Shape-memory polymer
Shape Memory Polymers (SMPs) are polymeric materials which have the ability to return from a deformed state (temporary shape) to their original (permanent) shape induced by an external stimulus (trigger), such as e.g. temperature change.[1]
Properties of shape memory polymers
In addition to temperature change, the shape memory effect of SMPs can also be triggered by an electric[2] or magnetic field [3], light [4] or a change in pH. As well as polymers in general, SMP also cover a wide property-range 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 [5].
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.
File:Strainrecovery.jpg File:Strainfixity.jpg
where N is the cycle number, File:Maxstrain.jpg is the maximum strain imposed on the material, and File:Nstrain.jpg and File:N1strain.jpg are the strains of the sample in to successive cycles in the stress-free state before yield stress is applied.
Shape memory effect can be described briefly as the following mathematical model.[6]
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 ,File:Permtemp.jpg, is the temperature that must 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(File:Transtemp.jpg) and are responsible for the temporary shape. In some cases this is the glass transition temperature (File:Glasstrans.jpg) and others the melting temperature (File:Melttrans.jpg). Exceeding File:Transtemp.jpg (while remaining below File:Permtemp.jpg) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. Below File:Transtemp.jpg, flexibility of the segments is at least partly limited. If File:Melttrans.jpg is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above File:Melttrans.jpg and subesequently cooled below File:Melttrans.jpg These crystallites form covalent netpoints which prevent the polymer from reforming it's 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[7]. The shape memory polymers are effectively visco-elastic [8] 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
where 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.[9]
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 it's 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.[9]
To prevent the slipping an flow of polymer chains, cross-linking can be used, both chemical or 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 polyethyleneterephrhalate (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 poly(Tetrahydrofuran).
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.
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 multifuctional (tri or higher functional) crosslinker or by subsequent crosslinking of a linear or branched polymer. They form insoluble materials which swell in certain solvents.[9]
Crosslinked polyurethane
This material can be made by using excess diisocyanate or by using a crosslinker such as glycerin, trimethylol propane. According to Buckley et al., introduction of covalent crosslinking improves in creep, increase in recovery temperature and recovery window. [10]
PEO based crosslinked SMPs
The PEO-PET block copolymers can be crosslinked by using maleic anhydride, glycerin or dimethyl 5-isopthalates 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.[11]
Light induced SMPs
Light activated shape memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change File:Glasstrans.jpg. 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.[12] For example, it has been reported that polymers containing cinnamic groups can be fixed into predetermined shapes by UV light illumination (> 260nm) and then recover their original shape when exposed to UV light of a different wavelength (< 260nm).[12] 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 which it would not be possible to use heat and is another active area of research. Some current efforts use conducting SMP composites using carbon nanotubes.[2] 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 tobe dependant on the filler content and the degree of surface modification of the MWNTs, with the surface modified versions exhibiting good energy conversion efficieny 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 oligo (e-capolactone)dimethacrylate/butyl acrylate composite with between 2-12% magnetite nanoparticles. Nickel and hybrid fibers have also been used wwth some degree of success.[2]
SMPs versus Shape Memory Alloys
Shape memory polymers differ from shape memory alloys [14] 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/Austenitic 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, and potential biocompatibility and biodegradability. [13]
Applications
Industrial applications
One of the first conceived industrial applications was in robotics [15] where shape memory (SM) foams were used to provide initial soft prehension 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 e.g. 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. [16] Polyurethane SMPs are also applied as an autochoke element for engines.[17]
Potential medical applications
SMPs are smart materials with potential applications as, e.g., intravenous cannula,[17] 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, e.g. minimally invasive (trough small incisions or natural orifices) implantation of a device in its small temporary shape which after activating the shape memory by e.g. temperature increase assumes its permanent (and mostly bulkier) shape. Certain classes of shape memory polymers possess an additional property: biodegradability. This offers the option to develop temporary implants. 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 allowing full functionality to be restored without the necessity for a second surgery to remove the implant in order to avoid long term negative effects (e.g. inflammation) . Examples of this development are e.g. vascular stents and surgical sutures (see for example www.mnemoscience.com). 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 too tight sutures and does support healing / regeneration. [18]
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.[19] 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 is testing wings which would change shape by 150%.[20]
Other potential applications
Some of the various other applications of SMPs includes clothing. For example, a shirt could be programmed to shorten sleeves[7] or increase the pore size of the clothing as temperature increases to increase breathability of the fabric and therefore regulate body temperature by increased ability of heat and water vapor to escape.[20]
See Also
News
The most recent developments are triple shape memory materials [21] which can store two shapes in memory.
