Technical Textile Yarns || Shape memory polymer yarns

© Woodhead Publishing Limited, 2010

429

13Shape memory polymer yarns

T. Wan, nanjing University of Information Science and Technology, P. R. China

Abstract: Shape memory polymers (SMPs) are smart materials that have the same recoverability and shape memory as niTi alloys. SMPs have great potential in the clothing and textiles industries due to their similarity to textile structures, as well as uses in the construction of medical devices, micro-electromechanical systems, etc. after introducing their remarkable thermal–mechanical properties, this chapter investigates the key aspects of synthesizing, manufacturing and programming SMP yarns, and goes on to consider how to expand the applications of shape memory polymers to other fields. Future trends in this area are identified. Emphasis is placed on recently developed shape memory nanocomposites and laminated hybrid composites, which have been designed to have enhanced shape memory recovery stress and two-way shape memory.

Keywords: shape memory polymer yarn, nanocomposites, shape memory effect, shape recovery, recovery stress.

13.1 Introduction

Shape memory polymers (SMPs) are a novel class of smart materials that can be induced to recover from a temporary deformed state to their original (permanent) shape by an external stimulus or trigger, such as a change in temperature (Lendlein and Kelch, 2002). a considerable number of SMPs with applications in the textile industry have been published or patented. Representative materials that are frequently cited in the literature include polynorbornene, poly(trans-isoprene), styrene–butadiene copolymers, ethylene–vinyl acetate copolymer, and some polyurethane elastomers (Otsuka and Wayman, 1998, Lendlein and Kelch, 2002; Wei et al. 1998; nakayama, 1990; Kim et al., 1996). SMPs are intrinsically capable of a shape memory effect, although the mechanisms are dramatically different from those of metal alloys. The unique behaviour of shape memory alloys is based on the temperature-dependent austenite-to-martensite phase transformation on an atomic scale. Thus a temporary shape can be fixed at a single temperature, and recovery occurs upon heating to beyond the martensitic transformation temperature. In contrast, SMPs achieve temporary strain fixing and recovery through a variety of physical means, the underlying large extensibility resulting from the intrinsic elasticity of polymeric networks.

430 Technical textile yarns


The shape memory effect (SME) of SMPs is characterized by shape recovery and shape fixity, and is unique for polymers with micro-phase-separated heterogeneous structures. It is due to thermodynamic incompatibility between hard segments (which relate to the maximum thermal transition temperature) and soft segments (which relate to the second highest thermal transition temperature). The domains of a hard segment act as net points, while chain segment domains in a soft reversible segment act as molecular switches. To display shape memory functionality for a particular application, the polymer network has to be temporarily fixed in a deformed state under specific conditions. This requires the deformed chain segments, which are under external stress, to be reversibly prevented from recoiling, and is achieved by introducing reversible net points such as molecular switches. The reversible phase, with the melting or glass transition temperature of the soft segments, holds the temporary deformation. The fixed phase involves the hard segments, which are covalently coupled to the soft segments and inhibit plastic flow through the presence of physical or chemical cross-linkage points between them, thus memorizing the permanent shape (Kim et al., 1996; Li et al., 1997; Lin and Chen, 1998a, 1998b; Takahashi et al., 1996; Lee et al., 2004). The efficiency of a shape memory polymer is empirically controlled by its composition, as defined by the polymer’s chemical structure, molecular weight, the morphology of the phase separation, phase composition, phase distribution, fraction and size of amorphous and crystalline domains (Lendlein and Kelch, 2002; Ohki et al., 2004). SMPs are an important smart material with potential in both academic and industrial settings, having low manufacturing costs, good processing ability, high shape recoverability, and a broader range of shape recovery temperatures than shape memory alloys (SMa) (Lendlein and Kelch, 2002; Wei et al., 1998; Hayashi et al., 1995). Compared with shape memory alloys, SMPs are likely to be more suited to textiles and clothing, as well as related products in actuators, sensors and biomaterials. One reason for this is that the mechanical properties of SMas can only be adjusted within a limited range (maximum deformation of about 8%), whereas SMPs are inherently recoverable even with a deformation of several hundred percent. However, although intensive research on SMPs has been conducted in both academia and industry in the past 20 years or so, the understanding of how to prepare SMP fibres is still in its infancy (Meng and Hu, 2008a). Compared with SMP bulks, SMP fibres have certain highly desirable mechanical properties as a result of the inherent molecular orientation of fibres (Meng and Hu, 2008a). Such properties contribute to the versatility of SMPs, allowing them to be used in applications ranging from the manufacture of biomedical materials to high performance sensors. In particular, their high recoverability broadens their applicability further in textile production – for the manufacture of woven fabrics such as fabrics, swimwear, sportswear,



ladies’ hosiery, corsetry and medical hosiery, where they are capable of responding dynamically to fluctuations in heat and moisture levels, thus ensuring improved comfort for the wearer. SMP fibres can be spun alone, or blended with other fibres.

