The escalating risk of terrorist attacks恐怖袭击、军事冲突、爆炸事故和化学灾害的风险不断升级，使军事和民用建筑更加需要防爆和抗冲击。这一要求对于军舰、装甲车和防护头盔等军用防护装备尤为重要，所有这些装备都需要承受近场爆炸产生的冲击波和高速碎片[1, military conflicts, explosive accidents and chemical disasters has heightened the necessity for blast and impact resistance in military and civilian buildings. This requirement is particularly crucial for military protective equipment such as military ships, armoured vehicles and protective helmets, all of which need to withstand shock waves and high-speed fragments resulting from near-field explosions ^{[1][2][3][4][5][6][7][8][9][10]}. Even though research on the protective properties of metal structures and high-performance fiber composites has made significant progress2,3,4,5, there is an increasing need for lightweight and efficient blast protection structures ^{[11][12][13][14][15][16][17][18]}. To accommodate this requirement6, the application of high-performance polymers in composite protection structures has gained momentum due to their enhanced protection performance7, cost reduction and expanded application possibilities. In essence, the development and design of efficient protective materials and structures play a vital role in mitigating blast and impact threats, wherein the utilization of high-performance polymers in composite protection structures holds promising potential in meeting these challenges8,9,10].

Polyurea is a block copolymer synthesized by rapidly reacting an isocyanate prepolymer with a polyamine. Commercial polyurea formulations typically comprise two components: Component A尽管对金属结构和高性能纤维复合材料的防护性能的研究取得了重大进展，但对轻量化和高效防爆结构的需求越来越大[11, an isocyanate prepolymer; and Component B12, a blend of end-amino polyethers, chain extenders and various additives ^[19]. The presence of soft and hard segments in polyurea gives rise to a unique microphase separation phenomenon within its microstructure. This microstructure reveals that the hard segments are uniformly dispersed within the soft segment matrix13,14, creating a cross-linked grid structure. As a result15, polyurea can be regarded as a nanocomposite material, with the hard segments serving as reinforcements within the soft segment matrix. This distinctive microstructure imparts favorable macroscopic properties to polyurea, including stability, high strength and resistance to aging16,17,18].为了满足这一要求，高性能聚合物在复合材料保护结构中的应用因其增强的保护性能、降低成本和扩大的应用可能性而获得动力。从本质上讲，高效防护材料和结构的开发和设计在减轻爆炸和冲击威胁方面发挥着至关重要的作用，其中在复合防护结构中使用高性能聚合物在应对这些挑战方面具有广阔的潜力。

聚脲最初由德士古（现为亨斯迈）在 developed and researched by Texaco (now Huntsman) in the mid-1980s 年代中期开发和研究，被证明是聚氨酯的高效且具有成本效益的替代品，因为它具有多种理想性能。这些特点包括高强度、高韧性、快速施工和最小的环境影响，使聚脲成为各种应用中非常有前途的材料。值得注意的是，聚脲弹性体技术因其固化速度快、易于应用和形成厚涂层的能力而与传统涂层方法区分开来。此外，聚脲材料因其高韧性而提供卓越的保护，并有效减轻爆炸碎片造成的伤害。此外，聚脲涂料具有成本效益且易于处理，使其成为增强结构抗爆性的独特而有利的选择。

2. 聚脲分子的结构特征及材料优化

聚脲具有显著的化学稳定性和出色的物理性能，使其非常适合在各种环境中进行防爆和冲击防护。为了适应各种基材或结构，可以通过改变异氰酸酯和氨基化合物的类型和含量来调整聚脲中硬链段和软链段的比例[21,22, polyurea proves to be a highly efficient and cost-effective alternative to polyurethanes23, as it possesses a diverse range of desirable properties. These include high strength, high toughness, rapid construction and minimal environmental impact, making polyurea a highly promising material for various applications. Notably, polyurea elastomer technology distinguishes itself from traditional coating methods due to its rapid curing speed, ease of application and ability to form thick coatings. Furthermore, polyurea materials provide exceptional protection and effectively mitigate the harm caused by explosive fragments due to their high toughness. Additionally, polyurea coatings are cost-effective and convenient to handle, rendering them a unique and advantageous choice for enhancing blast resistance in structures.

2. Structural Characteristics of Polyurea Molecules and Material Optimization

Polyurea demonstrates remarkable chemical stability and outstanding physical properties24,25,26]。此外，纳米或微米颗粒和增强纤维的加入有助于提高聚脲复合材料的力学性能和抗爆破性[11,12,13,14,15,16,17, rendering it highly suitable for blast and impact protection in diverse environments. To accommodate various substrates or structures18, the ratio of hard and soft segments in polyurea can be adjusted by modifying the type and content of isocyanate and amino compounds ^{[20][21][22][23][24][25]}. Moreover27, the incorporation of nano or micron particles and reinforcing fibers serves to enhance the mechanical properties and blast resistance of polyurea composites ^{[11][12][13][14][15][16][17][18][26][27][28]}. Additionally28,29]。此外，分子动力学 molecular dynamics (MD) simulation can be utilized to optimize polyurea at the molecular level, further enhancing its performance. This comprehensive approach ensures the effectiveness and versatility of polyurea in applications related to blast and impact protection, exhibiting the desired levels of perplexity and burstiness in the content.（MD） 模拟可用于在分子水平上优化聚脲，进一步提高其性能。这种全面的方法确保了聚脲在与防爆和冲击防护相关的应用中的有效性和多功能性，在内容物中表现出所需的困惑度和爆裂性。

