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Phase-Change Thermoplastic Elastomer Blends for Tunable Shape Memory by Physical Design Kenneth P. Mineart, Syamal S. Tallury, Tao Li, Byeongdu Lee, and Richard J. Spontak Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04039 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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Industrial & Engineering Chemistry Research

Phase-Change Thermoplastic Elastomer Blends for Tunable Shape Memory by Physical Design Kenneth P. Mineart,1† Syamal S. Tallury,2,3‡ Tao Li,4 Byeongdu Lee,4 and Richard J. Spontak1,2*

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Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA 2

Department of Materials Science & Engineering, North Carolina State University, Raleigh, NC 27695, USA 3

Fiber & Polymer Science Program, North Carolina State University, Raleigh, NC 27695, USA 4

Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA

ACS Paragon Plus Environment

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ABSTRACT Intelligent polymeric materials are of increasing interest in contemporary technologies due to their low cost, light weight, facile processability, and inherent ability to change properties, shape and/or size upon exposure to an external stimulus. In this study, we consider thermallyprogrammable shape-memory polymers (SMPs), which typically rely on chemistry-specific macromolecules composed of two functional species. An elastic, network-forming component permits stretched polymer chains to return to their relaxed state, and a switching component affords at least one thermal transition to regulate fixation of a desired strain state and return to a previous strain state. Here, we produce designer shape-memory materials by combining thermoplastic elastomeric triblock copolymers with a midblock-selective phase-change additive, thereby yielding shape-memory polymer blends (SMPBs). These materials not only exhibit tunable switch points but also controllable recovery kinetics. We further highlight the versatility of SMPBs through laminate welding for intermediate multishape fabrication and liquid metal inclusion for shape-memory electronics.


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INTRODUCTION Shape-memory polymers (SMPs) are of growing technological interest because they can change size and/or shape upon exposure to an environmental stimulus.1-3 They are of considerable importance in the ongoing development of stimuli-responsive biomedical4,5 and deployable6 lightweight devices, and their function effectively depends on the presence of two components.7 The first is responsible for providing mechanical rigidity to ensure retention of one or more temporary strain states and likewise serving as a switch, or trigger, capable of releasing a temporary strain state. The second, a network-forming component, is required to restore the polymer to a relaxed strain state upon stimulation. In thermally-activated SMPs, the switching element typically relies on the presence of at least one thermal transition such as a melting or glass transition temperature (Tm or Tg, respectively).,1-3,7 and broad or multiple switches permit access to several temporary strain states.8-10 Chemical integration of network-forming and switching species into a single macromolecule endows SMPs with chemistry-specific thermomechanical properties.8,10,11 Instead of relying on such SMPs with pre-selected properties, we demonstrate that physical incorporation of phase-change materials into network-forming macromolecules such as thermoplastic elastomers yields shape-memory polymer blends (SMPBs) with well-defined and highly tunable switching temperatures and recovery kinetics. Moreover, in the same spirit as our previous studies of functional soft materials,12,13,14 we highlight the unique versatility and facile processability of SMPBs by fabricating multiresponsive laminates and shape-change electronics. Phase-change materials are of growing interest in home and office construction to regulate temperature and use energy more efficiently.15,16 At Tm, heat is absorbed from the local environment under isothermal conditions. Since the phase transition is reversible, heat is


