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  Shape Memory Polymers & their applications

The research regarding Shape Memory Polymers (SMP), combining its super shape memory properties and improved strain resistance, continues its rapid growth and application in various fields, states O L Shanmugasundaram.

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 effect.

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 (permanent deformation).

  • 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

Extent of deformation (%) = up to 800%

Density / g cm^-3 = 0.9 to 1.1

Critical temperature / c = -10C to 100C

Recovery speeds minutes = <1second to several min.

Corrosion performance = excellent

Processing conditions = < 200C , low pressure

Can be biodegradable

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 to reshape.

  • 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 polymers.

  • 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 crystalline rubbers:

  • 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 thermoplastics:

  • 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 domains.

  • 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 co-polymer:

  • 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 altered.

  • 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 biodegradable.

  • Ex: styrene-trans butadiene-styrene triblock co-polymer.

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.

  • Ergonomic

    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.

  • SI Suits

    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.

  • Morphing aircrafts

    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.

  • Medical field

    Sutures


    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.

Conclusion

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 2008 issue.

Mr O L Shanmugasundaram
Lecturer,

Department of Textile Technology,
KSR College of Technology,
Tiruchengode,
Tamil Nadu 637 215
.
E-mail: mailols@yahoo.com.

published January , 2008
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