The Chain Flexibility Effects on the Self-assembly of Diblock Copolymer in Thin Film

TL;DR

This study explores how chain flexibility influences the self-assembly and orientation of diblock copolymer lamellae in thin films, revealing that rigidity and surface roughness significantly affect phase stability and transitions.

Contribution

It introduces a worm-like chain self-consistent field theory to analyze the effects of chain flexibility on lamellar orientation stability in confined thin films, highlighting new insights into phase behavior.

Findings

Increased chain rigidity favors perpendicular lamellae stability on flat surfaces.

Surface roughness enhances perpendicular phase stability for flexible chains.

Rigid chains show decreased perpendicular stability with increased substrate roughness.

Abstract

We investigate the effects of chain flexibility on the self-assembly behavior of symmetric diblock copolymers (BCPs) when they are confined as a thin film between two surfaces. Employing worm-like chain (WLC) self-consistent field theory, we study the relative stability of parallel (L$_{\parallel}$) and perpendicular (L$_{\perp}$) orientations of BCP lamellar phases, ranging in chain flexibility from flexible Gaussian chains to semi-flexible and rigid chains. For flat and neutral bounding surfaces (no surface preference for one of the two BCP components), the stability of the L$_{\perp}$ lamellae increases with chain rigidity. When the top surface is flat and the bottom substrate is corrugated, increasing the surface roughness enhances the stability of the L$_{\perp}$ lamellae for flexible Gaussian chains. However, an opposite behavior is observed for rigid chains, where the L$_{\perp}$ stability decreases as the substrate roughness increases. We further show that as the substrate roughness increases, the critical value of the substrate preference, $u^{*}$, corresponding to an L$_{\perp}$-to-L$_{\parallel}$ transition, decreases for rigid chains, while it increases for flexible Gaussian chains. Our results highlight the physical mechanism of tailoring the orientation of lamellar phases in thin-film setups. This is of importance, in particular, for short (semi-flexible or rigid) chains that are in high demand in emerging nanolithography and other industrial applications.