Functional shape memory textiles
This chapter introduces different mechanisms of shape memory materials (SMMs), strategies for applying SMMs to textiles and the applications of the resultant shape memory textiles. New functions of textiles created by integrating them with SMMs are also presented. The chapter mainly focuses on shape memory alloys (SMAs) and shape memory polymers (SMPs), which have been widely used in textiles. In addition to the present achievements of the SMM application research, the future of SMMs in textile applications is also discussed.
Shape memory materials (SMMs) can rapidly change their shapes (configuration or dimensions) under appropriate stimuli such as heat (Hu, 2007b), moisture/water (Yang et al., 2006), pH value (Feil et al, 1992), electricity (Asaka and Oguro, 2000), light (Jiang et al, 2006), magnetic field (Makhosaxana et al., 2000) and solvent (Siegal and Firestone, 1988). Smart and functional textiles have developed very quickly in recent decades. Textiles with new functions such as luminescent textiles (Kongolo, 2008), electronic display (Lee and Starner, 2008), heat sensing textiles (Zhu et al, 2009), self-cleaning textiles (Qi et al, 2007) and shape recovery textiles (Hu et al., 2007b) have been invented in the past decade. The ability of SMMs to sense and respond to environmental signals such as temperature, humidity and pH value inspires people to create smart textiles with self-regulating structures and performance.
SMMs include shape memory alloys (SMAs), shape memory polymers (SMPs) and shape memory ceramics. Shape memory ceramics have not been used in textiles because they are very brittle and difficult to process. This chapter will introduce the mechanisms of the shape memory effect (SME) of versatile SMAs and SMPs, the strategies for applying them to textiles, and the functions of SMMs in textiles. Existing problems with the application of SMMs to textiles are discussed and future applications of SMMs in textiles are presented.
The shape recovery of SMAs arises from the crystal lattice change of a specific martensite variant to its parent single crystal phase. Nickel-titanium (NiTi), copper-aluminum-nickel alloys and copper-zinc-aluminum-nickel are the three main types of SMAs. During the shape deformation and recovery cycles, SMAs show two phases; the austenite phase (A) with a body centered cubic structure at high temperature, and the martensite phase (M) with a tetragonal, orthorhombic or monoclinic crystal lattice structure at low temperature. NiTi alloys change from austenite to martensite upon cooling and change from martensite to austenite upon heating. The phase transformation from austenite to martensite upon cooling is not caused by the diffusion of atoms, but by shear lattice distortion. Upon cooling, the austenite transforms to martensite, resulting in the formation of several martensitic variants, up to 24 for NiTi. This arrangement of martensite variants does not show any macroscopic shape change to the materials. At a temperature below the martensite transformation finish temperature, however, deformation stress can cause the transformation of other variants of the martensite phase into a specific variant accompanied by macroscopic shape changes. The deformation strain is usually below 10% – any higher and it will cause a slippage of the lattice, which is unrecoverable. If the alloy is increased to a temperature above the starting temperature of the austenite phase, the specific martensite variant transforms into the lattice of the original austenite phase, which leads to the one-way shape recovery of the SMA. Figure 6.1(a) shows the shape deformation and recovery process of a one-way SME. In order to obtain a temporary shape in the one-way SME an external force has to be employed. Figure 6.1(b) shows the two-way SME of SMAs. The two-way SME can be imparted to SMAs by training so that SMAs can remember two different shapes at a low temperature and at a high temperature respectively. The two-way SME is caused by the residual stress in the SMAs after specific training processes using specific heat treatment, and complicated thermomechanical cycles. Two-way SMAs have a much lower recoverable strain and the recovery stress is vey low in comparison with that of one-way SME.
Another important property of SMAs, which has been widely applied, is pseudoelasticity. The pseudoelasticity mechanism of SMAs works as follows: At a temperature slightly above the martensite transformation starting temperature, outer mechanical stress can cause the phase transformation of SMAs from the austenite to the martensite phase. The martensite variant (stress induced martensite) grows in the direction most favorable for the applied stress, which causes the shape deformation of the SMA. At a temperature above the austenite phase, if the external stress is released, the material recovers its original shape because of the transformation of SMAs from the specific martensite phase to the austenite phase.
SMAs have been used in a number of applications from industrial applications such as aircraft, piping and robotics, to medical applications such as optometry, orthopedic surgery and dentistry and from everyday items such as glasses frames and coffee-pot thermostats to high-performance devices such as satellite antennas and vascular stents.
