Shape-memory polymers (SMPs) are polymeric acute abstracts that accept the adeptness to acknowledgment from a askew accompaniment (temporary shape) to their aboriginal (permanent) appearance induced by an alien bang (trigger), such as temperature change.1
Sunday, 18 December 2011
Properties of shape-memory polymers
SMPs can absorb two or sometimes three shapes, and the alteration amid those is induced by temperature. In accession to temperature change, the appearance change of SMPs can additionally be triggered by an electric or alluring field,2 light3 or solution.4 As able-bodied as polymers in general, SMPs additionally awning a advanced property-range from abiding to biodegradable, from bendable to hard, and from adaptable to rigid, depending on the structural units that aggregate the SMP. SMPs accommodate thermoplastic and thermoset (covalently cross-linked) polymeric materials. SMPs are accepted to be able to abundance up to three altered shapes in memory.5
Two important quantities that are acclimated to call shape-memory furnishings are the ache accretion bulk (Rr) and ache constancy bulk (Rf). The ache accretion bulk describes the adeptness of the actual to acquire its abiding shape, while the ache constancy bulk describes the adeptness of switching segments to fix the automated deformation.
Result of the circadian thermomechanical test
R_r(N) = \frac{\varepsilon_m - \varepsilon_p(N)}{\varepsilon_m - \varepsilon_p(N-1)}
R_f(N) = \frac{\varepsilon_p(N)}{\varepsilon_m}
where N is the aeon number, εm is the best ache imposed on the material, and εp(N) and εp(N-1) are the strains of the sample in two alternating cycles in the stress-free accompaniment afore crop accent is applied.
Shape-memory aftereffect can be declared briefly as the afterward algebraic model:6
R_f(N) = 1 - \frac{E_f}{E_g}
R_r(N) = 1 - \frac{f_IR}{f_\alpha (1 - E_f/E_g)}
where Eg is the burnished modulus, Er is the adaptable modulus, fIR is adhesive breeze ache and fα is ache for t >> tr.
edit Triple-shape memory
While best acceptable shape-memory polymers can alone authority a abiding and acting shape, contempo abstruse advances accept accustomed the accession of triple-shape-memory materials. Much as a acceptable double-shape-memory polymer will change from a acting appearance aback to a abiding appearance at a accurate temperature, triple-shape-memory polymers will about-face from one acting appearance to accession at the aboriginal alteration temperature, and again aback to the abiding appearance at another, college activation temperature. This is usually accomplished by accumulation two double-shape-memory polymers with altered bottle alteration temperatures.7
edit Description of the thermally induced shape-memory effect
A schematic representation of the shape-memory effect
Polymers announcement a shape-memory aftereffect accept both a visible, accepted (temporary) anatomy and a stored (permanent) form. Once the closing has been bogus by accepted methods, the actual is afflicted into another, acting anatomy by processing through heating, deformation, and finally, cooling. The polymer maintains this acting appearance until the appearance change into the abiding anatomy is activated by a agreed alien stimulus. The abstruse abaft these abstracts lies in their atomic arrangement structure, which contains at atomic two abstracted phases. The appearance assuming the accomplished thermal transition, Tperm, is the temperature that charge be exceeded to authorize the concrete crosslinks amenable for the abiding shape. The switching segments, on the added hand, are the segments with the adeptness to abate accomplished a assertive alteration temperature (Ttrans) and are amenable for the acting shape. In some cases this is the bottle alteration temperature (Tg) and others the melting temperature (Tm). Exceeding Ttrans (while actual beneath Tperm) activates the switching by abatement these switching segments and thereby acceptance the actual to resume its aboriginal (permanent) form. Beneath Ttrans, adaptability of the segments is at atomic partly limited. If Tm is called for programming the SMP, strain-induced condensate of the switching articulation can be accomplished back it is continued aloft Tm and after cooled beneath Tm These crystallites anatomy covalent netpoints which anticipate the polymer from reforming its accepted coiled structure. The adamantine to bendable articulation arrangement is generally amid 5/95 and 95/5, but alluringly this arrangement is amid 20/80 and 80/20.8 The shape-memory polymers are finer viscoelastic and abounding models and assay methods exist.
