Effect and Mechanism of a Triazinetrione Curing Agent on Mechanical Properties of Thermosetting Glycidyl Azide Polyme Cross-Linked Network

1. Introduction

Glycidyl azide polymer (GAP) is a typical energetic binder featured with reactive C-N₃ functional groups. Each mole of azide groups can provide a high positive formation enthalpy of 341-398 kJ·mol^-1, making GAP one of the most promising energetic binders for thermosetting nitramine-based composite propellants[1,2,3,4]. Despite its superior energetic characteristics, including low glass transition temperature, moderate mechanical sensitivity, and excellent chemical stability[5], GAP still exhibits obvious shortcomings in molecular structure. The strong polarity of the azide group(-N₃) significantly restricts the rotation freedom of molecular chains and increases the rigidity of the polymer backbone, which further weakens intermolecular forces in the crosslinked matrix. Consequently, the mechanical properties of traditional thermosetting GAP-based propellants are generally insufficient and difficult to meet the practical application requirements.

To improve the mechanical performance of GAP crosslinked networks, most current research strategies focus on regulating the crosslinked structure, such as adjusting crosslinking density, modifying the GAP prepolymer, and introducing conventional curing agents[6,7,8,9,10,11,12]. However, the chemical modification of azide-based prepolymers is often accompanied by complex synthetic processes and inevitably reduces the overall energy level of the binder system. In addition, due to the strong rigidity and polarity of azide groups, the hydrogen bonds formed by traditional curing agents in GAP-based thermosetting systems are usually sparse and unevenly distributed, resulting in limited improvement in the mechanical properties of the final crosslinked networks. Furthermore, the influence of crosslinking point structure on the mechanical behavior of thermosetting GAP networks has not been systematically studied and clarified in previous reports[13].Zhai et al. compared the effects of N-100 with a mixture of TMP and HDI on the mechanical properties of GAP crosslinked networks, and quantitatively analyzed the contribution of intermolecular hydrogen bonding at crosslinking points. When the R value ([NCO]/[OH]) was fixed at 1.0, the tensile strength of the GAP network crosslinked by N-100 was only 0.267 MPa, while that crosslinked by the HDI/TMP mixture reached 0.408 MPa (GAP hydroxyl value: 0.4647 mmol·g⁻¹, number-average molecular weight: 3757 g·mol^-1 )[14]. This result indicated that optimizing the structure and interaction mode of crosslinking points is an effective way to enhance the mechanical properties of GAP-based crosslinked networks.

1,3,5-Tris(2-hydroxyethyl)cyanuric acid (THEIC) is a low-cost, easily available, and highly functional fine chemical intermediate[15,16]. The triazine ring in THEIC contains three highly polar carbonyl groups, which can form multiple strong hydrogen bonds with urethane segments on different molecular chains, thereby effectively compensating for the negative effect of azide groups on hydrogen bond formation and distribution. Meanwhile, the unique ring tension of the triazine ring is also conducive to enhancing the rigidity, stability, and mechanical response of the crosslinked network[17]. In addition, the three hydroxyethyl groups in THEIC can react with isocyanate groups to form stable urethane bonds, providing additional hydrogen-bonding sites for the polymer network. It is well known that hydrogen bonding plays a critical role in adjusting the mechanical strength, toughness, and deformation recovery of polyurethane materials[18,19,20].

The bond energy of carbonyl-related hydrogen bonds in urethane structures is about 29.7 kJ·mol-1 , which is much lower than that in urea structures (63.1 kJ·mol-1 )[21,22]. Such low-energy hydrogen bonds can undergo reversible dissociation and re-association during deformation, which endows the polymer chains with sufficient mobility and orientation ability in the three-dimensional network. This dynamic interaction mechanism is beneficial to improving both tensile strength and elongation at break of the crosslinked system simultaneously.

In this work, a novel multifunctional isocyanate curing agent TCI was designed and synthesized via the reaction of low-cost THEIC with HDI. A series of GAP-based crosslinked networks were then prepared by using TCI and commercial N-100 curing agent respectively, forming crosslinked networks with different crosslinking point structures and intermolecular interaction modes. The results showed that the isocyanurate ring structure and additional urethane bonds in TCI significantly enhanced the interaction strength between crosslinking points, optimized the aggregation state of GAP molecular chains, and thus greatly improved the mechanical properties of the thermosetting GAP-based crosslinked network. This study provides a new and practical strategy for improving the mechanical performance of thermosetting azide-based energetic adhesives, which is of great significance for the engineering application of high-energy propellants.

