Project Details
Projekt
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 coupling limit
Applicants
Professor Dr.-Ing. Gunther Eggeler; Professor Dr.-Ing. Holger Steeb; Professor Dr. Fathollah Varnik
Subject Area
Mechanical Properties of Metallic Materials and their Microstructural Origins
Mechanics
Mechanics
Term
from 2014 to 2021
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 257549561
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.
DFG Programme
Priority Programmes
