Order and Phase Behavior of Thin Film of Diblock Copolymer-Selective Nanoparticle Mixtures: A Molecular Dynamics Simulation Study
LS Shagolsem and JU Sommer, MACROMOLECULES, 47, 830-839 (2014).
By means of molecular dynamics simulations, we study AB diblock copolymer and nanoparticle mixtures confined between two identical walls in slit geometry. The nanoparticles are selective to the minority A-block, while the walls are neutral to both copolymer and nanoparticle. We obtained the various structures of the copolymer nanocomposites and are summarized in a phase diagram constructed in diblock composition and nanoparticle concentration space. In comparison to the phase diagram in bulk, we observe a much wider lamellar region with a broad class of lamellar structures, and the phase boundaries are shifted with increasing nanoparticle concentration. We find that both vertically and horizontally oriented lamellar structures are realized. The vertically oriented lamellae are formed by slightly asymmetric and symmetric diblock copolymers at low nanoparticle concentrations and have a very limited region of stability in the phase space, whereas the horizontally oriented lamellae are formed by asymmetric copolymer at large nanoparticle concentrations. In the vertically oriented lamellae, the segregated nanoparticles at the polymer wall interfaces form nanoparticle monolayer above the A-domains and exclude A-monomers from this region. Consequently, the copolymer interface lines near walls are perturbed; also, the chains close to the walls are overstretched compared to the bulk. For horizontally oriented lamellae there is no overstretching of chains near the walls. The test of stability of the lamellar structures against the different thermodynamic pathways is also performed. Lastly, by considering the horizontal lamellae, we study the effect of nanoparticle concentration on the lamellar layer thickness.
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