In Situ Grafted Composite Nanoparticles-Reinforced Polyurethane Elastomer Composites with Excellent Continuous Anti-Impact Performance

Polyurethane elastomer (PUE) has attracted much attention in impact energy absorption due to its impressive toughness and easy processability. However, the lack of continuous impact resistance limits its wider application. Here, an amino-siloxane (APTES) grafted WS2-coated MWCNTs (A-WS2@MWCNTs) filler was synthesized, and A-WS2@MWCNTs/PUE was prepared by using the filler. Mechanical tests and impact damage characterization of pure PUE and composite PUE were carried out systematically. Compared with pure PUE, the static compressive strength and dynamic yield stress of A-WS2@MWCNTs/PUE are increased by 144.2% and 331.7%, respectively. A-WS2@MWCNTs/PUE remains intact after 10 consecutive impacts, while the pure PUE appears serious damage after only a one-time impact. The improvement of mechanical properties of A-WS2@MWCNTs/PUE lies in the interfacial interaction and synergy of composite fillers. Microscopic morphology observation and damage analysis show that the composite nanofiller has suitable interfacial compatibility with the PUE matrix and can inhibit crack growth and expansion. Therefore, this experiment provides an experimental and theoretical basis for the preparation of PUE with excellent impact resistance, which will help PUE to be more widely used in the protection field.


Introduction
Polyurethane elastomer (PUE), as a well-know (AB)n-type block linear polymer, has attracted much attention recently from industry and academia due to its light weight, high strength, and suitable recovery performance applications [1,2]. However, the problems such as low impact resistance, especially continuous impact, restrict its application as a high-performance impact protection material. In impact applications, PUE usually needs to resist impact and shear forces, which requires PUE to have excellent toughness and strength. The hard segments of PUE serve as tougheners, dissipating energy during deformation, resulting in a high toughness [3]. Nevertheless, the toughness obtained by energy dissipation is ineffective in resisting crack growth under cyclic impact. At this time, the strength of PUE plays a decisive role in hindering crack formation and propagation [4]. Due to the synergistic effect between nanofillers and polymer matrix, incorporating nanoparticles into polymers is an effective way to develop excellent polymer composites with strength and toughness [5].
Nanoparticles-reinforced polymer composites have gradually received more and more attention due to their specific and attractive mechanical properties and thus have unique

Preparation of Composite Materials
The overall experimental process and chemical reaction are shown in Figure 1. The raw materials of component A and component B were mixed in the weight ratio of 5:2 and then poured into the mold to remove bubbles. Pure PUE was made by compression molding.

Preparation of Composite Materials
The overall experimental process and chemical reaction are shown in Figure 1. The raw materials of component A and component B were mixed in the weight ratio of 5:2 and then poured into the mold to remove bubbles. Pure PUE was made by compression molding.

Preparation of WS 2 @MWCNTs Composite Nanoparticles
The synthesis method of composite nanoparticles (WS 2 @MWCNTs) was as follows: C 2 H 5 NS (3.2 g) was poured into 10 mL ultrapure water for magnetic stirring for 30 min, and then WCl 6 (4.45 g) was added into the dispersion solution for further reaction for 1 h. Then, MWCNTs (1.85 g) were weighed into the above solution and dispersed ultrasonically for 2 h. The mixed solution was poured into the reactor and reacted at 200 • C for 24 h. After the reaction, the mixed solution was washed and filtered several times with acetic acid and ultrapure water. The filtered solid was dried at 100 • C in a vacuum environment for 24 h to obtain the product WS 2 @MWCNTs.

Preparation of A-WS 2 @MWCNTs/PUE Composite Materials
The synthesis of APTES-WS 2 @MWCNTs (A-WS 2 @MWCNTs) was as follows: First, APTES (4 mL) was dropped into a mixed solution of ethanol (40 mL) and ultrapure water (10 mL) for magnetic stirring for 30 min. Then composite nanoparticles (1 g) were added to the solution after hydrolysis, adjust the PH between 4 and 5 with an appropriate amount of acetic acid, ultrasonicate for 2 h, and magnetic stirring at 60 • C for 6 h. After the reaction was completed, wash with ethanol and water until neutral. Finally, the product was vacuum dried at 60 • C for 12 h and then ground for later use. Finally, A-WS 2 @MWCNTs was added to B component after ultrasonic dispersion, and then A component was added, mixed evenly, and poured into the mold to form A-WS 2 @MWCNTs/PUE composite material (The cylinder model with a diameter of Φ 20 × 4 mm was used for static pressure test, and the cylinder model with diameter Φ 10 × 2 mm was used for SHPB test). The added information of nanofillers is shown in Table 1.

