Shape Memory Polymer

Shape Memory Polymer

Shape Memory Polymers (SMPs) are polymer materials which may be returned from a deformed state to their original shape or "permanent configuration" via an external stimulus. The external stimulus is usually temperature, as in the case of thermally activated SMPs, but can also be the application of an electric or magnetic field, light or a change in pH. An SMP is an example of a smart material. Projected applications include self-adjusting orthodontic wires and selectively pliable tools for small scale surgical procedures where metal-based shape memory alloys such as Nitinol are widely used. Additional applications include self-repairing structural components, such as automobile fenders in which dents are repaired by application of heat. [Brennan, Mairin "Chemical and Engineering News", 2001, "79", p. 5 ]

Shape memory polymers differ from shape memory alloys [Czichos H. Adolf Martens and the Research on Martensite. The Martensitic Transformation in Science and Technology (Ed. E. Hornbogen & N. Jost). pp 3–14, Informationsgesselschaft (1989)] in that a glass transition from a hard to a soft phase is responsible for the effect rather than metallic Martensitic/Austenitic transitions.

Polymers with shape memory have both a visible, current form and a stored, permanent form. Once the latter has been produced by conventional methods, the material is molded into a second, temporary form by skillful heating, deformation, and finally, cooling. The plastic maintains this shape until the permanent form is recalled by a predetermined external stimulus. The secret behind these materials lies in their molecular network structure, which contains meltable "switching segments." Raising the temperature activates the switching: the crystallized switching segments melt and the material resumes its original form. The materials is effectively visco-elastic [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)] and many models and analysis methods exist.

After an undesired deformation, such as a dent in the fender, these plastics "remember" their original shape. Heating them both improves and activates their "memory." In this way, the dent could then be removed with a hair-dryer, for example. In this case, the important form is its undamaged original shape. The impact results in a temporary form, which changes back to the original form upon heating -- in effect, the plastic repairs itself.

One of the first conceived industrial applications was in robotics [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] where shape memory foams were used to provide initial soft prehension in gripping which could be subsequently hardened by cooling making a shape adaptive grip. Since then the materials have seen widespread usage in the building industry (foam which expands with warmth to seal window frames), sports wear (helmets, judo and karate suits) etc. and in some cases with thermochromic additives for ease of thermal profile observation [Monkman. G.J. - Advances in Shape Memory Polymer Actuation - Mechatronics - Vol 10, No. 4/5, pp. 489–498 - Pergamon June/August 2000] .

External links

* [ TEMBO Shape Memory Polymer]
* [ Veriflex Shape Memory Polymer]
* [ Memory Polymer News]


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