聚合物动力学过程对其变形与破坏的影响机制研究

聚合物软材料因其良好的柔韧性、变形回复能力和环境适应性，被广泛应用于组织工程、软体机器人、柔性电子等诸多领域。其优异的力学性能不仅是实现未来新兴领域应用的基础，也对人类社会的生产生活发展具有重要意义。因此，深入研究聚合物软材料的宏观力学性能及其影响因素，对推动这一材料及其应用领域的发展至关重要。聚合物软材料的力学性能不仅取决于化学组分，还在很大程度上由其网络结构决定。聚合物网络动力学过程中所涉及的化学和力学问题，对理解影响软材料变形与破坏因素及其潜在理论机制至关重要。然而，目前有关聚合物动力学过程对其变形与破坏的影响机制相对较少。因此，深入研究聚合物动力学过程对其变形与破坏的影响机制对推动聚合物软材料及其应用领域的发展具有重要意义。本文从聚合物动力学过程出发，以利用可控聚合制备和具有不同相互作用的聚合物软材料为研究对象，对影响聚合物软材料的模量、流变、溶胀、断裂韧性、疲劳阈值及加载速率相关的疲劳行为进行了细致研究。本文的主要工作包括：

（1）基于可控聚合的方法构建了网络拓扑结构与宏观力学性能的关系。利用可逆加成断裂链转移（RAFT）聚合法制备了一系列具有可控力学性能的聚丙烯酰胺水凝胶，并揭示了这些力学性能变化背后的机制。结合实验和理论研究，深入探讨了链转移剂对网络拓扑结构的影响。不同的力学性能，如剪切模量、流变学响应和平衡膨胀比等，被归因于不同的网络拓扑结构，即悬挂链与有效弹性链的比例。聚合物网络结构的蒙特卡罗模拟结果与实验观察相一致，验证了拓扑结构在水凝胶力学性能中的作用。

（2）通过以RAFT聚合的聚丙烯酰胺（RAFT-PAAm）水凝胶和聚丙烯酸丁酯（RAFT-PBA）弹性体为代表的两种聚合物软材料，本文明确了网络拓扑结构变化对这些材料断裂与疲劳特性的影响。考虑到网络拓扑结构的改变带来的链长及弹性骨干的变化，结合蒙特卡罗模拟结果提出了描述RAFT聚合制备软材料疲劳阈值的修正Lake-Thomas模型，实验测得的疲劳阈值与该理论模型十分吻合。此外，通过RAFT-PBA弹性体在不同加载速率下的疲劳行为，提出了以悬挂链缠结为主要粘性贡献的材料的疲劳行为主要由加载时间决定，加载速率的影响较小。

（3）澄清了与黏弹性材料疲劳测试相关的两个常见误区：即表观阈值和通过积分稳态加载曲线得到的阈值，两者均高度依赖于加载速率。实验结果表明，物理键的贡献直接影响了这两个阈值随加载速率的变化。相比之下，通过积分卸载部分曲线得到的真实阈值是与加载速率无关的，并与静态疲劳阈值一致。此外，观察到的加载速率依赖的疲劳裂纹扩展“速度阶跃”现象直接与加载速率引起的弹性体的韧性-脆性转变相关。

（4）本研究探讨了链断裂与重组的竞争对软材料疲劳的影响。选取无化学交联的P(HEA-co-BA-co-AA)弹性体（链重组为主导）和有化学交联的聚丙烯酸羟乙酯（PHEA）弹性体（链断裂为主导）作为研究对象，研究了两种弹性体在不同加载速率下的疲劳损伤、动态疲劳裂纹扩展以及静态疲劳特性。结果显示，链重组主导的弹性体在低加载速率下表现出更出色的抗疲劳特性，而对于链断裂主导的弹性体，在高加载速率下展现出更卓越的抗疲劳能力。该研究阐明了链断裂与重组在聚合物软材料疲劳破坏中随加载速率变化的竞争关系，为设计具备更高抗疲劳性能的聚合物软材料提供了指导。

