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Abstract

Chemically triggered shape memory polymers: Molecular modelling and experiments Shape memory polymers are known for their capability to recover large shape changes. In contrast to metallic shape memory alloys, where the recovery of the original shape relies on a diffusionless thermodynamic phase transition, the shape memory effect in polymers has an entropic origin and is driven by dynamic processes. Upon heating above the glass transition temperature, stretched polymers become sufficiently mobile to leave the low-entropic elongated states and visit the higher entropic isotropic conformations. We provide direct evidence for these entropic forces via molecular dynamic simulations [1]. A special focus of the present talk is on the effect of small additive molecules which diffuse into the sample, and thus influence the dynamics of structural relaxation [2]. Experiments show a clear reduction of the switching temperature for shape recovery process upon diffusion of small molecules such as acetone, ethanol and water [3]. Molecular dynamics simulations of a bead-spring co-polymer model are qualitatively in line with these findings [1]. Moreover, it is shown via spectroscopy experiments that additive molecules dramatically alter the distribution of hydrogen bonds in the polymer matrix, thereby providing a mechanism for changing the shape memory switching temperature. [1] E. M. Zirdehi, H. Dumlu, G. Eggeler, F. Varnik, On the Size Effect of Additives in Amorphous Shape Memory Polymers, Materials 14, 327 (2021) [2] E. Zirdehi, F. Varnik, Non-monotonic effect of additive particle size on the glass transition in polymers, J. Chem. Phys. 150, 024903 (2019). [3] H. Dumlu, A. Marquardt, E. M. Zirdehi, F. Varnik, Y. Shen1, K. Neuking and G. Eggeler, A Mechanical Analysis of Chemically Stimulated Linear Shape Memory Polymer Actuation, Materials 14, 481 (2021)