WO2019168833A1

WO2019168833A1 - Tetrapolymers for electrocaloric devices

Info

Publication number: WO2019168833A1
Application number: PCT/US2019/019553
Authority: WO
Inventors: Qiming Zhang; Qing Wang; Xin Chen
Original assignee: The Penn State Research Foundation
Priority date: 2018-02-28
Filing date: 2019-02-26
Publication date: 2019-09-06

Abstract

Electrocaloric tetrapolymers which exhibit a temperature increase upon application of an electrical field or voltage, and a temperature decrease upon removal of an electrical field or voltage are disclosed. Such tetrapolymers are prepared from vinylidene fluoride (VDF), trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorotrifluoroethylene (CTFE) and/or chlorodifluoroethylene (CDFE) monomers and in particular mole ratios. The tetrapolymers are useful for electrocaloric cooling devices (i.e., refrigerators or heat pumps, which exhibit temperature changes and pump heat from the cold thermal load to a hot heat sink) based on these tetrapolymers.

Description

TETRAPOLYMERS FOR ELECTROCALORIC

DEVICES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No.

62/636,434 filed 28 February 2018 the entire disclosure of which is hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under Contract No. DE-

AC02-07CH11358 awarded by U.S. Department of Energy. The Government has certain rights in the invention.

TECHNICAL FIELD

[0003] The present invention relates to tetrapolymers prepared from vinylidene fluoride

(VDF), trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorotrifluoroethylene (CTFE) and/or chlorodifluoroethylene (CDFE) monomers and in particular mole ratios, which exhibit large electrocaloric cooling.

BACKGROUND

[0004] An electrocaloric effect (ECE) is a characteristic of certain materials in which the material shows a reversible temperature and entropy change under an applied electric field. Such materials were reported in U.S. Patent Application Publication No. 20110016885 to Zhang et al, for example. Advances in the past decade have led to the development of self-supporting polymer and nanocomposite films which exhibit giant ECE, i.e., an electrocaloric (EC) cooling of greater than 30 K near room temperature. Dielectric materials with such a large EC response is very attractive for developing EC cooling devices which are more energy efficiency and environmentally benign, compared with the traditional vapor compression cycle refrigeration technology.

[0005] In addition, easy control of the electric signal, low noise, and compact size of

EC coolers allow applications which are not possible with vapor compression based coolers, such as wearable cooling patches and benders to replace ice bags for injury treatment and cooling of biologic tissues and organs, compact air-conditions and thermal management systems placed in limited spaces such as on office desks or integrated into chairs for localized climate control, and for electronic gadgets such as lab-on-a-chips. Indeed, several recent experimental and simulation studies have shown advantages of solid state EC coolers.

[0006] While very large EC temperature cooling (> 30 °C) has been reported in self- supporting polymer and nanocomposite films, this large EC cooling is achieved under fields near the dielectric breakdown of the material, e.g., an electric field that will cause device failure such as 100 MV/m. Practical EC coolers are operated at fields well below the dielectric breakdown for long-term reliable use of the cooling devices. For the self-supporting EC polymer films, this field is below 100 MV/m.

[0007] Hence, there is a continuing need to develop materials exhibiting a large electrocaloric effect at applied electric fields suitable for practical EC cooling devices.

SUMMARY OF THE DISCLOSURE

[0008] Advantage of the present disclosure include certain tetrapolymers having a large electrocaloric cooling.

[0009] These and other advantages are satisfied, at least in part, by a tetrapolymer of formula (I): (VDFi_-x-TrFE_x)_y-CFE_z-(CTFE or CDFE)_k, or formula (II): (VDFi_-x-TrFE_x)_y-CFE_z- CTFEk or formula (III): (VDFi-_x-TrFE_x)y-CFE_z- CDFEk.

[0010] In each of formulae (I), (II) or (III), VDF is vinylidene fluoride, TrFE is trifluoroethylene, CFE is chlorofluoroethylene, CTFE is chlorotrifluoroethylene and CDFE is chlorodifluoroethylene; and wherein x is between and including 0.25 to 0.5, y is between and including 0.89 to 0.94, z is between and including 0.09 to 0.04, k is between and including 0.06 to 0.02, the sum of z+k is between and including 0.06 and 0.11 and the sum of y, z and k equals 1. In certain embodiments y is between and including 0.90 to 0.94 and/or z is between and including 0.08 to 0.04 and/or the sum of z+k is between and including 0.06 and 0.10.

