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[Takamasa Hirai](https://orcid.org/0000-0002-5577-8018), [Koichiro Uto](https://orcid.org/0000-0001-7091-0585), [Mitsuhiro Ebara](https://orcid.org/0000-0002-7906-0350), [Ken-ichi Uchida](https://orcid.org/0000-0001-7680-3051)

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[Elastocaloric effect of shape memory polymers in elastic response regime](https://mdr.nims.go.jp/datasets/7a9a3fcf-d0d7-4fbb-a6f7-523f5e088a04)

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Elastocaloric effect of shape memory polymers in elastic response regimePAPER • OPEN ACCESSElastocaloric effect of shape memory polymers inelastic response regimeTo cite this article: Takamasa Hirai et al 2023 J. Phys. Energy 5 034011 View the article online for updates and enhancements.You may also likeElastocaloric effects of carbon fabric-reinforced shape memory polymercompositesSeok Bin Hong, Yongsan An and Woong-Ryeol Yu-SMA foil-based elastocaloric cooling: frommaterial behavior to device engineeringF Bruederlin, H Ossmer, F Wendler et al.-A single long NiTi tube compressiveelastocaloric regenerator: experimentalresultsSiyuan Cheng-This content was downloaded from IP address 144.213.253.16 on 28/07/2023 at 07:07https://doi.org/10.1088/2515-7655/ace7f3/article/10.1088/2631-6331/ab0c4c/article/10.1088/2631-6331/ab0c4c/article/10.1088/2631-6331/ab0c4c/article/10.1088/1361-6463/aa87a2/article/10.1088/1361-6463/aa87a2/article/10.1088/1402-4896/acdf25/article/10.1088/1402-4896/acdf25/article/10.1088/1402-4896/acdf25J. Phys. Energy 5 (2023) 034011 https://doi.org/10.1088/2515-7655/ace7f3Journal of Physics: EnergyOPEN ACCESSRECEIVED30 January 2023REVISED29 June 2023ACCEPTED FOR PUBLICATION17 July 2023PUBLISHED27 July 2023Original content fromthis work may be usedunder the terms of theCreative CommonsAttribution 4.0 licence.Any further distributionof this work mustmaintain attribution tothe author(s) and the titleof the work, journalcitation and DOI.PAPERElastocaloric effect of shape memory polymers in elasticresponse regimeTakamasa Hirai1,∗, Koichiro Uto2,∗, Mitsuhiro Ebara2 and Ken-ichi Uchida11 Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan2 Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan∗ Authors to whom any correspondence should be addressed.E-mail: HIRAI.Takamasa@nims.go.jp andUTO.Koichiro@nims.go.jpKeywords: elastocaloric effect, shape memory polymer, elastic deformation, cross-linking, lock-in thermographyAbstractSolid-state cooling/heating technology based on the elastocaloric effect is one of the promisingalternatives to vapor compression systems. Large elastocaloric temperature modulation is oftengenerated through the non-linear strain-induced structural transition by applying large strainand/or stress to ferroelastic materials. Recently, an unconventional approach to expand theapplication possibilities of the elastocaloric effect was demonstrated by processing elastocaloricmaterials into kirigami structures, which was inspired by the art of paper cutting. Using thisapproach, only a small stretch of processed conventional plastics can locally provide more efficientperformance of elastocaloric temperature modulation than that of ferroelastic materials. To furtherimprove such a unique functionality, it is necessary to find plastic or polymeric materials showinglarge elastocaloric effects in the linear elastic response regime that can be driven by a MPa-orderweak stress application, where the non-linear structural transition is irrelevant. In this work, bymeans of a recently developed measurement technique for the elastocaloric effect based on thelock-in thermography, we found that shape memory polymers (SMPs) show prominentperformance for elastocaloric temperature modulation that is larger than conventional plastics.SMPs enable the control of crystallinity by changing the cross-linking agents, melting temperatureby changing the degree of polymerization, and