Small-Molecule-based Supramolecular Plastics Mediated by Liquid-Liquid Phase Separation

Abstract

Plastics are one of the most widely used polymeric materials. However, they are often undegradable and non-recyclable due to the very stable covalent bonds of macromolecules, causing environmental pollution and health problems. Here, we report that liquid-liquid phase separation (LLPS) could drive the formation of robust, stable, and sustainable plastics using small molecules. The LLPS process could sequester and concentrate solutes, strengthen the non-covalent association between molecules and produce a bulk material whose property was highly related to the encapsulated water amounts. It was a robust plastic with a remarkable Young's modulus of 139.5 MPa when the water content was low while became adhesive and could instantly self-heal with more absorbed water. Finally, responsiveness enabled the material to be highly recyclable. This work allowed us to understand the LLPS at the molecular level and demonstrated that LLPS is a promising approach to exploring eco-friendly supramolecular plastics that are potential substitutes for conventional polymers.

Keywords: Electrostatic Self-Assembly; Liquid-Liquid Phase Separation; Macrocycles; Supramolecular Materials.

Scheme 1

Illustration of molecular mechanism for the formation of MSPS materials that were self‐assembled by ADM macrocycles and CTAB surfactants in water.

Figure 1

a) Chemical structures of ADM macrocycle, CTAB and DHDAB surfactants. b) Time‐lapse bright field microscopy images of ADM (1 mM)/CTAB/1 : 4 mixture at 90 min after solution preparation. c) Evolution of the average size of droplets with time before full phase separation in ADM (0.1 mM)/CTAB/1 : 4 solution. (pH≈10, H₂O). d) TEM image of samples taken from ADM (1 mM)/CTAB/1 : 4 mixture at 15 min after solution preparation (inserted red arrow: nanosheets). e) Partial ¹H NMR spectra of ADM (10 mM)/CTAB/1 : n. (pD≈10, D₂O, 500 MHz). f) DOSY spectrum of ADM (10 mM)/CTAB/1 : 2. (pD≈10, D₂O, 600 MHz). g) Partial 2D NOESY spectra of ADM (10 mM)/CTAB/1 : 2. (pD≈10, D₂O, 600 MHz, the mixing time: 100 ms and relaxation delay: 2 s). h) Chemical structure illustration of interwoven ADM‐4CTAB MSPS materials in top view and side view, zoom‐in pictures show the details of host–guest and hydrogen bonding interaction between carboxylic groups, amide bonds and water molecules.

Figure 2

a) Strain sweeps and b) frequency sweeps showing loss (G′′, open data point) and storage moduli (G′, filled data points) of bulk ADM‐4CTAB MSPS materials with ≈5 %, ≈20 % and ≈45 % water content. c) Shear‐viscosity tests of ADM‐4CTAB MSPS materials with ≈45 % water content. d) Cyclic strain sweep of ADM‐4CTAB MSPS materials with ≈45 % water content.

Figure 3

a) Photograph of cylinder‐like ADM‐4CTAB MSPS materials specimens after cutting into several fragments and subsequent self‐healing process (dark bule specimen was colored by Brilliant blue R‐250). b) Brief schematic illustration of lap‐shear specimen preparation of ADM‐4CTAB MSPS materials. c) The lap‐shear strength of ADM‐4CTAB MSPS materials apply for different substrates (gray bar indicated the substrate plates have been broke before pulled apart). d) Photographs of macroscopic adhesive behavior of ADM‐4CTAB MSPS materials applied to steel substrates. e) Photograph of free‐standing ADM‐4CTAB MSPS materials after processing into specific shapes (bottom to up: art words, transparent film, spring). f) Tensile‐stress curves of free‐standing ADM‐4CTAB (≈5 % wc) MSPS materials and ADM‐4DHDAB MSPS materials (≈2 % wc). g) Photograph of free‐standing ADM‐4DHDAB MSPS materials after elongation and fracture.

Figure 4

a) Leakage content of ADM‐4CTAB and ADM‐4DHDAB MSPS materials in PBS solution (37 °C, shaker) and distilled water (room temperature). b)–d) Bright field microscopy images of ADM‐4CTAB MSPS materials (b) before, (c) after UV 365 nm irradiation for 60 s, (d) after remove UV light and stay in dark for 5 min (1 mM, H₂O, pH 10, room temperature). e) Cyclic temperature sweep measurements of ADM‐4CTAB MSPS materials with ≈20 % water content. f) The appearance of a piece of bulk ADM‐4CTAB MSPS materials reduction in water by adding 10 equiv DTT, and subsequent oxidation by loosening the cap in air. g) The chemical structure illustration of reversible redox of ADM macrocycle by DTT and oxygen. h) The contrastive ¹H NMR spectrum of ADM‐4CTAB MSPS materials after adding 10 equiv DTT, bottom: before adding DTT (replaced by ADM (10 mM)/CTAB/1 : 5 aqueous solution), medium: after adding DTT for 5 min, top: after adding DTT for 5 h (D₂O, 500 MHz, 10 mM for ADM, pD 10).