Scheme 1. (a) Schematic Figure of the composite route of TCI. (b) Schematic illustration of cross-linked GAP.

Scheme 1. (a) Schematic Figure of the composite route of TCI. (b) Schematic illustration of cross-linked GAP.

2. Materials and Methods

Materials: Both 1,6-Diisocyanatohexane (HDI, 99%) and 1,3,5-Tris(2-hydroxyethyl) isocyanatohexane (THEIC, 98%) and tetrahydrofuran ultra-dry solvent (THF, 99.9%) were purchased from Energy Chemical and used as received. Dimethylformamide (DMF; Tianjin Reagents Co. Ltd., AR grade) was used after being dried. Amorphous prepolymer poly (azide glycidyl ether) (GAP hydroxyl value 0.4769 mmol·g⁻¹, numberaverage molecular weight 4242 g·mol⁻¹, polydispersity 1.37) and N100 (isocyanate concentration 5.38 mmol·g⁻¹, numberaverage molecular weight 703 g mol⁻¹,viscosity 4.5 Pa·s) were provided by Luoyang Liming Chemical Research Institute provid. GAP was dried at 60℃ under vacuum for 5 h before usage.

Synthesis of TCI: One pot method was used to synthesize TCI. THEIC (2.612 g, 10 mmol) and HDI (6.728 g, 40 mmol) was dissolved into a mixture of 10 ml DMF and 10 ml THF. Then the mixed solution was poured into a 250 mL reaction flask equipped with a stir bar. The reaction was carried out at 42℃ under nitrogen atmosphere for 4 h, and cooled to room temperature. Solvents and residual HDI were removed by rotary steaming, and then a colorless viscous liquid was obtained, marked as TCI.

Preparation of thermosetting cross-linked networks: To explore the best dosage of the new curing agent, different molar ratios of isocyanate to hydroxyl (R value) were utilized in thermosetting cross-linked networks compositions (Table 1).

Taking N100-GAP1.0 for example, a mixture of TCI (1.43), GAP (8.57 g), TPB (0.03 g) and T12 (0.01g) was added into a 50 mL tetrafluoroethylene cup, and then it was placed in a centrifuge at 2500 r/min for 5 min to remove the bubbles. Sample PU-T1.0 was obtained by casting the resultant viscous fluid on Teflon plates and cured at 60℃for 7 days in the constant temperature water bath incubator. The structures of the prepared thermosetting cross-linked networks are shown in Figure 1.

Characterization. The ¹H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer in dimethyl sulfoxide-d6 (DMSO-d6), tetramethylsilane (TMS) was used as a reference.

Thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA/DSC1 STARe System) from 30℃ to 800 ℃ at a heating rate of 10 K/min under an nitrogen atmosphere.

Differential scanning calorimetry (DSC) was performed from -60 ℃ to 100 ℃ at a heating rate of 10 K/min under an argon atmosphere on a TGA/DSC1 STARe System.

Dynamic thermomechanical analysis (DMA) tests were conducted on a Mettler DMA/SDTA861e instrument at a heating rate of 5 K·min⁻¹ and a frequency of 1 Hz.

X-ray diffraction (XRD) patterns were recorded using a Xeuss 2.0 (Xenocs, France).

SAXS studies on polymer films were carried out on a SAXS instrument (Xeuss 2.0) equipped with a Cu microfocal X-ray source. Optical tube power is 30W and wavelength is 1.54189 Å with a two-dimensional hybrid photon counting detector (Pilatus 3R 300K, and single pixel size: 172 μm). The long period (L) was obtained by Bragg’s law: L=2π/q, where q is the value of the peak in the intensity curves (I(q) ∼ q). The scattering vector q = 4π (sin θ)/λ, where λ is the X-ray wavelength and θ is one-half scattering angle (2θ).

Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were recorded on a Nicolet iS5 (Thermo Fisher, USA) spectrometer with ATR accessory. The variable-temperature FTIR was performed by Nicolet 6700 equipped with a thermo-controller.

