![]() ![]() 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. 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. In the amorphous state, polymer chains assume a completely random distribution within the matrix. Thermodynamics of the shape-memory effect The shape-memory polymers are effectively viscoelastic and many models and analysis methods exist. 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. These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure. If T m is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above T m and subsequently cooled below T m. Below T trans, flexibility of the segments is at least partly limited. Exceeding T trans (while remaining below T perm) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. In some cases this is the glass transition temperature ( T g) and others the melting temperature ( T m). The switching segments, on the other hand, are the segments with the ability to soften past a certain transition temperature ( T trans) and are responsible for the temporary shape. The phase showing the highest thermal transition, T perm, is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape. The secret behind these materials lies in their molecular network structure, which contains at least two separate phases. The polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus. 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. Polymers exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (permanent) form. Description of the thermally induced shape-memory effect A schematic representation of the shape-memory effect 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. ![]() 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. 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. Result of the cyclic thermomechanical test R r ( N ) = ε m − ε p ( N ) ε m − ε p ( N − 1 ). 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. Two important quantities that are used to describe shape-memory effects are the strain recovery rate ( R r) and strain fixity rate ( R f). SMPs have demonstrated recoverable strains of above 800%. SMPs are known to be able to store up to three different shapes in memory. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. Like polymers in general, SMPs cover a wide range of properties from stable to biodegradable, from soft to hard, and from elastic to rigid, depending on the structural units that constitute the SMP. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, light or solution. SMPs can retain two or sometimes three shapes, and the transition between those is often induced by temperature change. Crystalline trans-polyisoprene is an example of a shape-memory polymer.When heated above its glass-transition or melting temperature Polymer that, after heating and being subjected to a plastic deformation, resumes its original shape
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