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Study on the interfacial interaction between ammonium perchlorate and hydroxyl-terminated polybutadiene in solid propellants by molecular dynamics simulation

  • Ge Dong , Hengzhi Liu , Lei Deng , Haiyang Yu , Xing Zhou EMAIL logo , Xianqiong Tang EMAIL logo and Wei Li EMAIL logo
Published/Copyright: March 15, 2022
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e-Polymers
From the journal e-Polymers Volume 22 Issue 1

Abstract

The interfacial interaction between the main oxidant filler ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB) matrix in AP/HTPB propellants were studied via an all-atom molecular dynamics simulation. The results of the simulation showed the effects of the microscopic cross-linked structure of the matrix, stretching rate during uniaxial stretching, and contact area between the filler and matrix on the mechanical properties, such as the stress and strain of the composite solid propellant. Among the aforementioned factors, the stretching rate considerably affects the mechanical properties of the solid propellant, and the maximum stress of the solid propellant proportionally increases with the stretching rate. When defects were introduced on the surface of the AP filler, the contact area between the filler and matrix affected the strain type of the matrix molecules. Owing to the interaction between the molecules and atoms, the strain behaviour of the matrix molecule changed with the change in its microscopic cross-linked structure during uniaxial stretching. Molecular dynamics simulations were used to explore the characteristics at the AP–HTPB interface in AP/HTPB propellants. The aforementioned simulation results further revealed the interfacial interaction mechanism of the AP–HTPB matrix and provided a theoretical basis for the design of high-performance propellants.

Keywords: AP/HTPB propellant; interface interaction; all-atom molecular dynamics

1 Introduction

Solid propellants are widely used as both the energy source and motor load-bearing components of solid rockets. Ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) composite solid propellants comprise a solid particle-filled polymer energetic composite material that is widely studied and used (1,2,3,4,5). The AP/HTPB composite solid propellant mainly comprises an AP solid filler and a macromolecule binder HTPB, of which the solid filler typically accounts for more than 80% of the total mass of the propellant. The interfacial interactions between the mixture matrix and solid filler are the key factors affecting the mechanical properties of the AP/HTPB propellants (6,7,8,9). Therefore, studying the mechanical behaviour between the filler and binder matrix is essential to understand the mechanism affecting the development and mechanical properties of the solid propellants.

Studies have shown that the main factors affecting the interfacial performance are the micro-molecular structure and viscoelastic behaviour of the matrix, crystal face type of the filler, contact area between the filler and the matrix, and temperature of the system (10,11,12,13,14,15,16). Because the matrix is a cross-linked polymer, the viscoelastic behaviour of the matrix at the matrix–filler interface significantly affects the mechanical properties of the composite solid propellant. Moreover, a high strain rate will produce a greater stress and strain. The network-like microstructures of the polymer molecules with a high degree of cross-linking can enhance the viscoelasticity and mechanical properties. Yang et al. (10) used molecular dynamics to study the tensile strength and strain behaviour of the interface between a highly cross-linked polymer and a Cu base. The results showed that the size of the simulated unit and strain rate can considerably affect the stress and strain behaviours of the polymer on the Cu base surface. The adsorption of the matrix on the filler interface in the solid propellant can be mainly attributed to the van der Waals (vdW) effect and hydrogen bonding. The intermolecular interaction and the interfacial adhesion energy between the filler and matrix increase with the increase in the contact area between them. Sun and Nan (12) studied the interfacial characteristics of different particle size fillers and HTPB films, and the results showed that the adhesion work between the filler and HTPB decreased with the increase in the filler particle size. Therefore, under the same load, there is a smaller contact area between the matrix and fillers with large particle sizes compared to those with small particle sizes, which promotes the desorption phenomenon. The adhesion behaviour of the matrix is considerably different on different crystal faces of the filler because of the hydrogen bonding and vdW interactions between the molecules and atoms. In AP crystal fillers, different group atoms are exposed in different crystal faces. Therefore, the strength of the molecule–atom interactions is different on different crystal faces, leading to different adhesion behaviour along the filler–matrix interface. Fu et al. (13) used molecular dynamics to simulate the mechanical properties of the HTPB with different aluminium crystal planes. The results showed that the binding energies between the different crystal planes as well as HTPB are different. Furthermore, the mechanical properties of the system showed a positive correlation with the binding energy. Walid and Liang (16) conducted uniaxial tensile experiments on HTPB composite solid propellant at different temperatures and strain rates to investigate the mechanical behaviour of the solid propellant at different tensile rates. The results showed that the mechanical properties of the solid propellants at different tensile rates were quite different, and the maximum stress as well as strain of the matrix increased proportionally with the increase in the tensile rate. Moreover, because the solid propellant is a non-linear viscoelastic material, decreasing the temperature has the same effect as increasing the strain rate.