SMAs, after an apparent plastic deformation, can return to their original shape, which is ascribed to the pseudoelasticity of SMAs. The recoverable strain in a certain temperature range can be up to 10%. Composite materials reinforced with SMA fibers having superior ductility and energy absorption capacity, also known as damping properties, due to the pseudoelasticity of SMAs (Soroushian, 1997) SMA reinforced fabrics with a good damping effect (energy absorption) can be used in armor textiles (Boussu and Petitniot, 2002). By incorporating SMAs into polyparaphenylene terephalamide and high tenacity polyethylene fabrics, the high-velocity impact resistance was substantially improved.
The pseudoelasticity of SMAs can also be used for the underwire of brassieres to improve comfort (Wu and Schetky, 2000). Shape memory under-wire is more comfortable because of the lower elastic modulus of SMAs compared to those made of other steel wires. Furthermore, it is reported that the SMA underwire is resistant to permanent deformation because washing and drying the brassieres can trigger the shape recovery of deformed underwire.
SMAs, after suitable training, can attain a two-way SME, meaning that the shape change of textile products with two-way SMAs can be reversible and repeatable. Stylios et al. (2005) (Stylios and Wan, 2007) used NiTi wires after a solution treatment at 650 °C for 60 min and an ageing treatment at 480 °C for 1 h and 40 min followed by air-cooling to fabricate shape memory yarn. The two-way shape memory training process is as follows:
(3) The constrained springs were thermomechanically trained for several cycles, at a temperature of 5 °C (in a martensitic state) before relaxing the force and again heating up to 100–300 °C, a temperature slightly higher than the austensite state finished temperature.
Yarns with trained NiTi SMA wires as their core component were developed. The SMA yarns were wrapped with polyester, viscose or polyamide. Stylios’ research group designed many yarns with different twist levels and structures (Stylios, 2006).
Many woven and knitted fabrics were prepared by using the yarns with SMA wires. Three aims can be achieved by using SMA wires (Chan and Stylios, 2003)
Various designs and structures can be used to incorporate SMA wires with textiles to prepare shape memory fabrics. Figure 6.2 shows the shape recovery of a shape memory fabric embedded with SMA wires (Oclaciro, 2010). The fabric is first compressed into a small size. Then with increasing temperature, the fabric became fluffy and unfolds.
Stylios and Wan (2007) demonstrated the shape recovery of a fabric with trained SMA wires which had a two-way SME. The trained SMA wires were blended with two kinds of polymer yarn. The fabric changed from a flat to a curled shape in 50 s when it was heated to a temperature of 50 °C which was slightly above the austenite transformation starting temperature. By cooling the fabric from above 50 °C to room temperature, the fabric recovered from the curled state to its original flat state. Figure 6.3 presents another shape memory fabric with two-way SMA wires (Leenders, 2010). The fabric was originally flat as shown in Fig. 6.3(a). When heated, the fabric rolled up as shown in Fig. 6.3(b). Finally, the fabric was cooled to room temperature which caused it to recover its original flat shape as shown in Fig. 6.3(c).
Various shape memory clothing items and accessories have been designed for their aesthetic interactive and functional effects on textiles. An Italian design house based in Florence, Corpo Nove, designed ‘lazy shirt’ fabric which was joined with SMA wires and nylon fiber. When outside temperatures are high, the shirtsleeves can quickly wind up from wrist to elbow. When the temperature drops the sleeves automatically return to their original shape (Marks, 2001).
Figure 6.4 shows another interesting design of shape memory dress (Leenders, 2010). The under layer of the dress is made of fabric embedded with SMA wires. As shown in Fig. 6.4, the under layer of the dress shrinks when the dress is heated using a hair dryer. The dress recovers its original length when the dryer is removed.
Figure 6.5 illustrates the external and internal layers of a fabric incorporated with springs made of two-way SMAs (Hu, 2007b). The two-way shape memory spring with a switching temperature of ~ 50 °C extends when subjected to a temperature above 50 °C. At a high temperature, above the switching temperature, the two layers separate from one another as the length of the shape memory spring increases. This increases the air gap between the two layers which acts as a barrier against flames and intense heat. At low or normal temperatures, the two layers approach each other because the length of the spring decreases at a temperature below its switching temperature.
Fabric with two-way SMA wires is not only useful in clothing for aesthetic and novel functions; it can also be used for practical purposes in many other textile products. Shape memory fabrics were used in the intelligent window curtain applications proposed by Stylios and Wan (2007; Stylios et al., 2005). The two-way shape memory curtains can self-regulate their structures and performances in response to the room temperature.
SMAs are electro-conductive intermetallic alloys. They can be heated using an electrical current via Joule heating. Shape change driven by an electrical current is easier and more efficient than that caused by direct heating. Figure 6.6 shows the shape recovery of a knitted fabric with SMA wires (Labs, 2010). The pre-designed hole, which is much larger at the beginning (see Fig. 6.6(a)), shrinks to a smaller size (see Fig. 6.6(b)) when the fabric is charged with an electrical current.