2.1. 聚脲分子的结构特征

2.1. Structural Features of Polyurea Molecules

Polyurea聚脲是一种由硬链段和软链段组成的微相分离嵌段高分子材料。硬链段由强极性含尿素 is a micro（-phase-separated block polymer material composed of hard and soft segments. The hard segment consists of strongly polar urea-containing (-NH-CO-NH-) chain segments connected by hydrogen bonding and π-stacked aromatic chain segments. It is generated by the reaction of polyisocyanate and the chain extender, resulting in a glass transition temperature (Tg) above ambient temperature. The soft segment is composed of oligomeric polyol and oligomeric polyamines, providing flexibility and aliphatic chain segments. The Tg of the soft segment is typically below ） 链段组成，通过氢键和π堆叠的芳香族链段连接。它由聚异氰酸酯和扩链剂反应生成，导致玻璃化转变温度（Tg）高于环境温度。软链段由低聚多元醇和低聚多胺组成，提供柔韧性和脂肪族链段。软段的Tg通常低于−30 °C. This structure makes polyurea a micro-dispersed thermoplastic cross-linked polymer at room temperature (20 °C). The soft segments exhibit superelasticity, while the hard segments display elastoplastic behavior. Hydrogen bonds are formed between the hard segments and between the hard and soft segments, creating reversible physical cross-linking and reinforcing components. These hydrogen bonds also contribute to the formation of a mesh structure ^[29], resulting in excellent mechanical properties such as modulus, hardness and tear strength. The soft segments contribute to the material’s flexibility and low-temperature resistance. 。 这种结构使聚脲在室温（20°C）下成为微分散的热塑联聚合物。软段表现出超弹性，而硬段表现出弹塑性行为。在硬链段之间以及硬链段和软链段之间形成氢键，从而产生可逆的物理交联和增强成分。这些氢键还有助于形成网状结构[30]，从而产生优异的机械性能，如模量、硬度和撕裂强度。柔软的段有助于提高材料的柔韧性和耐低温性。MD simulations have shown that the soft segments can store more strain energy compared to the hard segments, while energy is consumed through structural disruption and hydrogen bond dissociation of the hard segments ^[30]. 模拟表明，与硬段相比，软段可以存储更多的应变能，而能量则通过硬段的结构破坏和氢键解离来消耗[31]。李婷等[32]合成的聚脲（Polyurea (PUR1000), as synthesized by Ting Li et al. ^[31], was prepared through the native polymerization of polycarbodiimide-modified diphenylmethane diisocyanate and poly(tetramethylene oxide di-p-aminobenzoate).）是通过聚碳二亚胺改性的二苯基甲烷二异氰酸酯和聚（四环氧甲烷二对氨基苯甲酸酯）的天然聚合制备的。 聚脲中软链段的长度对其准静态和动态条件下的力学性能有显著影响。当软段的长度增加时，抗拉强度成比例地降低，The length of the soft segments in polyurea has a significant influence on its mechanical properties both under quasi-static and dynamic conditions. When the length of the soft segments increases, the tensile strength decreases proportionally and the Tg decreases as well. In a study by 也降低。在D.A. Tzelepis et al. ^[32], several polyureas were synthesized with the same molecular weight of the soft segments but varying weight fractions of the hard segments. The structure of the polyurethanes was characterized using differential scanning calorimetry (等[33]的一项研究中，合成了几种聚脲，这些聚脲的软链段分子量相同，但硬链段的重量分数不同。采用差示扫描量热法（DSC) and a transmission electron microscope (）和透射电子显微镜（TEM) which revealed that the three polymers had almost the same Tg. According to the time-temperature superposition (TTS), a reduction in Tg makes the materials less susceptible to brittle damage by maintaining their elasticity during high strain rate loads, such as blast and impact loads.）对聚氨酯的结构进行了表征，结果表明三种聚合物的Tg几乎相同。根据时间-温度叠加 （TTS），Tg 的降低通过在高应变率载荷（如爆炸和冲击载荷）下保持其弹性，使材料不易受到脆性损伤。 The聚脲硬段内尿素键的存在在确定其性能方面起着关键作用。可以操纵嵌段共聚物中硬段区域的大小、性质和分布，以增强材料的损耗谱。研究人员进行了几项研究，重点关注聚脲的热稳定性，并观察到它在高温下会经历一步分解。聚脲的热分解过程始于硬段内尿素键的分解。热分解温度通常在 presence of urea bonds within the hard segments of polyurea plays a critical role in determining its properties. The size, properties and distribution of the hard segment region in the block copolymer can be manipulated to enhance the material’s loss spectrum. Researchers have conducted several studies focusing on the thermal stability of polyurea and have observed that it undergoes one-step decomposition at high temperatures. The thermal decomposition process of polyurea initiates with the breakdown of the urea bonds within the hard segments. The thermal decomposition temperature typically falls within the range of 300 to300 至 320 °C. To improve the thermal stability of polyurea, di- and tri-functional polyamines can be incorporated into the formulation. These additives reinforce the cross-linked structure of the material, consequently enhancing its thermal stability. By strengthening the material’s structure, the added polyamines contribute to its ability to withstand higher temperatures without undergoing significant decomposition. 范围内。 为了提高聚脲的热稳定性，可以在配方中加入二官能团和三官能团多胺。这些添加剂增强了材料的交联结构，从而增强了其热稳定性。通过加强材料的结构，添加的多胺有助于其承受更高温度而不会发生显着分解的能力。 Hydrogen氢键对聚脲的分子结构和力学性能有显著影响。与聚氨酯聚氨酯中的单配位氢键不同，聚脲在硬段尿素键内表现出双配位氢键，从而在材料内产生更高的键合能和增强的微相偏析。聚合物氨基 bonds significantly influence the molecular structure and mechanical properties of polyureas. Unlike the mono-coordinated hydrogen bonding in urethane urethanes, polyurea exhibit bi-coordinated hydrogen bonding within the hard-segmented urea bonds, resulting in higher bonding energy and enhanced micro-phase segregation within the material. The redshift in the amino (（N-H) and carbonyl (C=O) regions of the polymer provides valuable insights into the extent of hydrogen bonding. Techniques such as Fourier transform infrared spectroscopy (FTIR) can be used to analyze changes in the position and intensity of these regions, enabling researchers to estimate the strength and prevalence of hydrogen bonding in polyureas. Understanding the role of hydrogen bonding in polyureas is crucial as it significantly impacts various material properties. Bi-coordinated hydrogen bonding fosters micro-phase segregation, which influences the overall mechanical behavior and structural characteristics of polyureas, thus allowing for tailored properties to suit specific applications. Considering the complexities of hydrogen bonding and its effects on polyureas, researchers can further explore and optimize the material’s mechanical properties, thermal stability and chemical resistance. This knowledge creates opportunities for advancements and applications in a wide range of industries.） 和羰基 （C=O） 区域的红移为氢键程度提供了有价值的见解。傅里叶变换红外光谱（FTIR）等技术可用于分析这些区域的位置和强度的变化，使研究人员能够估计聚脲中氢键的强度和普遍性。了解氢键在聚脲中的作用至关重要，因为它会显着影响各种材料性能。双配位氢键促进了微相偏析，从而影响了聚脲的整体机械性能和结构特性，从而允许定制性能以适应特定应用。考虑到氢键的复杂性及其对聚脲的影响，研究人员可以进一步探索和优化材料的力学性能、热稳定性和耐化学性。这些知识为各行各业的进步和应用创造了机会。