[0011] Advantageously, tetrapolymers of any one of formulae (I), (II) or (III) exhibit a peak dielectric constant greater than 50 at 100 Hz and, in some instances, lower than 65 at 100 Hz.

[0012] Another aspect of the present disclosure is a cooling device which comprises a tetrapolymer of any one of formulae (I), (II) or (III), wherein the tetrapolymer exhibits a temperature change upon application or removal of an electric field or voltage.

[0013] Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Reference is made to the attached drawings, wherein elements having the same reference numeral designations represent similar elements throughout and wherein:

[0015] Fig. 1 is a plot showing electrocaloric cooling exhibited by a terpolymer upon removal of an applied electric field.

[0016] Fig. 2 is a plot showing responses vs. applied field as measured at room temperature for tetetrapolymer of the present discloser compared to a corresponding terpolymer. FIG 2a shows EC responses vs. applied field measured at room temperature for a tetrapolymer of P(VDF-TrFE-CFE-CTFE) with the feed ratio of 56.1/33.2/5.9/4.8 mol% (and the tetrapolymer shows a peak dielectric constant of 55) and compared with that of a terpolymer P(VDF-TrFE-CFE) 59.2/33.6/7.2 mol% and FIG. 2b shows a comparison of the electrocaloric coefficient of the tetrapolymer and terpolymer vs applied field.

[0017] Fig. 3 is a plot showing EC adiabatic temperature cooling measured under different electric fields vs. temperature of (a) tetrapolymer number 1 (in Table 1) and (b) terpolymer.

[0018] Fig. 4 is a plot of dielectric properties vs. temperature measured at different frequencies of a tetrapolymer of the present disclosure. The plot shows the tetrapolymer exhibits typical relaxor ferroelectric behavior.

[0019] Fig. 5 is a plot showing a comparison of unipolar polarization-electric (P-E) loops under 50 MV/m AC field of 10 Hz at room temperature for a tetrapolymer according to the present disclosure compared to a terpolymer. The tetrapolymer shows much higher polarization level compared with the terpolymer. (Tetrapolymer #1).

[0020] Fig. 6 is a plot of EC cooling characteristics of a tetrapolymer according to an embodiment of the present disclosure. Fig. 6a is a plot of temperature vs electric field and Fig 7b is a plot of temperature change vs temperature generated by a tetrapolymer of P(VDF-TrFE- CFE-CTFE) (Tetrapolymer #5 in Table 1).

[0021] Fig. 7 illustrates comparative sizes of different monomers in the EC polymers. [0022] Fig. 8 is a plot of dielectric properties vs. temperature of the tetrapolymer #22, which is not a relaxor ferroelectric as shown by its sharp increase of dielectric constant with temperature.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0023] Recently, through theoretical analysis, syntheses and testing, it was surprisingly found that certain tetrapolymers (polymers having four different monomer units) can exhibit significantly higher electrocaloric response compared with similar terpolymers (polymers with three different monomer units). For example, we found that tetrapolymers prepared from vinylidene fluoride (VDF), trifluoroethylene (TrFE), chlorofluoroethylene (CFE), chlorotrifluoroethylene (CTFE) and/or chlorodifluoroethylene (CDFE), such as tetrapolymers of P(VDF-TrFE-CFE-CTFE), can exhibit significantly higher electrocaloric responses compared to terpolymers of P(VDF-TrFE-CFE) and P(VDF-TrFE-CTFE), as characterized by a large adiabatic temperature cooling under a lower electric field. An adiabatic temperature change (AT) is the change of temperature of a material under the application or removal of a magnetic field without exchanging heat from the surrounding environment. Terpolymers exhibiting ECE are known. See Xinyu Li, et al, Tunable Temperature Dependence of Electrocaloric Effect in Ferroelectric Relaxor P(VDF-TrFE-CFE) Terpolymer. Appl. Phys. Lett. 99, 052907-1-3 (2011); Vittorio Basso et al, Direct measurement of the electrocaloric effect in P(VDF-TrFE-CTFE) terpolymer films Appl. Phys. Lett. 103, 202904 (2013). However, it was unexpected that tetrapolymers prepared from certain monomers and with a certain narrow mole ratio could exhibit significantly higher electrocaloric responses.