The aforementioned experiments and simulations can be attributed to the in-depth study on the filler–matrix interface effects on the composite solid propellant, revealing the effect of several factors, such as the crystal face of the filler, stretching rate, microstructure of the matrix–filler contact area, on the mechanical properties of the composite solid propellant. However, filler–matrix interface interactions in solid propellants and the mechanism of these interactions have not been adequately reported. There is little in-depth research on the interfacial interactions between the AP oxidiser and HTPB binder matrix, which have a great effect on the mechanical properties of the solid propellants in practical applications. The progress of quantum mechanics and molecular mechanics enabled the exploration of mechanical properties of solid propellants at a microscopic level based on numerical simulations. Compared to the experimental methods, molecular dynamics simulation technology is economical and can investigate the filler–matrix interface behaviour in solid propellants at a microscopic level. In this study, a molecular dynamics method was used to simulate the uniaxial stretching process of the propellant and explore the effect of different factors on the mechanical properties of the propellant. The results showed that the microstructure of the matrix and stretching rate during the uniaxial stretching process can significantly affect the stress and strain in the matrix. This provides a theoretical basis for an in-depth understanding of the interactions at the interface between the AP oxidant and HTPB binder matrix in AP/HTPB propellants and the improvements of the mechanical properties of AP/HTPB propellants.

2 Materials and methods

AP crystals have tightly arranged structures and different ionic groups exposed on different crystal faces. The results of X-ray diffraction experiments and computational simulation were obtained by investigating the interaction between the (0 1 1) crystal plane of AP and the matrix molecule. First, the Materials Studio (MS) software16 was used to cut along the (0 1 1) crystal plane of the AP crystal and expand the unit cell to a suitable size (55 Å × 56 Å × 19 Å). In addition, defects were added to the surface structural model of the AP crystal to compare the characteristics of different filler–matrix contact areas. The matrix was obtained via a curing cross-linking reaction (the reaction mechanisms are shown in Figure 1) between HTPB, isophorone diisocyanate (IPDI), and tri(2-methyl-1-aziridine)-phosphorus oxide (MAPO). Figure 2 shows the molecular structure of the HTPB/IPDI/MAPO matrix molecule.

Figure 1 The reaction mechanisms of HTPB–IPDI and HTPB–MAPO.
Figure 1

The reaction mechanisms of HTPB–IPDI and HTPB–MAPO.

Figure 2 The molecular structure of the three components in the HTPB/IPDI/MAPO matrix molecule: (a) IPDI, (b) MAPO, and (c) HTPB. (d) The initial structure of the AP–HTPB interface model. The dots coloured grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H.
Figure 2

The molecular structure of the three components in the HTPB/IPDI/MAPO matrix molecule: (a) IPDI, (b) MAPO, and (c) HTPB. (d) The initial structure of the AP–HTPB interface model. The dots coloured grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H.

The hydroxyl groups at both ends of the HTPB reacted with the isocyanate group of the IPDI molecule and aaziridine ring group of the MAPO molecule, respectively. The solidified and cross-linked matrix molecule (molecular weight = 194,877) was constructed using the MS software, and the molecular ratio of HTPB:IPDI:MAPO was 10:10:1. Subsequently, the canonical ensemble, constant-number of particles (N), constant-volume (V), constant-temperature (T) ensemble [NVT] balance was performed on the matrix molecules at 293 K. The molecules spontaneously curled up into a cluster structure (Figure 2d) from the linear structure. All molecule–atom interactions were studied using the condensed phase optimised molecular potential of atomic simulation studies (COMPASS II) (17).

