Various shape memory effects of stimuli-responsive shape memory polymers

Different from one-step SME which can only remember its single original permanent shape, stepwise SMEs (multiple SMEs) can remember more shapes in addition to its original shape. Most stepwise SME polymers either have more than one switching thermal transition or have a single switching transition with a broad thermal transition range. It can be easily understood that if a SMP has well-separated thermal transitions as the switching transitions, the SMP may show stepwise multiple SME after a multi-step programming process. In addition to thermal transitions, any other switching transition listed in figure 2 may act as a switching transition for multi-step SME. Proper programming is normally required to achieve the multi-step SME [25, 27, 170, 227–229]. Stepwise SMPs have many advanced application potentials such as in assembly, morphing aircraft, material packaging, releasable fasteners, and many medical applications [25, 27, 170, 227–230].

3.1.1. Neat SMPs.

Grafting and blocking copolymers of different soft segments may induce more than one well-separated multiple phase in a single SMP. The multiple phases may provide more than one well-separated thermal transition as the shape memory switching transition. Bellin et al [227] first reported a SMP with two-step triple-SME by copolymerizing poly(ethylene glycol)mono-methylether-monomethacrylate (PEGMA) with poly(caprolactone)dimethacrylate (PCLDMA). At a suitable composition, the polymer had crystalline PCL domains and crystalline polyethylene glycol domains, which act as the two switching phases. By copolymerizing PCLDMA with cyclohexymethacrylate (CHMA), Bellin et al [231] prepared another SMP showing triple-SME. The melting transition of the PCL domains and the glass transition of the cyclohexymethacrylate domains acted as the two switching transitions of the triple-SMEs. Chen et al [232] obtained two separated melting transitions as the switches in shape memory polyurethane (SMPU) using two polyols PCL-10000 and poly(tetramethylene glycol)-2900 with different molecular weights as the soft segment. Paderni et al [233] demonstrated that the triple-SME can also be triggered by alternating magnetic field through non-contact activation in a stepwise increasing alternating magnetic field, if iron (III) oxide particles are added to the polymer [233, 234].

One phase in SMPs from the same soft segments may show two thermal transitions at two temperatures. For example, Pretsch [229] observed two-step SME in SMPU by employing the glass transition around −50 ° C and the melting temperature of soft segment above the ambient temperature as the two switching transitions. Similarly, Bothe et al [235] prepared a star-shaped POSS-polycaprolactone (PCL) polyurethane network. The glass transition at around −40 ° C and the melting transition at around 50 ° C of the PCL were employed as the well-separated switching transition for the two-step SME. One of the disadvantages of the above two-step SMPs is that the first switching transition temperature is very low, i.e., −40 to −50 °C, which is not suitable for most applications.

Not only can glass and melting transitions be used as the switching transitions of stepwise SMPs, nematic–isotropic transformations of nematic network can also be employed for this purpose. Qin and Mather [27] developed a glass-forming, polydomain nematic network by cross-linking a thermotropic unsaturated polyester. A glass transition at about 80 ° C and nematic–isotropic transitions at 160 ° C were employed as the two switching temperatures of the triple-SME. Similarly, Ahn et al [81] prepared a side-chain liquid crystal random terpolymer network. The molecular structure of the terpolymer is shown in figure 3(a). The acrylate end group forms cross-linking by curing at 120 ° C. The polymer network had well-separated glass transition temperature and clearing temperature for the triple-SME. Figure 3(b) shows the two-step shape recovery of the side-chain liquid crystal polymer networks, corresponding to the states of the molecular morphology at different temperatures. The study by Ahn and Kasi [236] on the stepwise liquid crystal polymers shows that a sufficient extent of motional decoupling between mesogen-rich and backbone-rich domains is the key factor for the outstanding stepwise SME in side-chain liquid crystal polymers.

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3.1.2. Polymer blends.

3.1.3. Polymer laminates.

3.1.4. Other SMP hybrids.

In fact, not only blends or laminates, but any hybrid SMPs with multiple switching transitions, may show stepwise SME after proper thermomechanical programming. For example, Luo and Mather [242] simply impregnated a PCL microfiber mat with shape memory epoxy resin which was cured later. The elastic modulus profiles indicated that the composite had two separated thermal transitions, corresponding to the glass transition of the epoxy and the melting of the PCL mat, respectively. Figure 4 shows the two-step triple-SME of the composite.

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3.1.5. Programming of stepwise SMEs.

