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

Asymmetric Star Polymers Synthesis and Properties 115
Fig. 9. Density profiles of the cores of stars of the same total molecular weight but different
number of equal branches as a function of the reduced distance Z 0 , linear chain f=2 (heavy),
f=4 (normal), f=8 (dashed), f=15 (dotted). The ratio r=2, defined in text (Sect. 3.2.1) represents
the ratio of the sizes of a branch at the left and right phase (reproduced with permission
from [79])
for z£0 are the sizes of the i-th branch of the miktoarm at the two phases and r i
their ratios. The concentration of miktoarm chains is r (number of chains per
volume n/V). The density profile of a homopolymer star which is the simplest
case of miktoarm star is illustrated in Fig. 9 as a function of the reduced distance
Z = 3z / f R( z )
2.
0 0 0
3.2.2
Experimental Results
Only a few studies have been devoted to the bulk properties of asymmetric
homopolymer stars. The main issue under investigation was, up to now, the selfdiffusion
and viscoelastic behavior of three-arm stars where the molecular
weight of the third arm was varied in order to observe the transition in the diffusion
from linear polymer to a polymer with a star architecture.
In the earlier study Fetters et al. [80] utilized a series of deuterated polybutadienes
ranging from a linear 2A-mer to a three-arm with a centrally located
third arm which had a molecular weight A' varying from 920 to 4.56·10 3 g/mol.
A linear hydrogenous 1,4 PBd of Mw=2·10 5 g/mol was used as the matrix. Diffusion
coefficients were measured using the thin film IR microdensitometry
technique. A monotonic decrease of the diffusion coefficient was observed as the
centrally placed side-arm mol. weight increased. The decrease was sharp for
MW(A')