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

106 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
to an increased number of heterocontacts. They noticed that the expansion was
greater as the molecular weight of the arms increased.
Teyssié and coworkers [68, 69] have studied the surface, interfacial and emulsifying
properties of A 2 B stars, where B is a polydiene, PS or poly(tert-butyl styrene),
(PtBuS) and A poly(ethylene oxide). The miktoarm stars were shown to
be better emulsifying agents for water-organic solvents mixtures than linear
block copolymers. Saturation of the interface was reached more quickly with the
miktoarm polymers. The differences were ascribed to the miktoarm architecture
which produces higher segment density near the interface of the two components.
These results are in agreement with the experiments conducted by Xie
and Xia on PS 2 PEO 2 stars [34].
Higashimura et al. [70] tried to elucidate the interactions between amphiphilic
miktoarm star molecules produced by cationic polymerization and
small molecules using NMR techniques. In their comparison between two different
architectures no distinct differences were observed for star-blocks and
miktoarm stars, both species being sufficiently capable of accommodating hydrophilic
molecules within their hydrodynamic volume.
Anastasiadis and coworkers [71] investigated the effect of macromolecular
architecture on the dynamic behavior of three-arm miktoarm stars of the types
A 2B and AA'B where A is PI and B is PS. AA'B stands for asymmetric miktoarm
stars where the two arms of identical chemical constitution have different molecular
weights. From a different point of view these two series of samples can be
considered as symmetric simple grafts (A 2B) and asymmetric simple grafts
(AA'B) where in the second case the B branch is situated at a position t=0.25 on
the backbone, where t is the ratio of the distance of the branching point from
one backbone end to the length of the backbone. They used the pulsed-field-gradient
NMR technique to probe the dependence of the self-diffusion coefficient,
D, on the concentration in semidilute and concentrated solutions in the common
good solvent toluene. For both types of samples the diffusion coefficient for the
polyisoprene rich grafts were consistently lower than those for the copolymers
with high styrene contents when compared at the same concentration due to the
different entanglement characteristics of the two components. In a DN 0.59 N e 0.41
vs f/f e representation, where N is the number of segments, N e the entanglement
length, f the volume fraction of copolymer and f e the volume fraction for entanglements
to occur, the data for the asymmetric I 2 S stars were slightly higher than
those for the symmetric I 2 S samples. Further analysis of the data showed that the
D dependence on c in both I 2S copolymers and P(S-b-I) diblock copolymers
could be collapsed onto a master curve when plotted as D(gN) 0.59 N e 0.41 vs f/fe,
a procedure that takes into account the difference in the entanglement characteristics
of the parent homopolymers and the different radii of gyration of stars
vs linear polymers. The exponential slowing down of the diffusivities expected
for star molecules was not observed due to the moderately high molecular
weights and concentrations studied.
Tsitsilianis and coworkers [41] presented results on the micellization behavior
of anionically synthesized amphiphilic miktoarm star copolymers with PS