Normally, to create a stepwise SME, the polymer has to be subjected to a multi-step programming process which corresponds to the multiple phase switching transitions of the SMP [194, 227, 228]. During the multiple-step programming, multiple temporary shapes are consequently fixed at a temperature below the respective switching transition temperatures as shown in figure 5(a) using external load. One-step programming as shown in figures 5(b) and (c) with a simplified programming process is more feasible or preferable in many applications. Several researchers have demonstrated that multiple-step programming is not a prerequisite for most SMPs to show stepwise SME. The stepwise SME created by one-step programming arises because, during the programming, the segments of all the different phases can be oriented and fixed at the programming temperature. Using a high-temperature one-step programming process shown in figure 5(b), Behl et al [243] created two-step triple-shape memory capability of a polymer network based on PCL and poly(cyclohexyl methacrylate) at a temperature below the switching transition temperature of both the PCL and poly(cyclohexyl methacrylate) phases. In comparison with the high-temperature one-step programming, the cold-temperature one-step programming shown in figure 5(c) does not need to heat the polymer to above the highest switching temperature. Zotzmann et al [244] showed that a cold-temperature one-step programming process at an ambient temperature (which was below the switching temperatures of a copolymer) could induce stepwise shape recovery. It was also found that the one-step cold-drawn programming temperature did not affect the shape recovery temperature and the proportioning of the recovery in the two steps. To achieve better shape controllability of the SMPs during the stepwise SME, a multiple-step programming is preferred.

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For applications such as sealant in expansion joint, SMP needs to be subjected to two-dimensional programming (tension in one direction and compression in the transverse direction) to solve the problem of interfacial debonding and sealant squeezing out of the channel problem in summer [197]. Li et al [194] shows that this simultaneous 2D programming [245] can be replaced by hybrid two-step 1D programming, i.e., tension programming in the longitudinal direction followed by compression programming in the transverse direction. They found that, depending on the prestrain level in the two directions, the thermosetting SMP shows some weak triple shape during shape recovery. They also found that the recovery sequence depends more on the prestrain level, not necessarily the reverse sequence of programming [194].

3.1.6. Summary remarks.

3.2.1. SMPs with a broad glass transition range.

3.2.2. SMPs with a broad melting transition range.

3.2.3. SMPs with a broad glass and a melting switching transition.

Although theoretically feasible, practically, it is impossible to achieve unlimited-step SME in a polymer with a broad thermal transition. The maximum number of steps a SMP can achieve so far is three-step (quadric-SME) reported by Xie et al [248]. In further research, to achieve a four-step quintuple-SME, Li et al [259] incorporated another additional melting transition into a polymer which already possessed a broad glass transition. They prepared a poly(methylmethacrylate)/polyethylene glycol semi-IPN which provides a broad glass transition temperature and an additional melting temperature. The four-step quintuple-SME in a single-shape memory cycle of the SMP is shown in figure 6.

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3.2.4. SMPs with a broad gradient glass transition.

Creating a linear glass transition temperature gradient within one SMP leads to glass transition distribution throughout the polymer. The linear glass transition temperature gradient can allow consequent recovery of the polymer in the linear gradient direction. DiOrio et al [204] obtained linear glass transition gradient in a SMP by UV precuring a thiol–ene-based photo-cross-linkable glassy thermoset and post-curing the thermoset on a temperature gradient plate under the same UV source. Figure 7 shows the consequent shape recovery of the SMP from right to left. The glass transition temperature gradient is marked by slicing along the sample to give 15 'fingers' along the bottom edge with a black dot at the end of each 'finger'. The shape recovery initiates at the left end and propagates to the right.

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3.2.5. Summary remarks.

Selective heating of predeformed SMPs or homogeneous heating of predeformed structurally inhomogeneous SMPs can lead to spatially controllable stepwise SME. Polymers are thermal insulators, which makes it possible to raise the temperature of SMPs in a local area while the other part of the polymer is not considerably affected. The recovery only occurs at the place where it is heated. Heating another area will trigger the recovery of that area. Localizing heating of SMPs to achieve spatially controllable stepwise shape recovery does not require complicated polymer structure design and/or synthesis. Homogeneous heating of predeformed structurally inhomogeneous SMPs to achieve stepwise SME needs complicated polymer structure design. For example, for a multi-component SMP with different switching transition temperatures at different areas, homogeneous heating of the polymer will trigger the recovery of the different components at different temperatures. Therefore, controllable stepwise SME can be achieved [264].

3.3.1. Selective joule heating of conductive CNT/SMP.

Spatially controllable stepwise SME has been demonstrated in many traditional one-step SMPs. If the SMP is filled with electrical conductive fillers, selective heating locally using electricity can control the shape recovery of the SMP. For example, Fei et al [265] prepared a CNT/poly(methylmethacrylate-co-butyl acrylate) composite with good electrical conductivity. Figure 8 shows the schematic of the consequent spatially controlled shape recovery of the SMP composite by selection of the place where the electrical voltage is applied or switching the power on/off.