orientation of the polymer chain segment by theshape memory effect. By utilizing the unique properties of SMPs, we manipulated theirelastocaloric performance. The experimental results reported here will highlight the potential ofsmart polymers for flexible and durable elastocaloric applications.1. IntroductionDemand for developing thermal management technologies is rapidly increasing with the remarkable increasein the number of electronic devices and in the amount of global energy use. Recently, for realizing anenvironment-friendly temperature control, a solid-state temperature modulation based on caloric effects hasattracted large interests as a promising alternative to widely used vapor compression systems that employgreenhouse-gas refrigerants [1]. The caloric effects are characterized by an adiabatic temperature change∆Tad and/or an isothermal entropy change∆Siso in a solid via the application and removal of an externalfield, such as a magnetic field for the magnetocaloric effect [2, 3], an electric field for the electrocaloric effect[4], a hydrostatic pressure for the barocaloric effect [5] and a strain/stress for the elastocaloric effect [6, 7].Among them, it is predicted that the temperature modulation based on the elastocaloric effect could be moreefficient than other caloric effects and other solid-state temperature modulation technologies (e.g.,thermoelectric effects) [7, 8]. To realize these advantages, extensive efforts have been exerted for developingpractical applications and exploring materials for the elastocaloric effect.© 2023 The Author(s). Published by IOP Publishing Ltdhttps://doi.org/10.1088/2515-7655/ace7f3https://crossmark.crossref.org/dialog/?doi=10.1088/2515-7655/ace7f3&domain=pdf&date_stamp=2023-7-27https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://orcid.org/0000-0002-5577-8018https://orcid.org/0000-0001-7091-0585https://orcid.org/0000-0002-7906-0350https://orcid.org/0000-0001-7680-3051mailto:HIRAI.Takamasa@nims.go.jpmailto:UTO.Koichiro@nims.go.jpJ. Phys. Energy 5 (2023) 034011 T Hirai et alA general formula of∆Tad and∆Siso for the elastocaloric effect is described as∆Tad =− T0ρcpˆ σ0(∂ε∂T)σdσ =−T0∆Sisocp, (1)where T0, ρ, cp, and ε are the base temperature, density, specific heat, and uniaxial strain, respectively, andequation (1) is compromised by assuming that the value of cp does not depend on T and σ [1]. Equation (1)shows that materials with large strain-induced∆Siso can have large cooling and heating capability. In general,the mechanism of strain-induced∆Siso is divided into three factors: (i) linear elastic deformation, (ii)non-linear structural transition, and (iii) atomic/molecular rearrangement, rotation, and/or deformation.The∆Siso contribution induced by (i) can be obtained in all solids within the range of the linear andreversible relationships between the stress σ and ε, where the magnitude of∆Tad is proportional to σ [9].The mechanism (ii) corresponds to the strain-induced martensitic, ferroelastic, or glass transition, resultingin the drastically large∆Siso, which is usually much larger than∆Siso due to the mechanisms (i) and (iii).The mechanism (iii) includes a variety of effects, such as the nano-scale atomic reordering after structuraltransition [10], electric polarization reordering by the piezoelectric effect [11], and rubber-like entropyelasticity observed in an elastomer [12]. In elastocaloric materials, shape memory alloys (SMAs), such asNiTi-based alloys [13–16] and Cu-based alloys [17–19], are most promising owing to the contributioncoming from the mechanism (ii). By using SMAs, the temperature modulation over 10 K has been achieved[13–16]. However, in such materials, the repetitive application of the large input stress of>0.5 GPa duringthe thermodynamic and mechanical cycles is required to drive the structural phase transition, which leads todevice performance degradation due to irreversible plastic deformation, microcracks, and