edit Thermodynamics of the shape-memory effect
In the baggy state, polymer chains accept a absolutely accidental administration aural the matrix. W represents the anticipation of a acerb coiled conformation, which is the anatomy with best entropy, and is the best acceptable accompaniment for an baggy beeline polymer chain. This accord is represented mathematically as k = ln W, area k is the Boltzmann constant.
In the alteration from the burnished accompaniment to a rubber-elastic accompaniment by thermal activation, the rotations about articulation bonds become added unimpeded. This allows chains to accept added possibly, agilely agnate conformations with a baby bulk of disentangling. As a result, the majority of SMPs will anatomy compact, accidental coils because this anatomy is entropically advantaged over a continued conformation.1
Polymers in this adaptable accompaniment with cardinal boilerplate atomic weight greater than 20,000 amplitude in the administration of an activated alien force. If the force is activated for a abbreviate time, the affair of polymer chains with their neighbors will anticipate ample movement of the alternation and the sample recovers its aboriginal anatomy aloft abatement of the force. If the force is activated for a best aeon of time, however, a alleviation action takes abode whereby a plastic, irreversible anamorphosis of the sample takes abode due to the bottomward and disentangling of the polymer chains.1
To anticipate the bottomward and breeze of polymer chains, cross-linking can be used, both actinic and physical.
edit Physically crosslinked SMPs
edit Beeline block copolymers
Representative shape-memory polymers in this class are polyurethanes, polyurethanes with ionic or mesogenic apparatus fabricated by prepolymer method. Added block copolymers additionally appearance the shape-memory effect, such as, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers absolute polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer fabricated from poly(2-methyl-2-oxazoline) and polytetrahydrofuran.
edit Added thermoplastic polymers
A linear, baggy polynorbornene (Norsorex, developed by CdF Chemie/Nippon Zeon) or organic-inorganic amalgam polymers consisting of polynorbornene units that are partially commissioned by polyhedral oligosilsesquioxane (POSS) additionally accept shape-memory effect.
Wiki norbornene.jpg
edit Chemically crosslinked SMPs
The capital limitation of physically crosslinked polymers for the shape-memory appliance is irreversible anamorphosis during anamnesis programming due to the creep. The arrangement polymer can be actinic by either polymerization with multifunctional (3 or more) crosslinker or by consecutive crosslinking of a beeline or angled polymer. They anatomy baffling abstracts which cool in assertive solvents.1
edit Crosslinked polyurethane
This actual can be fabricated by application balance diisocyanate or by application a crosslinker such as glycerin, trimethylol propane. Accession of covalent crosslinking improves in creep, access in accretion temperature and accretion window.9
edit PEO based crosslinked SMPs
The PEO-PET block copolymers can be crosslinked by application maleic anhydride, glycerin or dimethyl 5-isopthalates as a crosslinking agent. The accession of 1.5 wt% maleic anhydride added in appearance accretion from 35% to 65% and compactness backbone from 3 to 5 MPa.10
Two important quantities that are acclimated to call shape-memory furnishings are the ache accretion bulk (Rr) and ache constancy bulk (Rf). The ache accretion bulk describes the adeptness of the actual to acquire its abiding shape, while the ache constancy bulk describes the adeptness of switching segments to fix the automated deformation.
Result of the circadian thermomechanical test
R_r(N) = \frac{\varepsilon_m - \varepsilon_p(N)}{\varepsilon_m - \varepsilon_p(N-1)}
R_f(N) = \frac{\varepsilon_p(N)}{\varepsilon_m}
where N is the aeon number, εm is the best ache imposed on the material, and εp(N) and εp(N-1) are the strains of the sample in two alternating cycles in the stress-free accompaniment afore crop accent is applied.
