聚脲是一类具有软硬段交替结构的嵌段聚合物。由于热力学不相容性，软硬段相互排斥形成特征的纳米尺度微相分离结构。通常，硬段的刚性较强且存在分子间氢键，因此具有高于室温的玻璃化转变温度。玻璃态的硬域对橡胶态的软域基体起到了物理增强的作用。得益于特殊的微观结构，聚脲在单轴拉伸中表现出优异的力学性能，在冲击波载荷下表现出能量耗散性能。因此，聚脲能够应对局部战争中高速破片和爆炸冲击波的多重威胁，在军事防护领域得到了越来越多的关注。然而，由于微观结构的复杂性，宏观形变过程中聚脲分子尺度的结构演化机制尚不明确。本文建立了聚脲多尺度模拟方法，研究了单轴拉伸和冲击波加载两种形变过程中聚脲微观结构的演化过程，揭示了力学响应背后的分子机制。 针对聚脲特殊的多尺度结构，开发了适用于聚脲的全原子/粗粒化杂化分子模型，并与现有的全原子和粗粒化模型进行了比较。由于显式建模了氢键作用，杂化模型能够复现全原子模型中的氢键结构和链段运动能力的差异。这导致使用杂化模型的应力松弛模拟结果表现出更宽的松弛时间谱，与现有的粗粒化模型相比更符合实验结果。 基于杂化模型对聚脲单轴拉伸过程进行了分子动力学模拟。模拟得到的应力应变曲线与实验结果符合良好，捕捉到了聚脲特征的非线性力学响应。通过对微观应力与应变、硬域结构以及链段构象的分析，揭示了硬段自增强和软段应力适应释放这两种导致聚脲高韧性的分子机制，建立了微观结构与宏观性能之间的桥梁。 使用不同模拟方法对聚脲冲击波加载过程进行了模拟，计算了Rankine-Hugoniot关系，证明全原子分子动力学是更加合适的模拟方法。通过对微相结构贡献和能量转换路径的分析，发现冲击波能量耗散能力主要受到分子结构的影响。进一步的模拟研究揭示了原子的不规则堆砌和分子链扭转这两种冲击波动能转化为原子热运动动能的分子机制，为高冲击波耗散性能的材料设计提供了理论基础。
Polyurea are block copolymers with alternating hard and soft segments. Due to the thermodynamic incompatibility between hard and soft segments, the hard segments are aggregated and embedded in the soft matrix, forming a nanoscale segregated morphology. Typically, the hard segments are connected by hydrogen bonds between urea groups and therefore exhibit a glass transition temperature above room temperature. The glassy hard domains act as physical reinforcements to the rubbery soft domain matrix. Owning to the unique microstructure, polyurea exhibits excellent mechanical properties in uniaxial deformation and energy dissipation under shock wave loads. Therefore, polyurea helps protect against high-velocity fragments and explosive shockwaves in local wars, and has received more and more attention in the field of military protection. However, due to the complexity of the microstructure, the molecular-scale structural evolution of polyurea during macroscopic deformation remains elusive. In this paper, the multiscale simulation method of polyurea has been established. The structural evolution of polyurea during the two deformation processes of uniaxial elongation and shock wave loading is studied, and the molecular mechanism behind the mechanical response is revealed. Aiming at the multiscale structure of polyurea, a hybrid all-atom/coarse-grained (AA/CG) molecular model for polyurea is developed and compared with the existing AA and CG models. Since hydrogen bonds are modeled explicitly, the hybrid model reproduces the hydrogen bonding structure and the differences in the segment mobility in the AA model. This leads to a broader relaxation time spectrum in the stress relaxation results obtained from the hybrid model, which is in better agreement with the experimental data than the results obtained from existing CG model. The uniaxial elongation of polyurea is investigated through MD simulations based on the hybrid model. The stress-strain curves obtained from simulations are in good agreement with the experimental results, capturing the nonlinear mechanical response of polyurea. Through the analysis of microscopic stress and strain, hard domain structure and segment conformation, two molecular mechanisms of hard segment self-enhancement and soft segment stress adaptive release are revealed, bridging the gap between microstructure and macroscopic properties. The shock wave loading of polyurea is simulated by different simulation methods, and the Rankine-Hugoniot relationships are calculated, which proved that AA MD is a more suitable simulation method. By analyzing the contribution of microphase structure and the energy conversion path, it is found that the energy dissipation capacity of the shock wave is mainly affected by the molecular structure. Further simulation studies revealed the two main mechanisms of the conversion of shock wave energy into atomic thermal kinetic energy, which are the irregular packing of atoms and the twisting of molecular chains, providing a theoretical basis for the design of materials with high shock wave dissipation performance.
