Thermally induced shape-memory effect (polymers)
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.
The sample is heated to the transition temperature (TTrans) and within a short time the recovery of the permanent shape is observed.
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.
Metals and ceramics with thermally induced bidirectional shape-memory effect have had great application in medical implants, sensors, transducers, etc.
The ratio of the elements forming the phase separation largely regulates the TTrans transition temperature; this is much easier to control than in metallic alloys.
PET has a high degree of crystallinity and the formation of these crystals provides for the flow and rearrangement of the PEO chains as they are stretched at temperatures higher than their Tm.
The control of curing either by electromagnetic waves or with peroxides is very important since it increases the TTrans and decreases the crystallinity, determining factors in the shape-shape-memory effect.
The influence of the crosslinking of the molecules, the cooling rate and the crystallization behavior are specific to each system and impossible to enumerate since the synthesis possibilities are almost infinite.
If a polymer sample is stretched for a short time in the elastic range, when the load is removed, the sample will recover its original shape, but if the load remains for a sufficiently long period, the chains rearrange and the original shape is not recovered, the result is an irreversible deformation, also called relaxation process (in this case: creep).
In order for a polymer to exhibit the thermally induced shape-memory effect, it is necessary to fix the chains with anchor points to avoid these relaxation processes that inelastically modify the system.
In designing copolymers for thermally induced shape-memory effect it is very important to keep in mind that a slight change in chemical structure (cis/trans ratios, tacticity, molecular mass, etc.)
Its properties can be studied with differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA) techniques to determine its physicochemical arrangement.
Most of the applications of polymers with this effect are only suggestions for now, many possibilities have been proposed, but so far only a few have been used, the most important being medical devices and automotive elements, although the greatest success has been achieved with heat-shrinkable polyethylene, which is also an exception in the programming step, since it is processed in a different way.