Characterization Methods
Microscopic morphology of nanoparticles and microscopic damage of specimen surface as well as cross-section after impact observed by scanning electron microscopy (SEM, Zeiss, Sigma300) (Carl Zeiss, Baden-Württemberg, Germany). The phases of the composite nanoparticles were analyzed by X-ray diffraction (XRD, Bruker, D8 Advance, Karlsruhe, Germany) at a scanning angle of 10 • -80 • and a scanning speed of 10 • /min. The grafting of composite nanoparticles was characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 380, Waltham, MA, USA), and the powder samples were prepared by the pressing sheet method. Raman spectroscopy (Raman, Renishaw, inVia, London, UK) was used to characterize the area ratio of peak D to peak G of MWCNTs. A universal testing machine (Hua long, WDW-100C, Shanghai, China) was used for static compression tests with a compression rate of 1.2 mm/min and a compression thickness of 3.5 mm. Three parallel experiments were conducted for all samples to take the average value [18,19]. Aluminum Hopkinson Bar (made in National University of Defense Technology, Changsha, China) was used to measure the dynamic impact resistance of specimens [20][21][22][23]. An optical microscope (Olympus, BX53M, Tokyo, Japan) and ultra-depth of field 3D microscope system (Keyence, VHX-6000, Osaka, Japan) were used to observe the damaged microscopic appearance of the specimen after the impact.

Characterization of Composite Nanoparticles
As shown in Figure 2a, the crystal structure and phase purity of WS 2 @MWCNTs composite nanoparticles grown in situ are studied. The main diffraction peak observed is consistent with the main diffraction peak of hexagonal crystal system WS 2 . The diffraction peak corresponding to MWCNTs (002) is about 26 • [13]. Figure 2b is the Raman spectra of MWCNTs and WS 2 @MWCNTs. MWCNTs are coated with WS 2 , which reduces the effective vibrational groups of MWCNTs. The peak strength of WS 2 @MWCNTs decreases significantly. However, the area ratio between the D peak (Peak D represents the SP 3 hybridization of the C atom) and the G peak (Peak D represents the SP 2 hybridization of the C atom) does not change significantly. This means that MWCNTS is coated by WS 2 and will not destroy its own structure [24]. Figure 2c is the infrared spectrum of MWC-NTs, WS 2 @MWCNTs, and A-WS 2 @MWCNTs nanoparticles. WS 2 @MWCNTs grafted with APTES have strong Si-O-Si vibration at 1034 cm −1 , and C-H symmetric and asymmetric tensile vibration peaks at 2974 and 2872 cm −1 .
posite nanoparticles grown in situ are studied. The main diffraction peak observed is consistent with the main diffraction peak of hexagonal crystal system WS2. The diffraction peak corresponding to MWCNTs (002) is about 26° [13]. Figure 2b is the Raman spectra of MWCNTs and WS2@MWCNTs. MWCNTs are coated with WS2, which reduces the effective vibrational groups of MWCNTs. The peak strength of WS2@MWCNTs decreases significantly. However, the area ratio between the D peak (Peak D represents the SP 3 hybridization of the C atom) and the G peak (Peak D represents the SP 2 hybridization of the C atom) does not change significantly. This means that MWCNTS is coated by WS2 and will not destroy its own structure [24]. Figure 2c is the infrared spectrum of MWCNTs, WS2@MWCNTs, and A-WS2@MWCNTs nanoparticles. WS2@MWCNTs grafted with APTES have strong Si-O-Si vibration at 1034 cm −1 , and C-H symmetric and asymmetric tensile vibration peaks at 2974 and 2872 cm −1 .   Figure 3d,e. Compared with the surface of pure PUE, the surface microstructure of A-WS2@MWCNTs/PUE is rougher with obvious protrusions. However, the A-WS2@MWCNTs nanofiller wrapped in a polyurethane matrix has suitable compatibility. Figure 3f shows the microstructure  3a-c shows the microstructure of MWNCTs and WS 2 @MWCNTs. The in situ grown WS 2 nanosheets are coated with MWCNTs, which is beneficial to enhance the interface interaction between the nanofillers and the PUE matrix. The surface micro morphologies of pure PUE and 1.5%A-WS 2 @MWCNTs/PUE are shown in Figure 3d,e. Compared with the surface of pure PUE, the surface microstructure of A-WS 2 @MWCNTs/PUE is rougher with obvious protrusions. However, the A-WS 2 @MWCNTs nanofiller wrapped in a polyurethane matrix has suitable compatibility. Figure 3f shows the microstructure of 1.5% A-WS 2 @MWCNTs/PUE fracture. It can be seen that A-WS 2 @MWCNTs are uniformly distributed in the PUE matrix. It proves that the filler has suitable interface interaction with the polymer matrix and makes a great contribution to the improvement of antiimpact performance. of 1.5% A-WS2@MWCNTs/PUE fracture. It can be seen that A-WS2@MWCNTs are uniformly distributed in the PUE matrix. It proves that the filler has suitable interface interaction with the polymer matrix and makes a great contribution to the improvement of anti-impact performance.

