[论文解读] Micromechanical Analysis of Strength of Polymer Networks with Polydisperse Structures

Mohammad Jafari Tehrani|arXiv (Cornell University)|Oct 5, 2017

Polymer Nanocomposites and Properties被引用 4

一句话总结

本文提出了一种微机械模型，解释了在未填充和填充弹性体中，聚合物网络链段长度的多分散性如何控制弹性及力学失效行为。通过模拟链段在形变下的断裂以及与填料颗粒的可逆吸附，该模型表明短链段引发损伤并决定材料强度，而多分散性对填充橡胶的疲劳寿命和Payne效应行为具有关键影响。

ABSTRACT

The effect of network chain distribution on the mechanical behavior of elastomers is one of the long-standing problems in rubber mechanics. The classical theory of rubber elasticity is built upon the assumption of entropic elasticity of networks whose constitutive strands are of uniform length. The kinetic theories for vulcanization, computer simulations, and indirect experimental measurements all indicate that the microstructure of vulcanizates is made of polymer strands with a random distribution of length. The polydispersity in strand length is expected to control the mechanical strength of rubber as the overloaded short strands break at small deformations and transfer the load to the longer strands. The purpose of the contributions presented in this thesis is to present simple theories of rubber mechanics that take into account the length distribution of strands and its effect on elasticity and the onset of bulk failure in unfilled and filled elastomers. In unfilled system, the population of short chains is identified as the culprits for damage initiation. Upon deformation of a polydisperse network, shorter strands break at considerably smaller stretches compared to the longer ones. The network alteration continues concurrent with increasing deformation and controls the onset of mechanical failure. In the filled networks, the degradation in network mechanical behavior is assumed to be controlled by the adhesive failure of the short strands adsorbed onto the filler surface. The finite extensibility of the short adsorbed strands is a key determinant of mechanical strength.

研究动机与目标

• 理解网络链段长度的多分散性如何影响弹性体的力学行为。
• 研究短链段在未填充聚合物网络中引发损伤并控制极限强度的作用。
• 模拟填料-聚合物相互作用对填充弹性体力学响应的影响，重点关注可逆吸附与链段脱附行为。
• 通过短链段在高度拉伸状态下逐步失效的机制，解释填充橡胶中的Payne效应。
• 提供一个将微观结构（链段长度分布）与宏观力学响应及失效起始相联系的理论框架。

提出的方法

• 采用Bueche和Watson对硫化物提出的指数函数建模链段长度分布。
• 应用Langevin统计描述链段在形变下的熵弹性张力，识别出短链段为失效起始点。
• 使用一级动力学方程描述链段在填料表面过渡区内的可逆吸附/脱附过程。
• 在有限形变下模拟网络响应，追踪链段的逐步断裂及吸附链段的损失。
• 引入Morse势能以表示链段与填料表面之间的键能，简化界面粘附作用。
• 预测不同应变和填料含量下的刚度、非线性起始点及储能模量行为。

实验结果

研究问题

• RQ1 网络链段长度的多分散性如何影响未填充弹性体的弹性和极限强度？
• RQ2 短链段在多分散网络中损伤起始与渐进性失效过程中发挥何种作用？
• RQ3 聚合物链段在填料颗粒上的可逆吸附如何影响填充弹性体的力学行为？
• RQ4 该模型在多大程度上能够解释Payne效应，包括应变软化及填料含量依赖性？
• RQ5 多分散性如何影响聚合物硫化物在循环载荷下的疲劳寿命？

主要发现

• 由于更高的熵弹性张力，多分散网络中的短链段在远小于长链段的拉伸程度下即发生断裂，从而引发损伤。
• 未填充网络的极限力学强度与短链段的数量直接相关，后者充当薄弱环节。
• 形变下链段的逐步断裂导致历史依赖性损伤，从而降低循环载荷下的疲劳寿命。
• 在填充网络中，软化行为（即Payne效应）源于短而高度拉伸的吸附链段的逐次脱附与失效。
• 模型成功预测了储能模量的应变软化行为，以及填料含量增加导致的非线性起始点提前和刚度对填料浓度的依赖性。
• 理论预测支持了多分散性同时控制损伤起始与填充橡胶硫化物力学响应的假设。