polymers Article
Nanoindentation Investigation of Temperature Effects on the Mechanical Properties of Nafion® 117 Re Xia 1 , Hongjian Zhou 1 , Runni Wu 1 and Wen-Ping Wu 2, * 1 2
*
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China;
[email protected] (R.X.);
[email protected] (H.Z.);
[email protected] (R.W.) Department of Engineering mechanics, School of Civil Engineering, Wuhan University, Wuhan 430072, China Correspondence:
[email protected]; Tel.: +86-27-6877-5496
Academic Editor: Mohamed Khayet Received: 12 June 2016; Accepted: 14 September 2016; Published: 20 September 2016
Abstract: Operating temperature can be a limiting factor in reliable applications of Proton Exchange Membrane (PEM) fuel cells. Nanoindentation tests were performed on perfluorosulfonic acid (PFSA) membranes (Nafion® 117) in order to study the influence of the temperature condition on their mechanical properties. The hardness and reduced modulus of Nafion® 117 were measured within a certain temperature range, from 10 to 70 ◦ C. The results indicate that both hardness and elastic modulus show non-monotonic transition with the increase of the test temperature, with reaching peak values of 0.143 and 0.833 GPa at 45 ◦ C. It also found that the membranes have a shape memory effect and a temperature dependent shape recovery ratio. Keywords: perfluorosulfonic acid (PFSA) membranes; Nafion® 117; temperature effects; mechanical properties; nanoindentation
1. Introduction Perfluorosulfonic acid (PFSA) polymers, such as Nafion® membranes [1], are widely used as the solid electrolyte in Proton Exchange Membrane (PEM) fuel cells and waterelectrolyzers. Today, a major concern in PFSA membranes is to continue to improve the sustainability and to extend their service life, which can be significantly reduced by the combined effects of changing chemical and mechanical environments [2]. During operation, chemical degradation and mechanical failure due to hydration and temperature cycles may be the most prominent factors restricting the durability of PEMs [3–6]. Therefore, it is very important to understand their mechanical stability changes with respect to operating conditions (humidity, temperature, etc.). It has been found that the elastic modulus and the proportional limit stress of PFSA membranes decrease with increasing humidity and temperature, and that the change trend becomes weaker when the temperature increases to a certain value. Compared with humidity, higher temperature can bring about enhanced effects on lowering break stress and rising break strain [7,8]. The fatigue lifetime of the membranes also strongly depends on the test temperature, relative humidity and applied stress. Compared with the effect of increasing humidity, higher temperature can bring about enhanced effects on lowering break stress and raising break strain as well as reducing fatigue life [9]. However, humidity can have more complex affections on the mechanical properties of the membrane. For Nafion® 117, dry samples show a lower elastic modulus than hydrated ones, with temperature ranging from 50 to 100 ◦ C [10]. The different history of thermal treatment will cause different water uptakes and microstructures, which, in turn, may result in mechanical variation of a PFSA membrane [11]. The mechanical properties of a PFSA membrane are also affected by the deformation rate, and during a uniaxial tensile test, increasing the loading rate should strengthen the elastic modulus and the Polymers 2016, 8, 344; doi:10.3390/polym8090344
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proportional limit stress, as well as the hardening modulus [12–14]. Then, through forming composite membranes, not only conductivity and stiffness of PFSA membranes, but also the stability of these two properties can be significantly improved [15,16]. Meanwhile, a considerable amount of simulation research and micromechanical models have been elaborated during the last decade, which also efficiently enriches the understanding of PFSA degradation and failure mechanisms [17–19]. In the present work, we focus on the evolution of mechanical properties of PFSA membranes with rising environmental temperature. Using the method of nanoindentation, the hardness and reduced modulus of Nafion® 117 membranes were tested at temperatures between 10 and 70 ◦ C, and the effects of indentation depth and applied temperature on the mechanical response of the membrane was investigated. 