Vitrimer Dynamic Disulfide Polymer: Advanced Covalent Adaptable Networks For Recyclable Thermosets

Molecular Architecture And Dynamic Exchange Mechanisms Of Vitrimer Dynamic Disulfide Polymers

The fundamental design principle of vitrimer dynamic disulfide polymers centers on incorporating disulfide (–S–S–) linkages as dynamic covalent bonds within crosslinked polymer networks. Unlike dissociative CANs (e.g., Diels-Alder systems) where bonds break before reforming, disulfide-based vitrimers follow an associative exchange mechanism: new bonds form simultaneously as old ones break, preserving network integrity and crosslink density 1. This mechanism is governed by nucleophilic substitution or radical-mediated pathways, with activation energies typically ranging from 80 to 150 kJ/mol depending on the chemical environment and catalyst presence 2.

Polyolefin-based vitrimer systems exemplify this approach by integrating disulfide linkages into maleic anhydride-functionalized or glycidyl methacrylate-functionalized polyolefin backbones 1. The resulting semi-crystalline morphology combines the processability of polyolefins with the adaptive network topology of vitrimers. At temperatures above the topology freezing transition temperature (Tv)—typically 50–120°C for disulfide systems—the viscosity decreases following Arrhenius behavior (η = A·exp(Ea/RT)), enabling melt reprocessing without catalyst leaching 12. Below Tv, the material behaves as a conventional thermoset with elastic modulus values of 0.5–2.5 GPa, depending on crosslink density and crystallinity 1.

Epoxy-based vitrimer formulations further enhance mechanical performance by combining disulfide bonds with complementary dynamic chemistries. For instance, dual-exchange systems incorporating both imine (–C=N–) and disulfide bonds achieve glass transition temperatures (Tg) exceeding 80°C and storage moduli at room temperature of 1.8–3.2 GPa, while exhibiting stress relaxation times under 10 minutes at 150°C 3. The synergy between imine condensation (reversible at 60–100°C) and disulfide metathesis (active above 120°C) provides tunable relaxation dynamics across a broad temperature window, critical for applications requiring both dimensional stability during service and rapid reprocessing 3.

Bio-based vitrimer epoxy foams demonstrate the versatility of disulfide chemistry in sustainable formulations. By epoxidizing castor oil-derived fatty acids and curing with benzoxazine-disulfide hardeners synthesized via condensation of enolic molecules, formaldehyde, and cystamine, researchers have produced crosslinked networks with polybenzoxazine-disulfide moieties 45. These materials exhibit self-healing efficiencies exceeding 85% after 2 hours at 140°C and can be reprocessed at 160°C with retention of 90% original tensile strength after three cycles 4. The dynamic disulfide bonds undergo exchange reactions at elevated temperatures (≥130°C), enabling both autonomous healing of microcracks and full material recycling through melt-pressing 5.

## Synthesis Strategies And Crosslinker Design For Disulfide-Based Vitrimers

### Polyolefin Vitrimer Synthesis Via Disulfide Crosslinkers

The synthesis of polyolefin-based vitrimer dynamic disulfide polymers typically involves free-radical copolymerization of functionalized polyolefins with disulfide-containing crosslinkers 19. A representative protocol employs maleic anhydride-grafted polypropylene (PP-g-MA, grafting degree 0.5–2.0 wt%) reacted with bis(2-mercaptoethyl) disulfide in the presence of dicumyl peroxide (DCP, 0.3–1.0 wt%) at 180–200°C for 10–15 minutes under nitrogen atmosphere 1. The resulting material exhibits a melting temperature (Tm) of 160–165°C and a Tv of approximately 90°C, enabling reprocessing via compression molding at 200°C and 10 MPa for 5 minutes 1.

For ethylene-based copolymers, high-purity disulfide crosslinkers with at least two polymerizable groups (e.g., diallyl disulfide or bis(methacryloyl) disulfide) are incorporated during in-reactor polymerization or post-reactor compounding 9. The crosslinker concentration typically ranges from 0.1 to 5.0 mol% relative to polymer repeat units, with higher loadings increasing crosslink density and Tv but reducing ultimate elongation 9. Dynamic mechanical analysis (DMA) reveals that these materials maintain a rubbery plateau modulus of 1–10 MPa up to 180°C, with reversible crosslinking dissociation occurring above 150°C, facilitating recycling while preserving 85–95% of original mechanical properties after reprocessing 9.

