Time-resolved small-angle neutron scattering study of spinodal decomposition in deuterated and protonated polybutadiene blends. I. Effect of initial thermal fluctuations

Time-resolved small-angle neutron scattering (SANS) experiments have been performed on the self-assembling process of a binary mixture of deuterated polybutadiene and protonated polybutadiene at the critical composition. This mixture has an upper critical solution temperature type of phase diagram with the spinodal temperature at 99.2 °C. Specimens held in the single-phase state at an initial temperature (T_i) were quenched to a point inside the spinodal phase boundary at a final temperature (T_f) to induce phase separation via spinodal decomposition (SD). In order to examine the effect that thermal concentration fluctuations have on SD, three different initial temperatures, T_i= 102.3 °C, 123.9 °C, and 171.6 °C, were chosen while T_f was fixed at -7.5 °C. The time-dependent SANS structure factor, S(q,t;T_f), showed clear scattering peaks corresponding to the early and intermediate stages of SD. The time changes in the wave number q_m(t;T_f) and the intensity S _m(t;T_f) at the peak of S(q,t;Tf) followed different paths depending on the initial temperature. This fact evidences a definite effect of thermal concentration fluctuations on SD (i.e., a significant "memory" effect). A critical test of the linearized Cahn-Hilliard-Cook theory led to the conclusion that this theory can describe satisfactorily the early stage SD in the deep-quench region.