Low field nuclear magnetic resonance (LF-NMR) was measured using VTMR20-010 V-T (Niumai Corporation, China). The elastomer was chopped and placed in a 1 mL NMR bottle for testing at temperature of 30 ℃, 50 ℃ 70 ℃. Each sample was tested 5 times and the average value was taken.

Mechanical tests were performed on an electronic tensile machine (Instron 5966, USA). The samples were cut into a dog-bone shape (a testing measure of 12.0 × 2.0 × 1.0 mm³) for tests. The stretching rate was set at 100 mm min⁻¹.

The fractured morphologies were characterized by a JSM-5800 scanning electron mi-croscope (SEM, Hitachi, Japan). The sample was plated with platinum at 10 mA for 40 s, and then tested under vacuum.

3. Results

3.1. Synthesis of New Curing Agents and Curing of Elastomers

THEIC, an environmentally and thermally stable material, was used as a core building block to react with HDI in a one-step synthesis, affording a novel isocyanate-based curing agent designated as TCI. The reaction mechanism is as follows: it is easy for the reactive hydrogens of THEIC to have nucleophilic addition reactions with the isocyanate functional group (-N=C=O) of HDI, which leads to the synthesis of the desired product.

The successful synthesis of TCI was confirmed by ¹H NMR spectra (Supporting Information, Figure S1) and FTIR curves (Figure 1a). As shown in Figure 1a, the strong absorption peak at 2258 cm⁻¹of TCI corresponded to the characteristic absorption peak of -N=C=O. Due to the induction effect of the electron-withdrawing group triazine ring, the peak position of -N=C=O on the new curing agent TCI has redshifted compared with that at 2249 cm⁻¹of the raw material HDI. The new telescopic vibration peak of the N-H bond appeared at 3331 cm⁻¹and amide band at 1520 cm⁻¹ was found, verifying the formation of the urethane bond. Meanwhile, the characteristic absorption of the carbonyl group in the triazine ring at 1672cm^-1 and the C-N stretching vibration of the triazine ring at 1458cm^-1 verified that the triazine moiety was successfully incorporated into the molecular structure of TCI. Accordingly, the target curing agent TCI was successfully synthesized.

Figure 1b presents the FTIR spectra of the TCI-GAP and N100-GAP networks. The strong absorption peak of -N=C=O at 2258 cm^-1 could not be observed on the FTIR curves. An amide band was found at 1516 cm^-1 in both TCI-GAP and N100-GAP. The stretching vibration of amine (-NH) bond on urethane linkages appeared at 3350 cm^-1 , and the stretching vibration of amide carbonyl (-C=O) bond at 1721cm^-1 and 1690 cm^-1 , indicating the formation of new urethane groups. A doublet of the ether bonds (C-O-C) absorption bands at 1073 cm^-1 and 1103cm^-1 , methylene at 2873cm^-1 and 2925cm^-1 and azide group at 2090cm^-1 appeared in both elastomers and GAP prepolymer. These analyses show that the TCI-GAP and N100-GAP were successfully synthesized. No characteristic absorption of −N=C=O at 2258 cm^-1 was observed in either spectrum, indicating complete consumption of isocyanate groups during the curing reaction. Both networks displayed an amide II band at 1516 cm^-1, N-H stretching of urethane at 3350cm^-1 , and carbonyl (−C=O) stretching at 1721 and 1690 cm^-1 , confirming the formation of urethane linkages. These results confirm the successful preparation of TCI–GAP and N100–GAP networks.

The XRD patterns of TCI–GAP and N100–GAP samples are presented in Fig. 1c. A diffraction peak at 2θ = 22.0° for TCI–GAP and 2θ = 21.6° for N100–GAP verify the existence of partially ordered microstructures in both crosslinked elastomer networks. To quantitatively evaluate the molecular chain packing, the interplanar crystal spacing (d-spacing) was calculated using the Bragg equation[23], which reflects the intermolecular distance and internal free volume within the polymer matrix. As illustrated in Fig. 1d and Table 2, small-angle X-ray scattering (SAXS) results show that the average intermolecular chain distance is 0.424nm for TCI–GAP and 0.436nm for N100-GAP, confirming that TCI effectively reduces the intermolecular spacing relative to N100.