Static Mechanical Characterization
Cylindrical specimens (Φ 20 × 4 mm) are selected for static compression test. As shown in Figure 4a, the addition of 1.5% A-WS2@MWCNTs has the best effect on improving the anti-compression performance of PUE. When the filler addition exceeds 2%, the compression resistance of the composite PUE will be severely reduced. A-WS2@MWCNTs fillers are usually unevenly dispersed in the polymer matrix due to their high specific surface energy, which will cause the filler to accumulate and agglomerate under higher filler loading [25]. Figure 4b shows the effect of uncompounded nanoparticles on the compression performance of PUE at 1.5% mass fraction filling. The addition of uncompounded WS2 and MWCNTs alone is also able to enhance the static compression performance of PUE, which can be attributed to the homogeneous dispersion of nanofillers that has an enhancing effect on the mechanical properties of the polymer [8,13,26]. However, A-WS2@MWCNTs is still the most obvious improvement in static compression strength. Under the same compression conditions, pure PUE has been completely destroyed, while A-WS2@MWCNTs/PUE remains in its original state ( Figure 4d). Compared with pure PUE, the elastic modulus of A-WS2@MWCNTs composite PUE is increased by 127.1%, the maximum pressure is increased by 159%, and the 10% constant pressure strength is increased by 144.2% ( Figure 4c and Table 1). It is speculated that the reasons for the improvement of its mechanical properties are as follows: (I) Strong interfacial interaction between A-WS2@MWCNTs and PUE [27]. A-WS2@MWCNTs composite nanoparticle system has suitable compatibility with PUE. The organic polymers and inorganic nanoparticles form a strong interface force. During the compression process, the load force is transferred from the polymer chain to the rigid nanomaterial, thereby effectively increasing the