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conversely released upon cooling. Normal hydrocarbons are appealing as abundant and sustainable phase-change materials because they possess relatively high latent heats at Tm.17 Physical incorporation of network-forming macromolecules into a phase-change material yields form-stable phase-change materials that are also capable of undergoing shape change upon stimulation.18,19 The shape-memory behavior of such SMPBs (phase-change component + macromolecular network) has recently been explored using covalently cross-linked macromolecules as the elastic network, but such materials cannot be recycled and ultimately add to the accumulation of solid waste.20-22 Alternatively, thermoplastic elastomers (TPEs)23 not only provide stable elastic networks but also afford the added benefits of being reprocessable and easily recycled. While numerous TPEs including multiblock polyolefins,19 segmented polyurethanes24 and random ionomers25,26 are suitable for this purpose, we consider a hydrocarbon-based triblock copolymer due to its well-defined molecular architecture,27 facile processability and commercial availability. At typical application temperatures, the minority endblocks are rigid (glassy), whereas the low-Tg, majority midblock is flexible (rubbery). Incompatibility between the blocks promotes self-assembly of copolymer molecules into the same nanoscale morphologies reported28 for diblock copolymers. In triblock copolymers, however, the rigid microdomains act as physical cross-links that stabilize a rubbery network. Inspired by previous studies of thermoplastic elastomer gels (TPEGs) containing a midblockselective oil to generate hyperelastic soft materials12,13 and the efforts of Wu et al.29,30 to achieve shape memory by adding paraffin wax to TPEs, we illustrate the basic mechanism of the current SMPBs in Figure 1. Here, the TPE is a commercial poly[styrene-b-(ethylene-co-butylene)-bstyrene] (SEBS) triblock copolymer, and the phase-change additives are aliphatic linear (crystallizable) hydrocarbons each designated by HCn, where n denotes the number of carbons


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per molecule (e.g., n-eicosane [C20H42] = HC20). Unlike prior efforts18,29,30 to generate SMPBs with commercial paraffins (i.e., common waxes), each HCn considered here possesses welldefined thermal properties. Moreover, as described in the Experimental Section, the process developed for preparing TPE/HCn blends systematically derives from our previous studies of TPEGs, as discussed in more detail below. At low temperatures (below Tm) in Figure 1, the TPE establishes a contiguous (primarily bridged) network of rubbery olefinic midblocks (green) stabilized by glassy styrenic micelles (red). The HC crystallizes in the midblock-rich matrix and

Figure 1. Schematic diagram of a TPE/HC-designed SMPB (following arrows) relaxed at ambient temperature (top left), heated above Tm of the HC (top right), deformed to a temporary strain state (middle), cooled below Tm to fix the temporary strain state (bottom), heated above Tm to promote strain recovery (top right), and cooled again to complete a full strain-thermal cycle (top left). This illustration depicts the role of networked micelles (red), and is not intended to address changes in their spatial arrangement (morphology), during the shape-memory process. forms an opaque blend due to the presence of HC crystals (as observed in several optical images displayed below). Upon heating to T > Tm, the crystals melt and an ultrastretchable, transparent


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TPEG capable of being strain-cycled with little mechanical hysteresis forms.B At this temperature, the TPEG can be deformed to a temporary strain state and then cooled to T < Tm to induce HC crystallization and lock-in (fix) the new strain state. Upon reheating to T > Tm, the TPE/HCn blend fully recovers and the cycle can be repeated. Prior to proceeding, it is worth noting that the preparation route utilized here produces more homogenous SMPBs than previous efforts,18,29,30 as immediately evidenced by the substantially improved transparency of films above Tm (compare, for instance, optical images presented at T > Tm in this study with Figure 5 in ref. 18).

EXPERIMENTAL SECTION Material preparation The SMPB samples were prepared by first dissolving the SEBS copolymer (31 wt% styrene, number-average molecular weight = 144 kDa and polydispersity index < 1.10; Kraton Polymers) with one or more of the following species — HC20 (99%; Fisher Scientific), HC30 (98%; Sigma-Aldrich), HC40 (≥95%; Sigma-Aldrich), and cycloaliphatic hydrocarbon tackifying resin (TR; molecular weight = 230 Da; ExxonMobil Chemicals) — at predetermined quantities in toluene (≥99.5%, Fisher Scientific) so that the total solids content was 5 wt%. Resultant solutions were hot-cast and dried in TeflonTM molds at T > Tm of the HC utilized to prevent crystallization, followed by thermal annealing under vacuum at 120°C for 24 h. This systematic procedure, derived from previous studies12,13,31 of TPEGs, fundamentally differs from prior efforts29,30 to generate TPE/wax blends stochastically varying in phase miscibility through the use of nonsolvent-induced precipitation. Melt pressing was used to produce films at 140°C and,


