Exploring a naturally tailored small molecule for stretchable, self-healing, and adhesive supramolecular polymers

Abstract

Polymeric materials with integrated functionalities are required to match their ever-expanding practical applications, but there is always a trade-off between complex material performances and synthetic simplification. A simple and effective synthesis route is reported to transform a small molecule of biological origin, thioctic acid, into a high-performance supramolecular polymeric material, which combines processability, ultrahigh stretchability, rapid self-healing ability, and reusable adhesivity to surfaces. The proposed one-step preparation process of this material involves the mixing of three commercially available feedstocks at mild temperature without any external solvent and a subsequent cooling process that resulted in a dynamic, high-density, and dry supramolecular polymeric network cross-linked by three different types of dynamic chemical bonds, whose cooperative effects in the network enable high performance of this supramolecular polymeric material.

Fig. 1. One-step preparation of the poly(TA-DIB-Fe) copolymer network and its characterization.

(A) Schematic representation of the synthesis route of the copolymer network. (B) Photographs of TA powder, molten TA liquid, and poly(TA-DIB-Fe) copolymer solid. r.t., room temperature. (C) The transparent polymeric film prepared by 25-g poly(TA-DIB-Fe) cured in a Petri dish with a TA-to-iron(III) molar ratio of 18,000:1. (D) Photographs of poly(TA) and poly(TA-DIB-Fe) film samples synthesized by different TA-to-iron(III) molar ratios. (E) Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) mapping images of molecular ion fragments attributed to TA [TA-to-iron(III) molar ratio of 18,000:1]. (F) Frequency dependency of storage (solid dots, G′) and loss (hollow dots, G″) moduli of copolymer network with different iron(III) concentrations. (G) Temperature dependency of storage (solid dots, G′) and loss (hollow dots, G″) moduli and viscosity (blue dots, η) of copolymer network [TA-to-iron(III) molar ratio of 18,000:1]. Inset photographs manifest the transformation from solid to liquid due to the decrease in viscosity by heating.

Fig. 2. Mechanical properties of poly(TA-DIB-Fe) copolymer.

(A) A free-standing copolymer can be pressed without fragmentation and cannot be cut easily with a knife. (B) A rod-like copolymer can be stretched into a thin filament. (C) Optical microscopy image of a stretched copolymer filament (left) with a human hair (right) as a reference. (D) Stress-strain curve of the resulting network [TA-to-iron(III) molar ratio of 18,000:1] for a strain of 15,000%. (E) Photographs of the stretchable viscous copolymer adhered between the two glass slices. The copolymer was first preheated to the viscous state and then deposited onto glass surfaces. (F) Proposed energy dissipation mechanism for ultrahigh stretchability. (G) Stress-strain curves of the copolymer with different iron(III) concentrations. (H) Stress-strain curves of the copolymer at varied strain rates [TA-to-iron(III) molar ratio of 18,000:1]. (I) Sequential loading-unloading stress-strain curves without rest internals [TA-to-iron(III) molar ratio of 18,000:1]. The successful stretching without breaking proves the resistance of the copolymer to fatigue.

Fig. 3. Self-healing properties of the poly(TA-DIB-Fe) copolymer.

(A) Photographs depict that a scratch on the copolymer membrane can be autonomously cured after 6 hours at room temperature. (B) Optical microscopy image of a healed sample. (C) Photographs of a healed film before and after stretching. (D to F) The rod-shaped copolymer can be used to construct objects of different shapes. (G) Stress-strain curves of the healed samples [TA-to-iron(III) molar ratio of 18,000:1] with freshly cut interfaces. (H) Dependency of self-healing efficiency of the aged copolymers on time [TA-to-iron(III) molar ratios of 18,000:1 and 90,000:1]. Both the healing efficiencies are presented by the maximal strain (column) and the maximal tensile stress (dot) of the healed samples. (I) Proposed self-healing mechanism of the freshly cut and aged interfaces. Error bars show SD with n = 3 repeats.

Fig. 4. Application of poly(TA-DIB-Fe) copolymer as adhesive materials.

(A) Schematic representation of the adhesion procedure. (B) Shear strength of the copolymer for different TA-to-iron(III) molar ratios. The inset optical image illustrates the adhesive behavior of the copolymer on glass slices. The adhesion area was 2.0 cm × 2.0 cm, and the weight was 5 kg. (C) Comparison of shear strengths between poly(TA-DIB-Fe) copolymer [with a TA-to-iron(III) molar ratio of 50:1] and commercial adhesive materials (3M instant adhesive and double-sided tape) on nine different types of surfaces. PVC, polyvinyl chloride. (D) Shear strength versus temperature curves of the copolymer [TA-to-iron(III) molar ratio of 50:1] for the glass and Teflon surfaces. The shear strength for Teflon at below 20°C was too high to be measured because of the limited mechanical strength of the used Teflon film. (E) Shear strengths of the copolymer after 30 times cycling experiments [TA-to-iron(III) molar ratio of 150:1]. Error bars present the SDs with n = 5 repeats.