Polymethylhydrosiloxane-Coated Ammonium Dinitramide: A Simple Strategy to Moisture-Resistant and Insensitive Energetic Microspheres

Abstract

Ammonium dinitramide (ADN) is a promising, environmentally friendly oxidizer for next-generation solid propellants. However, its high hygroscopicity and sensitivity to mechanical stimuli significantly hinder its practical applications. Here, ADN–polymethylhydrosiloxane (ADN–PMHS) composite microspheres were prepared via solvent–antisolvent crystallization, followed by a simple surface coating step. The morphology, chemical structure, thermal behavior, hygroscopicity, and mechanical sensitivity of the coated microspheres were systematically characterized. Results revealed that PMHS formed a uniform, continuous hydrophobic layer on the ADN crystal surfaces, yielding well-dispersed spherical particles. At an optimal PMHS content of 0.5 wt % (ADN–0.5%PMHS), the composite exhibited 54.8% lower moisture uptake after 25 h at 60% relative humidity and a 1.6 times reduction in impact sensitivity compared with raw ADN. Moreover, the activation energy increased from 164.44 to 184.96 kJ/mol, indicating improved thermal stability, while the decomposition temperature remained nearly unchanged. These findings demonstrate that ADN–0.5%PMHS microspheres possess enhanced moisture resistance, mechanical insensitivity, and thermal stability, making them highly attractive for advanced, environmentally sustainable solid-propellant formulations.

1. Introduction

Ammonium dinitramide (ADN) has attracted considerable attention in recent decades as a high-performance oxidizer for propellants and explosives. Its distinct properties, which include high energy density and clean combustion by-products, position it as a promising candidate for environmentally friendly applications in propulsion systems and energetic materials. The combustion products of ADN are predominantly free of halogens, leading to a reduced environmental impact compared to conventional oxidizers like ammonium perchlorate (AP). Furthermore, its superior oxygen balance enhances energy output, providing a competitive advantage in high-performance formulations.

However, its pronounced hygroscopicity significantly restricts its application in solid propellants and explosives. As a highly hygroscopic compound, ADN readily absorbs moisture from the surrounding environment, , resulting in issues such as caking, structural degradation, and diminished energetic performance. These consequences undermine its handling, storage, and long-term stability, rendering it less suitable for industrial-scale use. Therefore, addressing the hygroscopicity of the ADN is a crucial step in unlocking its full potential. Three main approaches have been explored to address this issue: seeding, cocrystallization, and surface coating. Seeding modifies the ADN particle morphology by reducing the specific surface area of the crystal, but does not alter the chemical structure or address the underlying hygroscopicity. − Cocrystallization forms single-phase crystals with other components, altering the physical and chemical properties of the ADN. However, this approach often reduces desirable ADN properties, such as oxidation resistance, and remains challenging to implement in practice. − Surface coating provides the most versatile solution by creating a protective film around the ADN particles, isolating them from moisture while enhancing properties such as dispersibility, compatibility, and mechanical stability. − Among these methods, surface coating stands out as the most adaptable and widely applicable method to improve the performance of ADN in energy materials. According to the principles of coating techniques, the main approaches include the solvent evaporation method, ,,,, the solvent–nonsolvent (antisolvent) method, the melt crystallization method, and the atomic layer deposition (ALD) method. Among these, the solvent evaporation method is particularly attractive due to its simplicity, operational safety, and high efficiency in forming uniform coatings.

