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

116 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
ly(isoprene) stars, in order to study the crossover from linear to branched polymer
relaxation dynamics in highly entangled melts. The molecular weight of the
two equal arms, constituting the linear backbone was ~38 times larger than M e ,
the critical molecular weight for entanglements, where the M w of the third arm
was varied between 0.5 M e and 18 M e . Dynamic moduli were measured in shear
over a wide range of frequencies and temperatures. Time-temperature superposition
was observed for all samples. However, in the branched samples a significantly
larger modulus shift was required than in the linear ones. The shift factor
for the time scale depended more strongly on temperature for the asymmetric
stars. The viscosity, h o , increased by nearly a factor of three going from the linear
backbone sample to the sample with the shortest arm. The change in recoverable
compliance across the series was much smaller than the change in viscosity. The
diffusion coefficient, determined by forward recoil spectroscopy, decreases even
more rapidly. For a sample with only 2.2 entanglements on the third arm D was
reduced by a factor of 100. The transition from linear to branched dynamics
could be seen more clearly in the product h oD. It was concluded that even a single
mid-backbone arm with two to three entanglements is quite sufficient to
generate a branched polymer relaxation behavior.
Much experimental work has appeared in the literature concerning the microphase
separation of miktoarm star polymers. The issue of interest is the influence
of the branched architectures on the microdomain morphology and on
the static and dynamic characteristics of the order-disorder transition, the ultimate
goal being the understanding of the structure-properties relation for these
complex materials in order to design polymers for special applications.
In the first seminal morphological study of A 2 B miktoarm stars [81] where A
and B are PS or PI, by Hadjichristidis and collaborators, an SI 2 sample with
37 vol.% PS was found, by TEM, to be microphase separated as PS cylinders in
PI matrix in contrast to a lamellae structure expected for linear diblock copolymer
of the same composition. Later, more thorough studies [23, 82, 83] with a
larger number of samples and covering a wider range of compositions showed
that differences exist in the phase diagram for miktoarm star copolymers in
comparison to the linear diblocks. The boundaries for the microstructures usually
encountered in diblocks are shifted to higher compositions of the single
arm. At constant composition the miktoarm star copolymer has a morphology
with greater curvature than the diblock. This provides a means of relieving overcrowding
of the A 2 component and overstretching of the B component of the
miktoarm star. Analogous shifts were also observed in the case of A 3B (A=PI and
B=PS) miktoarm stars [83]. These findings are in qualitative agreement with the
theoretical predictions of Milner, which was developed after the first experimental
findings [81]. However some discrepancies in terms of the exact location of
the morphology boundaries do exist between theoretical prediction and experimental
results. An I 2S sample with 53 vol.% PS formed a tricontinuous cubic
structure where a lamellae structure was predicted by theory (Fig. 10). Another
I 2S sample with high PS content, (81% by vol.), showed microphase separation
without long range order. This randomly oriented wormlike morphology of