Direct observation of the percolation-to-cluster transition of an off-critical mixture of polybutadiene and polyisoprene was achieved by optical microscopy. According to optical evidence, the transition occurs in a slow and a fast step not revealed in parallel light scattering experiments.

The typical interconnected bicontinuous percolation structure of a critical 50/50 wt % polybutadiene (PB)/polyisoprene (PI) mixture was characterized in detail by the structure function of a light scattering experiment and by statistical image processing of optical microscope images. The time development of the interdomain distance and the local radius of the curved interface in the late stage of the spinodal decomposition are obtained for the first time for the same system by both methods and are found to be in good agreement. PB/PI blends are systems with a lower critical solution temperature (LCST). The critical point was varied within a wide temperature range by changing the 1,2 content of the PB. For optical microscopy, the structure was fixed by photo-cross-linking and the optical contrast was achieved in a second step by selective staining of the PI phase by exposing the sample to iodine vapor.

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