Using MS, the optimised matrix molecule was placed 2 Å above the AP crystal (Figure 2d). To ensure stable adsorption of the matrix molecules on the surface of the AP crystals, a 1 ns NVT ensemble simulation was performed using a time step of 1 fs. The Nose–Hoover temperature coupling was used to control the temperature at 333.15 K. The short-range vdW interactions were described using a 12-6 Lennard-Jones potential, and the cut-off radius was set to 12 Å. The particle–particle–particle–mesh method (18) was used to calculate the long-range electrostatic interactions. After the matrix molecules reached equilibrium, the effect of different factors on the mechanical properties of the AP/HTPB propellant was investigated by conducting a 1 ns NVT ensemble simulation at 298.15 K. The uniaxial stretching process was achieved by fixing the bottom most AP crystal along the XYZ directions and applying a constant strain rate of 125–145 Å along the Z axis on the matrix molecule. The aforementioned molecular dynamics simulations were completed using LAMMPS-07.08.2019 software package (19).

3 Results and discussion

3.1 Adsorption behaviour of the matrix molecules on the AP crystal surface

The temperature was increased to 333.15 K, and a molecular dynamics simulation of the NVT ensemble was performed on the AP–HTPB interface model. The results of the simulation showed that the matrix molecules can be stably adsorbed on the AP surface (Figure 3). Figure 3 shows that the molecular chains of the matrix at the AP–HTPB matrix interface were arranged in layers, and a structural stratification was prominently close to the interface. This can be attributed to the following: (1) the vdW interaction between the AP and matrix molecules and (2) the hydrogen bond between C and H in the ammonium ion, which was the main group exposed in the AP (0 1 1) crystal plane, as C and H are the most abundant elements in the HTPB matrix. These hydrogen bonds and vdW forces cause C and H in the matrix molecules to diffuse to the AP–HTPB matrix interface, yielding the distribution of the matrix–polymer molecular chains along the AP–HTPB matrix interface. However, the hydrogen bond and vdW interaction weaken, causing the orderliness of the molecular chain of the matrix to decrease because the matrix molecules move away from the AP–HTPB matrix interface. Therefore, the molecular chains of the matrix are arranged in a more orderly manner and layered closer to the AP–HTPB matrix interface. Figure 4 shows the radial distribution function (RDF) of C in the matrix molecule and H in AP (the cyan thin line) as well as RDF of H in the matrix molecule and N in AP (the red thin line). The results of the RDF indicated that there is a strong hydrogen bonding interaction between the matrix molecules and AP, which in turn promotes the orderly distribution of the matrix molecular chains at the AP–HTPB matrix interface.

Figure 3 The adsorption of the matrix molecules on AP, illustrating the configuration distribution at the AP–HTPB matrix interface (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).
Figure 3

The adsorption of the matrix molecules on AP, illustrating the configuration distribution at the AP–HTPB matrix interface (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).

Figure 4 RDF of C in the matrix molecule and H in AP and the RDF of H in the matrix molecule and N in AP.
Figure 4

RDF of C in the matrix molecule and H in AP and the RDF of H in the matrix molecule and N in AP.

Subsequently, the adsorption energy, ΔE Mat–AP, of the HTPB matrix on the AP (0 1 1) crystal plane was calculated using Eq. 1 to be −839.52 kcal·mol−1:

(1) Δ E Mat − AP = E Mat + AP − E Mat − E AP

where E Mat+AP is the energy of the AP–HTPB matrix interface system, E Mat is the energy of the HTPB matrix, and E AP is the energy of AP. The absolute value of the adsorption energy was relatively large. This confirms that the matrix molecules can be stably adsorbed on the AP (0 1 1) crystal plane.

