Shape Memory Polyurethanes

Although their syntheses and morphological characteristics differ, all shape memory polymers (SMPs) are two-component materials capable of returning to their original shape in the presence of an appropriate stimulus. While one component exhibits elastic behavior throughout the application range, the other component can undergo transitions with the help of the correct stimulus. Figure 1 shows the operating mechanism of thermally responsive shape memory polymers. At low temperature, both the elastic and transition components are hard (Figure 1a). When the transition temperature is exceeded, the transition component becomes soft and can be easily deformed (Figure 1b). The elastic component has been deformed and stores elastic energy within itself. Cooling below the transition temperature will cause the transition component to harden. If the deformation is held during cooling (Figure 1c), the deformed shape will be preserved even after the load is removed (Figure 1d). The reason for this is that the transition component is solid below the transition temperature and prevents elastic recovery in the elastic component. The temporary shape imposition represents an example of the standard procedure for shape memory polymers. When reheated above the transition temperature, the transition components soften and lose the ability to hold the deformed elastic component. As the elastic component releases the elastic energy it has retained, the free portions of the shape memory polymer return to their original state (Figure 1e). This mechanism applies to two-component structures. The addition of extra components or segments enables improvement in performance characteristics (strength, electrical conductivity, multiple transitions, etc.). The shape memory effects of other shape memory polymers basically operate through a similar mechanism. The driving force in shape memory polymers is elastic energy storage in the elastic component (Huang et al., 2010a).

2. Shape Memory Polymer Applications

Shape memory polymers and composites are encountered with a quite broad application range, from smart textiles and sensor applications to mandrel (reusable) production. (Everhart and Stahl 2005, Everhart et al., 2006, Dietsch and Tong 2007, Snyder et al., 2010, Hu J 2007, Leng and Du 2010). Thermally responsive shape memory polymers will be examined with examples demonstrating their significant potential. The Braille alphabet, known as the writing system for the blind or visually impaired, consists of raised dots that enable blind or partially sighted individuals to read and write by touch (Figure 2). This alphabet consists of a 3x2 matrix containing raised points. These points are typically created using a needle and plate system or automatically written (impressed) onto plastic or paper sheets using thermoset technique (which is difficult to remove). There is a need for a next-generation Braille alphabet where writing errors can be easily corrected and rewriting is possible. A plate developed from polyurethane shape memory polymer has been created to assist the visually impaired (SMP Technologies, Japan). As shown in Figure 3, any point errors that may occur can be corrected using point heaters. Corrections made afterwards can be applied to the same area. Additionally, the plate has the property of being recoverable repeatedly when heated in its entirety. Due to the low cost of shape memory polymers, affordable and accessible innovations can be implemented for visually impaired individuals. Screws are generally used in the assembly of electrical tools or as implants in medical applications to hold two components together. Commercially, screws of different sizes require holes of different sizes. Polymeric screws developed with shape memory capability can be used in all holes whether threaded or unthreaded. Furthermore, a screwdriver is not required for tightening. Images of this design, known as active assembly, are shown in Figure 4. The increasing use of electronic devices such as cell phones, video players, and personal computers in daily life poses an environmental threat. This threat must be reduced by decreasing material consumption, recycling, or reusing. Practically speaking, there is a need to develop disassembly systems with low transportation costs rather than classical systems. Deployable structures including wheels, poles, solar sails, solar arrays, and antennas must be compact due to limited space within launch rockets and must enter expanded configurations upon deployment. Additionally, modern unmanned aerial vehicles have different configurations to achieve high performance for different missions. The ability to reconfigure during flight is called "morphing". "Morphing wings" must be used because changing wing shape and/or size depending on flight speed can significantly reduce drag (Figure 5). The most promising applications of shape memory polymers are in the biomedical engineering field. SMPs have been regarded as promising materials for degradable and functional cardiovascular implants and other therapeutic applications (Jung et al., 2010a, Karp and Langer 2007). One of the most attractive micro-devices studied intensively in recent years is a stent (its working principle is shown in Figure 6) (Yakacki et al., 2007). The advantage of degradable SMP stents is that they require no follow-up procedure for removal (Chen et al., 2007a). Surgical sutures made from such SMPs do not need to be removed later (Lendlein and Langer 2002). Both biodegradable and non-biodegradable SMPs have been used for controlled drug release (Wache et al. 2003, Chen et al. 2009b, Wischke et al. 2009, Zhang et al. 2010). Today, SMPs are being used in endovascular thrombectomy devices (Wilson et al., 2007a), active microfluidic reservoirs (Gall et al. 2004), ocular implants (Song et al. 2010), and self-deploying neural electrodes (Sharp et al. 2006).