Innovative composite material systems for ADN have been primarily developed by applying hydrophobic coatings to the surface of ADN particles. ,,,,,, These coatings, typically made from polymeric materials, effectively transform ADN from a hydrophilic to a hydrophobic state. The fabrication process for these composite material systems is relatively straightforward and involves mild reaction conditions. Additionally, the incorporation of polymeric materials significantly enhances the mechanical tolerance and properties of solid fuel systems. Therefore, utilizing composite material systems is a key strategy for improving the characteristics of ADN and broadening its potential applications in energy materials. ,− Researchers have explored various methods for modifying ammonium dinitramide coatings based on this strategy. These methods include microencapsulation processes, ,,, physical adsorption, ,, and chemical cross-linking. ,, A range of coating materials, such as polymers, carbon nanomaterials, metal oxides, and energetic organic crystals, have been employed to enhance the properties of ADN. Most studies have focused on the effects of these coating materials on the hygroscopicity and mechanical sensitivity of ADN. However, there is still a significant gap in the systematic investigation of the energy release performance of coating-modified ADN materials when integrated into propellant systems. This limitation highlights the need for comprehensive studies to evaluate the effect of coating modifications on the energetic properties of ADN in practical applications. A highly effective method for reducing the hygroscopicity of ADN involves the use of core–shell microcapsules. Initially, organic polymers were the preferred materials for coating ADN. Heintz et al. utilized fluidized bed technology to coat ADN with polyacrylate, hydroxyl-terminated polybutadiene (HTPB), or glycidyl azide polymers. While this approach improved the mechanical compatibility and stability of the coated ADN particles, it did not address their hygroscopicity. In another study, Oliveira Silva et al. applied a thin layer of HTPB to ADN particles using a simple precipitation technique, which involved dissolving the HTPB resin and curing agent in n-hexane. However, they did not provide any information regarding the hygroscopicity of the resulting product. Inorganic materials have also shown potential for coating ADN. For instance, Gong et al. applied a thin aluminum film to ADN using atomic layer deposition, achieving coating thicknesses of several hundred nanometers. While the morphology of the coated ADN remained stable after 48 h at 25 °C and 70% relative humidity (RH), its hygroscopicity did not improve. In another study, Yan et al. utilized the Pickering emulsion method to coat ADN, using alkylated graphene oxide (GO) nanoparticles as a particulate surfactant. This approach reduced the moisture absorption of the coated ADN to some extent. However, during solvent evaporation, the coated ADN tended to aggregate into particles of uneven size. In addition, Li et al. conducted a study on the fabrication of core–shell energy microspheres composed of ammonium dinitramide@monoaminopropyl heptaphenylsilsesquioxane (ADN@POSS-NH₂). Their results indicated that the hydrophobicity of the final product was enhanced, which significantly reduced the moisture absorption. Under conditions of 75% relative humidity and a temperature of 20 °C, the ADN@POSS-NH₂ particles remained nearly solid even after 7 days. However, the authors did not further investigate the compatibility of ADN@POSS-NH₂ with other propellant components, nor did they analyze how process and technological factors could affect the adoption of this high-energy material in rocket-propellant manufacture, even though the POSS-NH₂ coating constitutes a substantial fraction of the microspheres (≈5 wt %). More recently, Zhang et al. reported a double-shell composite, ADN@GO@NC, prepared via Pickering emulsification using two-dimensional carbon nanomaterials as stabilizers. Incorporation of graphene oxide (GO) and nitrocellulose (NC) increased the thermal decomposition onset of ADN@GO@NC by 3.5 °C and reduced moisture uptake by 91.5% relative to raw ADN. Critical friction and impact sensitivities were reduced by factors of 1.35 and 1.57, respectively. Although this composite shows excellent antihygroscopic performance, its synthesis is complex and requires tight control of process variables (for example, emulsification conditions and stabilizer concentration). Finally, although incorporation of ADN-based energetic microspheres into propellant formulations can produce stable combustion with rapid, sustained energy release, achieving precise control of net energetic output in mixed solid-propellant matrices remains challenging.

Polymethylhydrosiloxane (PMHS) is a versatile organosilicon polymer characterized by its unique combination of chemical and physical properties, making it particularly suitable for surface coating and hydrophobic applications. − Its siloxane backbone provides excellent thermal stability, chemical inertness, and mechanical flexibility, allowing it to maintain performance under harsh environmental conditions. Moreover, the presence of reactive −Si–H bonds enables PMHS to undergo cross-linking or hydrosilylation reactions with a wide range of organic and inorganic substrates, resulting in the formation of durable, water-repellent, and thermally stable surface layers. , In addition, its low surface energy and viscoelastic behavior promote uniform coverage on ADN crystals, reducing the number of coating defects that might otherwise permit moisture ingress. Unlike rigid coatings that are prone to fracture during thermal cycling, PMHS coatings remain elastic, ensuring long-term durability in dynamic environments. Consequently, numerous studies have demonstrated the effectiveness of PMHS as a moisture-resistant coating across a wide range of materials. −

In this study, polymethylhydrosiloxane was utilized as a protective coating material for ammonium dinitramide, resulting in the formation of a microspherical composite referred to as ADN–PMHS. The ADN–PMHS microspheres were synthesized through a combined solvent–antisolvent crystallization and solvent evaporation process, enabling uniform coating and controlled particle morphology. The work systematically investigates the moisture resistance, mechanical sensitivity, and thermal decomposition compatibility of ADN with the PMHS coating. The overarching goal is to establish a pathway for developing advanced energetic materials based on ADN derivatives with potential applications in next-generation solid propellants and explosives.

2. Experimental Section

2.1. Materials

Raw ADN (ADN-R) was synthesized at the Institute of Explosives and Propellants. Polymethylhydrosiloxane (PMHS, ≥99.5%) was supplied by Sigma-Aldrich. Ethyl acetate (analytically pure, ≥99.5%), dichloromethane (DCM, analytically pure, ≥99.5%), and ethanol (analytically pure, ≥99.5%) were purchased from Aladdin Scientific.