Figure 6.7 shows a garment incorporated with SMA wires and a power source (Xslabs, 2010). The designed gaps on the garment can open and close as a result of the increasing and decreasing temperature of the SMA wires by Joule heating. The power is supplied by the power source embedded within the garment.
Although many shape memory textiles with different structures and functions have been designed and developed by incorporating SMAs, there are still some minor problems. First, because of the significant mechanical difference of SMA wires and traditional textile fibers, SMA wires have a tendency to protrude from the garment. Complicated textile structures are therefore difficult to accomplish. Second, due to their stiffness and limited extensibility, the knitting of SMA wires is difficult. Finally, if not designed properly, the SMA wires in shape memory fabrics significantly affect the soft hand feel of the garment.
In comparison with SMAs, SMPs have the advantages of being lightweight, low cost and easy to process as well as possessing high shape deformability, high shape recoverability and tailorable switching temperatures. The disadvantages of SMPs compared with SMAs, however, are low mechanical and recovery strengths. According to the switch mechanisms of SME, SMPs can be divided into the following categories: (a) those with an SME based on conventional glass or melting transition (Liu et al, 2007a), (b) those with an SME based on anisotropic chain conformation change (Ahir et al, 2006; Qin and Mather, 2009), (c) those with an SME based on thermally reversible Diels-Alder cycloaddition (Ishida and Yoshie, 2008), (d) those with an SME based on carbon nanotube (CNT)/SMP composites (Vaia, 2005) and finally (e) those with an SME based on in-direction heating such as electricity (Koerner et al., 2004), light (Langer and Tirrell, 2004; Small IV et al, 2005) or a magnetic field (Cuevas et al, 2009). SMPs can be used in smart textiles and apparels (Meng et al, 2007a, intelligent medical devices (Nagahama et al, 2009; Wischke et al, 2009), heat shrinkable packages for electronics (Charlesby, 1960), sensor and actuators (Kunzelman et al, 2008; Lan et al., 2008), smart water vapor permeability materials (Mondal et al., 2006), self-deployable structures in spacecraft (Campbell et al., 2006), micro-systems (Eddington and Beebe, 2004), damping materials (Yang et al., 2004), self-peeling reversible adhesives (Xie and Xiao, 2008), vehicle components (Lendlein, 2006), toys (Hayashi et al., 2004), hair treatments (Lendlein and Ridder, 2007) and chemical feeding in chemical reactions (Laroche et al, 2002).
Until now, the SMPs applied in textiles were mostly based on conventional glass or melting transitions. The molecular mechanism of the SME of an SMP, based on glass or melting transition, as a switch, is shown in Fig. 6.8. These SMPs usually have a physical cross-linking structure, crystalline/amorphous hard phase, or chemical cross-linking structure and a low temperature transition of crystalline, amorphous or liquid crystal phase. They are processed or thermally set to have an ‘original’ shape. Generally, in the permanent shape, internal stress is zero or very low. If the SMP is subject to deformation, the large internal stress can be stored in the cross-linking structure by cooling the polymer below its switch transition temperature (glass transition or melting temperature). The deformed temporary shape is thus fixed because of the elastic modulus sharp increase once the glass transition or melting temperature is reached. By heating the polymer above the switch transition temperature, the SMP recovers to its permanent shape as a result of releasing internal stress stored in the cross-linking structure. The network to store the internal stress, which is used as a shape recovery driving force, may be a physical cross-linking structure, crystalline/amorphous hard phase or chemical cross-linking structure. The ‘molecular switch’ to resist the release of internal stress may be a crystallization, glass transition or liquid crystal phase. It is clear then that SME is a result of polymer morphology structures, but not of the properties of specific polymers.
Liu et al. (2007a) divided SMP based on glass or melting transition into four categories: class I, covalently cross-linked glassy thermoset networks as SMPs (glass transition as a switch); class II, covalently cross-linked semi-crystalline networks as SMPs (melting transition as a switch); class III, physically cross-linked glassy copolymers as SMPs (glass transition or melting transition as a switch); and class IV, physically cross-linked semi-crystalline block copolymers as SMPs (glass transition or melting transition as a switch). From the view of the application, the prerequisite for thermal sensitive polymers as SMPs is that their switch transitions are above the environment temperature into which they are applied.
SMPs can be used for garment finishing and fiber spinning using various techniques to impart the SME of SMPs to textiles. Wan and Stylios (2004) finished fabrics with shape memory polyurethane (SMPU). The finishing solution was fabricated by dissolving SMPU chips in dimethylacetamide. The coating was conducted using a traditional finishing method. The SME of the treated fabrics could be trained from being flat at a high temperature to being bent at a low temperature (Stylios and Wan, 2007).
