Annealing and Defect Trapping in Lamellar Phases of Triblock Terpolymers

Equilibrium morphologies in molten ABC triblock terpolymers are much more difficult to attain than in AB diblocks. Here, we show that even the simplest lamellar structures exhibit high sensitivity to preparation conditions and that strongly trapped structural defects inherent to ABC triblock architecture cannot be removed by long annealing. Poly(styrene)-block-poly(butadiene)-block-poly(methyl methacrylate) (SBM) triblock terpolymers with nearly symmetric composition and low molar mass are studied by combining transmission electron microscopy with dynamical mechanical analysis and small-angle X-ray scattering. We find that annealing can induce a transition from a lamellar structure in which S and M end blocks are mixed together to a lamellar structure where all three components S, B, and M are segregated. The presence of B “loops” and B “bridges” is at the origin of characteristic defects which are difficult to eliminate. In a blend of SBM triblock terpolymer with a poly(styrene)-block-poly(butadiene) (SB) diblock having the same block length, diblock chains are completely incorporated in the lamellar structure obtained after solvent evaporation. However, annealing induces a macroscopic phase separation of SB and SBM chains. We propose a scenario for the separation mechanisms based on highly confined diffusion of SB chains within the SBM ordered structures.