1.1 “Gels” in General, and “Cryogels” in Particular
There are many definitions of what the gels are [2–6], and some of these definitions
are rather complicated because they include numerous significant properties of the
polymeric systems classified as gels. However, at a qualitative level one can say
that gels are systems consisting of polymer and immobilized solvent in which
macromolecular chains are connected to each other via chemical or physical
bonds, forming a 3D network. The definition can be made simpler still: gels are
swollen spatial polymeric networks. If the immobilized solvent is water, then we
deal with hydrogels; if the solvent is an organic liquid, then organogels are
considered. Certainly, various intermediate variants are also possible when the
solvents are water–organic mixtures. Depending on the nature of the interchain
bonds in the nodes of the polymeric 3D network, gels are commonly classified as
follows: covalent (i.e., chemically crosslinked) gels; ionically or ion-chelately
crosslinked gels; and noncovalent or physical gels. “Mixed” variants also exist.
The gel’s bulk morphology (homophase or heterophase) is determined by the
chemical structure of the constituent polymers and by the method of gel preparation. In this context, cryogels are gel systems whose formation occurs in moderately
frozen solutions or colloidal dispersions of precursors potentially capable of gelling
[7, 8]. Therefore, the occurrence of gel formation in the frozen precursor-containing
system is the necessary specific feature that distinguishes cryotropic gelation from
gel formation at temperatures above the freezing point of the feed [9].
1.2 “Cryotropic Gelation”
The word combination “cryotropic gelation” connects directly with the proper
processes resulting in the formation of diverse cryogels. The term’s construction
is similar to that of the terms “chemotropic” (caused by chemical reactions),
“ionotropic” (caused by ionic bonding), and thermotropic gel formation (caused
by heating, as in the gelation of aqueous solutions of methylcellulose upon warming
above the lower critical solution temperature, LCST). Hence, cryotropic gelation,
derived from the Greek kryos (frost) and tropos (cause), is gel formation caused by
the cryogenic treatment (freezing–frozen storage–thawing) of the precursor system.
The solvent crystallization in this process acts as a trigger enabling the subsequent
gelation phenomena.
1.3 “Positive” and “Negative” Temperature
These terms are defined as, respectively, the temperatures above and below the
freezing/melting point of the initially liquid system (see [1]). In addition, in the
course of further discussions on nonaqueous systems we will also operate, when
Basic Principles of Cryotropic Gelation
51
There are many definitions of what the gels are [2–6], and some of these definitions
are rather complicated because they include numerous significant properties of the
polymeric systems classified as gels. However, at a qualitative level one can say
that gels are systems consisting of polymer and immobilized solvent in which
macromolecular chains are connected to each other via chemical or physical
bonds, forming a 3D network. The definition can be made simpler still: gels are
swollen spatial polymeric networks. If the immobilized solvent is water, then we
deal with hydrogels; if the solvent is an organic liquid, then organogels are
considered. Certainly, various intermediate variants are also possible when the
solvents are water–organic mixtures. Depending on the nature of the interchain
bonds in the nodes of the polymeric 3D network, gels are commonly classified as
follows: covalent (i.e., chemically crosslinked) gels; ionically or ion-chelately
crosslinked gels; and noncovalent or physical gels. “Mixed” variants also exist.
The gel’s bulk morphology (homophase or heterophase) is determined by the
chemical structure of the constituent polymers and by the method of gel preparation. In this context, cryogels are gel systems whose formation occurs in moderately
frozen solutions or colloidal dispersions of precursors potentially capable of gelling
[7, 8]. Therefore, the occurrence of gel formation in the frozen precursor-containing
system is the necessary specific feature that distinguishes cryotropic gelation from
gel formation at temperatures above the freezing point of the feed [9].
1.2 “Cryotropic Gelation”
The word combination “cryotropic gelation” connects directly with the proper
processes resulting in the formation of diverse cryogels. The term’s construction
is similar to that of the terms “chemotropic” (caused by chemical reactions),
“ionotropic” (caused by ionic bonding), and thermotropic gel formation (caused
by heating, as in the gelation of aqueous solutions of methylcellulose upon warming
above the lower critical solution temperature, LCST). Hence, cryotropic gelation,
derived from the Greek kryos (frost) and tropos (cause), is gel formation caused by
the cryogenic treatment (freezing–frozen storage–thawing) of the precursor system.
The solvent crystallization in this process acts as a trigger enabling the subsequent
gelation phenomena.
1.3 “Positive” and “Negative” Temperature
These terms are defined as, respectively, the temperatures above and below the
freezing/melting point of the initially liquid system (see [1]). In addition, in the
course of further discussions on nonaqueous systems we will also operate, when
Basic Principles of Cryotropic Gelation
51