老化诱致导电高聚物复合材料压阻效应衰变规律的研究

Aging, one of the bottleneck constraints in the application of conductive polymer composites, affects their conductive properties and leads to the attenuation of the piezoresistive effect. The possible mechanisms include the aging damage, cross-linking, and fracture of molecular chains. These mechanisms result in the change of potential barriers between conductive inclusions, and further affect the constitutive relationships of the materials. In the viewpoint of mechanics, such a problem is on the attenuation of piezoresistive effect that is caused by the aging constitutive relation. In this proposal, the evolution of damage and configuration of molecular chains are studied. Not only damage and the viscoelastic strain, but also the number of free radical of molecular chains, are selected as inner variables for establishing the Helmholtz's free energy function. By means of this method, aging is introduced in a thermal dynamics frame for studying the constitutive relations of the materials. The influences of aging damage, mechanical properties, cross-linking and fracture of materials on their conductivity and piezoresistivity are experimentally investigated. In combination with the study of constitutive relations, a new model of the aging-induced attenuation of piezoresistive effect is proposed.

制约导电高聚物复合材料长期应用的主要瓶颈之一在于材料的老化。老化既影响材料的导电性能，又影响其压阻效应。影响机制包括老化过程中导电夹杂与基体变形的不协调形成的损伤，以及高分子链的交联与断裂。两者都会增加导电夹杂之间的势垒，从而弱化其导电性能；两者都会影响材料的本构关系，从而导致压阻效应的衰变。从力学的角度看，正是由于老化带来了材料力学性能的改变，从而导致了压阻效应衰变。本项目在理论方面，从老化的损伤和分子链构型的演变入手，不仅选择老化损伤、粘性应变，而且选择直接反映老化的分子链自由基数为内变量，以构造Helmholtz自由能密度函数，将老化问题自然地纳入热力学框架来构建导电复合材料的本构关系。在实验方面，通过开展老化实验考察材料的老化损伤、材料性能、分子链构型的变化对材料的导电性与压阻效应的影响。结合本构关系的研究，给出压阻老化衰变的理论模型。

本项目研究了老化条件下导电高聚物的力学性能和导电性能的演化规律。取得了3方面的创新性成果：1）导电高聚物的势垒（电学性能指标）与材料的松弛时间（力学性能指标）之间服从幂律关系。这是全新的结果，该结果的本质是揭示了材料的粘性效应对材料导电性能的作用。2）得到了两个重要的关系：第一，材料势垒随老化时间线性减小；第二材料的能量耗散随老化时间线性减小。这两点是定量地揭示了老化过程对材料的电学性能和力学性能的影响。3）发现了材料的导热系数和热膨胀系数并非常数，而是环境温度的线性函数。这一发现具有重要意义，表明热传导过程是一个非线性过程。项目研究成果对于导电高聚物的材料设计与分析有重要的指导作用。