Characterizing interface shape evolution in immiscible polymer blends via 3D image analysis.

The coordinate transformation (CT) method was applied to measure the local curvature of the interface of an immiscible polymer blend made of fluorescently labeled polystyrene (FLPS) and styrene-ran-acrylonitrile copolymer (SAN). The CT method involves the local parametrization of the interface by a quadratic polynomial to compute the local values of the mean (H) and the Gaussian (K) curvatures. Distributions of the curvatures at different annealing times were obtained by measuring H and K at many (typically 10(7)) points on the interface. Coarsening of a symmetric (50/50 w/w FLPS/SAN) and a nonsymmetric (35/65 w/w) blend was monitored. For the symmetric blend, two regimes of surface evolution were identified: in the early stage, the probability densities of the curvatures at various times were successfully scaled by a time-dependent characteristic length, i.e., interface area per unit volume (Q). This behavior has been previously observed in blends with morphologies created by a different mechanism, namely spinodal decomposition. In the late stage, the dynamic scaling failed and the time evolution of the interface slowed down. For the nonsymmetric blend, the domains of the minor phase (FLPS) were more elongated and they eventually broke up producing a composite microstructure with islands of drops within cocontinuous domains. We defined a "scaled" genus (G) to quantify the topology evolution of the blends during coarsening. Loss of connectivity was evidenced by a decrease of G with time for the nonsymmetric blend, while a constant value of this parameter indicated no change in topology during coarsening for the symmetric blend.