Complex Spherical Phases in a Zwitterion-tethered Diblock Copolymer Melt

A recent surge of work has taken place over recent years concerning complex spherical morphologies in block polymers, including Frank-Kasper phases. The stability of such phases has been shown to rely heavily on conformational asymmetry which captures differences in statistical segment lengths between the constituent blocks. While differing polymer chemistries offer a small range of statistical segment lengths, conformational asymmetry can be enhanced via architectural and polymer dispersity modifications. Using these ideas, the number of neutral (non-ionic) block polymers exhibiting complex spherical phases is growing quickly while continuing to remain exceptionally rare for ion-containing block polymers. This may be due in part to the strong impact electrostatic interactions have on the phase behavior of block polymers. Since continuous morphologies such as Frank-Kasper phases are highly desirable in the design of ion-containing block polymers, it is important that we understand the relationships between conformational asymmetry, electrostatics, and the overall self-assembly of such systems.

In this work, the Frank-Kasper A15 phase is observed, preceded by other metastable micellar phases, in two zwitterion-tethered diblock polymers of differing charge fraction, both characterized with higher molecular weight (> 15 kDa) than what is commonly observed in complex spherical assemblies. The system of inquiry contains architectural asymmetry, wherein the volumetric majority block comprises short neutral and zwitterionic side chains. Using dielectric relaxation spectroscopy, small angle X-ray scattering, rheology, and atomic force microscopy, relationships between zwitterionic behavior and the overall self-assembly will be presented.