Simulation-Aided Design
and Synthesis of Hierarchically
Porous Membranes
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Abstract
Free-standing silica membranes with hierarchical porosity
(ca.
300 nm macropores surrounded by 6–8 nm mesopores) and controllable
mesopore architecture were prepared by a dual-templating method, with
the structural design aided by mesoscale simulation. To create a two-dimensional,
hexagonal macropore array, polymeric colloidal hemisphere arrays were
synthesized by a two-step annealing process starting with non-close-packed
polystyrene sphere arrays on silicon coated with a sacrificial alumina
layer. A silica precursor containing a poly(ethylene) oxide–poly(propylene
oxide)–poly(ethylene) oxide (PEO–PPO–PEO) triblock-copolymer
surfactant as template for mesopore creation was spin-coated onto
the support and aged and then converted into the free-standing membranes
by dissolving both templates and the alumina layer. To test the hypothesis
that the mesopore architecture may be influenced by confinement of
the surfactant-containing precursor solution in the colloidal array
and by its interactions with the polymeric colloids, the system was
studied theoretically by dissipative particle dynamics (DPD) simulations
and experimentally by examining the pore structures of silica membranes
via electron microscopy. The DPD simulations demonstrated that, while
only tilted columnar structure can be formed through tuning the interaction
with the substrate, perfect alignment of 2D hexagonal micelles perpendicular
to the plane of the membrane is achievable by confinement between
parallel walls that interact preferentially with the hydrophilic components
(PEO blocks, silicate, and solvent). The simulations predicted that
this alignment could be maintained across a span of up to 10 columns
of micelles, the same length scale defined by the colloidal array.
In the actual membranes, we manipulated the mesopore alignment by
tuning the solvent polarity relative to the polar surface characteristics
of the colloidal hemispheres. With methanol as a solvent, columnar
mesopores parallel to the substrate were observed; with a methanol–water
mixed solvent, individual spherical mesopores were present; and with
water as the only solvent, twisted columnar structures were seen