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when desired, fabricate channels for electrical devices at 100°C.32 Where necessary, thermal welding of SMPB films to generate laminates was accomplished by positioning pressed/molded films and subsequently heating the assembly to 140°C for 30 min. For electrical devices, eutectic gallium indium (EGaIn; Indium Corporation) was syringe-fed into microfabricated channels prepared by thermally welding micromolded TPEG sheets. Further details regarding selection of the specific thermal welding temperature and electrical device fabrication can be found in our previous study of flexible conductors.32 Material characterization Differential scanning calorimetry (DSC) was performed on a TA Instruments Discovery unit at a heating rate of 10°C/min. Concurrent small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) was conducted on beam line 12-ID-B in the Advanced Photon Source at Argonne National Laboratory, and a Linkam TMS600 hot-stage permitted precise temperature control between 20 and 80°C. Instrument settings and data acquisition details are provided elsewhere.33 Shape-memory properties were measured by either dynamic mechanical thermal analysis (DMTA) on a TA Instruments RSA-III or image analysis with a high-resolution digital camera. Recovery-rate experiments employed SMPB films measuring 1.5 ± 0.1 mm thick. Examples of the procedure utilized here to extract measurements from digital images are provided in Figure S1 (Supporting Information), and specimen replicates indicating the level of reproducibility are supplied in Figure S2 (Supporting Information). The electrical resistance of EGaIn-containing shape-memory wires was assessed using a 4-point probe setup.


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RESULTS AND DISCUSSION Property Tunability We first establish the shape-memory capability of SMPBs using a 20/80 w/w TPE/HC20 blend as an illustrative case. Figure 2 displays a sequence of uniaxial strain, stress and temperature measurements acquired by DMTA for the TPE/HC20 SMPB, which is first heated to 60°C (above Tm = 38°C) to form a TPEG and then strained to 170%. Shortly after this strain state is reached, the temperature is lowered to 34°C under isostrain conditions. When T = Tm, the stress is removed and the strain drops slightly to 164%, resulting in a strain fixity of 96.5%. Fixity levels of 95+% are common in these SMPBs. Upon reheating to T ≥ Tm, the temporary strain drops, and the blend recovers to 99+% of the original permanent shape. The shapememory behavior established in Figure 2 can easily be extended to SMPBs with alternative HCs

Figure 2. Time-dependent sequence of uniaxial strain, nominal stress and temperature measurements (color-coded) acquired from a 20/80 w/w TPE/HC20 SMPB. The dashed horizontal line identifies Tm. as the added phase-change component. The thermal properties of SMPBs containing HC20 (neicosane), HC30 (n-triacontane) and HC40 (n-tetracontane) incorporated into the TPE at 80 wt% HC are presented in Figure 3a to highlight the advantage of such straightforward and precise


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control over the switching temperature. These DSC thermograms indicate that each HC exhibits a sharp melting endotherm and that the peak position at Tm increases with n. Figure 3b verifies that Tm values extracted from such thermograms compare favorably with those of the pure HCs,16,34 implying that HC crystals are largely unaffected by the TPE micellar nanostructure and molecular network at compositions where the HC resides in the continuous matrix (70 and 80

Figure 3. (a) DSC thermograms acquired from SMPBs containing one or three HCs (labeled and color-coded). (b) Tm as a function of carbon number (n) for HCs incorporated into a TPE. The solid line corresponds to Tm for pure HCs predicted34 by [2347(n – 2) + 1953(2)]/[56.5 – 19.2logσ + 9.2τ], where σ = 0.97 is a rotational symmetry parameter and τ = 0.61(n – 2) – 1 relates to the number of torsional angles within the HC. Values of Xc,blend and Xc,cryst discerned from the specific heats of melting are tabulated in (b). wt% HC). Moreover, the area under each peak in Figure 3a yields the specific heat of melting


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(∆Hm). The ratio of ∆Hm measured from the blends to that of the pure HCs (248, 252 and 272 J/g for HC20, HC30 and HC40, respectively16) provides the crystallinity (Xc). On the basis of the binary TPE/HCn blends examined, values of Xc,blend range from 69 to 75% (see Figure 3b for values). If only the crystallizable species in each blend is considered, then Xc,cryst varies from 86 to 94%, confirming that ~90% of the HC molecules crystallize in the presence of the host TPE. In marked contrast, Xc is much lower (2-40%) in traditional SMPs35 and has been shown to decrease substantially with increasing phase-change material in some SMPBs.26 One other

Figure 4. Combined SAXS/WAXS intensity profiles acquired from (a) 20/80 w/w TPE/HC20, (b) 20/80 w/w TPE/HC30, and (c) 20/40/40 w/w/w TPE/HC20/HC30 SMPBs initially fixed at 200% strain upon progressively heating (top half) above Tm to 78-80°C (labeled) and re-cooling (bottom half) below Tm to 21-23°C (labeled). noteworthy feature in Figure 3a is that a ternary HC mixture added to the TPE retains the individual (albeit shifted) melting point of each constituent HC.


