aim of extracting appropriate descriptors of the structural transformations occurring
at the relevant length scale and the kinetic parameters. Chemical hydrogels of PVA
are not included in the present context, in spite of their importance and interesting
properties in several applications. Some excellent reviews on this subject may be
found in the literature [1, 12, 13]. The last section is devoted to the conclusions and
outlook.
It is worth noting that understanding the factors that govern the cryogelation
mechanism of homogeneous polymer solutions is useful both in fundamental
studies of the phenomena subtending the formation of macroporous gels in PVA
and polymers in general and from an applicative standpoint. Such knowledge can
enable control of the final architecture of cryogels by simply tuning the conditions
of preparation, with the aim of obtaining materials with enhanced properties for
tailored applications.
2 General Considerations
Polymer gels are mesoscopic systems that include a liquid phase (solvent) and a
crosslinked three-dimensional (3D) polymer network intimately swollen by the
solvent (solvate) [14–16]. The two phases cannot be distinguished at macroscopic
level, meaning that the gels at macroscale are homogeneous systems.
Gels, are generally formed starting from a solution of a low or high molecular
mass precursor (sol) as a result of a crosslinking process that is able to create a
macroscopic 3D network (gel). Because the presence of crosslinks reduces the
mobility of the molecular species that have reacted to become part of the network,
the solvent may end up entrapped within the network-gel in high amounts instead of
being rejected. It is worth noting that, although the solvent may largely exceed the
polymeric component by weight, gels do not behave as viscous polymer solutions,
but rather behave as elastic solids or semisolids able to withstand their own weight,
even over long time scales.
The liquid component of a gel may either be a pure solvent of the initial
precursor or a polymer-poor liquid phase. The mobility of the solute and solvent
molecules in these liquid domains is high, as in a solution. Consequently, gels
combine the properties of a solid network structure and a liquid solution at the
mesoscale level, giving rise to materials with unique properties that are useful for a
variety of applications.
In particular, polymer gels (depending on the chemical constitution of the
components, processing conditions, and phase composition) may feature a gamut
of properties that range from those of hard and tough materials to those of
elastomers, that is, they are able to undergo large deformation upon application
of pressure and to quickly recover the initial shape and dimensions when the
external stress is removed [12–18]. In all cases, the high dimensional stability of
gels is ensured by the presence of a network, which can be built by either physical
or chemical crosslinks, giving rise to either physical or chemical gels.
162
C. De Rosa et al.
at the relevant length scale and the kinetic parameters. Chemical hydrogels of PVA
are not included in the present context, in spite of their importance and interesting
properties in several applications. Some excellent reviews on this subject may be
found in the literature [1, 12, 13]. The last section is devoted to the conclusions and
outlook.
It is worth noting that understanding the factors that govern the cryogelation
mechanism of homogeneous polymer solutions is useful both in fundamental
studies of the phenomena subtending the formation of macroporous gels in PVA
and polymers in general and from an applicative standpoint. Such knowledge can
enable control of the final architecture of cryogels by simply tuning the conditions
of preparation, with the aim of obtaining materials with enhanced properties for
tailored applications.
2 General Considerations
Polymer gels are mesoscopic systems that include a liquid phase (solvent) and a
crosslinked three-dimensional (3D) polymer network intimately swollen by the
solvent (solvate) [14–16]. The two phases cannot be distinguished at macroscopic
level, meaning that the gels at macroscale are homogeneous systems.
Gels, are generally formed starting from a solution of a low or high molecular
mass precursor (sol) as a result of a crosslinking process that is able to create a
macroscopic 3D network (gel). Because the presence of crosslinks reduces the
mobility of the molecular species that have reacted to become part of the network,
the solvent may end up entrapped within the network-gel in high amounts instead of
being rejected. It is worth noting that, although the solvent may largely exceed the
polymeric component by weight, gels do not behave as viscous polymer solutions,
but rather behave as elastic solids or semisolids able to withstand their own weight,
even over long time scales.
The liquid component of a gel may either be a pure solvent of the initial
precursor or a polymer-poor liquid phase. The mobility of the solute and solvent
molecules in these liquid domains is high, as in a solution. Consequently, gels
combine the properties of a solid network structure and a liquid solution at the
mesoscale level, giving rise to materials with unique properties that are useful for a
variety of applications.
In particular, polymer gels (depending on the chemical constitution of the
components, processing conditions, and phase composition) may feature a gamut
of properties that range from those of hard and tough materials to those of
elastomers, that is, they are able to undergo large deformation upon application
of pressure and to quickly recover the initial shape and dimensions when the
external stress is removed [12–18]. In all cases, the high dimensional stability of
gels is ensured by the presence of a network, which can be built by either physical
or chemical crosslinks, giving rise to either physical or chemical gels.
162
C. De Rosa et al.