3.2 Mechanical behaviour of the AP–HTPB matrix interface

Mechanical properties of the solid propellants are one of the criteria determining the quality of the propellant materials. Uniaxial tensile experiments performed at a constant strain rate are effective in evaluating the mechanical properties of the solid propellants. In this study, all-atom molecular dynamics (AAMD) of the AP–HTPB propellants were used to simulate a structural model of the AP filler–HTPB matrix interface undergoing uniaxial stretching process to explore the different factors affecting the stress and strain characteristics at the AP–HTPB matrix interface.

3.2.1 Effect of physical cross-linking structure of the HTPB matrix on the mechanical behaviour of the AP–HTPB matrix interface

In the interface model of the AP–HTPB matrix, the HTPB matrix is adsorbed on the surface of the AP crystal. During the uniaxial stretching process, a constant strain rate was applied only to a part of the atoms of the HTPB matrix. Therefore, the microstructure of the HTPB matrix molecule near the interface affects the stress and strain behaviour of the interface between the AP filler and HTPB matrix.

Figure 5 shows that the HTPB matrix can be divided into two parts and reveals the importance of forming a network-like physical cross-link between them. The uniaxial stretching process was simulated with and without a network-like physical cross-linking structure between the two parts of the HTPB matrix, and the stretching rate was 0.105 Å·ps−1. Figure 6 shows the simulation snapshots of the uniaxial stretching process of the HTPB matrix molecules with a network-like physical cross-linking structure. As the uniaxial stretching process progresses, the cylindrical HTPB matrix molecule gradually deforms, and different parts exhibit different strain behaviours. The pulling region of the HTPB matrix molecules moved along the Z axis under the effect of a constant stretching rate, and their structures did not show any considerable strain. Owing to the interaction between the HTPB matrix molecule and AP, the HTPB matrix molecule still maintained an interfacial adsorption state at the interface of the AP–HTPB matrix. Because the HTPB matrix is a type of polymer material with high viscoelasticity, a filamentous structure appears at the physical cross-linking of HTPB matrix molecules with the progress of the uniaxial tension. Therefore, under the combined action of the pulling force and attraction of AP on the HTPB matrix molecules at the interface, the matrix molecules were gradually elongated, but no fracture phenomenon was observed in the molecular structure of the HTPB matrix. Figure 7 shows a simulated snapshot of the uniaxial stretching process of the system without a network-like physical cross-linked structure within the HTPB matrix. The HTPB matrix molecules exhibited the same strain behaviour as the HTPB matrix molecules in the physical cross-linked structure case. But the HTPB matrix molecules broke after a time of approximately 335 ps, and the fracture site was located at the physical cross-linking of the HTPB matrix. In this case, the matrix is held together with only the vdW interactions because the network-like physical cross-linking structure between the two parts of the HTPB matrix is absent. Stress gradually increased as the pulling region moved along the Z axis. Therefore, when the stress exceeded the critical value of the vdW interaction, the HTPB matrix molecular fracture occurred. The aforementioned results of the simulation showed that the microstructure of the HTPB matrix molecule near the interface can considerably affect the strain behaviour of the AP–HTPB matrix interface during the uniaxial stretching.

Figure 5 Schematic of the uniaxial stretching process of the model of the AP filler and HTPB matrix interface.
Figure 5

Schematic of the uniaxial stretching process of the model of the AP filler and HTPB matrix interface.

Figure 6 Simulation snapshots of the AP–HTPB matrix interface model with a network-like physical cross-linked structure (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).
Figure 6

Simulation snapshots of the AP–HTPB matrix interface model with a network-like physical cross-linked structure (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).

Figure 7 Simulation snapshots of the AP–HTPB matrix interface model without a network-like physical cross-linked structure (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).
Figure 7

Simulation snapshots of the AP–HTPB matrix interface model without a network-like physical cross-linked structure (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).