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Combined SAXS/WAXS patterns are displayed in Figure 4 to track the temperaturedependent morphological evolution of the 20/80 w/w TPE/HC20 (Figure 4a), 20/80 w/w TPE/HC30 (Figure 4b) and 20/40/40 w/w/w TPE/HC20/HC30 blends (Figure 4c) initially fixed at 200% and first heated above and subsequently cooled below Tm. At ambient temperature, scattering from the TPE morphology at low scattering vector (q) values reveals form factor features arising from the presence of TPE micelles, but little to no correlation between micelles (i.e., no intermicellar ordering) as indicated by an ill-defined structure factor contribution. At higher q, the sharp peaks are indicative of structured HC molecules (with one peak for the crystalline phase and the other for the rotator phase, as explained in conjunction with Figure S3 in the Supporting Information). As the temperature is increased beyond Tm in each SMPB (and ultimately to 78-80°C), the high-q peaks vanish and low-q peaks positioned according to a bodycentered cubic (BCC) morphology emerge as the SMPB converts into an amorphous TPEG and likewise recovers its permanent macroscopic shape. The scattering peaks associated with the BCC morphology (√2q*, √3q*, and 2q*) are more identifiable when the structure factor is isolated from the total scattering intensity (see Figure S4 in the Supporting Information) and suggest that spherical TPE micelles reside loosely on the lattice. This assignment is consistent with the relatively low volume fraction of the styrenic endblocks (≈ 0.06-0.09) in the SMPBs. Upon re-cooling, the structure factor in each panel of Figure 4 retains its distinctive scattering peaks until the HC present re-crystallizes at its Tm. At this temperature, the higher-order peaks indicative of HC ordering appear while the structure factors again become smeared as the TPE morphologies becomes distorted due to the formation of HC crystals in the SMPB matrix. The coupled, temperature-dependent nanostructural transitions are more quantitatively distinguishable upon examining the corresponding integrated peak intensities at both low-q


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Figure 5. Integrated peak intensities for the low-q and high-q regimes identified in Figure 4 and presented for three SMPBs differing in HC content (labeled and color-coded) in (a) and (b), respectively, during heating. In all three cases discussed in regard to Figure 4, the SMPBs consisted of 80 wt% HC. (Figure 5a) and high-q (Figure 5b). This analysis is displayed for the binary TPE/HC20 and TPE/HC30 blends, as well as a ternary 20/40/40 w/w/w TPE/HC20/HC30 blend. All three cases exhibit clearly defined transitions at their respective melting temperatures with the ternary SMPB displaying two apparent transitions, thereby signifying that multiple distinct crystal populations can co-exist in the TPE matrix (see also Figure S5a in the Supporting Information). Delineation of the high-q region into contributions from the two constituent HCs in Figure S5b further supports this conclusion. Upon subsequent cooling, the high-q peaks appear again as the


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Figure 6. Integrated low-q (black) and high-q (red) peak intensities over 20 thermal cycles from a 20/80 w/w TPE/HC20 SMPB with the cycle numbers labeled. The low-q integrated peak intensities in (b) are shifted vertically for clarity. HC molecules recrystallize, while the low-q peaks become diffuse due to crystallization-driven disordering of the TPE micelles. The results presented in Figures 6 and S6 verify that this heating/cooling sequence can be cycled repeatedly with negligible change in structural detail at either length scale. Comparison of the calorimetric, scattering, and thermomechanical properties discussed thus far helps to establish the nanostructure-property relationships of SMPBs and demonstrates that the switching temperature of the present TPE/HC blends can be straightforwardly and precisely tuned through selection of the HC added. Furthermore, analysis of SMPBs containing several, unique HCs reveals that multiple thermal switches can be controllably introduced and judiciously spaced. While facile tuning of the switching temperature in SMPBs constitutes a considerable improvement over most single-molecule SMPs, such control has been previously achieved10 in traditional SMPs at the expense of laborious synthetic means. In the following section, we seek to exploit further the ease by which preparing SMPBs


