Shape-memory polymer

SMPs can retain two or sometimes three shapes, and the transition between those is induced by temperature. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, light or solution. As well as polymers in general, SMPs also cover a wide property-range from stable to biodegradable, from soft to hard, and from elastic to rigid, depending on the structural units that constitute the SMP. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. SMPs are known to be able to store up to three different shapes in memory. SMPs have demonstrated recoverable strains of above 800%. Two important quantities that are used to describe shape-memory effects are the strain recovery rate (Rr) and strain fixity rate (Rf). The strain recovery rate describes the ability of the material to memorize its permanent shape, while the strain fixity rate describes the ability of switching segments to fix the mechanical deformation. where N is the cycle number, εm is the maximum strain imposed on the material, and εp(N) and εp(N-1) are the strains of the sample in two successive cycles in the stress-free state before yield stress is applied. Shape-memory effect can be described briefly as the following mathematical model: where Eg is the glassy modulus, Er is the rubbery modulus, fIR is viscous flow strain and fα is strain for t >> tr. While most traditional shape-memory polymers can only hold a permanent and temporary shape, recent technological advances have allowed the introduction of triple-shape-memory materials. Much as a traditional double-shape-memory polymer will change from a temporary shape back to a permanent shape at a particular temperature, triple-shape-memory polymers will switch from one temporary shape to another at the first transition temperature, and then back to the permanent shape at another, higher activation temperature. This is usually achieved by combining two double-shape-memory polymers with different glass transition temperatures or when heating a programmed shape-memory polymer first above the glass transition temperature and then above the melting transition temperature of the switching segment. 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. In the amorphous state, polymer chains assume a completely random distribution within the matrix. W represents the probability of a strongly coiled conformation, which is the conformation with maximum entropy, and is the most likely state for an amorphous linear polymer chain. This relationship is represented mathematically by Boltzmann's entropy formula S = k ln W, where S is the entropy and k is Boltzmann's constant.