2. Materials and Experiments Nafion® PFSA membranes are non-reinforced films, based on Nafion® PFSA polymer, a perfluorosulfonic acid/polytetrafluoroethylene (PTFE) copolymer in the acid (H+ ) form. Commercially available DuPont’s Nafion® N-117 (Wilmington, DE, USA) was used in this study, with a typical thickness of 183 µm, basis weight of g/m2 and conductivity of 0.083 S/cm. Membranes that were used without any pretreatment. Quasi-static nanoindentation was carried out on the samples using a nanoindenter (Triboscope TI950, Hysitron Inc., Minneapolis, MN, USA) equipped with a commercial heating stage. The heating stage consists of an upper stage component and a bottom stage component, and each of has an electrical heating element and liquid cooling system. The sample is clamped between the upper stage component and the bottom stage component. Thermocouples embedded in the upper stage and bottom stage are used for temperature feedback to maintain a constant sample temperature. The temperature control stage allows for temperatures ranging from −20 to 400 ◦ C. A three-sided pyramidal diamond tip (Berkovich tip) with a face angle of 142.3◦ and a radius of curvature of ~100 nm was selected. A peak load of 100 µN was applied at a rate of 20 µN/s and then the indenter was maintained at that load for 5 s (hold-at-the-peak method), followed by unloading at a constant speed for 5 s. In order to investigate the temperature effects on the mechanical properties of Nafion® 117, the load temperature in the test chamber was set at a range of 10 to 70 ◦ C, with an increment value of 10 ◦ C. The temperature was increased to the set point at a rate of 2 ◦ C/min, and then allowed to stabilize for an hour, keeping both the tip and samples at the same temperature during the nanoindentation tests. Environment humidity was at a relative humidity of 58%. A minimum of 9 indents were collected for each sample at each temperature point. Hardness, H and elastic modulus (reduced modulus Er ) were calculated from the load-displacement data obtained by nanoindentation, based on the Oliver-Pharr method [20]. The elastic modulus of membranes is related to the reduced modulus by: 1 (1 − ν2 ) (1 − νi2 ) = + Er E Ei
(1)
where E and ν are the elastic modulus and Poisson’s ratio of the material, respectively, and Ei and νi those of the indenter. Then, the elastic modulus of membrane was calculated by using ν = 0.5 for Nafion® 117 and Ei = 1140 GPa, νi = 0.07 for the diamond indenter. However, for low modulus polymers (ν = 0.3~0.5), the correction is lower than 0.1% for the second term and the first one only modified the absolute values, resulting in the insignificant difference between Er and E. 3. Results and Discussion Figure 1 shows representative indentation load-depth curves (P–h curves) of Nafion® 117 membranes at different temperatures, T, with the same peak load of 100 µN. Nanoindentation curves shifted to the left, and maximum depth reduced due to increasing hardness. The horizontal segment at the peak load represents that the indenter continues to penetrate into the membranes, giving
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an Polymers indication the result of creep, even at 10 ◦ C. If weakening the effect of creep is desired, a long 2016, of 8, 344 3 ofhold 9 period, asaswell well a indenter fast indenter unloading are required [21,22].P–h Through P–h curves period, as aasfast unloading rate, arerate, required [21,22]. Through curves comparison, period, as one wellcan as find a fast unloading rate,and areresidual required [21,22]. Through P–hdifferences curves comparison, thatindenter theindentation maximum indentation depths and residual depths show obvious one can find that the maximum depths depths show obvious comparison, onevarying can findconditions. that the maximum indentation depths and residual depths show obvious differences temperature conditions. Although all curves reveal that the maximum with varyingwith temperature Although all curves reveal that the maximum penetration depth differences varying conditions. Although curves reveal that the maximum penetration atmaximum T = 10 °C and the maximum depth at Tlowest = 70 °C, both the lowest occurred at Tdepth =with 10 ◦occurred C and thetemperature residual depth at Tresidual = 70all◦ C, both the penetration depth ◦ penetration depth occurred at T = 10 °C and the maximum residual depth at T = 70 °C, both the lowest penetration residual depth at T = 40 °C. and residualdepth depthand were observed at Twere = 40 observed C. penetration depth and residual depth were observed at T = 40 °C.
® ® 117membrane Figure 1. 1.Comparison vs. curves of Nafion different Figure Comparisonof load vs. vs. displacement displacement curves Figure 1. Comparison ofof load load displacement curvesof of Nafion Nafion®117 117 membrane membraneat at atdifferent different temperatures, from 1010to toto70 7070◦°C. temperatures, from °C. temperatures, from 10 C.