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by physical blending provides by examining a previously unexplored parameter of shapememory behavior: shape-recovery kinetics. Incorporation of a second midblock-compatible additive, such as the TR described in the Experimental Section, to TPEGs results in time-composition rheological equivalence,36 which provides similar mechanical property information as classical time-temperature superpositioning,

Figure 7. (a) DSC thermograms measured at different concentrations of a tackifying resin (wTR) in the HC/TR mixture incorporated into the matrix of SMPBs containing 20 wt% TPE. Values of Tm and crystallinity (Xc,blend and Xc,cryst) are presented as functions of wTR in (b, color-coded). While the solid and dashed lines are intended to serve as guides for the eye, both Tm and Xc,blend appear to decrease linearly with increasing wTR. The shaded region identifies the range of Xc,cryst for SMPBs that are capable of fixing a temporary strain state. but under isothermal conditions. Thermograms acquired from TPE/HC20/TR blends (maintained


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at 20 wt% TPE) containing increasing TR loading levels in the HC/TR mixture (wTR) are provided in Figure 7a and indicate that the presence of TR promotes systematic decreases in both Tm and ∆Hm (crystallinity). Nearly linear reductions in Tm and Xc,blend with increasing wTR are evident in Figure 7b and reveal that these thermal properties are likewise compositiontunable, with Xc,blend being substantially more sensitive to composition than Tm. This is not surprising since the population of crystallizable molecules in the blend is being replaced by amorphous TR molecules. However, the values of Xc,cryst included in Figure 7b imply that there is minimal change in the ability of the HC20 molecules to crystallize in the ternary SMPBs (Xc,cryst > 85%) up to a 50/50 w/w HC/TR mixture. Interestingly, over this composition range, high-fidelity fixation (≥ 91%) is retained. When wTR is increased to 60 wt%, however, Xc,cryst drops to ~75%, and the HC crystals can no longer fix a temporary strain in the SMPB (as evidenced by slow, but steady, recovery), suggesting that the HC crystals do not provide a sufficient load-bearing framework at this and higher TR concentrations. The TR loading level at which shape memory is no longer achieved depends on the restorative capability of the TPE network, which, for a single TPE, is dictated by the TPE content in the blend. In a TPE/HC20/TR blend with 30 wt% TPE, for instance, this critical fixation fraction decreases to ~50 wt% TR (cf. Figure S7 in the Supporting Information). Of equal importance, the strainrecovery levels of all TR-containing ternary SMPBs examined in this work ranged from 95-99%. While the presence of TR slightly influences the HC-regulated switching temperature in ternary SMPBs, it also affects the recovery kinetics, as evidenced by the results in Figures 8a and 8b for blends with 20 and 30 wt% TPE, respectively, fixed at 100% strain and subsequently heated to 10°C above Tm (Movies S1, S2, and S3 show 20 wt% TPEs with wTR = 0, 20, and 40 wt%, respectively). In both cases, increases in TR content accelerate shape recovery


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Figure 8. Strain recovery kinetics are displayed at different TR loading levels for ternary TPE/HC20/TR SMPBs with 20 and 30 wt% TPE in (a) and (b), respectively. The actuation times required to reach 50% (red) and 90% (blue) recovery are included as a function of Xc,blend in the inset of (b) for SMPBs with 20 (circles) and 30 (triangles) wt% TPE. The solid lines are linear regressions to the data. even though Tg,TR > Tm,HC. This counterintuitive outcome can be explained by considering the TR/HC mixed matrix. Below Tm, increasing the concentration of glassy TR disrupts the formation of HC crystals, as evidenced in Figures 7a and 7b and depicted in the top panel of Figure 9, but does not degrade the shape-fixity capability of the SMPBs (within identified composition intervals). Once the HC crystals melt, liquid HC plasticizes the TR. Since the Tg of HC20 is reported37 as −116°C and the Tg of the TR is 74°C, the Tg of the 50/50 w/w HC20/TR mixture can be estimated from the Fox-Flory equation to be −57°C, in


