polypropylene is crystallizable, while atactic polypropylene is non-crystallizable.
Such an asymmetry of crystallization driving forceEpbetween two components
makes a positive contribution to the mixing free energy, and results in the latter
larger than zero. Therefore, the mixing state becomes unstable, and the blend may
spontaneously perform phase separation. Indeed in reality, such blends have been
observed immiscible in their melt states (Hu and Mathot 2003 ).
The statistical thermodynamic theory makes a mean-field treatment to the
crystallization energyEp. However, in practice, thermal fluctuations for parallel
ordering of local chains generally exist in the melt states of blends. Such an
additional anisotropic contribution of thermal fluctuations will make the practical
parallel-packing interactions larger than the mean-field estimation on the basis of
the isotropic liquid, which leads to a deviation from the theoretical predictions of
the phase separation binodals.
In Fig.11.3, we made a comparison between the binodals obtained from
dynamic Monte Carlo simulations (data points) and from mean-field statistical
thermodynamics (solid lines). First, one can see that even with zero mixing
interactionsB¼0, due to the contribution ofEp, the binodal curve is still located
above the liquid–solid coexistence curve (dashed lines). This result implies that the
phase separation of polymer blends occurs prior to the crystallization on cooling.
This is exactly the component-selective crystallizability-driven phase separation, as
discussed above. Second, one can see that, far away from the liquid–solid coexis-
tence curves (dashed lines), the simulated binodals (data points) are well consistent
Fig. 11.3 Comparisons of phase diagrams of symmetric polymer blends (same chain lengths
32 monomers, only one component crystallizable) obtained from simulations (data point with the
same labeled sequences as thesolid lines) and from mean-field theory (solid linesfor binodals, and
dashed linesfor liquid–solid coexistence lines) in the cubic lattice space 32^3. The x-axis is the
volume fraction of crystallizable component, and the y-axis is the reduced temperature. The data
labeled near the solid lines are the reduced energy parameterB/Ec, and all the curves have
Ep/Ec¼1 (Ma et al. 2007 ) (Reprinted with permission)
11.2 Enhanced Phase Separation in the Blends Containing Crystallizable Polymers 227