Chirality in Block Copolymer Melts: Mesoscopic Helicity from Inter-Segment Twist

TL;DR

This paper investigates how segment-level chirality influences the thermodynamics and mesoscopic structures in block copolymer melts, revealing a critical dependence on the cholesteric pitch that leads to chiral mesophases.

Contribution

It introduces a self-consistent field theory analysis showing the emergence of chiral mesophases depending on the cholesteric pitch in block copolymer melts.

Findings

Achiral mesophases at large pitch

Chiral phases appear below a critical pitch

Identification of a cylinder-to-helix transition

Abstract

We study the effects of chirality at the segment scale on the thermodynamics of block copolymer melts using self consistent field theory. In linear diblock melts where segments of one block prefer a twisted, or cholesteric, texture, we show that melt assembly is critically sensitive to the ratio of random coil size to the preferred pitch of cholesteric twist. For weakly-chiral melts (large pitch), mesophases remain achiral, while below a critical value of pitch, two mesocopically chiral phases are stable: an undulated lamellar phase; and a phase of hexagonally-ordered helices. We show that the non-linear sensitivity of meso-scale chiral order to preferred pitch derives specifically from the geometric and thermodynamic coupling of the helical mesodomain shape to the twisted packing of chiral segments within the core, giving rise to a second-order cylinder-to-helix transition.