Abstract
Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additive-to-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials.
Highlights
This way, the shape memory effect is not deteriorated by structural relaxation
The following findings shall be demonstrated: (1) The simple copolymer model presented above is capable of entropy-driven shape memory behavior, (2) adding small molecules to the sample plays a role similar to increasing temperature with regard to shape recovery and (3) the temperature, at which significant shape recovery takes place, first decreases for smaller additive particles but increases as the particle size falls below roughly half a monomer diameter
A way to highlight the entropic effect in shape memory polymers is to monitor the chain conformation
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Crucial to shape memory effect in polymers is the change of chains’ conformation upon deformation and the tendency to recover a high entropic state during structural relaxation These high entropic conformations have been recently studied for quiescent copolymers of different architectures at low temperatures and under poor-solvent conditions [18,19,20]. SMPs, the additive-effect often manifests itself in a shift of the Tg and the onset of shape recovery at a lower temperature as compared to the pure sample [37] This solvent-induced effect on SME has been investigated experimentally by many researchers [37,38,39,40,41,42,43,44,45,46,47,48]. At a constant number concentration, the temperature at which significant relaxation towards the original shape takes place changes in a non-monotonic way with the additive diameter
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