In a blend consisting of symmetric SBS block
copolymers, it is possible to produce a particular
morphology by mixing a definite amount of each of
the compositionally complementary block copolymers
[30] However, the situation changes dramatically
if the architectures of the block copolymers are
different. Spontak et al. showed that an equimolar
blend of a diblock copolymer and an octablock
copolymer—both of identical composition and molecular
weight—was immiscible, leading to the formation
of macrophase separated grains of one block
copolymer embedded in a matrix of the other [109].
The macrodomains were clearly separated by a welldefined
macroscopic interface. More recently, binary
blends of SBS block copolymers having lamellar and
cylindrical morphologies were investigated [110].
The lamellar block copolymer had a highly asymmetric
architecture, the molecular weight ratio R of
outer PS blocks being more than 5 while the
cylindrical block copolymer was symmetrical
(Rz1). It was found that the blend components
phase-separate even when these copolymers have
comparable molecular weight (z100,000 g/mol). For
example, at lower concentration of cylindrical block
copolymer, grains of the former were observed in the
matrix of a lamellar block copolymer [110].These
results clearly indicated that macrophase separation
can be induced by the asymmetric architecture of one
of the blend partners.
The styrene/butadiene block copolymers used in
this study are ST2 and LN4 (Fig. 4): ST2 is
thermoplastic whereas LN4 shows thermoplastic
elastomeric properties at room temperature. The star
block copolymer ST2 (total styrene volume contentZ
0.74) possesses lamellae-like morphology, while the
linear copolymer LN4 (total styrene volume contentZ0.65)
has a morphology consisting of dispersed
PS domains in a matrix of PS-co-PB random
copolymer (Section 4.1). Based on the results from
different microscopic and thermal methods, it was
concluded that these blends represent a partially
miscible system. The equilibrium morphology of
these blends is described elsewhere [111,112]. Fig. 21
shows TEM images of a blend containing 60 wt%
LN4. Comparison of the morphology of the pure
blend components (Fig. 4) with that of the blend
(Fig. 21) reveals the following:
(a) The blend shows morphology typical of an
incompatible polymer blend (particle–matrix
structure). The macrophase-separated dispersed
phase obviously shows a lamellar morphology.
(b) The tiny PS domains embedded in the PB
lamellae of ST2 (Fig. 4d) do not appear in the
blend-lamellae. The matrix in the blend appears
to consist of co-continuous network of hard
and soft phases. Hence, the morphological details
of the blend do not resemble the structure of its
components.