Shear-induced parallel and transverse alignments of cylinders in thin films of diblock copolymers
YL Chen and Q Xu and YF Jin and X Qian and R Ma and J Liu and DX Yang, SOFT MATTER, 14, 6635-6647 (2018).
Coarse-grained Langevin dynamics simulations were performed to investigate the alignment behavior of monolayer films of cylinder- forming diblock copolymers under steady shear, a structure of significant importance for many technical applications such as nanopatterning. The influences of shear conditions, the interactions involved in the films, and the initial morphology of the cylinder- forming phase were examined. Our results showed that above a critical shear rate, the cylinders can align either along the shearing direction or transverse (log-rolling) to the shearing direction depending on the relative strength between the interchain attraction in the cylinders (epsilon(AA)) and the surface attraction of the confining walls with the film (epsilon(BW)). To understand the underlying mechanism, the microscopic properties of the films under shear were systematically investigated. It was found that at low epsilon(AA)/epsilon(BW), the majority blocks of the diblock polymer that are adsorbed on the confining walls prefer to move synchronously with the walls, inducing the cylinder-forming blocks to align along the flow direction. When epsilon(AA)/epsilon(BW) is above a threshold value, a strong attraction between the cylinder-forming blocks restrains their movement during shear, leading to the log-rolling motions of the cylinders. To predict the threshold epsilon(AA)/epsilon(BW), we developed an approach based on equilibrium thermodynamics data and found good agreement with our shear simulations.
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