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

Asymmetric Star Polymers Synthesis and Properties 109
onic polymers compared to the amine precursors. Increased values of the polydispersity
parameter µ 2 /G 2 in dynamic light scattering experiments indicated
that the aggregates were polydisperse supporting the high values of polydispersity
calculated from osmometry and static light scattering. The strongly negative
k D values for the trifunctional stars indicated the existence of strong hydrodynamic
interactions. These samples showed also low aggregation numbers and
only slight increase in the hydrodynamic radius, R h , compared to their precursors.
This behavior is the result of intramolecular association at low concentrations.
For the difunctional stars the phenomena were less pronounced, although
intramolecular association is also possible in this case.
Assuming that the aggregates of the monofunctional stars can be described as
regular stars (more precisely as umbrella star polymers) and the precursors as
the arms the authors calculated aggregation numbers from the ratio of
(R h) Zw/(R h) N, where (R h) Zw is the hydrodynamic radius of the zwitterioncapped
polymers in cyclohexane and (R h) N is the same radius for the amine precursor
in the same solvent, using the literature data for stars of different functionalities.
The results showed that the aggregates behave hydrodynamically as
star polymers with functionality equal to 2N w, where N w is the weight average
aggregation number determined by static light scattering. They concluded that
the two unfunctionalized arms anchored at the periphery of the aggregates are
responsible for the overall size of the associates.
Low intrinsic viscosities and high k H (Huggins constant) values were determined
by dilute solution viscosity measurements, indicative of large hydrodynamic
interactions, supporting the conclusions drawn from light scattering that
intramolecular association does occur at low concentration. Comparative examination
of R v and R h values showed that R v