[0024] Hence, the present disclosure is directed to tetrapolymers of certain monomers and in a narrow range of monomer ratios which exhibit a very large electrocaloric cooling effect. Such tetrapolymers can be represented by formula (I):

(VDF i-x-TrFE_x)y-CFEz-(CTFE or CDFE)_k (I)

[0025] wherein VDF is vinylidene fluoride, TrFE is trifluoroethylene, CFE is chlorofluoroethylene, CTFE is chlorotrifluoroethylene and CDFE is chlorodifluoroethylene and wherein x is between and including 0.25 to 0.5, y is between and including 0.89 to 0.94, z is between and including 0.09 to 0.04, k is between and including 0.06 to 0.02, the sum of z and k is between and including 0.06 and 0.11, and the sum of y, z and k equals 1. Preferably, y is in the range of between 0.90 to 0.94 and/or z is between and including 0.08 to 0.04 and/or the sum of z and k is between and including 0.06 and 0.10. [0026] In an aspect of the present disclosure, the tetrapolymer is a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene-chlorotrifluoroethylene having a certain ratio of monomers. Such a tetrapolymer can be represented as formula (II):

(VDF i-x-TrFE_x)y-CFEz-CTFEk (II)

where x is between and including 0.25 to 0.5, y is between and including 0.89 to 0.94, z is between and including 0.09 to 0.04, k is between and including 0.06 to 0.02, the sum of z and k is between and including 0.06 and 0.11, and the sum of y, z and k equals 1 (i.e., y+z+k = 1). Preferably, y is in the range of between 0.90 to 0.94 and/or z is between and including 0.08 to 0.04 and/or the sum of z and k is between and including 0.06 and 0.10.

[0027] In another aspect of the present disclosure, the tetrapolymer is a polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene-chlorotrifluoroethylene having a certain ratio of monomers. Such a tetrapolymer can be represented as formula (III):

(VDF i-x-TrFEx)_y-CFEz- CDFE k (III)

[0028] where x is between and including 0.25 to 0.5, y is between and including 0.89 to

0.94, z is between and including 0.09 to 0.04, k is between and including 0.06 to 0.02, the sum of z and k is between and including 0.06 and 0.11, and the sum of y, z and k equals 1 (i.e., y+z+k = 1). Preferably, y is in the range of between 0.90 to 0.94 and/or z is between and including 0.08 to 0.04 and/or the sum of z and k is between and including 0.06 and 0.10.

[0029] Such tetrapolymers are useful in cooling devices, such as air conditioner, refrigerator. The electrocaloric effect of tetrapolymers of the present disclosure can be in thermal connection with a cooling load or heat sink of the device and can be used to pump heat from the cold thermal load to a hot heat sink in cooling devices. The tetrapolymers of the present disclosure can also be used in dehumidifiers in which the tetrapolymer exhibits a temperature change upon application or removal of an electric field or voltage.

[0030] In certain aspects of the present disclosure, the tetrapolymer according to formulae (I), (II) or (II) exhibit an electrocaloric cooling (- DT) larger than 2 K at 50 MV/m electric field at room temperature and/or an electrocaloric cooling (- DT) larger than 4 K at 70 MV/m electric field at room temperature and/or an electrocaloric cooling (- DT) larger than 7 K at 100 MV/m electric field at room temperature, e.g., an electrocaloric cooling (- DT) greater than 3.5 K under 50 MV/m and larger than 5 K under 70 MV/m electric field at and around room temperature.

[0031] In electrocaloric polymers, applying an electric field causes a temperature rise above ambient. As the electric field is removed, the electrocaloric polymer temperature is lowered. Presented in Figure 1 is an example of an electrocaloric cooling of an electrocaloric terpolymer (a P(VDF-TrFE-CFE) terpolymer) as the applied field of 100 MV/m is reduced to zero. The peak cooling temperature is equal to the adiabatic electrocaloric cooling temperature DT, which is approximately 6 K.

[0032] Figure 2 shows plots comparing electrocaloric characteristics of certain tetrapolymers according to the present disclosure to electrocaloric characteristics of terpolymers. The plots show a comparison of DT at approximately room temperature (about 23 °C) (as well as isothermal entropy change AS) of a tetrapolymer P(VDF-TrFE-CFE-CTFE) (molar percent of each of the monomers of 56.1/33.2/5.9/4.8 mol% and having a peak dielectric constant of 55 at 100 Hz) and a corresponding terpolymer made with three of the same monomers and similar molar percentages, P(VDF-TrFE-CFE) (59.2/33.6/7.2 mol%). As shown by the data in Figure 2(a), the tertrapolymer surprisingly generates a significantly higher electrocaloric effect over the whole electric field range of from about 30 to about 100 MV/m compared to the corresponding terpolymer. The increased EC cooling is especially significant at electric fields below 60 MV/m. For example, the terpolymer generates only an EC cooling of about 1 K under 50 MV/m. In contrast, the tetrapolymer generates an EC cooling of 3.8 K under the same field, a 380% increase.