2.2. Preparation of ADN–PMHS Microspheres

The general procedure for producing ADN–PMHS microspheres is illustrated in Scheme . First, 1.0 g of ADN-R was dissolved in a mixed solvent of ethyl acetate (5 mL) and ethanol (5 mL) to obtain a homogeneous ADN solution. This solution was introduced dropwise into 500 mL of dichloromethane (DCM, antisolvent) under ultrasonic agitation and maintained for 3 h, leading to the formation of spherical ADN (ADN-S) particles via antisolvent precipitation. Separately, 0.005 g of polymethylhydrosiloxane was dissolved in 5 mL of DCM with ultrasonic treatment for 15 min to obtain a uniform PMHS solution. This PMHS solution was then added dropwise to the ADN suspension at a controlled rate under continuous stirring (500 rpm) at ambient temperature. After complete addition, the mixture was stirred for an additional 2 h under the same conditions. The resulting product was collected by rotary evaporation of all solvents and subsequently dried under vacuum at 40 °C for 24 h, yielding the ADN–PMHS microspheres.

1. Preparation Process of ADN–PMHS Microspheres.

2.3. Characterization and Measurements

2.3.1. Chemical Structure

Fourier transform infrared (FTIR) spectroscopy (Spectrum Two, PerkinElmer, USA) was employed to analyze the compositional features of the samples. The spectra were recorded in the range of 400–4000 cm^–1. Nuclear magnetic resonance (NMR) spectroscopy (Bruker Avance 600 MHz, Germany) was used to determine the chemical structure. The ¹H NMR spectra were reported in δ (ppm) relative to tetramethylsilane (TMS), with δ (acetone-d ₆) = 2.05 ppm used as the internal reference. All samples were vacuum-dried at 40 °C for 24 h prior to characterization.

2.3.2. Morphology

Surface morphology, particle dimensions, and microstructure were examined by using thermal field emission scanning electron microscopy (SEM, JSM-IT800, JEOL, Japan). The crystal structure was characterized by powder X-ray diffraction (PXRD, Panalytical X’Pert Pro MRD, The Netherlands), employing Cu Kα radiation (λ = 1.54178 Å) at 25 °C. The operating conditions were set at 40 kV and 15 mA. Diffraction data were collected in the range of 10–60° (2θ) with a step size of 0.03° and a counting time of 0.5 s per step.

2.3.3. Thermal Stability

The thermal decomposition behavior of the samples was analyzed by using a differential scanning calorimeter (DSC, 204F1 Phoenix, Netzsch, Germany). Measurements were carried out with a sample mass of approximately 0.2 mg under a nitrogen atmosphere (flow rate: 25 mL/min). Heating rates of 5, 10, 15, and 20 °C/min were applied over the temperature range of 50–350 °C.

Vacuum stability tests (VST) were conducted to evaluate the chemical stability and compatibility of the energetic materials under vacuum conditions. The experiments were performed on a Stabil Vacuum tester at 100 °C for 40 h. Each sample was tested in triplicate, and the average value was reported.

2.3.4. Mechanical Sensitivity

Impact sensitivity was measured using a Cast hammer apparatus under standard conditions (hammer weight: 10 kg; drop height: 25 cm). For each test, 0.05 ± 0.005 g of sample was used, and 25 trials were conducted. The impact sensitivity was expressed as the percentage of explosions observed of the total number of trials.

2.3.5. Water Contact Angle

Wettability was evaluated by measuring the water contact angle of the samples. The ADN and ADN–PMHS powders were pressed into smooth tablets prior to measurement. Contact angles were determined using an optical contact angle meter and interface tensiometer (KRÜSS DSA25S, Germany) via the sessile drop method.

2.3.6. Hygroscopicity

Hygroscopicity was determined by using the weighing method. Raw ADN and ADN–PMHS complexes were placed in a constant-humidity and constant-temperature chamber (Premium Climatic Chamber, JSRH-150CPL, JS Researcher Inc., Korea). The samples were periodically removed and weighed to monitor mass changes over time.

3. Results and Discussion

3.1. Chemical Structure of Microspheres

FTIR spectroscopy was employed to identify the characteristic functional groups of PMHS and ADN within the microspheres (Figure ). The characteristic absorption peaks in the spectra were analyzed to confirm the incorporation of PMHS into the modified ADN microspheres at the molecular level. All modified ADN samples displayed peaks corresponding to those of unmodified ADN-R, indicating that the structural framework of ADN remained intact after modification. Specifically, the absorption peak at 3215 cm^–1, attributed to raw ADN, corresponds to the N–H stretching vibrations of the ammonium ion (NH₄ ⁺). The nitro group (–NO₂) exhibited absorption peaks at 1516 and 1341 cm^–1, corresponding to its asymmetric and symmetric stretching vibrations, respectively. In addition, the peak at 1015 cm^–1 was assigned to the characteristic stretching mode of the central nitrogen atom within the dinitramide anion (N­(NO₂)₂ ^–), consistent with previously reported data. These characteristic peaks confirm the successful incorporation of ADN into the microspheres. Furthermore, the characteristic absorption peak of PMHS at 2162 cm_, ^–1 associated with the stretching vibrations of Si–H groups, was also observed in the spectra of ADN–PMHS microspheres. The appearance of this peak verifies the successful coating of the ADN particles with PMHS.

