Simulating the Shape‐Memory Behavior of Amorphous Switching Domains of Poly(L‐lactide) by Molecular Dynamics

Abstract

The thermally induced shape-memory effect of polymers is typically characterized by cyclic uniaxial thermomechanical tests. Here, a molecular-dynamics (MD) simulation approach of such a cyclic uniaxial thermomechanical test is presented for amorphous switching domains of poly( L -lactide) (PLLA). Uniaxial deformation of the constructed PLLA models is simulated with a Parinello-Rahman scheme, as well as a pragmatic geometrical approach. We are able to describe two subsequent test cycles using the presented simulation approach. The obtained simulated shape-memory properties in both test cycles are similar and independent of the applied deformation protocols. The simulated PLLA shows high shape fi xity ratios ( R f ≥ 94%), but only a moderate shape recovery ratio is obtained ( R r ≥ 30%). Finally, the structural changes during the simulated test are characterized by analysis of the changes in the dihedral angle distributions.

Influence of the addition of water to amorphous switching domains on the simulated shape-memory properties of poly(l-lactide)

“…[34]. For the dry PLLA system (PLAw0, open squares) a good agreement with the experimental specific volume data was observed at temperatures >450 K, where PLLA is completely molten.…”

“…It should be emphasized that our models present exclusively the amorphous parts of PLLA and not crystalline parts of this semicrystalline material (for more details see Ref. [34]). After the final equilibration simulation at T high , densities of the unstretched (ε m ¼ 0%) samples were calculated.…”

“…Furthermore, diverse theoretically modeling approaches have been reported to simulate or investigate the SME of polymers, including different constitutive models [23e28], but also molecular simulation methods [29e33]. In this context we recently introduced a molecular dynamic modeling approach, which could successfully describe two subsequent thermomechanical cycles of amorphous PLLA switching segments [34,35]. As in our former studies, we assume that the crystalline domains in the semicrystalline thermoplastic PLLA represent the physical netpoints that only transfer stress to the viscoelastic amorphous domains and provide the permanent shape.…”

“…Additionally, the same procedure consisting of two subsequent test cycles was carried out with ε m ¼ 50% and 150% for the dry and the 2 wt% water swollen system. Following the previous studies with PLLA [34,35], as reference temperatures T low ¼ 250 K and T high ¼ 500 K were selected, as preliminary studies conducted at T high ¼ 450 K did not result in any shape recovery over a simulation time up to some nanoseconds. Finally, the relaxation kinetics during unloading at T low ¼ 250 K and the recovery kinetics at T high ¼ 500 K were analyzed by application of a Voigt model approach.…”

A molecular dynamics investigation of the deformation mechanism and shape memory effect of epoxy shape memory polymers

“…Instead of real SMPs, a virtual polyisoprene was constructed in their model, in which entropy dependent shape storage and recovery was confirmed. Recently, Ghobadi et al [32] presented an atomistic modeling approach to predict the shape-memory behavior of poly(L-lactide) and its cyclic uniaxial thermomechanical test.…”