Static Mechanical Characterization
Cylindrical specimens (Φ 20 × 4 mm) are selected for static compression test. As shown in Figure 4a, the addition of 1.5% A-WS 2 @MWCNTs has the best effect on improving the anti-compression performance of PUE. When the filler addition exceeds 2%, the compression resistance of the composite PUE will be severely reduced. A-WS 2 @MWCNTs fillers are usually unevenly dispersed in the polymer matrix due to their high specific surface energy, which will cause the filler to accumulate and agglomerate under higher filler loading [25]. Figure 4b shows the effect of uncompounded nanoparticles on the compression performance of PUE at 1.5% mass fraction filling. The addition of uncompounded WS 2 and MWCNTs alone is also able to enhance the static compression performance of PUE, which can be attributed to the homogeneous dispersion of nanofillers that has an enhancing effect on the mechanical properties of the polymer [8,13,26]. However, A-WS 2 @MWCNTs is still the most obvious improvement in static compression strength. Under the same compression conditions, pure PUE has been completely destroyed, while A-WS 2 @MWCNTs/PUE remains in its original state (Figure 4d). Compared with pure PUE, the elastic modulus of A-WS 2 @MWCNTs composite PUE is increased by 127.1%, the maximum pressure is increased by 159%, and the 10% constant pressure strength is increased by 144.2% (Figure 4c and Table 1). It is speculated that the reasons for the improvement of its mechanical properties are as follows: (I) Strong interfacial interaction between A-WS 2 @MWCNTs and PUE [27]. A-WS 2 @MWCNTs composite nanoparticle system has suitable compatibility with PUE. The organic polymers and inorganic nanoparticles form a strong interface force. During the compression process, the load force is transferred from the polymer chain to the rigid nanomaterial, thereby effectively increasing the compressive strength of the composite material. (II) The enthalpy and entropy interaction of amino-functionalized WS 2 @MWCNTs in the nanoparticle-polymer composites can guide the arrangement and distribution of nanoparticles [28], and the overall density of the matrix increases, which is beneficial to the improvement of mechanical properties [27]. (III) The hydroxyl groups contained in the composite nanofiller form hydrogen bonds with the carbonyl groups of polyurethane in polyurethane. It produces physical cross-linking, which limits the movement of polyurethane molecular chains during compression and thus improves the compressive strength of polyurethane.
linking, which limits the movement of polyurethane molecular chains during compression and thus improves the compressive strength of polyurethane.
In particular, it should be pointed out that the influence of static compression test and dynamic impact test on the deformation of PUE lies in the difference of strain rate. As a typical strain rate material, PUE will have a strain rate effect and glass transition effect under a high strain rate, which is not available in static compression tests [29,30]. Therefore, the static compression test is different from the dynamic impact test. In order to further study the continuous anti-impact performance of the composite PUE, a series of dynamic impact tests are necessary.

Dynamic Impact Test
A series of dynamic impact tests of PUE at different strain rates are carried out by using a split Hopkinson pressure bar (SHPB). SHPB is mainly composed of impact rod, input rod, and output rod. All rods are made of aluminum alloy.
In the SHPB test, the size of the sample has a great influence on the final reliable data. For the softer polymer materials, a smaller ratio of length to diameter (L/D = 0.2) is beneficial to avoid the end effect and inertia effect and keep the uniform deformation and stress balance. In addition, the uniform application of Vaseline on the rod end is beneficial to reduce the friction effect on the test. Because the change of sample size with time cannot In particular, it should be pointed out that the influence of static compression test and dynamic impact test on the deformation of PUE lies in the difference of strain rate. As a typical strain rate material, PUE will have a strain rate effect and glass transition effect under a high strain rate, which is not available in static compression tests [29,30]. Therefore, the static compression test is different from the dynamic impact test. In order to further study the continuous anti-impact performance of the composite PUE, a series of dynamic impact tests are necessary.

Dynamic Impact Test
A series of dynamic impact tests of PUE at different strain rates are carried out by using a split Hopkinson pressure bar (SHPB). SHPB is mainly composed of impact rod, input rod, and output rod. All rods are made of aluminum alloy.
In the SHPB test, the size of the sample has a great influence on the final reliable data. For the softer polymer materials, a smaller ratio of length to diameter (L/D = 0.2) is beneficial to avoid the end effect and inertia effect and keep the uniform deformation and stress balance. In addition, the uniform application of Vaseline on the rod end is beneficial to reduce the friction effect on the test. Because the change of sample size with time cannot be measured accurately, and the change of sample size after impact is small, the dynamic stress, strain, and strain rate are calculated by using the initial cross-sectional area and length of the sample. According to the one-dimensional wave propagation theory, the strains recorded by the incident, transmitted, and reflected waves are used to derive the stress, strain, and strain rate over time. They have the following relationships: .
In the formula, σ s (t), ε s (t), and ε s (t) are the stress, strain, and strain rate of the sample under test with time; ε r (t) and ε t (t) are recorded strains with time for the input and output rods, respectively; A s0 and L s0 are the initial cross-sectional area and length of the tested specimen; E 0 is the Young's modulus of bars; A 0 is the cross-sectional area of bars; and C 0 = ρ E0 /ρ 0 (ρ 0 is the density of bars) is the wave velocity in bars.
The energy absorbed by a material under a high-speed impact is defined as the strain energy (U) per unit volume (V) and is equal to the area of the stress-strain curve measured from ε 0 to ε 1 . It can be expressed in terms of strain energy density (u), which can be expressed by the following equation: where σ x is the normal stress in the bar, ε x is the normal strain, and ε 1 is the normal strain corresponding to elongation x 1 [31,32].

