Shape-Memory Polymers
From the engineering point of view, the material properties of polymers are much simpler to modify than those of metals/alloys. In addition, the cost (both material
FIGURE 6.10 Triple-SME in an NiTi SMA strip upon heating. Since there is no apparent visible deflection during subsequent cooling, the material does not have the two-way SME.
and processing costs) of polymers is traditionally much lower. A number of SMPs have been discovered and well recorded in literature, while new ones are still emerging every week, if not every day. In addition to the above-mentioned advantages, the SMPs are much lighter, have a much higher (at least higher order) recoverable strain than the SMAs, and can be triggered at the same time for the recovery of shape by various stimuli and even multiple stimuli. In addition to heat, light (UV and infrared light) and chemical (moisture, solvent, and pH change) are two of these types of stimuli. In addition, many SMPs are naturally biocompatible and biodegradable. As a result, we have more degrees of freedom to manipulate SMPs to meet the needs of a specific application.
The thermoplastic polyurethane SMP was originally invented by Dr. S. Hayashi at the Mitsubishi Heavy Industry Nagoya R&D Center in Japan and has been successfully marketed for over 15 years. The same SMP was developed into open-cell foams for space missions and biomedical applications based on the concept of cold hibernated elastic memory (CHEM) proposed by Dr. W. Sokolowski at Jet Propulsion Laboratory, United States. Figure 6.11 demonstrates the possibility of delivering a coil, consisting of a thermo-moisture responsive SMP, to a jellyfish. This concept can be extended to, for example, the delivery of a tiny machine to a living cell for future cell surgery or to a tiny microbe for special function/operation within a microbe. Biomedical application is currently emerging as a promising field for SMPs, although surface patterning (to modify various surface-related properties, such as reflection, surface tension, etc.) is another area. Figure 6.12 is a zoom-in view of the patterned
FIGURE 6.11 Delivery of an SMP coil into a jellyfish by injection, (a) Original coiled shape; (b) After being straightened at high temperature; (c) Ready for injection; (d) Recovered shape.
FIGURE 6.12 Zoom-in view (three-dimensional) of patterned surface atop SMP produced by laser after one single exposure.
surface above the SMP, which is produced by a laser beam after a single exposure through a microlens, compared to patterning on top of SMAs, in particular for different shaped/size patterns. Indentation-polishing-heating (IPH) patterns are made, while those at the top of the SMAs are usually reversible between two shapes during thermal cycle; those at the top of the SMPs are fixed and permanent. A further step is to integrate micro-/nano-sized w'rinkles with such surface patterns for dramatically improved performance, such as self-cleaning, cell adhesion, water splitting, and light extraction, etc.
The underlying mechanism for small- and medium-sized enterprises in SMPs is the dual-segment/domain structure (one is often hard/elastic, while the other may be soft/ductile or rigid depending on whether the stimulus is correct). The former is knowrn as the elastic segment, and the latter is the transition segment. Take an example of the thermo-responsive SMP. The SME mechanism is shown in Figure 6.13. As we can see, the SMP is typically much weaker at high temperatures than at low temperatures. This mechanism differs from the well-known and highly predictable reversible martensitic transformation between the high-temperature austenite phase (which is hard and rigid) and the low-temperature martensite phase (which is soft and flexible) in SMAs.
Since SMPs have a much greater recoverable strain and, as a rule, a wider recovery temperature range, it is possible to have more than one intermediate shape in the multi-SME through a proper programming procedure, as Xie has recently demonstrated. We have shown that during a constrained recovery (i.e. heat with a temporary SMP shape fixed), the maximum reaction force/stress should appear at the temperature that the SMP is deformed (which should be within the Xie transition temperature range). This feature shows the TME in the SMPs. The exact mechanism behind this function is the step-by-step release of the elastic energy stored in the elastic section during programming, which varies from that of the SMAs.
FIGURE 6.13 Illustration of the mechanism of the SME in thermo-responsive SMP. (a) Hard at low temperature; (b) Easily deformed at high temperature; (c) Hard again after cooling; (d) Temporary (deformed) shape after constraint removed; (e) Shape recovery upon heating.
Conversely, multi-SME in SMPs can be accomplished by setting various shape recovery conditions, e.g. different stimuli or multiple changes across different temperature ranges and even a gradient transition temperature. As such, the programmed recovery can be realized in a well-controlled manner.
SMP composites have remarkably broadened the potential applications of SMPs; in addition to the use of various types of fillers (including different types of clay, SiC nanoparticles, etc.) to strengthen SMPs. Heating of thermo-responsive SMPs can also be achieved by heating Joules (by means of filling with different forms of conductive inclusions), by induction heating (by means of dissipation of energy by hysteresis by applying an alternating magnetic/electrical field, etc.), and also by radiation. Figure 6.14 shows scanning electron microscope images that show the dispersion of Ni powder (3-7 pm in diameter) in SMP/carbon black composite polyurethane. As reported by Leng with an additional 0.5 vol percent of Ni powders, and after forming chains (under a weak magnetic field before curing), the electrical resistance of SMP/carbon black composites reduces by more than ten times, so that low electrical voltage heat can be achieved, just like SMAs.
As compared to SMAs, SMPs usually soften in the presence of the right stimulus; thus, most SMPs (with the exception of a few) are not appropriate for cyclical action and cannot be conditioned to have the so-called two-way SME (which is the ability to repeatedly move between two shapes depending on how the stimulus is applied).
On the other side, the shape recovery of SMPs can be followed by color change, excellent clarity, reversible adhesion/peeling, and even self-healing. The SME has been shown to be similar to SMAs in submicron size SMP (Figure 6.15). While it has been difficult to manufacture high-quality porous SMAs so far, it is always easy to produce SMP foams using many conventional polymer foaming techniques. As
FIGURE 6.14 Enhanced electrical conductivity in an SMP/carbon black composite through alignment of Ni powder (SEM image), (a) SMP/carbon black only; (b) With randomly distributed Ni; (c) Chained Ni.
reported in the complete form recovery, 94 percent of pre-compressed polyurethane SMP foam is detected even after a few months of storage.
