Dynamics of fluctuations and spinodal decomposition in polymer blends

Abstract

We discuss here some qualitative features of the concentration fluctuations in binary polymer melts A+B, A and B being long, flexible chains, with a Flory interaction parameter χ which can be positive (favoring segregation), or negative. In the one phase domain, the fluctuations of long wavelength (2π/q) are expected to relax by a reptation process. At shorter wavelengths (qR0≳1, where R0 is the size of one chain) equilibration takes place by local adjustment of each chain inside a fixed ’’tube’’: the predicted relaxation rates τq−1 are proportional to q6 for χ?0. (For a negative χ, opposing segregation, one expects τq−1∼q4). In the two phase domain, where the trend towards segregation is strong (χN≫1, N being the number of monomers per chain), one should observe a very anomalous type of spinodal decomposition: in the early stages, the growing fluctuations have a wavelength comparable to the thickness of the interfacial region, and small compared with R0: the growth is realized through local adjustment. During the later stages (Ostwald ripening) the size R(t) of the growing droplets should change from a t1/5 behavior (when R≲R0) to a t1/3 behavior (in the classical limit R≳R0).