domains, up to the ultimate layered morphology. The transient morphologies
depend not only on the initial concentration and temperature, which in turn define
the relative amount of the two phases in equilibrium based on the lever rule [19],
but also on the degree of evolution of the phase separation and, hence, on the time
of observation [25].
At temperatures lower than T 1 , the intersection point B shifts towards higher
concentrations up to reach the Berghmans point BP at T 2 (see Fig. 4). At this point,
the polymer-rich phase vitrifies, blocking the progress of the phase separation. A
first direct consequence of vitrification is that at and below the temperature
corresponding to BP, the system ends up in a metastable state characterized by
the same phase morphology prevailing at the stage of vitrification, instead of
attaining the ultimate thermodynamically stable state characterized by the segregation of the two phases in layers [17, 25].
A second consequence of vitrification is that morphological development is
arrested and all further compositional changes cease, and the glass transition
becomes invariant with composition. If, at the moment of the arrest of LL phase
separation, the vitrified phase has become connected throughout the macroscopic
sample volume, the solution converts into a gel [17, 25]. Possible limiting morphologies of these gels are illustrated as an example in Fig. 5.
Gels may also arise from a combination of LL phase separation coupled with
crystallization [17, 25]. This is a more complex situation and occurs when one or
more metastable states lie “buried” beneath the state of ultimate stability in the
phase diagram [17, 25]. A hierarchy of metastability arises in this case due to the
possibility of several metastable phases. These metastable phases, once formed, can
evolve faster than the transition leading to the ultimate stable phase and can
dominate the whole transformation process.
As an example, Fig. 4b illustrates the case of a fully miscible polymer solution,
in which polymer crystallization takes place at equilibrium values of temperatures
delineated by the curve c. Hence, for this system, at temperatures below the curve c,
the crystalline state corresponds to the state of ultimate stability. However, the
system in Fig. 4b also displays a metastable LL phase separation, as indicated by
curve b, lying below the crystallization curve c. Also shown is the glass transition
temperature T g of the polymer as a function of polymer concentration intersecting
the binodal at the Berghmans point BP. A first set of metastable states may arise
because of the high undercooling ΔT that is generally required for crystallization to
take place at practical rates. Besides crystallization, other sets of metastable states
may also arise for this system, depending on the undercooling, according to a
hierarchy. Such scheme fits quite well what occurs in the case of poly(phenylene
ether) in cyclohexanol, as an example [38]. In Fig. 4b, route 1 shows that, starting
from an initially homogeneous solution, crystallization takes place at low
undercooling ΔT 1 if sufficient time is allowed. At low undercooling, only crystallization takes place. In route 2, the undercooling ΔT 2 is larger than ΔT 1 and crosses
the binodal. Depending on the magnitude of the undercooling, either de-mixing
occurs first and crystals are formed in the polymer-rich phase (point C) or the
polymer-rich phase vitrifies at and below the Berghmans point (point D), and both
170
C. De Rosa et al.
depend not only on the initial concentration and temperature, which in turn define
the relative amount of the two phases in equilibrium based on the lever rule [19],
but also on the degree of evolution of the phase separation and, hence, on the time
of observation [25].
At temperatures lower than T 1 , the intersection point B shifts towards higher
concentrations up to reach the Berghmans point BP at T 2 (see Fig. 4). At this point,
the polymer-rich phase vitrifies, blocking the progress of the phase separation. A
first direct consequence of vitrification is that at and below the temperature
corresponding to BP, the system ends up in a metastable state characterized by
the same phase morphology prevailing at the stage of vitrification, instead of
attaining the ultimate thermodynamically stable state characterized by the segregation of the two phases in layers [17, 25].
A second consequence of vitrification is that morphological development is
arrested and all further compositional changes cease, and the glass transition
becomes invariant with composition. If, at the moment of the arrest of LL phase
separation, the vitrified phase has become connected throughout the macroscopic
sample volume, the solution converts into a gel [17, 25]. Possible limiting morphologies of these gels are illustrated as an example in Fig. 5.
Gels may also arise from a combination of LL phase separation coupled with
crystallization [17, 25]. This is a more complex situation and occurs when one or
more metastable states lie “buried” beneath the state of ultimate stability in the
phase diagram [17, 25]. A hierarchy of metastability arises in this case due to the
possibility of several metastable phases. These metastable phases, once formed, can
evolve faster than the transition leading to the ultimate stable phase and can
dominate the whole transformation process.
As an example, Fig. 4b illustrates the case of a fully miscible polymer solution,
in which polymer crystallization takes place at equilibrium values of temperatures
delineated by the curve c. Hence, for this system, at temperatures below the curve c,
the crystalline state corresponds to the state of ultimate stability. However, the
system in Fig. 4b also displays a metastable LL phase separation, as indicated by
curve b, lying below the crystallization curve c. Also shown is the glass transition
temperature T g of the polymer as a function of polymer concentration intersecting
the binodal at the Berghmans point BP. A first set of metastable states may arise
because of the high undercooling ΔT that is generally required for crystallization to
take place at practical rates. Besides crystallization, other sets of metastable states
may also arise for this system, depending on the undercooling, according to a
hierarchy. Such scheme fits quite well what occurs in the case of poly(phenylene
ether) in cyclohexanol, as an example [38]. In Fig. 4b, route 1 shows that, starting
from an initially homogeneous solution, crystallization takes place at low
undercooling ΔT 1 if sufficient time is allowed. At low undercooling, only crystallization takes place. In route 2, the undercooling ΔT 2 is larger than ΔT 1 and crosses
the binodal. Depending on the magnitude of the undercooling, either de-mixing
occurs first and crystals are formed in the polymer-rich phase (point C) or the
polymer-rich phase vitrifies at and below the Berghmans point (point D), and both
170
C. De Rosa et al.