制备条件对水凝胶力学性质的影响机理研究

水凝胶等软物质材料具备诸多优点，例如优异的柔韧性，较强的环境适应性，灵活可调的力学性能以及良好的生物相容性等，其可在微小的外界环境刺激下（例如细微的温度或载荷变化）产生较大的变形。在未来的科学和工程领域发展中，作为一个重要前沿方向，软材料会在人类社会的生产和生活中起到举足轻重的作用。因此，研究水凝胶等软物质材料的基本力学性质以及影响这些力学性质的因素和背后的理论机理，对于推动软物质材料的广泛应用具有至关重要的意义。对于传统的固体结构性材料，其力学性质不仅与构成该材料的化学成分有关，更与其内部微观结构等因素有关，大量的研究表明结构性材料的力学性质会受到诸多因素的影响，例如合成该材料时的初始制备条件，后续的加工过程以及微观结构等。然而对于水凝胶等软物质材料，目前关于除了其材料构成组分以外的其他因素对其力学性质影响以及机理的研究还非常有限，例如软材料内部聚合物网络的微观结构，制备软材料时的初始条件以及随后的制备流程等因素，都会对其力学性质产生较为显著的影响，这对水凝胶等软材料在各科学和工程领域的应用带来了挑战。从微观结构的层面出发，研究软材料的制备条件和制备流程对其微观网络结构产生的影响，从而研究微观结构的改变对软材料的力学性质产生的影响及其背后的物理机理，是软材料力学研究的重要方向之一。本文以聚丙烯酰胺为研究对象，分别研究了初始制备条件和制备流程的改变对聚丙烯酰胺水凝胶的弹性刚度，断裂韧性，疲劳阈值以及溶胀性能的影响，并从微观网络结构的层面解释了这些影响因素背后的物理机理。文中的主要研究工作包括：

（1）从实验和理论上研究分析了初始制备条件和制备流程的改变对聚丙烯酰胺水凝胶弹性模量的影响，建立了可以精准描述水凝胶的弹性模量随其初始制备条件和制备流程变化关系的理论标度律模型，实现了在各种制备状态以及溶胀和退溶胀状态下精确预测水凝胶的弹性模量，并且提出了一种当化学交联剂不足时拓扑交联点补偿化学交联点的理论思路。实验结果表明，通过溶剂交换至组分完全相同的水凝胶会表现出差异很大的力学行为，水凝胶的弹性刚度在很大程度上依赖于其制备条件和制备流程。通过该模型可以实现对水凝胶弹性刚度的灵活调控。

（2）通过实验测量和理论分析，研究了聚丙烯酰胺水凝胶的断裂韧性和疲劳阈值随其初始制备条件和制备流程的变化关系，建立了精准描述初始制备条件和制备流程对水凝胶疲劳阈值影响机理的理论模型，提出了悬挂链和网络缺陷导致的粘弹性是造成退溶胀状态水凝胶断裂韧性剧烈增加主要原因，成功地解释了初始制备状态以及溶胀和退溶胀状态下水凝胶的断裂韧性随其初始体积分数的变化规律，阐明了制备条件和制备流程对水凝胶断裂韧性的影响机理。实验结果表明水凝胶的断裂韧性和疲劳阈值在很大程度上会受其制备条件和制备流程的影响，尤其是对于初始体积分数很低接近临界成胶点的水凝胶，退溶胀会导致其断裂能的急剧增加，甚至接近一些双网络水凝胶的断裂能。这部分研究在一定程度上揭示了聚合物凝胶的制备条件与力学性质之间的耦合关系，这对于未来设计制造高韧性的新型软材料及其在各领域中的广泛应用都具有重要意义。

（3）从微观网络结构的层面出发，通过实验和理论系统地分析研究了聚丙烯酰胺水凝胶的溶胀性能与初始制备条件之间的耦合关系，提出了一种新的计算混合自由能的模型，代替了Flory-Huggins模型，并改进了Flory-Rehner溶胀理论中的弹性自由能部分，建立了描述水凝胶溶胀性能随初始制备条件变化关系的完整理论模型。实验结果表明，在不同交联剂/单体摩尔比的制备条件下合成的一系列水凝胶会表现出差异很大的溶胀性能。通过对不同范围内的交联剂/单体摩尔比下水凝胶的聚合物网络结构进行分析研究，建立了精准预测平衡溶胀率随制备浓度以及交联密度变化关系的理论模型。在以上关于平衡溶胀率研究的基础上，通过调整溶剂的渗透压来改变水凝胶与溶剂分子相互作用的净渗透压，研究分析了在非平衡状态下时水凝胶的溶胀行为随制备浓度和交联密度的变化关系，建立了在非平衡状态下描述水凝胶溶胀和退溶胀行为的完整理论模型。这部分研究对于水凝胶等软材料在卫生用品等工业领域中的应用具有重要的价值，例如对于一些高吸水性的聚合物软材料产品，利用该溶胀模型理论上可以通过控制初始制备条件来使材料的吸水性达到最优化。

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