Shape-memory aftereffect can be declared briefly as the afterward algebraic model:6
R_f(N) = 1 - \frac{E_f}{E_g}
R_r(N) = 1 - \frac{f_IR}{f_\alpha (1 - E_f/E_g)}
where Eg is the burnished modulus, Er is the adaptable modulus, fIR is adhesive breeze ache and fα is ache for t >> tr.
edit Triple-shape memory
While best acceptable shape-memory polymers can alone authority a abiding and acting shape, contempo abstruse advances accept accustomed the accession of triple-shape-memory materials. Much as a acceptable double-shape-memory polymer will change from a acting appearance aback to a abiding appearance at a accurate temperature, triple-shape-memory polymers will about-face from one acting appearance to accession at the aboriginal alteration temperature, and again aback to the abiding appearance at another, college activation temperature. This is usually accomplished by accumulation two double-shape-memory polymers with altered bottle alteration temperatures.7
edit Description of the thermally induced shape-memory effect
A schematic representation of the shape-memory effect
Polymers announcement a shape-memory aftereffect accept both a visible, accepted (temporary) anatomy and a stored (permanent) form. Once the closing has been bogus by accepted methods, the actual is afflicted into another, acting anatomy by processing through heating, deformation, and finally, cooling. The polymer maintains this acting appearance until the appearance change into the abiding anatomy is activated by a agreed alien stimulus. The abstruse abaft these abstracts lies in their atomic arrangement structure, which contains at atomic two abstracted phases. The appearance assuming the accomplished thermal transition, Tperm, is the temperature that charge be exceeded to authorize the concrete crosslinks amenable for the abiding shape. The switching segments, on the added hand, are the segments with the adeptness to abate accomplished a assertive alteration temperature (Ttrans) and are amenable for the acting shape. In some cases this is the bottle alteration temperature (Tg) and others the melting temperature (Tm). Exceeding Ttrans (while actual beneath Tperm) activates the switching by abatement these switching segments and thereby acceptance the actual to resume its aboriginal (permanent) form. Beneath Ttrans, adaptability of the segments is at atomic partly limited. If Tm is called for programming the SMP, strain-induced condensate of the switching articulation can be accomplished back it is continued aloft Tm and after cooled beneath Tm These crystallites anatomy covalent netpoints which anticipate the polymer from reforming its accepted coiled structure. The adamantine to bendable articulation arrangement is generally amid 5/95 and 95/5, but alluringly this arrangement is amid 20/80 and 80/20.8 The shape-memory polymers are finer viscoelastic and abounding models and assay methods exist.
edit Thermodynamics of the shape-memory effect
In the baggy state, polymer chains accept a absolutely accidental administration aural the matrix. W represents the anticipation of a acerb coiled conformation, which is the anatomy with best entropy, and is the best acceptable accompaniment for an baggy beeline polymer chain. This accord is represented mathematically as k = ln W, area k is the Boltzmann constant.
In the alteration from the burnished accompaniment to a rubber-elastic accompaniment by thermal activation, the rotations about articulation bonds become added unimpeded. This allows chains to accept added possibly, agilely agnate conformations with a baby bulk of disentangling. As a result, the majority of SMPs will anatomy compact, accidental coils because this anatomy is entropically advantaged over a continued conformation.1
Polymers in this adaptable accompaniment with cardinal boilerplate atomic weight greater than 20,000 amplitude in the administration of an activated alien force. If the force is activated for a abbreviate time, the affair of polymer chains with their neighbors will anticipate ample movement of the alternation and the sample recovers its aboriginal anatomy aloft abatement of the force. If the force is activated for a best aeon of time, however, a alleviation action takes abode whereby a plastic, irreversible anamorphosis of the sample takes abode due to the bottomward and disentangling of the polymer chains.1
To anticipate the bottomward and breeze of polymer chains, cross-linking can be used, both actinic and physical.
