This work compares the time dependence of isothermal shape recovery in thermoset and thermoplastic shape memory polyurethanes (SMPUs) with comparable glass transition temperatures. In each case, tensile tests have been used to quantify the influence of various thermo-mechanical programming parameters (deformation temperature, recovery temperature, and stress and storage times following the deformation step) on strain recovery under zero load (free recovery) and stress recovery under fixed strain (constrained recovery). It is shown that the duration of the recovery event may be tuned over several decades of time with an appropriate choice of programming parameters, but that there is a trade-off between the rate of shape recovery and the recoverable stress level. The results are discussed in terms of the thermal characteristics of the SMPUs in the corresponding temperature range as characterized by modulated differential scanning calorimetry and dynamic mechanical analysis, with the emphasis on the role of the effective width of the glass transition temperature and the stability of the network that gives rise to the shape memory effect.

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Electrospun thermoplastic polyurethane (TPU) nanofibers are known to contract considerably (40%) on heating up to 90 8C. This study investigates this thermomechanical behavior and the TPU shape memory capabilities. The shape memory effect was first studied in TPU films as a model system by applying classical thermomechanical cycles (programming and recovery). The films were able to fix the applied deformation during long-term storage at room temperature, well above the material's calorimetric glass transition temperature and in the absence of a percolated structure of hard domains. Structural analysis (Fourier transform infrared, differential scanning calorimeter, and dynamic mechanical analysis) revealed broad thermal transitions indicating the presence of a mixed phase of hard segments dispersed in the soft segment matrix. Using a linear viscoelastic model together with timetemperature superposition, the shape memory effect was attributed to the thermoviscoelastic properties of TPU. In particular, the mixed phase was found to give rise to a very broad relaxation spectrum dominated by long relaxation times, which explains the suppression of strain recovery at room temperature. Finally, the electrospinning process was examined and was found to be similar to a programming cycle characterized by the strong elongation flow accompanied by massive solvent evaporation, whereas the contraction effect was interpreted as the recovery phase in a shape memory perspective. Thus, the contraction of electrospun TPU mats may be considered to be an electrospinning-induced shape memory effect.

Thermally activated shape memory polymers (SMPs) recover from a secondary shape induced by mechanical deformation to a primary equilibrium shape when they are heated to their actuation temperature. In certain applications, for example in the biomedical field, it may be necessary to control the rate of shape recovery under isothermal conditions, which requires knowledge of the time-dependent response of the SMP. In the present work, the time dependence of isothermal shape recovery has been investigated for polyurethane-based SMPs with two different molecular architectures. The results are discussed in terms of a linear thermo-viscoelastic model for the time and temperature dependence of the shape memory response at small strains, using data from single constant frequency dynamic mechanical analysis (DMA) temperature sweeps. This approach is based on the establishment of an approximate relationship between the storage modulus, the loss modulus and the shift factor, aT(t), usually derived from time–temperature superposition of isothermal data obtained at different temperatures. The DMA data are thus shown to provide an approximate measure of the relaxation and retardation time spectra, which may in turn be used to predict the shape memory response to a simple programming-isothermal shape recovery sequence. This procedure is argued to permit rapid quantitative comparison of the shape memory performance of different materials, with minimal experimental characterization, and is hence potentially a useful tool for designing materials for specific applications.

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