Notably, the triazine ring in TCI introduces considerable steric hindrance, which suppresses the formation of long-range ordered hard domains, as reflected by the weaker XRD diffraction intensity of TCI-GAP compared with N100-GAP. As a conventional crosslinker, the flexible aliphatic structure of N100 promotes the assembly of ordered hard segments. In contrast, the additional urethane linkages and polar carbonyl groups in TCI strengthen hydrogen-bonding interactions at crosslinking junctions, which counteracts the steric hindrance effect. Consequently, TCI-GAP exhibits a smaller average chain distance, demonstrating that hydrogen-bonding interactions play a dominant role over steric effects in regulating intermolecular packing.

3.2. Curing Kinetics of TCI

Figure 2a and Figure 2b display the time-dependent conversion curves of two reaction systems—TCI-GAP (without catalyst) and N100-GAP (catalyzed by a 3:1 mixture of TPB and T12, 0.4 wt%) at different temperatures (40,50, and 60℃). The curves clearly show that in the initial reaction stage, the TCI-GAP system reaches a conversion ratio of 71.6% at 40℃ after 600 min, whereas the N100–GAP system only achieves 60.7% at 70℃ over the same time period. These results confirm that the novel curing agent TCI possesses high curing reactivity, which effectively compensates for the low reactivity of the secondary hydroxyl groups in GAP.

The TCI–GAP curing system was analyzed using a second-order kinetic model, and the plot of α/(1−α) versus time is presented in Figure 2c. The good linear correlation confirms that the curing reaction of TCI–GAP follows second-order kinetics. Fitting by the Arrhenius equation yields the following relationship:

lnk = −1.743/T + 0.1604

For the conventional curing agent N100, the N100–GAP system was evaluated using a first-order kinetic model. The linear relationship observed in the plot of ln(1−α) versus time (Figure 2d) verifies that the N100–GAP curing reaction conforms to first-order kinetics. The corresponding Arrhenius equation is determined as:

lnk = −8.568/T + 18.97

Calculations show that the activation energies (Eₐ) of the TCI–GAP and N100–GAP systems are 14.49 kJ·mol^-1 and 71.23 kJ·mol^-1 , respectively. The remarkably lower activation energy of TCI–GAP further demonstrates that the TCI-based system exhibits a significantly higher curing reaction rate than the N100-based system.

3.3. Mechanical Properties and Fractography Analysis of Elastomers

Subsequently, the mechanical properties of the two elastomeric networks were evaluated at different R values. As shown in Figure 3a and Figure 3b, the tensile strength of the TCI-GAP elastomer exhibited superior performance with increasing R value. At an R value of 1.3, TCI-GAP achieved a tensile strength of 1.74 MPa and an elongation at break of 297.7%, whereas N100-GAP showed a tensile strength of 0.85 MPa and an elongation of 105.8% under the same condition. At R = 1.0, the tensile strengths of TCI-GAP and N100-GAP were 0.83 MPa and 0.66 MPa, respectively, while TCI-GAP’s elongation at break was nearly double that of N100-GAP.

This performance disparity is attributed to TCI’s structural advantages over the classical curing agent N100: the triazine ring in TCI contains three additional carbonyl groups, and each branch chain features two freely rotatable methylene groups. The cyclic isocyanurate structure enhances tensile strength via ring strain, while the extra urethane linkages promote hydrogen bonding between crosslinking points. The methylene groups extend the load-bearing chain segments in the network and facilitate hydrogen bond formation between adjacent urethane groups, thereby improving both deformability and load capacity.

Morphological analysis of the tensile fracture surfaces (Figure 3c and Figure 3d) reveals distinct characteristics: the TCI-GAP fracture surface is rough, indicative of ductile failure, with numerous spherical particles of varying sizes. These particles are hypothesized to arise from the aggregation of crosslinking points driven by the triazine ring’s conjugated structure and hydrogen bonding. The triazine ring’s electron-withdrawing effect catalyzes the reaction between neighboring -NCO groups and hydroxyl groups, forming spherical domains. Close observation of the particle distribution (marked by red circles) shows that these structures deflect crack propagation paths, increasing energy dissipation during fracture and further enhancing the elastomer’s toughness.

