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

Abstract

It 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 Mn = 52,000 g mol−1. 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 εm was varied between 50% and 150%. All simulated PLLA models showed high shape fixity ratio of Rf ≥ 84%. The shape-memory properties obtained at εm = 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 Rr = 61%–64%. In contrast PLLA with 2 wt% water exhibited higher values of Rr = 76% during the first test cycle, but a lower Rr = 52% in the subsequent second cycle. Furthermore, increasing the applied εm resulted in a decrease of Rr from 82% to 42% for the dry PLLA, whereas PLLA with 2 wt% water did not show a dependence of Rr on εm. 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.