Structure self‐assembling in the spinodal decomposition (SD) of polymer blends in its late stage has been explored for a near‐critical mixture of polybutadiene and polyisoprene by a time‐resolved light scattering technique, with a particular emphasis on the time evolution of the interface structure. By analysis of a scaled structure factorF(x, t) ≡I(q, t)qm(t)3over wide ranges of a reduced scattering vectorx≡q/qm(t) and time, it was found relevant to divide the late stage of SD into two stages, I and II. Here,I(q, t) denotes the scattered intensity as a function of the scattering vectorqand timet. In the intermediate stage preceding the late one,F(x, t) became sharper with its peak atx= 1 increasing witht. However, as time elapsed,F(x, t) turned out to be universal fort, first in the range ofxsmaller than about 2 and then over the entire range ofxaccessible by the present experiment. The time interval in which the former occurred is defined as late stage I and the one in which the latter was realised is called late stage II. In late stage I, the average thickness of phase‐phase interfaces decreases towards an equilibrium value and the time evolution of the interfacial area densityΣ(t) does not scale withqm(t),i.e.the exponentsγandαin the power lawsΣ(t) ~t−γandqm(t) ~t−αdo not coincide (actually,α<γ). Late stage II corresponds to the process in which these exponents become equal and the interface thickness reaches equilibrium. Such conditions probably ensure the establishment of a complete dynamical scalin
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