The transmission electron microscope (TEM) images of TCI-GAP and N100-GAP exhibit phase separation (Figure 3e,f). The darker region(hard segment) is wrapped with the brighter part(soft segment) as continuous.Comparing the TEM images of N100-GAP with TCI-GAP, it can be clearly observed that the number and distribution of cross-linking points in the two materials vary greatly.The N100-GAP shows a poor mechanical property of synthesized with N100. Due to the small number and uneven distribution of internal cross-linking points, crack propagation cannot be efficaciously delayed, resulting in brittle fracture. While the internal crosslinking points in TCI-GAP, which is solidified by TCI (with the additional urethane linkage and triazine ring), are numerous and more evenly distributed. They can effectively disperse external forces, which correlates with excellent mechanical properties of TCI-GAP.

3.4. Hydrogen Bonding Interactions

In addition to chemical network architecture, hydrogen-bonding interactions also play a critical role in determining the mechanical properties of the elastomeric networks. To gain insight into the evolution of functional groups and hydrogen bonds under tensile deformation, in-situ ATR-FTIR measurements were performed on the two elastomers before and after 10% elongation, with the corresponding spectra presented in Figs. 4a and 4b.

Tensile deformation reduces the intermolecular chain spacing and enhances the local density of triazine rings within the TCI–GAP network. The decreased chain distance facilitates the conversion of free ester carbonyl groups into loosely associated hydrogen-bonded carbonyl moieties, leading to a reduction in the stretching frequency of free carbonyl groups. Consequently, the urethane carbonyl peak of TCI–GAP at 1690.2 cm^-1 shows a blue shift to 1692.3 cm^-1 upon stretching. Meanwhile, the ordered ester carbonyl groups are influenced by the enhanced electron-withdrawing effect of the triazine rings, which increases the stretching vibration frequency of the carbonyl groups and causes the peak at 1722 cm^-1 to exhibit a red shift to 1720 cm^-1.

Figure 4. (a, b) FTIR spectra of TCI-GAP and N100-GAP before and after stretching. (c, d) Fitting curves of carbonyl peaks and (e, f) the proportions of each type of carbonyl for TCI-GAP and N100-GAP before and after stretching.

Figure 4. (a, b) FTIR spectra of TCI-GAP and N100-GAP before and after stretching. (c, d) Fitting curves of carbonyl peaks and (e, f) the proportions of each type of carbonyl for TCI-GAP and N100-GAP before and after stretching.

Figure 5. Different hydrogen bonded carbonyl types.

Figure 5. Different hydrogen bonded carbonyl types.

To probe the dynamic evolution of hydrogen bonding under tensile deformation, peak-fitting analysis was conducted on the FTIR spectra of both elastomers before and after 10% elongation (Figs. 4c, d). The carbonyl region (1600–1750 cm^-1) was deconvoluted into six distinct peaks, corresponding to three hydrogen-bonding states (ordered, disordered, and free) of both urea and urethane carbonyl groups. The relative contributions of these peaks are summarized in Figs. 4e and f, with detailed peak positions and assignments provided in Tables S1 and S2 (Supporting Information).

At 10% elongation, TCI-GAP exhibited far more pronounced alterations in carbonyl group distributions compared to N100-GAP. Specifically, the fraction of hydrogen-bonded carbonyls in TCI-GAP increased significantly from 63.6% to 75.8%, whereas N100-GAP showed only a marginal rise from 64.9% to 67.8%. This discrepancy can be attributed to two key structural features of TCI:

(1) Ester Carbonyl Groups: The proportion of hydrogen-bonded ester carbonyls in TCI-GAP increased from 69.8% to 75.1%, in contrast to a modest rise from 72.4% to 74.1% for N100–GAP. This improvement stems from the fact that TCI possesses three additional freely rotatable carbamate groups compared to N100, which provide more accessible ester carbonyl sites to form robust hydrogen bonds under tensile stress.

(2) Urea Carbonyl Groups: The fraction of hydrogen-bonded urea carbonyls in TCI–GAP underwent a dramatic increase from 41.3% to 80.1%, whereas N100-GAP showed only a slight rise from 45.3% to 49.2%. The substantial formation of hydrogen bonds in TCI-GAP under external force is likely due to the unique electronic environment of the triazine ring. Unlike the flexible aliphatic chains in N100, the urea carbonyl groups on the conjugated triazine ring in TCI-GAP are less shielded by alkyl chains and are thus geometrically predisposed to form strong hydrogen bonds, thereby effectively reinforcing the elastomeric network.