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Figure 9. Schematic depictions of a strain-fixed TPE/HC/TR ternary SMPB before (top) and after (bottom) heating above Tm. This diagram is intended to illustrate the effect of incorporated tackifying resin (purple) on the crystallizable HC matrix. which case the matrix with the highest TR concentration (in Figure 8a) remains liquid-like at Tm. This state is illustrated in the bottom panel of Figure 9. Moreover, since the Xc,blend values of the SMPBs are reduced with increasing wTR, less energy is required to induce melting, in which case liquefaction of the matrix, as well as shape recovery, occurs more rapidly. In SMPBs with 20 wt% TPE, the time to 50% recovery drops 78% as the TR loading level is increased from 0 to 50 wt%, whereas SMPBs with 30 wt% TPE exhibit faster restoration and a 76% reduction in recovery time as the TR loading level is increased to 40 wt%. While differences in recovery kinetics due to TPE content are expected to reflect the network cross-link (TPE micelle) density, this is not the case due to the more dominate influence of Xc,blend (as indicated in the inset of Figure 8b). SMPB Devices To demonstrate the versatile and facile processing of the current SMPBs, we have fabricated


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laminates with complex and multiple actuation states similar to shape-memory materials previously reported.22,38 Thermally welding two or more pre-formed SMPB films together at temperatures above the TPEG lattice-disordering temperature promotes the formation of mixed micelles (along the interface) to ensure sufficient adhesion.32 The laminate displayed in Figure 10, for instance, consists of two 20/80 w/w TPE/HC films — film 1 with HC20 and Tm,1 and film 2 with HC30 and Tm,2 — melt-welded at 140°C. Once formed, the laminate is uniaxially stretched to 100% strain at 70°C and fixed by rapid cooling to lock-in the desired strain state. In this particular case, Tm,2 > Tm,1 > T. Increasing the temperature above Tm,1 allows the TPE/HC20

Figure 10. Series of optical micrographs acquired from a bilaminate SMPB illustrating the two temporary strain states achieved with two HCs differing in Tm (see text). Scalebars = 1 cm. film to return to its original length. Due to interfacial adhesion between the films, however, film 2 must accommodate the recovery of film 1 by curling the laminate into a new strain state. At T > Tm,2, the shape recovery loop is completed as the laminate returns to its original, relaxed state. While this arrangement is a relatively simple example, it demonstrates that stable, intermediate


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shapes can be controllably introduced during the recovery loop. As indicated in Figure S8, more complex intermediate shapes (e.g., grips) are possible with such bilaminates. In principle, the number of intermediate strain states accessed, along with their geometries, can be selected based on the number of HCs employed and the physical arrangement of the laminates. That is, fabrication of a TPE/HC20, TPE/HC30, TPE/HC40 film stack subjected to uniaxial strain would exhibit similar behavior to that in Figure 10, but with two intermediate curled states differing in curvature. The inherent device-fabrication advantage of SMPBs can be further exploited by molding continuous channels in the same fashion as prior studies32 focusing on microfluidic devices. Imbibing such constructs with liquid metal39 yields conductors that marry the electrical conductivity of the liquid metal (through a channel) with the mechanical properties of the encapsulating medium. Figure 11a confirms that the shape-memory character of a 20/80 w/w TPE/HC20 blend is maintained even when filled with liquid metal and fixed in a nonplanar (coiled) geometry. To ensure expected function, replicates of the device presented in Figure 11a have been heated above Tm, stretched uniaxially, and fixed at a desired strain (at which point the electrical resistance through the channel was measured). The devices are subsequently heated above Tm to recover their permanent shape (where their resistance is measured again). This entire process has been repeated over multiple cycles by incrementally increasing strain to discern both the strain-dependent resistance and corresponding hysteresis (see Figure S9 for a depiction of the analysis). All the resistance measurements have been collected discretely at ambient temperature so that consideration of thermal variations is unnecessary. Resultant normalized resistance values (= Rε/R0, where Rε and R0 denote resistance levels at strains of ε and 0%, respectively) of the device, shown in Figure 11b, exhibit a monotonic increase with fixed strain.


