Processive Pathways to Metastability in Block Copolymer Thin Films

Abstract

Block copolymers (BCPs) self-assemble into intricate nanostructures that enhance a multitude of advanced applications in semiconductor processing, membrane science, nanopatterned coatings, nanocomposites, and battery research. Kinetics and thermodynamics of self-assembly are crucial considerations in controlling the nanostructure of BCP thin films. The equilibrium structure is governed by a molecular architecture and the chemistry of its repeat units. An enormous library of materials has been synthesized and they naturally produce a rich equilibrium phase diagram. Non-equilibrium phases could potentially broaden the structural diversity of BCPs and relax the synthetic burden of creating new molecules. Furthermore, the reliance on synthesis could be complicated by the scalability and the materials compatibility. Non-equilibrium phases in BCPs, however, are less explored, likely due to the challenges in stabilizing the metastable structures. Over the past few decades, a variety of processing techniques were introduced that influence the phase transformation of BCPs to achieve a wide range of morphologies. Nonetheless, there is a knowledge gap on how different processive pathways can induce and control the non-equilibrium phases in BCP thin films. In this review, we focus on different solvent-induced and thermally induced processive pathways, and their potential to control the non-equilibrium phases with regards to their unique aspects and advantages. Furthermore, we elucidate the limitations of these pathways and discuss the potential avenues for future investigations.

Revealing the Kinetic Phase Behavior of Block Copolymer Complexes Using Solvent Vapor Absorption–Desorption Isotherms

“…Polystyrene- block -poly­(4-vinylpyridine) (PS- b -P4VP)-based supramolecular BCP complexes are a convenient choice to study the self-assembly kinetics due to the ability of nitrogen in the pyridine ring to effectively create hydrogen bonds with electrophiles such as 3-pentadecylphenol (3-PDP) (Figure a,b). The use of thermal annealing to understand the complex kinetics of supramolecular self-assembly is often limited by weak secondary interactions and thermal volatility of the small molecule. , Therefore, solvent vapor annealing (SVA) presents an alternative processive pathway for tuning morphologies and expanding the energy landscape of BCPs. − Solvent vapor molecules diffuse into one or both blocks and increase the chain mobility by acting as plasticizers. While the plasticization effect enhances the self-assembly kinetics, this further evolves the complexity of the self-assembly.…”

Substrate Neutrality for Obtaining Block Copolymer Vertical Orientation

“…However, the use of BCPs for patterning requires orientational control using a templated substrate, a process formally known as directed self-assembly (DSA). The BCPs are subjected to either thermal or solvent annealing or a combination of both for the lattice structure to propagate on the template [10][11][12]. The templated substrate is critical to DSA and it serves two purposes: (i) vertical orientation, where phase-segregated blocks face the top surface (the central theme of this review); and (ii) alignment of the lattice along the substrate for long-range order [1,2,5,13].…”