can seenininFigure Figure11that that there there is is an evident evident recovery thethe depth It It can bebeseen recoveryeffect effectofofthe theindents indentsinin depth It can be seen in Figure 1 that there is an evident recovery effect of the indents in the depth direction, whichmost mostcommonly commonlyoccur occur in in aa nanoindentation nanoindentation test (SMA). direction, which testofofshape shapememory memoryalloys alloys (SMA). direction, which most commonly occur in a nanoindentation test of shape memory alloys (SMA). Then, Then, comparingthe theindentation indentation surface surface in in the (at(at 2020 °C), it it Then, bybycomparing the membrane membrane before beforeand andafter aftertest test °C), ◦ C), byconfirms comparing the indentation surface in the membrane before and after test (at 20 it confirms thatthere thereisisno noapparent apparentresidue residue of of indentations, indentations, as Nafion confirms that asshown shownininFigure Figure2.2.Similarly, Similarly, Nafion that there is no apparent residue of indentations, as shown in Figure 2. Similarly, Nafion nanofibers nanofibers were found to exhibit excellent shape memory performance when deformed nanofibers were found to exhibit excellent shape memory performance when deformedand and were found to exhibit excellent shape memory performance when deformed and recovered above the recovered above thetransition transitiontemperature, temperature, having shape [23]. The degree recovered above the having shaperecovery recoveryratios ratiosabove above90% 90% [23]. The degree transition temperature, having shape recovery ratios above 90% [23]. The degree of indent recovery in indent recoveryininthe thedepth depthdirection direction can can be be determined a recovery of of indent recovery determined quantitatively quantitativelybybydefining defining a recovery theratio depth direction can be determined quantitatively by defining a recovery ratio [24–26]: [24–26]: ratio [24–26]: δ max −−δδr r = δδmax (2) (2) max − δ r δδdδ d== δδmax (2) d δ max max
where the depth recovery ratio δ d a is a measure of recoverable work done during indentation,δr δisr the where of recoverable work done during indentation, where the the depth depth recovery recoveryratio ratioδdδ is ismeasure a measure of recoverable work done during indentation, δ r d is the remnant at unloading, is the depth at the maximum indentation load. δ d remnant depth atdepth unloading, and δmaxand is the at the maximum indentation load. δd approaches δ mdepth ax is the remnant depth at unloading, and δ m ax is the depth at the maximum indentation load. δ d zero when there is when no recovery indentation equal to maximum while it iswhile 1 when approaches zero there is(i.e., no recovery (i.e., is indentation is equal to penetration), maximum penetration), approaches zero there is noisAll recovery (i.e., istoequal to maximum penetration), while the recovery is complete. data related to the recovery are ratio listedare in listed Tablein 1 and the itelastic is 1 when thewhen elastic recovery complete. Allindentation data related the ratio recovery Table it is 1 when the elastic recovery is complete. All data related to the recovery ratio are listed in Table 1 and theinvariations in indent recovery ratios attemperatures elevated temperatures are variations indent recovery ratios at elevated are shown inshown Figurein3.Figure 3. 1 and the variations in indent recovery ratios at elevated temperatures are shown in Figure 3.
Figure Indentationsurface surfaceof of the the membrane membrane (a) Numbers 0–40–4 ◦ C. Figure 2. 2.Indentation (a) before before and and(b) (b)after aftertest testatat2020°C. Numbers represent the marking-points of five indentations. represent marking-points Figure 2. the Indentation surfaceofoffive theindentations. membrane (a) before and (b) after test at 20 °C. Numbers 0–4
represent the marking-points of five indentations.
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Table 1. Variation in recovery ratio of Nafion® 117 membranes with different temperatures. Temperature (◦ C)
10
20
30
40
50
60
70
Depth at peak load, δmax (nm) Remnant depth, δr (nm) Depth recovery Polymers 2016, ratio, 8, 344 δd
520.5 ± 11.6 176.4 ± 9.5 66.1%
328.0 ± 9.4 85.5 ± 5.1 73.9%
342.2 ± 9.4 97.4 ± 6.9 71.5%
294.2 ± 6.4 109.0 ± 4.5 62.9%
346.2 ± 8 162.5 ± 3.7 53.1%
416.5 ± 6.4 220.0 ± 5.7 47.2%
479.0 ± 9.4 267.5 ± 12 4 of44% 9
Figure 3. Variation in depth recovery ratio with elevated temperatures.
Figure 3. Variation in depth recovery ratio with elevated temperatures. Table 1. Variation in recovery ratio of Nafion® 117 membranes with different temperatures.
