Description of The Thermally Induced Shape-memory Effect
Polymers exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (permanent) form. Once the latter has been manufactured by conventional methods, the material is changed into another, temporary form by processing through heating, deformation, and finally, cooling. The polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus. The secret behind these materials lies in their molecular network structure, which contains at least two separate phases. The phase showing the highest thermal transition, Tperm, is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape. The switching segments, on the other hand, are the segments with the ability to soften past a certain transition temperature (Ttrans) and are responsible for the temporary shape. In some cases this is the glass transition temperature (Tg) and others the melting temperature (Tm). Exceeding Ttrans (while remaining below Tperm) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. Below Ttrans, flexibility of the segments is at least partly limited. If Tm is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above Tm and subsequently cooled below Tm. These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure. The hard to soft segment ratio is often between 5/95 and 95/5, but ideally this ratio is between 20/80 and 80/20. The shape-memory polymers are effectively viscoelastic and many models and analysis methods exist.
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