hysteresis energyloss [11, 15, 20, 21]. The natural rubber also shows the large elastocaloric temperature change due tostrain-induced crystallization [22, 23], but at least several hundred percent change of ε is required togenerate a temperature change comparable to SMAs [12, 24], which restricts the miniaturization of systems.Recently, a different approach to manipulate the magnitude and distribution of the elastocalorictemperature change has been proposed and demonstrated based on the ‘kirigami’ processing, inspired by theart of paper cutting [25]. A programmable cutting pattern on a solid sheet artificially introduces anon-uniform spatial distribution of the internal stress against the application of uniaxial strain, generatingfocused elastocaloric heating and cooling sources at designed positions. In addition, the kirigami patterngives ultrahigh stretchability and flexibility to solids, which can dramatically reduce the required stress forstretching. By processing the kirigami pattern on a polystyrene (PS) sheet, one of the conventional plastics,the performance of elastocaloric cooling/heating can locally exceed the values in previous reports usingSMAs even though the applied strain (stress) is a few percents (a few MPa), where the dominant contributionof the elastocaloric effect of PS is the mechanism (i) without the glass transition. Despite the small∆Tad inconventional polymers, such a unique functionality driven a by MPa-order weak stress without hysteresisenergy loss during mechanical cycles will open up unconventional applications for locally selective, flexible,and durable temperature modulators based on the elastocaloric effect and may utilize omnipresentmechanical energy that is currently wasted. To improve the perspective of elastocaloric kirigami temperaturemodulators, it is important to uncover materials with large elastocaloric performance in the region of elasticdeformation because, in that region, the performance at a focused cooling/heating source in akirigami-patterned sheet directly depends on the performance in the unpatterned sheet. Note that themagnitude of the elastocaloric temperature modulation normalized by the stress in the unpatterned PS sheetis∼1.5× 10−8 K Pa−1, which realizes the local temperature change of 0.4 K at typical positions by applyinguniaxial strain (stress) of 1.0% (4.0 MPa) in the kirigami-patterned PS sheet [25].In this study, we have investigated the elastocaloric effect in the region of reversible elastic deformationusing unpatterned shape memory polymer (SMP) sheets (figure 1). SMPs, a group of a smart polymer, arecheaper, more flexible, and easier to be manufactured and processed than SMAs [26], so that SMPs are ofsignificance in flexible/wearable electronics [27] and biomedical research fields [28]. Recently, theelastocaloric effect arising from the mechanism (ii) in an SMP has been reported [29], but the effect in thelinear elastic response regime with small strain and stress has not been investigated yet. Although there aremany SMPs, in this work, we used SMPs composed of poly(ε-caprolactone), one of the conventionalsemicrystalline SMPs [30, 31]. Hereafter, poly(ε-caprolactone) is abbreviated as PCL. By precisely designingthe nanoarchitectonics of PCL, such as the number of branches and chain length, various physical properties,such as thermal, mechanical, and even shape memory properties, can be tuned [31–33]. We found that theperformance of elastocaloric temperature change in PCL-based SMPs is larger than that in otherconventional plastics. By changing the cross-linking agent and/or the degree of polymerization of PCL, theelastocaloric performance can be further enhanced; the maximum performance obtained here is twice aslarge as that in PS. In addition, we found that the performance of SMPs can also be improved by using the2J. Phys. Energy 5 (2023) 034011 T Hirai et alFigure 1. Schematic of the elastocaloric effect in tetra-branched