Structurally Asymmetric Porous Carbon Materials with Ordered Top Surface Layers from Nonequilibrium Block Copolymer Self-Assembly

Abstract

Inorganic materials with asymmetric pore structures provide increased accessibility and flux, making them attractive for applications in energy conversion and storage, separations, and catalysis. Non-equilibrium-based block copolymer structure-directed self-assembly approaches provide routes to obtaining such materials. We report a one-pot synthesis using the co-assembly and non-solvent-induced phase separation (CNIPS) of poly­(isoprene)-b-poly­(styrene)-b-poly­(4-vinylpyridine) (ISV) triblock terpolymer and phenol formaldehyde resols. After heat-treatment, carbon materials with asymmetric pore structures result. They have a mesoporous top surface atop a porous support with graded porosity along the film normal. The walls of the macroporous support are also mesoporous, providing an additional structural hierarchy and increased specific surface area. We demonstrate how successfully navigating the pathway complexity associated with the nonequilibrium approach of CNIPS enables switching from disordered to ordered top surfaces in the as-made organic–organic hybrids and resulting carbon materials after thermal treatments. To that end, a combination of ex situ transmission small-angle X-ray scattering (SAXS) of the membrane dope solutions, in situ grazing-incidence SAXS (GISAXS) after dope solution blading and during solvent evaporation, and scanning electron microscopy (SEM) of the final membrane structures was used. We expect the final porous carbon materials exhibiting a combination of asymmetric, hierarchical pore structures and well-defined mesoporosity throughout the material to be of interest for a number of applications, including batteries, fuel cells, electrochemical double-layer capacitors, and as catalyst supports.

GISAXS: A versatile tool to assess structure and self‐assembly kinetics in block copolymer thin films

“…In general it is good practice to combine the global information obtained with scattering methods with nanoscale imaging methods that reveal the local structure and defects. 8,10,11 GISAXS scattering patterns are closely related to patterns in fiber diffraction, 49 both representing 2D powders with either a preferential plane or a preferential axis, respectively. Indexing is a crucial step in the identification of the BCP structure, the preferential alignment of the BCP domains, as well as possible swelling or shrinking of the BCP unit cell due to solvent annealing or drying.…”

“…The same holds also vice versa: lying cylinders and standing lamellae look very similar in AFM, but can be distinguished via the GISAXS structure and form factors. In general it is good practice to combine the global information obtained with scattering methods with nanoscale imaging methods that reveal the local structure and defects 8,10,11 …”

“…An important application emerged at the time: using block copolymer morphology as a template to create nanostructured inorganic materials 8–11,74,75 . Here an organic precursor is dissolved preferentially in one of the blocks.…”

“…However, a direct conversion of this material by pyrolysis which crosslinks the resol additive and eventually transforms it into carbon, did not yield the desired regular surface pore structure which ensures high membrane throughput and small pore polydispersity. Hence the SNIPS process was again invoked to freeze in the structure in the surface layer 10 . Two methods of solution preparation were compared: In method A ISO powder was added to the resol solution.…”

“…A characteristic feature of block copolymer solutions is that microphase separation sets in already, when there is still a significant amount of solvent in the swollen blocks. 3 Modern applications of BCPs are often based on thin films and range from 2D nanoscale patterning templates [4][5][6][7] and 3D scaffolds for solar cells, catalysts and battery electrodes [8][9][10] to ultrafiltration membranes. [11][12][13][14] Smooth thin films with thicknesses ranging from a few 10 nm up to a 1 μm are readily prepared using spin coating where a drop of solution is placed on a spinning substrate and the spreading film thins under the action of the centrifugal force and evaporation.…”