MadSci Network: Engineering

Re: How do shape memory alloys and shape memory polymers work?

Date: Thu Jun 15 17:34:21 2000
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Engineering
ID: 959168455.Eg

   These are materials which are stable at two or more temperature states. 
While in these different temperature states, they have the potential to be 
different shapes once their 'Transformation temperatures' (Tx) are 
reached.  Shape Memory Alloys (SMA) and Shape Memory Polymers (SMP) are 
materials with very different shape changing characteristics. While 
exposed to their Tx, devices made from SMAs have the potential to provide 
force such as in the case of actuators. Devices made from SMPs in 
contrast, while exposed to their Tx, provide mechanical property loss as 
in the case with releasable fasteners. This phenomena whether SMA or SMP 
is called Shape Memory Effect (SME).

Shape memory polymers
	Typical polymers posses what is called a glass transition 
temperature (Tg) i.e. the temperature below which they are glassy and 
above which they are rubbery.  The shape memory effect in polymers is a 
novel physical property exhibited best by amorphous polymers whose glass 
transition temperature is marginally higher than room temperature and 
whose transition from glass to rubber is particularly sharp.  In this 
case, strain energy may be stored in the polymer by quick mechanical 
deformation (e.g. by stretching) at T > Tg, such that t << t, followed by 
cooling below Tg.  Here, t is the deformation time and t is a 
characteristic relaxation time marking the transition from rubber-like to 
liquid-like behavior. Recovery of the strain, or shape memory, is 
exhibited upon reheating the sample above Tg, allowing a return of the 
stretched polymer chains to more equilibrium, coiled stuctures.  Reversal 
of this process is only possible, by repeating the same cycle or by 
incorporating the polymer into a device featuring an elastic spring or 
another shape memory polymer with a distinct glass transition 
temperature.  A major application of shape memory in polymers has been as 
heat-shrinkable tubing.


Shape memory alloys:
NiTi shape memory metal alloy can exist in a two different temperature-
dependent crystal structures (phases) called martensite (lower 
temperature) and austenite (higher temperature or parent phase). Several 
properties of austenite NiTi and martensite NiTi are notably different.
When martensite NiTi is heated, it begins to change into austenite.  The 
temperature at which this phenomenon starts is called austenite start 
temperature (As). The temperature at which this phenomenon is complete is 
called austenite finish temperature (Af). When austenite NiTi is cooled, 
it begins to change onto martensite. The temperature at which this 
phenomenon starts is called martensite start temperature (Ms). The 
temperature at which martensite is again completely reverted is called 
martensite finish temperature (Mf).
Composition and metallurgical treatments have dramatic impacts on the 
above transition temperatures. From the point of view of practical 
applications, NiTi can have three different forms: martensite, stress-
induced martensite (superelastic), and austenite. When the material is in 
its martensite form, it is soft and ductile and can be easily deformed 
(somewhat like soft pewter). Superelastic NiTi is highly elastic (rubber-
like), while austenitic NiTi is quite strong and hard (similar to 
titanium). The NiTi material has all these properties, their specific 
expression depending on the temperature in which it is used.


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