Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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Giant Reversible Barocaloric Effects in Nitrile Butadiene Rubber around Room Temperature E. O. Usuda,*,†,‡ W. Imamura,†,§ N. M. Bom,∥,⊥ L. S. Paixão,† and A. M. G. Carvalho†
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Laboratório Nacional de Luz Síncrotron (LNLS), Centro Nacional de Pesquisa em Energia e Materiais (CNPEM),CEP 13083-100 Campinas, SP, Brazil ‡ Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo (UNIFESP), CEP 00972-270 Diadema, SP, Brazil § Faculdade de Engenharia Mecânica, Universidade Estadual de Campinas (UNICAMP), CEP 13083-860 Campinas, SP, Brazil ∥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), 08860 Belaterra, Barcelona, Spain ⊥ The Institute of Photonic Sciences (ICFO), 08193 Castelldefels, Barcelona, Spain S Supporting Information *
ABSTRACT: Elastomers have shown to be promising barocaloric materials, being suitable candidates for solid-state cooling devices. Moreover, this family of polymers presents additional advantages, such as their low cost and long fatigue life. In this context, we investigated the barocaloric effects in nitrile butadiene rubber (NBR) in a large range around room temperature. Applied pressures up to 390 MPa on NBR yield giant reversible isothermal entropy change (ΔST) and adiabatic temperature change (ΔTS), reaching the maximum absolute values of 59(6) J kg−1 K−1 at 303 K and 16.4(2) K at 323 K, respectively, for a pressure change of 390 MPa. Besides, both ΔTS and ΔST have shown to be rather reversible in a wide range of temperatures. For the first time, it was reported the influence of the glass transition on ΔST. Our study evidences the potential of NBR for room-temperature cooling applications based on the barocaloric effect. KEYWORDS: barocaloric effect, mechanocaloric effect, nitrile butadiene rubber, polymer, elastomer, solid-state cooling
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INTRODUCTION Nitrile butadiene rubber (NBR) is a well-known polymer from the family of elastomers with remarkable mechanical and chemical properties.1 NBR is an amorphous unsaturated copolymer formed by chains composed of butadiene (C4H6) and acrylonitrile (C3H3N). Also, the glass transition and mechanical properties of NBR change depending on the composition (degree of the acrylonitrile part) and quantities of the fillers (e.g., carbon black). Nevertheless, the glass transition temperature is usually around 243 K for pure NBR.2 The rubber-like behavior (elasticity), the resistance to chemical agents (e.g., oils and acids), and low cost make the NBR an interesting material for applications in many fields, such as automotive and aeronautic industry, as well as for making disposable equipment.1 In recent years, elastomers have been found to be promising materials for applications in solid-state cooling.3−6 Despite the favorable properties of NBR, this elastomer has not been explored in view of its cooling potential to date. Current refrigeration technology is based on vaporcompression cycles, which brings environmental and energetic issues. Aiming to solve these problems, solid-state cooling devices appear as promising options.7−9 This technology is based on materials that present caloric effects (also called i© XXXX American Chemical Society
caloric effects), in other words, materials exhibiting a thermal response when exposed to an external field. The nature of the external field can be magnetic, electric, or mechanical. Therefore, we can categorize the caloric effects as magnetocaloric, electrocaloric, or mechanocaloric effects. The latter can be subdivided in barocaloric, driven by isostatic pressure, and elastocaloric effects, driven by uniaxial tension. Over the past two decades, the research on caloric materials has seen a fast growth due to the discoveries of the giant magnetocaloric effect in Gd5Si2Ge2 compound in 1997,10 the giant electrocaloric effect in PbZr0.95Ti0.05O3 in 2006,11 and the giant barocaloric effect in Ni−Mn−In shape memory alloy in 2010.12 However, the studies on caloric materials date from 1805, when J. Gough reported a temperature change in natural rubber under rapid stretching.13 Decades later, W. Thomson predicted the caloric effects using thermodynamic considerations.14,15 With regard to the barocaloric effect, it is the least studied caloric effect so far. Only a small number of materials with giant barocaloric effect have been reported in the literature, Received: March 11, 2019 Accepted: June 20, 2019 Published: June 20, 2019 A
DOI: 10.1021/acsapm.9b00235 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials such as shape-memory alloys (SMA), 12 compounds,16 fluorites,17−22 magnetic materials,23−25 ferrielectric/ferroelectric materials,26,27 superionic conductors,28,29 plastic crystals,30,31 and organic−inorganic hybrids.32−34 Polymers also exhibit great potential as barocaloric materials. In 1982, the giant barocaloric effect was measured in poly(methyl methacrylate).35 More recently, interesting results were reported for elastomers: vulcanized natural rubber (VNR)4 and polydimethylsiloxane (PDMS)5 showed a giant barocaloric effect around room temperature. Furthermore, the supergiant barocaloric effect was measured in acetoxy silicone rubber.6 One advantage of elastomers is that they exhibit giant effects around room temperature even in the absence of phase transitions. Although this behavior was also reported for superionic conductors,29 elastomers present a very high and wide table-like barocaloric effect3−5 under lower pressures (up to 400 MPa). Such a barocaloric effect is related to rearrangements of amorphous chains and changes in conformational entropy when pressure is varied. In this context, NBR appears as a new option of barocaloric material for application. Here, we investigate the barocaloric effect in NBR in a large range around room temperature at moderate pressures (up to 390 MPa). Direct measurements of temperature change (ΔTS) yielded giant values. Volumetric strain (εV) versus temperature (T) data allowed us to calculate the isothermal entropy change (ΔST). Moreover, we observed both ΔTS and ΔST results are clearly hindered by the glass transition.
