Memory effects in homopolymer blends during annealing

We perform light scattering and direct optical experiments on a homopolymer blend of poly(methylphenylsiloxane) (PMPS) with polystyrene (PS). The system is subjected to the three-step process. The system is first quench to low temperature (T-1) and allowed to separate for 5 h; next it is heated to a high temperature (T-0) to the one-phase region where it mixes for a couple of minutes (1-10 min) and then quenched back to T-1 and observed for 5 h. We note that annealing at T-0 can be quantitatively studied by the analysis of the scattering intensity summed over a linear array of photodiodes. This quantity is very sensitive to the structure exisiting in the system. If the system is properly annealed, it has a noisy behavior and while the structure inside the system persists, it behaves very regularly. Moreover, one can observe the differences in the scattering intensity between the first and the second quench at very short wavevectors, indicating that large domains survived the annealing process for short annealing time (less than 4 min). However, the average area of the domains per unit volume is the same as obtained from the tail of the scattering intensity, indicating that small domains dominating in the system do not survive the mixing process even if it is very short (2 min). Finally, the direct observation under the microscope reveals that they dissolve in such a way that their size changes at the end of the process of dissolution, when as we suspect the size of the interface becomes comparable to the size of the dissolving domain. Domains inside the domains are also observed at short times after the second quench. In general, our methods allow the quantitative estimate of the annealing time for polymer mixtures and thus can save a lot of time, especially if we have to repeat the same measurements many times and we need to anneal the samples between measurements.