13.2 Thermo-mechanical behaviour of shape memory polymers (SMPs)

a large volume of literature regarding the general thermo-mechanical behaviour of SMPs exists (Tobushi et al., 1998; Lendlein and Kelch, 2002). Materials with the unique ability of being able to recover their original, permanent shape from a fixed temporary shape only when triggered by some specific external stimulus, are classified as demonstrating a shape memory effect (SME). In particular, SMPs are able to change shape in response to variations in temperature. For SMPs of the polyurethane series, the glass transition temperature (Tg) may be set to approximate room temperature, and their defining characteristics of shape recovery, shape fixation, etc., may be quite distinct at temperatures above and below this critical Tg value. In Fig. 13.1, an SMP material based on a post-polymerized aB-polymer network is shown as an example of thermally induced SME. In SMP materials, the unconstrained recoverable material strain limits are of the order of 100%, whereas in shape memory metals or ceramics, this value is about 10% and 1% strain, respectively. a typical schematic of the thermo-mechanical behaviour of SMP is illustrated in Fig. 13.2. Thermo-mechanical processes, such as hot compression, extrusion or injection moulding, can be used to process thermoplastic SMPs to give them a memorized permanent shape. at temperatures higher than the transition temperature Tg, in the first step of the cycle, SMPs are mechanically deformed into the desired temporary shape. The temporary shape is then fixed by some constraint while lowering the temperature in the second step. as the temperature decreases, the stress needed to maintain the shape gradually diminishes. after the SMP chain segments have been frozen in a temporary position by thermally reversible interactions between the molecular chains, the constraint is then removed and the induced shape is fully retained in the third step. Here, the strain fixity rate is defined as the difference in the shapes found after stages 1 and 3, and it can be a criterion for assessing shape memory performance because large strain fixity rates imply enhanced micro-phase separation in SMPs. The rigidity of polymer chains in the soft segment is found to decrease upon heating in stage 4, and the frozen stress balance breaks so that the SMP returns to its original, permanent shape. This cycle is then repeated (Lin and Chen, 1998a, 1998b; Otsuka and Wayman, 1998; Lendlein and Kelch, 2002; Lendlein et al., 2001). Since the hard segments act like net points and remain rigid throughout



the memorizing process (stage 2 of Fig. 13.2), it is the variation of rigidity in the soft segment with temperature that determines both the deformation to the temporary shape and the recovery to the permanent shape. Depending on the molecular arrangement of the soft segments, the transition in rigidity of polymer molecules is governed by the glass transition temperature (Tg); and accordingly, as illustrated in Fig. 13.3, a drastic change in elastic modulus is observed in the vicinity of Tg. above Tg, these materials are soft, whilst below Tg their hardness increases rapidly until they become rigid and eventually brittle. Thus, SMPs with different mechanical properties can be developed by controlling Tg of the soft segment, the level of micro-phase separation, and the structure of the hard segment, leading to a variety of potential applications (Shim et al., 2006; Gall et al., 2002; Beloshenko et al., 2003; Hosoda et al., 2004; Hampikian et al., 2006).

t = 0 s

t = 2 s

t = 8 s

t = 15 s

t = 18 s

t = 20 s

13.1 Series of photographs showing the macroscopic shape memory effect of an AB-polymer network. The pictures show the transition from temporary to permanent shape at 70°C within 20 s.