Hu et al. (2007a; Liem et al, 2007; Liu et al., 2008) treated cotton and wool fabrics using an SMPU water-borne emulsion. The water-borne emulsion was prepared through solution polymerization following the steps as shown in Fig. 6.9 (Hu, 2007a; Liu et al., 2005). First, SMPU oligomers were prepared with poly (propylene glyols) (PPG), 4, 4,-diphenylmethane diisocyanate (MDI) and dimethylolpropionic acid (DMPA). Then, the –NCO was end-capped with methyl ethyl ketoxime (MEKO). Triethylamine was added to neutralize the free carboxylic groups. The acetone and water mixture was dropped into the reactor to prepare SMPU aqueous emulsions. Finally, the aqueous emulsion was used to finish cotton fabrics by a pad→pre-dry→cure process. During curing at a high temperature, the released isocyanates reacted to form cross-linking structures on cotton fabrics following the reaction as shown in Fig. 6.10. To improve the hydrophilicity of the finished textile products, hydrophilic segments such as polyethylene glycol with a molecular weight of 200–600 can be incorporated in SMPUs.
The SMP-treated cotton fabrics showed wrinkle-free properties due to the shape recovery effect of the SMPs (Li et al, 2004). Fabrics, especially cotton fabrics, become very wrinkled during wearing or storing due to the de-bonding and slippage of hydrogen bonding. As shown in Fig. 6.11, the cotton fabric treated with SMP can recover its original flat shape within 1 min simply by steam blowing, while the untreated fabric cannot. Compared with traditional wrinkle-free finishing by formaldehyde agents such as DMDHEU (dimethyloldihydroxyethylene urea), the advantage of SMPU finishing is that it has no harmful formaldehyde. Unlike another kind of wrinkle-free finishing technique which uses polycarboxylic acid finishing agents such as BTCA (1, 2, 3, 4-butane tetra-carboxylic acid), SMPU finishing does not significantly decrease the mechanical strength and whiteness of the fabric. SMPU finishing of cotton increases the mechanical strength of cotton fabrics to some extent. Repeated washing experiments showed that the wrinkle-free effect of SMPU emulsion treated fabric can last for hundreds of laundry cycles.
The ability of SMPs to hold shape well means that cotton fabrics treated with SMPU emulsion have a high crease and pattern retention ability and that aesthetic designs can be easily achieved. Figure 6.12 shows the crease retention effect of a cotton fabric treated with SMPs. In Fig. 6.12, one cotton fabric is treated by SMP; another one is not. Both fabrics have a crease shape in the centre at the beginning by ironing. After washing them in hot water with a temperature around 60 °C, the crease shape on the untreated fabric disappears, while the crease shape on the treated fabric remains. Figure 6.13 shows the superior pattern retention of an SMP designed fabric. Even after many laundering cycles the pattern is retained.
We also treated wool fabrics with the SMPU emulsion using a similar reaction mechanism as that used in the shape memory finishing of cotton fabrics. Wool fabrics and sweaters have serious felting and shape shrinkage problems because of the scale friction on wool fiber surfaces. Resin coating, chlorination shrink proofing and oxidation anti-felting can significantly reduce these problems. The SEM images of the SMPU-treated wool fibers are presented in Fig. 6.14. The wool garment treated with SMP emulsion has a better dimension stability than that of the untreated garment because the SMPU covers the wool fiber scales and, as a result, reduces the wool’s directional frictional effect after the finishing process. As shown in Fig. 6.15, the untreated wool garment shrinks after a certain amount of washing cycles whereas the treated garment maintains its original size.
6.15 Improved dimensional stability of SMP-treated wool fabrics after washing. The SMP-treated garment maintains its original size (left garment); the untreated garment shrinks significantly after washing (right garment).
In addition, the finishing of wool fabrics significantly reduces the felting effect because the coated SMPU resin reduces the friction between wool fibers. The textures of untreated fabric and treated fabric after shape memory finishing are shown in Fig. 6.16. The texture of the treated wool fabric is still clear after 25 washes; in contrast, the texture of the untreated wool fabric shows serious felting after five washes. Both were washed following standard AATCC wool washing procedures.
