ma300500z_si_001.pdf (156.53 kB)
Long-Range Ordering of Symmetric Block Copolymer Domains by Chaining of Superparamagnetic Nanoparticles in External Magnetic Fields
journal contribution
posted on 2012-12-11, 00:00 authored by Vinay Raman, Arijit Bose, Bradley D. Olsen, T. Alan HattonA new method to achieve long-range
orientational order in symmetric
diblock copolymer nanodomains through the alignment and chaining of
superparamagnetic nanoparticles in a magnetic field is investigated
computationally and theoretically. The effects of nanoparticle size,
volume fraction, and magnetization strength are explored using the
hybrid particle field (HPF) technique for particles that selectively
segregate into one domain of a symmetric diblock copolymer assembly.
A critical selectivity of the particles for one nanodomain is observed,
above which strong alignment results and below which comparatively
disordered structures are formed. The 2D simulations reveal that,
for a given nanoparticle volume fraction, only a nanoparticle size
commensurate with the block copolymer domain spacing yields well-aligned
nanostructures. Nanoparticles significantly larger than the domain
spacing break the symmetry of the lamellar phase and result in poor
alignment, while high defect densities are observed for smaller particles
owing to colloidal jamming within the preferred domains. Small field
strengths produce a high degree of alignment in simulations, but as
corroborated by scaling calculations, high magnetization strengths
are required to lower the equilibrium defect density for such nematic–isotropic
phase transitions in lamellar thin films. Simulations also elucidate
a window of optimal nanoparticle volume fractions over which alignment
is achieved. For low nanoparticle volume fractions, only local alignment
is observed, while high volume fractions lead to an order–order
phase transition from lamellae to a hexagonal phase.