Morphological characteristics of lamellar ABA triblock copolymers: A self-consistent field treatment

Abstract

Symmetric ABA triblock copolymers consist of two contiguous monomer sequences of one species (A) separated by a middle sequence of a chemically dissimilar species (B). If the blocks are incompatible, the copolymers order into the same periodic morphologies as their AB diblock analogs. Unlike the single-grafted blocks of an AB copolymer, however, the B midblock of an ABA copolymer molecule is grafted at both ends and may adopt either a bridged or looped conformation. Since the mechanical properties of ordered ABA copolymers are dependent on midblock conformation, monomer fraction, f, chain length, N, and on the interaction parameter, χ, a theoretical formalism capable of incorporating these characteristics into the free energy potential of lamellar copolymers has been developed. Predictions obtained in this work reveal that an equilibrium fraction of bridged midblocks exists, decreasing with N but varying little with f. Since the extent of bridging could conceivably be controlled through shear-induced molecular alignment during microphase ordering, the effect of midblock bridging on morphology is also examined, as is the assumption that, in the strong-segregation regime, a looped midblock possessing N B monomers adopts a hairpin conformation, behaving as two single-grafted blocks of N B 2 monomers.