VISVESWARAIAH TECHNOLOGICAL UNIVERSITY BELGAUM, KARNATAKA
A SEMINAR REPORT ON “SHAPE MEMORY POLYMERS”
A seminar report submitted in the partial fulfillment of the requirement of the completion of the requirements for Bachelor of Engineering in MECHANICAL ENGINEERING
SUBMITTED BY: ANUL KUMAR KOKRADY .D 1GA07ME004
DEPARTMENT OF MECHANICAL ENGINEERING GLOBAL ACADEMY OF TECHNOLOGY Rajarajeshwari Nagar, Ideal Home Township, Bangalore-560098.
Certificate This is to certify that the seminar work entitled “SHAPE MEMORY POLYMERS” is a bonafide work carried out by ANUL KUMAR KOKRADY th
.D (1GA07ME004) student of 8 semester Dept. of Mechanical Engineering, in
the partial fulfillment of the requirement of the completion of the requirements for Bachelor of Engineering in Mechanical Engineering of the Visveswaraiah Technological University, Belgaum during the year 2011. The seminar report has been approved as it satisfies the academic requirements in respect of project work prescribed for the the Bachelor of Engineering Engineering Degree.
Signature of H.O.D
Acknowledgement
I take this responsibility to convey my deep sense of gratitude to all those who have been kind enough to offer their advice and provide assistance when needed which has lead to the successful completion of the seminar report. I extend my sincere thanks to our principal Dr. NARENDRA VISHWANATH for his cooperation. I am grateful to our HOD, Dr. RAJGOPAL M.S and D.V RAVI KUMAR, Asst prof. for their constant guidance and support. I would like to thank all the faculty members of mechanical department with whose timely help the completion of my report work has been possible. Lastly, I also thank my friends and parents for all help and support given by them throughout.
ABSTRACT Shape-memory polymers (SMP) are an emerging class of intelligent materials which have the capability of changing their shape from a temporary shape to a permanent shape when subject to an appropriate stimulus such as temperature, light, electric field, magnetic field, pH etc. A variety of polymers ranging from completely amorphous to semi-crystalline have been extensively studied from the point of view of shape-memory polymer (SMP) while the primary underlying origin for shape recovery is the desire for polymer segments to adopt a random-coil conformation in order to maximize its entropy, the temperature at which the transition occurs could be either glass transition temperature (Tg) or melting temperature(Tm) depending on the morphology of polymers. In all SMPs the permanent shape is fixed under thermodynamically equilibrium condition either by chemical crosslinks or by physical crosslinks, such as formation of crystallites or hydrogen-bonding etc. On the other hand the temporary shape is formed at a temperature above Tg or Tm under an externally applied stress. Cooling the sample below the Ttrans in the presence of the applied stress leads to fixation of the temporary shape. The polymer chains in the temporarily formed shape are kinetically trapped and as a consequence when heated above Ttrans retracts its original shape driven by configurational entrop y.
In this report several approaches that have been adopted to generate SMPs and highlight examples wherein the external trigger to cause the switching from the temporary shape to the permanent one is done using heat, electricity and light .Also applications of SMPs in several fields is discussed.
1.INTRODUCTION
Science and technology have made amazing developments in the design of electronics and machinery using standard materials, which do not have particularly special properties (i.e. steel, aluminum, gold). Imagine the range of possibilities, which exist for special materials that have properties scientists can manipulate. Some such materials have the ability to change shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost instantly when near a magnet; these materials are called smart materials.
Smart materials have one or more properties that can be dramatically altered. Most everyday materials have physical properties, which cannot be significantly altered; for example if oil is heated it will become a little thinner, whereas a smart material with variable viscosity may turn from a fluid which flows easily to a solid. A variety of smart materials already exist, and are being researched extensively. These include piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory alloys,shape memory polymers. All these above mentioned smart materials are used in various fields. Among these materials shape memory polymer is the latest addition .Untill now smp‘s are not be ing used completely or it is in the development stage . But from the current research and developments it is prooving to be the most promising variety of smart materials, Shape-memory polymers are an emerging class of polymers with applications spanning various areas of everyday life. Such applications can be found in, for example, smart fabrics, heat-shrinkable tubes for electronics or films for packaging3, self-deployable sun sails in spacecraf t4, self-disassembling mobile phones5, intelligent medical devices6, or implants for minimally invasive surgery7 and 8. These examples cover only a small number of the possible applications of shape-memory technology, which shows potential in numerous other applications. In this review, the fundamental aspects of the shape-memory effect are presented.
