11.6 Interplay of Phase Transitions in Diblock Copolymers
The examples presented above are limited in the intermolecular multi-component
systems. For polymers, there are even many intramolecular multi-component
systems, such as diblock copolymers with one end containing different chemical
species than the other end, grafted copolymer, star-shape copolymers, and statisti-
cal copolymers. Complex self-assembly process of these copolymers can form
hierarchical structures with certain flexibility in their environmental responses. It
is not difficult to find such examples in the bio-functional macromolecular systems.
The role of block junction in the interplay between phase transitions of two
polymer blocks can be revealed by its comparison to the parallel polymer blends
(Ma et al. 2011 ). Figure11.11shows that in the symmetric binary blends of 16-
mers, the melting points keep constant whenB/Ec>0 due to phase separation, and
decrease whenB/Ec<0 with the decreasing ofB/Ecdue to homogeneous mixing.
The melting point lines are shifted down in parallel with the lowerEp/Ecvalues,
because the latter dominates the thermodynamic stability of the crystals. In the
symmetric dilbock copolymers of 32-mers, the critical segregation strength shifts to
0.1. This shift is relevant to the additional free energy penalty for microphase
separation of diblock copolymers, either due to the interface formation of
microdomains or due to the stretching of chain conformations. WhenB/Ec>0.1,
the melting points increase slowly withB/Ec. This behavior can be attributed to the
significant amount of microdomain interfaces, at which the melting points are
raised as demonstrated in Fig.11.7, and the extent of raising depends on the
segregation strength.
The microphase separation and crystallization of diblock copolymers can com-
pete with each other on cooling. If crystallization occurs first, we observe only
Fig. 11.11 Melting points of symmetric binary blends and diblock copolymers for threeEp/Ec
values. The blends contain the chain length 16 monomers in parallel to the block length of diblock
copolymers. Thestraight linesare drawn to guide eyes (Ma et al. 2011 ) (Reprinted with
permission)
11.6 Interplay of Phase Transitions in Diblock Copolymers 235