1.

Chemical structure of ADN-R and ADN–0.5%PMHS microspheres by FTIR (A) and ¹H NMR (B).

The ¹H NMR spectrum further confirmed the presence of PMHS in the microspheres (Figure ). Distinct characteristic peaks were observed at chemical shifts of 0.22 and 4.72 ppm, which correspond to the proton environments of the Si–CH₃ and Si–H groups, respectively. Comparative analysis of the ¹H NMR spectra of ADN-R and ADN–PMHS revealed a downfield shift in the resonance of the ammonium (NH₄ ⁺) protons, shifting from 7.48 ppm in ADN-R to 7.36 ppm in ADN–PMHS. This spectral shift can be ascribed to hydrogen-bond interactions between PMHS and ADN, which perturb the local electronic distribution around the NH₄ ⁺ group.

In addition, a notable difference was observed in the chemical shift of water protons: 3.36 ppm in ADN-R versus 3.86 ppm in ADN–PMHS. The integrated peak area for water in ADN-R (3.36 ppm) was substantially larger than that in ADN–PMHS (3.86 ppm), indicating a higher water content in the uncoated ADN-R sample. These results suggest that the PMHS coating reduces moisture uptake, most likely due to its hydrophobic nature, thereby improving the moisture resistance of ADN.

The PXRD analysis of ADN-R, spherical ADN (ADN-S), and ADN–PMHS microspheres (Figure ) revealed that the characteristic diffraction peaks of spherical ADN were retained in the ADN–0.5%PMHS microspheres. The corresponding 2θ values were observed at 15.1°, 17.8°, 27.1°, 27.7°, 29.3°, and 30°, indicating that the crystalline structure of ADN was preserved during the coating process. Complementary SEM analyses further reveal a pronounced morphological transformation: the needle-like crystals of ADN-R are converted into uniform spherical particles in the ADN–PMHS microspheres (Figure A–C). Collectively, these findings indicate that PMHS encapsulation modifies only the external particle morphology, while preserving the intrinsic structural integrity of ADN.

2.

PXRD pattern and SEM images of raw ADN (A), spherical ADN (B), and ADN–0.5%PMHS microsphere (C).

3.2. Crystal Morphology of Microspheres

The crystal morphology of ADN-S and ADN–PMHS microspheres was examined using SEM, and the corresponding images are shown in Figure A,B. In the case of ADN–PMHS (Figure B), a uniform and continuous thin polymer layer is observed, encapsulating the surface of the ADN spheres, confirming successful coating. Elemental mapping of nitrogen (N), oxygen (O), and silicon (Si) further substantiates the coating integrity (Figure B1–B4). The N and O signals validate the presence of ADN as the core component, whereas the Si distribution clearly delineates the PMHS coating localized on the particle surface. Together, these morphological and elemental analyses confirm that PMHS forms a uniform shell around the ADN, a feature essential for enhancing its stability and moisture-resistance properties.

3.

SEM images of ADN-S (A), ADN–0.5%PMHS (B), and elemental mappings of ADN–0.5%PMHS (B1–B4).

The influence of the PMHS concentration on microsphere morphology is presented in Figure . As the PMHS content increases from 0 to 0.5%, the particles remain discrete, spherical, and well-defined (Figure A–C), indicating a uniform coating and minimal interparticle adhesion. However, at higher PMHS concentrations (0.8–1.2%), the particles exhibit pronounced aggregation and cluster formation (Figure D–F). The increased extent of agglomeration is attributed to the growing prevalence of interparticle interactions as the amount of PMHS increases.

4.

SEM images of ADN-S (A) and ADN–PMHS microspheres ((B) ADN–0.2%PMHS, (C) ADN–0.5%PMHS, (D) ADN–0.8%PMHS, (E) ADN–1%PMHS, and (F) ADN–1.2%PMHS).

These qualitative observations agree well with the quantitative particle size results summarized in Table and illustrated in Figure . As the PMHS content increases, both the median and mean particle sizes exhibit a general upward trend, reflecting the combined effects of thicker polymer encapsulation and particle coalescence at higher polymer loadings. The sample containing 0.5 wt % PMHS exhibits the narrowest particle size distribution, as evidenced by the minimal difference between its median and mean diameters (Figure C). In contrast, samples with higher PMHS concentrations (1.0–1.2 wt %) show broader and less uniform size distributions due to significant aggregation and the formation of clustered structures.