One-Time Impact Test
The engineering stress-strain curves of pure PUE and A-WS 2 @MWCNTs/PUE under the impact force of 0.2 MPa are shown in Figure 5a. In the initial elastic deformation stage, the stress-strain relationship is linear, and its slope is dynamic Young's modulus, which represents the material's ability to resist deformation. Compared with pure PUE, the dynamic Young's modulus of A-WS 2 @MWCNTs/PUE is increased by 167.2%. After the elastic deformation stage, a nonlinear transition occurs at the dynamic yield stress point. Then a large strain occurs under the platform stress until the material densifies, accompanied by a significant increase in stress. In this process, the material has no obvious softening phenomenon, so the dynamic yield stress is consistent with the platform stress, which is a key parameter for evaluating the material's strain energy absorption and impact resistance [33][34][35]. Compared with pure PUE, the dynamic yield stress of A-WS 2 @MWCNTs/PUE is increased by 331.7%. Moreover, compared with the dynamic yield stress of PUE or PUA prepared by predecessors, this research also has great advantages (  Figure 5b shows the influence of the addition of different nanomaterials on the impact resistance of PUE. The addition of A-WS 2 @MWCNTs and MWCNTs improves energy absorption. However, the addition of A-WS 2 reduces the energy absorption of the PUE. It is attributed to the fact that WS 2 nanosheets only increased the toughness of PUE but not the strength of PUE. Therefore, only the addition of A-WS 2 @MWCNTs can enhance both the toughness and strength of PUE. In addition, Figure 5c shows that pure PUE suffers irreversible damage after impact. Due to the concentration of stress, the middle part has been completely destroyed. However, A-WS 2 @MWCNTs/PUE does not show any damage.  Figure 6a shows the stress-strain curve (0.02-0.25 MPa) under the 10 successive impacts. As the impact load increases, the maximum stress of A-WS2@MWCNTs/PUE continues to rise. In the initial low-velocity impact stage, A-WS2@MWCNTs/PUE relies on its own high strength to resist impact. When the impact load is loaded to 0.25 MPa, it is found that the maximum stress no longer rises, but the strain rate has increased greatly. In this process, A-WS2@MWCNTs rely on their own suitable toughness to undergo plastic deformation to increase energy absorption (Figure 6b). The increase in toughness can be attributed to the strain rate effect and glass transition characteristics of PUE (strain rate material) [38]. Figure 6c-d illustrates the continuous impact resistance enhancement mechanism of A-WS2@MWCNTs. Excellent interfacial interaction between nanoparticles and the PUE matrix is a prerequisite for enhanced mechanical properties, which promotes the force transfer from the PUE matrix to the nanoparticles. In addition, energy absorption resulting from plastic deformation of chain segments, synergistic toughening between nanoparticles and PUE matrix, van der Waals forces, and synergistic effect between nanoparticles are also important reasons for the improvement of the impact resistance of A-WS2@MWCNTs/PUE [39].  Figure 6a shows the stress-strain curve (0.02-0.25 MPa) under the 10 successive impacts. As the impact load increases, the maximum stress of A-WS 2 @MWCNTs/PUE continues to rise. In the initial low-velocity impact stage, A-WS 2 @MWCNTs/PUE relies on its own high strength to resist impact. When the impact load is loaded to 0.25 MPa, it is found that the maximum stress no longer rises, but the strain rate has increased greatly. In this process, A-WS 2 @MWCNTs rely on their own suitable toughness to undergo plastic deformation to increase energy absorption (Figure 6b). The increase in toughness can be attributed to the strain rate effect and glass transition characteristics of PUE (strain rate material) [38]. Figure 6c,d illustrates the continuous impact resistance enhancement mechanism of A-WS 2 @MWCNTs. Excellent interfacial interaction between nanoparticles and the PUE matrix is a prerequisite for enhanced mechanical properties, which promotes the force transfer from the PUE matrix to the nanoparticles. In addition, energy absorption resulting from plastic deformation of chain segments, synergistic toughening between nanoparticles and PUE matrix, van der Waals forces, and synergistic effect between nanoparticles are also important reasons for the improvement of the impact resistance of A-WS 2 @MWCNTs/PUE [39].