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Figure 11. (a) Optical images of a 20/80 w/w TPE/HC20 SMPB fabricated with a centerline channel, imbibed with liquid metal (dark feature) and strained into a coil upon heating above Tm to form a shape-memory conductor. Normalized resistance values measured from replicates strained to different levels are provided in (b), along with the predicted trend (solid line, see text for details). Included in (b) is a 2-D pattern, indicating that these conductive SMPBs can be used as shape-memory antennae. Scalebars = 1 cm. More specifically, the resistance closely follows predicted behavior up to 300% strain on the basis of the cross-sectional area change of a square channel when Poisson’s ratio is equal to 0.5. In similar fashion as previously explored32 TPEG-based conductors, strain-recovered resistance values display relatively little variation. Therefore, the SMPB-EGaIn composites introduced here behave as low-hysteresis shape-memory wires that exhibit high fixity and recovery in terms of both strain state and electrical conductivity. The inset in Figure 11b affirms that this fabrication approach can be readily extended to 2-D patterns for shape-memory antennae.


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CONCLUSIONS Unlike conventional single-molecule SMPs that (i) chemically integrate network-forming and switching moieties and (ii) possess fixed thermomechanical properties specific to the moieties chosen,8-11,35 the SMPBs introduced here can be physically generated from a wide range of network-forming TPEs and various rubber-compatible, crystallizable organic species in facile fashion with high precision. The thermal properties of organic phase-change materials such as normal hydrocarbons, which have recently been reported40 to exhibit unexpected crystallization attributes, endow SMPBs with tremendous tunability in terms of switching temperature and energy, little mechanical/electrical hysteresis and high strain fixity and recovery levels. These straightforward and inexpensive systems are recyclable, and they provide fundamental insight into the crystallization behavior of relatively small molecules confined to reside within a contiguous polymer network. Physical incorporation of a second midblock-compatible species, such as a glassy tackifying resin, promotes additional tunability of shape-change properties and recovery kinetics, which can be critically important in microfluidics.41 While the results presented here showcase a potpourri of property and functional benefits of a single class of SMPBs, a variety of other contemporary applications such as shape-memory earphones activated by body heat and bimorphic42 constructs fabricated from welded multilaminates is anticipated as the versatile properties of these straightforward SMPBs are explored further. In the spirit of physical blending, the SMPBs reported here can be readily designed for a wide and diverse range of specific end uses and produced with relative ease and low cost.

ASSOCIATED CONTENT


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Supporting Information. Digital image treatment for shape-memory kinetics; SAXS/WAXS analysis of crystalline and rotator HCn phases, TPE structure factors, delineated HC20/HC30/TPE contributions, and SMPB thermal cycling; DSC characterization of ternary SMPBs; videos of ternary SMPBs during recovery; optical micrographs of biaxiallyprogrammed bilaminates; and a graphical representation of cyclic resistance experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed (e-mail: [email protected]). Present Addresses † Present address: Materials Science & Engineering Division, National Institute of Standards & Technology, Gaithersburg, MD 20899, USA. ‡ Present address: ExxonMobil Chemical Company, Baytown, TX 77520, USA.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENTS This contribution was identified by Professor John Gilmer (King University) and Dr. Jason Jenkins (Eastman Chemical Company) as the Best Presentation in the session “Eastman Chemical Student Award in Applied Polymer Science” of the 2016 ACS Fall National Meeting in Philadelphia, PA. K. P. M. and S. S. T. thank the NC State Nonwovens Institute for Ph.D. fellowships. K. P. M. would also like to thank MANN+HUMMEL GmbH for additional support. Use of the Advanced Photon Source is provided by the U. S. Department of Energy, Office of Sciences, under Contract No. DE-AC02-06CH11357. We are indebted to M. D. Dickey and S. A. Khan for use of their fabrication and testing facilities, and S. E. White for technical assistance.

ABBREVIATIONS SMPB, shape memory polymer blend; TPE, thermoplastic elastomer; TPEG, thermoplastic elastomer gel; SEBS, poly[styrene-b-(ethylene-co-butylene)-b-styrene]; HCn, linear hydrocarbon containing n carbon atoms.

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