Dual Dynamic Bonds Give Bio-Based Epoxy Vitrimers Remarkable Self-Healing Power

Plastics that mend their own cracks have long been a dream of materials science, but most self-healing polymers still depend heavily on petroleum-based chemistry. A team of researchers at Chiba Institute of Technology in Japan, led by Ryota Takeda, Kaito Sugane and Mitsuhiro Shibata, has now reported a major step toward greener alternatives. In a study published in Polymer Bulletin, they describe bio-based epoxy vitrimers built almost entirely from renewable feedstocks that can heal themselves repeatedly, recovering up to 86 percent of their original tensile strength. The secret lies in combining two different types of dynamic covalent bonds within the same network, a design choice that proved decisively better than relying on either bond type alone.

Vitrimers are a special class of polymer networks that behave like conventional thermosets at service temperatures yet can rearrange their internal topology when heated. Unlike thermoplastics, which simply melt, vitrimers exchange covalent bonds in a controlled, associative manner, so the material flows and reforms without losing its network integrity. This bond-exchange capability is what enables self-healing and reprocessing. Chemists have exploited a variety of dynamic bonds for this purpose, including disulfide exchange, transesterification, imine formation, boronic esters and vinylogous urethanes. The Japanese team focused on vinylogous urethane bonds, which form when acetoacetate groups react with primary amines and can exchange through transamination reactions with free amino groups still present in the network.

The researchers assembled their vitrimers from four largely bio-based building blocks. Polyglycerol polyglycidyl ether, a multi-functional epoxy resin with a bio-based content above 99 percent, served as the crosslinking backbone. Glycerol tris(acetoacetate), synthesized by reacting glycerol with tert-butyl acetoacetate at 135 degrees Celsius for 24 hours, provided the acetoacetate groups needed for vinylogous urethane formation. Cystamine, a disulfide-containing diamine, introduced the second dynamic bond type, while dimer diamine, a flexible fatty-acid-derived diamine supplied by Cargill, acted as a soft, renewable co-monomer. By varying the molar ratio of cystamine to dimer diamine across four formulations, designated VUE-C/D-0/4 through VUE-C/D-3/1, the team could systematically tune the balance between stiffness, flexibility and dynamic bond content.

Characterization confirmed that the intended chemistry had taken place. Fourier-transform infrared spectroscopy showed the acetoacetate carbonyl bands weakening dramatically after curing, while new absorptions characteristic of vinylogous urethane bonds appeared at 1649 and 1604 reciprocal centimeters, alongside a broad hydroxyl stretch from the epoxy-amine reaction. Chloroform extraction experiments yielded gel fractions exceeding 91 percent for all films, evidence of well-formed crosslinked networks. The cured products emerged as transparent brown films with biomass contents of approximately 81 weight percent, a remarkably high figure for epoxy thermosets and a substantial improvement over earlier vinylogous urethane vitrimers that relied on petroleum-based polyamines.

Thermal and mechanical measurements revealed how the cystamine-to-dimer-diamine ratio shaped the final materials. Glass transition temperatures, measured by differential scanning calorimetry, ranged from 10 to 32 degrees Celsius and rose steadily as more cystamine replaced the flexible dimer diamine. Tensile modulus and strength followed the same trend, because the shorter, more rigid cystamine segments stiffened the network. Interestingly, the apparent crosslinking density derived from dynamic mechanical analysis moved in the opposite direction, decreasing with increasing cystamine content. The researchers attribute this counterintuitive result to residual primary amino groups and network heterogeneity, which complicate the interpretation of rubbery-state modulus measurements and show that glass-transition behavior cannot be explained by crosslink density alone.

The most striking results came from stress relaxation experiments, which probe how quickly the network can rearrange its topology. Tested at temperatures from 100 to 140 degrees Celsius, the dual-dynamic formulation VUE-C/D-1/3 relaxed far faster than a control material, VUE-H/D-1/3, in which cystamine was replaced by hexamethylene diamine so that only vinylogous urethane bonds remained. At 140 degrees Celsius, the dual-dynamic vitrimer reached its characteristic relaxation time in just 111.5 seconds, compared with 375.5 seconds for the control. Arrhenius analysis showed the dual-dynamic material had a higher activation energy of 71.1 kilojoules per mole versus 41.9 for the control, meaning its relaxation accelerated more steeply with temperature, a signature of two cooperative exchange mechanisms at work.

Self-healing tests translated this faster relaxation into concrete performance. Rectangular specimens were cut in half, pressed back together and hot-pressed at 140 degrees Celsius for three hours under a modest 0.5 megapascals of pressure. Every formulation healed successfully, with all healed samples strong enough to support a 200-gram load, and each film could be healed at least three times in succession. The dual-dynamic VUE-C/D-1/3 achieved the highest first-cycle healing efficiency, recovering 86 percent of its original tensile strength, while the single-dynamic control managed only 63 percent. Healing efficiencies declined over successive cycles for all films, and within the cystamine series the 1/3 ratio outperformed both 2/2 and 3/1, indicating that beyond an optimal composition the added network stiffness outweighs the benefit of more dynamic bonds.

The study also honestly documents the limits of the technology. When the researchers attempted bulk reprocessing by hot-pressing fragmented samples at 160 or 180 degrees Celsius under 30 megapascals, the pieces fused together but never formed homogeneous, flat specimens suitable for mechanical testing, and the recovered maximum stresses fell below 15 percent of the original values. Infrared spectra revealed that the vinylogous urethane carbon-carbon double bond band at 1604 reciprocal centimeters nearly vanished during this harsh treatment, showing that the dynamic chemistry itself had been chemically altered. By contrast, the same band remained intact after the gentler self-healing conditions, demonstrating that the network can heal under mild conditions that preserve its structure, even if full-scale recycling remains out of reach for now.

The implications reach well beyond one laboratory formulation. Epoxy thermosets are ubiquitous in coatings, adhesives, electronics and composites, and their inability to be repaired or recycled generates enormous waste. By showing that pairing vinylogous urethane and disulfide bonds measurably accelerates network rearrangement and boosts healing efficiency in a network that is roughly 81 percent biomass by weight, the Chiba Institute of Technology team has provided a clear design principle for the next generation of sustainable thermosets. The work also cautions that more dynamic bonds are not automatically better, since stiffness, residual amine content and network heterogeneity all conspire to determine real-world performance. As the field of bio-based vitrimers matures, studies like this one, which carefully isolate the contribution of each dynamic bond type, will be essential guides for engineers seeking materials that are simultaneously strong, renewable and, quite literally, able to heal themselves.

Subject of Research: Self-healing bio-based epoxy vitrimers with dual dynamic vinylogous urethane and disulfide bonds

Article Title: Self-healing bio-based epoxy vitrimers containing dual dynamic vinylogous urethane and disulfide bonds

Article References: Takeda, R., Sugane, K., & Shibata, M. (2026). Self-healing bio-based epoxy vitrimers containing dual dynamic vinylogous urethane and disulfide bonds. Polymer Bulletin, 83(12), Article 661. https://doi.org/10.1007/s00289-026-06726-6

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06726-6

Keywords: bio-based epoxy, vitrimer, self-healing, vinylogous urethane, disulfide bonds, dynamic covalent bonds, stress relaxation, renewable polymers, glycerol tris(acetoacetate), cystamine, dimer diamine, reprocessability