The critical points of the C1, C2 and C3 systems correspondingly are close to the
melting point, in the region of spontaneous crystallization temperatures below the
melting point, and far below the melting point. Thus, when a homogeneous solution
with the concentration 0.125 is cooled from high temperatures, the C1 solution will
firstly perform phase separation, generating a two-phase coexisting structure.
Figure11.4also demonstrates the onset temperatures of crystal nucleation
obtained from simulations. One can see that the C1 solution performs crystal
nucleation at the highest temperature. This behavior is because the higher concen-
tration corresponds to a higher melting point, providing a larger supercooling for
the initiation of crystallization at the same temperatures. In this sense, the
concentrated phase resulted from phase separation contains the highest priority
for crystal nucleation. The data points obtained from simulations form the kinetic
phase diagrams for the onsets of crystal nucleation versus concentration curves,
which become horizontal below the theoretical spinodal lines. The horizontal lines
indicate that due to the prior phase separation, crystal nucleation in this region share
the same concentrations of the polymer-rich phases to meet the thermal fluctuation
energy for the spontaneous initiation of crystallization. At any specific concentra-
tion of this region, the onsets of crystal nucleation on the horizontal lines are higher
than the corresponding results extrapolated from the kinetic phase diagram in the
region above the crossover with the theoretical spinodal lines, indicating crystal
nucleation triggered by the prior spinodal decomposition. The horizontal region of
the C2 solutions is relatively narrow, while no horizontal region occurs in the C3
solutions. Above the crossover, the onsets of crystal nucleation increase with of
polymer concentrations, following the same trends in the curves of equilibrium
melting points (Zha and Hu 2007 ).
If the homogeneous C2 and C3 solutions with the concentration of 0.125 are
quenched to a low temperature of 1.5Ec/kfor isothermal crystallization, their
structure factors as a function of time are shown in Fig.11.5a, b. According to
the introduction in the Sect.9.2, Fig.11.5ashows the typical evolution of structure
Fig. 11.5 Simulation results for the time evolution of structure factors on the isothermal crystal-
lization of homogeneous C2 (a) and C3 (b) solutions with the concentration 0.125 at the
temperature 1.5Ec/k(Time periods are labeled in parallel sequences with thecurves, in the unit
of Monte Carlo cycles). The dot segments represent the initial state (Zha and Hu 2007 ) (Reprinted
with permission)
11.3 Accelerated Crystal Nucleation in the Concentrated Phase 229