1. Median Size and Mean Size of Spherical ADN and ADN-Based Complex Particles.

samples	median size (μm)	mean size (μm)
ADN-S	7.2164	7.689
ADN–0.2%PMHS	8.038	8.265
ADN–0.5%PMHS	9.277	9.296
ADN–0.8%PMHS	9.034	9.940
ADN–1.0%PMHS	9.473	10.278
ADN–1.2%PMHS	9.736	10.618

5.

Particle size distribution diagram of the samples: ADN-S (A), ADN–0.2%PMHS (B), ADN–0.5%PMHS (C), ADN–0.8%PMHS (D), ADN–1%PMHS (E), and ADN–1.2%PMHS (F).

This morphological transition can be attributed to hydrogen-bonding interactions between the flexible hydrogens of the −Si–H groups in PMHS and the nitrogen atoms of the –N­(NO₂)₂ groups in ADN (Figure B). At low PMHS content, these hydrogen bonds primarily occur between PMHS and the surface of individual ADN particles, promoting uniform dispersion by effectively encapsulating each microsphere. In contrast, at higher PMHS levels, the excess −Si–H groups form additional interparticle hydrogen bonds, linking adjacent microspheres together and ultimately leading to aggregation into larger clusters. These findings highlight the importance of optimizing the PMHS concentration to balance coating uniformity and prevent undesirable particle agglomeration, which can influence the performance and stability of the coated ADN material.

9.

Hygroscopic mechanism of ADN (A), mechanism of ADN coating with PMHS to form ADN–PMHS microspheres and antihygroscopic (B), water absorption mechanism of ADN–PMHS with high PMHS content (>0.8%) (C).

3.3. Thermal Stability

The thermal decomposition behavior of the prepared samples was investigated by using differential scanning calorimetry (DSC) at heating rates of 5, 10, 15, and 20 °C/min, as shown in Figure . The DSC curve of the ADN-S sample exhibited a distinct endothermic peak at approximately 91 °C, corresponding to the phase transition from crystalline solid to liquid. This was followed by a pronounced exothermic peak at around 180 °C, which represents the complete thermal decomposition of ADN, producing nitrogen dioxide (NO₂), nitric oxide (NO), ammonia (NH₃), and water (H₂O). A clear trend was observed with increasing PMHS content: the peak exothermic decomposition temperature of ADN-based samples progressively decreased as the PMHS concentration increased from 0 to 1.2 wt %. At a heating rate of 20 °C/min, the peak decomposition temperatures were 199.36 °C for ADN-S, 197.31 °C for ADN–0.5%PMHS, and 194.00 °C for ADN–1.2%PMHS.

6.

DSC curve at different heating rates of ADN-S (A) and ADN–PMHS microspheres ((B) ADN–0.2%PMHS, (C) ADN–0.5%PMHS, (D) ADN–0.8%PMHS, (E) ADN–1.0%PMHS, and (F) ADN–1.2%PMHS).

This decrease in the decomposition temperature is attributed to the presence of the PMHS coating, which alters the heat transfer dynamics and energy release behavior within the microspheres. The hydrophobic PMHS layer likely facilitates the localized accumulation of thermal energy, thereby lowering the activation energy for decomposition and promoting the earlier onset of ADN degradation. These findings indicate that the PMHS coating plays a critical role in modifying the thermal stability and decomposition kinetics of the ADN.

The apparent activation energy (E _a), pre-exponential factor (A), and thermal explosion critical temperature (T _b) were determined at different heating rates using the Kissinger equation (eq ), the Roger equation (eq ), and the Arrhenius equation (eq ). −

$$\ln \frac{{\beta }_{\mathrm{i}}}{T_{\mathrm{pi}}^2}=\ln \frac{\mathit{AR}}{E_{\mathrm{a}}}-\frac{E_{\mathrm{a}}}{{\mathit{RT}}_{\mathrm{pi}}}$$ 1

$$T_{\mathrm{p}0}=T_{\mathrm{pi}}-b{\beta }_{\mathrm{i}}-c{\beta }_{\mathrm{i}}^2$$ 2

$$T_{\mathrm{b}}=\frac{E_{\mathrm{a}}-\sqrt{E_{\mathrm{a}}^2-4R{E}_{\mathrm{a}}{T}_{\mathrm{p}0}}}{2R}$$ 3

where β_i is the heating rate; A is pre-exponential factor; E _a is the apparent activation energy, kJ/mol; R is the thermodynamic constant, R = 8.314 J/(mol K); T _pi is the decomposition peak temperature; T _p0 is the decomposition peak temperature when the heating rate tends to 0, K; b and c are constants; and T _b is the thermal explosion critical temperature, K. The calculated results are summarized in Table and Figure .

