2D Molecular Weaving
‘Self-assembly of a layered two-dimensional molecularly woven fabric’ David P. August, Robert A. W. Dryfe, Sarah J. Haigh, Paige R. C. Kent, David A. Leigh, Jean-François Lemonnier, Zheling Li, Christopher A. Muryn, Leoni I. Palmer, Yiwei Song, George F. S. Whitehead and Robert J. Young, Nature, 588, 429-435 (2020). Full Article.
Textiles, fabrics consisting of woven fibres, are some of the most important materials in everyday life.1 The weaving of one-dimensional strands—ranging from threads with diameters measured in millimetres (reeds, plant fibres, etc) to those of a few microns (wool, cotton, synthetic polymers, etc)—into two-dimensional fabrics has underpinned technological progress through the ages. To date, however, the weaving of strands at the molecular level2 has largely been limited to coordination polymers and DNA.3 Covalent organic frameworks (COFs) featuring entanglements in three-dimensions have also been described.4 However, these were generated from short rigid building blocks and resulted in isotropic crystalline materials interlaced in all three dimensions rather than the discrete 2D layers of flexible woven strands that give conventional textiles and fabrics their characteristic flexibility, thinness, anisotropic strength, porosity and other properties. Despite being proposed on a number of occasions,5 the direct, bottom-up, assembly of molecular building blocks into linear organic polymers woven in two-dimensions has, until now, remained an elusive target.
Weaving and knotting are closely related processes, both require the formation of orderly entanglements in strands. The presence of such entanglements is increasingly being recognized as important in scientific fields as diverse as liquid crystals,6 magnetism,7 optical beams,8 superfluids,9 proteins,10 nanocarbons11 and molecular braiding12. In all of these cases the ordered mechanical entanglements have significant physical effects. Accordingly, we decided to apply the know-how we had gained from the tying of molecular knots12,13 to the weaving of polymer chains.
Molecular Weaving & Tessellation
We recently found14 that the crystal structure of a 3×3 molecular grid we were developing for a molecular 74 (endless) knot synthesis (Figure 1) had a layered structure with all of the grid ligand strands running parallel or orthogonal to the other strands in the same layer. This solid-state arrangement meant that connecting the strand end groups between grids could result in a 2D interwoven polymer.
Fig. 1. a Structure of the molecular building blocks and their self-assembly into a 3x3 woven grid. b-d X-ray crystal structure of [Fe916](BF4)18.
We carried out a polymerization reaction using slowly crystallizing woven grid ‘tiles’ (Figure 2 & Video 1). After removal of the ion templates, the result was a layered, wholly-organic, 2D-molecularly-woven fabric, in which warp and weft single-chain polymer strands remain associated solely through in-layer mechanical entanglements.
Figure 2. Self-assembly of a 3x3 interwoven grid and polymerization to form layers of a 2D-molecularly-woven fabric.
Video 1. Self-assembly of a 3x3 interwoven grid and polymerization to form layers of a 2D-molecularly-woven fabric. [Video credit: Stuart Jantzen, www.biocinematics.com]
Structure of the 2D-Molecularly-Woven Fabric
Images of the layered molecularly-woven fabric were obtained using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) (Figure 3). These images show flat areas of material that extend for thousands of grid lengths in two-dimensions (10-100s µm), but the individual layers are only 4 nm thick (Video 2). The long-range order of the woven chains in the material was evidenced by a range of other techniques, such as birefringence and wide-angle X-ray scattering (WAXS). The synthetic strategy means that some imperfections in the weave will inevitably occur from chains being incorrectly connected within layers, but every strand is woven within an individual discrete layer 4 nm thick. The molecularly-woven fabric’s thread count (strand density in two dimensions, corresponding to the sum of the number of threads both length-wise and width-wise per inch) is 40–60 million. In comparison, the finest Egyptian linen has a thread count of 1500.
Figure 3. Scanning Electron Microscopy (SEM) images (left and middle) of the woven material showing a multi-layer flake along with an Atomic Force Microscopy (AFM) image (right) of a single, individual, layer only 4 nm thick.
Video 2. Animation of Atomic Force Microscopy (AFM) analysis of layered sheets illustrating how they extend for thousands of grid lengths in two-dimensions in layers only 4 nm thick. [Video credit: Stuart Jantzen, www.biocinematics.com]
When the layers of the molecularly-woven fabric were deposited on a polyester substrate that could be stretched, the molecularly-woven material was found to tear along ordered, geometric, lines like a macroscopic fabric and to delaminate as layers slid across over each other (Video 3). This further confirmed that the polymerization occurred within layers, with little or no crosslinking between them.
Video 3. Animation of the delamination and tearing of the layered 2D-molecularly-woven fabric. [Video credit: Stuart Jantzen, www.biocinematics.com]
Properties induced by Molecular Weaving
To explore how weaving on the molecular scale alters material properties, an identical but unwoven polymer was prepared by simply polymerizing ligand 2 without first pre-organizing the strands with metal ions. This gave a material formed from constitutionally identical polymer strands but in a random spaghetti-like tangle, 3 (Figure 1). By pressing on the woven and unwoven polymers with an AFM tip, their relative stiffness could be compared. The molecularly-woven fabric was almost twice as strong as the non-woven material, even though the polymers have the same chemical composition (Video 4).
Video 4. Animation of the measurement of the Young’s Modulus (stress/strain) of the woven and unwoven polymers by AFM. [Video credit: Stuart Jantzen, www.biocinematics.com]
Fishing for Ions with a Molecular Net
The mesh of the 2D-molecularly-woven fabric is ~2 nm in diameter. When incorporated into a polymer-supported membrane the 2D woven polymer can act as a net, significantly slowing the passage of ions larger than 2 nm, while they pass through membranes incorporating the spaghetti-like 1D polymer much more rapidly (Figure 4 & Video 5). For ions smaller than 2 nm, the rate of diffusion through the membrane was the same for both materials.
Figure 4. Ion permeation experiments showing that the mesh of the 2D-molecularly-woven fabric traps large ions (>2 nm) while letting smaller (<2 nm) ions through. The chemically-identical non-woven polymer lets through both sizes of ions.
Video 5. Animation of ion permeation experiments showing how the mesh of the 2D-molecularly-woven fabric traps large ions (>2 nm) while letting smaller (<2 nm) ions through. The chemically-identical non-woven polymer lets through both sizes of ions. [Video credit: Stuart Jantzen, www.biocinematics.com]
‘The ultimate aspiration…’
In 1992 Daryle H. Busch, the ‘Father’ of metal template synthesis who discovered template effects in the 1960s, predicted the possibility of ‘molecular weaving’ from ‘orderly entanglements’ based on metal coordination complexes.5a Busch later suggested5b that ‘The ultimate aspiration of chemists working on interlocked structures might be to weave molecules as if they were macroscopic threads’. The weaving of polymer chains in two (and potentially three) dimensions by the tessellation of pre-woven tiles is a key step in the realization of Busch’s vision. The ability to weave polymer chains in two-dimensions—forming molecularly woven fabrics—marks the intersection of three major research fields: polymer science,15 two-dimensional materials16 and molecular nanotopology.17
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