The shape memory effect in SMPs is distinct from that in SMas. at temperatures above its Tg, an SMP will only return to its original shape provided there are no external forces applied. Unlike SMas, externally triggered shape reversal in SMPs can be described as metamorphic, in that the polymer exhibits a gradual change in shape during transformation. The SME is trained through a series of cyclic thermo-mechanical processes. An example training process in yarns of specimen SMPs is as follows:

Stress

Fixed shape

Temperature

Strain

slsm

sm

emep

em

eu

Ttrans

Th

Tl

1

3

2

4

Permanent shape

Temporary shape

13.2 Typical thermo-mechanical behaviour of SMP. (Seok et al., 2007).

Glassy region

Tg region

Rubbery region

Tg

Temperature

1010

109

108

107

106

E (

Pa)

13.3 Variations of SMP elastic modulus with temperature (Sokolowski et al., 2007).



1. Specimens are stretched at a high temperature, Thigh = Ttrans + 20°C, to an elongation of 100% strain.

2. Deformed specimens are cooled down to a low temperature Tlow (room temperature), whereupon the deformations are fixed.

3. at Tlow the external force on the specimens is removed.4. The deformation is recovered by heating the samples back up to Thigh.5. The cycle reverts to step 1 and repeats.

The shape memory effect in SMPs is described by the shape memory properties of shape fixity (SF), shape recovery ratio (RR) and recovery stress. The relationship between stress and strain is recorded in the cycles. as illustrated in Fig. 13.2, let em, eu and ep, respectively denote the maximum strain in the cyclic tensile tests, the residual strain after unloading at Tm–20 °C, and the residual strain after shape recovery; the shape fixity (Rf) and the shape recovery ratios (Rr) can be calculated using:

Rf = eu/em × 100% [13.1]

Rr = (em − ep)/em × 100% [13.2]

The unique characteristics and properties of SMPs make these materials highly attractive for applications in many commercial settings. Their advantages over SMas are summarized below:

∑ Lightweight, typical density about 1.13–1.25 g/cm3 compared to 6.4–6.5 g/cm3 for niTi

∑ Wide range of allowable glass transition temperatures, Tg = –70°C to +70°C, allowing for applications in a variety of thermal environments

∑ Shape recovery of up to 400% of plastic strain, compared to 7–8% for SMa

∑ Reversible changes in elastic modulus between the glassy and rubbery states of the polymers (can be up to 500 times the original value)

∑ Excellent biocompatibility allowing for biomedical applications∑ Reversible changes characteristically from moisture-permeable to

waterproof during transition from rubbery to glass state∑ Ease of processing, including moulding, extrusion and conventional

machining∑ Low cost, –10% of existing shape memory alloys (SMa)∑ Characterized by low recovery forces, i.e. low actuating forces, and

cannot be utilized in high power actuators.

13.3 Manufacture of shape memory polymer (SMP)-based yarns

Before being used in mass-production yarns, SMPs were synthesized for specific applications. For example, thermoplastic shape memory polyurethanes with



various hard segments were synthesized with a pre-polymerization method using poly-diols, di-isocyanate and a chain extender. The SME of shape memory polyurethane fibre is characterized by its micro-phase-separated heterogeneous structure, which is composed of a hard segment phase and a soft segment phase. The design required will dictate the choice of SMP yarn formation (e.g. extruder or injection moulding) and state (solution or molten). a few key processes for manufacturing SMP-based yarns are introduced below.

13.3.1 Melt spinning

a raw resin pellet was dried in a vacuum for 8 h in a hopper circulation oven at 80°C until moisture levels decreased to 0.03%. If the resin is not dried, the polymer’s viscosity becomes too low when melted, causing deformation by foaming, flashing and dropping at the nozzle. Shape memory polyurethane filaments were spun using a 20 mm single extruder with high-purity nitrogen protection. The winding-up speed was from 10 m min−1 to 50 m min−1, with an overfeed speed of 10 m min−1. The laminar air temperature was 22°C. The extruder head pressure was 5.00 MPa and the spin pack pressure was 22.00 MPa (Meng and Hu, 2008a, 2008b; Stylios and Wan, 2007). The temperature profile for processing an SMP filament of 0.4–0.6 mm diameter using an extruder of diameter 1 mm is as follows:

∑ Rear (feed zone) T: 170–180°C∑ Centre (compression) T: 175–185°C∑ Front (metering zone) T: 170–180°C.

The key to this operation is to control the viscosity of the SMP in the nozzle of the extruder, while maintaining uniform melting of the polymer. The viscosity of SMPs is more temperature-dependent than that of traditional polymers, requiring stricter temperature regulation and processing controls. In order to control the diameter of the SMP filament, the extrusion rate also has to be regulated.