Though shape memory finishing of fabrics can add new functions to fabrics, there are still some problems which have to be tackled. To obtain good shape memory functions, the coating cannot be too thin. In contrast, SMPU coating which is too thick will significantly decrease the soft hand feel of cotton and wool fabrics. Intrinsically, fabrics of different structures and fabrication specifications have different degrees of elasticity and fixability which affects the shape memory behavior of different SMPU-treated fabrics. For a specific function such as a wrinkle-free effect, high shape recovery ability and high elasticity of the SMPU are required. However, for a shape retention effect, high shape fixability is preferable. There are several areas of the shape memory finishing of fabrics process which would benefit from further study such as the influence of fabric structure on the properties of the SMPU-treated fabrics. The molecular structures of cellulose after cross-linking with SMPU need to be investigated. The reaction extent of hydroxyls on cellulose fabrics and the methods used for treating specific fabrics need to be established. The percentage of hydroxyls to be cross-linked by SMPU before the treated fabric obtains optimal SME and hand feel needs to be determined. Finally, the idea that the unsatisfied functions of SMPU-treated fabrics may be partially due to the friction of rigid fibers and fabric structures needs to be examined.
Hu et al. (Hu et al, 2007c, 2008b; Meng and Hu, 2007b; Meng et al, 2007; Zhu et al., 2006) developed different SMPU filaments by using polyol as the soft segment and small size diols and MDI as the hard segment and by using different spinning methods. The switching temperature of the SMP fibers was at around room temperature (~ 29–64 °C). The polyols used included PBA (poly (buthylene-adipate)) or PEA (poly (ethylene adipate)), PCL (polycaprolactone) and PHA (poly (hexylene adipate)). The polyol species, molecular weight and content significantly affect the spinnability of SMPU. The SMP fibers prepared by wet spinning and melt spinning are shown in Fig. 6.17(a) and (b) respectively. Figure 6.18 (Zhu et al., 2006) shows the stress–strain curves of an SMPU fiber compared with other synthetic fibers. The tenacity of SMPU fibers is in the range of ~ 6–14cN/tex with maximum strains in the range of ~ 35–204%. The stressstrain curve of SMPU fibers (PU56–90, PU56–120) is located between the high modulus fiber such as nylon and the high elasticity fiber such as Lycra fiber (Spandex fiber of Du Point). In comparison with SMPU films, SMPU fibers have lower shape fixability, higher shape recovery and higher recovery stress as a result of molecular orientation in SMPU fibers brought about during spinning processes. The recovery ratios of SMPU fibers can be as high as 100% (Zhu et al., 2006). By incorporating CNTs into the SMPU fibers, Hu et al. (Meng et al, 2007) developed electro-responsive SMP fibers which could change their shapes under electrical stimulation by Joule heating. At present, the voltage required to produce enough heat to trigger the shape recovery is around 100 volts. For safety reasons, the conductivity of fibers has to be further improved so that a low voltage can trigger shape recovery.
Figure 6.19 shows an SMP hollow fiber prepared by melt spinning by Hu et al. (Meng et al., 2009a). The internal diameter of the hollow fiber can noticeably change and recover under thermal stimulation as a result of SME. Because the changes of the internal diameter of hollow fibers affect the physical properties of textile products, smart SMPU hollow fibers can be used for thermal management in garments, or as stuffing for pillows and mattresses, which can adapt to body contours for comfort just like the function of memory foams. Furthermore, this kind of hollow fiber has the potential to be used in smart filtration, controlled drug-release and liquid transportation.
Shape memory yarns such as blended, warped or core yarns of SMP fibers and natural, regenerated or synthetic fibers have been developed. The shape memory properties of shape memory fabrics were investigated by Hu and her colleagues. Three kinds of knitted fabrics were prepared using (a) 100% shape memory fiber, (b) shape memory core yarns (50% cotton and 50% shape memory fiber) and (c) two-ply yarns (100% cotton). The basic fabricating parameters of the shape memory fabrics are shown in Table 6.1. Shape memory properties were determined using a bagging recovery test (Li, 2007; Liu et al., 2007b). First, a bag shape was produced on every fabric by an Instron machine. Then, the bagged fabric was heated in the oven at a constant temperature of 30 °C and 75 °C respectively for 3 min. Non-recoverable bagging height was measured every minute after heating. Strain fixity (%) and strain recovery ratio (%) were calculated using the bagging height. The strain recovery curves of shape memory fabrics are shown in Figs 6.20–6.22.
The knitted fabrics containing 100% shape memory fiber and shape memory core yarn (50% cotton and 50% shape memory fiber) showed a superior thermal sensitive shape memory performance in comparison with the knitted fabric composed only of two-ply yarns (100% cotton). The recovery ratio for knitted fabrics composed of 100% shape memory fibers had a shape recovery ratio of approximately 100% after heating to 75 °C which is above the switching temperature of shape memory fibers. The fabrics with 50% shape memory fibers had a recovery ratio above 90% after heating to 75 °C.
