Qing (2018) A 'frozen volume' transition model and working mechanism for the shape memory effect in amorphous polymers. Smart Materials and Structures, 27 (6). 065023.

By means of combining the influence of temperature and strain rate, and based on the cooperative Eyring model, a phenomenological formulation for the shape memory effect and temperature memory effect of an amorphous shape memory polymer (SMP) is proposed. The internal stress and stored mechanical energy are correlated to shape/temperature memory behaviours below and above the glass transition temperature. The working mechanism and fundamentals for the chemo-responsive shape memory behaviour in SMPs are further investigated in terms of the activation enthalpy parameter for inductive depression. Simulation using the proposed model is compared with the experimental results reported in the literature. Predictions are also made using the proposed model. This phenomenological framework is expected to provide a powerful tool for investigating the underlying thermomechanics that originate in the movement of cooperative segments and segmental relaxations in SMPs.

Shape memory polymers (SMPs) are thermo-rheologically complex materials showing significant temperature and time dependences. Their segments often undergo cooperative phase transitions and conformational relaxations simultaneously along with shape memory effect (SME). In this study, a cooperative domain model is proposed to describe the composition dependence, multiple phase transitions and conformational relaxations of SMPs within their glass transition zones. Variations in local-area compositions and cooperative domains of the amorphous SMPs cause significant differences in their segmental relaxation. At a fixed domain size, both intermolecular activation energy and relaxation time significantly influence the SME and thermomechanical properties of the SMPs. Finally, the model is successfully applied to predict the shape memory behavior of SMPs with one stage SME and triple-SME, and the theoretical results have been validated by the experimental ones. This model could be a powerful tool to understand the working mechanisms and provide a theoretical guidance for the designs of multi-SME in SMPs.

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A thermodynamic model for tunable multi-shape memory effect and cooperative relaxation in amorphous polymers. Smart Materials and Structures, 28 (2). 025031.