poly(ε-caprolactone) (PCL)-based shape memory polymers(SMPs) crosslinked by benzoyl peroxide (BPO) or dithiothreitol (DTT), where the structural formulae of two SMPs are alsoshown.shape memory effect, a unique characteristic of SMP, where the shape of SMP can be easily changed andfixed. The thermal management functionalities of SMPs and the systematic dataset will boost both basic andapplication studies in the communities of not only the caloric effects but also smart polymers.2. Materials andmethods2.1. Sample preparationTetra-branched (4b) PCL was synthesized by ring opening polymerization of ε-caprolactone (CL) with thetetravalent initiator, pentaerythritol [31, 34]. The PCL with various degrees of polymerization (x = 10–70)(abbreviated as 4bxPCL) was designed by adjusting the ratio of pentaerythritol to CL during thepolymerization. Thereafter, hydroxyl end groups of 4bxPCL were modified using acryloyl chloride tointroduce polymerizable acrylate groups onto the branch ends. The 4bxPCL modified with acrylate groupswere cross-linked by thermal polymerization with benzoyl peroxide (BPO) and Michael addition reactionwith dithiothreitol (DTT) (figure 1). The acrylated 4bxPCL was dissolved at 45 wt.% in xylene containing2-fold molar excess BPO to the end-group of the polymer. The solution was injected between glass slideswith a∼0.2 mm thick Teflon spacer. Subsequent thermal polymerization was conducted at 353 K for 16 h toobtain the cross-linked 4bxPCL (4bxPCL-BPO). In cross-linking with DTT, N,N-dimethylformamide wasused as a solvent instead of xylene, and a solution was prepared by adding 0.5-fold molar amount of DTTagainst end-groups of polymer and a catalytic amount of triethylamine, and cross-linked 4bxPCL(4bxPCL-DTT) was obtained in the same manner as described above. The actual values of x were estimatedat x = 11, 21, 32, 52, and 67 (x = 11, 21, 32, and 52) for 4bxPCL-BPO (4bxPCL-DTT) sheets by protonnuclear magnetic resonance spectroscopy. The thickness t of the 4bxPCL-BPO and -DTT samples is in therange of 0.1–0.2 mm. After fabrication, all the 4bxPCL SMP sheets are cut out into a strip with the width wof 6 mm. For comparison, we prepared five commercial plastic sheets: PS, polypropylene, polyvinyl-chloride,high density polyethylene (HDPE), and polyethylene-terephthalate with t = 1.0 mm and w = 6 mm, whichare available from NaRiKa Corp., Japan. For the lock-in thermography (LIT) measurements (see section 2.3),the surface of the samples was coated with insulating black ink with an emissivity of>0.94 (JSC-3,JAPANSENSOR Corporation) to endure high and uniform thermal radiation.2.2. Differential scanning calorimetryThe crystallinity ratio Xc and the melting temperature Tm, corresponding to the crystal-amorphoustransition of SMPs, of the 4bxPCL-BPO and -DTT samples were estimated using the differential scanningcalorimetry (DSC). All samples were first equilibrated at 353 K and then cooled to 253 K, followed byperforming the measurements during the heating process to 353 K at a rate of 5 K min−1 under nitrogen3J. Phys. Energy 5 (2023) 034011 T Hirai et alFigure 2.Measurement of the elastocaloric effect based on the lock-in thermography (LIT). By applying a sinusoidal-waved strainwith the amplitude∆ε, offset ε0, and frequency f, the sample temperature is sinusoidally modulated from the base temperatureT0 with the same f by the elastocaloric effect. Captured thermal images are transformed into lock-in amplitude (A) and phase (ϕ)images. During the LIT measurements, the∆σ-∆ε curve is measured using the load cell.atmosphere. The value of Xc was calculated from the enthalpy change quantified by DSC thermograms andthe melting enthalpy of 100% crystalline PCL and that of Tm was defined as the temperature at the peak topof DSC thermograms [35, 36]. To check the reproducibility and estimate the magnitude of errors, the DSCmeasurements were conducted three times for each sample.2.3. LITmeasurement of elastocaloric effectThe