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

122 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos
the incompatible blocks. A broad relaxation process in the vicinity of the normal
mode relaxation time of PI could be identified. It was shown to arise from the
mobile junction points at the interface. By fitting the experimental data with Havriliak-Negami
functions the parameters characterizing the process were determined
and the width of the interphase was estimated. The experimentally determined
value of 5–6 nm for the interfacial width compared well with the theoretically
calculated one (3.6 nm), based on a theory developed for linear block copolymers.
The dependence of the experimentally observed ODT on the ordering temperature
(final quenching temperature) was shown recently by rheology measurements
on SI 3 and SI low molecular weight samples [91]. This dependence resembles
the dependence of the melting temperature in semicrystalline
homopolymers on the crystallization temperature. The existence of an equilibrium
transition temperature (T ODT˚) was demonstrated and a procedure, analogous
to the known Hofmann-Weeks plot for crystalline polymers, was proposed
as a means to calculate T ODT˚. The phenomenon was attributed to the fact
that grains when produced at different temperatures have different sizes.
The rheological behavior of S 2I 2 miktoarm star copolymers was studied by
McLeish and coworkers [92]. An independence of the rheological characteristics
on temperature, for the temperature range between 100 and 150 ˚C was observed
for a sample having 20 wt% PS. In this system stress relaxation may occur
via molecular diffusion along the interface in two ways. First the entangled
polyisoprene arms can retract as in the case of the star polymers, with the junction
points remaining at the interface. Second, the PS arms can collapse and then
the whole molecule can reptate in the PI phase. From the available results the authors
could not distinguish between the two mechanisms.
Tsitsilianis [93] used DSC to study the phase behavior of (polybutylmethacrylate)
n (polystyrene) n and (polybutylacrylate) n (polystyrene) n miktoarm stars
with varying n. The T g of the PS phase was found to be much lower than in the
homopolymer and the width of the transition was increased. In some cases an
intermediate T g was observed due to the extended interphase region in these
materials, as a result of mixing of the different arms in the vicinity of the star
cores.
The compatibilizing ability of miktoarm stars comprised of polystyrene and
Nylon-6 arms emanating from a phosphazene core were investigated in detail by
Miyata et al. [50]. For binary blends of poly(2,6-dimethylphenylene oxide)
(PPO) and a miktoarm star with about four PS arms (M n,arm~62,000) and two
poly(e-caprolactam) arms (M n,arm~26,000), having more than 10 wt% miktoarm
star, the PPO T g values were apparently shifted to lower temperatures and
the T g decreased with increasing the amount of miktoarm star polymer. For all
binary blends a single T g was observed indicating miscibility of the two polymers
(PS is miscible with PPO over a considerable range of molecular weights in
all proportions). The T g values were lower for the cases of high PS/Nylon-6 ratios
in the miktoarms. For a ternary blend of PPO/Nylon-6/miktoarm with 45/45/10
composition respectively only one T g was observed corresponding to the