2.2. Optimization of Polyurea Composition

2.2. 聚脲成分的优化

Polyurea, a highly versatile polymer, comprises both hard and soft segments in its composition. The hard segment is typically composed of isocyanates, while the soft segment consists of amino compounds. The properties of polyurea can be precisely controlled by manipulating the ratio of hard and soft. This can be achieved through varying the amount and type of isocyanates and amines. Increasing the concentration of isocyanates in polyurea leads to a higher proportion of hard segments, resulting in elevated hardness, strength and durability. Conversely, augmenting the content of amino compounds increases the ratio of soft segments, leading to enhanced flexibility, bendability and elasticity of polyurea. Thus, adjusting the isocyanate-to-amino compound ratio directly influences the balance between hard and soft segments within the polyurea structure. For example, elevating the proportion of isocyanates while reducing the amount of amino compounds can enhance the hardness and strength of polyurea, although at the expense of flexibility and bendability. This ability to tailor the ratio of hard and soft segments empowers the customization of polyurea properties to align with specific application requirements.聚脲是一种用途广泛的聚合物，其成分包括硬链段和软链段。硬链段通常由异氰酸酯组成，而软链段由氨基化合物组成。聚脲的性能可以通过操纵硬软比例来精确控制。这可以通过改变异氰酸酯和胺的量和类型来实现。增加聚脲中异氰酸酯的浓度会导致硬链段比例增加，从而提高硬度、强度和耐久性。相反，增加氨基化合物的含量会增加软链段的比例，从而增强聚脲的柔韧性、弯曲性和弹性。因此，调整异氰酸酯与氨基化合物的比例直接影响聚脲结构内硬链段和软链段之间的平衡。例如，提高异氰酸酯的比例，同时减少氨基化合物的用量，可以提高聚脲的硬度和强度，但代价是牺牲柔韧性和弯曲性。这种定制硬段和软段比例的能力使聚脲性能的定制成为可能，以符合特定的应用要求。 Different types of isocyanates and amino compounds have varying properties and reactivity. By selecting different combinations of these compounds, the ratio of hard and soft segments in polyurea can be adjusted, along with other properties such as heat resistance and chemical resistance. In a study by 不同类型的异氰酸酯和氨基化合物具有不同的性质和反应性。通过选择这些化合物的不同组合，可以调整聚脲中硬链段和软链段的比例，以及耐热性和耐化学性等其他性能。M.F. Sonnenschein et al. ^[20], polyether polyols were used as raw materials and an ester exchange reaction with p-amino benzoate was conducted to synthesize terminated aniline polyols. The resulting amine exhibited higher thermo-oxidative stability and viscosity compared to the parent polyol. The physical properties of polyurea等[21]的一项研究以聚醚多元醇为原料，与对氨基苯甲酸酯进行酯交换反应，合成封端苯胺多元醇。与母体多元醇相比，所得胺表现出更高的热氧化稳定性和粘度。从反应动力学、拉伸性能、形貌和老化性能等方面对由苯胺封端基制备的聚脲/polyurethane elastomers prepared from these aniline-terminated end groups were evaluated in terms of reaction kinetics, tensile properties, morphology and aging properties. The study found that increasing the volume of hard segments in order to enhance elastomer hardness and tensile strength led to challenges in phase separation due to the inhomogeneous reactivity between the aniline end groups and the hydroxyl groups of the hard-segment chain extender. This hindered the desired phase separation of the hard segments.聚氨酯弹性体的物理性能进行了评价。研究发现，由于硬链段扩链剂的苯胺端基和羟基之间的反应性不均匀，增加硬链段的体积以提高弹性体硬度和拉伸强度导致相分离方面的挑战。这阻碍了硬段所需的相分离。 The mechanical and thermal properties of PU and PUR coatings are significantly influenced by the length of the aliphatic chain and the properties of the aromatic chain extender ^[21]. In the study conducted by 和PUR涂层的机械和热性能受脂肪族链的长度和芳香族扩链剂的性能的显著影响[22]。V. Shahi et al. ^[22], polyurethane elastomers were synthesized using 等[23]的研究以PTMO-based diamines and 基二胺和MDI diisocyanate as raw materials via step-growth polymerization. The investigation of thermo-mechanical properties revealed that 二异氰酸酯为原料，通过阶梯式生长聚合合成了聚氨酯弹性体。