temperature modulation generated by the elastocaloric effect was measured using the LIT technique[37]. In the LIT measurement, we recorded thermal images of a sample surface by applying a periodicoscillation of the input signal using an infrared camera and extracted a temperature change oscillating withthe same frequency f as the input frequency [25, 38–42] (figure 2). Since the LIT method enablescontact-free temperature measurements with high temperature and spatial resolutions and separation of thetemperature change due to a target effect and a background signal, it is suitable to measure the elastocalorictemperature change in the small strain regime, compared with other methods for measuring the caloriceffects [39]. The procedure of the LIT measurement for this study is as follows (see also figure 2). First, thesamples were clamped at the distance L0 = 15 mm on a motor-driven tensile machine with a load cell and alinear encoder, where the center between the two clamps was adjusted to the center of the viewing area of theinfrared camera by using a custom-made ball screw. Second, a sinusoidal strain with the amplitude∆ε, offsetε0, and f = 1 Hz was applied to the sample along its longitudinal direction, where∆ε=∆l/L0 andε0 =∆ε+ 0.5% with∆l being the amplitude of the sinusoidal elongation. Finally, the thermal imagesrecorded were transformed into the lock-in amplitude A (>0) and phase ϕ (0◦ ⩽ ϕ < 360◦) images throughFourier analysis, in which A informs the magnitude of the elastocaloric temperature modulation and ϕ thesign of the temperature modulation as well as the time delay due to the thermal diffusion. In previous study,it was shown that the value of ϕ due to the elastocaloric effect in unpatterned plastic sheets at 1 Hz is nearly180◦ and the contribution of thermal diffusion is negligible [25]. Unless otherwise specified, the integrationtime of each LIT measurement tint is 60 s. The performance of the elastocaloric temperature modulation wasthus quantified by |∆T|/∆σ, where∆T is the elastocaloric temperature change calculated as∆T = Acosϕand∆σ is the stress for stretching the sample, calculated as∆σ =∆F/wt with∆F being the change in theload monitored using the load cell. All the LIT measurements were conducted at room temperature andunder air atmosphere.4J. Phys. Energy 5 (2023) 034011 T Hirai et al3. Results and discussion3.1. Crystallinity andmelting temperature of 4bxPCL SMPsFigures 3(a) and (b) show the x dependence of Xc (Tm) for 4bxPCL-BPO and -DTT samples. In both SMPs,the decreasing trend of Xc and Tm was observed with a decrease in x. When comparing the two SMPs withsame x, the value of Xc of 4bxPCL-DTT and 4bxPCL-BPO was almost the same except for the sample withx = 11, while Tm of 4bxPCL-DTT was larger than that of 4bxPCL-BPO. Figure 3(c) shows the absolute valueof DSC heat flow per unit mass |jH|, proportional to specific heat of each SMP, at 300 K. The magnitude of jHof 4bxPCL-BPO was larger than that of 4bxPCL-DTT and, in both 4bxPCL SMPs, the maximum |jH| wasobtained in the sample with a minimum x (i.e., x = 21 for 4bxPCL-BPO and x = 11 for 4bxPCL-DTT).3.2. Elastocaloric effect in 4bxPCL SMPsIn figure 4(a), we show the A and ϕ images of the 4b11PCL-DTT sheet at∆ε= 1.5% as an example of theresult of the LIT measurement. The temperature modulation signal with uniform A and ϕ of∼180◦ wasclearly observed in the sample. As shown in figure 4(b), the magnitudes of A and ϕ are independent of tint,confirming the reversible temperature modulation during the LIT measurement. Figure 4(c) shows the∆εdependence of A and∆σ for the 4b11PCL-DTT sheet. These linear relationships indicate that theelastocaloric effect obtained here is dominantly attributed to the mechanism (i), similar to the conventionalplastic sheets [25]. In all of the samples used in this study, we obtained ϕ∼ 180◦, indicating that heat isabsorbed (∆T < 0) when stretching the sample. We confirmed the linear∆ε dependence of