Shape memory polymer Shape memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state to their original shape induced by an external stimulus, such as temperature change. Most SMPs can retain two shapes, and the transition between those is induced by temperature. In some recent SMPs, heating to certain transition temperatures allows to fix three different shapes. In addition to temperature chan ge, the shape change of SMPs can also be triggered by an electric or magnetic field, light or solution.. SMPs include thermoplastic and thermoset polymeric materials. Shape-memory polymers are dual-shape materials belonging to the group of ‗actively moving‘ polymers. They can actively change from a shape A to a shape B. Shape A is a temporary shape that is obtained by mechanical deformation and subsequent fixation of that deformation. This process also determines the change of shape shift, resulting in shape B, which is the permanent shape. In shape-memory polymers reported so far, heat or light has been used as the stimulus. Using irradiation with infrared light, application of electric fields, alternating magnetic fields, or immersion in water, indirect actuation of the shape-memory effect has also been realized. The shape-memory effect only relies on the molecular architecture and does not require a specific chemical structure in the repeating units. Therefore, intrinsic material properties, e.g. mechanical properties, can be adjusted to the need of specific applications by variation of molecular parameters, such as the type of monomer or the comonomer ratio. The shape-memory effect is not an intrinsic property, meaning that polymers do not display this effect by themselves. Shape memory results from a combination of polymer morphology and specific processing and can be understood as a polymer functionalization. By conventional processing, e.g. extruding or injection molding, the polymer is formed into its initial, permanent shape B. Afterwards, in a process called programming, the polymer sample is deformed and fixed into the temporary shape A. Upon application of an external stimulus, the polymer recovers its initial permanent shape B. This cycle of programming and recovery can be repeated several times, with different temporary shapes in subsequent cycles. In comparison with metallic shape-memory alloys, this cycle of programming and recovery can take place in a much shorter time interval and polymers allow a much higher deformation rate between shapes A and B.
Figure 2. Transition from the temporary shape (spiral) to the permanent shape (rod) for a shape-memory network that has been synthesized from poly(_-caprolactone) dimethacrylate (1) and butylacrylate (2; co-monomer content: 50 wt%; see Section 2.6.2). The switching temperature of this polymer is 46_C. The recovery process takes 35 s after heating to 70_C.
Features of shape memory polymers 1. Super elasticity (high deformability) above the transition temperature to avoid residual strain (permanent deformation). 2. Rapid fixing of temporary shape by immobilizing the polymeric chains without creep . 3. SMPs possess two material phases. The glass and the rubber phases. In the glass phase, the material is rigid and cannot be easily deformed. 4. When the temperature is greater than ―glass transition temperature‖, the material enters the rubber phase and becomes easily deformable.
Properties of SMPs Most SMPs can retain two shapes, and the transition between those is induced by temperature. In some recent SMPs, heating to certain transition temperatures allows to fix three different shapes. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, light[ or solution. As well as polymers in general, SMPs 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 ab le to store up to three different shapes in memory. 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.
where N is the cycle number, εm is the maximum strain imposed on the material, and ε p(N) and ε p
(N-1) are the strains of the sample in two successive cycles in the stress-free state before yield
stress is applied. Shape memory effect can be described briefly as the following mathematical model:
where E g is the glassy modulus, E r is the rubbery modulus, f IR is viscous flow strain and f is α
strain for t >> t r.
Result of the cyclic thermomechanical test
Triple-Shape Memory
While most traditional shape-memory polymers can only hold a permanent and temporary shape, recent technological advances have allowed the introduction of triple-shape memory materials. Much as a traditional two-shape memory polymer will change from a temporary shape back to a permanent shape at a particular temperature, triple-shape memory polymers will switch from one temporary shape to another at the first transition temperature, and then back to the permanent shape at another, higher activation temperature. This is usually achieved by combining two double-shape memory polymers with different glass transition temperatures.
Figure showing a SMP with triple shape memory.
General properties of SMPs Extent of deformation (%)
= up to 800%
Density / g cm^-3
= 0.9 to 1.1
Critical Temparature
= -10 degree C to 100 degree C
Recovery speeds (in mins)
= < 1 second to several minutes
Corrosion performance
= excellent
Processing conditions
= <200degree C, low pressure
Can be biodegradable Low cost
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 as k = ln W , where k is the Boltzmann constant. In the transition from the glassy state to a rubber-elastic state by th ermal 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. 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 its 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.