Rubbery polymers, renowned for their excellent flexibility, deformability, and adaptability to various environments, have found extensive applications in diverse fields such as tissue engineering, soft robotics, and flexible electronics. The outstanding mechanical properties of these materials serve as the foundation for emerging applications and hold significant importance for advancing human society in both production and daily life. Therefore, unveiling the influencing factors of macroscopic mechanical properties of rubbery polymers is crucial for driving material development and application forward. The chemical and mechanical issues involved in the kinetic processes of polymer network, which are crucial to understanding the factors affecting the mechanical properties of rubbery polymers and their potential theoretical mechanisms. However, studies on the effects of kinetic processes on deformation and fracture of rubbery polymers are still relatively scarce. Therefore, in-depth research on the effects of kinetic processes on deformation and fracture of rubbery polymers is of great importance to the development of rubbery polymers and their applications. This work takes polymer kinetic processes as the starting point and focuses on rubbery polymers prepared by controlled polymerization with different interactions. We systematically investigate the effects of network structure on the modulus, rheology, swelling, fracture toughness, fatigue threshold, and loading rate-dependent fatigue behavior of rubbery polymers. The main contributions of this thesis are as follows:
(1) The relationship between network topology and macroscopic mechanical properties is constructed based on the method of controlled polymerization. A series of polyacrylamide hydrogels with controllable mechanical properties were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and the mechanisms behind the changes in these mechanical properties were revealed. The effect of chain transfer agents (CTA) on network topology was investigated in depth by combining experimental and theoretical studies. Different mechanical properties, such as shear modulus, rheological response, and equilibrium swelling ratio, were attributed to different network topologies, namely the ratio of dangling chains to effective elastic chains. The Monte Carlo simulation results of the polymer network structure agreed with the experimental observations, which verified the role of topology in the mechanical properties of hydrogels.
(2) Examining two representative polymer soft materials, namely RAFT-PAAm hydrogels and RAFT-PBA elastomers, the influence of changes in network topology on fracture and fatigue properties has been clarified. By considering the changes in chain length and elastic backbone caused by changes in network topology, and combining Monte Carlo simulation results, a modified Lake-Thomas model is proposed to describe the fatigue threshold of soft materials prepared by RAFT polymerization. Experimental fatigue threshold values closely match the predictions of this theoretical model. Combined with the fatigue behavior of RAFT-PBA elastomer under different loading rates, it is proposed that the fatigue behavior of materials with suspended chain entanglement as the main viscosity contribution is mainly determined by the loading time, and the impact of the loading rate is small.
(3) Two common misunderstandings related to fatigue testing of viscoelastic materials are clarified: the apparent threshold and the threshold obtained by integrating the steady-state loading curve, both of which are highly dependent on the loading rate. Experimental results indicate that the contribution of physical bonds directly affects the variation of these two thresholds with loading rate. In contrast, the true threshold obtained by integrating the unloaded portion of the curve is loading rate-independent and consistent with the static fatigue threshold. Additionally, the observed "speed jump" phenomenon in fatigue crack propagation, dependent on loading rates, is directly related to the ductile-brittle transition induced by loading rates in the elastomer.
(4) The effect of chain scission and rearrangement on the fatigue of rubbery polymer was studied. Taking P(HEA-co-BA-co-AA) elastomer without chemical cross-linking (chain rearrangement dominant) and chemical cross-linking PHEA elastomer (chain scission dominant) as the research objects, the two properties were studied under different loading rates. Fatigue damage, dynamic fatigue crack growth, and static fatigue characteristics of various elastomers. The study found that elastomers dominated by chain rearrangement have better fatigue resistance at low loading rates. Compared, elastomers dominated by chain scission show better fatigue resistance at high loading rates. This study elucidates the competitive relationship between chain scission and rearrangement in the loading rate-dependent fatigue failure of elastomers, guiding the design of rubbery polymers with higher fatigue resistance.

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