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3.3.2. Selective focused light heating of gold nanoparticle/SMPs.

Gold nanoparticles have a high level of interaction with light especially with infrared light. Focused light absorbed by the gold nanoparticles in gold nanoparticle/SMP composites can heat the polymer up locally and trigger the shape recovery locally. Since light can be easily manipulated, the spatial stepwise SME can be controlled by manipulating the light. Once the light is turned off, the heating caused by light is halted; the shape recovery may be stopped at any stage.

Zhang et al [266] loaded PCL-surface-functionalized gold nanoparticles in a branched oligo(ε-caprolactone) (bOCL) cross-linked with hexamethylene diisocyanate (referred to as XbOCL). The chemical structure of the SMP (XbOCL) and polymer-functionalized gold nanoparticles are shown in figure 9(a). Figure 9(b) shows the stepwise shape recovery process of the prestretched gold nanoparticles-loaded XbOCL film by separate laser exposures on four sections of the film.

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3.3.3. Selective light heating of SMPs with printed patterns.

If selected areas of a prestretched SMP are coated with light-absorption materials such as dark ink or carbon black, unfocused light will only heat up the predefined area where it is coated. In this way, localized heating at the predefined coated areas is possible without using focused light. Liu et al [264] demonstrated localized self-folding of a prestretched shape memory film on which predefined hinge patterns were created using a desktop printer as shown in figure 10. The black ink pattern provided localized absorption of light. The sheet under the predefined inked regions (i.e., hinges) recovers, and thereby causes the planar sheet to fold into a three-dimensional object.

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3.3.4. Selective alternative magnetic field heating of multi-composites.

Figure 11(a) shows a SMP multi-composite consisting of a Fe₃O₄/SMP region and a CNT/SMP region separated by the neat SMP prepared by He et al [267]. The Fe₃O₄ and CNT nanoparticles have different selective radio frequency heating properties. The 13.56 kHz magnetic field only heats the filler particles Fe₃O₄; therefore, it only heats up and recovers the Fe₃O₄/SMP region. The 296 MHz magnetic field only heats up the filler CNTs; therefore, it only heats up and recovers the CNT/SMP region. Finally, in an oven at a temperature above the switching transition of the SMP, the neat SMP region recovers. The well-controlled stepwise shape recovery of the multi-composite is shown in figure 11(b).

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3.3.5. High-intensity focused ultrasound-induced selective heating.

High-intensity focused ultrasound (HIFU) was originally used as an extracorporeal tool for the treatment of tumors based on its thermal effects. It can also be used to heat up SMPs locally since polymers can absorb the mechanical energy generated by viscous shearing oscillation exerted by focused ultrasound. Li et al [12] demonstrated the HIFU triggered shape recovery of a SMP as shown in figure 12 by localized heating in the circled area. The shape recovery process can be controlled by selecting the ultrasound exposure time, intensity and the position of its action.

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3.3.6. Summary remarks.

Many semicrystalline SMPs have been demonstrated to be able to show SCE if they are subjected to a constant tension load. The reason for two-way SCE is that cooling-induced crystallization of semicrystalline polymer films under a tensile load results in elongation; and subsequent heating triggers shape recovery leading to contraction. Chung et al [268] first revealed the SCE in cross-linked shape memory semicrystalline poly(cyclooctene). During cooling under constant stress, the crystallites form in the loading direction, leading to extension of the SMP. When heated to a temperature above the melting transition of the polymer, the SMP recovers, causing contraction. The film showed the highest two-way shape-changing strain at 44% with a recovery ratio of 85% under a stress of 0.4 MPa. Westbrook et al [269] developed a 1D constitutive model to describe the SCE of semicrystalline SMPs based on the fact that the deformation states within the individual stretch-induced crystallization phases formed at different times are different.

Inspired by this research, many researchers have observed the two-way SCE on many semicrystalline SMPs under appropriate constant tensile load, such as semicrystalline SMPU [270], polyhedral oligomeric silsesquioxane (POSS)/PCL networks [271], cross-linked PCL-based polyesterurethane [272], linear, three- and four-arm star PCL functionalized with methacrylate [30], and poly(ethylene-co-vinyl acetate) [28]. Zotzmann et al [273] achieved not only two-way SCE but also two-step SME on a multi-phase polymer network with two different crystalline phases at the two switching transitions. The constant tensile stress is the key factor to achieve the two-way SCE. To obtain long-term stress for the semicrystalline SMP, Kang et al [274] coated a semicrystalline SMP fiber with elastomer, which can apply tensile stress to the SMP fiber.

