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

Asymmetric Star Polymers Synthesis and Properties 103
According to the simulation results the mean conformation achieved by the
miktoarm stars in solution is characterized by a significant deformation along
the arms, mainly near the star center where the possibility of interactions is
high and a change in the relative orientation of the vectors c anci cB (cie-
termined fiom their angle) extends the center of mass separat~on in or2er to
diminish the repulsions. The results show that both mechanisms have a similar
influence on the increase of the < G: , > . The values of the ratio showever
corresponding to an artificial segreggtpd model (also considered in this work)
are very large compared to the value of a single chain in various solvents. It is
concludeci that in dilute solutions the miktoarm chain does not segregate intramolecularly.
As in the previous case experimental results are not available.
Vlahos et al. [65] have determined the dimensionless ratio s,;
by intrinsic
viscosity analysis for the A,B and A,B miktoarm stars in various solvent conditions.
Considering the ratios gs* and gSg
to be close to 1 for the particular
miktoarms studied in this work they arrived at the following equations:
F
h s h s h
F
F
h s h s h
F
Monte Carlo calculations based on the lower bound method were applied to
estimate the Flory parameter Ffor
asymmetric stars, whereas, for the symmetric
stars, experimental results on three- and four-arm homopolymer stars were
used. The obtained values of s for different macroscopic states together with
those obtained from RG and MC calculations are presented in Table 2. p is the
ratio of the lengths of the B and A arms (p=NH/NA).
Table 2. The ratio s,
of miktoarm stars 122,651
Coxlxlorl good MC RG Exp.
A,B (p-1) 1.294k0.005 1.249
A,B (p=1.7) 1.289k0.006 1.238
A4 (p=2) 1.342k0.010 1.316
A,B (p-3.9) 1.241k 0.017 1.262
Selective ( qfor
A)
A2B (p=l) 1.419k0.005 1.374
A,
F
F
F
F