Tailoring the time-dependent recovery of shape memory polymers
On application of heat, shape memory polymers (SMPs) are able to revert to a primary shape from a secondary shape induced by mechanical deformation. It may be desirable to induce shape recovery at controlled rates under quasi-isothermal conditions, e. g. in certain biomedical applications in which actuation occurs at body temperature, requiring knowledge of the time dependent response of SMPs. In the present work, the time dependence of isothermal shape recovery has been investigated for two shape memory polyurethanes ( SMPUs) with different molecular architectures. These are discussed in terms of a simple linear thermo-viscoelastic model for the time and temperature dependence of the shape memory response at small deformations, based on data obtained from a single constant frequency dynamic mechanical analysis (DMA) temperature sweep. This approach is based on the establishment of an approximate relationship between the storage modulus E'(T), the loss factor tan delta(T) and the shift factor, a(T)(T), more usually derived from time-temperature superposition of isothermal data obtained at different temperatures. The DMA data are thus shown to be sufficient to describe the relaxation and retardation time spectra. As well as providing a useful phenomenological description of the shape memory effect, the model derived from the DMA data permits quantitative comparison of materials, and may hence be used as guidelines for materials design for specific applications. This is demonstrated for the two SMPUs considered in the present work, whose viscoelastic spectra vary significantly in width, affording considerable insight into the distribution of segmental mobility within the polymer network.