A and∆σ below∆ε < 3%. To check endurance of the elastocaloric material, we performed repetitive LIT measurements inthe 4b11PCL-DTT sheet. In figure 4(d), A and∆σ at∆ε= 2.0% are plotted against the repetitive stretchingnumber Nrep up to 10 000, where the LIT measurements with the accumulation time of 60 s and interval of40 s were repeated by NLIT = 100 times. The magnitudes of A and∆σ were found to be independent of Nrep,confirming the durability of our elastocaloric material up to 10 000-repeated strains of 2.0%. Figure 4(e)depicts the relationship between |∆T| and∆σ at∆ε= 1.0% for the 4bxPCL SMP and plastic sheets. Sincethe data at upper (larger |∆T|) and left (smaller∆σ) positions in this graph correspond to higherperformance of the elastocaloric temperature modulation, we found that 4bxPCL SMPs are betterelastocaloric materials than the conventional plastics. Note that |∆T|/∆σ of the PS sheet with t = 1.0 mm is1.4× 10−8 K Pa−1, which is comparable to that of the PS sheets with t = 0.2 and 0.3 mm [25], showing thatthe difference in t is irrelevant to the present results. Figure 4(f) shows the x dependence of |∆T|/∆σ for the4bxPCL-BPO and -DTT samples. The |∆T|/∆σ values of the PCL-based SMP sheets were larger than that ofthe HDPE sheet (∼2.0× 10−8 K Pa−1), the maximum among five conventional plastic sheets. In both SMPsheets, the values of |∆T|/∆σ were almost the same in the range of x ⩾ 32, but below x = 32, the value of|∆T|/∆σ increased with decreasing x. The maximum |∆T|/∆σ observed here was estimated to be∼3.0× 10−8 K Pa−1 in the 4b11PCL-DTT sheet, which leads to∆Siso ∼ 7× 10−4 J g−1K−1. To compare theperformance of elastocaloric materials, we can use the material-level dimensionless elastocaloric efficiency ζ[43, 44], defined as the ratio between the useful cooling/heating (=ρcp|∆T|) and the work input, i.e., thearea between the∆σ-∆ε curve and horizontal axis [13]:∆ε∆σ/2 in an elastic response. Here, we note thatthe material-level coefficient of performance COPmat is also widely used for comparing elastocaloricmaterials but inappropriate for our samples; since the work input in COPmat is defined by the area of the∆σ-∆ε curve enclosed by loading and unloading curves (=¸σdε), it is almost zero in the reversible elasticregime, leading to the divergence of COPmat. Although the magnitude of |∆T| and |∆T|/∆σ of thePCL-based SMP sheets are still smaller than those of SMAs, ζ is estimated to be∼5 for our samples whenρ= 1.1× 106 g m−3 and cp = 1.8 J g−1K−1 are used [45, 46]. This ζ value is comparable to that of SMAs [13,47]. Moreover, assuming that the 4b11PCL-DTT sheet is patterned into the same kirigami structure asshown in the PS sheets in [25] and the similar increase ratio of focused stress is obtained within the linearelastic response regime, the local |∆T|/∆σ value will be enhanced around 4× 10−7 K Pa−1. This value isnearly an order of magnitude larger than that of SMAs, corresponding the local |∆T| of over 1.0 K only at∆ε= 2.0% (note that the total |∆T|/∆σ averaged over the kirigami-patterned sample is not enhanced).Here, we discuss the origin of the x dependence of |∆T|/∆σ in 4bxPCL SMPs. When we consider onlythe mechanism (i), equation (1) is simply reformulated as |∆Tad|/∆σ = |T0α/c|, where α and c are the linearexpansion coefficient and volumetric heat capacity, respectively [9]. According to the results of the DSCmeasurements (see section 3.1), c should increase with decreasing x. Thus, the dominant contribution of theenhancement of |∆T|/∆σ with decreasing x is the increase in α. In the vicinity of Tm, the thermomechanicalproperties including αmay be modulated as well as the mechanical properties [48, 49]. However, thedecrease in Tm also causes the reduction of Xc and durability. The combination of small x and large Xc arecrucial for enhancing |∆Tad|/∆σ at room temperature. Owing to the DTT cross-linking, the high rate of5J. Phys. Energy 5 (2023) 034011 T Hirai et alFigure 3. (a) x