### Epoxy-Disulfide Vitrimer Formulations

Epoxy vitrimer formulations leverage the reactivity of epoxide groups with disulfide-containing curing agents to construct dynamic networks. A dual-exchange epoxy monomer incorporating both imine and disulfide functionalities can be synthesized by reacting 4,4'-diaminodiphenyl disulfide with salicylaldehyde (molar ratio 1:2) in ethanol at 60°C for 4 hours, followed by epoxidation using epichlorohydrin and sodium hydroxide 3. This monomer is then cured with conventional epoxy resins (e.g., diglycidyl ether of bisphenol A, DGEBA) and amine hardeners at 120°C for 2 hours, then post-cured at 150°C for 1 hour 3. The resulting vitrimer exhibits a Tg of 82–95°C (DSC, 10°C/min heating rate), tensile strength of 65–78 MPa, and Young's modulus of 2.1–2.8 GPa 3.

For adhesive applications, curable compositions combine epoxy resins with disulfide-amine crosslinkers such as bis(4-aminophenyl) disulfide at stoichiometric ratios (epoxy:amine = 1:0.5–1.0) 14. The addition of 0.5–2.0 wt% zinc acetate or triphenylphosphine as transesterification catalysts accelerates stress relaxation, reducing the characteristic relaxation time (τ*) from >1000 seconds to <100 seconds at 160°C 14. These formulations achieve lap shear strengths of 18–25 MPa on aluminum substrates and can be debonded on-demand by heating to 180°C for 10 minutes, enabling non-destructive disassembly 14.

### Bio-Based Disulfide Vitrimer Synthesis

The synthesis of bio-based vitrimer epoxy foams begins with epoxidizing castor oil-derived ricinoleic acid using hydrogen peroxide (30% aqueous solution) and Amberlite IR-120 cation exchange resin as catalyst at 60°C for 6–8 hours, achieving epoxidation degrees of 85–95% 4. Concurrently, a benzoxazine-disulfide hardener is prepared via Mannich condensation of phenol (or cardanol), formaldehyde, and cystamine dihydrochloride (molar ratio 2:4:1) in ethanol at 80°C for 12 hours 45. The epoxidized castor oil and hardener are mixed at a weight ratio of 10:1, degassed under vacuum, and cured at 120°C for 3 hours, then post-cured at 160°C for 2 hours to form a crosslinked foam with density of 0.3–0.5 g/cm³ 4.

The resulting material demonstrates a compressive modulus of 5–12 MPa and compressive strength of 0.8–1.5 MPa at 10% strain 5. Thermogravimetric analysis (TGA) shows onset decomposition temperature (Td,5%) at 280–310°C, indicating excellent thermal stability 5. The dynamic disulfide bonds enable self-healing: samples with 2 mm cuts heal to 85–92% of original tensile strength after heating at 140°C for 2 hours under 0.1 MPa contact pressure 4. Reprocessing via grinding and compression molding at 160°C for 15 minutes yields recycled foams retaining 88–94% of virgin material properties after three cycles 5.

## Mechanical Properties And Stress Relaxation Behavior

The mechanical performance of vitrimer dynamic disulfide polymers is characterized by a unique combination of high room-temperature strength and thermally activated stress relaxation. Polyolefin-based disulfide vitrimers exhibit tensile strengths of 15–35 MPa, elongation at break of 200–600%, and Shore A hardness of 70–90, depending on polyolefin type (polypropylene vs. polyethylene) and crosslink density 19. Dynamic mechanical analysis reveals a storage modulus (E') of 500–2000 MPa at 25°C, decreasing to 1–10 MPa above Tv as disulfide exchange becomes kinetically favorable 1.

Stress relaxation experiments quantify the viscoelastic response: samples subjected to constant strain (typically 5–10%) at elevated temperatures (120–180°C) exhibit exponential stress decay with characteristic relaxation times (τ*) ranging from 10 seconds to 30 minutes, depending on temperature and disulfide bond density 212. The activation energy (Ea) for stress relaxation in polyolefin disulfide vitrimers is typically 90–120 kJ/mol, consistent with the energy barrier for disulfide metathesis 12. This Arrhenius-type behavior enables precise control of reprocessing conditions: at 200°C, viscosity drops to 10³–10⁴ Pa·s, suitable for compression molding or extrusion 1.