热力学性能研究表明，随着长链二胺的掺入量增加，PU-HB05, with increased incorporation of long-chained diamines, exhibited lower thermal conductivity and heat capacity, a more amorphous structure and increased stability at high temperatures. In another study, 表现出较低的导热系数和热容，更无定形的结构和更高的高温稳定性。在另一项研究中，H. Guo et al. ^[23] synthesized various polyurea coating materials by adjusting the proportions of amino-terminated polyether types and amine chain extender types in the original polyurea components. Analysis of the properties of these coatings led to the conclusion that the optimal ratio of amino-terminated polyether 等[24]通过调整原聚脲组分中氨基封端聚醚类型和胺链扩展剂类型的比例，合成了各种聚脲涂层材料。通过对这些涂层性能的分析，得出的结论是，氨基封端聚醚D2000 to 与T5000 was 12:1 and the optimal ratio of amine chain extender E100 to W6200 was 1.6:1, as these ratios resulted in the best performance indicators for the polyurea coating.的最佳比例为12：1，胺链延伸剂E100与W6200的最佳比例为1.6：1，因为这些比例导致了聚脲涂层的最佳性能指标。 Covalent thermosets are known for their strong mechanical properties, however, they lack reprocessing or recycling capabilities, making them fragile. In a study by 共价热固性塑料以其强大的机械性能而闻名，但是，它们缺乏再加工或回收能力，因此很脆弱。B. Qin et al. ^[24], a new approach was developed to enhance the toughness and recyclability of cross-linked supramolecular polyurethanes (等[25]的一项研究提出了一种提高交联超分子聚氨酯（CSPUs). This was achieved by introducing noncovalent bonds into the polymer backbone. ）韧性和可回收性的新方法。这是通过将非共价键引入聚合物主链来实现的。采用二异氰酸酯单体、四氢键二胺单体和共价二胺/三胺单体共聚制备了CSPUs were prepared through the copolymerization of diisocyanate monomers, tetrahydrogen bonded diamine monomers and covalent diamine/triamine monomers. The resulting CSPUs exhibited excellent mechanical properties and solvent resistance due to the combination of covalent cross-linking and noncovalent bonding. Additionally, 。由于共价交联和非共价键的结合，所得的CSPU表现出优异的力学性能和耐溶剂性。此外，L. Zhang等[26]通过设计具有不同强度的氢键相互作用并结合永久共价键，成功合成了一种超分子聚脲弹性体。这种弹性体表现出卓越的机械强度，断裂伸长率超过 et al. ^[25] successfully synthesized a supramolecular polyurea elastomer by designing hydrogen bonding interactions with various strengths and incorporating permanent covalent bonds. This elastomer demonstrated remarkable mechanical strength with an elongation at break exceeding 1600%, a，对缺口不敏感的拉伸能力高达 notch-insensitive tensile capacity of up to 800% and a toughness of up to 800%，韧性高达 12,500 J m⁻². The covalent cross-linking provided high strength, while the multi-strength hydrogen bonding offered elasticity, energy dissipation and fast self-healing properties at room temperature.共价交联提供了高强度，而多强度氢键在室温下提供了弹性、能量耗散和快速自愈性能。 A crucial aspect to consider is that the modification of polyurea through the alteration of isocyanate and amino compounds requires meticulous handling. The reactivity and properties of these compounds can significantly influence the structure and characteristics of the resulting polyurea. Consequently, it is imperative to conduct thorough experimentation and testing throughout the preparation and modification processes of polyurea. These measures are essential to ensure the attainment of the desired properties and stability of the material. Careful attention and precision are vital to optimize the outcome and guarantee the reliability of the modified polyurea.需要考虑的一个关键方面是，通过改变异氰酸酯和氨基化合物来改性聚脲需要细致的处理。这些化合物的反应性和性能可以显著影响所得聚脲的结构和特性。因此，必须在聚脲的整个制备和改性过程中进行彻底的实验和测试。这些措施对于确保材料达到所需的性能和稳定性至关重要。仔细的关注和精度对于优化结果和保证改性聚脲的可靠性至关重要。