2. Thermal Decomposition Kinetic Parameters and Vacuum Stability of Spherical ADN and ADN-Based Complexes.

samples	E a (kJ/mol)	log A	T b (K)	T p0 (K)	vacuum stability (mL/g)
ADN-S	164.44	16.65	455.18	444.70	2.10
ADN–0.2%PMHS	170.69	17.40	456.27	446.13	2.05
ADN–0.5%PMHS	174.57	17.89	457.00	447.05	2.02
ADN–0.8%PMHS	179.66	18.53	459.31	449.55	1.98
ADN–1%PMHS	181.60	18.88	459.41	449.75	1.97
ADN–1.2%PMHS	184.96	19.24	459.20	449.72	1.95

7.

Activation energy (A); T _b, thermal explosion critical temperature; and T _p0, decomposition peak temperature when the heating rate tends to 0 (B) of ADN-S and ADN–PMHS microspheres.

An analysis of the data in Figure A and Table reveals that the activation energy of the ADN–PMHS microspheres increases progressively with the increasing PMHS content. This trend indicates that higher PMHS loadings reduce the reactivity of ADN, as a greater energy input is required to initiate its decomposition. Consequently, the sensitivity of the ADN–PMHS system decreases, enhancing its thermal safety. These findings highlight the importance of selecting an optimal coating thickness (PMHS content) to achieve a balance between improved safety and maintaining sufficient reactivity for the target application.

Notably, the activation energies of ADN–0.8%PMHS, ADN–1.0%PMHS, and ADN–1.2%PMHS exhibit only minor variations, suggesting that beyond this concentration range, additional PMHS does not significantly affect the energetic barrier to decomposition. This negligible change can be attributed to the saturation of hydrogen-bonding interactions between the PMHS coating and the ADN surface. Once these interactions reach their maximum capacity, further increases in the PMHS content provide little to no additional stabilization effect.

Similarly, both the critical thermal explosion temperature and the extrapolated zero-heating-rate decomposition temperature were observed to increase with the PMHS content (Figure B). However, when the PMHS content exceeded 0.5 wt %, these increases became negligible, indicating that the system approaches a stabilization plateau. This behavior is consistent with the trend in activation energy, reinforcing the interpretation that an optimal PMHS concentration exists, beyond which additional coating no longer contributes meaningfully to thermal stabilization.

Furthermore, the vacuum thermal stability of both ADN-S and ADN–PMHS samples remained largely unchanged, indicating that the PMHS coating does not significantly affect the intrinsic thermal stability of ADN under vacuum conditions.

3.4. Mechanical Sensitivity

To evaluate the safety performance of the samples, their impact sensitivities were measured, and the results for the ADN-R, ADN-S, and ADN–PMHS microspheres are shown in Figure . The ADN–PMHS microspheres exhibited significantly lower impact sensitivities than raw ADN. Increasing the PMHS content progressively reduced sensitivity, with values decreasing from 64% for ADN-R to 40% for ADN–0.5%PMHS and 32% for ADN–1.2%PMHS. These improvements can be attributed to the spherical morphology and uniform PMHS coating, which together mitigate localized stress concentrations and reduce interparticle mechanical interactions.

8.

Impact susceptibility of raw ADN, spherical ADN, and ADN–PHMS microspheres.

Collectively, these findings underscore the substantial desensitization effects achieved through spheroidization and surface coating, demonstrating that the PMHS-coated ADN microspheres exhibit enhanced safety characteristics while maintaining functional energetic performance.

Hot spot formation, a primary trigger for explosive initiation, is effectively mitigated in the coated samples due to the uniform dispersion of ADN particles within the PMHS coating layer. This well-organized structure facilitates the dissipation of mechanical energy as a portion of the applied impact energy is absorbed by the flexible PMHS coating, thereby reducing localized stress concentrations and lowering the probability of hot spot generation.

Interestingly, further increases in the PMHS content beyond a certain level did not result in a significant reduction in impact sensitivity. This phenomenon can be explained by the equilibrium point reached in the coating’s ability to absorb and dissipate impact energy. Once this threshold is achieved, the intrinsic sensitivity of the ADN core becomes the dominant factor controlling hot spot formation. As a result, additional coating thickness beyond the optimal level provides diminishing returns in improving the mechanical desensitization of the system.

3.5. Hygroscopicity Test

The moisture absorption behavior of raw ADN and ADN–PMHS microspheres is shown in Figure A. Raw ADN exhibits pronounced moisture uptake, which can be attributed to its strong tendency to rapidly attract water molecules through electrostatic interactions and hydrogen bonding upon initial exposure to a humid environment. , This initial interaction leads to the formation of a thin saturated solution layer on the particle surface. During the hygroscopic process, water molecules diffuse through this surface layer and penetrate the bulk of the ADN particles, resulting in continuous moisture absorption. Over time, this process culminates in the formation of a fully saturated aqueous ADN solution, as illustrated schematically in Figure A.

10.

Water absorption curves of raw ADN and ADN–PMHS microspheres at 25 °C and 60% relative humidity (A) and water contact angles (B–H).

