Self-Assembly of Diblock Copolymer on Substrates Modified by Random Copolymer Brushes

Abstract

We model the self-assembly of a diblock copolymer thin film in contact with a random copolymer brush using self-consistent field theory employing a quenched distribution for the brush chains. We focus on the regime of parameters where the diblock copolymers exhibit lamellar morphologies, and study the alignment behavior of the lamellar morphologies on the grafted substrates. Our results reveal a templating of the self-assembly morpology by the brush chains. We find two novel features of this templating behavior: The ends of the grafted chains rearrange themselves to create a more favorable interface, an effect which is present in both the parallel and perpendicular morphologies, and is enhanced with increasing blockiness of the sequences of the random copolymer. In addition, the brush chains may splay laterally in perpendicular morphologies and enrich the interface even further in the favorable component. The latter feature leads to nontrivial free energy differences between the parallel and perpendicularly aligned lamellae on the grafted surface. We explicitly find the parametric window for the stability of perpendicular lamellae and compare against the trends suggested by surface energies of the pure homopolymeric components. Such comparisons indicate that viewing the grafted surface purely in terms of the surface energies of the components of the diblock copolymer may not necessarily capture the stabilities of the parallel and perpendicular morphologies.

Field-theoretic simulations of block copolymers : design and solvent annealing.

“…Interestingly, Han et al reported thermally annealed samples of thick films (∼500nm or ∼14 d 0 ) which exhibited C ⊥ , supporting the idea that truly neutral conditions promote perpendicular alignment as well, although these conditions require great care to achieve [14]. The stability of C ⊥ in these results may be partly explained by a recent study demonstrating that random copolymer brushes actually enhance the stability of L ⊥ over neutral, neat surfaces, a result we expect applies to cylinders as well [33]. A comprehensive understanding of micro-phase separation requires addressing a broad range of possible influences, but one of the most puzzling and difficult to test experimentally is the effect of solvent evaporation rate.…”

Thermal Approaches to Perpendicular Block Copolymer Microdomains in Thin Films: A Review and Appraisal

“…Another assumption made about grafted random copolymers is that they act as hard walls during thermal annealing of the overlying BCP. However, recent studies have shown that the underlying grafted random copolymer undergoes significant restructuring and interpenetration into the overlying BCP layer to maximize A–A and B–B contact, obviously a phenomenon that does not occur in homopolymer brushes . Furthermore, the studies show that “randomness” or “blockiness” of the monomer sequence in random copolymer also influences the BCP orientation.…”

“…Vaporized DVB was condensed onto and subsequently polymerized on the film surface via free radical polymerization (FRP), while the high flux of free radicals onto the film surface caused the BCP surface layer to become chemically grafted to the resulting cross‐linked PDVB mat ( Figure a). This effectively created a top coat whose interface with the underlying BCP is the grafted, immobilized BCP itself which can function like a neutral layer (recall that blocky random copolymers may also act as efficient neutral layers) . The potential of iCVD top coats was further demonstrated by photo‐patterning the top coat into arbitrary shapes (Figure B).…”

Controlling interfacial interactions for directed self assembly of block copolymers

“…Relative stability of parallel ( ) and perpendicular ( ) lamellar domain orientations ( AB diblock) as a function of substrate interactions ΔF=F∥−F⊥, where and are the free energies of each orientation.…”