Simulations of shear-induced morphological transitions in block copolymers
A Nikoubashman and RA Register and AZ Panagiotopoulos, SOFT MATTER, 9, 9960-9971 (2013).
In this review, we focus on simulation studies performed to provide a molecular-level understanding of shear-induced morphological transitions and domain alignment of block copolymers in the bulk and in thin films. Block copolymers are highly relevant for many scientific and industrial applications due to their ability to form uniform domains of controllable shape at nanometer length scales. In the bulk, morphologies depend on the constituent block interactions and their volume fractions, while for thin films the surface properties and film thickness also play important roles. Spontaneously formed samples do not usually have the long-range order required in many applications. Long-range order can be induced by external guidance, for example using electric fields, surface patterns, or shear forces. In particular, shearing of both bulk systems and thin films is an excellent method for achieving long-range order, and remarkable progress has been made in experimental techniques for controlling pattern formation and transferring them to materials of interest, e.g., metal nanowires. Many simulation studies of pattern formation in copolymer systems have been performed, but simulations using explicit representations of the chains and incorporating shear have only been attempted in recent years. Because of length-and time- scale limitations, most simulations of shear alignment are performed on coarse-grained models. We survey the methods used for obtaining parameters for coarse-grained models to represent specific block copolymer systems, and the simulation algorithms utilized to impose shear. The simulations are in general agreement with experiments on the relative ease of alignment of lamellar, cylinder-forming, and sphere- forming systems, and provide insights into alignment mechanisms. Both simulations and experiments display a strong dependence of alignment quality on film thickness and substrate-polymer interactions. The review closes with a summary of unresolved questions for future research.
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