13.3.2 Wet spinning

Details of the wet spinning process are described in Ji et al. (2006) and Zhu et al. (2006). The concentration of solids in the final polyurethane solution (PU solution) in DMF (N,N-dimethylformamide) was adjusted in the range of 20–30 wt% to satisfy the associated viscosity requirements under which the PU solution is spun into fibres on the equipment after filtration and degassing at 100°C, as shown in Fig. 13.4. With the equipment shown in Fig. 13.4(a), an SMP solution was extruded from a spinneret with 30 pinholes each 0.08 mm in diameter into a coagulation bath at a speed of 6



m min–1 under pressure exerted by the compressed nitrogen. The coagulated fibres were then extracted onto a group of rolls at 10 m min–1 and passed through a water bath where they were rinsed. Next, the rinsed fibres were passed through a hot chamber to undergo drying. The dried fibres were subsequently wound up by a winder. To eliminate the internal stress stored in the spinning process, a heat treatment was carried out, during which the fibres were passed through a heated oven (see Fig. 13.4(b)) of length 2 m, maintained at a temperature of 120°C.

13.3.3 Transfer of SME to a generic fibre after the finishing process

The subtle interaction between SMPs and cellulose fibres within fabrics remains a critical issue for understanding their thermal–mechanical properties, and thus their shape memory behaviour in cotton fibres. Various finishing processes for transferring the SME to cotton or generic polymer yarn

Compressed nitrogen

SMPu solution Rinsing Drying

Winding

Spinneret

Coagulation bathTo recovery

Drawing roll 1

Drawing roll 2

Hot oven

Initiate fibre

Taking up winder

(a)

(b)

13.4 Schematic diagrams of (a) the wet spinning equipment, and (b) heat setting of shape memory fibres (Ji et al., 2006, Zhu et al., 2006).



have been proposed (Li et al., 2004; Liem et al., 2007). a highly viscous resin adhesive was prepared by dissolving polyurethane in toluene or N,N-dimethylformamide. The shape memory polyurethane powder and the resin adhesive were mixed in a certain ratio. The cotton or polymer yarn was then immersed in the SMP solution or the mixture, passed through rolls by means of the rollers, and subjected to a pad–dry–cure process. after padding twice between two rubber rollers at a pressure of 3 MPa, the fabrics were chemically treated with finishing agents. The wet fabric was then transferred to an oven, dried at 50°C in a vacuum chamber for 6 h to degas in order to remove the solvent, and then cured at 120°C for 3 min.

13.4 Applications

13.4.1 Breathability, clothing comfort, wrinkle recovery, etc.

Shape memory woven cotton fabrics/garments and shape memory fibres have been under development ever since smart textiles and clothing applications for SMPs were described by Hu et al. in 1998 (Hu et al., 2004; Hu, 2006). Shape memory fabrics treated with SMP have excellent hand, shape retention, dimensional stability, durability, wrinkle resistance, flat appearance, bagging recovery, comfort for the wearer, and ease of care, even under water and at high temperatures. But although SMPs already have applications in the textile industry, their technological potential has not been fully exploited. all comfortable clothing demonstrates interactions with the human body, such as variable breathability in response to varying temperature. When the temperature of the skin differs by more than 3.0°C from the ideal body temperature, the wearer will begin to feel uncomfortable (Tao, 2001; Mattila, 2006). In a temperature-regulating clothing system, phase change materials (PCMs) are used to regulate temperature fluctuations. The phase transition temperature should be in the range of 10–50°C (Bryant, 1999; Colvin, 1999). When the temperature is higher, PCMs absorb and store heat energy with no temperature increase; when the material cools down, the latent heat is released to the human body. An SMP fibre, a temperature-regulating fibre with a large latent heat-storage capacity of about 100 J/g, was fabricated by melt spinning (Meng and Hu, 2008b). The PEG soft segment phase transfer of the SMP between crystalline and amorphous states results in heat storage and release. At temperatures above the PEG phase melting transition, the SMPs remain solid because the hydrogen-bonded hard segments restrict the free movement of soft segments. Since air has very low conductivity, heat insulation in newly developed materials is achieved by trapping as much air as possible in the microspaces between and within fibres, thus minimizing heat losses by convection. A



