G. Muller et al., HOMOGENEOUS NUCLEATION AND GROWTH IN THE CRITICAL 3-DIMENSIONAL ISINGREGIME OF A BINARY POLYMER BLEND, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 55(6), 1997, pp. 7267-7282
Phase separation in a binary blend of deutero-polystyrene and polyphen
ylmethylsiloxane was studied with time-resolved light scattering (smal
l-angle light scattering) for a nearly critical concentration at tempe
ratures between 0.2 and 2.6 K below the binodal. It was shown by addit
ional neutron small-angle scattering experiments that all quenches end
in the metastable region, i.e., in the gap between binodal and spinod
al of T-B -T-S congruent to 4 K. A Ginzburg number of 0.48+/- 0.14 was
found much larger than in simple binary mixtures, indicating that the
rmal concentration fluctuations are very strong and that the three-dim
ensional (3D) Ising model is valid. It was shown that strong thermal c
oncentration fluctuations determine the phase separation, leading to h
omogeneous nucleation and growth in the metastable region as proposed
by Binder [Phys. Rev. A 29, 341 (1984)]. Scattering patterns similar t
o those of spinodally decomposed structures were observed in a time re
gion of the intermediate and the late stage. Power laws were obtained
for the time dependence of position Q(m)(t) and intensity I-m(t) of th
e scattering maximum in the late stage. Furthermore, the data were ana
lyzed with scaling concepts based on the evolution of self-similar str
uctures. The time-dependent structure factor scales in the range of x=
Q/Q(m) less than or equal to 2 for all times in the late stage and all
temperatures studied, following the relation S(Q,t)=Q(m)(t)F-d(x). Bu
t instead of the Euclidean dimension d=3, an exponent of d(f)=2.43 +/-
0.05 was found. This is tentatively explained by a precipitation proce
ss where the compactness of the domains decreases with time. In the ra
nge x>2 a second characteristic length was observed, namely, an appare
nt interface thickness l(l)=7000+/-200 Angstrom independent of time an
d quench depth. This was interpreted by a waviness of the interface se
parating the coarsening domains. Furthermore, scaling was also observe
d for the position Q(m)(t) and intensity I-m(t) of the scattering maxi
mum with the appropriate values for the collective diffusion coefficie
nt and the correlation length in the two-phase region. Their values we
re determined by photon correlation spectroscopy (PCS) and small-angle
neutron scattering in the one-phase region by an extrapolation into t
he two-phase region. Finally, by the PCS measurements an extra relaxat
ion process with a characteristic time from 0.1 to 1 sec was discovere
d, which was related to density fluctuations.