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Using External Fields to Control the Location of Nanoparticles In Block Copolymers: Experiments and Simulations

Vibha Kalra1, Jinwoo Lee2, Sergio Mendez1, Fernando Escobedo1, and Yong L. Joo1. (1) School of Chemical and Biomolecular Engineering, Cornell University, 120 Olin Hall, Ithaca, NY 14853, (2) Department of Chemical Engineering and School of Environmental Science & Engineering, Pohang University of Science and Engineering, xxx xxx xxx, Pohang, xxx, South Korea

Nanocomposite nanofiber mats (100-400 nm diameters) with internal self assembly used to tailor nanoparticle location have been fabricated. The fibers are obtained using coaxial electrospinning with symmetric poly(styrene-b-isoprene)/magnetite NP as the core and silica (synthesized using sol gel technique) as the shell. Silica shell provides thermal stability to the fibers and helps to anneal the materials at temperatures higher than polymer glass transition to obtain equilibrium, confined self assembly without destroying the fiber morphology. The magnetite NPs (4 nm) used in this work, are surface coated with oleic acid to provide slight selectivity towards isoprene domain while preserving their magnetic moment. All nanofiber samples are characterized using scanning electron microscopy (SEM), small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). A systematic study on effect of NP:polymer volume ratio on material self assembly has been conducted. One of our key findings is that strong deformation (~10000 s-1) and fast solvent evaporation (200 nl/s) during electrospinning enables selective, uniform dispersion of magnetite NPs in the isoprene domain for as high as 20 wt% NPs with respect to isoprene in spite of strong magnetic inter-particle interactions. Magnetic properties are measured using superconducting quantum interference device (SQUID) magnetometer and all nanocomposite fiber samples exhibit superparamagnetic behavior.

To further understand the effect of flow conditions on nanoparticle location in block copolymers, we have performed coarse grained molecular dynamics simulations using dissipative particle dynamics (DPD) thermostat. We modeled and simulated the behavior of symmetric diblock copolymer/nanoparticle systems under simple shear flow. We considered three systems such that; a) NPs have selective interactions with one block, b) NPs have equal interactions with both blocks and c) NP-NP interactions are dominant. We find that shear has a significant effect on prevention of NP aggregation in cases where NP-NP attraction is dominant and this effect of shear is a strong function of both nanoparticle size and polymer chain length. These results qualitatively match with our experimental results on PS-b-PI/magnetite NP nanofibers.