Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape when induced by an external stimulus (trigger), such as temperature change.
Properties of shape-memory polymers SMPs can retain two or sometimes three shapes, and the transition between those is often induced by temperature change. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, light or solution. 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. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. SMPs are known to be able to store up to three different shapes in memory. SMPs have demonstrated recoverable strains of above 800%.
Two important quantities that are used to describe shape-memory effects are the strain recovery rate (Rr) and strain fixity rate (Rf). The strain recovery rate describes the ability of the material to memorize its permanent shape, while the strain fixity rate describes the ability of switching segments to fix the mechanical deformation.
where is the cycle number, is the maximum strain imposed on the material, and and are the strains of the sample in two successive cycles in the stress-free state before yield stress is applied.
Shape-memory effect can be described briefly as the following mathematical model:
where is the glassy modulus, is the rubbery modulus, is viscous flow strain and is strain for .
Triple-shape memory 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. Much as a traditional double-shape-memory polymer will change from a temporary shape back to a permanent shape at a particular temperature, triple-shape-memory polymers will switch from one temporary shape to another at the first transition temperature, and then back to the permanent shape at another, higher activation temperature. This is usually achieved by combining two double-shape-memory polymers with different glass transition temperatures or when heating a programmed shape-memory polymer first above the glass transition temperature and then above the melting transition temperature of the switching segment.
Description of the thermally induced shape-memory effect
Polymers exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (permanent) form. 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 polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus. The secret behind these materials lies in their molecular network structure, which contains at least two separate phases. 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. The switching segments, on the other hand, are the segments with the ability to soften past a certain transition temperature (Ttrans) and are responsible for the temporary shape. In some cases this is the glass transition temperature (Tg) and others the melting temperature (Tm). Exceeding Ttrans (while remaining below Tperm) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. Below Ttrans, flexibility of the segments is at least partly limited. If Tm is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above Tm and subsequently cooled below Tm. These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure. The hard to soft segment ratio is often between 5/95 and 95/5, but ideally this ratio is between 20/80 and 80/20. The shape-memory polymers are effectively viscoelastic and many models and analysis methods exist.
Thermodynamics of the shape-memory effect In the amorphous state, polymer chains assume a completely random distribution within the matrix. W represents the probability of a strongly coiled conformation, which is the conformation with maximum entropy, and is the most likely state for an amorphous linear polymer chain. This relationship is represented mathematically by Boltzmann's entropy formula S = k ln W, where S is the entropy and k is the Boltzmann constant.
In the transition from the glassy state to a rubber-elastic state by thermal activation, the rotations around segment bonds become increasingly unimpeded. This allows chains to assume other possibly, energetically equivalent conformations with a small amount of disentangling. As a result, the majority of SMPs will form compact, random coils because this conformation is entropically favored over a stretched conformation.
Polymers in this elastic state with number average molecular weight greater than 20,000 stretch in the direction of an applied external force. If the force is applied for a short time, the entanglement of polymer chains with their neighbors will prevent large movement of the chain and the sample recovers its original conformation upon removal of the force. If the force is applied for a longer period of time, however, a relaxation process takes place whereby a plastic, irreversible deformation of the sample takes place due to the slipping and disentangling of the polymer chains.
To prevent the slipping and flow of polymer chains, cross-linking can be used, both chemical and physical.
Physically crosslinked SMPs
Linear block copolymers Representative shape-memory polymers in this category are polyurethanes, polyurethanes with ionic or mesogenic components made by prepolymer method. Other block copolymers also show the shape-memory effect, such as, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran.
Other thermoplastic polymers A linear, amorphous polynorbornene (Norsorex, developed by CdF Chemie/Nippon Zeon) or organic-inorganic hybrid polymers consisting of polynorbornene units that are partially substituted by polyhedral oligosilsesquioxane (POSS) also have shape-memory effect.
Another example reported in the literature is a copolymer consisting of polycyclooctene (PCOE) and ) (PNBEDCA), which was synthesized through ring-opening metathesis polymerization (ROMP). Then the obtained copolymer was readily modified by grafting reaction of NBEDCA units with polyhedral oligomeric silsesquioxanes (POSS) to afford a functionalized copolymer . It exhibits shape-memory effect.