External links
References
- ^ Lendlein, A., Kelch, S. Shape-memory polymers. Angew. Chem. Int. Ed. Engl. 41, 2034-2057 (2002).
- ^ a b c Liu Y et al., Review of electro-active shape-memory polymer composite, Compos Sci Technol (2009), doi:10.1016/j.compscitech.2008.08.016
- ^ Mohr, R. et al. Initiation of shape-memory effect by inductive heating of magnetic nanoparticles in thermoplastic polymers. Proc. Natl. Acad. Sci. U.S.A. 103, 3540-3545 (2006). Copyright (2006) National Academy of Sciences, U.S.A.
- ^ Lendlein, A. et al. Light-induced shape-memory polymers. Nature 434, 879-882 (2005).
- ^ Toensmeier, P.A.,"Shape memory polymers reshape product design". Plastics Engineering. FindArticles.com. 02 Apr, 2009. http://findarticles.com/p/articles/mi_hb6619/is_3_61/ai_n29164103/
- ^ Kim B.K.; Lee S.Y.; Xu M. Polymer, 1996, 37, 5781
- ^ a b Shanmugasundaram, O.L.,"Shape Memory Polymers & their applications". The Indian Textile Journal. 04 Apr, 2009. http://www.indiantextilejournal.com/articles/FAdetails.asp?id=776
- ^ Tobushi. H., Hashimoto T., Hayashi S. & Yamada E. Thermomechanical Constitutive Modeling in Shape Memory Polymer of Polyurethane Series. Journal of Intelligent Material Systems and Structures. Vol 8, Technomic (August 1997).
- ^ a b c Cite error: The named reference
Lendleinwas invoked but never defined (see the help page). - ^ Buckley CP.; Prisacariu C.; Caraculacu A. Polymer, 2007, 48, 1388
- ^ Park C.; Lee J.Y.; Chun B.C.; Chung Y.C.; Cho J.W.; Cho B.G. J. Appl Polym Sci., 2004, 94, 308
- ^ a b Havens, E., Snyder, E.A., Tong, T.H.,Light-activated shape memory polymers and associated applications, Proc. SPIE 5762, 48 (2005), DOI:10.1117/12.606109
- ^ a b Liu Y et al., Review of progress in shape-memory polymers, J. Mater. Chem, 2007, 17, 1543-1558
- ^ Czichos H. Adolf Martens and the Research on Martensite. The Martensitic Transformation in Science and Technology (Ed. E. Hornbogen & N. Jost). pp 3–14, Informationsgesellschaft (1989).
- ^ Brennan, Mairin Chemical and Engineering News, 2001, 79, p. 5.
- ^ Monkman. G.J. & P.M. Taylor Memory Foams for Robot Grippers Robots in Unstructured Environments Proc. 5th Intl. Conf. on Advanced Robotics, pp. 339 342, Pisa, June 1991.
- ^ a b H. Tobushi et al. Sci. Technol. Adv. Mater. 9 (2008) 015009 "Shape recovery and irrecoverable strain control in polyurethane shape-memory polymer" free download
- ^ Lendlein, A., Langer, R.: Biodegradable, Elastic Shape Memory Polymers for Potential Biomedical Applications, Science 296, 1673-1675 (2002).
- ^ Monkman. G.J. - Advances in Shape Memory Polymer Actuation - Mechatronics - Vol 10, No. 4/5, pp. 489–498 - Pergamon June/August 2000.
- ^ a b Toensmeier, P.A.,"Shape memory polymers reshape product design". Plastics Engineering. FindArticles.com. 02 Apr, 2009. http://findarticles.com/p/articles/mi_hb6619/is_3_61/ai_n29164103/
- ^ Bellin, I., Kelch, S., Langer, R. & Lendlein, A. [1]. Proc. Natl. Acad. Sci. U.S.A. 103, 18043-18047 (2006. Copyright (2006) National Academy of Sciences, U.S.A.