further contribution to the insulating effect arises from the air entrapped between the fabric and the skin. This is akin to the principle of keeping warm by wearing multilayered clothing in cold weather in order to trap more air. Wind speed and wind chill factors associated with low temperatures must also be considered in the design of clothing, because the movement of air disturbs the boundary air layer of the clothing, thus changing its insulation capability (Walker, 1983). The prototype design has a layer of shape memory yarn incorporated between adjacent layers of clothing. The films can be made on an extrusion/calendering line and the laminates can be compression-moulded using conventional equipment. To promote the circulation of air and moisture vapour within the interstitial space, as well as to reduce the weight of the film, holes and cut-outs can be made in the laminate. When the temperature of the outer layer of clothing has dropped sufficiently, the shape memory layer should shrink linearly by about 3% and become rigid, while the conventional elastomer remains largely unaltered, thus the polyurethane film broadens the air gaps between layers of clothing. as a result, an out-of-plane deformation of the laminate is expected, and non-evaporative heat loss can be controlled. Of course, the deformation should be reversible if the outer layers of clothing become warmer again. One factor that determines garment quality is the ability to recover from wrinkling, i.e. to retain a smooth appearance and to avoid crease retention after repeated home laundering (Xu and Cuminato, 1998). Fabric and garment manufacturers have made considerable efforts to improve anti-wrinkling properties by overcoming the ruggedness of fabric surfaces (Cheng and Kai, 1998). according to the shape memory principles described above, external stimuli such as cooling and heating may affect the shape of materials, and this is used to improve smoothness, crease retention and wrinkle recovery by incorporating a thermally induced SME into woven wool garments. Woven wool fabrics treated by this method show enhanced smoothness, crease retention and wrinkle recovery after washing and tumble drying. In fact, garment wrinkling can even be recovered completely and quickly using a travel hairdryer to provide hot air.

13.4.2 Engineering fabric aesthetics

With a surge of designers, technologists and engineers keen for mutual cooperation, textile design and manufacture have developed beyond the use of traditional materials, techniques and methodologies, allowing the incorporation of specialized skills and unconventional materials. Research has been conducted into textiles with shape memory attributes conferred by using engineering SMPs incorporated into a woven structure (Stylios and Wan, 2007; Chan et al., 2002). These textiles can change shape and interact with



the surroundings, in contrast to the biomedical and engineering applications that the SMPs were principally developed for, such as constructing ligaments and vascular stents and in robotics. Figures 13.5 and 13.6 illustrate the shape memory performance of various composite textiles densely and uniformly woven with SMP yarn of 0.4 mm diameter. Conventional nylon yarns or polymer yarns of assorted fine wire

(a) (b) (c)

13.5 Shape memory recovery of SMP composite loosely woven fabric with SMP yarn at 50°C with recovery time (a) 0 s, (b) 30 s and (c) 60 s.

(a) (b)

13.6 Shape memory recovery of SMP composite loosely woven fabric at 50°C with recovery time (a) 0 s and (b) 30 s.



with differing performance were blended and woven with SMP yarn in these samples. The SMP yarn was woven spaciously and loosely across the fabric weft to allow room for the SME to take place. The initial planar shape of the SMP composites was fixed by exerting an external stretch force when the sample was placed into a frozen state. The SME then deformed the fabric from its original, flattened shape at low temperature to a rough and embossed matrix with convex edges upon contraction at high temperature. In this case, complete contraction occurred because of the dramatic decrease in the elastic modulus of the SMP yarn when the environmental temperature rose above Tg. These shrunken fabrics exhibit rough and embossed edges, the convex edges appearing predominantly in regions consisting only of SMP yarns. The recovery process is metamorphic, so the polymer experiences a gradual shape variation during transformation. When blended with various kinds of flexible and light yarns, fabric designs based on SMP yarns can display interesting aesthetic appeal, as shown in Figs 13.7 and 13.8, depending on the fabric design and SMP-specific training. However, since the SME in existing SMP yarns is only one way, it is difficult to perform an invert shape variation for these composite textiles. This is solved by adding a reinforcement material with a high elastic modulus to the SMP matrix. There is interest in more complex shape variations, such as polymer fabric reinforcements with high elastic modulus crosslinking with SMP yarn in a fabric structure, as demonstrated in elastic memory materials used in spacecraft (Meink et al., 2001).