[0033] To reflect the electric field level on the EC response, an electrocaloric coefficient, i.e., DT/E, is presented in Figure 2(b). As can be seen, DT/E of the tetrapolymer is significantly higher than that of the P(VDF-TrFE-CFE) terpolymer, at fields below 70 MV/m, which is in fact the field range used in a recent high performance EC polymer cooler, employing the EC P(VDF-TrFE-CFE) terpolymer, by Ma et al. (Rujun Ma, Ziyang Zhang, Kwing Tong, David Huber, Roy Kombluh, Yongho Sungtaek Ju, Qibing Pei, Sci. 357, 1130 (2017)).

[0034] The EC cooling of the tetrapolymer P(VDF-TrFE-CFE-CTFE) 56.1/33.2/5.9/4.8 mol% under different electric fields vs measuring temperature is presented in Figure 3(a), which shows that the tetrapolymer generates a high EC response over a broad temperature range (Tetrapolymer #1 in Table 1). In comparison, the corresponding EC cooling from the terpolymer is also presented in Figure 3(b). As shown by the data in Figure 3, the tetrapolymer exhibits significantly higher EC cooling (a larger (- DT)) at lower applied fields (less than 100 MV/m) across a large temperature range (e.g., between and including 10 °C to 45 °C).

[0035] Figure 4 presents the dielectric properties of a tetrapolymer (#1 in Table 1) measured at different frequencies vs. temperature. As shown, the tertrapolymer #1 exhibits typical ferroelectric relaxor behavior, broad dielectric constant peak (peak value of 55 at 100 Hz), which position moves to higher temperature as the measuring frequency increases. (See L. E. Cross, Ferro. 1987, 76, 241; Q. M. Zhang, V. Bharti, X. Zhao. ScL, 1998, 280, 2101). This is similar to the terpolymer 59.2/33.6/7.2 mol%.

[0036] In ferroelectric materials, the EC response is closely related to the polarization level. The polarization response of the tetrapolymer 56.1/33.2/5.9/4.8 mol% under uni-polar electric fields was characterized and presented in Figure 5 which is the polarization response measured under 50 MV/m AC field of 10 Hz and the comparison with that of the terpolymer. When comparing the polarization levels of the two EC polymers, the tetrapolymer displays higher polarization as shown in Figure 5.

[0037] By varying the compositions, the EC cooling generated can be changed. For example, presented in Figure 6 is an EC cooling for another tetrapolymer. Table 1 is a compiled data of the EC cooling of several tetrapolymers. The data of sample #1 are presented in Figs 2 to 6. And data of the sample #5 are presented in Fig. 6. The composition for the sample #3 is P(VDF-TrFE-CFE-CTFE) 48/38/10.5/3.5 mol%. It was discovered that the content of CFE and CTFE in the tetrapolymers #2, #3, #6 and #7 is too high (e.g., greater than 0.09 mol% and 0.06 mol%, respectively) so that the EC cooling is much smaller than that of the samples #1 and #5.

[0038] Table 1 below shows certain P(VDF-TrFE-CFE-CTFE) tetrapolymers. The compositions of the samples 1 and 3 were determined by a combination of nuclear magnetic resonance (NMR) and element analysis method. The sample 1 has x=0.372, y= 0.893, z=0.059, k= 0.048, which values fall within Formula (I). For the sample #3, x=0.44, y= 0.86, z=0.105, k = 0.035, which values fall outside of Formula (I). For the other tetrapolymers, the initial feed ratio of the monomers is shown in Table 2. Because of the synthesis system (a laboratory synthesis system) used, the synthesis reaction could not be controlled precisely and the final composition after the synthesis varies somewhat from the initial feed ratio.

Table 1. Examples of the performance of several tetrapolymers.

Note: tetrapolymer samples 2, 3, 6, and 7 each had peak dielectric constants of less than 50 at 100 Hz and had relatively less EC cooling compared to samples 1 and 5. No compositional analysis was performed for the sample 2, 5, 6, and 7.