Methods of activating SMP’s 1.Thermally induced shape memory polymer. 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, T perm , is the temperature that must be 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 (T trans) and are responsible for the temporary shape. In some cases this is the glass transition temperature (T g ) and others the melting temperature (T m). Exceeding T trans (while remaining below T perm) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. Below T trans, flexibility of the segments is at least partly limited. If T m is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above T m and subsequently cooled below T m These crystallites form covalent netpoints which prevent the polymer from reforming its 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.[9] The shape memory polymers are effectively viscoelastic and many models and analysis methods exist.
A schematic representation of the shape memory effect
2 .Light-induced SMPs
Light activated shape memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change T g . 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.[13] For example, it has been reported that polymers containing cinnamic groups can be fixed into predetermined shapes by UV light illumination (> 260 nm) and then recover their original shape when exposed to UV light of a different wavelength (< 260 nm).[13] Examples of photoresponsive switches include cinnamic acid and cinnamylidene acetic acid
A schematic representation of reversible LASMP crosslinking
3. Electro-active SMPs The use of electricity to activate the shape memory effect of polymers is desirable for applications where it would not be possible to use heat and is another active area of research. Some current efforts use conducting SMP composites with carbon nanotubes.[14] short carbon fibers (SCFs).[15][16] carbon black, metallic Ni powder. 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 to be dependent on the filler content and the degree of surface modification of the MWNTs, with the surface modified versions exhibiting good energy con version efficiency 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 and 12% magnetite nanoparticles. Nickel and hybrid fibers have also been used with some degree of success
Shape memory polymers vs Shape memory alloys Shape memory polymers differ from shape memory alloys[18] 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 The major differences between SMPs and SMAs
Property
SMPs
SMAs
0.9 – 1.1
6 – 8
up to 800%
<8%
1 – 3
50 – 200
1 – 3
150 – 300
Transition temperatures(°C)
– 10..100
– 10..100
Recovery speed
1s – few minutes
<1s
Processing conditions
<200 °C ,low pressure
>1000 °C ,high pressure
Costs
<$10/lb
~$250/lb
Density (g/cm ) Extent ofdeformation Required stress for deformation (MPa) Stress generated upon recovery (MPa)
Classification of SMP’s SMP‘s can be classified into four major categories based on their ―differences in fixing mechanism‖ and origin of ―permanent shape elasticity‖
1.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
2.Chemically cross linked semi crystalline rubbers: Semi crystalline networks are fixed to their secondary shapes b y crystallization instead of vitrification. Shape recovery speeds of these materials are much faster. This class of materials include, liquid crystal elastomers and hydroogeis 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
3.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. Recently multiblock co-polymers consisting of multiple polymers are also developed. Electrospinning technology is used to fabricate shape memory fibres . Ex:miscible blend of thermoplastic polyurethane with phenoxy resin.
4.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 from physical cross links by hydrogen bonding or crystallization. These cross links withstand moderately high temperatures. The crystallisable soft segments from the thermally reversisble phase They are biocompatible and biodegradable. Ex. Styrene-trans butadiene-styrene triblock co-polymer.
Applications Industrial applications
One of the first conceived industrial applications was in Robotics where shape memory (SM) foams were used to provide initial soft pretension in gripping. These shape memory 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 so me cases with thermochromic additives for ease of thermal profile observation. Polyurethane S MPs are also applied as an autochoke element for engines.
Potential industrial applications
Further potential applications include self-repairing structural components, such as e.g. automobile fenders in which dents are repaired b y application of temperature. 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%.
Medical applications
Most medical applications of SMP have yet to be developed, but devices with SMP are now beginning to hit the market. Recently, this technology has expanded to applications in orthopedic surgery, such as the Morphix suture anchor and ExoShape graft fixation device, by MedShape Solutions. Potential medical applications
SMPs are smart materials with potential applications as, e.g., intravenous cannula, self-adjusting orthodontic wires and selectively pliable tools for small scale surgical procedures where currently metal-based shape memory alloys such as Nitinol are widel y used. Another application of SMP in the medical field could be its use in implants: for example minimally invasive, trough small incisions or natural orifices, implantation of a device in its small temporary shape. S hape memory technologies have shown great promise for cardiovascular stents, since they allow a small stent to be inserted along a vein or artery and then expanded to prop it open. After activating the shape memory by temperature increase or mechanical stress, it would assume its permanent shape. Certain classes of shape memory polymers possess an additional property: biodegradability. This offers the option to develop temporary implants. In the case of biodegradable polymers, 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 full functionality would be restored without the necessity for a second surgery to remove the implant (to avoid inflammation). Examples of this development are vascular stents and surgical sutures. 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 overtightened sutures and does support healing and regeneration.
