HOMOGENEOUS NUCLEATION AND GROWTH IN THE CRITICAL 3-DIMENSIONAL ISINGREGIME OF A BINARY POLYMER BLEND

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.