3. Shape Memory Polyurethanes

Polyurethanes are known to consist of long linear chains composed of two types of interconnected segments (Figure 7). Flexibility varies with the ratio of soft segments (typically diisocyanate-linked low-melting-point polyester or polyether chains), elastic and relatively hard components (typically diurethane bridges formed from the reaction of a diisocyanate with a small molecule glycol chain extender). When an appropriate diisocyanate and polyol combination or soft segment/hard segment ratio is selected, desired properties such as elasticity, crystallization temperature range, and melting point can be easily adjusted. The soft segments of polyurethanes can be used as transition components for shape memory effects. Many shape memory polyurethanes have been developed to date (Chen et al., 2010). Shape memory polyurethane foams are produced and their structures are clarified using different techniques to increase stress ratios (and thus recovery capability after deformation) and reduce density (Chung and Park 2010). Physical aging has been studied by many research groups because it is particularly important for medical applications (Lorenzo et al. 2009). Thermally responsive woven stents have been numerically investigated (Kim et al. 2010). Electrically conductive shape memory polyurethanes have been obtained by mixing with various conductive filler types (Gunes et al., 2009). In one study, after mixing with magnetic particles and applying a magnetization field of 4.4 kA/m at 50 Hz frequency, heating successfully triggered shape recovery in a shape memory polyurethane-magnetic particle composite (Razzaq et al., 2007). Additionally, shape recovery films that are electrically conductive, optically transparent, and mechanically strong have been developed by incorporating multi-walled carbon nanotubes (MWCNT) surface-modified photochemically to make them light-responsive (Jung et al. 2010b). Different types of fillers such as silica, carbon nanofiber (CNF), carbon black, and clay have been added as fillers for shape memory polyurethane composites (Kuriyagawa et al., 2010). Highly branched polyurethane-MWCNT composites have been reported in the literature to exhibit improved biodegradability properties compared to pure ones (Deka et al. 2010). Cytocompatibility testing based on hemolysis of red blood cells has demonstrated the absence of cytotoxicity. Most of these shape memory polyurethanes are thermally responsive materials, but water-responsive materials have also been obtained through modification using polyhedral oligomeric silsesquioxane or pyridine. Dr. S. Hayashi (Hayashi 1990, Hayashi et al. 1995) discovered shape memory polyurethane by following the basic synthesis process below: Dr. Hayashi's polyurethane is currently the most popular polyurethane successfully commercialized and researched. Two-part SMP plates are partially cut and bent 180 degrees in the planar direction at room temperature (approximately 23°C). They are then placed on a hot plate so they can thermally return to their original shape. At the end of the heating process, both plates almost completely recover their original flat shapes. Recently, micro-beads, micro-springs, fine wires, thin and ultra-thin films were produced from this SMP. Based on electron microscope images, all are said to have great potential for micro- and nano-devices (Huang et al. 2010a). Shape memory polyurethanes of this type are generally ductile at low temperatures compared to other SMPs. Therefore, programming can be performed at low or high temperatures, as shown in Figure 8a. At high temperatures (Tg + 15°C), the deformation during tension is nearly uniform across the sample cross-section, but at low (room) temperatures where deformation is not homogeneous, we can clearly observe neck formation and propagation during stretching. This is a point to keep in mind. After heating to Tg + 15°C, both samples take on their original bone shapes (Figure 8b). Hybrid polyurethane SMP composites were investigated by Liang and colleagues (1997). In recent years, just as shape memory alloys have been made, carbon black and nickel powder have been blended into this SMP to increase Joule heating, since such heating is more practical in engineering applications. Figure 9 demonstrates the wing morphing (morphing wing) concept by passing an electrical current through a conductive SMP piece (carbon black was used as the conductive filler). To increase the strength of this polyurethane SMP, attapulgite clays (heat-treated and untreated), which occur in nature in nanofiber form and are weak in electrical conductivity, have been used as low-cost alternatives (Pan et al., 2008). As seen in Figure 10, the composite demonstrates excellent shape memory properties. Although this SMP was initially designed to be only heat-responsive (Yang et al. 2004), it was found that moisture can significantly reduce Tg. This means that shape recovery can be achieved by water immersion rather than lowering the Tg temperature. This property is also applicable to both electrically conductive and non-conductive materials (Yang et al. 2005a). Figure 11 shows the recovery process of polyurethane SMP-attapulgite composite when immersed in water at room temperature. Some application areas of polyurethane-based SMPs with superior compatibility properties are listed below (SMP Technologies, Japan): 1. Shape memory polymers that can be easily disassembled with heat. 2. Intravascular injection tube (maintains rigid phase during injection, converts to flexible form under the skin and provides comfortable use). 3. Barrier-free containers (for example; enabling eating, tooth brushing, and use of scissors and razors thanks to superior shape characteristics). 4. Inflatable materials used in aerospace. 5. Toy hair (filaments with various deniers and colors available). 6. Brassières (SMP-foam materials have wide dynamic tan δ and J-curves (tension-elongation curves). This property is very similar to the human body.) 7. Nose pads for eyeglasses (These pads have the same mechanical properties as the nose), 8. Netting used for wigs (tan δ and tension-elongation curve properties are very similar to human scalp skin), 9. Membranes (micro-brown motion enables transfer of water vapor molecules in pore-free polymer membranes), 10. Textiles (superior water impermeability, breathability, and anti-clumping properties have been provided), 11. New vegetable containers (As temperature increases, perspiration increases and vapor needs to be removed. SMP films maintain suitable conditions within the container.) Serhat Oran / Doctoral Student - Department of Polymer Engineering - Yalova University Reyhan Özdoğan / Doctoral Student - Department of Polymer Engineering - Yalova University Kevser Özdemir / Doctoral Student - Department of Polymer Engineering - Yalova University Associate Professor Dr. Mehmet Atilla Taşdelen / Department of Polymer Engineering - Faculty of Engineering - Yalova University

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