142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

Asymmetric Star Polymers Synthesis and Properties 119
Fig. 11a,b. Bright field TEM micrographs of: a a linear tetrablock copolymer; b a topologically
asymmetric miktoarm starblock copolymer of the same composition and molecular
weight (see text) (reproduced with permission from [62])
iting a three-fold symmetry. The authors suggested that the PDMS phase was continuous
in three dimensions. The other two phases were not distinguishable. Because
of the equal fractions of the three components the authors hypothesized
that the other two microphases should also be continuous in three dimensions. All
the examined simple models failed to describe quantitatively the observed morphology
because of the molecular complexity. It has to be mentioned that tilting
experiments gave images of other kinds of symmetry, which could not be identified
in terms of simple geometrical arrangements. The crystallinity of the PDMS
phase was found by DSC to be lower in the miktoarm case than in the PS-PDMS
case, due to the increased steric hindrance close to the central branch point.
Thomas and coworkers [62] studied the structures formed by strongly segregated
compositionally symmetric four-arm inverse star block copolymers and
their linear counterparts. These macromolecules having the general formula
(PS M-b-PI aM) n-(PI M-b-PS aM) n where M~20,000, n=1, 2 and a=1, 2, 4 (a is the ratio
of the outer block molecular weight to that of the inner block, see Fig. 1c) can
be considered as topologically asymmetric star copolymers. Transmission electron
microscopy (TEM) and small angle X-ray scattering (SAXS) were used to
characterize the microdomain morphology of these materials. Samples with n=
1, 2 a=1, 2 formed highly ordered lamellae structures. For a=4 (sample SB4) a
transition from a lamellar structure for n=1 to a triconnected cubic structure
(n=2) was observed (Fig. 11). Comparison of TEM data with simulated images