Initial stages of nucleation in phase separating polymer blends

The initial stages of nucleation during liquid-liquid phase separation in m ixtures of high molecular weight polymers was studied by time-resolved smal l angle neutron scattering. Phase separation was induced either by decreasi ng temperature or by increasing pressure. One of the blend components was l abeled with deuterium to obtain sufficient scattering contrast between the components. The general features of nucleation were independent of quench d epth and the nature of the quench (temperature quench versus pressure quenc h). The early stages of nucleation consisted of amplification of concentrat ion fluctuations. During this stage, the scattered intensity (I) in the low scattering vector (q) limit was consistent with the Ornstein-Zernike equat ion. This enabled the determination of the characteristic length scale of t he growing fluctuations, xi. The I vs q behavior at intermediate scattering vectors (q > 1/xi) could be described by a power law (I similar to q(-d)). We demonstrate the existence of a time-temperature superposition principle during nucleation: The time dependence of d at different quench depths cou ld be superimposed by a lateral shift of the data along the time axis (log scale). In analogy to the shift factor for viscoelastic behavior of polymer s, we define a nucleation shift factor, a(N), which describes the slowing d own of nucleation kinetics with decreasing quench depth. Similarly, nucleat ion after pressure quenches can be described by a time-pressure superpositi on law. For each quench, we find that the scattering intensity is independe nt of time in the high q regime (q > q(merge)). This implies the absence of growing structures with length scales smaller than xi(crit)=1/q(merge) dur ing nucleation. This aspect of nucleation is consistent with classical theo ries which predict the existence of a critical nucleus size. As expected, x i(crit) increases with decreasing quench depth. (C) 1999 American Institute of Physics. [S0021-9606(99)50437-6].