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

Asymmetric Star Polymers Synthesis and Properties 107
and PEO branches in THF. Although THF is not very selective for PS increased
values of apparent weight average molecular weights were obtained. The micelles
formed in THF had molecular weights two orders of magnitude larger
than the M w of the unimer. The authors observed an increase in the depolarization
ratio by increasing polymer concentration indicative of the formation of
large multimolecular micelles with PEO cores.
Micelle formation was also observed, by the same authors [72], in the case of
miktoarm stars with equal number of polystyrene and poly(tert-butyl acrylate)
branches in methanol a good solvent for PtBuA and a precipitant for PS. For a
sample of the type PS 15 PtBuA 15 of molecular weight M w =218,000 and 70 wt%
tBuA, the association number was estimated to be 27 by light scattering. It was
assumed that the multimolecular micelles were spherical in shape with PtBuA
chains in the corona and PS in the core. The DVB cores of the unimers were assumed
to be located at the core-corona interface forming a third domain with
high segment density.
In the same study viscometric experiments in acetone indicated the existence
of a conformational transition around 35 ˚C. In that temperature region a sudden
decrease in the intrinsic viscosity and a maximum for the Huggins constant,
k H, were observed. Although static light scattering envelopes were curved downwards
at low angles, indicating the existence of a small number of high molecular
weight associated species, extrapolation from higher angles gave molecular
weights very close to the unimer molecular weight. The authors concluded that
these phenomena could only be interpreted by a transition from a segregated to
a nonsegregated conformation.
The micellar structures formed in solutions of well defined (PE) 2 (PEP) 2 ,
PE(PEP) 2 and (PE) 2 PEP type polyethylene (PE)-poly(ethylene-alt-propylene)
(PEP) miktoarm stars in n-decane were investigated by Richter et al. [73] by
small angle neutron scattering using the contrast variation technique. The solvent
is selective for the PEP branches. The formation of multimolecular structures
was also influenced by the ability of PE to crystallize. The PE chains were
75% deuterated to enhance scattering contrast and their MW was kept constant
and the MW of PEP was varied in order to study the influence of composition
and architecture on the micellar properties. The miktoarm stars formed multimolecular
micelles with flat dense, disk-like, crystalline PE cores and soft coronas
of PEP “hairs” sticking out on both sides of the core. Using model fitting procedures
structural parameters like core and corona thickness were estimated.
It was confirmed that the molecular architecture does not modify the global
characteristics (shape) of the micelles but only its geometrical parameters. The
average thickness of the core was nearly constant and independent of the MW of
the PEP brush for the PE 2PEP 2 case. For PE(PEP) 2 and (PE) 2PEP the thickness
decreased with increasing MW of PEP. It was found that for all PEP molecular
weights the chains are more extended if the PE chains are connected to two PEP
arms instead of one. The difference was more pronounced when the PEP molecular
weights were higher. On the other hand the thicknesses of the cores were
higher for the system (PE) 2PEP. In comparing the systems (PE) 2(PEP) 2 and