Thermodynamic description and modeling of two‐way shape‐memory effect in crosslinked semicrystalline polymers

Recently received results have shown that crosslinked semicrystalline polymers are able to exhibit not only one‐way irreversible shape‐memory effect (SME) but also two‐way reversible SME, which is revealed as an anomalous elongation of a specimen under load during non‐isothermal crystallization and subsequent contraction during heating. Up to now, there is no well‐based physical theory, which is able to explain this phenomenon. The present study deals with theoretical explanation and description of two‐way SME in crosslinked semicrystalline polymers. The proposed approach is based on the theory of the stress‐induced crystallization but allows calculating the free energy change of a specimen deformed under constant load and cooled down below crystallization temperature at constant cooling rate, i.e. in non‐isometric and non‐isothermal conditions, respectively. The presented theory was confirmed for a covalently crosslinked high‐density polyethylene. The analysis of the free energy change performed in case of different crosslink density and deformation predicts the possible crystal morphology (extended or folded chains) and orientation of crystallites generating at cooling under load. It is shown that aforementioned anomalous elongation may be observed when the orientation of chain folds in crystals is parallel to or makes a sharp angle with the stretch direction. The stress–strain–temperature relationship derived from the free energy change was used to fit the experimental findings of the two‐way SME, which were received by the authors for crosslinked high‐density polyethylene for the first time. The fitting curves have shown well agreement with the experimental values.