Shape memory materials are those that
have the ability to "Memorise" a macroscopic (permanent) shape, be
manipulated and "Fixed" to a temporary shape under specific
conditions of temperature and stress. They later relax to the original,
stress-free condition under thermal, electrical or environmental command.
The thermal treatment affects the physical responses such as shrinkage
stress, stress relaxation and strain recovery rate of polymers.
Polymers are capable of shape memory effect with basic principle that
elevated temperature deformations caused by applied load can be fixed during
cooling. Work performed on the sample is stored as latent strain energy if
the recovery is prohibited by crystallisation, ie, cooling and fixing.
Shape memory polymers undergoing deformation at higher temperatures,
"Retain" the deformed shape when cooled and return to their
original configuration when heated above "Glass transition
temperature". Such types of materials capable of undergoing thermal
shape-transition are a division of smart or intelligent materials.
What is shape memory polymer?
Shape memory polymers (SMP) can be stimulated by temperature, pH (the
level of acidity or alkalinity), chemicals and light. They are able to sense
and respond to external stimuli in pre-determined shape.
In terms of chemical structure, SMP's can be considered as phase segregated
linear block co-polymers having a hard segment and a soft segment. The hard
segment acts as a frozen phase and the soft segment acts as the reversible
phase. The ratio by weight of the hard segment: soft segments are between
about 5:95 and 95:5, preferably between 20:80 and 80:20. The reversible
phase transformation of the soft segment is responsible for the shape memory
The polymer materials have various characteristics such as from hard glass
to soft rubber. Shape memory polymers however, have the characteristic of
both of them and their elasticity modulus show reversible change with the
transition temperature. The picture below shows the transition of a shape
memory polymer from the secondary shape to the primary shape as the
temperature increases, within a time period of 45 seconds.
Features of Shape Memory Polymers
A sharp transition that can be used to promptly fix the secondary shape
at low temperatures and trigger shape recovery at high temperatures.
Super elasticity (high
deformability) above the transition temperature to avoid residual strain
Rapid fixing of temporary shape by
immobilising the polymeric chains without creep.
SMPs possess two material phases.
The glass and the rubber phases. In the glass phase, the material is
rigid and cannot be easily deformed.
When the temperature is greater than
"Glass transition temperature", the material enters the soft
rubber phase and becomes easily deformable.
Properties of SMP
Ø Extent of deformation (%) = up to
Ø Density / g cm^-3 = 0.9 to
Ø Critical temperature / °c
= -10°C to 100°C
Ø Recovery speeds minutes = <1second
to several min.
Ø Corrosion performance = excellent
Ø Processing conditions = < 200°C
, low pressure
Ø Can be biodegradable
Ø Low cost
Classification of SMP:
Shape memory polymers can be classified into four major categories based
on their "Differences in fixing mechanism" and origin of
"Permanent" shape elasticity.
Chemically cross linked glassy thermostat:
Thermostat polymers, primary shape
is covalently fixed. So, once processed, these materials are difficult
These polymers show quiet complete
shape fixation by vitrification and demonstrate fast and complete shape
recovery due to sharp glass transition temperature.
They have the advantage of being
castable and optically transparent.
It has the disadvantage that, the
transition temperature cannot be easily varied and there is difficulty
of processing because of high viscosity of high molecular weight
So, thermostat polymers are
processed by solvent casting like extrusion, injection molding and
compression molding instead of more desirable thermal processing.
Ex: Vinylidene co-polymer consisting
of two monomers methyl methacrylate and butyl methacrylate.
Chemically cross linked semi
Semi crystalline networks are fixed
to their secondary shapes by crystallisation instead of vitrification.
Shape recovery speeds of these materials are much faster.
This class of materials include,
liquid crystal elastomers and hydrogels.
The shape can be returned to the
primary shape promptly upon reheating above its melting point.
Besides thermal heating, recovery in
this material was successfully triggered using an electric current at
very low voltage.
Ex: chemically cross linked trans-
polyisoprene , trans polyoctenamer.
Physically cross linked
The thermoplastics have a relatively
low shape recovery when compared with bulk polymer.
Generally melt miscible blend of
thermoplastics are used. Here, the crystalline or rigid amorphous
domains in thermoplastics may serve as physical cross links.
Advantage of this is, as they are
physically cross linked, they are processable above T high of hard
Recently multiblock co-polymers
consisting of multiple polymers were also developed.
Electrospinning technology was used
to fabricate shape memory fibres.
Ex: miscible blend of thermoplastic
polyurethane with phenoxy resin.
Physically cross linked block
Block co-polymers can be processed
and shaped above their melting point and attaining their glass
transition temperature is not necessary.
The polymers generally have hard and
soft domain areas. By adjusting the domain ratios, the properties can be
The hard segments form physical
cross links by hydrogen bonding or crystallisation. These cross links
withstand moderately high temperatures.
The crystallisable soft segments
form the thermally reversible phase.
They are biocompatible and
Ex: styrene-trans butadiene-styrene
Applications of SMP
Shape memory polymers find its
application in various fields due to its special and unique properties.
Shape Memory Fabric
The shirt with long sleeve could be programmed so that the
sleeves shorten as room temperature becomes hotter. The fabric can be
rolled up, pleated, creased and returned to its former shape by applying
heat. Ex: blowing air through hair dryer.
The violin is made from the combination of shape memory
polymer and carbon fibres. The shape memory polymer used here is "Veriflex".
Itdesigned to help to reduce the neck and shoulder pain of the player,
as it can be reshaped as desired by the player.
The suit was developed to help the sailors on the oceans and
sea. It adapts to the temperature variations and maintains a person's
body temperature constant. The membrane gives optimal breathability in
any given atmospheric condition.
Developing and demostrating morphing materials and
technologies that are necessary to construct deployable morphing
aircrafts and other innovative adaptive structures critical to air force
are taking place.
In many operations which involve stitches inside the human
body, a second operation is done to remove the internal stitches. In
such cases when biodegradable SMPs are used they dissolve gradually and
need not be removed as their composition is harmless.
The research regarding SMP, combining
its super shape memory properties and improved strain resistance, continues
its rapid growth and application in various fields. In this context, this
"Extraordinary invention" of biocompatible and biodegradable
polymers with shape memory properties is just a development in an important
group of "New materials" in the 21st century.
Note: For detailed version of this
article please refer the print version of The Indian Textile Journal January
Mr O L Shanmugasundaram
Department of Textile Technology,
KSR College of Technology,
Tamil Nadu 637 215.