Craze Evolution, Crack Formation, and Destruction Mechanism
The OM, SEM, and ultra-depth of field 3D microscope system are used to observe and analyze the surface, cross-section, and overall morphology of the sample. Figure 7 shows the surface micro morphologies of pure PUE and A-WS2@MWCNTs/PUE after impact. Figure 7a,b is the surface morphologies of pure PUE It can be seen that the middle part of the pure PUE has suffered serious perforation damage. The reason is that the shear band formed under the combined action of the circumferential stress and the radial stress moves to the center, causing stress concentration damage. However, as shown in Figure 7c,d, there is no concentrated shear band in the center of A-WS2@MWCNTs/PUE, and a few cracks on the surface do not continue to expand in the radial direction. Under SEM observation, it is found that there is no macroscopic continuous expansion of collapse and spalling, but a partial plastic deformation zone appears. This plastic zone retards the expansion of macroscopic cracks and improves the impact toughness.

Craze Evolution, Crack Formation, and Destruction Mechanism
The OM, SEM, and ultra-depth of field 3D microscope system are used to observe and analyze the surface, cross-section, and overall morphology of the sample. Figure 7 shows the surface micro morphologies of pure PUE and A-WS 2 @MWCNTs/ PUE after impact. Figure 7a,b is the surface morphologies of pure PUE. It can be seen that the middle part of the pure PUE has suffered serious perforation damage. The reason is that the shear band formed under the combined action of the circumferential stress and the radial stress moves to the center, causing stress concentration damage. However, as shown in Figure 7c,d, there is no concentrated shear band in the center of A-WS 2 @MWCNTs/PUE, and a few cracks on the surface do not continue to expand in the radial direction. Under SEM observation, it is found that there is no macroscopic continuous expansion of collapse and spalling, but a partial plastic deformation zone appears. This plastic zone retards the expansion of macroscopic cracks and improves the impact toughness.