The two-way SCE of liquid crystal elastomers results from the coupling effect between the ordering changing of mesogenic moieties and the elastic properties of the elastic network. The original research on liquid crystal elastomers was focused on monodomain liquid crystal polymer because it was believed that monodomain liquid crystal polymer could show higher shape-changing amplitudes. Preparation of liquid crystal monodomain requires two processes: synthesis of liquid crystalline elastomer and aligning uniformly the mesogens over the whole sample. Later, Burke [275], Qin and Mather [27] found that glass-forming polydomain nematic network could also show SCE. A large shape-changing stain was also observed on glass-forming polydomain nematic network. This research simplifies the fabrication process of two-way shape change liquid crystal polymer because it does not require the alignment procedure as do monodomain liquid crystal elastomers.

A maximum spontaneous elongation up to 500% has been achieved in main-chain liquid crystal elastomer in a fiber form by Ahir et al [276]. The large central block is made of a main-chain nematic polymer which has large spontaneous elongation along the nematic direction. To further improve the nematic structure orientation, Krause et al [277] prepared nanofibers of main-chain liquid crystal elastomers. Photoinduced cross-linking was formed during the electro-spinning process. Exceptional mechanical properties with outstanding shape-changing effect were obtained. Normally, in comparison with side-chain liquid crystal elastomers, main-chain liquid crystal elastomers have higher shape change. It is believed that combining a mesogen side chain and main chains in one elastomer can increase the shape change amplitude.

Laminates, layered materials or any hybrid materials made of materials of different thermomechanical properties may change their geometry upon changes in environmental temperature. The mechanism is that heating creates unbalanced mechanical stress. The unbalanced stress may arise from different coefficients of thermal expansion, different modulus changes, and SME (if shape memory materials are used). The layered materials can be both SMPs, or a SMP layered with non-SMP, or both non-SMPs.

Langer and Lendlein [278] in their patent first outlined the method of preparing SCPs using SMPs by a laminate method. Chen et al [279] prepared a shape changing material by laminating a SMP with an elastic polymer. As well as SMPs, shape memory alloys have also been used for this purpose. Tobushi et al [280] fabricated a two-way shape-changing laminate with shape memory alloys and SMPs. Ghosh et al [281] prepared a two-way shape changing bias system by embedding shape memory alloy (SMA) wires in a SMP which had a glass switching transition between the martensite transformation finish temperature and austenite phase transformation finishing temperature of the SMA. Actually, if any two dissimilar materials are laminated, the interfacial stress will vary at the interface if the environmental temperature changes, which may drive the shape change of the laminate. These two-way shape memory laminates do not depend on the switching transition of SMPs. The main disadvantages of these two-way shape memory laminates compared with those made of SMPs are slow shape-changing speed.

Two-way shape-changing laminates can be easily miniaturized using advanced processing techniques such as chemical vapor deposition and photolithographic technique. For example, Kalaitzidou et al [282, 283] fabricated a layered tube composed of a polydimethylsiloxane film and nanometer-thick gold which was coated on the polydimethysiloxane film. Figure 13(a) shows the micro-shape and thermal-active two-way SCE of the composite rolls. Upon cooling, the polydimethysiloxane film shrinks more than gold, which produces a compression at the polydimethysiloxane side to fold the tube. Reversely, upon heating, the layered composite unfolds. Similarly, LeMieux et al [284] made a bimorph actuator by coating plasma-polymerized polystyrene on one side of an atomic force microscope silicon tip by a plasma enhanced chemical vapor deposition process. Figure 13(b) shows the schematic of the bimaterial cantilever and the actual optical image of the microcantilever bending. The actuator has ultrathermal sensitivity.

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The principle of the thermal-active SCE of the laminate materials can be extended to water-active SCE of laminate materials if the laminated materials have a component of a water-solvable/swellable material such as hydrogels. In fact, any polymers whose elastic modulus can be changed by water or any chemical may be used to prepare water/chemical-active two-way shape-changing laminates, provided that the coupled materials have good interface interaction. For example, Saha et al [285] fabricated water-responsive microcylinders as shown in figure 14. The cylinders are comprised of a water-swellable hydrogel compartment as well as an inert compartment and thus undergo substantial bending in water. The asymmetric expansion creates surface stresses resulting in significant and controllable bending of the microcylinders. The microcylinders are fabricated by electrohydrodynamic co-jetting followed by microsectioning [285–288].