As shown in Figure A, the ADN–PMHS microspheres exhibit a markedly lower moisture uptake compared to ADN-R. This reduction can be attributed to the formation of hydrogen bonds between the nitrogen atoms of the –N­(NO₂)₂ groups in ADN and the hydrogen atoms of the −Si–H groups in PMHS, resulting in the uniform coating of PMHS around the ADN particles (Figure B). Owing to the inherent hydrophobicity of the siloxane backbone and the presence of water-repelling methyl groups, ,,,, this coating acts as a protective barrier with low surface energy, thereby significantly suppressing moisture adsorption.

At a PMHS content of 0.2%, the decrease in moisture uptake is limited, which can be attributed to the insufficient thickness and uneven distribution of PMHS on the particle surface. In contrast, increasing the PMHS content to 0.5% results in a dramatic reduction of moisture absorption, from 67.1 to 12.3% within 25 h at 25 °C and 60% RH, indicating the formation of a uniform and continuous hydrophobic coating. Increasing the concentration to 0.8% yields no further improvement, suggesting that a complete hydrophobic barrier has already been formed.

At PMHS contents >1.0%, the moisture absorption increases again. This trend is likely associated with (i) enhanced hydrogen-bonding interactions between water molecules and the more mobile hydrogen atoms of excess −Si–H groups, ,, and (ii) increased free volume within the thicker polymeric coating, creating nanovoids that facilitate moisture diffusion, similar to the behavior of silica gel (Figure C). These observations indicate that an optimal PMHS content of approximately 0.5–0.8% provides maximum hygroscopic suppression.

The hydrophobic enhancement imparted by PMHS was further evaluated by water contact angle measurements (Figure B). Raw ADN displayed a contact angle of approximately 0° with the rapid spreading of the water droplet, indicating strong hydrophilicity. In contrast, ADN–PMHS microspheres exhibited significantly increased contact angles. At 0.2% PMHS, the contact angle reached 35.7°, increasing to 68.4° at 0.5% and 74.1° at 0.8%. However, the contact angle decreased slightly to 62° at 1.0% PMHS and remained unchanged at 1.2%, reflecting the saturation effect at higher coating concentrations. These results are consistent with the moisture absorption trends, confirming that 0.5% PMHS provides the optimal balance between the hydrophobicity and moisture-resistance performance.

To place the performance of the PMHS coating in context, data from previously reported ADN coating systems were compiled and are summarized in Table . PMHS demonstrates highly effective moisture suppression using a simple solvent evaporation method. At only 0.5 wt % coating, ADN–PMHS exhibits 12.3% water absorption after 25 h at 60% RH, substantially outperforming conventional organic coatings such as HTPB, poly­(vinyl butyral) (PVB), and poly­(ethylene glycol) (PEG), which exhibit 34–39% moisture uptake at 75% RH despite similar or higher coating contents. Several polymer-coated ADN materials with higher coating amounts (e.g., G-ADN, CA-coated ADN, polystyrene (PS)-coated ADN at 5 wt %) still show considerably higher moisture absorption (18–40%), confirming the efficiency of PMHS at minimal loading.

3. Antihygroscopic Results of Various Coatings Applied to ADN Using Different Coating Methods .

samples	coating amount (%)	time (h)	relative moisture (%)	temperature (oC)	water absorption (%)	coating method
ADN/HTPB	0.2	144	75	30	34.15	solvent volatilization
ADN/PVB	0.2	144	75	30	35.29	solvent volatilization
ADN/PEG	0.2	144	75	30	39.21	solvent volatilization
ADN/SA	0.2	96	62	25	9.52	solvent volatilization
ADN/TNT	0.2	96	62	25	18.10	solvent volatilization
ADN/PMMA	0.5	1	62	25	4.50	solvent volatilization
ADN/EC	0.5	1	62	25	5.80	solvent volatilization
ADN–PMHS	0.5	25	60	25	12.30	solvent volatilization
ADN/NC	3	6	50	20	3.75	solvent volatilization
ADN/PS	5	4	65		30.00	solvent volatilization
ADN/HTPB	5	4	65		18.00	solvent volatilization
G-ADN	5	24	60	25	40.00	solvent volatilization
F-ADN	5	24	60	25	9.43	solvent volatilization
H-ADN	5	24	60	25	12.12	solvent volatilization
ADN@POSS-NH2	5	168	75	20		solvent volatilization
ADN@Al		48	70	25	40.75	atomic layer deposition technology
ADN@AmGO	1	0.08	50	25	0.42	Pickering emulsions (solvent–antisolvent)
ADN/CA	5	3	75	20	18.75	water-in-oil emulsification–diffusion method
ADN@GO@NC	5.3	24	60	25	5.64	Pickering emulsion

^aHTPB: hydroxyl-terminated polybutadiene; PVB: poly­(vinyl butyral); PEG: poly­(ethylene glycol); SA: stearic acid; TNT: trinitrotoluene; PMMA: poly­(methyl methacrylate); EC: ethyl cellulose; NC: nitrocellulose; PS: polystyrene; G: glycidyl azide polymer (GAP); F: fluorine rubber (F₂₆₀₂); H: hydroxyl-terminated block copolyether (HTPE); POSS-NH₂: monoaminopropyl heptaphenylsilsesquioxane; AmGO: dodecylamine-functionalized graphene oxide; CA: copper alginate; GO: graphene oxide.