To further compare the effects of the network-like physical cross-linking structures of the matrix molecules on the mechanical behaviour of the AP–HTPB matrix interface, the maximum stresses of the AP–HTPB interface models with different physical cross-linking structures were calculated (Figure 8). In case of the HTPB matrix molecule with the physical cross-linked structure, the stress at the AP–HTPB matrix interface reached its maximum value (110 MPa), then began to decrease and finally stabilised (green curve). However, the maximum stress at the non-physical cross-linked matrix molecular interface model was 120 MPa and it decreased until it reached 0 MPa at 35 Å (red curve), which is caused by the fracture of the structure of the HTPB matrix. The AP–HTPB matrix interface models with two microstructure matrix molecules exhibited the same trend in the stress change with similar maximum stress values. The maximum stress occurred in the elastic part of the HTPB matrix because it is mainly dominated by the vdW interactions between molecules and atoms. Therefore, the stress curves of the two models overlap within a displacement of 0–14 Å.

Figure 8 Stress–displacement curves of different interface structural models during uniaxial tension.
Figure 8

Stress–displacement curves of different interface structural models during uniaxial tension.

The aforementioned results of the simulation showed that the microstructure of the HTPB matrix at the interface has a considerable effect on the strain behaviour of the AP–HTPB matrix interface and a lesser effect on the maximum stress of the system. In addition, the extended part of the HTPB matrix molecule was always located at the inner half of the matrix. The interactions between the internal atoms of the HTPB matrix are both bonding and non-bonding (dominated by vdW forces) interactions. Thus, the attraction forces between the HTPB matrix molecules and AP weakens when they are away from the AP–HTPB matrix interface. Thus, there is almost no interaction between the HTPB matrix molecules at the one-half position and AP. Moreover, under the combined action of the opposing pulling and attractive forces, the HTPB matrix molecules stretched from the one-half position to both ends.

3.2.2 Effect of stretching rate on the mechanical behaviour of AP–HTPB matrix interface during uniaxial stretching

The previous simulation results showed that the maximum stress of the AP–HTPB matrix interface model is less affected by the microstructure of the HTPB matrix. Based on the AP–HTPB matrix interface model with a network-like physical cross-linked HTPB matrix internal structure, the effect of different stretching rates on its maximum stress and strain behaviours was further explored. To study the effect of the stretching rate on the stress of the AP–HTPB matrix interface model, different stretching rates, including 0.105 and 0.0105 Å·ps−1, were applied to the pulling region of the HTPB matrix (the snapshots of simulation results are shown in Figure 9).

Figure 9 Simulated snapshot of the AP–HTPB matrix interface models under different stretching rates (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).
Figure 9

Simulated snapshot of the AP–HTPB matrix interface models under different stretching rates (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).

At different stretching rates, the matrix molecules near the AP–HTPB matrix interface exhibited similar strain behaviour; the matrix molecules gradually extend from the one-half position to both ends under the combined action of traction and attraction forces. However, because of the different stretching rates, there were differences in the speed and mode of the strain of the matrix molecules. At a tensile rate of 0.105 Å·ps−1, the strain of the matrix occurred after 0.3 ns. With the progress of the uniaxial tension, the strain of the matrix became more pronounced. Because the matrix molecules exhibited a Poisson effect, they change from a columnar to a dumbbell-shaped structure after 1.0 ns. That is, when the matrix molecules were subjected to a one-dimensional stress, it simultaneously generated strains along the longitudinal and transverse directions. Therefore, when the HTPB matrix molecules were stretched along the longitudinal direction, their lateral contraction occurs. Thus, reducing the stretching rate to 0.0105 Å·ps−1 decreased the strain rate of the HTPB matrix and considerably changed its strain style. Furthermore, for a stretching time below 4.5 ns, the HTPB matrix molecules began to strain from the one-half position. However, the HTPB matrix molecules exhibited a larger axial strain and a smaller lateral contraction. As the uniaxial stretching process continued, the structure of the HTPB matrix molecules did not change to the dumbbell-shaped structure and the fracture phenomenon occurred from inside the matrix. In addition, the structure of the HTPB matrix molecules experienced an outward elliptical fracture with the one-half position as the centre of the circle. The fracture of the HTPB matrix molecule into two parts started from the inside of the molecule, and there was still a filamentous structure connecting the two parts of HTPB matrix molecule. The aforementioned phenomenon suggested that the stretching rate can affect the change in stress of the HTPB matrix molecule near the AP–HTPB matrix interface. At a larger stretching rate, the HTPB matrix molecules expanded along the stretching direction and the structure break started from the outside to the inside of the molecule. Conversely, at a small stretching rate, the internal structure of the HTPB matrix molecules experienced an elliptical rupture from the inside to the outside. The rupture of the HTPB matrix molecules was the result of several interactions including traction, attraction, and vdW interactions. However, the aforementioned two opposite mechanisms of structural ruptures (from the outside to the inside and vice versa) were closely dependant on the stretching rate. Thus, to deeply explore the effect of the stretching rate on the stress of the AP–HTPB matrix interface, this study further analysed the changes in stress during uniaxial stretching (Figure 10).