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Azobenzenes and their derivatives undergo reversible trans–cis isomerization upon the irradiation of light of suitable wavelength [289]. The isomerization causes the angle change of the two isomers accompanied by significant change in molecular length between 9.0 Å (trans) and 5.5 Å [290, 291] as shown in figure 15. If azobenzenes are coupled into liquid crystal elastomers, significant photoinduced deformation can be achieved. Due to the photoisomerization of azobenzene, the liquid crystal elastomers experience a reduction in alignment order and even liquid crystal to isotropic transition, leading to macroscopic shape change [290, 292–296]. Different liquid crystal elastomer molecular structures with azobenzene as dopants, chemically bonded pendants or photo-crosslinkers have been studied to improve the response speed and deformation strain [215, 297–299]. In comparison with other photoactive molecules, azobenzenes can produce larger geometrical changes.

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Several outstanding review papers [300–305] have been published regarding the synthesis, photomechanical effect and application strategies of azobenzene-based liquid crystal elastomers. Light, in comparison with other stimuli such as electricity, can be easily manipulated in terms of intensity, polarization, and phase [306]. Through proper molecular design, polymer structure design, and stimulus design, complex SCE such as twisting, bending [307], swimming [308, 309], rotation [310], inchworm walk and controlled vibration [294, 308] have been achieved in liquid crystal elastomers [311, 312] and their composites [86, 294, 300–303, 311, 313–317]. Recently, oscillations were demonstrated in monodomain azobenzene liquid crystal polymer cantilevers when the cantilever was sequentially exposed to the front and back surface of the sample as the film inertially passed through the laser beam [294, 308] as shown in figure 16 [308]. The polymer was synthesized by copolymerizing two azobenzene liquid crystal monomers in the aligned nematic phase. At the start, the azobenzene mesogens are aligned vertically. The azobenzene mesogens on the irradiated surface reorient due to the formation of trans-azobenzene orthogonal to the polarization direction of the laser light. During the upstroke, the front surface of the samples is exposed. During the down-stroke, the back surface is exposed. Therefore, controlled vibration of the cantilever is achieved. Spatial variation of the domain orientation utilizing command surfaces and photo-patterning (holographic and masking) can enable more complicated moving effects.

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Azobenzene-based photoactive SMP does not have to have a liquid crystal structure. For example, Lee et al [85, 318] prepared a light-active linear azobenzene-functionalized polyimide fiber which was spun from an amorphous to semicrystalline structure. As shown in figure 17 [85], exposure of the cantilever to blue-green irradiation (λ = 442 nm) produced polarization directed forward and reverse bending. The SCE effect is similar to that observed on glassy azobenzene liquid crystal network polymers [318, 319]. The crystallinity is the dominant factor influencing azobenzene photoisomerization and the SCE.

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Photoactive azobenzene-based polymers can be used for recordable and erasable memories [86, 311, 315], bioseparation [155], microrobots or optical microtweezers [315, 320], remotely triggered drug delivery systems [321, 322], remote-controllable micro/nanoactuators and sensors [323], cardiovascular therapeutic device [324, 325], artificial muscles [313], energy harvesting [294], and display devices [294, 301].

Photoactive polymers in the nanoscale have special functions such as photo-switchable nanoarchitectures [326, 327], and photoactive polymer membranes with photo-regulated permeability [328]. Photoactive azobenzenes/inorganic nanoporous membranes are made by immobilizing azobenzene derivatives to inorganic nanoporous membranes such as silica frameworks. The light-controlled conformation of the azobenzene in the composites can change the pore size and correspondingly the transport behavior of the composite membrane [329]. Continuous excitation at a wavelength, which both the trans and cis azobenzene absorb, can lead to continuous photoisomerization and continual dynamic wagging [330]. The azobenzenes attached to the interiors of the inorganic substrate pores templated by surfactant [331, 332] can act as both impellers and gatekeepers [333, 334]. Azobenzenes/inorganic nanoporous composites can be used as photoactive nanovalves [330, 335], drug delivery nanovehicles [336], microfluidic devices [337], photonic nanoparticle devices [338], and micro-optical-controlled soft microrobots [339].

Anthracene and its derivatives can undergo 4 + 4 dimerization upon irradiation of UV light. Figure 18(A) shows the photodimerization of 9-anthracene carboxylic acid (9-AC) under UV light of proper wavelength. Al-Kaysi and Bardeen [340] demonstrated the large reversible SCE of a crystal nanorod made of 9-AC compound material using UV light as shown in figure 18(B).

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Kondo et al [341] incorporated the anthracene into a polymer with anthracene as the side group and made a photo-responsive fiber from the polymer. Figure 19(A) shows the molecular structure of the polymer. Under UV irradiation, the bending angle of the fiber increases with increasing temperature as shown in figure 19(C), which is related to the modulus decrease of the fiber.

