The thermally induced unidirectional shape-shape-memory effect is an effect classified within the new so-called smart materials. Polymers with thermally induced shape-memory effect are new materials, whose applications are recently being studied in different fields of science (e.g., medicine), communications and entertainment. There are currently reported and commercially used systems. However, the possibility of programming other polymers is present, due to the number of copolymers that can be designed: the possibilities are almost endless.
General information
Polymers with thermally induced shape-memory effect are those polymers that respond to external stimuli and because of this have the ability to change their shape. The thermally induced shape-memory effect results from a combination of proper processing and programming of the system. This effect can be observed in polymers with very different chemical composition, which opens a great possibility of applications.
Description of the effect on polymers In the first step the polymers are processed by means of common techniques, such as injection or extrusion, thermoforming, at a temperature (THigh) at which the polymer melts, obtaining a final shape which is called "permanent" shape. The next step is called system programming and involves heating the sample to a transition temperature (TTrans). At that temperature the polymer is deformed, reaching a shape called "temporary". Immediately afterwards the temperature of the sample is lowered. The final step of the effect involves the recovery of the permanent shape. The sample is heated to the transition temperature (TTrans) and within a short time the recovery of the permanent shape is observed. This effect is not a natural property of the polymer, but results from proper programming of the system with the appropriate chemistry. For a polymer to exhibit this effect, it must have two components at the molecular level: bonds (chemical or physical) to determine the permanent shape and "trigger" segments with a TTrans to fix the temporary shape.
Characteristics of the effect on polymers Metals exhibit a bidirectional shape-memory effect, maintaining one shape at each temperature. Polymers recover their shape only once. Polymers can change their shape with elongations up to 200% while metals have a maximum of 8-10% elongation. Recovery in metals and ceramics involves a change in crystal structure, while recovery in polymers is due to the action of entropic forces and anchor points. Polymers can be designed according to the desired application, they can be: biodegradable, drug delivery systems (medicinal), antibacterial, etc. The transition temperature is designed with "trigger" segments, which makes temperature adjustment easier than in ceramics, since they depend on equiatomic quantities.
Functioning It should first be noted that the first inelastic mechanism of these polymers is the mobility of the chains and the conformational rearrangement of the groups. Then the effect on semi-crystalline and amorphous polymers must be distinguished. In both cases, anchor points must be created that act as "triggers" for the effect. In the case of amorphous polymers, these will be the knots or "tangles" of the chains, and in the case of semi-crystalline polymers, the crystals themselves will form these anchor points. By modifying the shape of the material under minimal critical stress, the chains slide and a metastable structure is created, which increases the organization and order of the chains (lower entropy), when the deformation load is eliminated, the anchor points provide a storage mechanism for macroscopic stresses in the form of small localized stresses and decreasing entropy. In the glassy state the rotational motions of the molecules are frozen and impeded, as the temperature increases and the glassy state is reached, these motions thaw and rotations and relaxations occur, the molecules take the form that is entropically most favorable to them, the one with the lowest energy. These movements are called relaxation process and the formation of "random strings" to eliminate stresses is called shape-memory loss. A polymer will exhibit the shape-memory effect if it is susceptible to being stabilized in a given state of deformation, preventing the molecules from slipping and regaining their higher entropy (lower energy) form. This can be achieved almost entirely by creating crosslinking or vulcanization, these new bonds act as anchors and prevent the relaxation of the chains, the anchor points can be physical or chemical.
Comparison with metals and ceramics The unidirectional shape-shape-memory effect was first observed by Chand and Read in 1951 in a Gold-Cadmium alloy and in 1963 Buehler described this effect for nitinol, which is an equiatomic Nickel-Titanium alloy. This effect in metals and ceramics is based on a change in the crystal structure, called martensitic phase transition. The disadvantage of these materials is that it is an equiatomic alloy and deviations of 1% in the composition modify the transition temperature by approximately 100 K. Some metals and ceramics present the effect bidirectionally, which means that at a certain temperature there is a shape and this can be changed by changing the temperature, but if the first temperature is recovered, also the first shape is recovered. This is achieved by training the material for each shape at each temperature. Metals and ceramics with thermally induced bidirectional shape-memory effect have had great application in medical implants, sensors, transducers, etc. Many present a risk however due to their high toxicity.
Phases in the system To obtain the effect, it is necessary to achieve a phase separation, one of these phases works as the trigger for the temporary form, using a transition temperature that can be Tm or Tg and in this effect is called TTrans. A second phase has the higher transition temperature and above this temperature the polymer melts and is processed by conventional methods.
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