Biomedical applications of shape-memory polymers
An attractive application area for shape-memory polymers is their use in active medical devices. First examples include a laser-activated device for the mechanical removal of blood clots .The device is inserted by minimally invasive surgery into the blood vessel and, upon laser activation, the shape-memory material coils into its permanent shape, enab ling the mechanical removal of the thrombus (blood clot). Another example of a medical challenge to be addressed is obesity, which is one of the major health problems in developed countries. In most cases, overeating is the key problem, which can be circumvented by methods for curbing appetite. One solution may be biodegradable intragastric implants that inflate after an approximate predetermined time and provide the patient with a feeling of satiety after only a small amount of food has been eaten. Shape-memory foams have been proposed as a measuring device to survey the shape of a human ear canal, so a hearing aid can be fitted properly. The material is a commercially available polyurethane foam. The foam shows full recovery after 83% compression. Another application of shape-memory polymers is in stents for the prevention of strokes. Here, coils of a composite consisting of tantal and a polyetherurethane have been studied. Tantal is needed as a radio-opaque filler for diagnostic detection. The filling does not affect the shape recovery behavior but lowers and the maximum recovery stress. Other potential applications
Some of the various other applications of SMPs include clothing. For example, a shirt could be programmed to shorten sleeves 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.
Biodegradable shape-memory polymers
As well as responding to different stimulations, biodegradability would be beneficial for man y medical applications. The combination of shape-memory capability and biodegradability is an example of multifunctionality in a material. This type of multifunctionality is especially advantageous for medical devices used for minimally invasive surgery. The polymers allow the insertion of bulky implants in a compressed shape into the human body through a small incision. When stimulated within the body, they turn into their application-relevant shape. Another example is a biodegradable shape-memory polymer as an intelligent suture for wound closure. Upon actuation of the shape-memory effect, the material is able to apply a defined stress to the wound lips.In both applications, removal of the implant in follow-up surgery is not necessary, as the implant degrades within a predefined time interval.
(a) A smart surgical suture self-tightening at elevated temperatures (left). A thermoplastic shapememory polymer fiber was programmed by stretching to about 200% at a high temperature and fixing the temporary shape by cooling. After forming a loose knot, both ends of the suture were fixed. The photo series shows, from top to bottom, how the knot tightened in 20c when heated to 40C(b) Degradable shape-memory suturefor wound closure (right). The photo series from the animal experiment shows (top to bottom) the shrinkage of the fiber while the temperatureincreases from 20 to 41C.
The Future for Shape Memory Polymers
Whatever applications they are needed for new materials will play a key role in the development of new technologies in the next century. ‗In the field of medical engineering, number of new
technologies can only b realised if the biocompatible materials require can be developed. In this context, this ‗extraordinary invention‘ of biocompatible and b iodegradable polymers with shape
memory properties is just on development in an important group of ‗new materials‘ for the 21st century.Shape memory polymers once developed successfully might out perform other conventional materials. Degradable shape-memory polymers provide interesting advantages over metal implants. On the one hand a follow-on surgery to remove the implant can be avoided, and on the other hand such medical products can be introduced into the body by minimally invasive procedures through a small incision. In this way, patients benefit from a more gentle treatment and costs in health care can be reduced. In this sense, shape-memory implant materials have the potential to influence decisively the way medicinal products are designed in the future. Fighter planes with morphing technology could save the lives of the pilots during wars. Potential applications for shape-memory polymers exist in almost every area of daily life: from selfrepairing auto bodies to kitchen utensils, from switches to sensors, from intelligent packing to tools.
Conclusion
The field of actively moving polymers – polymers that are able to perform movements by themselves – is progressing rapidly. Shape-memory polymers play a key role within this field. Fundamental shape-memory research is focusing on the implementation of stimuli other than heat to actuate shape-memory polymers, or to actuate them remotely. First examples include the light-induced stimulation of shape-memory polymers or the use of alternating magnetic fields for remote actuation. It is assumed that these methods of stimulation will open up new fields of application. An important application area for shape-memory polymers is in active medical devices and implants, and initial demonstrations have been presented. The application requirements can be complex in this area. Therefore, a trend toward the development of multifunctional materials can be seen. Furthermore, there is a strong deman d for actively moving materials able to perform complex movements. These requirements could be fulfilled by materials that are able to perform two or more predetermined shifts.
