Time-resolved small-angle neutron scattering study of spinodal decomposition in deuterated and protonated polybutadiene blends. II. Q-dependence of Onsager kinetic coefficient

The spinodal decomposition (SD) of a critical mixture of deuterated and protonated polybutadiene of nearly equal chain lengths was investigated. This mixture has an upper critical solution temperature type phase diagram and the spinodal temperature at the critical point is 99.2 °C. Phase separation was induced by quenching a single-phase specimen at an initial temperature, T _i (=102.3, 123.9, and 171.6 °C), to a final temperature, T _f (=-7.5, 1.1, and 10.5 °C). The subsequent SD was followed by time-resolved small-angle neutron scattering. The Onsager coefficient, Λ(q;T_f), as a function of wave number q and T_f, derived from experimental growth rates, R(q;T_f), of the Fourier mode of concentration fluctuations and estimation of S_T(q;T_f), was compared to the reptation model theories of Pincus and Binder. Experimental Λ(q;T_f) was found to give a q-dependence greater than that given by the theories. Here, S_T(q;T_f) denotes the virtual structure factor at T_f inside the spinodal region. The reduced wave number Q_m(τ) and intensity S̃_m(τ) at the peak of the scattering structure factor in the early and intermediate stages of SD were found to be scalable in terms of a reduced time τ when T_i was fixed and T_f was varied, but not when T_f was fixed and T_i was varied. The failure of the scaling law in the latter instance may be attributed to the fact that the concentration fluctuation at the onset of SD has a different memory of the thermal concentration fluctuation in the single-phase region depending on T_i, which affects the subsequent SD over an extended period of time.