On the size effect of additives in amorphous shape memory polymers
E. Mahmoudinezhad Zirdehi, H. Dumlu, G. Eggeler, F. Varnik.
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.
(a) Molecular structure of ESTANE, consisting of strongly and weakly interacting ('hard' and 'soft') segments as indicated (Courtesy of Axel Marquardt). In the simplified model, each hard segment is represented as a type A (red) particle and each soft segment by a type B (blue) sphere. The arrangement and number of A and B particles follows the sequence of hard and soft segments in the experimental sample. (b) A snapshot of a simulated single chain. (c-e) Snapshots of the simulation box containing 400 chains which shows a heterogeneous distribution of hard (red) and soft (blue) segments.