalkylated polymer chains through the homopolymerization of the
cross-linking agent (Crini et al. 1998a). Two years later, these conclusions were
confirmed by HRMAS experiments (Crini et al. 2000). In 2012, Wilson’s group
reported similar interpretations using NMR experiments (Mohamed et al. 2012).
8.2.3 Swelling Properties of Cyclodextrin-Epichlorohydrin
Polymers
Various types of materials can be obtained with physical textures and mechanical
properties that can be varied by giving different shapes, such as gels/hydrogels or
“small balls” (beads, resins). At the end of the 1960s, Professor Wiedenhof was the
first to demonstrate that materials can easily be prepared as irregularly shaped particles or regular “balls” and that they had a remarkably high swelling capacity in
water, depending on the conditions of synthesis, especially the degree of crosslinking. Under particular synthesis conditions, such as heterogeneous two-phase
synthesis in the presence of a blowing agent, it is possible to obtain a well-defined
spherical size and shape and a uniform and controlled distribution (Bertini et al.
1999; Vecchi et al. 1998). Other forms of such sponges or foams insoluble in water
and in many other solvents can also be obtained, depending on the intended application (Crini and Morcellet 2002; Crini 2005a).
Among the most studied materials are gel polymers that can swell in water and
absorb up to several times their weight. They simultaneously have properties characteristic of both liquids and solids. Their swelling properties become useful for the
complexation of contaminants because they promote diffusion processes in the
polymer network (Crini et al. 1998a). The macromolecular network also has a structure that is mainly amorphous with very few or complete absence of crystalline zone
(Crini et al. 1998b, 2000; Vecchi et al. 1998). This amorphous character represents
an additional advantage in wastewater treatment as it favors adsorption processes
(Crini 2005a). Indeed, it is also important to note that the flexibility of the molecular
chains makes them easily entangled with each other, resulting in a non-porous structure with a very low specific surface area (Crini and Morcellet 2002; Crini 2005a).
Professor Szejtli was the first to study in detail the precise role of the solvent (water,
organic solvents, or a mixture of both) in the formation of non-porous or porous gels
and beads (Szejtli 1982). Since then, all highly porous cyclodextrin polymers have
been synthetized in organic phase using customized cross-linkers, including epichlorohydrin (Morin-Crini et al. 2013, 2018a). Literature methods to produce
G. Crini
sample is mostly amorphous, and cross-linking is not homogeneous. The amorphous character is caused by the loss of cyclodextrin crystallinity during the crosslinking reaction. The structure is heterogeneous and presents different regions with
different mobility properties. For the first time, NMR studies have shown cyclodextrin gels are composed of a relatively dense, rigid, and hydrophobic cross-linked
core and a more hydrophilic surface, less cross-linked containing long and highly
mobile hydroxyalkylated polymer chains through the homopolymerization of the
cross-linking agent (Crini et al. 1998a). Two years later, these conclusions were
confirmed by HRMAS experiments (Crini et al. 2000). In 2012, Wilson’s group
reported similar interpretations using NMR experiments (Mohamed et al. 2012).
8.2.3 Swelling Properties of Cyclodextrin-Epichlorohydrin
Polymers
Various types of materials can be obtained with physical textures and mechanical
properties that can be varied by giving different shapes, such as gels/hydrogels or
“small balls” (beads, resins). At the end of the 1960s, Professor Wiedenhof was the
first to demonstrate that materials can easily be prepared as irregularly shaped particles or regular “balls” and that they had a remarkably high swelling capacity in
water, depending on the conditions of synthesis, especially the degree of crosslinking. Under particular synthesis conditions, such as heterogeneous two-phase
synthesis in the presence of a blowing agent, it is possible to obtain a well-defined
spherical size and shape and a uniform and controlled distribution (Bertini et al.
1999; Vecchi et al. 1998). Other forms of such sponges or foams insoluble in water
and in many other solvents can also be obtained, depending on the intended application (Crini and Morcellet 2002; Crini 2005a).
Among the most studied materials are gel polymers that can swell in water and
absorb up to several times their weight. They simultaneously have properties characteristic of both liquids and solids. Their swelling properties become useful for the
complexation of contaminants because they promote diffusion processes in the
polymer network (Crini et al. 1998a). The macromolecular network also has a structure that is mainly amorphous with very few or complete absence of crystalline zone
(Crini et al. 1998b, 2000; Vecchi et al. 1998). This amorphous character represents
an additional advantage in wastewater treatment as it favors adsorption processes
(Crini 2005a). Indeed, it is also important to note that the flexibility of the molecular
chains makes them easily entangled with each other, resulting in a non-porous structure with a very low specific surface area (Crini and Morcellet 2002; Crini 2005a).
Professor Szejtli was the first to study in detail the precise role of the solvent (water,
organic solvents, or a mixture of both) in the formation of non-porous or porous gels
and beads (Szejtli 1982). Since then, all highly porous cyclodextrin polymers have
been synthetized in organic phase using customized cross-linkers, including epichlorohydrin (Morin-Crini et al. 2013, 2018a). Literature methods to produce
G. Crini