13.4.3 Applications in the medical field

Since El Feninat et al. (2002) and Lendlein and Kelch (2005) reported a PCL-based biodegradable polymer and demonstrated its potential in medical applications, biodegradable SMPs have been the focus of considerable research. Different clinical devices contact or are inserted into the human body. The combination of shape memory capability and biodegradability possessed by certain SMPs is especially advantageous for medical devices requiring shape restoration and/or self-deployment, where the emphasis is on minimally invasive surgery (Ratna and Karger-Kocsis, 2008). The polymers provide a convenient means of inserting bulky implants in a compacted state (a temporary shape). They are inserted into the human body in string form through a small incision. When influenced by body heat, they expand back into their original, permanent shape, as shown schematically in Fig. 13.9. The biodegradability of the implant ensures that it will dissolve completely in the body over time, for instance a stent in a blocked artery. Biodegradable SMPs are especially useful in situations where a medical structure incorporated into the body is not intended to be permanent. While scaffolding devices



for assisting in bone and tissue repair might call for biodegradable SMPs, the option of permanent prosthetic implants is also possible. The use of biodegradable SMPs as an intelligent suture for wound closure provides an interesting example (Sokolowski et al., 2007). SMPs used in surgical sutures could allow optimized tightening of the knot. The suture could be applied loosely in its temporary shape, and when the temperature is raised to above Tg, the knot would shrink and tighten, applying optimum force, as shown in Fig. 13.10. This technique is effective for minimizing scar formation and reduces the risk of foreign infection. These intelligent polymers also have potential for probing neurons in the

(a)

(b)

(c)

13.7 Shape memory recovery of SMP composite loosely woven fabric with flexible yarn at 50°C with recovery time (a) 0 s, (b) 30 s and (c) 60 s.



13.8 Shape memory recovery of SMP composite woven fabric samples with varied fabric design at 50°C with recovery time 30 s.

Temporary shape

Permanent shape

13.9 Representation of recovery of a string-like material to a tubular device (Ratner and Karger-Kocsis, 2008).

20°C 37°C 41°C

13.10 The medical possibilities of a shape memory polymer (SMP) were recently demonstrated in the form of a self-tightening knot (Sokolowshi et al., 2007).



brain or engineering a tougher spinal bone. another application of SMPs is targeted drug delivery, where SMP matrices release drugs by a change in structure. Often a hydrolysis reaction will occur, resulting in cleavage of bonds and release of the drug as the matrix breaks down into its constituent, biodegradable components.

13.4.4 Some further applications

The scope of these smart polymers can be extended to the automotive, electronics and aviation industries, all of which are likely to reap innovative benefits from their intrinsic shape-changing properties. In the automotive industry, SMPs can feasibly be used in vehicle bumpers, fascia panels, or other parts of the exterior car body sheath which are vulnerable to crashes and deformations and consequently the formation of dents. any indentation/damage due to involvement in a crash or accident suffered by a car sheath manufactured from SMPs will result in a temporary form. Upon reheating, the SMP components would change back to the undamaged original form, allowing for rapid and effective repair. SMP yarns are expected to play a major role in the development of morphing wing technology in the aviation industry, where the ambition is to produce planes which can respond to dynamic changes in flight conditions by adjusting their shape continuously so as to maximize efficiency at all times. Wings can be constantly adjusted to compensate for gusts or patches of lift, so the aircraft of the future will be in a constant state of flux, altering wing shape and control surfaces, and narrowing or flaring its engine exhausts. The incorporation of structural or functional polymers in micro-electro-mechanical systems (MEMS) raises new issues and challenges. Recent work has concluded that certain MEMS sensor applications will benefit greatly from the use of SMPs (Gall et al., 2004). Liu (2007) provides a comprehensive review of the recent state of polymer-based MEMS, including materials, fabrication processes, and representative devices such as microfluidic valves and tactile sensors. The fundamental challenges include the dispersion of nanoscale reinforcements in polymer matrix materials, the fabrication of micron-scale MEMS from nanocomposites by micro-casting and photopolymerization, and the realization and characterization of functional shape memory properties at micron scales. If successful, research on SMP-based nanocomposites in MEMS will impact upon microscale actuation in complicated environments for many biomedical and microsensor applications. The potential applications of such devices include microvalves, micropumps, microgrippers, microswitches, etc. The expansion of human and robotic exploration dictates the need for novel and complex modes of communication. Visually, these would include layered architecture that supports hyperspectral imaging, synthetic aperture



radar, high-definition television, and telemedicine. For antennas as large as those needed, critical performance specifications include low aerial density, high packaging efficiency, accurate surface geometry to ensure high reflector efficiency, and reliable deployment. Johns Hopkins University and the NASA Glenn Research Center are currently developing inflatable membranes with a rigid torus, and a hybrid shape memory composite that incorporates a fixed central reflector that serves as a backup antenna in case of deployment problems with the main aperture (Lin et al., 2006). The objective is to develop large gossamer antennas for use in deep space, e.g. missions to the Moon, or Mars, which require them to be cost-effective, have very low mass, and have improved deployment reliability, all while maintaining accurate dimension tolerances.