Epoxy-disulfide vitrimers demonstrate superior mechanical properties due to the rigid aromatic backbone and high crosslink density. Tensile testing yields strengths of 60–85 MPa, Young's moduli of 2.0–3.5 GPa, and elongation at break of 3–8% 312. The incorporation of carbon fiber reinforcement further enhances performance: carbon fiber-reinforced polymer (CFRP) composites with disulfide-crosslinked epoxy matrices achieve flexural strengths of 800–1200 MPa and interlaminar shear strengths (ILSS) of 60–85 MPa 12. Critically, these composites retain recyclability—after dissolution in dimethyl sulfoxide (DMSO) at 120°C for 4 hours, recovered carbon fibers maintain 95–98% of original tensile strength (3.5–3.8 GPa) and can be reused in new composite fabrication 12.

The dual-exchange epoxy vitrimers combining imine and disulfide bonds exhibit tailored relaxation dynamics: at 100°C, imine exchange dominates with τ* ≈ 300 seconds, while at 150°C, disulfide metathesis accelerates relaxation to τ* ≈ 50 seconds 3. This multi-modal relaxation enables applications requiring both creep resistance at moderate temperatures and rapid reprocessing at higher temperatures. Fracture toughness (KIC) values of 1.2–1.8 MPa·m^0.5 have been reported, significantly higher than conventional epoxy thermosets (0.6–1.0 MPa·m^0.5), attributed to the energy dissipation through reversible bond exchange during crack propagation 3.

## Thermal Stability And Reprocessing Characteristics

Thermal stability is a critical parameter for vitrimer dynamic disulfide polymers, particularly for high-temperature applications. Thermogravimetric analysis (TGA) of polyolefin-based disulfide vitrimers shows onset decomposition temperatures (Td,5%) of 280–320°C in nitrogen atmosphere, with maximum degradation rates occurring at 400–450°C 19. The disulfide bonds themselves are stable up to approximately 200°C under inert conditions, but can undergo homolytic cleavage at 180–220°C in the presence of oxygen, necessitating processing under nitrogen or argon 1.

Differential scanning calorimetry (DSC) reveals the thermal transitions governing reprocessability. Polyolefin disulfide vitrimers exhibit melting endotherms at 155–165°C (for polypropylene-based systems) or 120–135°C (for polyethylene-based systems), corresponding to the crystalline phase melting 19. The Tv, determined from the temperature-dependent viscosity inflection point in rheological measurements, typically occurs 20–40°C below Tm, enabling reprocessing in the semi-crystalline state where disulfide exchange is active but crystalline domains provide dimensional stability 1.

Epoxy-disulfide vitrimers, being amorphous, exhibit a single Tg ranging from 60°C to 110°C depending on crosslink density and aromatic content 31214. The Tv for these systems is typically 30–50°C above Tg, placing the reprocessing window at 120–180°C 3. Isothermal TGA at 150°C for 4 hours shows weight loss of less than 0.5%, confirming stability during typical reprocessing operations 12. However, prolonged exposure (>10 hours) at 180°C can lead to 2–5% weight loss due to partial disulfide bond cleavage and volatile sulfur compound formation 12.

Reprocessing protocols have been optimized for various vitrimer types. Polyolefin disulfide vitrimers are typically ground to <2 mm particles, then compression molded at 200°C and 10–15 MPa for 5–10 minutes 19. Mechanical testing of reprocessed samples shows retention of 85–95% tensile strength and 80–90% elongation at break after the first cycle, with gradual degradation to 70–80% of original properties after five cycles 9. The decrease is attributed to cumulative oxidative degradation and chain scission rather than loss of dynamic crosslink functionality 9.

Epoxy-disulfide vitrimers require higher reprocessing temperatures (160–180°C) and pressures (15–25 MPa) due to their higher Tg and crosslink density 312. Alternatively, solvent-assisted recycling in DMSO or N-methyl-2-pyrrolidone (NMP) at 100–120°C for 2–6 hours enables disulfide bond cleavage and network dissolution, facilitating fiber recovery in composite applications 12. The recovered epoxy oligomers can be re-crosslinked with fresh disulfide-amine hardeners, achieving 75–85% of virgin material properties 12.

## Self-Healing Mechanisms And Quantitative Performance

The self-healing capability of vitrimer dynamic disulfide polymers arises from the thermally activated mobility of disulfide bonds, which enables crack closure and interfacial rebonding. The healing process involves three stages: (1) surface contact and wetting, (2) disulfide bond exchange across the crack interface, and (3) network equilibration to restore mechanical continuity [4