[0039] More than 30 compositions of PVDF based tetrapolymers have been synthesized and tested (there are already many studies have been conducted on PVDF based copolymers and terpolymers, see Q. M. Zhang, S.-G. Lu, Xinyu Li, Lee Gomy, and Jiping Cheng, Polymer- Based Electrocaloric Cooling Devices. US Patent. No. 8,869,542; Bret Neese, Baojin Chu, Sheng-Guo Lu, Yong Wang, E. Furman, and Q. M. Zhang, Large Electrocaloric Effect in Ferroelectric Polymers Near Room Temperature. Science, 321, 821-823 (2008); Xinyu Li, Sheng-Guo Lu, Xiang-Zhong Chen, Haiming Gu, Xiao-shi Qian and Q. M. Zhang. Pyroelectric and electrocaloric materials, J. Mater. Chem. C. 1, 23-37 (2013)).

[0040] Table 2 below provides the initial feeding ratios of several prepared tetrapolymers. For the synthesis reactor used to prepare the various tetrapolymers, due to the constraints of the synthesis system, the final tetrapolymer compositions are different from the initial feed ratio. Therefore, in order to determine the precise molar percentage composition of a tertrapolymer (there are four different monomers), a combined NMR and element analysis was carried out. The composition of two samples, e.g., #1 and #3 were analyzed; composition #1 showed good performance and composition #3 showed poor performance.

Table 2. Initial feed ratio of different monomers for tetrapolymers synthesized

Notes: The initial feed ratio of the samples #1 to #3 was controlled by a system, which is different from the others. Tertrapolymer sample number 10 was prepared with a hexafluoropropyleneu monomer but this tetrapolymer had poor performance. Due to synthesis system used, there are large errors in the feeding ratio, especially for CFE and CTFE which have feeding ratio error to 50% (5 mol% can be in the range of 2.5 mol% to 7.5 mol%), due to small amount used. In addition, there are large differences in the reactivity among different monomers, and consequently the final composition of the product using the synthesis system is also different from the initial feeding ratio.

[0041] The possible mechanism for the larger ECE in the tetrapolymers over the terpolymers could be due to an increased dipolar randomness in the non-polar phase: Electrocaloric effect is the temperature and entropy change in a dielectric material as an applied field changes. It is believed that to increase the electrocaloric effect, one should increase the polymer dipole randomness in the non-polar phase and it is believed that one way to do this is to increase the number of polar-entities. According to phase rule, the available polar entities are determined by available external variables such as composition, stresses, electric fields in the polymer. Increasing such variables should result in a larger number of available polar-entities in a given polymer. Here we increase the composition variable from 3 (in terpolymer) to 4 (in tetrapolymer) with the hope to increase the ECE. However, simply increasing the number of different monomer units from a terpolymer to a tetrapolymer was not sufficient to increase the ECE as shown by many samples such as samples 2, 3, 6, 7 in Table 2 (in fact, most samples in Table 2 do not show high ECE). Only a certain combination of monomers to form a tetrapolymer and in certain monomer ratios achieved a large ECE.

[0042] It is believed that the effects of CFE and CTFE, as well as other monomers such as CDFE (chlorodifluoroethylene) on the ferroel electric and electrocaoric properties of P(VDF- TrFE) based tetrapolymer (as well as the terpolymer) could be understood from their monomer sizes. Figure 7 illustrates relative sizes of monomers useful to prepare tetrapolymers according to the present disclosure. In our earlier works, we have made use of the bulky size of CFE to act as a defect to convert the normal ferroelectric P(VDF-TrFE) into a relaxor ferroelectric (and achieve a high ECE) (see Q. M. Zhang, S.-G. Lu, Xinyu Li, Lee Gomy, and Jiping Cheng, Polymer-Based Electrocaloric Cooling Devices. US Patent. No. 8,869,542; Bret Neese, Baojin Chu, Sheng-Guo Lu, Yong Wang, E. Furman, and Q. M. Zhang, Large Electrocaloric Effect in Ferroelectric Polymers Near Room Temperature. Science, 321, 821-823 (2008); Xinyu Li, Sheng-Guo Lu, Xiang-Zhong Chen, Haiming Gu, Xiao-shi Qian and Q. M. Zhang. Pyroelectric and electrocaloric materials, J. Mater. Chem. C. 1, 23-37 (2013). Compared with CFE, CTFE monomer has three fluorine (F) atoms and one chlorine (Cl) atom, and hence has a much larger size than that of CFE. Due to its large size, it is believed CTFE could not be included in the crystalline phase of the polymer and is not as effective as CFE in converting P(VDF-TrFE) into a relaxor.

