a b s t r a c tIt is well known that the environmental conditions e.g. humidity can alter the shape-memory properties of polymers. In this work we applied an atomistic molecular dynamics simulation approach to model the influence of the addition of 1 wt% and 2 wt% water on the simulated shape-memory behavior of the amorphous switching domains of poly(L-lactide) (PLLA) with a molecular weight of M n ¼ 52,000 g mol À1 .For the dry as well as the water swollen PLLA-models, two subsequent uniaxial thermomechanical test cycles have been calculated, whereby the applied uniaxial deformation ε m was varied between 50% and 150%.All simulated PLLA models showed high shape fixity ratio of R f ! 84%. The shape-memory properties obtained at ε m ¼ 100% for the dry PLLA and the models containing 1 wt% water were almost identical with a shape recovery ratio in the first and second test cycle around R r ¼ 61%e64%. In contrast PLLA with 2 wt% water exhibited higher values of R r ¼ 76% during the first test cycle, but a lower R r ¼ 52% in the subsequent second cycle. Furthermore, increasing the applied ε m resulted in a decrease of R r from 82% to 42% for the dry PLLA, whereas PLLA with 2 wt% water did not show a dependence of R r on ε m . We anticipate that these observations can be attributed to differences in the initial structure of the various simulated PLLA models e.g. the different distribution of the free volume elements.
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.
An atomistic molecular dynamics simulation approach is applied to model the influence of urethane linker units as well as the addition of water molecules on the simulated shape‐memory properties of poly[(rac‐lactide)‐co‐glycolide] (PLGA) and PLGA‐based copolyester urethanes comprising different urethane linkers. The shape‐memory performance of these amorphous packing models is explored in a simulated heating–deformation–cooling–heating procedure. Depending on the type of incorporated urethane linker, the mechanical properties of the dry copolyester urethanes are found to be significantly improved compared with PLGA, which can be attributed to the number of intermolecular hydrogen bonds between the urethane units. Good shape‐memory properties are observed for all the modeled systems. In the dry state, the shape fixation is found to be improved by implementation of urethane units. After swelling of the copolymer models with water, which results in a reduction of their glass transition temperatures, the relaxation kinetics during unloading and shape recovery are found to be substantially accelerated.
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