microns for the relative organization of clusters of knots; and so on. Gels may
include macro- and/or microscale heterogeneities such as macro- and micropores
inside their structure. The length scales and the kind and degree of heterogeneities
in a gel have a strong influence on its final properties. Therefore, the design of
preparation/processing methods for achieving gels with tailored morphology able to
ensure prefixed properties is a research area of great applicative interest.
From a fundamental point of view, the achievement of this goal needs the
establishment of precise relationships between the functionality of the gel and its
fundamental constituents, their chemical nature, their geometric arrangement, and
their mobility. Of primary interest in basic research in the field of polymer gels are
(1) definition of the structural organization of gel components on different length
scales and (2) the structural and morphological transformations of a gel in the initial
state (preparative ensemble), during its performance under application of external
stimuli on the proper time scale (applicative ensemble), and in the final state (end
use ensemble) [24]. In the present context we focus on gels in the preparative
ensemble.
2.1 Physical Gels in the Preparative Ensemble: Main
Processes Creating Networks
Physical gels may originate from a large number of processes able to create
junctions (Fig. 1). Each one of these processes may act alone or in synergy with
the other processes to induce the sol–gel transitions (gelations). Gelation is not a
first-order phase transition and does not lead to a state of thermodynamic equilibrium [16]. Instead, the kinetics of the process and the way a crosslinking process is
activated play key roles in the transition [17, 25].
As shown in Fig. 1, sol–gel transitions may be easily driven by the entanglements established among the polymer chains in a semidilute or concentrated
homogeneous solution , i.e., at concentrations higher than that occupied by each
polymer coil within its own pervaded volume, corresponding to the overlap concentration [19]. This means that polymer random coils should overlap in order to
establish an entangled network [24] and, hence, to form a gel.
Besides entanglements, a second kind of crosslinking process leading to gelation
involves physical associations such as polar–polar interactions, ionic forces, colloid
interactions, hydrogen bonding, or complex associations such as formation of
multiple helices and/or coiled-coils in biopolymers, self-assembly with formation
of micellar aggregates in amphiphilic block copolymers, and so on.
As shown in Fig. 1, gelation may be also driven by phase transitions as for
instance crystallization. Under specific conditions, formation of crystals from a
homogeneous solution can give rise to junctions and then to a gel. This happens, for
example, with poly(vinyl chloride) [15], isotactic polystyrene, polyethylene [15,
26–31], and many other polymers [32]. In these gels, the junctions are constituted
164
C. De Rosa et al.
include macro- and/or microscale heterogeneities such as macro- and micropores
inside their structure. The length scales and the kind and degree of heterogeneities
in a gel have a strong influence on its final properties. Therefore, the design of
preparation/processing methods for achieving gels with tailored morphology able to
ensure prefixed properties is a research area of great applicative interest.
From a fundamental point of view, the achievement of this goal needs the
establishment of precise relationships between the functionality of the gel and its
fundamental constituents, their chemical nature, their geometric arrangement, and
their mobility. Of primary interest in basic research in the field of polymer gels are
(1) definition of the structural organization of gel components on different length
scales and (2) the structural and morphological transformations of a gel in the initial
state (preparative ensemble), during its performance under application of external
stimuli on the proper time scale (applicative ensemble), and in the final state (end
use ensemble) [24]. In the present context we focus on gels in the preparative
ensemble.
2.1 Physical Gels in the Preparative Ensemble: Main
Processes Creating Networks
Physical gels may originate from a large number of processes able to create
junctions (Fig. 1). Each one of these processes may act alone or in synergy with
the other processes to induce the sol–gel transitions (gelations). Gelation is not a
first-order phase transition and does not lead to a state of thermodynamic equilibrium [16]. Instead, the kinetics of the process and the way a crosslinking process is
activated play key roles in the transition [17, 25].
As shown in Fig. 1, sol–gel transitions may be easily driven by the entanglements established among the polymer chains in a semidilute or concentrated
homogeneous solution , i.e., at concentrations higher than that occupied by each
polymer coil within its own pervaded volume, corresponding to the overlap concentration [19]. This means that polymer random coils should overlap in order to
establish an entangled network [24] and, hence, to form a gel.
Besides entanglements, a second kind of crosslinking process leading to gelation
involves physical associations such as polar–polar interactions, ionic forces, colloid
interactions, hydrogen bonding, or complex associations such as formation of
multiple helices and/or coiled-coils in biopolymers, self-assembly with formation
of micellar aggregates in amphiphilic block copolymers, and so on.
As shown in Fig. 1, gelation may be also driven by phase transitions as for
instance crystallization. Under specific conditions, formation of crystals from a
homogeneous solution can give rise to junctions and then to a gel. This happens, for
example, with poly(vinyl chloride) [15], isotactic polystyrene, polyethylene [15,
26–31], and many other polymers [32]. In these gels, the junctions are constituted
164
C. De Rosa et al.