Equilibrium behavior of symmetric ABA triblock copolymer melts

Melts of ABA triblock copolymer molecules with identical end blocks are exa mined using self-consistent field theory (SCFT). Phase diagrams are calcula ted and compared with those of homologous AB diblock copolymers formed by s nipping the triblocks in half. This creates additional end segments which d ecreases the degree of segregation. Consequently, triblock melts remain ord ered to higher temperatures than their diblock counterparts. We also find t hat middle-block domains are easier to stretch than end-block domains. As a result, domain spacings are slightly larger, the complex phase regions are shifted towards smaller A-segment compositions, and the perforated-lamella r phase becomes more metastable in triblock melts as compared to diblock me lts. Although triblock and diblock melts exhibit very similar phase behavio r, their mechanical properties can differ substantially due to triblock cop olymers that bridge between otherwise disconnected A domains. We evaluate t he bridging fraction for lamellar, cylindrical, and spherical morphologies to be about 40%-45%, 60%-65%, and 75%-80%, respectively. These fractions on ly depend weakly on the degree of segregation and the copolymer composition . (C) 1999 American Institute of Physics. [S0021-9606(99)51139-2].