Shape memory fabrics made of SMP fibers can be used in textiles and clothing to create self-adapting textiles with self-regulating structures and performance in response to environmental temperature variation. Apparel prototypes of shape memory fabrics have been made using knitted and woven SMP fibers (Hu et al., 2009; Liu et al, 2007b). The garments made of SMP fibers can be enlarged to adapt to various wearers’ figures (Li, 2007; Liu et al., 2007b). Vertical pressure test results shown in Fig. 6.23 show that, in comparison with fabrics made of Lycra (spandex fibers) or PA/ Lycra, the garments made of SMP fibers have a relatively low vertical tension stress. The low tension stress can be ascribed to the shape fixability of SMP fibers to temporary shapes, which diminishes the pressure sensation for wearers. The fabric made with SMP fibers can be used to improve the comfort sensation of textile products such as intimate apparel (Hu et al., 2008b, 2009).
6.23 Pressure (vertical direction to fabric surface) of enlarged fabrics to ‘wearer’ (testing cylinder) (SMF1, SMF2, SMF3 and SMF4 are fabrics made of shape memory fibers with different composition, Lycra 70D is a fabric made of Lycra fibers of 70 denier, PA/Lycra is a fabric made of polyamide fibers and Lycra fibers).
Figure 6.24 (Bonanni, 2010) shows another application of shape memory textiles: they can be made to fit to individual figure size which takes advantage of the SME of shape recovery fibers. Figure 6.24(a) is a columnar shaped memory fabric which can be facially fabricated. First, the diameter of the columnar shaped memory fabric is enlarged to a size larger than the diameter of the model’s body so that the columnar fabric can be easily put on the model’s body, as shown in Fig. 6.24(b). By using a hair dryer to heat the fabric, the fabric recovers and becomes the same shape as the model’s body, as shown in Fig. 6.24(c). Thus a perfect match can be obtained, and different body shapes can be accommodated and shown to their best advantage using shape memory fabrics.
6.24 Easy wear and perfect match of shape memory garment. The diameter of the columnar shape memory fabric is enlarged to a size larger than the diameter of the model’s body (a) so that the columnar fabric can be easily put on the model’s body (b); upon heating the columnar fabric recovers, thus a perfect match is obtained (c).
The biological properties of shape memory fabrics with a switching temperature of around body temperature were preliminarily evaluated (Meng et al, 2009b). The results showed that the shape memory fabric was not cytotoxic, hemolytic, insensitive or irritating. With a better compatibility with human bodies than shape memory films or bulks, SMPs in fiber/fabric form may also find applications in biomedical areas such as wound dressing, scaffold materials and orthodontics.
In addition to the self-adaptability of shape fabrics which feel good and are a perfect match for body shape, shape memory fabrics can be used for wrinkle-free effects, and many kinds of artistic designs. Depending on the desired properties, shape memory fabrics can be used on the collars and cuffs of shirts which need a rigid fixed shape, on the elbows and knees of jackets and trousers which need to be hard-wearing, and on denim, velvet and cord which need help holding their shape.
SMP fibers, yarns and fabrics have also been developed by Stylios et al. (2005; Vili, 2007). Yarns of different conventional fibers along with SMP fibers were woven spaciously and loosely along the weft to allow room for the SME to take place. Contract (shape recovery) occurs when the environmental temperature is over the glass transition temperature of the SMP. Thus, fabric with SMP yarns can be aesthetically appealing. Shape memory fabrics can also be used for temperature and moisture management of the human body.
It has been found that if the filaments are used solely to make fabrics by knitting or weaving, the fabrics do not have a good hand feel. For this reason SMPU filaments are preferred for use along with other fibers such as cotton. If the filaments are used as core spun yarn, fasciated yarn or wrap yarns with cotton, the SME of the fabrics made of the yarns is not pronounced due to the restriction of the cotton fibers on the movement of the SMP fibers. The elasticity structure of knitting fabrics decreases the shape fixity of the fabrics, while the structure of woven fabrics hinders the shape recovery of the fabrics. Furthermore, during processing at room temperature, SMPU fibers in fabrics are elongated to some extent due to the low glass transition temperature (slightly above room temperature) of SMPU fibers. This temporary elongation needs to be taken into consideration during the design of shape memory fabrics because it can cause shrinkage.
Shape memory nano-fibers have special properties due to their large surface area to volume ratio. Hu et al. (Zhuo et al., 2008) coated nano SMPU fiber meshes onto textiles by electrospinning. After the SMPU nano-fibers were coated on fabrics, many unique properties were found on the fabrics due to the surface properties and orientation structure of the nano-fibers, such as good hand feel, good water vapor permeability, waterproofing ability and unusual SME. Krause et al. (2007) prepared nano-fibers and thin film of main-chain liquid crystal elastomers, which showed a shape response temperature change. The photo cross-linkable liquid crystal elastomers were cross-linked during the electrospinning process. The prepared nano-fibers had a uniform director alignment along their long axis. SMEs were observed in this kind of material.
