(351a) Fabrication of Hierarchical Sorbents By a Combined 3D-Printing and in-Situ Phase Separation Process from Carbon Nanotube-Enriched Polymer Solutions

Sorbent platforms with structures tailored at the molecular through device scales are needed to meet the demand for separation systems that can isolate trace analytes from complex mixtures in an energy-efficient manner. Non-solvent induced phase separation is a common technology used to create thin polymer membrane-based sorbents, which are attractive due to their rapid mass transfer characteristics. However, membrane sorbents suffer from relatively low binding capacities. While smaller pores and thicker membranes can increase the surface area and number of binding solute sites, these changes will inevitably decrease the permeability of the membranes. Addressing this tradeoff and translating lab scale discoveries of binding chemistries to functioning devices requires the development of manufacturing methods that provide control over sorbent structures at the molecular through meter scales. Here, direct-ink-writing (DIW) 3D printing is combined with a surface-segregation and vapor-induced phase separation (SVIPS) process to create hierarchically-structured sorbents that satisfy this criterion.

Composite inks containing polysulfone, polystyrene-block-poly(acrylic acid) (PS-PAA), and carbon nanotubes were extruded to form woodpile structures with micrometer-scale channels between the printed filaments. Simultaneously, because the DIW process occurred within a controlled humidity environment, the SVIPS process generates a fully-interconnected network of nanoscale pores within the filaments as they are extruded to form the sorbent matrix. The ink formulation is critical to enabling the combined DIW and SVIPS method. For example, shear and Dripping-onto-substrate (DOS) rheometry measurements were used to demonstrate the influence of carbon nanotubes on the shear and extensional viscosities of the ink. Specifically, adding carbon nanotubes resulted in more pronounced shear thinning behavior, increased the extensional viscosity, and delayed the pinch-off time, which enable the deposition of more regular micrometer-scale patterns. Additionally, quantifying the thermodynamic properties of the ink demonstrates their impact on the gelation time scale within different humidity environments. The ability to engineer these natural time scales through the ink formulation and modify the printing conditions to match the processing time with these time scales provides insights for designing protocols that can be extended to different ink systems, as well as providing for better control over the hierarchical sorbent structures.

In addition to creating high surface area nanoporosity, the SVIPS mechanism drives the PAA blocks to the surface of the pore walls, thereby providing functional handles that allow the sorbent chemistry to be further tailored. In this work, sequential carbodiimide coupling reactions were used to introduce branched-polyethylenimine (PEI) and then terpyridine (Terp) functionality, which increased the density of binding sites and affinity towards transition metal ions, respectively. The metal ion capture performance that results from this molecular design is demonstrated through the selective recovery of Co²⁺ from solutions containing cobalt and lithium dissolved at pH 1. At the device-scale, the microstructure of the sorbents results in high throughput devices (i.e., hydraulic permeabilities of ~10⁵ L m^-2 h^-1 bar^-1) that demonstrate the efficient (>95%) removal of copper from 1 ppm feed solutions during dynamic flowthrough experiments.

In conclusion, incorporating 3D printing technique with SVIPS casting enables the creation of sorbents with well-controlled hierarchical structures that achieve high binding capacity, affinity, and permeability simultaneously. Ultimately, the successful design, printing, and control of porous nanostructure provide new strategies for the fabrication of next-generation sorbents.