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Coumarin and its derivatives undergo photoinduced reversible dimerization reaction upon photo-irradiation as shown in figure 20(a) [342]. The photochemical dimerization reaction in a polymer can cause migration of the photodimerization moieties and bending effect of the polymer. He et al [342] prepared a coumarin-based polymer by partially functionalizing poly(4-vinyl pyridine) with 7-(carboxymethoxy)-4-methylcoumarin. The coumarin moieties dimerization occurs upon irradiation with UV light of wavelength above 310 nm. The reversible cleavage of the cyclobutane bridge happens on irradiation with UV light of wavelength below 260 nm. The photodimerization on the surface exposed to UV light leads to large bending toward the UV light as shown in figure 20(b). The degree of the photoactive bending of the polymer is comparable with that of photoactive liquid crystal elastomer made of azobenzene-based polymers.

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The UV reversible photodimerization of cinnamic acid group as shown in figure 21 can lead to macroscopic shape changing of the polymer. Several types of one-step SMPs based on cinnamate have been reported. Two-way SCE of cinnamate-based polymers has not been studied intensively. Cinnamate moieties may be incorporated into the polymer in the grafted branches or in the main chain [86, 311, 320, 343–345]. Lendlein et al [9] grafted cinnamic acid into the backbone of hydroxyethyl acrylate hydrogels. Wu et al [89] prepared a biodegradable polymer from soft diol(polycaprolactone diol), biodegradable hard diol (poly-(l-lactic acid) diol), and N,N-bis(2-hydroxyethyl) cinnamamide with cinnamamide groups as the pendent groups. Both the cinnamate polymers showed outstanding UV-light-induced one-step SME. The one-step SMPs are first deformed and the temporary shape is fixed by the photodimerization of the cinnamate moieties. Upon light irradiation of another proper wavelength, the reversible cross-linking cleaves, leading to the shape recovery of the polymers [9, 87, 88, 345].

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Shi et al [346] and Thi et al [347] developed a bio-copolymer poly[(3,4-dihydroxycinnamicacid)-co-(4-hydroxycinnamic acid)] with hyper-branched structure from cinnamic acid. The copolymers formed nanoparticles by self-organization. They observed significant diameter change and rapid recovery of the nanoparticles with whole cinnamate groups upon the photo-crosslinking and photo-cleavage of the cinnamate groups with UV irradiation.

CNTs [348, 349] absorb near-infrared irradiation. Ahir and Vaia [348, 350] filled CNTs into polydimethylsiloxane elastomer and observed two-way SCE in the composite. The expansion or contraction of the composite depends on the extent to which the composite is strained. If the material is slightly pulled, it expands when it is exposed to infrared light. Conversely, if the material is subjected to a strain greater than 10%, it contracts under identical exposure to infrared light. This process is completely reversible and persists after numerous cycles. The mechanism of two-way SCE has not been fully revealed although it is believed to be related to the photothermal effect.

The above materials are elastomer materials filled with near-infrared irradiation fillers. Ugur et al [351] achieved the near-infrared irradiation-induced SCE of a pure polymer film made of cross-linked polyarylamide nanofibers. The cross-linked multi-block polyarylamide contained 'coil-like' polyethylene oxide and 'rod-like' polyarylamide blocks as shown in figure 22. The polyarylamide self-assembles into fibrillar morphology on silicon substrates and the resulting films exhibit a high degree of optical anisotropy. The reversible SCE can be repeated for numerous cycles. A study on better understanding of the mechanisms is underway; it is suspected that the two-way SCE is related to the photothermal effect of the highly aligned fibrillar structure.

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Scott et al [352, 353] developed a light-active covalently cross-linked network which underwent photo-cleavage in its backbone chain to allow chain re-arrangement. The monomers used to create the cross-linking network are shown in figure 23. The switch mechanism of the reversible backbone cleavage is addition–fragmentation chain transfer. Reaction diffusion of radicals through the cross-linked matrix occurs initially by the reaction of a radical with an in-chain functionality, forming an intermediate, which in turn fragments, reforming the initial functionality and radical [354]. Figure 23(D) shows the addition–fragmentation process through the polymer backbone of the photoactive network [352] and figure 24 shows the photoinduced actuation accordingly [352]. Irradiating the sample on the unirradiated side allows elimination of the introduced stress and a shape change in the other direction. If the sample is irradiated on both sides in an alternating pattern, oscillatory SCE can be observed. Long et al [355] have proposed a photomechanics model for this light-activated polymer by integrating four coupled phenomena, i.e., photophysics, photochemistry, chemo-mechanical coupling, and mechanical deformation. The simulation results compared nicely with the experiment results.

