Shape-memory polymer
[5] 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.
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.
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.
The phase showing the highest thermal transition, Tperm, is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape.
These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure.
[1] Polymers in this elastic state with number average molecular weight greater than 20,000 stretch in the direction of an applied external force.
Another example reported in the literature is a copolymer consisting of polycyclooctene (PCOE) and poly(5-norbornene-exo,exo-2,3-dicarboxylic anhydride) (PNBEDCA), which was synthesized through ring-opening metathesis polymerization (ROMP).
[15] The main limitation of physically crosslinked polymers for the shape-memory application is irreversible deformation during memory programming due to the creep.
The addition of 1.5 wt% maleic anhydride increased in shape recovery from 35% to 65% and tensile strength from 3 to 5 MPa.
[17] While shape-memory effects are traditionally limited to thermosetting plastics, some thermoplastic polymers, most notably PEEK, can be used as well.
They have a high capacity for elastic deformation (up to 200% in most cases), much lower cost, lower density, a broad range of application temperatures which can be tailored, easy processing, potential biocompatibility and biodegradability,[24] and probably exhibit superior mechanical properties to those of SMAs.
[26] One of the first conceived industrial applications was in robotics where shape-memory (SM) foams were used to provide initial soft pretension in gripping.
Since this time, the materials have seen widespread usage in, for example, the building industry (foam which expands with warmth to seal window frames), sports wear (helmets, judo and karate suits) and in some cases with thermochromic additives for ease of thermal profile observation.
Due to the shape changing capability, SMPs enable the production of functional and responsive photonic gratings.
[30] By using modern soft lithography techniques such as replica molding, it is possible to imprint periodic nanostructures, with sizes of the order of magnitude of visible light, onto the surface of shape memory polymeric blocks.
By taking advantage of the polymer's shape memory effect, it is possible to reprogram the lattice parameter of the structure and consequently tune its diffractive behavior.
[31] By doping SMPs with highly scattering particles such as titania it is possible to tune the light transport properties of the composite.
By configuring both the amount of scatters and of the organic dye, a light amplification regime may be observed when the composites are optically pumped.
[18] Additionally, SMPs are now being used in various ophthalmic devices including punctal plugs, glaucoma shunts and intraocular lenses.
SMPs are smart materials with potential applications as, e.g., intravenous cannula,[29] self-adjusting orthodontic wires and selectively pliable tools for small scale surgical procedures where currently metal-based shape-memory alloys such as Nitinol are widely used.
In the case of biodegradable polymers, after the implant has fulfilled its intended use, e.g. healing/tissue regeneration has occurred, the material degrades into substances which can be eliminated by the body.
[35] SMPs have also potential for use as compression garments[36] and hands-free door openers, whereby the latter can be produced via so-called 4D printing.
Currently, the Defense Advanced Research Projects Agency DARPA is testing wings which would change shape by 150%.
[6] The realization of a better control over the switching behavior of polymers is seen as key factor to implement new technical concepts.
[39] Recently, a new manufacturing process, mnemosynation, was developed at Georgia Tech to enable mass production of crosslinked SMP devices, which would otherwise be cost-prohibitive using traditional thermoset polymerization techniques.
[40] Mnemosynation was named for the Greek goddess of memory, Mnemosyne, and is the controlled imparting of memory on an amorphous thermoplastic materials utilizing radiation-induced covalent crosslinking, much like vulcanization imparts recoverable elastomeric behavior on rubbers using sulfur crosslinks.
The customizable mechanical properties of traditional SMPs are achievable with high throughput plastics processing techniques to enable mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable glass transition temperatures.
Shape memory polymers may serve as technology platform for a safe way of information storage and release.
[41] Overt anti-counterfeiting labels have been constructed that display a visual symbol or code when exposed to specific chemicals.
[45][46] This phenomenon allows these composites to be potentially used to create deployable structures[47] such as booms,[48] hinges,[49] wings[50][51] etc.