Compared with other organic-based coatings, PMHS also demonstrated improved long-term moisture suppression. Although poly­(methyl methacrylate) (PMMA) and ethyl cellulose (EC) show lower initial water absorption (4.5 and 5.8%), these measurements were taken after only 1 h and therefore do not represent long-term stability. In contrast, PMHS maintains durable hydrophobic performance over longer exposure periods, likely due to the low polarity of its siloxane backbone and its methyl-rich surface, which collectively hinder water diffusion.

Relative to advanced coating technologies, PMHS achieves competitive moisture suppression with significantly simpler processing. Pickering-emulsion-derived coatings such as ADN@AmGO and ADN@GO@NC achieve very low water absorption (<1–6%), but they require multistep fabrication and higher coating masses (1–5.3 wt %). In contrast, inorganic coatings such as ADN@Al produced via atomic layer deposition exhibit a poor moisture resistance (40.75%). Overall, PMHS stands out as one of the most practical and effective polymeric coatings for improving the hygroscopic stability of ADN, offering a combination of low material usage, simple preparation, and robust moisture-resistance performance.

4. Conclusions

In this study, a straightforward and effective approach was developed for the fabrication of the ADN–PMHS microspheres. Spherical ADN crystals were first produced via solvent–antisolvent crystallization and subsequently encapsulated with PMHS via solvent evaporation. The influence of the PMHS content on the morphology, structure, and properties of the composites was systematically investigated. SEM images revealed that the needle-like crystals of raw ADN were transformed into uniform spherical particles with an adherent PMHS coating. PXRD confirmed that the crystalline structure of the ADN was preserved during fabrication. The presence of the PMHS layer was further verified by FTIR, ¹H NMR, and energy-dispersive X-ray (EDX) mapping analyses. Thermal analysis showed that the decomposition temperature of the microspheres was nearly identical to that of raw ADN, indicating stable thermal behavior. Hygroscopicity tests demonstrated a 54.8% reduction in moisture uptake compared with that of raw ADN, while contact angle measurements confirmed enhanced hydrophobicity due to PMHS coverage. Sensitivity tests revealed that the PMHS coating significantly decreased both the impact sensitivity of ADN-R.

Overall, the encapsulation of ADN with PMHS via solvent evaporation affords a simple yet highly efficient strategy for producing ADN-based microspheres with improved morphology, reduced hygroscopicity, enhanced surface hydrophobicity, and diminished mechanical sensitivity. These enhanced properties underscore the strong potential of ADN–PMHS microspheres as promising oxidizer materials for next-generation solid-propellant formulations.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c10688.

• The SI is divided into three main sections: 1. Chemical structure of microspheres: this section provides spectroscopic and crystallographic evidence confirming the successful coating of ammonium dinitramide (ADN) with polymethylhydrosiloxane (PMHS) and the preservation of ADN’s intrinsic crystal structure; FTIR spectra: PMHS (Figure S1), raw ADN (ADN-R) (Figure S2), and the optimal composite sample, ADN–0.5%PMHS (Figure S3); ¹H NMR spectra for ADN-R (Figure S4) and ADN–0.5%PMHS (Figure S5); XRD patterns for raw ADN (Figure S6), spherical ADN (ADN-S) (Figure S7), and the coated composite ADN–0.5%PMHS (Figure S8). 2. Crystal Morphology of Microspheres: this section utilizes scanning electron microscopy (SEM) images to document the physical transition of the ADN particles and the quality of the coating; SEM images of raw ADN (ADN-R), typically needle-like (Figure S9), spherical ADN (Figure S10), coated ADN–0.2%PMHS (Figure S11), ADN–0.5%PMHS (Figure S12), ADN–0.8%PMHS (Figure S13), ADN–1%PMHS (Figure S14), and ADN–1.2%PMHS (Figure S15). 3. Hygroscopicity Test: this dedicated section provides visual confirmation of the material’s enhanced moisture resistance through surface wettability analysis; water contact angle: contains images illustrating the water contact angle measurements for raw ADN (ADN-R) (Figure S16) and coated samples at varying concentrations: ADN–0.2%PMHS (Figure S17), ADN–0.5%PMHS (Figure S18), ADN–0.8%PMHS (Figure S19), ADN–1%PMHS (Figure S20), and ADN–1.2%PMHS (Figure S21); measurements are supported with relevant literature references (PDF)

The authors declare no competing financial interest.