[0043] Due to the fact that CTFE monomers are excluded from the crystallites, adding

CTFE also reduces crystallinity. In the tetrapolymers, it is believed that including CTFE in the polymer would reduce crystallite size (and a broad x-ray peak of the tetrapolymer has indeed been observed). This could increase the randomness in the polymer as well as promote electric field induced crystalline/amorphous transition which should generate a larger ECE than the polar/non-polar phase change in the crystalline phase. This could be one of the reasons for the very large ECE in the tetrapolymer P(VDF-TrFE-CFE-CTFE) at fields < 100 MV/m. Based on these considerations, it is expected that the tetrapolymer P(VDF-TrFE-CFE-CDFE) should have better EC response than the P(VDF-TrFE-CFE-CTFE) tetrapolymers. Because CDFE monomer is larger than CFE but smaller than CTFE, it should have less effect than CTFE in reducing the crystallinity of the tetrapolymer compared with CTFE, while retaining other effects such as a large ECE at lower electric fields than that of the pure terpolymer. It can also be seen that the effects of CTFE and CDFE of reducing the crystallinity also limit the mol% of CTFE and CDFE in the tetrapolymers. From the studies of the compositions of P(VDF-TrFE-CFE) terpolymer, the monomer mol% of CTFE and CDFE should be less than or equal to 6 mol%, but in order to increase the polar-species, its composition is larger than or equal to 2 mol%.

[0044] In another aspect of the present disclosure, the CTFE in the tetrapolymer can be replaced by CDFE. That is, in another aspect of the present disclosure, the tetrapolymer has a composition in range of (VDFi-_x-TrFE_x)y-CFEz- CDFEk where x is in the range of 0.25 to 0.5, y in the range of 0.89 to 0.94, z in the range of 0.09 to 0.04, and k in the range of 0.06 to 0.02, z+k is in the range of 0.06 to 0.11 and y+z+k = 1. Preferably, y is in the range of between 0.90 to 0.94 and z+k is in the range of 0.06 to 0.10. Such tetrapolymers are useful in cooling devices, such as air conditioner, refrigerator, and also in a dehumidifier in which the tetrapolymer exhibits a temperature change upon application or removal of an electric field or voltage. In certain aspects of the present disclosure, the tetrapolymer exhibits an electrocaloric cooling (- DT) larger than 2 K at 50 MV/m electric field at room temperature and/or an electrocaloric cooling (- DT) larger than 4 K at 70 MV/m electric field at room temperature and/or an electrocaloric cooling (- DT) larger than 7 K at 100 MV/m electric field at room temperature, e.g., an electrocaloric cooling (- DT) greater than 3.5 K under 50 MV/m and larger than 5 K under 70 MV/m electric field at and around room temperature.

[0045] The performance of these tetrapolymers can be first screened by the data of dielectric properties vs. temperature and frequency. In general, a tetrapolymer with the peak dielectric constant less than 50 (at 100 Hz) will not exhibit a very high ECE. Hence it is preferable that a tetrapolymer according to any one of formulae (I), (II) or (II) exhibits a peak dielectric constant greater than 50 at 100 Hz. Table 3 below provides peak dielectric constant data at 100 Hz for several poorer performing tetrapolymers.

Table 3. Examples of the performance of several poorer performing tetrapolymers.

Notes: Table 2 provides initial feeding ratio of the tetrapolymers for the tertapolymers of Table 3. However, as previously noted, the actual composition depends on both the feed ratio and control of the synthesis condition (which could vary between different synthesis runs). [0046] In addition to a peak dielectric constant value, the shape of the dielectric constant vs. temperature (and frequency) can also serve as an indication of the EC performance. Figure 8 presents the dielectric properties of tetrapolymer sample 22. The relatively sharp dielectric peak of sample 22 shown in Figure 8 (as indicated by a steep increase in the dielectric constant with temperature) indicates that the tetrapolymer is not a relaxor ferroelectric (which should have a broad dielectric constant peak as shown in Figure 4).

EXAMPLES

[0047] The following examples are intended to further illustrate certain preferred embodiments of the invention and are not limiting in nature. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances and procedures described herein.

