Shape memory polymer
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Shape-memory polymers (SMPs) are polymeric
smart material Smart materials, also called intelligent or responsive materials, are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, moisture, electric or magnetic ...
s 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 A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
and
thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer ( resin). Curing is induced by heat or suitable radiation ...
(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 (''R''r) and strain fixity rate (''R''f). 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. :R_r(N) = \frac :R_f(N) = \frac where N is the cycle number, \varepsilon_m is the maximum strain imposed on the material, and \varepsilon_p(N) and \varepsilon_p(N-1) 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: :R_f(N) = 1 - \frac :R_r(N) = 1 - \frac where E_g is the glassy modulus, E_r is the rubbery modulus, f_ is viscous flow strain and f_ is strain for t >> t_r.


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 The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...
(''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 In statistical mechanics, Boltzmann's equation (also known as the Boltzmann–Planck equation) is a probability equation relating the entropy S, also written as S_\mathrm, of an ideal gas to the multiplicity (commonly denoted as \Omega or W), the ...
''S'' = ''k'' ln ''W'', where ''S'' is the
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
and ''k'' is Boltzmann's 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 The molar mass distribution (or molecular weight distribution) describes the relationship between the number of moles of each polymer species (Ni) and the molar mass (Mi) of that species. In linear polymers, the individual polymer chains rarely have ...
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
polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from ...
s, polyurethanes with ionic or mesogenic components made by
prepolymer In polymer chemistry, the term prepolymer or pre-polymer, refers to a monomer or system of monomers that have been reacted to an intermediate-molecular mass state. This material is capable of further polymerization by reactive groups to a fully c ...
method. Other
block copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are ...
s 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 Polytetrahydrofuran, also called poly(tetramethylene ether) glycol or poly(tetramethylene oxide), is a chemical compound with formula ()''n''OH2 or HO-(-(CH2)4O-)''n''-H. It can be viewed as a polymer of tetrahydrofuran, or as the polyether deriv ...
.


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 oligo silsesquioxane (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 In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
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 Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...
, trimethylol propane. Introduction of covalent crosslinking improves in creep, increase in recovery temperature and recovery window.


PEO based crosslinked SMPs

The PEO-PET block
copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s can be crosslinked by using
maleic anhydride Maleic anhydride is an organic compound with the formula C2H2(CO)2O. It is the acid anhydride of maleic acid. It is a colorless or white solid with an acrid odor. It is produced industrially on a large scale for applications in coatings and pol ...
, 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.


Thermoplastic shape-memory

While shape-memory effects are traditionally limited to
thermosetting plastic In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
s, some
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
polymers, most notably
PEEK Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PAEK) family, used in engineering applications. The polymer was first developed in November 1978, later being introduced to the market by Vic ...
, 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 Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
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 Cinnamic acid is an organic compound with the formula C6H5-CH=CH- COOH. It is a white crystalline compound that is slightly soluble in water, and freely soluble in many organic solvents. Classified as an unsaturated carboxylic acid, it occurs na ...
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 A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon na ...
, 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 Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
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 Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
nanoparticles.
Nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
and hybrid fibers have also been used with some degree of success.


Shape-memory polymers vs. shape-memory alloys

Shape-memory polymers differ from shape memory alloys (SMAs) by their glass transition or melting transition from a hard to a soft phase which is responsible for the shape-memory effect. In shape-memory alloys
martensitic Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties Marte ...
/
austenitic Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 ...
transitions are responsible for the shape-memory effect. There are numerous advantages that make SMPs more attractive than shape memory alloys. 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, and probably exhibit superior mechanical properties than SMAs.


Applications


Industrial applications

One of the first conceived industrial applications was in robotics where shape-memory (SM) foams were used to provide initial soft pretension in gripping. These SM foams could be subsequently hardened by cooling, making a shape adaptive grip. 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 Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an excellent example of this phenomenon, but thermochromism also has more practical uses, such as baby bottles which change to a differen ...
additives for ease of thermal profile observation. Polyurethane SMPs are also applied as an autochoke element for engines.


Application in photonics

One field in which SMPs are having a significant impact is photonics. Due to the shape changing capability, SMPs enable the production of functional and responsive photonic gratings. 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. As a result of the refractive index periodicity, these systems diffract light. 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. Another application of SMPs in photonics is shape changing random lasers. By doping SMPs with highly scattering particles such as titania it is possible to tune the light transport properties of the composite. Additionally, optical gain may be introduced by adding a molecular dye to the material. 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. Shape memory polymers have also been used in conjunction with nanocellulose to fabricate composites exhibiting both chiroptical properties and thermo-activated shape memory effect.


Medical applications

Most medical applications of SMP have yet to be developed, but devices with SMP are now beginning to hit the market. Recently, this technology has expanded to applications in
orthopedic surgery Orthopedic surgery or orthopedics ( alternatively spelt orthopaedics), is the branch of surgery concerned with conditions involving the musculoskeletal system. Orthopedic surgeons use both surgical and nonsurgical means to treat musculoskeletal ...
. Additionally, SMPs are now being used in various ophthalmic devices including punctal plugs, glaucoma shunts and intraocular lenses.