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is characteristic of the CO stretching vibration. The band at 1462 cm−1 is related to the −CH2− deformation vibration, and the band at 964 cm−1 is attributed to the C−H deformation vibration. The experimental setup and procedures used in the present work are described in detail in the Supporting Information and elsewhere.3−5,38 Volumetric strain vs temperature experiments were performed through isobaric processes; i.e., the temperature was varied, in a rate of 4 K/min, between 213 and 333 K under constant pressures within the 4−390 MPa range. Figure 2 shows volumetric strain vs temperature curves for four isobaric cycles (see the Supporting Information for full data of volumetric strain−temperature measurements).
Figure 2. Complete cycles of volumetric strain vs temperature measurements for pressures of 52, 130, 234, and 390 MPa.
MATERIALS AND METHODS
We used a commercial vulcanized NBR sample reinforced with carbon black fillers provided by Elastim Co. The material was supplied in a long cylindrical shape with a diameter of 12 mm. The sample with 8 mm in diameter and 20 mm in length was formed from the supplied material. The density of the sample, measured with a pycnometer, is 1390(10) kg m−3. We characterized the NBR sample via Fourier transform infrared spectroscopy (Figure 1) using a PerkinElmer spectrometer (model Spectrum Two). The broad peak at 3350 cm−1 is due to N−H stretching and O−H symmetric stretching vibrations. The bands at 2916 and 2848 cm−1 are due to stretching vibrations of the methylene group. At 2233 cm−1, the peak is related to the stretching vibration of −CN. The 1730 cm−1 band
Direct ΔTS measurements were performed by applying/releasing pressure (maximum values within the range of 26−390 MPa) in a quasi-adiabatic condition (rate >1.5 GPa/s for decompression and up to 0.1 GPa/s for compression). When the temperature in the sample is stable at the set point, a compressive stress is rapidly applied, resulting in a sharp increase in temperature. The load is kept constant until the temperature decreases to the initial value. Finally, the stress is quickly released, causing an abrupt decrease in the sample’s temperature. Figure 3 presents typical ΔTS measurements.
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RESULTS AND DISCUSSION ΔTS data as a function of the initial temperature for compression and decompression processes are shown in Figure 4a. Both processes present very close results in the compared temperature range (293−324 K). At 390 MPa, we observe a maximum |ΔTS| value of 16.4(2) K (at 323 K) for decompression. This giant barocaloric ΔTS surpasses or is comparable to the best barocaloric intermetallics reported so far.39 Normalizing this result by the applied pressure (|ΔTS/ Δp|), we have a high value of 42(2) K GPa−1. In Table 1, we compare our results with other promising barocaloric materials. For the two lowest pressures, the ΔTS curves present a slight ascending behavior as the temperatures increases. On the other hand, for 86 MPa and above, we observe a stronger dependence of the ΔTS on temperature: the curves show a smooth increase of the ΔTS values, tending to saturate at higher temperatures (plateau). This behavior can be attributed to the influence of the glass transition of the NBR, since the mobility of the chains is significantly reduced as the material changes from the rubbery state (plateau) to the glassy state (steep region and below). Therefore, ΔTS values tend to decrease below the glass transition temperature (Tg).
Figure 1. FT-IR spectrum of NBR filled with carbon black. The bands are in accordance with the literature.36,37 B
DOI: 10.1021/acsapm.9b00235 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
In Figure 4b, the temperature and pressure are shown as a function of time for sequential adiabatic compression− decompression cycles. Note that the procedure is different from the measurements of ΔTS (Figure 3). Here, the sample does not return to the initial temperature after the pressure is applied. The behavior of the temperature change shows a rather reversible process, since the temperature change in compression is very close to the temperature change in decompression, with a difference