[0048] Tetrapolymers were synthesized via a suspension polymerization using a 300 mL stainless steel reaction vessel. 100 mL of de-ionized water and 0.15 g potassium peroxodisulfate initiator were added to the vessel which was subsequently sealed and degassed via a vacuum pump and cooled using liquid nitrogen bath. Gases of four monomers (VDF, TrFE, CFE, and CTFE/CDFE) were separately pumped into the reaction vessel at liquid nitrogen temperature using a gas controller allowing for control of the amount of each monomer. Upon entering the vessel the gaseous monomers would condense and solidify. The amount of monomer was controlled by modulating the feeding time. After that, the vessel with all the monomers was sealed and heated to 90 °C and stirred at 600 rpm for 4 hours. Once the reaction was complete (after 4 hrs), the product was washed by vacuum filtration with both de-ionized water and methanol, and then dried at 80 °C for 48 hours. It should be noted that the synthesis system used has certain limitation, including: (i) we could not control the feeding ratio of each monomer (e,g, for CFE and CTFE (which were in small cylinders), the error in the feed ratio can be at ±50% (5 mol% could be in between 2.5 mol% to 7.5 mol%); (ii) the reactivity of each monomer is different from each other, and thus the composition of the final product can be different from the initial feeding ratio; (iii) with 4 different monomers in the tetrapolymer, there are also large errors in the composition from the element analysis. Hence, a more precise way to determine the performance of a tetrapolymer is to measure certain properties. These properties included: (i) determining the teetrapolymer’s dielectric response (we found that there is a good correlation between the dielectric response (the peak dielectric constant should be between 50 and 65, too low or too high will generate poor ECE response)); (ii) directly measure the EC response. In high quality tetrapolymer synthesis, one should continuously feed each monomer (in this case, use 4 channels) with controlled rate (so that the final product has uniform composition), rather than a batch process.

[0049] The EC property of tetrapolymer films was characterized by a specially designed calorimeter. In general, the heat generated by the tetrapolymer films was compared with the heat generated by a standard reference resistor R, from which AS' is determined. When a voltage,

V, with a pulse time duration, t, applies to the resistor heater, it would produce a joule heat Qh = (V²/R)t. The heat Qh was also detected as Ah by a heat flux sensor directly attached to the sample surface. Now, applying another voltage VECE on the tetrapolymer films to generate ECE, the heat signal (cooling and heating) recorded by heat flux sensor was recorded as AECE. Then the QECE could be deduced from equation: QECE/AECE= Qh/Ah. The cooling (and heating) per unit sample mass Q’ECE is equal to QECE/M, m is the mass of tetrapolymer film. AS' is deduced from Q ECE = T^AS and Q’ECE = C^AT, where T and C are the environment temperature and specific heat of the tetrapolymer.

[0050] Only the preferred embodiment of the present invention and examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Thus, for example, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances, procedures and arrangements described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A tetrapolymer of formula (I):

(VDF i-x-TrFE_x)y-CFEz-(CTFE or CDFE)_k (I) wherein VDF is vinylidene fluoride, TrFE is trifluoroethylene, CFE is

chlorofluoroethylene, CTFE is chlorotrifluoroethylene and CDFE is chlorodifluoroethylene; and

wherein x is between and including 0.25 to 0.5, y is between and including 0.89 to 0.94, z is between and including 0.09 to 0.04, k is between and including 0.06 to 0.02, the sum of z and k is between and including 0.06 to 0.11 and the sum of y, z and k equals 1.

2. The tetrapolymer of claim 1 having formula (II):

(VDF i-x-TrFEx)_y-CFEz-CTFEk .

3. The tetrapolymer of claim 1 having formula (III):

(VDF i-x-TrFE_x)y-CFEz- CDFEk .

4. The tetrapolymer of any one of claims 1-3, wherein y is between and including 0.90 to 0.94 and/or z is between and including 0.08 to 0.04 and/or the sum of z and k is between and including 0.06 to 0.10.

5. The tetrapolymer of any one of claims 1-3, wherein the tetrapolymer exhibits an electrocaloric cooling (- AT) larger than 2 K under 50 MV/m electric field at and around room temperature.

6. The tetrapolymer of any one of claims 1-3, wherein the tetrapolymer exhibits a peak dielectric constant greater than 50 and lower than 65 at 100 Hz.

7. A cooling device which comprises a tetrapolymer of any one of claims 1-3, wherein the tetrapolymer exhibits a temperature change upon application or removal of an electric field or voltage.