Membranes with Highly Ordered Straight Nanopores by Selective Swelling of Fast Perpendicularly Aligned Block Copolymers

Abstract

Membranes with uniform, straight nanopores have important applications in diverse fields, but their application is limited by the lack of efficient producing methods with high controllability. In this work, we reported on an extremely simple and efficient strategy to produce such well-defined membranes. We demonstrated that neutral solvents were capable of annealing amphiphilic block copolymer (BCP) films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) with thicknesses up to 600 nm to the perpendicular orientation within 1 min. Annealing in neutral solvents was also effective to the perpendicular alignment of block copolymers with very high molecular weights, e.g., 362 000 Da. Remarkably, simply by immersing the annealed BCP films in hot ethanol followed by drying in air, the originally dense BCP films were nondestructively converted into porous membranes containing highly ordered, straight nanopores traversing the entire thickness of the membrane (up to 1.1 μm). Grazing incident small-angle X-ray spectroscopy confirmed the hexagonal ordering of the nanopores over large areas. We found that the overflow of P2VP chains from their reservoir P2VP cylinders and the deformation of the PS matrix in the swelling process contributed to the transformation of the solid P2VP cylinders to empty straight pores. The pore diameters can be tuned by either changing the swelling temperatures or depositing thin layers of metal oxides on the preformed membranes via atomic layer deposition with a subnanometer accuracy. To demonstrate the application of the obtained porous membranes, we used them as templates and produced centimeter-scale arrays of aligned nanotubes of metal oxides with finely tunable wall thicknesses.

Strategies for Inorganic Incorporation using Neat Block Copolymer Thin Films for Etch Mask Function and Nanotechnological Application

“…For example, using 500 nm porous PS-b -P2VP fi lms Yin et al [ 98 ] created titania nanotubes as shown in Figure 7 d and e. An important process step for this accomplishment is related to the "reactivity" of the PS-b -poly-x-vinylpyridine (x = 2 or 4) containing BCPs. As mentioned in Section 3.1, when exposed to selective solvents, PVPs swell and form pores.…”

Nanopatterns Produced by Directed Self-Assembly in Block Copolymer Thin Films

“…Such metastability makes it possible to force the alignment of the nanostructures without tuning the energy balance, and the glassy nature of the final film insures the long-term absence of evolution of the pattern. Solvent swelling was proven to be a useful process to reach perpendicular alignment of cylinders in different systems [61][62][63][64] (cf. Fig.…”

“…4.5). This process is however still considered difficult to reproduce, mainly because reproducibility requires a precise mastering and controlling of many experimental [65], annealing time with characteristic times sometimes shorter than 1 min [63], vapor pressure and the rate of vapor removal [39,66], glass transition temperature. All these can affect, often in coupled ways, the resulting metastable morphologies.…”

Fit-for-purpose block polymer membranes molecularly engineered for water treatment

“…For example, thin films of block polymers can be prepared by coating a solution onto a solid substrate followed by complete solvent evaporation and annealing to an equilibrium nanostructure. In order to induce porosity, the thin film may undergo selective swelling in the melt followed by extraction of the swelling agent [38][39][40] or a sacrificial moiety may be etched to create the percolating network of pores necessary for filtration. 35,[41][42][43] The nanostructured thin film is then transferred onto a mechanically stable support for transport characterization.…”

“…Since this seminal study, several other efforts have used this approach to generate key insights into the use of nanoporous block polymer thin films based on cylindrical morphology as filters. 38,40,[50][51][52][53][54][55] Scalable fabrication of high performance block polymer membranes The self-assembled nanostructure of block polymers at equilibrium demonstrated their promise as next-generation membranes. However, the translation of these materials to functional membrane systems required more than a well-ordered nanostructure on the surface.…”