On the effect of thermo-mechanical and chemo-mechanical coupling on the structural and functional properties of shape memory polymers (SMPs) II: Effect of superimposed mechanical stresses during chemically affected actuation and exploring the strong coupl

在DFG优先项目SPP 1733中，来自材料科学与工程、连续介质力学和聚合物物理三个学科的研究人员联合开展了这一项目，探索形状记忆聚合物(SMP)在工程执行器应用中的潜力。SMP致动器利用单向效应(1WE)，材料首先变形为高应变。然后，新的几何形状保持稳定，直到温度升高到临界值以上。然后这种材料就会恢复到原来的形状。此外，对于SMP执行器，需要考虑化学机械相互作用。SMP执行器可以在化学环境中很好地工作，在这种环境中，像水这样的小分子可以扩散到SMP(物理老化)中。这会影响所有功能材料属性。在第一个资助期，研究表明，1WE效应可以被化学触发。分子尺度研究(聚合物物理学)强调了颗粒大小在这里的作用。在考虑在机械工程应用中使用SMP致动器之前，需要了解和控制这种类型的化学/机械耦合。在过去三年中，该项目建立了一个实验数据库，用于将丙酮、乙醇和水分子扩散到Estane ETE 75DT3型商业参考SMP中。建立了程控线性无约束SMP执行器的化学开关动力学模型。第一个资助期的实验结果为分子(聚合物物理)和宏观(连续介质力学)长度尺度的新模型的设计提供了基础。目前对SPP 1713第二个资助期的提议有三个新的科学目标：第一，将考虑不同水平的叠加应力如何影响化学触发的1WE。必须提供实验基准数据，并且需要从分子和连续介质力学的角度来理解叠加机械应力的影响。其次，当小分子扩散到SMP中时，它们建立了随时间演变的浓度分布。存在相关的瞬变情况，其中表面区的小分子浓度高到足以触发单向效应，而材料的内部仍然不受影响。这导致材料中的应力和应变分布不均匀，并随着时间的推移而变化。这一演变将从实验和理论上加以阐述。第三，在简单的实验条件下建立了对所有相关基本过程的基本理解之后，有必要处理以多轴应力状态和逐渐变化的应力和温度为特征的更复杂的情景。还必须确定哪些工艺会导致功能材料性能的退化，并最终延长SMP执行器组件的可开发使用寿命。

Within the DFG priority programme SPP 1733, this joint project between researchers from three disciplines, materials science and engineering, continuum mechanics and polymer physics explores the potential of shape memory polymers (SMP) for engineering actuator applications. SMP actuators exploit the one way effect (1WE), where a material is first deformed to high strains. The new geometry then remains stable, until the temperature is raised above a critical value. The material then recovers its original shape. In addition, for SMP actuators, chemical mechanical interactions need to be considered. SMP actuators may well operate in chemical environments, where small molecules like water can diffuse into the SMP (physical aging). This affects all functional material properties. In the first funding period, it was shown that 1WE effect can be chemically triggered. Molecular scale studies (polymer physics) highlight the role of particle size here. This type of chemical/mechanical coupling needs to be understood and controlled, before a use of SMP actuators in mechanical engineering applications can be considered. In the past three years, the project has established an experimental database for the diffusion of acetone, ethanol and water molecules into a commercial reference SMP of type Estane ETE 75DT3. The kinetics of chemical switching of a programmed linear unconstrained SMP actuator was established. The experimental results from the first funding period provided a basis for the design of new models on the molecular (polymer physics) and macroscopic (continuum mechanics) length scale.The present proposal for the second funding phase of SPP 1713 has three new scientific objectives: First, it will be considered, how different levels of superimposed stresses, reflecting the presence of end loads during actuation, affect the chemically triggered 1WE. Experimental benchmark data have to be provided and the effect of a superimposed mechanical stress needs to be understood from a molecular and a continuum mechanics perspective. Second, as small molecules diffuse into a SMP, they establish concentration profiles which evolve with time. There are relevant transient cases where the concentration of the small molecules in the surface region is high enough to trigger the one way effect, while the inner part of the material is still unaffected. This causes heterogeneous stress and strain distributions in the material which evolve with time. This evolution will be addressed experimentally and theoretically. Third, after a basic understanding of all relevant elementary processes has been established under simple experimental conditions, there is a need to address more complex scenarios, characterized by multiaxial stress states and by gradually changing stresses and temperatures. It also must be established which processes lead to a degradation of functional material properties and finally the exploitable service life of SMP actuator components.