13.5 Future trends

We have already seen that SMPs are characterized by their remarkable recoverability and SME. However, most SMPs exhibit a one-way shape memory, and lack two-way memory capability (Lendlein and Kelch, 2002). SMPs also have relatively low recovery stress, usually 1–3 MPa compared to 0.5–1 GPa for shape memory metal alloys (Lendlein and Kelch, 2002). This has become the main limiting factor in many applications, especially in cases where SMPs need to overcome a large resisting stress during shape recovery.

13.5.1 SMP nanocomposite combined with nano reinforcement phase

In order to significantly increase the shape recovery stress of SMPs destined for structural applications, some reinforcements, such as organoclay, carbon nanofibre (CNF), silicon carbide (SiC) and carbon black (CB), have recently been selected as fillers to enhance the stiffness of SMPs (Gall et al., 2000, 2002; Bhattacharya and Tummala, 2002). Figure 13.11 illustrates the shape recovery stresses of SMP multi-walled carbon nanotube (MWNT) fibres with different MWnT contents. When compared with those of pure SMP fibres, the recovery stresses of SMP-MWNT fibres with 1.0–3.0 wt% MWNT content are much higher due to better interface properties, and can also attain maximum values much more quickly. It has also been noted that properly incrementing the weight fraction of reinforcements significantly improves storage elastic modulus, and the CnT/SMP nanocomposites showed a good SME in modulus strength (Ash et al., 2001; Bhattacharya and Tummala, 2002; Gall et al., 2002). While the recovery stress of pure SMP fibre is primarily attributed to characteristics of the hard segment phase, for SMP fibres with a



reinforcement phase it may also be ascribed to strong interactions between the reinforcement phases and the hard segment. Composite stiffness and recoverable strain levels were found to depend strongly on the volume fraction of the discontinuous reinforcement. However, when the MWnT content is continuously increased to 5.0 wt%, the shape recovery stress begins to fall. This may be due to the inhomogeneous distribution of MWnT and the deteriorating surface quality of reinforcement composites, which can disturb the fundamental polymer networks responsible for shape memory functions. additionally, it has been observed that incorporation of particles also changes the crystallization behaviour and activation energies of relaxation processes in polymers. The addition of other particles to SMPs, such as carbon nanotubes, carbon particles, conductive fibres and nickel zinc ferrite ferromagnetic particles, alters not only their stiffness and recoverable strain levels but also their electrical and magnetic properties. In this case, shape recovery can be triggered by various external stimuli, not only heat (Hu et al., 2005; Liu et al., 2007; Lendlein et al., 2005) but also light (Jiang et al., 2006; Lendlein et al., 2005), joule heating in an electric field (Leng et al., 2007; Koerner et al., 2004; Paik et al., 2006; Schmidt, 2006), induction heating in a magnetic field (Buckley, 2006), etc. Although a lot of research focusing on stimuli-responsive SMPs and their composites has been conducted in this area to date, few conclusive, mutually corroborating results have been obtained.

3.0 wt% MWNT

1.0 wt% MWNT

5.0 wt% MWNT

0 wt% MWNT7.0 wt% MWNT

0 2 4 6 8Time (min)

0.0030

0.0025

0.0020

0.0015

0.0010

0.0005

0.0000

Str

ess

(cN

/dte

x)

13.11 Shape recovery stress of SMP fibres with different MWNT (multi-walled carbon nanotubes) contents (Gall et al., 2000).



