Dynamic mechanical analysis for rapid assessment of the time-dependent recovery behavior of shape memory polymers
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, a(T)(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.