Chemically crosslinked SMPs The main limitation of physically crosslinked polymers for the shape-memory application is irreversible deformation during memory programming due to the creep. The network polymer can be synthesized by either polymerization with multifunctional (3 or more) crosslinker or by subsequent crosslinking of a linear or branched polymer. They form insoluble materials which swell in certain solvents.
Crosslinked polyurethane This material can be made by using excess diisocyanate or by using a crosslinker such as glycerin, trimethylol propane. Introduction of covalent crosslinking improves in creep, increase in recovery temperature and recovery window.
PEO based crosslinked SMPs The PEO-PET block copolymers can be crosslinked by using maleic anhydride, glycerin or dimethyl 5-isophthalates as a crosslinking agent. The addition of 1.5 wt% maleic anhydride increased in shape recovery from 35% to 65% and tensile strength from 3 to 5 MPa.
Hard phaseCrosslinkerTr (°C)Rf(5)(%)Rf(5)(%)PETGlycerol/dimethyl 5-sulfoisophthalate11–3090–9560–70PETMaleic anhydride8–1391–9360AA/MAA copolymerN,N'-methylene-bis-acrylamide90 99MAA/N-vinyl-2-pyrrolidoneEthyleneglycol dimethacrylate90 99PMMA/N-vinyl-2-pyrrolidoneEthyleneglycol dimethacrylate45, 100 99
Thermoplastic shape-memory While shape-memory effects are traditionally limited to thermosetting plastics, some thermoplastic polymers, most notably PEEK, can be used as well.
Light-induced SMPs Light-activated shape-memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change Tg. Photo-crosslinking is achieved by using one wavelength of light, while a second wavelength of light reversibly cleaves the photo-crosslinked bonds. The effect achieved is that the material may be reversibly switched between an elastomer and a rigid polymer. Light does not change the temperature, only the cross-linking density within the material. For example, it has been reported that polymers containing cinnamic groups can be fixed into predetermined shapes by UV light illumination (> 260 nm) and then recover their original shape when exposed to UV light of a different wavelength (< 260 nm). Examples of photoresponsive switches include cinnamic acid and cinnamylidene acetic acid.
Electro-active SMPs The use of electricity to activate the shape-memory effect of polymers is desirable for applications where it would not be possible to use heat and is another active area of research. Some current efforts use conducting SMP composites with carbon nanotubes, short carbon fibers (SCFs), carbon black, or metallic Ni powder. These conducting SMPs are produced by chemically surface-modifying multi-walled carbon nanotubes (MWNTs) in a mixed solvent of nitric acid and sulfuric acid, with the purpose of improving the interfacial bonding between the polymers and the conductive fillers. The shape-memory effect in these types of SMPs have been shown to be dependent on the filler content and the degree of surface modification of the MWNTs, with the surface modified versions exhibiting good energy conversion efficiency and improved mechanical properties.
Another technique being investigated involves the use of surface-modified super-paramagnetic nanoparticles. When introduced into the polymer matrix, remote actuation of shape transitions is possible. An example of this involves the use of composite with between 2 and 12% magnetite nanoparticles. Nickel and hybrid fibers have also been used with some degree of success.
Humidity-driven SMPs
Actuation of SMPs is most commonly achieved by thermal activation. The actuation could also typically be achieved through immersion at high or low temperatures.
In some cases, humidity is used to reduce the glass transition temperature (Tg) of the material, thereby lowering the temperature required for actuation. More recently, studies have shown that SMPs can exhibit shape-memory behavior entirely at room temperature, without requiring changes in thermal conditions for programming or actuation.
The mechanism of humidity-driven actuation has been associated with the creation of hydrogen bonds within the polymeric network, as observed by Fourier-transform infrared spectroscopy (FTIR). Hydrogen bonds are believed to increase the flexibility of polymer chains, allowing for motion of the shape-memory structure. In particular, chitosan-based composites have demonstrated reversible actuation at ambient humidity, highlighting their potential for sustainable and low-energy applications in smart materials.
Shape-memory polymers vs. shape-memory alloys