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The mechanism of the SCE of semicrystalline polymers indicates that all the semicrystalline SMPs can exhibit two-way SCE more or less under proper stress situation. The proper tensile stress has to be applied to the polymer. In addition, the SCE of semicrystalline SMPs can only have elongation/contraction. Due to the limitation of the crystallites formation in the loaded direction, the length extension of the semicrystalline SMP is not high. All of the above limitations can significantly affect the applications of semicrystalline two-way SMPs.

Without a complex synthesis process, the laminating or hybridization method is one of the most promising methods to prepare shape-changing materials. There are many options for selecting the compounding materials. The compounding materials are not limited to polymers. Metals can also be used for this purpose. The SCE of laminated or hybrid materials is easily tailored by selecting suitable materials couples, adjusting the thickness of the layers, and changing the processing conditions.

Four main types of photo-responsive two-way SMPs have been developed, i.e., azobenzene-based polymers, anthracene-based polymers, coumarin-based polymers, and polymers with addition–fragmentation chain transfer reaction. Complex movements of photoactive polymer films and laminated films have been obtained from photo-responsive two-way SMPs especially with azobenzene based liquid crystal elastomers, such as oscillating, twisting, swimming [308, 309, 356], rotation [310], and inchworm walk [314]. So far, the photoactive SCE of these SCPs, except for the azobenzene-based polymers, has not been fully investigated. In addition, the photothermal-effect-induced two-way SCE should be studied in detail because the photothermal effect is common and easy to achieve. The research on photothermal-effect-induced two-way SCE may lead to a large number of novel two-way shape-changing materials.

Of the two-way SCPs, liquid crystal elastomers have attracted a high level of research attention because they can provide high shape-changing strain and high mechanical output. Their promising applications as artificial muscles, soft actuators and nonlinear optical devices are still under investigation and some good results have been obtained [82, 357]. For practical applications, researchers [358–360] are investigating in more detail the properties of liquid crystal elastomers, such as chemical and mechanical fatigue, and environmental stability.

Most of the movements of present SMPs are simple shrinkage and bending. For more advanced applications, complex movements with controllable steps, controllable strains, controllable deformation speed, and exact deformation stress are preferred. The moving effect of stimuli-active moving polymers is affected by many factors such as the thermomechanical programming and recovery process in addition to the molecular structures. The localized stimulation method has provided a facile and efficient method to achieve a well-controlled, actively moving process. Further studies need to be conducted to investigate the controllability of SME by localizing stimulation. For many advanced applications of SMPs such as sensors and actuators, accuracy of shape recovery is equally important. Although some effort has been made to understand the accuracy of SMPs [361], there is a lack of systematic studies on the influences of environmental conditions on the whole shape recovery process of actively moving polymers.

In recent years, modeling of SMPs has accelerated, motivated by the need for an efficient design tool for increasingly sophisticated applications [24, 362–365]. The modeling of SMPs falls into two main approaches: the viscoelastic approach [24, 53, 218, 261, 263, 366–368, 373], and the phase change approach [217, 369–371]. Although a number of models have been set up and used to explain and predict the complicated SME of SMPs [372], each of them is only applicable to a certain type of SMP such as thermoset or thermoplastic, thermal active or athermal active, and dual SME or stepwise SME. SMPs are versatile; a more general model is urgently needed which can capture the most important behaviors of SME while being simple and applicable.

Effort should be directed into accurate molecular simulations and physics-based mathematical modeling to obtain the optimal actively moving effect and predicting the shape memory process. Through the effort of researchers and scientists, this goal will be achieved, although the complicated structures of various SMPs make it a tough task for theoretical studies [24, 218, 373–385, 355]. Recently, Nguyen has given an excellent review on this topic [386].

At present, one of the critical challenges facing SMPs for some engineering applications is their low mechanical strength and low mechanical output. Due to the soft nature of polymers, the mechanical output of SMPs is far less than that of their metallic counterparts, although SMPs can have high recoverable strain up to several hundred per cent. Even worse, the mechanical properties of thermal-responsive SMPs at a temperature above the switching transition temperature drop significantly. Improving the molecular orientation of stimuli-active moving polymers can significantly improve their mechanical properties. Some studies have demonstrated the effectiveness of this method. Also, cold-drawing programming of semicrystalline SMPs can increase the recovery stress [218].