Electrospun non-woven meshes have minimal resistance to moisture vapor diffusion and maximal efficiency in trapping aerosol particles in comparison with conventional textiles (Peppas and Kim, 2006). Protective clothing with nano-fiber coating can be fabricated using SMPs. Fabrics coated with nano-fibers from SMPs can neutralize hazard agents without decreasing the air and water vapor permeability of the fabrics. The nano non-woven structure can also provide a strong resistance to the penetration of harmful agents in aerosol forms. SMP nano-fibers can be used not only in textile areas, but also in tissue engineering (Ulijn et al., 2007), scaffolding (Chen and Ma, 2004) and wound dressing (Ignatova et al., 2006).
SMP foams have been proposed for use in aerospace applications (Tobushi et al., 2006), weight reducing (Marco and Eckhouse, 2006), drug delivery (Wache et al., 2004) and measuring tools of complex cavities (Huang et al., 2006). Figure 6.25 shows examples of textile products using memory foams. The memory pillow is filled with low resilience polyurethane memory foams developed by Bayer. It can adjust its shape to the contour of the neck and shoulders of the body. SMP foams can also be used as memory mattresses to provide comfort and support for the user’s body. SMP foams have also been used to prepare insoles, which can effectively improve shoe fitting. Wearing high-heeled shoes can cause many problems for women, such as calluses, painful bunions and spine deformities (Gefen et al, 2002). Dr Scholl’s Company produces Dr Scholl’s® Memory FitTM (Work) Customizing Insoles (Fig. 6.25) (Scholl, 2010) using memory foams for the balls of the feet or from heel to toe, which can adapt to everybody’s unique foot shape with every step and can help tackle the above-mentioned problems effectively.
SMPs can absorb impact energy due to their good damping properties once they reach the glass transition temperature (switching temperature) (Yang et al., 2004). AlliedSignal Inc. (Lim et al., 2002) developed an automotive seatbelt fabric using SMP fibers (Securus fiber) to help increase occupant safety utilizing the damping effect of the SMPs. It is reported that the Securus fiber can absorb energy from the body’s forward motion and improves the safety of passengers during a crash. First, the seatbelt holds the passenger securely in place, then elongates slightly and cushions the body as the belt absorbs the energy of the body’s forward motion. The Securus fibers are melt spun from shape memory poly (ethylene terephthalate)-poly (caprolactone) block copolymers.
SMPU used as a coating or film in garments can offer temperature-dependent water vapor permeability to improve the comfort of the garment (Hyashi, 1993). SMPUs for breathable fabrics have a glass transition temperature of about human body temperature. The water vapor permeability of the SMPU dense films changes to correspond with the wearer’s temperature variations. When the body temperature is above the glass transition temperature of the polyurethane dense film, the free volume of the film increases significantly which increases the transmission speed of water vapor molecules with an average diameter of 3.5 Å through the SMPU films (Hu et al, 2002). Thus, the heat and vapor from perspiration are transported away from the body into the environment, and comfort can be obtained. When the body temperature is below the glass transition temperature of the SMPU, the free volume decrease, which prevents air and water molecules passing through. Consequently, the film keeps the body warm. By employing hydrophilic segments such as dimethylpropionic acid, diol terminated poly (ethylene oxide) (Jeong et al, 2000; Mondal and Hu, 2006, 2007) in SMPUs, the overall water vapor permeability of dense SMPU films can be improved. The overall water vapor permeability of SMPs can also be significantly increased by forming microfoams in the SMPs. Thermal-responsive SMPU film can be coated, laminated or interlined in traditional fabrics.
The shape recovery of deformed SMPs can be triggered by water or moisture due to the plasticizing effect of water molecules, which increase the flexibility of macromolecule chains (Huang and Yang, 2005; Leng et al., 2008). If an SMP has a hydrophilic or water soluble ingredient, the shape recovery can be accelerated (Chen et al., 2007; Jung et al, 2006). A pyridine unit, which is responsive to moisture, can be used to improve the moisture absorption of polyurethane. Hu et al. (Chen et al., 2009) introduced a pyridine unit into SMPU by N-bis(2-hydroxylethyl) isonicotinamine (BINA) and prepared moisture-responsive SMPU. High strain recovery with fast recovery speed was obtained. The synthesis route of SMPU with pyridine unit is presented in Fig. 6.26. First, 1, 6-hexamethylene diisocyanate (HDI) reacts with BINA to form a pre-polymer. Then the pre-polymer is extended with 1, 4-butanediol (BDO). It can be deduced that SMPs sensitive to their suitable solvents can be created just as hydrophilic SMPs can be created to be sensitive to water/ moisture. Lv et al. (2008) observed the DMF (N,N′-dimethylformamide) (a good solvent of SMPU)-responsive SME of SMPUs.
