S-for-O substitution strategy for high-strength room-temperature rapid self-healing polyurethane elastomers

Professors Kan Yue and Rongrong Hu of South China University of Technology collaboratively addressed the inherent contradiction in high-performance polyurethane (PU) elastomers—the difficulty of simultaneously achieving high strength and rapid self-healing at room temperature. Through a molecular-level "S-for-O" substitution strategy, they successfully constructed a PU-S elastomer with a synergistic strong and weak hydrogen bond network. The study found that replacing traditional urea chain extenders with thiourea units induces the formation of a disordered, asymmetric, and dense multi-point hydrogen bond network, significantly improving chain segment mobility and accelerating hydrogen bond exchange, while also exhibiting high mechanical properties. Based on this mechanism, the PU-S elastomer can achieve efficient self-healing at room temperature without external stimulation. After 24 hours of room-temperature healing, the material achieved a 92% toughness recovery rate, a tensile strength of 23.7 MPa, and an elongation at break of 1080%, overcoming the bottleneck of traditional high-strength elastomers' inability to self-heal at room temperature. This work establishes a new pathway from molecular design to regulate hydrogen bond dynamics to achieve a balance between high strength and room-temperature self-healing, providing new insights for sustainable high-performance flexible materials. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Polyurethane (PU) elastomers, due to their tunable hard-soft segment structure and excellent mechanical properties, are considered a recyclable alternative to traditional bulk plastics and have great application potential in fields such as flexible electronics. However, pursuing high strength requires the introduction of dense, strong hydrogen bonds, leading to the formation of ordered rigid microdomains in the hard segments, which restricts chain segment migration. Therefore, most high-strength PU elastomers cannot self-heal at room temperature and often rely on external stimuli. The key to solving this contradiction lies in the design of chain extenders. Traditional urea chain extenders have strong and ordered hydrogen bonds. To address this, the team proposed an "S-for-O" substitution strategy, replacing the urea moiety with thiourea. Due to the low electronegativity and large radius of sulfur atoms, thiourea can induce the formation of a weak and asymmetric multi-point hydrogen bond system, breaking the ordered stacking of hard domains and constructing a dynamic network. This strategy provides a new molecular design approach for achieving a balance between high strength and rapid self-healing at room temperature.

Highlights of this article:

1. “S-for-O” replacement strategy regulates hydrogen bond networks.
The research team replaced the traditional urea chain extender (CH₂) with its sulfur-containing analogue (TCH₂), achieving a "S-for-O" substitution at the molecular level. Due to the larger atomic radius and lower electronegativity of sulfur atoms, the thiourea motif tends to form weaker, more asymmetric hydrogen bonds (such as twisted "zigzag" hydrogen bonds and urea-thiourea cross-interactions). This disordered hydrogen bond configuration effectively prevents the formation of highly crystalline hard domains, constructing a dynamic supramolecular network with synergistically strong and weak hydrogen bonds. FT-IR spectroscopy confirmed that the proportion of hydrogen-bonded carbonyl groups in PU-S decreased to 60.8% (compared to 75.7% in PU-O), indicating a more dynamic hydrogen bond exchange capacity.

2.  Achieving a perfect balance between high strength and rapid self-healing at room temperature.
Thanks to its dynamically disordered hydrogen bond network, PU-S elastomer exhibits superior comprehensive mechanical properties: a tensile strength of 23.7 MPa, an elongation at break of 1080%, and an initial tensile modulus comparable to the control group (5.81 MPa). Regarding self-healing properties, cut PU-S strips, after healing for 24 hours at 25 °C, can withstand a 5 kg load without breaking. Quantitative testing shows a toughness recovery efficiency of up to 92%, with tensile strength recovering to 19.5 MPa. In contrast, the control sample PU-O, under the same conditions, only achieved a healing efficiency of 27%, even after heating to 50 °C, only increasing to 52%. Furthermore, PU-S demonstrates excellent recyclability; after three recycling cycles, the tensile strength remains at approximately 90%.

3.  Revealing the molecular mechanism of rapid self-repair.
Through variable-temperature FT-IR, stress relaxation, DMA, rheological, and DFT calculations, the team has revealed the mechanism behind PU-S's excellent self-healing performance:

• Thermal responsiveness: The C=O bands of PU-S undergo more significant displacement upon heating, indicating that its hydrogen bond network is more thermally reversible.
• Low activation energy: Stress relaxation experiments show that the activation energy of PU-S is only 41.8 kJ/mol, which is much lower than that of PU-O (106.7 kJ/mol), indicating that its chain segments are more likely to overcome hydrogen bond restrictions and diffuse.
• Rheological evidence: At 25 °C, the G' and G'' of PU-S cross over in the low-frequency region, indicating that dynamic hydrogen bond exchange occurs within the timescale relevant to the healing experiment.
• Theoretical calculations: DFT calculations show that the binding energy of the thiourea-thiourea (TCH-TCH) dimer is -13.05 kcal/mol, which is less than half that of the urea-urea (CH-CH) dimer (-29.61 kcal/mol), theoretically confirming that the hydrogen bonds of thiourea are more easily dissociated and recombined.

Summary and Outlook:

This work successfully modulated the dynamic exchange capacity of hydrogen bonds in polyurethane elastomers using a simple "S-for-O" molecular design strategy. This strategy, requiring no complex synthetic steps or external stimuli, endowed PU-S elastomers with high strength (23.7 MPa), high toughness (1080% elongation at break), and highly efficient room-temperature self-healing capabilities (92% healing efficiency after 24 hours). This strategy demonstrates that controlling hard-segment hydrogen bond dynamics at the molecular level is an effective way to resolve the contradiction between the mechanical robustness and self-healing properties of polymer materials, providing a new design approach for developing reliable and sustainable flexible electronic and soft robotic materials.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.