Craze Evolution, Crack Formation, and Destruction Mechanism
The OM, SEM, and ultra-depth of field 3D microscope system are used to observe and analyze the surface, cross-section, and overall morphology of the sample. Figure 7 shows the surface micro morphologies of pure PUE and A-WS2@MWCNTs/PUE after impact. Figure 7a,b is the surface morphologies of pure PUE. It can be seen that the middle part of the pure PUE has suffered serious perforation damage. The reason is that the shear band formed under the combined action of the circumferential stress and the radial stress moves to the center, causing stress concentration damage. However, as shown in Figure 7c,d, there is no concentrated shear band in the center of A-WS2@MWCNTs/PUE, and a few cracks on the surface do not continue to expand in the radial direction. Under SEM observation, it is found that there is no macroscopic continuous expansion of collapse and spalling, but a partial plastic deformation zone appears. This plastic zone retards the expansion of macroscopic cracks and improves the impact toughness.   Figure 8 shows SEM images of the cryo-fracture surface of the pure PUE samples and the A-WS 2 @MWCNTs/PUE samples after impact. Figure 8a,b shows the shear failure zone and part of the spalled fragments in the direction of pure PUE impact loading. This proves that pure PUE has been completely destroyed under impact loading. On the contrary, as follows from these SEM images (Figure 8c,d), A-WS 2 @MWCNTs are apparently rooted within the PUE matrix, which ascertains suitable adhesion between the matrix and the filler. This phenomenon clearly indicates that the pull-out mechanism of the nanocomposites hinders the crack extension and promotes the improvement of the composite toughness and energy absorption capacity.
Materials 2021, 14, x FOR PEER REVIEW 11 Figure 8 shows SEM images of the cryo-fracture surface of the pure PUE samples the A-WS2@MWCNTs/PUE samples after impact. Figure 8a,b shows the shear failure and part of the spalled fragments in the direction of pure PUE impact loading. This pr that pure PUE has been completely destroyed under impact loading. On the contrar follows from these SEM images (Figure 8c,d), A-WS2@MWCNTs are apparently ro within the PUE matrix, which ascertains suitable adhesion between the matrix and filler. This phenomenon clearly indicates that the pull-out mechanism of the nanocom sites hinders the crack extension and promotes the improvement of the composite to ness and energy absorption capacity.  WS2@MWCNTs/PUE under the ultra-depth of field three-dimensional microsystem impact. The center of the pure PUE is severely damaged, and a ring-shaped macrosc shear band appears around the hole, which indicates that the external force on the extends from the periphery to the center and finally causes the perforation damage a center. On the contrary, many discontinuous micro-cracks appear on the surface o WS2@MWCNTs. The micro-cracks stop spreading at the distribution of nanoparticle ducing the stress concentration in the center and protecting the PUE from damage. also proves that A-WS2@MWCNTs nanofiller has the function of inhibiting crack pr gation. Figure 9a-d shows the surface morphology of pure PUE and 1.5%A-WS 2 @MWCNTs/ PUE under the ultra-depth of field three-dimensional microsystem after impact. The center of the pure PUE is severely damaged, and a ring-shaped macroscopic shear band appears around the hole, which indicates that the external force on the PUE extends from the periphery to the center and finally causes the perforation damage at the center. On the contrary, many discontinuous micro-cracks appear on the surface of A-WS 2 @MWCNTs. The micro-cracks stop spreading at the distribution of nanoparticles, reducing the stress concentration in the center and protecting the PUE from damage. This also proves that A-WS 2 @MWCNTs nanofiller has the function of inhibiting crack propagation.  [40]) and the Mohr-Coulomb (flexible polymer impact loading experiments [41]). This model helps explain the preventing crack propagation role of A-WS2@MWCNTs in the impact process. PUE is loaded by radial normal, σr, and circumferential normal stresses, σθ, under the impact of SHPB. In the stress element of the θ-Z plane, the σθ can be decomposed into two-component stresses on any stress plane, namely shear stress, τn, and normal stress, σn. Under the action of two-component stress, a radia shear band is produced. The motion of radial shear bands can be explained in terms of elliptic criteria: In elliptic criteria, τ0 is the critical shear strength; σ0 is the critical dissociation strength of PUE, which depends on the material. When the maximum value of the function f (τn σn) increases to the critical function value of PUE, the joint action of shear stress τn and normal stress σn occurs, and the radial shear zone starts to form and propagate along the radial direction. Therefore, according to the ellipse criterion, the radial shear band is driven by circumferential tensile stress, σθ.
On the other hand, in the stress element of the R-Z plane, there is a pure shear stress τ in the Z direction perpendicular to the R direction. According to ⃑⃑⃑ = + ⃑⃑⃑ , radia normal stress σr can be changed to synthetic stress τr, according to Mohr-Coulomb criterion: A simple damage model ( Figure 10) of PUE under impact is established by observing the microscopic morphology of the specimens and referencing the elliptic criterion (metal glass stamping experiments [40]) and the Mohr-Coulomb (flexible polymer impact loading experiments [41]). This model helps explain the preventing crack propagation role of A-WS 2 @MWCNTs in the impact process. PUE is loaded by radial normal, σ r , and circumferential normal stresses, σ θ , under the impact of SHPB. In the stress element of the θ-Z plane, the σ θ can be decomposed into two-component stresses on any stress plane, namely shear stress, τ n , and normal stress, σ n . Under the action of two-component stress, a radial shear band is produced. The motion of radial shear bands can be explained in terms of elliptic criteria: In Mohr-Coulomb criterion, | | is the absolute value of the combined stress of radial normal stress, τr is the critical shear fracture strength of the PUE, τ0 is the critical shear strength of the material, and μ is the material constant of PUE. Therefore, when the resultant stress τr is in contact with the critical shear fracture line, the circumferential shear band begins to form and expand in the circumferential direction [40]. Therefore, the synthetic pure shear stress τr causes the formation and movement of the circumferential shear band. In conclusion, the radial shear band is caused by σθ, while the circumferential shear band is excited by pure shear stress τr. Different deformation mechanisms determine the failure mode of the material, and the formation and movement of the double shear band lead to the final central failure of PUE. The essence of the PUE deformation process is the In elliptic criteria, τ 0 is the critical shear strength; σ 0 is the critical dissociation strength of PUE, which depends on the material. When the maximum value of the function f (τ n , σ n ) increases to the critical function value of PUE, the joint action of shear stress τ n and normal stress σ n occurs, and the radial shear zone starts to form and propagate along the radial direction. Therefore, according to the ellipse criterion, the radial shear band is driven by circumferential tensile stress, σ θ .
On the other hand, in the stress element of the R-Z plane, there is a pure shear stress τ in the Z direction perpendicular to the R direction. According to τ r = τ + σ r , radial normal stress σ r can be changed to synthetic stress τ r , according to Mohr-Coulomb criterion: In Mohr-Coulomb criterion, |τ r | is the absolute value of the combined stress of radial normal stress, τ r is the critical shear fracture strength of the PUE, τ 0 is the critical shear strength of the material, and µ is the material constant of PUE. Therefore, when the resultant stress τ r is in contact with the critical shear fracture line, the circumferential shear band begins to form and expand in the circumferential direction [40]. Therefore, the synthetic pure shear stress τ r causes the formation and movement of the circumferential shear band.
In conclusion, the radial shear band is caused by σ θ , while the circumferential shear band is excited by pure shear stress τ r . Different deformation mechanisms determine the failure mode of the material, and the formation and movement of the double shear band lead to the final central failure of PUE. The essence of the PUE deformation process is the competition process between σ θ and τ r . However, during the deformation of A-WS 2 @MWCNTs/PUE, A-WS 2 @MWCNTs increases the critical shear strength τ 0 and the critical dissociation strength σ 0 , which makes radial shear band difficult to form and move. Similarly, the formation and expansion of circumferential shear bands are impeded. Finally, the two shear bands do not extend deep, and some areas form a discontinuous plastic deformation zone, which effectively improves energy absorption. In addition, for the dominant plastic deformation τ r , A-WS 2 @MWCNTs can effectively prevent the regional damage caused by the accumulation of plastic deformation bands through its own deformation, which is also due to the excellent compatibility and dispersion of A-WS 2 @MWCNTs.