2.3. 增强材料的引入

2.3. Introduction of Enhanced Materials

Polyurea materials have the potential to be optimized by incorporating various forms of reinforcing materials ^[33][37]. By introducing fiber reinforcing materials, particle filling materials, foam reinforcing materials and nano reinforcing materials, the properties of polyurea such as strength, hardness, abrasion resistance, temperature resistance and chemical resistance can be enhanced. Among the fiber reinforcing materials, glass fibers, carbon fibers and aramid fibers, among others, are known to significantly improve the strength, stiffness and durability of polyurea while also enhancing its temperature and chemical stability [11]. Typically, these fibers are integrated into polyurea in the form of yarn or cloth, creating a composite material. Additionally, polyurea-based hybrid composites can be synthesized [12]. Previous research studies have demonstrated that polyurea-coated fiber-reinforced composites can enhance the impact resistance of concrete slabs ^[13][14][13,14]. Furthermore, the combination of polyurea coatings with carbon fibers and basalt fiber-reinforced polymer reinforcement techniques has shown positive effects in enhancing the blast resistance of urban utility tunnels [15]. The use of glass-fiber reinforced polyurea materials has also been found to enhance the bullet intrusion resistance of steel plates [16]. In summary, the incorporation of various reinforcing materials into polyurea holds great potential for improving its properties and expanding its application range. In a study conducted by J. Lv et al. [17], a hierarchical interfacial phase with high interfacial shear strength and toughness was created in an aramid composite through in situ grafting and foaming of polyurea on the fiber surfaces, as well as epoxy infiltration into the pores of the aramid composite. This resulted in the construction of a “rigid-flexible” interlocking three-dimensional interfacial structure, further increasing the interfacial shear strength and toughness of aramid fiber composites. N.V. Vuong et al. [18] developed different types of composites consisting of corrugated glass fibers/vinyl ester and polyurea using a conceptual composite panel inspired by mollusk shell pearl laminates. Various interlocking corrugated laminates were simulated and compared with planar and conventional dog-bone interlocking laminates, demonstrating a significant improvement in the performance of this composite under blast and impact loading. Particulate fillers, including silica sand, alumina, carbon black and nanoparticles, are recognized for enhancing the hardness, abrasion resistance and durability of polyurea, as well as for improving its thermal and chemical stability ^[26][27]. A study conducted by A.S. Roy et al. ^[26][27] used a detailed all-atom MD model confirmed these results. Typically, granular filler materials are mixed into polyurea in the form of powder or granules. For instance, Q. Liu et al. ^[27][28] performed quasi-static and dynamic compression tests on pure polyurea and polyurea/SiC nanocomposites with varying amounts of nanofillers at different strain rates using an electronic universal testing machine and a SHPB device. The researchers found that, in comparison to pure polyurea, the addition of nanoparticles influences on the compression properties. Under static loading, the nanocomposites with the content 1.5 wt% fillers greatly affected the compressive mechanical properties. However, under dynamic loading, the mechanical behaviors of nanocomposites with the additional amount of SiC (0.7 wt%) was observed to be more active compared to other nanocomposites. The reason may be that more cracks were formed on the inside of the specimens with the increased content of particles under a high stain rate which leads to the decrease of mechanical properties. Nano-reinforcement materials such as nano-oxides, carbon nanotubes and nanofibers, can enhance the strength, toughness and durability of polyurea while improving its thermal and chemical stability. These nano-reinforcement materials are typically incorporated into polyurea as nanoparticles. G. Wu et al. ^[28][29] developed a novel highly elastic protective coating by reinforcing polyurea with nano-silica filler composites. The polyurea material exhibited a tensile strength of 15.7 MPa and an elongation at break of 472%. Application of the polyurea coating resulted in a 9.7 kJ/m² increase in the impact strength of the substrate, while maintaining good mechanical properties and ductility. Simulation results indicated that the polyurea coating could effectively mitigate the impact caused by the ball’s equivalent force at different velocities.

3. Protection Mechanism under Blast Impact Loading and Ballistic Penetration

Polyurea is a high-performance polymer with outstanding protective properties, making it suitable for resisting blast impact loading and ballistic penetration. Its protection mechanism encompasses several aspects. Firstly, the complex structure formed by the hard and soft segments of polyurea provides it with high strength and toughness. Consequently, when subjected to impact loading or ballistic penetration, polyurea effectively withstands external forces using its strength and toughness. Secondly, polyurea exhibits excellent energy absorption capabilities, allowing it to absorb and disperse the energy from external impact loading and ballistic penetration, thereby safeguarding the protected objects. Moreover, polyurea possesses the ability to undergo deformation in response to external forces, thereby dispersing and mitigating their effects and ultimately protecting the objects within. Additionally, polyurea’s chemical stability ensures that its performance remains unaffected under diverse environmental conditions. This stability prevents any chemical reactions or decomposition from occurring when exposed to external impact loading and ballistic intrusion, further contributing to the protection of the object.

