Examples of physical crosslinks are microcrystalline aggregates, entangled
chains, ionic interactions between charged atoms or groups of atoms, and hydrogen
bonds. Physical gels are also reversible because the physical crosslinks can be
created and removed by appropriate physical stimuli (e.g., pH, contact with a
nonsolvent, etc.), and, typically, temperature. For this reason, they are often
named “thermoreversible.”
In chemical gels, the crosslinks are covalent bonds connecting the arms of the
3D network. As a result of covalent bonding, chemical gels are irreversible.
Whereas chemical gels are generally strong, physical gels may be strong or
weak, depending on the strength of the interactions involved in the crosslinks.
Strong physical gels have strong physical crosslinks between polymer chains that
are effectively permanent for a given set of experimental conditions [15–17, 19]. As
an example, strong physical junctions may consist of glassy or microcrystalline
nodules, triple helices, or coiled-coils as, for instance, in the case of hydrogels
formed from fibrous proteins or biomacromolecules.
Weak physical gels have reversible links formed from temporary weak interactions between chains, of the order of kT. These associations have finite lifetimes,
breaking and reforming continuously. Examples of weak physical bonds are hydrogen bonds, ionic associations, and micellar aggregates of block copolymers.
The outstanding physical properties of gels associated with their fast stimuli
response have noticeably stimulated research for a long time. Polymeric gels have
been developed that are “smart,” i.e., able to quickly respond to different external
stimuli [15, 20]. For instance, Tanaka’s gels have valuable applications because
they can expand and contract up to 1,000 times their original volume in response to
external stimuli. These gels could be used as artificial muscles, set in motion by a
specific electric pulse [21]. More importantly, the polymers in the gels can capture
or expel specific substances as they grow or shrink, so that the gels could be used,
for example, as super-sponges to absorb and immobilize toxic waste, or as molecular filters of various sorts [18, 20–23]. Among the smart gels, there are materials
that imitate proteins by recognizing conditions and responding to their environment. For example, smart gels can be fine-tuned to draw humidity from the air when
it is over a certain level at a given temperature. Other gels can release insulin when
the glucose level in the water-rich phase drops below a given point [15, 18]. Moreover, it is worth remembering that gels represent an important intermediate step in
polymer processing for obtaining highly oriented fibers showing high strength and
high modulus (e.g., ultrahigh-strength polyethylene fibers Dyneema and
Spectra) [17].
The outstanding properties of gels are directly related to the chemical nature of
their constituents, their reciprocal arrangement and interactions, and their mobility
in the space. Structure and mobility in a gel, in turn, are fixed by the preparative
conditions and processing. Although a gel may be assimilated to a single giant
branched macromolecule, the structure of its parts covers different orders of
hierarchical organization on different length scales. Roughly, the hierarchical
apparatus of gels ranges from a few nanometers for the size of knots; to hundreds
of nanometers for the organization of matter between first neighboring knots; to
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
163
chains, ionic interactions between charged atoms or groups of atoms, and hydrogen
bonds. Physical gels are also reversible because the physical crosslinks can be
created and removed by appropriate physical stimuli (e.g., pH, contact with a
nonsolvent, etc.), and, typically, temperature. For this reason, they are often
named “thermoreversible.”
In chemical gels, the crosslinks are covalent bonds connecting the arms of the
3D network. As a result of covalent bonding, chemical gels are irreversible.
Whereas chemical gels are generally strong, physical gels may be strong or
weak, depending on the strength of the interactions involved in the crosslinks.
Strong physical gels have strong physical crosslinks between polymer chains that
are effectively permanent for a given set of experimental conditions [15–17, 19]. As
an example, strong physical junctions may consist of glassy or microcrystalline
nodules, triple helices, or coiled-coils as, for instance, in the case of hydrogels
formed from fibrous proteins or biomacromolecules.
Weak physical gels have reversible links formed from temporary weak interactions between chains, of the order of kT. These associations have finite lifetimes,
breaking and reforming continuously. Examples of weak physical bonds are hydrogen bonds, ionic associations, and micellar aggregates of block copolymers.
The outstanding physical properties of gels associated with their fast stimuli
response have noticeably stimulated research for a long time. Polymeric gels have
been developed that are “smart,” i.e., able to quickly respond to different external
stimuli [15, 20]. For instance, Tanaka’s gels have valuable applications because
they can expand and contract up to 1,000 times their original volume in response to
external stimuli. These gels could be used as artificial muscles, set in motion by a
specific electric pulse [21]. More importantly, the polymers in the gels can capture
or expel specific substances as they grow or shrink, so that the gels could be used,
for example, as super-sponges to absorb and immobilize toxic waste, or as molecular filters of various sorts [18, 20–23]. Among the smart gels, there are materials
that imitate proteins by recognizing conditions and responding to their environment. For example, smart gels can be fine-tuned to draw humidity from the air when
it is over a certain level at a given temperature. Other gels can release insulin when
the glucose level in the water-rich phase drops below a given point [15, 18]. Moreover, it is worth remembering that gels represent an important intermediate step in
polymer processing for obtaining highly oriented fibers showing high strength and
high modulus (e.g., ultrahigh-strength polyethylene fibers Dyneema and
Spectra) [17].
The outstanding properties of gels are directly related to the chemical nature of
their constituents, their reciprocal arrangement and interactions, and their mobility
in the space. Structure and mobility in a gel, in turn, are fixed by the preparative
conditions and processing. Although a gel may be assimilated to a single giant
branched macromolecule, the structure of its parts covers different orders of
hierarchical organization on different length scales. Roughly, the hierarchical
apparatus of gels ranges from a few nanometers for the size of knots; to hundreds
of nanometers for the organization of matter between first neighboring knots; to
Kinetic Analysis of Cryotropic Gelation of Poly(Vinyl Alcohol)/Water. . .
163