Shape memory transition

Thermoresponsive self-folding films can be designed using continuous thermal expansion, melting, shape-memory transition or polymers which demonstrate LCST (Low Critical Solution Temperature) behavior in solutions. Kalaitzidou et al. used continuous volume expansion with temperature and demonstrated thermoresponsive rolling-unrolling of polydimethylsiloxane-gold bilayers tubes at 60-70 °C [24a, 24b] which is due to different temperature expansion coefficients. 

Thermal techniques, in nondestructive evaluation, 17 420-421. See also Heat entries Heating entries Thermal-transfer printing, 9 242, 338 Thermal transfer processes, 19 320 Thermal transition, in shape-memory polymers, 22 357-358, 359t, 360, 361-362... 

A type of material known as shape memory alloy (SMA) can perform this trick. SMAs are more complicated than electrorheological fluids and the other smart materials previously described in this chapter. An SMA does not only react or respond to environmental conditions, it also has a memory that enables it to return to a specific structure, or sometimes switch between two different structures. After the material has been set, it can recover from a deformation that would be permanent in other materials. When the temperature is raised by an amount that depends on the specific material, it snaps back into shape automatically. The memory is based on phase transitions, as described in the sidebar on page 120. 

Shape memory polymers are defined by their ability to store and recover strains when subjected to a particular thermo-mechanical cycle. Shape-memory polymers can recover their original shape by being heated above their transition temperature, which are defined by different phases in the materials. In particular, the shape-... 

Goo et al. investigated the actuation durability of a conducting shape memory polyurethane/MWNT (CSMPU) actuator and concluded that the number of cycles at breaking decreased, as the actuation temperature increased (108). The possible reason is that more material degradation of CSMPU can be induced due to rapid and large movement of polymer chains as the actuation temperature increases. For a CSMPU actuator, the authors confirmed that an actuation temperature that is higher than the transition temperature produces a rapid response but low durability. 

Figure 3.25. Hysteresis loop associated with the phase transitions of shape-memory alloys.

Melting is the transition of a material from a solid to a liquid. Transitions from one phase to another also can take place within a solid. A solid can have two phases if it has two possible crystal structures. It is the ability to undergo these changes in crystalline structure that gives shape-memory alloys their properties. 

There are a number of displacive transitions mentioned in this book. The order-disorder transformation of hydrogen atoms in hydrogen bonds in ferroelectric ceramics (Section 11.3.5) is one example. Displacive transitions that involve a change from an ordered arrangement of atoms to a random arrangement are commonly found in alloys. A subgroup of such order-disorder transitions, martensitic transitions, which can be used to produce shape-memory alloys, are considered in Sections 8.3.2 and 8.3.3. 

Shape-memory alloys show a thermoelastic martensitic transformation. This is a martensitic transformation, as described above, but which, in addition, must have only a small temperature hysteresis, some 10s of degrees at most, and mobile twin boundaries, that is, ones that move easily. Additionally, the transition must be crystallographi-cally reversible. The importance of these characteristics will be clear when the mechanism of the shape-memory effect is described. 

---