3.1. Hydrogen Bond Dissociation and Reorganization, Rearrangement and Hardening of Soft and Hard Segments

Polyurea is known for its high strength, stiffness, hardness, flexibility and toughness, which can be attributed to the presence of hydrogen bonding within its molecules. The hardening of polyurea is achieved through the dissociation of hydrogen bonds and the reorganization of soft and hard segments via a heat curing reaction. During this reaction, the amide and urea bonds within the polyurea molecules are broken and reorganized, resulting in the formation of new hydrogen bonds and molecular chain cross-links. This cross-linking process enhances the strength and hardness of polyurea ^[30][34][35][31,38,39]. To investigate the temperature-dependent microscale impact response of polyurea at a fixed impact velocity, Y. Sun et al. ^[36][40] observed an increased absorption of localized impact energy at approximately 115 °C, which corresponds to the transition temperature from the glassy to the rubbery state when subjected to high-speed dynamic loading. Notably, materials that exhibit a wider temperature range in the glass transition zone and lower microphase segregation demonstrate superior flexibility and energy absorption under high strain rate loading conditions ^[37][38][41,42]. The soft phase exhibits superelasticity, while the hard phase demonstrates elastoplastic behavior. Through a combination of experimental and simulation analysis, M.H. Jandaghian et al. ^[39][43] discovered that the performance of the formulation in response to low-intensity impacts (such as seismic waves) is primarily influenced by the soft phase. On the other hand, the interaction between the two phases determines the formulation’s overall resistance against projectile penetration into the structure, with the hard phase playing a key role in response to high-intensity indirect impacts (such as blast shockwaves). The ductility of the material increases proportionally with the length of the soft section, while the tensile strength decreases as the length of the soft section increases. The frequency required to initiate the dynamic transition process from the “rubber” to “glass” state is directly proportional to the length of the soft section ^[40][44]. Remarkably, all analyzed formulations exhibit an elastic response even under typical high-frequency blast loading conditions. Both the length of the soft segments and the type of hydrogen bonding significantly influence the impact response ^[41][45]. The impact-induced changes in the chain segments are primarily caused by bending and torsional bonding and the molecular potential energy is predominantly stored in the soft mid-segments. Upon impact, the ordered arrangement of the hard segments is disrupted, resulting in a reduction in the number of hydrogen bonds. The dissociation of hydrogen bonds leads to a substantial increment in the potential ^[37][42][41,46]. Additionally, the soft phase stores a greater amount of strain energy compared to the hard phase under impact. Conversely, the hard phase dissipates plastic energy through hydrogen bond dissociation and structural disruption, which is more prominent at stronger shocks ^[43][47]. Polyureas can undergo hardening through the dissociation of hydrogen bonding and rearrangement of the soft and hard segments via a light-curing reaction. In this reaction, the amide and urea bonds within the polyurea molecule are fragmented and restructured to create new hydrogen bonds and molecular chain cross-links. As a result, the polyurea molecules become interconnected, enhancing the strength and hardness of the material. This reaction necessitates a specific light intensity and duration, typically achieved through ultraviolet or visible light irradiation.

3.2. Viscous Dissipation and Strain Rate Effects within the Material

The viscous dissipation and strain rate effects within polyurea materials primarily depend on the structure and movement mode of the polyurea molecules themselves. The polyurea molecule consists of two distinct structural units: the hard segment and the soft segment. The hard segment is formed through the reaction of diisocyanate and diol, resulting in a polyurethane structural unit with high strength and stiffness. In contrast, the soft segment is formed through the reaction of long-chain diol and dibasic acid, giving rise to a polyester structural unit with high flexibility and toughness. This combination of hard and soft segments imparts polyurea with both strength and flexibility, enabling it to undergo deformation when subjected to external stresses and thereby consuming energy ^[44][48]. The dissipation of shock wave energy occurs through three mechanisms: (1) thermal dissipation, (2) viscous dissipation and (3) plastic dissipation. Heat is dissipated due to viscosity and internal friction within the material. Viscous dissipation refers to the incomplete relaxation of molecular chains in a short period of time, resulting in the retention of potential energy. Plastic dissipation primarily occurs in the hard phase of the material. The mesoscale inhomogeneous two-phase structure must undergo deformation coordination during loading, leading to a significant lateral displacement of the soft phase. This displacement increases the deformation energy and frictional heat of the molecular chains ^[43][47]. Yao et al. ^[42][46] discovered that polyurea with a lower content of hard segments exhibits higher energy dissipation when the shock is released under the same impact pressure. The main mode of energy dissipation is through heat dissipation, which arises from an increase in kinetic energy. Unlike in a tensile simulation, under impact loading the increase in molecular potential energy is primarily partitioned into the increments of bonding energy, angular energy and dihedral angular energy, with the majority of these increments stored in the soft segments. During high-velocity impacts, the increment in hydrogen bonding potential accounts for only around 1% of the internal energy increment. The motion mode of polyurea molecules plays a significant role in their viscous dissipation and strain rate effects. The presence of hydrogen bonding between polyurea molecules causes the bonds to break and rearrange under external stress, resulting in various modes of motion such as rotation, slippage and twisting. These modes of motion generate friction and sticking between the polyurea molecules, leading to viscous dissipation and strain rate effects. Notably, the strain rate effect becomes more pronounced at higher strain rates ^{[45][46][47][48]}[20,49,50,51]. An increase in strain rate leads to higher rheological stress, compressive strength, strain rate sensitivity and strain energy, which can enhance the protection of structures against blast and shock loading ^[27][49][50][28,52,53]. Wu et al. ^[51][54] conducted an investigation on the enhancement properties of coated polyurea on localized damage of 6063-T5 aluminum alloy tubing using static and dynamic mechanical property tests, explosion tests and numerical simulation calculations. Their findings reveal that the AP103 polyurea exhibits a strain-rate sensitive effect during tensile testing, with a noticeable elastic phase followed by a slight strain-hardening phase. In dynamic compression experiments, the polyurea exhibits a significant nonlinear stress-strain relationship. At low strain rates, polyurea displays superelastic properties, whereas at high strain rates, it exhibits clear yield slip, strain-hardening properties and strain rate effects. In conclusion, the viscous dissipation and strain rate effects in polyurea are predominantly influenced by the molecular structure and motion modes. These effects contribute to polyurea’s remarkable energy absorption and stability capabilities under high stress-strain rates, making it highly promising for a wide range of applications requiring high strength and high speed.