Recently, Nair et al [387] reported a two-stage reactive non-stoichiometric thiol–acrylate SMP. After shape recovery, a second stage reaction by UV irradiation can polymerize remaining acrylate functional groups to form a more stable chemical cross-linking system. In this way, the SMP can maintain a high mechanical strength at a temperature above the switching transition temperature. This strategy still does not improve the mechanical output or recovery stress of the SMP during the shape recovery process. Another research direction may be to explore novel intrinsically athermal-active shape memory structures for which direct or indirect heating is not required to trigger the SME. Stimulus does not heat the polymer; instead it triggers molecular re-arrangement of the polymer without decreasing the modulus.

Another problem of most present SMPs is that they can only operate at low temperature, normally in the range of ambient temperature to 100 ° C. The low thermal stability and low operating temperature have limited SMPs' wider structural applications such as in satellites, aerospace, and transportation. Aromatic polyimides have high thermal stability (>300 °C) and are intrinsically flame resistant. Koerner et al [388] studied the SME of an aerospace-grade polyimide CP2 which is derived from 2,2-bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane (6FDA) and 1,3-bis(3-5 aminophenoxy)benzene (APB) [389]. The polyimide CP2 was slightly cross-linked so that the polyimide had proper thermomechanical properties for the SME. The CP2 had a narrow glass transition (<10 °C) at 220 ° C which could act as the switching temperature for SME. The polymer exhibited excellent high-temperature shape memory performance with excellent extensibility (>200%), and high room temperature modulus (2–3 GPa). The significantly high glass transition also affords this polymer with little to no creep at room temperature. Yoonessi et al [390] prepared a polyimide from bis phenol A dianhydride and 2,2-bis[4-(4-aminophenoxy)phenyl] propane. They studied the SME of the polyimide and the corresponding graphene polyimide nanocomposites. Both the neat polyimide and polyimide nanocomposites exhibited SME at 230 ° C with the glass transition as the switching transition.

Light as an energy source is environmentally friendly. As a stimulus, it can be controlled remotely, instantly and precisely. Many studies have been conducted on photoactive moving polymers. More research is needed for photoactive moving polymers. Particularly, photoactive moving polymers may play an important role as environmentally friendly energy harvesting materials [300]. Some problems, such as fatigue resistance and environmental stability of photoactive moving polymers, need to be further investigated [300].

Most of the present stimuli-responsive SMPs are responsive to a specific stimulus in a certain range. Future research will also include studying multiple-stimulus-active moving polymers by combining different stimulus-active structures into one single polymer. In this way, the SMPs may change their properties to adapt to the overall environmental conditions such as temperature, light, humidity, electricity signals, and pH.

Blending, in situ polymerization, grafting, and laminating not only improve the properties but also increase the SME versatility of SMPs such as stepwise SME and two-way SCE. For SCPs based on cross-linking semicrystalline polymers, a tensile load is necessary. For SCPs based on thermal or photoactive isomerization, high deformation strain cannot be produced except in liquid crystal structures. However, for practical applications, liquid crystal SCPs are still costly, in addition to their unstable shape-changing behavior. It seems that the lamination method is one of the most promising methods to prepare shape-changing materials. First, the laminating process is not complicated. In addition, the properties of shape-changing laminates are tailorable by simply changing the stacking sequence and ply thickness.

Although SMPs have been studied intensively, reports on SMP micro- and nanoscale devices are few [391, 392]. SMPs in microscale will have many applications such as tetherless microgrippers, microvalves, data storage, and soft robots. The smart tetherless microgrippers will be able to capture and release clusters of cells controlled by temperature, chemicals, and biological conditions. The present research has been mostly focused on creating surface wrinkles or patterns on SMPs [3, 393–399]. These SMPs with controllable microwrinkles and patterns may be used as molecular detection, optical devices, filters, and sorters. The future research may be to microminiaturize the present various bulk SMPs and develop micro or nano SMP devices, in addition to investigating the SME of SMPs at a micro- or nanoscale. It is believed that research in this direction will boost more advanced application opportunities for SMPs.

SMPs have been studied for half a century. The research on the molecular structures, mechanisms and principles has been conducted intensively. Now, researchers have better understanding of the SMEs of various SMPs. The research is entering a stage of practical engineering design and applications of various SMPs. SMPs have been proposed for use in many areas. Numerous patents related to SMPs have been filed in the past 10 years. In the past 5 years, several new companies developing SMP products have been established and several SMP products have been commercialized by researchers such as Yakacki et al [400] from Medshape. It is believed that the next 10 years will see an explosion in growth of SMP products. Although the applications of one-step SMPs have been widely studied, the applications of stepwise, spatially controllable and two-way SCPs have not been extensively explored. The next decade will see tremendous application-oriented research on complicated, stepwise, spatial, controllable, and two-way SMPs, in addition to research on the polymers themselves.