Conclusions
In this experiment, the A-WS 2 @MWCNTs composite filler was prepared by in situ growth and in situ grafting to improve the impact resistance of PUE. Through a static compression test, dynamic impact test, microscopic morphology observation, and failure mechanism analysis, we can draw the following conclusions:

1.
Compared with pure PUE, the static compressive strength and dynamic yield stress of composite PUE are increased by 144.2% and 331.7%, respectively. The addition of composite filler enhances the strength and toughness of PUE and avoids the damage and softening caused by stress concentration and heat concentration under the impact effect. Most notably, the composite PUE remains intact under the successive shocks, while the pure PUE is destroyed by a single impact. The suitable impact resistance of A-WS 2 @MWCNTs/PUE is attributed to the suitable interface interaction between nanoparticles and PUE and the synergy between nanoparticles.

2.
According to the surface morphology analysis, polyurethane is prone to perforation damage in the central area under high-speed impact. However, A-WS 2 @MWCNTs/ PUE has a large plastic deformation zone, which absorbs external energy, delays the growth of macroscopic cracks, and prevents damage to the specimen. In addition, the morphology of the section shows that A-WS 2 @MWCNTs is rooted in the PUE matrix, indicating that the adhesion between the matrix and the filler is suitable. This indicates that the pull-out mechanism of the nanocomposites impedes the crack propagation and promotes the toughness and energy absorption capacity of the composites.

3.
It can be concluded that the deformation process of PUE is a competitive process between σ θ and τ r by studying the damage mechanism of PUE. For A-WS 2 @MWCNTs/ PUE, A-WS 2 @MWCNTs increases the critical shear strength τ 0 and the critical dissociation strength σ 0 , making it difficult for the radial shear band to form and move. Similarly, the formation and expansion of circumferential shear zones are also hindered. Finally, the two shear bands do not extend deeply, and some regions form a discontinuous plastic deformation zone, which effectively improves energy absorption.

4.
Based on the characteristics of A-WS 2 @MWCNTs/PUE with high impact strength, it is worthy of being widely used in the field of impact protection. In addition, the damage formation process of PUE material is analyzed, and the preventing effect of nanofiller on crack propagation is explained, which lays a theoretical foundation for the subsequent development of impact-resistant nanocomposite PUE.