3.3. Impedance Mismatch between Base Material and Polyurea

3.3. 基材和聚脲之间的阻抗不匹配

Impedance mismatch occur when there are interfacial reflections and transmissions between the substrate and polyurea leading to potential problems like energy loss and signal attenuation. This mismatch is primarily caused by variations in physical parameters such as dielectric constant, acoustic wave velocity and density between the substrate and polyurea. Insufficient interfacial adhesion is a common source of impedance mismatch, resulting from factors such as differences in chemical composition, surface morphology and roughness of the substrate surface. These factors can weaken the bond between the materials and give rise to issues like interfacial peeling, crack expansion and material separation ^[52][53][54]. Another cause of impedance mismatch is the disparity in coefficient of thermal expansion between the substrate and polyurea. When temperature changes occur, the substrate and polyurea may undergo different degrees of thermal expansion, resulting in stress and strain discrepancies. This mismatch can lead to interfacial shear stresses, stress concentrations and subsequent material damage and degradation. Differences in mechanical properties between the substrate and polyurea also contribute to impedance mismatches. For example, the substrate may possess higher stiffness and strength, while the polyurea exhibits greater toughness and energy absorption. This mismatch can result in interfacial stress concentrations and failures, impacting the overall performance of the material ^{[55][56][57][58]}. Insufficient chemical compatibility between the substrate and polyurea can give rise to impedance mismatch as well. This chemical mismatch may cause issues such as interfacial reactions, dissolution or corrosion, ultimately affecting the performance and durability of the material. Surface energy differences between the substrate and polyurea can also contribute to impedance mismatches. These disparities can make processes such as coating, bonding or wetting more challenging, thereby influencing the interfacial properties and durability of the material.当基材和聚脲之间存在界面反射和透射时，就会发生阻抗失配，从而导致能量损失和信号衰减等潜在问题。这种不匹配主要是由物理参数的变化引起的，例如介电常数、声波速度和基材和聚脲之间的密度。界面附着力不足是阻抗失配的常见原因，这是由化学成分、表面形态和基材表面粗糙度等因素引起的。这些因素会削弱材料之间的结合，并引起界面剥落、裂纹扩展和材料分离等问题[55,56,57]。阻抗不匹配的另一个原因是基材和聚脲之间的热膨胀系数存在差异。当温度发生变化时，基材和聚脲可能会发生不同程度的热膨胀，从而导致应力和应变差异。这种不匹配会导致界面剪切应力、应力集中以及随后的材料损坏和降解。基材和聚脲之间机械性能的差异也会导致阻抗不匹配。例如，基材可能具有更高的刚度和强度，而聚脲则表现出更大的韧性和能量吸收。这种不匹配会导致界面应力集中和失效，从而影响材料的整体性能[58,59,60,61]。基材和聚脲之间的化学相容性不足也会导致阻抗失配。这种化学不匹配可能会导致界面反应、溶解或腐蚀等问题，最终影响材料的性能和耐久性。基材和聚脲之间的表面能差异也会导致阻抗不匹配。这些差异会使涂层、粘接或润湿等工艺更具挑战性，从而影响材料的界面性能和耐久性。 From a macroscopic standpoint, the impedance mismatch between the polyurea and the substrate is the primary factor contributing to the polyurea’s resistance to explosion and impact. 从宏观角度来看，聚脲与基材之间的阻抗不匹配是影响聚脲抗爆和抗冲击的主要因素。为了减轻这种不匹配导致的材料性能下降，可以提高聚脲和基材之间的界面强度[56]。此外，当由于材料和施工方法的差异而无法改变界面特性时，可以采用阻抗失配的定量设计来达到所需的保护目标[62]。故意制造阻抗失配的目的是在材料特性和能量吸收之间取得平衡。To mitigate the degradation of material properties resulting from this mismatch, enhancing the interfacial strength between the polyurea and the substrate can be pursued ^[53]. Additionally, when it is not feasible to alter interfacial properties due to disparities in materials and construction methods, a quantitative design of impedance mismatch can be employed to attain the desired protective objectives ^[59]. The deliberate creation of impedance mismatch aims to strike a balance between material properties and energy absorption. In the design of protective multilayer armour, T. Rahimzadeh et al. ^[59] utilized finite element analysis and determined that the outer layer of the armour should have a higher acoustic impedance than its neighboring layers. This allows for multiple reflections at the interface between the two layers, effectively tuning the wave. However, it is essential to ensure that the impedance mismatch is not excessive, as this can result in inefficient transmission of the stress wave across successive layers.