To further elucidate the influence of chemical structure on hydrogen bond stability, temperature-dependent FTIR measurements were performed (Figs. 6a, b). The carbonyl stretching regions (1600–1750 cm^-1) of urea and urethane bonds serve as sensitive probes for monitoring hydrogen-bonding interactions in these two systems (Figs. 6c, d). Peak fitting of the carbonyl curves was employed to quantify the distribution of various hydrogen-bonding states at 30, 50, and 70℃ (Figs. 6e, f).

As temperature increased from 30 to 70℃, both elastomers displayed a decrease in ordered urethane carbonyls, accompanied by a corresponding increase in disordered and free states. However, TCI–GAP exhibited exceptional stability in the distribution of urea carbonyl groups (variation ≤ 1.5%), indicating a highly resistant hydrogen-bonding network against thermal disruption. In contrast, N100-GAP experienced significant reductions in both ordered urethane and urea carbonyl fractions with rising temperature. This superior thermal stability of TCI-GAP originates from its structural advantages: the additional urethane linkages and triazine ring in TCI facilitate the formation of a dense and interconnected hydrogen-bonding network. Specifically, the triazine ring in TCI-GAP restricts chain rotation and locks urea carbonyl groups in optimal orientations for hydrogen bonding, minimizing thermal dissociation and thereby maintaining mechanical integrity at elevated temperatures.

Figure 6. (a, b)Temperature dependent FTIR spectra of TCI-GAP and N100-GAP upon heating from 30℃ to 120℃. (c, d) Fitting curves of carbonyl peaks and (e, f) the proportions of each type of carbonyl for TCI-GAP and N100-GAP at various temperatures.

Figure 6. (a, b)Temperature dependent FTIR spectra of TCI-GAP and N100-GAP upon heating from 30℃ to 120℃. (c, d) Fitting curves of carbonyl peaks and (e, f) the proportions of each type of carbonyl for TCI-GAP and N100-GAP at various temperatures.

Figure 7. (a) Storage modules (b) Loss factors of TCI-GAP and N100-GAP; (c) LF-NMR curves at various temperatures; (d) DSC thermograms of TCI-GAP and N100-GAP.

Figure 7. (a) Storage modules (b) Loss factors of TCI-GAP and N100-GAP; (c) LF-NMR curves at various temperatures; (d) DSC thermograms of TCI-GAP and N100-GAP.

3.5. Dynamic Mechanical Properties, Thermal Behavior, and Network Mobility Analysis

Dynamic mechanical analysis (DMA) was employed to reveal the influence of crosslinking point structure on the segmental mobility of the GAP-based networks. As presented in Figure 7a, a structural transition occurred at approximately -50℃, accompanied by a gradual decrease in the storage modulus. Below -40℃, N100-GAP exhibited a higher storage modulus than TCI-GAP, which can be ascribed to its denser hydrogen-bonding network at low temperatures. As the temperature rose above -40℃, the weak hydrogen bonds began to dissociate, promoting the relaxation and movement of molecular chain segments. Within the temperature range of -40 to 80℃, TCI-GAP consistently displayed a higher storage modulus relative to N100–GAP. This phenomenon is attributed to the rigid triazine ring structure within the TCI crosslinking junctions, which imparts intrinsic rigidity to the crosslinked network and effectively resists thermal softening. The synergistic effect of dynamic hydrogen-bonding interactions and the rigidity of the triazine ring contributes to the excellent thermomechanical stability of TCI-GAP over a wide temperature range.

As illustrated in Figure 7b, the α-transition peak, corresponding to the glass transition temperature (T_g), was observed at -27℃ -27.5℃ for TCI-GAP and -28.6℃ for N100-GAP. The 1.1℃ higher T_g of TCI-GAP verifies the stronger intermolecular interactions and more restricted segmental motion imposed by the TCI crosslinker. Furthermore, TCI-GAP exhibited a higher loss factor (tanδ) than N100-GAP, indicating greater resistance to intermolecular slippage under deformation.

