3 April 2013 The shape memory effect in crosslinked polymers: effects of polymer chemistry and network architecture
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The thermal shape memory effect in polymeric materials refers to the ability of a sample to retain a deformed shape when cooled below Tg, and then recover its initial shape when subsequently heated. Although these properties are thought to be related to temperature-dependent changes in network structure and polymer chain mobility, a consistent picture of the molecular mechanisms which determine shape memory behavior does not exist. This, along with large differences in the shape memory cycling response for different materials, has made model development and specific property optimization difficult. In this work we use coarse-grained molecular dynamics (MD) simulations of the thermal shape memory effect to inform micro-macro relationships and systematically identify the salient features leading to desirable shape behavior. We consider a simulation test set including chains with increasing levels of the microscopic restrictions on chain motion (the freely-jointed, freely-rotating, and rotational isomeric state chain models), each simulated with both the NPT and NVT ensembles. It is found that the NPT ensemble with attractive interactions between monomers enabled is the most appropriate for simulating the temperature-dependent mechanical behavior of a polymer using coarse-grained MD. Of the different models, the freely-jointed chain system shows the most desirable shape memory characteristics; this behavior is attributed to the ability of the particles in this system to pack closely together in an energetically favorable configuration. A comparison with experimental data demonstrates that the coarse-grained simulations display all of the relevant trends in mechanical behavior during constant strain shape memory cycling. We conclude that atomistic detail is not needed to represent a shape memory polymer, and that multi-scale modeling techniques may build on the mechanisms embodied in the simple coarse-grained model.
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Jacob D. Davidson, Jacob D. Davidson, Yali Li, Yali Li, N. C. Goulbourne, N. C. Goulbourne, } "The shape memory effect in crosslinked polymers: effects of polymer chemistry and network architecture", Proc. SPIE 8689, Behavior and Mechanics of Multifunctional Materials and Composites 2013, 86890K (3 April 2013); doi: 10.1117/12.2011538; https://doi.org/10.1117/12.2011538

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