dependence of the crystallinity ratio Xc. (b) x dependence of the melting temperature Tm. (c) Absolute value ofheat flow per unit mass |jH| at 300 K measured by differential scanning calorimetry for the 4bxPCL-BPO and 4bxPCL-DTTsamples. In (c), the data for the 4b11PCL-BPO sample is absent because its Tm is close to room temperature.Figure 4. (a) A and ϕ images of the 4b11PCL-DTT sheet at∆ε= 1.5%. Green arrows show the direction of the applied uniaxialstrain. (b) Dependence of A and ϕ on the integration time of the LIT measurement tint at∆ε= 1.5%. The magnitude and errorbars of the A and ϕ signals are the average and standard deviation of the data in the area surrounded by the white dashedrectangle in (a), respectively. (c)∆ε dependence of A (upper panel) and∆σ (lower panel) for the 4b11PCL-DTT sheet.(d) Stretching repetition number Nrep and LIT-measurement number NLIT dependences of A and∆σ for the 4b11PCL-DTTsheet at∆ε= 2.0%. (e) Relation between the elastocaloric temperature modulation |∆T|= |Acosϕ| and∆σ at∆ε= 1.0%. Inaddition to the PCL-based SMP sheets, the data for the conventional plastic sheets including polystyrene (PS), polypropylene(PP), polyvinyl-chloride (PVC), high density polyethylene (HDPE), and polyethylene-terephthalate (PET) sheets with thethickness t of 1.0 mm are shown. The data for the 4b11PCL-BPO sample is absent because it is too fragile to measure theelastocaloric effect due to Tm ∼ room temperature. (f) Degree of polymerization x dependence of |∆T|/∆σ for the 4bxPCL-BPOand -DTT sheets. The dotted line shows |∆T|/∆σ for HDPE, which exhibit the maximum |∆T|/∆σ value among the fiveconventional plastics.ordered crystal structure can be kept even at room temperature in the 4bxPCL SMPs with small x, resultingin the large |∆Tad|/∆σ.3.3. Modulation of elastocaloric temperature change using shape memory effectTo further investigate the elastocaloric properties of SMPs, we performed the same measurements on theSMP sheets uniaxially deformed by the shape memory effect. Figure 5(a) shows the images of the PCL-basedSMP sheets before and after deformation of which the procedure is as follows. First, the sheet clamped on thetensile machine was heated up to Tm using a blow-dryer. Then, while heating, the sheet was quickly stretchedby a uniaxial deformation strain η, which was controlled by moving the clamps of the tensile machine.6J. Phys. Energy 5 (2023) 034011 T Hirai et alFigure 5. (a) Photograph of the 4b11PCL-DTT sheets before and after the deformation using the shape memory effect. η is thedeformation strain. (b) A images of the 4b52PCL-BPO sheets at η = 0%, 50%, 100%, 150%, 300%, and 500%. (c)[(e)] ηdependence of A (upper panel) and∆σ (lower panel) for the 4b52PCL-BPO (4b52PCL-DTT) sheet. The magnitude and errorbar of A are the average and standard deviation of the data in the area surrounded by the white dashed rectangle in (b),respectively. (d)[(f)] η dependence of |∆T|/∆σ for the 4bxPCL-BPO (4bxPCL-DTT) sheets for x= 21 (squares), 52 (circles),and 67 (triangles) [x= 11 (diamonds), 21 (squares), and 52 (circles)].Finally, by cooling down the sheet to RT while holding the elongated sheet, the deformation was persistentlyfixed.Figure 5(b) shows the A images of the 4b52PCL-BPO sheet at several η values. Even after thedeformation, the distribution of the A signal in the SMP sheets is uniform. Importantly, the magnitude of theA signal depended on η. The upper panel of figure 5(c) shows the A signal as a function of η at∆ε= 1.0% inthe 4b52PCL-BPO sheet. The magnitude of A was enhanced with increasing η below η = 150%, where A atη = 100%–150% were about 1.4 times larger than that at η = 0%, but was decreased toward zero withfurther increasing η above 150%. However, the η dependence of∆σ at∆ε= 1.0% was quite different fromA: the clear increase in∆σ was obtained only around η = 150% as shown in the bottom panel of figure 5(c).Such individual η dependences enhanced the |∆T|/∆σ factor (light