Figure 10 Stress–displacement curves at different tension rates during uniaxial tension.
Figure 10

Stress–displacement curves at different tension rates during uniaxial tension.

At different tensile rates, the AP–HTPB matrix interface exhibited the same stress change; the stress of the system gradually increased to its maximum value and decreased until it stabilised. In addition, under different stretching rates, the curves of the stress rising stage of the AP–HTPB matrix interface are completely overlapping because the vdW interactions between the atoms in the AP–HTPB matrix interface was the dominant force before the stress reached its maximum value. The strain behaviour of the HTPB matrix near the AP–HTPB matrix interface was not clear, and the HTPB matrix molecules still maintained a columnar configuration. However, with the longitudinal expansion and transverse shrinkage of the HTPB matrix molecules, the microstructure of the HTPB matrix changed from columnar to dumbbell-shaped, and the stress at the AP–HTPB matrix interface began to decrease. Moreover, the slope of the drop phase of the stress curve was the same under different stretching rates. The maximum stress of the AP–HTPB matrix interface at a tensile rate of 0.105 Å·ps−1 was 112 MPa, which is greater than the maximum stress value (96 MPa) at a tensile rate of 0.0105 Å·ps−1. This is because strain occurred faster in the AP–HTPB matrix interface model at higher tensile rates, affording a larger maximum stress of the system.

The aforementioned results showed that the stretching rate of the uniaxial stretching process can considerably affect the maximum stress value of the AP–HTPB matrix interface, but it does not affect the changing trend of the stress. Moreover, the stretching rate cannot considerably affect the overall strain behaviour of the HTPB matrix, but it caused different strain rates and styles of the HTPB matrix.

3.2.3 Effect of interface contact area on the mechanical behaviour of the AP–HTPB matrix interface

The above results showed that the stretching rate has a relatively low effect on the strain behaviour of the HTPB matrix molecule near the AP–HTPB matrix interface, but the maximum stress increases with the increase in the stretching rate. To explore the impact of the contact area between the AP and HTPB matrix on the stress and strain of the AP–HTPB matrix interface, defects occupying one-fourth and one-half of the AP volume were introduced into the AP–HTPB matrix interface model. Figure 11 shows snapshots of the AP–HTPB matrix interface model at different moments during the uniaxial stretching process.

Figure 11 Simulated snapshot of the AP–HTPB matrix interface model with different surface defects (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).
Figure 11

Simulated snapshot of the AP–HTPB matrix interface model with different surface defects (grey: C, red: O, blue: N, green: Cl, pink: P, and hidden areas: H).