13.5.2 SMP hybrid combined with non-SMP

as described above, permanent shape memory in the SMPs themselves relies on the hard segment phase crosslinking within the structure, while the soft segment phase helps set the frozen temporary shape below Tg. Thus, after shape memory training, SMPs acquire the ability to remember the permanent shape above Tg, but not the temporary frozen shape below Tg. By hybridizing shape memory materials with other functional or structural materials, smart composites can be fabricated which integrate a multitude of specialized functions or properties of the separate, constituent materials to achieve complex responses to environmental changes. Tremendous potential for creating new paradigms for material structural interaction lies in these unique material systems. In polymer laminates consisting of one-way shape memory SMP film from PHAG5000-based shape memory polyurethane and elastic polymer film from PBAG600-based polyurethane, not only can a two-way shape memory effect be achieved, but the reversible deformation is also controllable (Chen et al., 2008). The PHaG5000-based shape memory polyurethane has a crystalline soft segment phase and amorphous hard segment, and shows a good one-way shape memory effect, while the PBaG600-based polyurethane shows an amorphous soft segment phase. as the temperature increases (decreases), the modulus of elasticity of the material decreases (increases). However, as shown in Fig. 13.12, the distinction is that the PBaG600-based polyurethane recovers its modulus gradually as the temperature decreases, while around Tg the PHaG5000-based shape memory polyurethane shows an obvious hysteresis effect in its modulus recovery, which is due to the presence of a soft segment phase. This difference results in the loss of balance in rigidity or stiffness upon cooling. The modulus of a substrate layer is usually much higher than that of an active layer of SMP. As shown in Fig. 13.12, upon heating, the elastic modulus of both layers decreased with the temperature; whereas upon cooling, the modulus of the substrate layer began to increase significantly at 60°C, while the modulus of the active layer increased little until the temperature cooled down to below 25°C. Moreover, the modulus of the substrate layer was usually much higher than that of the active layer. The bending force of the material increases with modulus of elasticity, while the elastic strain is determined by the deformation angle (Ping et al., 2005). Therefore, the resulting bending force of the substrate layer can be complementary to the recovery force of the active layer in the SMP laminate during the recovery process, and in particular during the cooling process. On the basis of this principle, upon heating, the whole SMP laminate bends toward the active layer due to the recovery force of SMP, and a temporary shape (high temperature shape) is preserved when the recovery force is equal to the bending force of the substrate layer. Upon cooling, the bending substrate layer acts as a bias



spring and leads to a reverse shape change. Then the whole SMP laminate recovers to another temporary shape (lower temperature shape). Similarly, a bi-component fibre designed with a crescent cross-section can produce the desired ‘two-way’ shape memory effect, and can thus be used for manufacturing comfortable apparel (Fred et al., 2005). With reference to Fig. 13.13, the inner (B) and outer (A) components of the fibre cross-section comprise polymer components with the higher (B) and lower (a) thermal expansion coefficients. By selecting and coupling two polymers with significantly different thermal expansion coefficients, a fibre system that is highly sensitive to temperature change will be formed. When the temperature is increased, the cross-section opens up as the inner component experiences a greater expansion than the outer component, and then closes when the temperature is decreased again. Competing thermally induced expansions induce net thermal and elasticity effects in the fibre, allowing the system to remember all temperature-dependent permanent shapes. This system will form a channel cross-section at higher temperatures and resemble a closed hollow fibre at lower temperatures. Compared with ordinary fabric systems, garments constructed from these bi-component fibres will have superior comfort properties in all weather conditions.

Heating curve of substrate

Cooling curve of substrate

Heating curve of SMP

Cooling curve of SMP

–150 –100 –50 0 50 100 150Temperature (°C)

12.0

11.5

11.0

10.5

10.0

9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0

5.5

Ben

din

g m

od

ulu

s (l

og

(E¢))

HeatingCooling

Cooling

Heating

13.12 Bending modulus vs temperature (Chen et al., 2008).



13.6 Conclusion

The various desirable attributes and properties that are often unique to SMPs, such as biocompatibility, dependence on temperature fluctuations, and ability to exhibit the shape memory effect, give them potential applications in fields as diverse and disparate as the textile industry, the manufacturing industry, engineering and biomedical science. SMPs have numerous fundamental advantages over SMas, and the key manufacturing processes for SMP-based yarns, such as melt spinning and wet spinning, can be used to achieve particular desired results. The immediate hope for the future lies in the development of novel shape memory nanocomposites and laminated hybrid SMP composites, in which dissimilar and often unconventional components are bound together to realize new and imaginative effects, ultimately enhancing shape memory recovery stress and two-way shape memory properties. as the use of SMPs in surgical sutures and biodegradable medical equipment suggests, in SMPs we have an example of how, when inspired by perseverance and scientific ingenuity, the discovery of a new product with unexplored properties can lead to the birth, development and eventual diversification of a new field of science. The future is promising.

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