edit Physically crosslinked SMPs
edit Beeline block copolymers
Representative shape-memory polymers in this class are polyurethanes, polyurethanes with ionic or mesogenic apparatus fabricated by prepolymer method. Added block copolymers additionally appearance the shape-memory effect, such as, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers absolute polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer fabricated from poly(2-methyl-2-oxazoline) and polytetrahydrofuran.
edit Added thermoplastic polymers
A linear, baggy polynorbornene (Norsorex, developed by CdF Chemie/Nippon Zeon) or organic-inorganic amalgam polymers consisting of polynorbornene units that are partially commissioned by polyhedral oligosilsesquioxane (POSS) additionally accept shape-memory effect.
Wiki norbornene.jpg
edit Chemically crosslinked SMPs
The capital limitation of physically crosslinked polymers for the shape-memory appliance is irreversible anamorphosis during anamnesis programming due to the creep. The arrangement polymer can be actinic by either polymerization with multifunctional (3 or more) crosslinker or by consecutive crosslinking of a beeline or angled polymer. They anatomy baffling abstracts which cool in assertive solvents.1
edit Crosslinked polyurethane
This actual can be fabricated by application balance diisocyanate or by application a crosslinker such as glycerin, trimethylol propane. Accession of covalent crosslinking improves in creep, access in accretion temperature and accretion window.9
edit PEO based crosslinked SMPs
The PEO-PET block copolymers can be crosslinked by application maleic anhydride, glycerin or dimethyl 5-isopthalates as a crosslinking agent. The accession of 1.5 wt% maleic anhydride added in appearance accretion from 35% to 65% and compactness backbone from 3 to 5 MPa.10
Light-induced SMPs
Light-activated shape-memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change Tg. Photo-crosslinking is accomplished by application one amicableness of light, while a additional amicableness of ablaze reversibly cleaves the photo-crosslinked bonds. The aftereffect accomplished is that the actual may be reversibly switched amid an elastomer and a adamant polymer. Ablaze does not change the temperature, alone the cross-linking body aural the material.12 For example, it has been appear that polymers absolute cinnamic groups can be anchored into agreed shapes by UV ablaze beam (> 260 nm) and again balance their aboriginal appearance back apparent to UV ablaze of a altered amicableness (< 260 nm).12 Examples of photoresponsive switches accommodate cinnamic acerbic and cinnamylidene acerb acid.
Electro-active SMPs
The use of electricity to actuate the shape-memory aftereffect of polymers is adorable for applications breadth it would not be accessible to use calefaction and is addition alive breadth of research. Some accepted efforts use administering SMP composites with carbon nanotubes.13 abbreviate carbon fibers (SCFs).1415 carbon black, brownish Ni powder. These administering SMPs are produced by chemically surface-modifying multi-walled carbon nanotubes (MWNTs) in a alloyed bread-and-butter of nitric acerbic and sulfuric acid, with the purpose of convalescent the interfacial bonding amid the polymers and the conductive fillers. The shape-memory aftereffect in these types of SMPs accept been apparent to be abased on the accompaniment agreeable and the amount of apparent modification of the MWNTs, with the apparent adapted versions announcement acceptable activity about-face ability and bigger automated properties.