blue circle data points in figure 5(d)) to∼3.0× 10−8 K Pa−1 at η = 100%, a similar value obtained in the 4b11PCL-DTT sheet with η = 0%.Although the enhancement of |∆T|/∆σ around η = 100% was also observed in the 4b67PCL-BPO sheet, the|∆T|/∆σ of the 4b21PCL-BPO sheet monotonically decreased with the increase in η (figure 5(d)). In the4bxPCL-DTT samples, as shown in figures 5(e) and (f), the significant enhancement of A was not obtainedand only the decrease in |∆T|/∆σ with η increase was confirmed even in the sample with x= 52, whereasthe∆σ-η relation showed the similar behavior.Finally, we discuss the origin of the modulation of the elastocaloric properties induced by the shapememory effect. The dominant contribution is expected to be different from the mechanisms discussed insection 3.2, because uniaxial deformation via the shape memory effect induces the orientation of polymerchains and the change in the order from isotropic to anisotropic network with almost no change incrystallinity of PCL [50]. Previous reports show that the orientation of polymer chains monotonicallyreduces α in other polymers [51–53]. If this scenario is applicable to PCL-based SMPs, the dramaticreduction of A at large η can be explained, while the enhancement of A around η = 100%–150% is stillunclear. Thus, additional∆Siso induced by the mechanism (iii) corresponding to the conformation ofpolymer chains may exist. To clarify the microscopic mechanism and optimize |∆T|/∆σ in SMPs, detailedanalyses of their structures are necessary.4. ConclusionsIn conclusion, we investigated the elastocaloric effect of the PCL-based SMPs with two different cross-linkingagents in the regime of the linear elastic deformation. By visualizing the elastocaloric temperature changeusing the LIT technique, we found that the elastocaloric performance, |∆T|/∆σ, in the PCL-based SMPs washigher than that in the conventional plastics and further enhanced by changing the cross-linking agent and7J. Phys. Energy 5 (2023) 034011 T Hirai et aldecreasing x. On the other hand, only in the PCL-based SMPs with BPO cross-linking and large x, theenhancement of |∆T|/∆σ was demonstrated using the shape memory effect. These results reveal thepotential of SMPs as elastocaloric materials and invigorate applications and materials science studies towardthe realization of ultralow-stress-driven, flexible, and durable elastocaloric kirigami temperature modulators.Data availability statementAll data that support the findings of this study are included within the article (and any supplementary files).AcknowledgmentsThe authors thank R Yamamoto and Y Sasaki for technical supports. This work was partially supported byCREST ‘Creation of Innovation Core Technologies for Nano-enabled Thermal Management’ (No.JPMJCR17I1) and ERATO ‘Magnetic Thermal Management Materials’ (No. JPMJER2201) from JST, Japan;Grant-in-Aid for Transformative Research Areas(A) (No. JP20H05877) from JSPS, Japan; Innovative Scienceand Technology Initiative for Security (No. JPJ004596), ATLA, Japan; the Thermal and Electric EnergyTechnology Foundation; and the Canon Foundation.ORCID iDsTakamasa Hirai https://orcid.org/0000-0002-5577-8018Koichiro Uto https://orcid.org/0000-0001-7091-0585Mitsuhiro Ebara https://orcid.org/0000-0002-7906-0350Ken-ichi Uchida https://orcid.org/0000-0001-7680-3051References[1] Moya X, Kar-Narayan S and Mathur N D 2014 Caloric materials near ferroic phase transitions Nat. Mater. 13 439–50[2] Wada H and Tanabe Y 2001 Giant magnetocaloric effect of MnAs1-xSbx Appl. Phys. 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Introduction 2. Materials and methods 2.1. Sample preparation 2.2. Differential scanning calorimetry 2.3. LIT measurement of elastocaloric effect 3. Results and discussion 3.1. Crystallinity and melting temperature of 4bxPCL SMPs 3.2. Elastocaloric effect in 4bxPCL SMPs 3.3. Modulation of elastocaloric temperature change using shape memory effect 4. Conclusions References