In the three interface models, the HTPB matrix molecules exhibited completely different strain behaviours during the uniaxial stretching. The system maintained its integrity because of AP, whereas the HTPB matrix molecules stretched longitudinally from the cross-linked area and simultaneously shrunk laterally. However, the two parts of HTPB matrix molecules were still connected via a filamentous structure. Although the HTPB matrix molecules generated large strain under the combined action of tension and attraction forces along the opposite directions, no structural fracture of the HTPB matrix molecules occurred owing to the bonding and non-bonding interactions between the atoms. When the contact area between AP and HTPB matrix decreased to three-fourth of the original area, the strain behaviour of the HTPB matrix molecules changed. As the pulling region moved along the Z axis, the HTPB matrix molecules began to separate from the cross-linked area. Similarly, because of the interaction between the molecules and atoms, the two parts of the HTPB matrix molecules were not completely separated. However, during the uniaxial stretching process, a fracture phenomenon occurred in the structure of the matrix molecules from the inside to the outside, which is different from the longitudinal tension and transverse shrinkage previously mentioned. When the contact area between the AP and HTPB matrix was three-fourth of the original, the style of the strain of the HTPB matrix molecules changed to be from the inside to the outside. This is because reducing the interface contact area between the AP and HTPB matrix weakens the interaction between AP and the matrix and consequently reduces the external force acting on the HTPB matrix molecules. Therefore, the HTPB matrix molecules near the AP–HTPB matrix interface exhibited completely different strain behaviours. To further confirm the aforementioned phenomenon, the contact area between AP and the HTPB matrix were reduced to half the original area. The simulation snapshot showed that the strain style of the HTPB matrix molecules near the AP–HTPB matrix interface changed again. The structure of the HTPB matrix molecules did not change from that at the cross-linking position, but desorption of the HTPB matrix molecules occurred at the AP–HTPB matrix interface. At the part of the interface that was not in contact with the AP surface, HTPB matrix molecules were directly separated under the effect of the pulling force. The HTPB matrix molecules in contact with the AP interface gradually extended and finally peeled off completely from the interface. Moreover, the HTPB matrix molecules always maintained a columnar structure throughout the desorption process. A further decrease in the interface contact area weakened the interaction between AP and the HTPB matrix molecules compared to the action force between the matrix molecules. Therefore, as the HTPB matrix molecules moved upward, they entirely detached from the AP surface. The results of the aforementioned uniaxial stretching process indicated that changing the contact area of the AP–HTPB matrix interface can change the strength of the interaction between AP and the HTPB matrix and considerably affect the strain style of the matrix near the AP–HTPB matrix interface.

The stresses of the systems with different interface contact areas showed the same trend with maximum stress values of approximately 115 MPa (Figure 12). Because the vdW interactions inside the HTPB matrix molecules were dominant, the HTPB matrix molecule near the AP–HTPB matrix interface generated less strain, which was generated in the same method. Therefore, the rise phases of the stress curves (0–0.1 ns) of different AP–HTPB matrix interface models completely coincide. As the stress reached its maximum value, the stress curves of the first two interface models still basically overlapped. However, when the time was greater than 0.2 ns, the stress of the interface model, in which the contact area between AP and the HTPB matrix was one-half of the original, gradually decreased to 0 MPa. From these results combined with those derived from the previous simulation snapshot, it can be concluded that the HTPB matrix molecules of the first two types of interface models generate similar strains along the opposite directions. However, the HTPB matrix molecules in the interface model with a contact area of one-half of the original area were completely desorbed from the AP surface during the uniaxial stretching process (time exceeding 0.3 ns). In addition, the HTPB matrix molecules always maintained a columnar structure, i.e. the HTPB matrix molecules detached from the AP surface did not generate any strain. Therefore, the stress of the interface model finally decreased to 0 MPa.

Figure 12 Stress–displacement curves of the AP–HTPB matrix interface model with different surface defects during uniaxial stretching (V = 0.105 Å·ps−1).
Figure 12

Stress–displacement curves of the AP–HTPB matrix interface model with different surface defects during uniaxial stretching (V = 0.105 Å·ps−1).

The aforementioned results of the simulation indicated that changing the contact area between the AP and the HTPB matrix does not affect the maximum stress and the changing trend of the stress at the AP–HTPB matrix interface. However, reducing the contact area between AP and the HTPB matrix can weaken the interactions between AP and the matrix; hence, the HTPB matrix molecules exhibit different strain behaviours. However, the strain of the HTPB matrix will be affected only when the interaction between AP and HTPB matrix is reduced to a certain value.