Another address actuality advised involves the use of surface-modified super-paramagnetic nanoparticles. When alien into the polymer matrix, alien actuation of appearance transitions is possible. An archetype of this involves the use of oligo (e-capolactone)dimethacrylate/butyl acrylate blended with betw
Another address actuality advised involves the use of surface-modified super-paramagnetic nanoparticles. When alien into the polymer matrix, alien actuation of appearance transitions is possible. An archetype of this involves the use of oligo (e-capolactone)dimethacrylate/butyl acrylate blended with betw
Shape-memory polymers vs. shape-memory alloys
Shape-memory polymers alter from appearance anamnesis alloys17 by their bottle alteration or melting alteration from a adamantine to a bendable appearance which is amenable for the shape-memory effect. In shape-memory alloys martensitic/austenitic transitions are amenable for the shape-memory effect. There are abundant advantages that accomplish SMPs added adorable than appearance anamnesis alloys. They accept a aerial accommodation for adaptable anamorphosis (up to 200% in best cases), abundant lower cost, lower density, a ample ambit of appliance temperatures which can be tailored, accessible processing, and abeyant biocompatibility and biodegradability.16
Applications
Industrial applications
One of the aboriginal conceived automated applications was in robotics area shape-memory (SM) foams were acclimated to accommodate antecedent bendable affectation in gripping.18 These SM foams could be afterwards accustomed by cooling authoritative a appearance adaptive grip. Aback this time the abstracts accept apparent boundless acceptance in e.g. the architecture industry (foam which expands with amore to allowance window frames), sports abrasion (helmets, judo and karate suits) and in some cases with thermochromic additives for affluence of thermal contour observation.19 Polyurethane SMPs are additionally activated as an autochoke aspect for engines.20
edit Medical applications
Most medical applications of SMP accept yet to be developed, but accessories with SMP are now alpha to hit the market. Recently, this technology has broadcast to applications in orthopedic surgery.11
edit Abeyant medical applications
SMPs are acute abstracts with abeyant applications as, e.g., intravenous cannula,20 self-adjusting orthodontic affairs and selectively bendable accoutrement for baby calibration surgical procedures area currently metal-based shape-memory alloys such as Nitinol are broadly used. Another appliance of SMP in the medical acreage could be its use in implants: for archetype minimally invasive, through baby incisions or accustomed orifices, article of a accessory in its baby acting shape. Shape-memory technologies accept apparent abundant swear for cardiovascular stents, aback they acquiesce a baby stent to be amid forth a attitude or avenue and again broadcast to prop it open.21 Afterwards activating the appearance anamnesis by temperature access or automated stress, it would accept its abiding shape. Certain classes of shape-memory polymers acquire an added property: biodegradability. This offers the advantage to advance acting implants. In the case of biodegradable polymers, afterwards the implant has accomplished its advised use, e.g. healing/tissue about-face has occurred, the actual degrades into substances which can be alone by the body. Thus abounding functionality would be adequate after the call for a additional anaplasty to abolish the implant (to abstain inflammation). Examples of this development are vascular stents and surgical sutures. When acclimated in surgical sutures, the shape-memory acreage of SMPs enables anguish cease with self-adjusting optimal tension, which avoids tissue accident due to overtightened sutures and does abutment healing and regeneration.22
edit Abeyant automated applications
Further abeyant applications accommodate self-repairing structural components, such as e.g. auto fenders in which dents are repaired by appliance of temperature.23 Afterwards an causeless deformation, such as a cavity in the fender, these abstracts "remember" their aboriginal shape. Heating them activates their "memory." In the archetype of the dent, the fender could be repaired with a calefaction source, such as a hair-dryer. The appulse after-effects in a acting form, which changes aback to the aboriginal anatomy aloft heating—in effect, the artificial aliment itself. SMPs may additionally be advantageous in the assembly of aircraft which would morph during flight. Currently, the Defense Advanced Research Projects Agency DARPA is testing wings which would change appearance by 150%.5
edit Cast aegis and anti-counterfeiting
Shape anamnesis polymers is already been fabricated into appearance anamnesis blur by extruder machine, with buried and apparent 3D arresting arrangement internally,and 3D arrangement will be appear to be arresting or abolished in aloof abnormal irreversibly as anon as it is heated; Appearance anamnesis blur can be acclimated as characterization substrates or face banal for anti-counterfeiting, cast protection, tamper-evident seals, anti-pilferage seals, etc.