When body temperature is unstable and leads to excess perspiration, wearers will feel uncomfortable. Figure 6.27 shows a smart shirt named ‘Sphere React Shirt’ with rear vents which can open up to allow perspiration and heat to escape when the wearer sweats. The vents automatically close once the wearer reaches a dry state. It may be very effective to use water/moisture-responsive textiles which can change macro-shape or micro-structures to achieve such functions for the moisture and heat management of human bodies. Figure 6.28 shows the schematic sketch of a moisture-responsive SMP for the intelligent management of sweat and heat from human bodies. The fabric made of single or laminated moisture-responsive SMPs has many small windows on it. If the wearer becomes hot and gets sweaty after excessive activity, the windows on the fabric open to allow heat and sweat to be released. In a cold situation without sweat, the windows close and keep the wearer warm and comfortable.
The US Patent 6627673 of Kimberly-Clark company (Topolkaraev and Soerens, 2003) disclosed a humidity activated laminate textile for disposable and reusable product applications. The humidity-responsive laminate textile contains an elastomeric polymer and a moisture absorbing polymer. Upon exposure to a high humidity environment, the disposable product transforms to a desired configuration which can guard against leakage. The laminated polymer is deformed at a dry state. In a high moisture situation, the modulus of the material decreases and triggers the shape change of the material because of the moisture absorbed by the moisture absorbing polymer. Thus, this material can change to a desired shape and/or texture during use. The products can therefore be used for disposable and reusable products such as disposable diapers, training pants, incontinence products and feminine care products.
By integrating SMMs into textile structures, we can obtain new functions of textiles for aesthetic design, moisture/temperature management and protection again extreme climates. Textiles with SMMs can move or change shape to achieve different 3D forms of garments for aesthetic appeal. Curtains with SMMs can open and close automatically to accommodate environmental stimulation. The micro-or macro-structure change of adapting clothing in response to stimuli is a good means for heat and moisture management of human bodies for comfort. The change of fabric structures can also be used for protecting against extreme environments.
At present, most SMMs used in textiles are sensitive only to one specific stimulus signal, namely thermal stimulus with specific stimulus range. In future, SMMs with multiple stimuli-active functions may be achieved by integrating different stimuli-response materials into one material together. Therefore, the SMMs may be able adapt themselves to different environmental conditions with varying thermal, light, electricity, pH and humidity signals.
In addition to shape change and recovery properties, stimuli-responsive textiles can offer a deformation force which may be used as an aid for physiological performance. Orthopedic suppliers may use such textiles for corrective aids. SMPs with shape change polymers in textiles may also be used for the release of perfume, nutrition and drugs to human skin. These stimuli-responsive textiles will open new opportunities for smart textiles in medicine and medical applications.
The SME of all SMPs which have been used in textiles are one-way. This means that the shape changes of all the shape memory textiles developed are non-reversible and cannot be repeated. Several technologies have been reported to develop SMPs with a two-way SME. Chung et al. (2008) reported a two-way SME in cross-linked shape memory poly (cyclooctene). Cooling-induced crystallization under a tensile load results in elongation. Subsequent heating melts the network yields which then contract and recover their shape. However, a tensile load is necessary for the two-way SME and it is difficult to control. Ahir et al. (2006) and Qin and Mather (2009) achieved two-way SME through anisotropic chain conformation change of liquid-crystalline polymers. Unfortunately, this process requires a temperature of above 100 °C which makes the material unsuitable for clothing applications. Hu et al. (Chen et al., 2008) obtained two-way SME by laminating an SMP with an elastic polymer. This laminate material is more suitable for textile applications. The two-way SME was ascribed to the release of an elastic strain of the SMP layer upon heating, and the recovery of the elastic strain induced by the bending force of the elastic polymer layer upon cooling. Hu’s research group is also developing bi-component fibers with one component as an SMP and another component as an elastic fiber, and studying the two-way SME of the conjugate fibers.
The study of SMMs for textile applications currently remains largely unexplored. Even though polymer materials can have a good compatibility with textiles, textile applications pose strict requirements on the original properties of SMMs such as making them safe, lightweight, highly stable, easy to process and with a soft hand feel. Cleaning and washing also need to be taken into consideration for some clothing applications. Extensive and intensive work needs to be conducted to investigate in detail the property parameters of SMMs and to integrate their properties with textile products.
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