Potential medical applications

SMPs are
smart material Smart materials, also called intelligent or responsive materials, are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, moisture, electric or magnetic ...
s with potential applications as, e.g., intravenous cannula, 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. Another application of SMP in the medical field could be its use in implants: for example minimally invasive, through small incisions or natural orifices, implantation of a device in its small temporary shape. Shape-memory technologies have shown great promise for cardiovascular stents, since they allow a small stent to be inserted along a vein or artery and then expanded to prop it open. After activating the shape memory by temperature increase or mechanical stress, it would assume its permanent shape. Certain classes of shape-memory polymers possess an additional property:
biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
. This offers the option to develop temporary implants. 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. Thus full functionality would be restored without the necessity for a second surgery to remove the implant. Examples of this development are vascular
stent In medicine, a stent is a metal or plastic tube inserted into the lumen of an anatomic vessel or duct to keep the passageway open, and stenting is the placement of a stent. A wide variety of stents are used for different purposes, from expandab ...
s and surgical sutures. When used in surgical sutures, the shape-memory property of SMPs enables wound closure with self-adjusting optimal tension, which avoids tissue damage due to overtightened sutures and does support healing and regeneration. SMPs have also potential for use as
compression garment Compression garments are pieces of clothing that fit tightly around the skin. In medical contexts, compression garments provide support for people who have to stand for long periods or have poor circulation. These come in varying degrees of co ...
s and hands-free door openers, whereby the latter can be produced via so-called 4D printing.


Potential industrial applications

Further potential applications include self-repairing structural components, such as e.g. automobile fenders in which dents are repaired by application of temperature. After an undesired deformation, such as a dent in the fender, these materials "remember" their original shape. Heating them activates their "memory". In the example of the dent, the fender could be repaired with a heat source, such as a hair-dryer. The impact results in a temporary form, which changes back to the original form upon heating—in effect, the plastic repairs itself. SMPs may also be useful in the production of aircraft which would morph during flight. Currently, the Defense Advanced Research Projects Agency
DARPA The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Ad ...
is testing wings which would change shape by 150%.Toensmeier, P.A. (2 April 2009
"Shape memory polymers reshape product design"
Plastics Engineering.
The realization of a better control over the switching behavior of polymers is seen as key factor to implement new technical concepts. For instance, an accurate setting of the onset temperature of shape recovering can be exploited to tune the release temperature of information stored in a shape memory polymer. This may pave the way for the monitoring of temperature abuses of food or pharmaceuticals. Recently, a new manufacturing process, mnemosynation, was developed at
Georgia Tech The Georgia Institute of Technology, commonly referred to as Georgia Tech or, in the state of Georgia, as Tech or The Institute, is a public research university and institute of technology in Atlanta, Georgia. Established in 1885, it is part of ...
to enable mass production of crosslinked SMP devices, which would otherwise be cost-prohibitive using traditional thermoset polymerization techniques. Mnemosynation was named for the Greek goddess of memory,
Mnemosyne In Greek mythology and ancient Greek religion, Mnemosyne (; grc, Μνημοσύνη, ) is the goddess of memory and the mother of the nine Muses by her nephew Zeus. In the Greek tradition, Mnemosyne is one of the Titans, the twelve divine chil ...
, and is the controlled imparting of memory on an amorphous thermoplastic materials utilizing radiation-induced covalent crosslinking, much like
vulcanization Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to includ ...
imparts recoverable elastomeric behavior on rubbers using sulfur crosslinks. Mnemosynation combines advances in ionizing radiation and tuning the mechanical properties of SMPs to enable traditional plastics processing (
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...
,
blow molding Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes. In general, there are three main types of blow molding: extrusion blow molding, injectio ...
, injection molding,
resin transfer molding Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave (industrial) curing process commonly used by the aerospace manufacturers for manufacturing composite material. Out of autoclave (OOA) is a pro ...
, etc.) and allows thermoset SMPs in complex geometries. 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.


Brand protection and anti-counterfeiting

Shape memory polymers may serve as technology platform for a safe way of information storage and release. Overt anti-counterfeiting labels have been constructed that display a visual symbol or code when exposed to specific chemicals. Multifunctional labels may even make counterfeiting increasingly difficult. Shape memory polymers have already been made into shape memory film by extruder machine, with covert and overt 3D embossed pattern internally, and 3D pattern will be released to be embossed or disappeared in just seconds irreversibly as soon as it is heated; Shape memory film can be used as label substrates or face stock for anti-counterfeiting,
brand protection Brand protection is the process and set of actions that a right holder undertakes to prevent third parties from using its intellectual property without permission, as this may cause loss of revenue and, usually more importantly, destroys brand eq ...
, tamper-evident seals, anti-pilferage seals, etc.


Multifunctional composites

Using shape memory polymers as matrices, multifunctional
composite material A composite material (also called a composition material or shortened to composite, which is the common name) is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or ...
s can be produced. Such composites can have temperature dependant shape morphing (i.e. shape memory) characteristics. This phenomenon allows these composites to be potentially used to create deployable structures such as booms, hinges, wings etc. While using SMPs can help produce one-way shape morphing structures, it has been reported that using SMPs in combination with shape memory alloys allows creation of more complex shape memory composites that is capable of two-way shape memory deformation.


See also

*
Memory foam Memory foam consists mainly of polyurethane with additional chemicals that increase its viscosity and density. It is often referred to as " viscoelastic" polyurethane foam, or low-resilience polyurethane foam ( LRPu). The foam bubbles or ‘cells ...
*
Smart material Smart materials, also called intelligent or responsive materials, are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, moisture, electric or magnetic ...
*
Shape-memory alloy In metallurgy, a shape-memory alloy (SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It may also be called memory metal, memory alloy, smart metal, smart alloy, or muscle wire. P ...


References

{{Authority control Polymers Smart materials