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
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