Low-field nuclear magnetic resonance (LF-NMR) was utilized to analyze the transverse relaxation time (T₂) of hydrogen atoms in both elastomers at varying temperatures (Fig. 6c)[23,24,25]. The T₂ value is inversely related to molecular mobility, where a shorter T₂ indicates more restricted chain motion. The three characteristic peaks in the LF-NMR spectra correspond to hydrogen protons located in (1) hard domains, (2) soft domains, and (3) small molecular fractions. Notably, TCI-GAP exhibited a shorter relaxation time for hard domains, confirming the stronger hydrogen-bonding interactions at crosslinking junctions. Combined with the higher tanδ value observed in DMA, these results demonstrate that segmental slippage in TCI-GAP is more effectively suppressed than in N100-GAP.

With increasing temperature, both elastomers exhibited prolonged T₂ relaxation times in the soft domain, indicating enhanced segmental mobility. However, N100-GAP showed more significant T₂ fluctuations, implying a more obvious reduction in confinement of molecular chains with rising temperature. This tendency is consistent with the temperature-dependent dissociation of hydrogen bonds shown in Figs. 5e and f, where the weaker hydrogen-bonding network in N100-GAP leads to more severe thermal softening.

The glass transition temperatures determined by DSC were -39.0℃ for TCI-GAP and -41.5℃ for N100-GAP (Figure 7d), which are consistent with the DMA results. No obvious endothermic or exothermic peaks corresponding to hard domain aggregation were observed for either TCI-GAP or N100-GAP in the DSC curves from -60 to 100℃, indicating the absence of distinct hard domain regions. Despite the lack of prominent hard domain aggregation, TCI-GAP still achieved significantly improved tensile strength, which is mainly attributed to the enhanced hydrogen-bonding interactions and rigid triazine-containing crosslinking structure.

4. Conclusions

A novel multifunctional isocyanate curing agent (TCI) was successfully designed and synthesized via a facile one-step reaction using cost-effective 1,3,5-tris(2-hydroxyethyl)cyanuric acid (THEIC) and hexamethylene diisocyanate (HDI). A series of glycidyl azide polymer (GAP)-based energetic elastomers were prepared using TCI and the conventional curing agent N100, and the effects of crosslinking structure, hydrogen-bonding interactions, and network morphology on the curing behavior, mechanical properties, and thermal stability of the elastomers were systematically investigated.

The results demonstrated that the TCI curing agent possessed significantly higher curing reactivity toward GAP compared with N100. The curing reaction of TCI-GAP conformed to second-order kinetics with an activation energy as low as 14.49 kJ·mol^-1, which was much lower than that of N100-GAP (71.23 kJ·mol^-1). Benefiting from the unique triazine ring structure and additional urethane groups, TCI enabled the formation of dense, reversible hydrogen-bonding networks in the GAP matrix, which effectively reduced the intermolecular chain distance and optimized the crosslinking network structure.

Benefiting from the synergistic effect of rigid cyclic structure and dynamic hydrogen-bonding interaction, TCI-GAP exhibited remarkably enhanced mechanical properties. At an R value of 1.3, the tensile strength and elongation at break of TCI-GAP reached 1.74 MPa and 297.7%, respectively, representing significant improvements relative to N100-GAP (0.85 MPa and 105.8%). Morphological characterization confirmed that TCI-GAP possessed denser and more uniform crosslinking junctions, which could effectively hinder crack propagation and achieve typical ductile fracture.

In-situ ATR-FTIR and temperature-dependent FTIR analyses revealed that the TCI-GAP network exhibited highly stable and responsive hydrogen-bonding interactions. Under tensile deformation, the proportion of hydrogen-bonded carbonyl groups increased significantly from 63.6% to 75.8%, realizing efficient energy dissipation. Meanwhile, the hydrogen-bonding structure constructed by TCI showed excellent thermal stability, and the change ratio of urea carbonyl was less than 1.5% in the range of 30-70℃. DMA and LF-NMR results further verified that the TCI-crosslinked network possessed higher storage modulus, more restricted segmental motion, and better thermomechanical stability.

This work provides a highly feasible and efficient strategy for optimizing the crosslinking network structure of GAP-based energetic elastomers by constructing rigid cyclic crosslinking points and dynamic hydrogen-bonding systems. The novel curing agent TCI exhibits outstanding application potential in high-performance energetic binders and composite solid propellants.