Shape memory polymers

A shape memory effect (SME) is the ability for a material to be deformed in a temporary stable shape and to have its initial shape restored by heating above a switching transition. A shape memory polymer (SMP) must exhibit a sharp transition between a glassy domain and a rubbery plateau. The glassy modulus has to be at least 100 time higher than the rubbery one. The transition between the glassy and rubbery domains is the switching temperature. The polymer is deformed above the transition by loading and cooled below the transition. When the load is removed, the temporary shape is stable and the initial shape is recovered by heating above the transition. The values of the rubbery modulus and the transition temperature can be controlled according to three main approaches : entanglement of very high molecular weight chains, physical and chemical crosslinking. Shape memory polymers can withstand very important deformations (up to 600%) but the stored stress is low (1-4 Mpa) compared with the shape memory alloys (deformation 2-5%; stored stress 700 Mpa). Biomedical and technological applications are expected for these materials.