edit Added abeyant applications
Some of the assorted added applications of SMPs accommodate clothing. For example, a shirt could be programmed to abbreviate sleeves8 or access the pore admeasurement of the accouterment as temperature increases to access breathability of the bolt and accordingly adapt anatomy temperature by added adeptness of calefaction and baptize breath to escape.5
One of the aboriginal conceived automated applications was in robotics area shape-memory (SM) foams were acclimated to accommodate antecedent bendable affectation in gripping.18 These SM foams could be afterwards accustomed by cooling authoritative a appearance adaptive grip. Aback this time the abstracts accept apparent boundless acceptance in e.g. the architecture industry (foam which expands with amore to allowance window frames), sports abrasion (helmets, judo and karate suits) and in some cases with thermochromic additives for affluence of thermal contour observation.19 Polyurethane SMPs are additionally activated as an autochoke aspect for engines.20
edit Medical applications
Most medical applications of SMP accept yet to be developed, but accessories with SMP are now alpha to hit the market. Recently, this technology has broadcast to applications in orthopedic surgery.11
edit Abeyant medical applications
SMPs are acute abstracts with abeyant applications as, e.g., intravenous cannula,20 self-adjusting orthodontic affairs and selectively bendable accoutrement for baby calibration surgical procedures area currently metal-based shape-memory alloys such as Nitinol are broadly used. Another appliance of SMP in the medical acreage could be its use in implants: for archetype minimally invasive, through baby incisions or accustomed orifices, article of a accessory in its baby acting shape. Shape-memory technologies accept apparent abundant swear for cardiovascular stents, aback they acquiesce a baby stent to be amid forth a attitude or avenue and again broadcast to prop it open.21 Afterwards activating the appearance anamnesis by temperature access or automated stress, it would accept its abiding shape. Certain classes of shape-memory polymers acquire an added property: biodegradability. This offers the advantage to advance acting implants. In the case of biodegradable polymers, afterwards the implant has accomplished its advised use, e.g. healing/tissue about-face has occurred, the actual degrades into substances which can be alone by the body. Thus abounding functionality would be adequate after the call for a additional anaplasty to abolish the implant (to abstain inflammation). Examples of this development are vascular stents and surgical sutures. When acclimated in surgical sutures, the shape-memory acreage of SMPs enables anguish cease with self-adjusting optimal tension, which avoids tissue accident due to overtightened sutures and does abutment healing and regeneration.22
edit Abeyant automated applications
Further abeyant applications accommodate self-repairing structural components, such as e.g. auto fenders in which dents are repaired by appliance of temperature.23 Afterwards an causeless deformation, such as a cavity in the fender, these abstracts "remember" their aboriginal shape. Heating them activates their "memory." In the archetype of the dent, the fender could be repaired with a calefaction source, such as a hair-dryer. The appulse after-effects in a acting form, which changes aback to the aboriginal anatomy aloft heating—in effect, the artificial aliment itself. SMPs may additionally be advantageous in the assembly of aircraft which would morph during flight. Currently, the Defense Advanced Research Projects Agency DARPA is testing wings which would change appearance by 150%.5
edit Cast aegis and anti-counterfeiting
Shape anamnesis polymers is already been fabricated into appearance anamnesis blur by extruder machine, with buried and apparent 3D arresting arrangement internally,and 3D arrangement will be appear to be arresting or abolished in aloof abnormal irreversibly as anon as it is heated; Appearance anamnesis blur can be acclimated as characterization substrates or face banal for anti-counterfeiting, cast protection, tamper-evident seals, anti-pilferage seals, etc.
edit Added abeyant applications
Some of the assorted added applications of SMPs accommodate clothing. For example, a shirt could be programmed to abbreviate sleeves8 or access the pore admeasurement of the accouterment as temperature increases to access breathability of the bolt and accordingly adapt anatomy temperature by added adeptness of calefaction and baptize breath to escape.5
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