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

Asymmetric Star Polymers Synthesis and Properties 121
overall degree of polymerization, of sample SI 2 near the ODT were higher than
in the case of diblocks of the same composition and almost independent of temperature.
The broad distribution of relaxation times (longer than the relaxation
times of individual chains) observed by rheology was attributed to a broad distribution
of grain sizes. At very long times the rheological behavior of the samples
was proposed to be governed by the relaxation of grains which rearrange
with rates related to their sizes. Quenching experiments were used to elucidate
the mechanism governing the ordering process. The rheological measurements
after each quench from the melt indicated that ordering proceeds via heterogeneous
nucleation. Quenching to final temperatures near the ODT made the kinetics
very slow with times which depended strongly on the final quench temperature.
Deeper quenches produced only moderate changes in the kinetics.
Compared with diblocks the kinetic times were slower for miktoarm stars in
deeper quenches. For shallow quenching there were no measurable differences.
The width of the kinetically accessible metastable region was also larger for the
star copolymers. External perturbations, i.e., continuous oscillating shear, produced
no changes in the ordering process.
Dielectric spectroscopy was also used by the same group in order to study the
local and global dynamics of the PI arm of the same miktoarm star samples [89].
Measurements were confined to the ordered state, where the dynamics of the PI
chain tethered on PS cylinders were observed in different environments since in
the SIB case the faster moving PB chains are tethered in the same point as the PI
arm. The distribution of segmental relaxation times were broader for SI 2 than
SIB. The effect was less pronounced at higher temperatures. The PI normal
mode time was found to be slower in SIB, when compared to SI 2 although both
arms had the same molecular weight. Additionally, the normal mode relaxation
time distributions of the PI chains tethered to PS cylinders in the miktoarm
samples were narrower than in P(S-b-I) systems of lamellar structure.
The characteristics of the order-disorder transition in two SI 3 miktoarm stars
were also investigated by Floudas et al. using SAXS and rheology [78]. Due to the
high molecular weights diisooctylphthalate (DOP) was used as the nonpreferential
solvent to bring the transition into the experimentally accessible temperature
window. All SAXS parameters were discontinuous at the ODT revealing a
first-order transition. The interaction parameter exhibited a weak temperature
dependence consistent with fluctuation effects near the transition. The experimental
results agreed qualitatively with the theoretical spinodal curves cN total =
F(f A) for AB n miktoarm copolymers. For shallow quenches rheological measurements
indicated a nucleation and growth ordering mechanism with an Avrami
exponent of about three, reminiscent of undiluted block copolymers.
Floudas and coworkers [90] employed dielectric spectroscopy to probe the interfacial
width in lamellae forming non-linear block copolymers of the type (Bb-A)
3B and (B-b-A) 3B(A-b-B) 3, where A is PI and B is PS. Their experiments
were conducted at temperatures below the ODT and below the glass transition
of the PS “hard phase”. In this temperature region the global chain dynamics of
PI bridges were used to provide an estimate of the “dynamic” interface between