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

Asymmetric Star Polymers Synthesis and Properties 105
[9]. They used size exclusion chromatography with a multiangle laser light scattering
detector and viscometry in an attempt to characterize the structure of the
material produced by this two step method. Depending on the flow rate, different
forms of chromatograms were obtained. At very low flow rates, i.e., of
0.1 ml/min a true size separation could be achieved in contrast to the normal
flow rates, where the separation mechanism could be influenced by diffusion
and/or entanglement effects. Plots of the weight average molecular weight, M w ,
or radius of gyration, R g , against elution volume for the asymmetric stars had a
“U” shape, in contrast to the expected linear decrease of M w or R g with increasing
elution volume. It was concluded that the material was a complex mixture of
asymmetric and linked stars. In the case where the asymmetric star peak was resolved
the branching factors g and g', (g=R g, star/R g, linear) were obtained. Their
values were found to be lower than the ones obtained for regular star precursors,
indicating a more compact structure for the asymmetric molecules.
The hydrodynamic properties of miktoarm stars of the types A 2B and A 2B 2
where A, B=PS, PI, PBd were studied by Hadjichristidis and coworkers [22] in
solvents good for both kinds of arms or theta for one of them and good for the
other. Using the techniques of dilute solution viscometry and dynamic light
scattering they tried to probe the conformation of these molecules in solution
and extract some information on the effect of heterocontacts on the overall size
of the molecule. The experimentally determined [h] and R h values for the copolymers
were compared with the theoretical values. The latter were calculated
from data on linear homopolymers in the same solvents, corrected for the star
structure. The final theoretical value was an average value weighted according to
composition of those calculated for the homostars.
Experimental [h] and R h values were found to be consistently larger than the
theoretical ones, irrespective of the solvent quality, the differences being larger
for the A 2 B 2 case. It was suggested that a small expansion of the miktoarm stars
occurs either in a good or in a selectively q solvent, due to the increased repulsive
interactions between A and B chains when they are tethered to the same point.
As an extension of this work viscometry was used to study the conformation
of A 3 B stars (A=PI and B=PS) in the common good solvent toluene [65]. In this
case the experimental data, together with the ones obtained from the previous
work, were used to extract the dimensionless ratio s G . This ratio expresses
quantitatively the effects of heterointeractions between unlike segments on the
conformational properties of the copolymers. The s G values for the A 3 B case
were higher than for the A 2B case; it seems that this is due to the increased segment
density of A units in the vicinity of B units for A 3B. The experimental values
compared rather well with the ones obtained from renormalization group
theory and Monte Carlo calculations taking into account the uncertainty in the
asymmetry correction coefficient used in the calculations (see Table 2).
Tsitsilianis et al. [14] also reached the conclusion, from size exclusion chromatography
measurements, that A nB n type copolymers, where A is PS and B is
poly(tert-butyl acrylate), are more expanded than the corresponding homostars,
due to the increased density of two different kinds of segments which leads