Molecular engineering of supramolecular polymer adhesive with confined water and a single crown ether

Abstract

Here, we report a water-induced supramolecular polymer adhesive formed from confined water and an intrinsically amphiphilic macrocyclic self-assembly in a nanophase-separated structure. The selenium-containing crown ether macrocycle, featuring a strong hydrophilic hydrogen-bond receptor (selenoxide) and a synergistic hydrophobic selenium-substituted crown core, confines water within a segregated, interdigitated architecture. While water molecules typically freeze around 0 °C, the confined water in this supramolecular polymer remains in a liquid-like state down to -80 °C. Previous studies suggested that multiple crown ether units are required to generate confined water; however, in this case, a single unit is sufficient to control the formation and disappearance of confined water, driving supramolecular polymerization. Typically, the DC conductivity of water follows an Arrhenius temperature dependency (ln σ _DC ∝ 1/T). In contrast, this new crown ether unit maintains water in confined states, exhibiting Vogel-Fulcher-Tammann behavior (ln σ _DC ∝ 1/(T - T ₀)) at temperatures above the glass transition. Moreover, this water-induced supramolecular polymer demonstrates remarkable adhesion to hydrophilic surfaces, maintaining strong adhesion even at low temperatures. These findings illustrate how a single small macrocycle can control the complex structure and functionality of water in supramolecular systems.

Fig. 1. Properties of selenoxide-modified crown ether macrocycles containing structural water. Through precise structural control, the modified macrocyclic structures acquire structural water components, leading to their incorporation into supramolecular assemblies with high-viscosity adhesive properties, as reported previously. (a) Chemical structures of C7SeO and its control C7. (b) Design principle of enhanced hydrogen bonding behaviour of selenoxide groups compared to ether groups. (c) Illustration of macrocycle-structural water-based supramolecular polymers, where water molecules act as a comonomer for supramolecular polymerization. (d) The dry C7SeO sample absorbs water from the ambient humidity (from a white fluffy to yellow sticky solid). (e) Pictures of the C7SeO samples featuring varying water content [C7SeO_n-H₁ is used to abbreviate samples with C7SeO : water (H) ratio of n : 1 (w/w)]. (f) The macroscopic adhesive properties of C7SeO₁₀-H₁ materials with a hydrophilic glass surface (with an adhesion area measuring 2.0 × 2.5 cm²). The material, located within a glass-adhesive-glass sandwich, is marked with a white dotted box. (g) Photograph of the hydrogel filament drawn from the C7SeO₁₀-H₁ reservoir. The scale bar is 0.5 cm.

Fig. 2. (a) Dependence of the DC conductivity σ_DCversus 1/T for C7SeO_n-H₁ materials with different water contents. (b) Temperature dependence of the dielectric loss (log ε″) at a frequency of 1000 Hz for C7SeO₁₀-H₁ for the heating and subsequent cooling cycle. The cooling curve (empty rhombus) shows a behaviour similar to that of the heating curve (solid rhombus). (c) Dielectric loss versus frequency for a C7SeO₁₀-H₁ adhesive at the indicated temperatures. (d) Heating rate versus inverse glass transition temperature estimated by fast scanning calorimetry for C7SeO_n-H₁. Dashed lines are fits of the Vogel–Fulcher–Tammann (VFT) equation to the data.

Fig. 3. (a) Illustrative fitting of FTIR spectra within the 2600–3800 cm⁻¹ range for C7SeO₁₀-H₁ at 25 °C. (b) Schematic depiction of three distinct types of OH configurations in the water network. (c) Percentage distribution of ice-like, distorted, and free OH stretching components derived from the fitting of FTIR spectra for C7SeO₁₀-H₁.

Fig. 4. (a) Equilibrium snapshot of the C7SeO₁₀-H₁ system from MD simulations at 240 K, exhibiting a large water ball and some dispersive water molecules or clusters among the C7SeO molecules. Water ball/clusters are coloured by light blue. C7SeO molecules are presented by grey line, and Se and O atoms in selenoxide groups are specially shown in yellow and red sticks, respectively. (b) Images showing (I and II) the size of the simulated water ball in C7SeO₁₀-H₁ and (III) a diagram of a confined water ball architecture, illustrating the water core and the interfacial water layer. (c) Structure of (①) the interface and (②) the interior of the water ball, and (③–⑥) structure of example dispersive water clusters forming hydrogen bonds with C7SeO molecules. H, C and N atoms in C7SeO molecules are shown in white, grey and blue sticks. Hydrogen bonds are indicated by light blue dashed lines. N_w represents the number of water molecules in the cluster. (d) Radial distribution functions (RDFs) of the O atoms in selenoxide groups (denoted by OSe) and H atoms in water molecules (g_OSe–HW), and the corresponding coordination numbers (CN_OSe–HW). CN_OSe–HW amounts to approximately 1.3 at the maximum of the first coordination distance, indicating that each selenoxide group, on average, forms 1.3 hydrogen bonds with water molecules. (e) RDFs of the O atoms in selenoxide groups and O atoms in water molecules (g_OSe–OW), and O atoms in ether group (denoted by OE) and O atoms in water molecules (g_OE–OW), respectively, and the corresponding CN_OSe–OW and CN_OE–OW. Notably, C7SeO molecules shown in the figures include only those forming hydrogen bonds with water molecules.

Fig. 5. Application of C7SeO₁₀-H₁ materials as adhesive materials at extremely low temperatures. (a) Digital image of lap shear tests performed at −40 °C. Cartoon representation of the adhesion procedure. (b) Macroscopic adhesive behaviour of C7SeO₁₀-H₁ materials on a 304 stainless steel surface at various low temperatures. Water was used in these pictures. (c) Lap shear strengths of C7SeO₁₀-H₁ and its control C7₁₀-H₁.