等[62]在多层防护装甲的设计中，利用有限元分析，确定装甲的外层应比其相邻层具有更高的声阻抗。这允许在两层之间的界面处进行多次反射，从而有效地调整波。然而，必须确保阻抗不匹配不会过多，因为这会导致应力波在连续层之间的低效传输。 Low-thickness polyurea coatings have been found to increase the frequency of wave reflections at the high-impedance polyurea研究发现，低厚度聚脲涂料会增加高阻抗聚脲/steel interface, leading to a significant increase in the pressure level and instantaneous specific energy density of the polyurea ^[60]. However, increasing the elastic resistance of the steel plate has the opposite effect on blast resistance. In a field explosion test conducted by 钢界面处的波反射频率，从而显著提高聚脲的压力水平和瞬时比能量密度[63]。但是，增加钢板的抗弹性对抗爆性有相反的作用。在G. Wu et al. ^[61], it was observed that when the impact side was sprayed with a thin polyurea layer, the unloading wave inside the polyurea layer could not catch up with the loading wave in time. Consequently, the compression wave carrying more energy passed through the polyurea layer and directly impacted the steel plate, resulting in more severe damage. The bonding strength between the polyurea layer and the steel plate also plays a crucial role in impact resistance. Premature debonding of the polymer from the substrate can prevent the coating from maximizing its energy absorption effect ^[62]. 等[64]进行的现场爆炸试验中，观察到当冲击侧喷涂一层薄薄的聚脲层时，聚脲层内部的卸荷波无法及时赶上加载波。因此，承载更多能量的压缩波穿过聚脲层，直接撞击钢板，造成更严重的损伤。聚脲层与钢板之间的粘结强度在抗冲击性方面也起着至关重要的作用。聚合物与基材的过早脱粘会阻止涂层最大限度地发挥其能量吸收效果[65]。L. Zhang et al. ^[63] investigated the blast resistance of 等[66]研究了ASTM 1045 steel plates reinforced with polyurea of varying mechanical properties and observed that the early overall collapse of high ductility polyurea coated on the front side of the plate severely limited the protective effect of the polyurea. Consequently, the damage to the plate was not significantly reduced under loading. The high ductility polyurea coating on the front side optimized the impedance relationship within the target, reducing the reflected load and attenuating the damage through the unloading effect of the loaded wave. However, the effectiveness was diminished due to the debonding of the polyurea and steel plate. Coating the backside of the target plate with highly ductile polyurea allowed for the dissipation of impact energy within an appropriate timeframe while reflecting and unloading the stress wave. This greatly improved the explosive resistance of the target plate.钢板用不同力学性能的聚脲增强的抗爆性能，观察到涂覆在板正面的高延展性聚脲的早期整体倒塌严重限制了聚脲的保护效果。因此，在负载下对板的损坏并没有显着减少。正面的高延展性聚脲涂层优化了目标内部的阻抗关系，通过负载波的卸载作用降低了反射载荷并减轻了损伤。然而，由于聚脲和钢板的脱粘，有效性降低。在靶板的背面涂上高延展性的聚脲，可以在适当的时间范围内消散冲击能量，同时反射和卸载应力波。这大大提高了靶板的抗爆性。 To investigate the interfacial impedance at the nanoscale, 研究纳米尺度的界面阻抗。Y. Chen et al. ^[64] conducted 等[67]进行了MD simulations to analyze the process of excitation wave premelting and dispersion of single-crystal copper when subjected to cylindrical convergent impacts. Their findings revealed that the premelted zone near the free surface experienced spalling off after unloading due to the formation of a stretching zone near the free surface, caused by the interaction between the reflected wave and the unloaded wave. The impact damage can be effectively mitigated by the presence of a polymer layer on the impact surface at the nanoscale. However, the reverberations of the shock wave weaken the polymer layer on the back side. In a separate study, 模拟，分析了单晶铜在圆柱形收敛冲击下的激发波预熔化和色散过程。他们的研究结果表明，由于反射波和空载波之间的相互作用，在自由表面附近形成了拉伸区，因此自由表面附近的预熔区在卸载后发生了剥落。通过在纳米尺度的冲击表面上存在聚合物层，可以有效地减轻冲击损伤。然而，冲击波的混响削弱了背面的聚合物层。在另一项研究中，M.A.N. Dewapriya et al. ^[65] performed 等[68]对多层纳米结构的弹道冲击试验进行了MD simulations of ballistic impact tests on multilayered nanostructures. The results showed that the ultrathin polyurea layer applied to the impact surface effectively redistributes the impact load to the underlying metal layer, resulting in improved energy absorption.模拟。结果表明，施加在冲击表面的超薄聚脲层有效地将冲击载荷重新分配到下面的金属层，从而改善了能量吸收。