Shape memory polymers (SMPs) are polymers that recover a certain “set” shape, or configuration state, after application of large amounts of strain. Typically the polymers are crosslinked through physical and chemical bonding. (Chemical crosslinks refer to covalent bonds between polymer chains, while physical crosslinks refer physical interactions of polymer chains, more specifically to aggregation of four or more polymer chains.) The cause of such large recoverability is based in the change in material properties above and below the glass transition temperature, Tg. At temperatures above Tg, the state is said to be rubbery. In the rubbery state, there are a high number of configurations that the polymer chains are able to assume. This high amount of configurational energy is due to the fact that physical crosslinks or molecule interaction is negligible. When mechanical deformation is applied, the number of available configurations decreases due to the spatial change, therefore the entropic energy decreases. Typical, shapes are set in SMPs by heating above Tg, applying mechanical load and then, with the load still applied, lowering the temperature to below Tg. After the temperature is lowered, the load is released. When the temperature is below Tg, the material state is said to be glassy, meaning the available configurations are significantly decreased. Most configurational change is an internal energy dominated change, meaning it occurs due to bond stretching. Once, the temperature is elevated above Tg, entropic energy increases and the polymer can recover its “set” shape. The photos below depict the shape change from the “set” shape to the temporary shape, then back to the “set” shape again. The photos are from: http://www.azom.com/article.aspx?ArticleID=6038.
Also, a video that depicts the shape memory polymer behavior is given here: http://www.youtube.com/watch?v=vuoorVtYWgk
Lui, Y.P., Gall, K., Dunn, M.L., Greenberg, A.R., Diani, J., 2006. Thermomechanics of shape memory polymers: Uniaxial experiments and constitutive modeling. Int. J. Plasticity 22 (2), 279-313.
Qi, H.J., Nguyen, T.D., Castro, F., Yakacki, C.M., Shandas, R. 2008. Finite Deformation thermo-mechanical behavior of thermally induced shape memory polymers. J. Mech. Phys. Solids 56, 1730-1751.