Polydispersity-driven shift in the lamellar mesophase composition window of PEO-PB-PEO triblock copolymers

The influence of broad and continuous center block polydispersity on the melt-phase self-assembly of OBO triblock copolymers (O = poly(ethylene oxide) and B = poly(1{,}4-butadiene)) is reported for a series of samples derived from tandem chain transfer ring-opening metathesis polymerization (ROMP-CT) and anionic ring-opening polymerization (AROP). By virtue of the polymerization techniques employed in these syntheses{,} the midblocks exhibit polydispersity indices Mw/Mn = 1.75{,} whereas the end blocks have relatively narrow dispersities Mw/Mn [less-than-or-equal] 1.25. Using a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to characterize these materials and their narrow dispersity homologues derived from anionic polymerization{,} we demonstrate that the combination of both chain length and composition polydispersity inherent in these polydisperse triblocks shifts the composition window of stability for the lamellar mesophase. Furthermore{,} these studies reveal that polydispersity in the center B segments of these OBO triblocks results in large lamellar domain spacing increases{,} enables stable coexistence of two morphologies in a single sample{,} and frustrates lattice ordering in a composition-dependent manner.