Among the various interactions between molecules and atoms, the vdW interaction is very important. The change in the vdW interaction energy between the atoms during the uniaxial stretching was further analysed (Figure 13). The trend of the change in the stress of the AP–HTPB matrix interface is opposite to the trend of the change in the vdW interaction energy, the vdW interaction energy decreased with the increase in the stress and vice versa. This is because when the HTPB matrix molecules generated greater strain, its microstructure became loose. At this point, the vdW interactions between atoms became weak and the stress of the system increased with the increase in the strain. Conversely, when the structure of the HTPB matrix molecules was basically unchanged (such as when the interface model with only one-half of AP was in contact with HTPB matrix molecules), the vdW interactions between the atoms increased and the stress of the system rapidly decreased. In addition, the amount of the vdW interaction energy of the three AP–HTPB matrix interface models was consistent with the AP content.

Figure 13 Variation in the vdW interaction energy between the molecules and atoms in the defect-free AP–HTPB matrix interface model (V = 0.105 Å·ps−1).
Figure 13

Variation in the vdW interaction energy between the molecules and atoms in the defect-free AP–HTPB matrix interface model (V = 0.105 Å·ps−1).

4 Conclusion

In this study, AAMD simulations were used to study the interfacial interactions of AP–HTPB matrix in the composite solid propellants. The effect of the microstructure of the matrix molecule, stretching rate, and interfacial contact area on the mechanical properties of the AP–HTPB matrix interface were investigated. The results showed different factors had different effects on the mechanical properties. Among them, the microstructure of the HTPB matrix has a considerable effect on the strain behaviour of the AP–HTPB matrix interface and a less effect on the maximum stress of the system. However, the stretching rate of the uniaxial stretching process can considerably affect the maximum stress value of AP–HTPB matrix interface, but it cannot change the overall strain behaviour of the HTPB matrix molecules. By reducing the surface area of AP, its interaction with the HTPB matrix can be weakened. Therefore, the HTPB matrix molecules exhibit different strain behaviours. Finally, the change in the vdW interaction energy of the AP–HTPB matrix interface was analysed, which showed an opposite trend of the change in the stress of the system. The mechanism of the interactions at the AP–HTPB matrix interface was studied from a microscopic point of view. This provides a theoretical basis for the design of propellant formulations and further improvement of the mechanical properties of the AP/HTPB propellants.

  1. Funding information: This work was funded by Open Research Fund Program of Science and Technology on Aerospace Chemical Power Laboratory (STACPL220181B05), Hunan Provincial Natural Science Foundation of China (2019JJ50622), High-level Talent Gathering Project in Hunan Province (2019RS1059), PhD Research Startup Foundation of Xiangtan University (19QDZ06), and Basic Research Project (XXXX-2019-083).

  2. Author contributions: Ge Dong: writing – original draft, conceptualisation, methodology, and software; Hengzhi Liu: writing – original draft, conceptualisation, methodology, and software; Lei Deng: writing – review and editing and validation; Haiyang Yu: formal analysis and investigation; Xing Zhou: resources and supervision; Xianqiong Tang: data curation, visualisation, and funding acquisition; Wei Li: project administration and funding acquisition. Ge Dong and Hengzhi Liu contributed equally.

  3. Conflict of interest: Authors state no conflict of interest.

  4. Data availability statement: The data have been given in the article.

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Received: 2021-10-12
Revised: 2021-11-09
Accepted: 2021-12-04
Published Online: 2022-03-15

© 2022 Ge Dong et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2022-0016
Keywords for this article
AP/HTPB propellant; interface interaction; all-atom molecular dynamics
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. The effect of isothermal crystallization on mechanical properties of poly(ethylene 2,5-furandicarboxylate)
  3. The effect of different structural designs on impact resistance to carbon fiber foam sandwich structures
  4. Hyper-crosslinked polymers with controlled multiscale porosity for effective removal of benzene from cigarette smoke
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  6. Effect of polyurethane/polyvinyl alcohol coating on mechanical properties of polyester harness cord
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  22. Study on the self-assembly of aromatic antimicrobial peptides based on different PAF26 peptide sequences
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