It can be Temperature seen that(°C)the recovery behaviors of membranes differentiate significantly when the 10 20 30 40 50 60 70 ◦ C (about room temperature), the membrane has a larger recovery Depth at peak load, δ mAt (nm) test temperature varies. 20–30 520.5 ± 11.6 328.0 ± 9.4 342.2 ± 9.4 294.2 ± 6.4 346.2 ± 8 416.5 ± 6.4 479.0 ± 9.4 ax Remnant depth, (nm) 176.4 ± 9.5 ± 5.1 97.4 ± 6.9 at 109.0 162.5 ± 3.7 220.0 ± 5.7 of 267.5 ± 12 during unloading andδthe maximum ratio,85.573.9%, occurs 20± ◦4.5C. With an increase temperature, r ◦ Depth recovery ratio, δd 66.1% 71.5%then a 62.9% 53.1% 44%of recovery the recovery gradually decreases (except at 73.9% 10 C), and relatively small 47.2% proportion occurred at 70 ◦ C, as shown in Figure 3. Temperature-dependent recovery ability may result from It can be seen that the recovery behaviors of membranes differentiate significantly when the test the change in phase state of the membranes. It is well known that the shape memory effect in SMA temperature varies. At 20–30 °C (about room temperature), the membrane has a larger recovery is the result of thermoelastic martensitic transformation from the cubic (B2) to the monoclinic (B190 ) during unloading and the maximum ratio, 73.9%, occurs at 20 °C. With an increase of temperature, structure in polymers, Fulcher et al. [30] suggestsmall that proportion the shapeofrecovery the[27–29]. recovery However, gradually decreases (except at 10 °C), and then a relatively recovery ability may arise from at a rearrangement long3.molecular chains, so that new ability bondsmay areresult formed occurred 70 °C, as shown inof Figure Temperature-dependent recovery fromand the a fixed phaseemphasized state of the membranes. It is well known the shape memory effectmemory in SMA is effect the shape ischange held. inThey that the best method tothat characterize the shape is to result of thermoelastic martensitic transformation from the cubic (B2) to the monoclinic (B19′) activate the rubbery state of the polymer at an elevated deformation temperature above the glass structure [27–29]. However, in polymers, Fulcher et al. [30] suggest that the shape recovery ability transition point. may arise from a rearrangement of long molecular chains, so that new bonds are formed and a fixed Figure the relationship between modulus contact shape4 isshows held. They emphasized that the besthardness/reduced method to characterize the shapewith memory effectdepth is to hc for different valuestheofrubbery the teststate temperatures. Clearly, the indenter contact depth atabove a peak activate of the polymer at an elevated deformation temperature theload glass100 µN point. does nottransition have a monotonic relation with rising temperature, and comes to a turning point where the 4 shows the relationship modulus contact depth hc for180 nm. deformationFigure temperature reaches 40 ◦ C.between At 40 ◦hardness/reduced C, depth is lowered to awith minimum of about different values of the test temperatures. Clearly, the indenter contact depth at a peak load 100 μN 330 nm Below this point, a rise in temperature decreases the contact depth, with a higher value of about does not have a monotonic relation with rising temperature, and comes to a turning point where the ◦ ◦ at 10 C. In the range of 40 to 70 C, depth increases with increasing temperature, reaching a maximum deformation temperature reaches 40 °C. At 40 °C, depth is lowered to a minimum of about 180 nm. ◦ C. value ofBelow aboutthis 390 nm aatrise 70 in point, temperature decreases the contact depth, with a higher value of about 330 Hardness reduced modulus, as°C, function of various are depicted in Figure 5. nm at 10and °C. In the range of 40 to 70 depth increases withtemperatures, increasing temperature, reaching a about 390 anm at 70 °C. It can bemaximum seen thatvalue theyofrepresent similar variation tendency with elevated temperatures, as shown in Hardness reduced as also function of various are depicted in Figure 5. Figure 4. The critical and point of themodulus, transition occurs at 40 ◦temperatures, C, in accordance with that of penetration It can be seen that they represent a similar variation tendency with elevated temperatures, as shown depth. In this situation, a higher hardness and reduced modulus are obtained, at values close to in Figure 4. The critical point of the transition also occurs at 40 °C, in accordance with that of 0.088 and 0.62 GPa. Both properties are strengthened when the temperature rises from 10 to 40 ◦ C, penetration depth. In this situation, a higher hardness and reduced modulus are obtained, at values and then weaken theGPa. testBoth temperature is increased difference that10the close to 0.088when and 0.62 properties are strengthenedfurther. when theThe temperature risesisfrom to lowest hardness the weaken highestwhen temperature of 70 ◦ C, with an approximate value of 0.025 GPa, 40occurs °C, andatthen the test temperature is increased further. The difference is that thewhereas lowestmodulus hardness occurs at the temperature °C, with approximate valuewith of 0.025 GPa, the reduced reaches a highest minimum at 10 ◦of C 70 with 0.27an GPa. Compared the hardness, whereas the reduced modulus reaches a minimum at 10 °C with 0.27 GPa. Compared with the using the measured modulus is likely to have a larger discrepancy due to polymers creep. Only hardness, the measured modulus is likely to have a larger discrepancy due to polymers creep. Only an elastic contact mechanics model to analyze the indenter results of viscoelastic polymers usually using an elastic contact mechanics model to analyze the indenter results of viscoelastic polymers leads tousually an overestimation in elastic modulus To confirm the transition of Nafion properties, leads to an overestimation in elastic [31]. modulus [31]. To confirm the transition of Nafion we conducted extra three group tests under the temperature conditions near the peak value properties, we conducted extra three group tests under the temperature conditions near the peak zone ◦ C).(35, value 45 andthe 55 °C). Clearly, the results prove similar changing of hardness (35, 45 and 55zone Clearly, results further provefurther similar changing trends oftrends hardness and reduced and reduced modulus, as shown in the insets of Figure 5a,b. Then, the corresponding peaks move to modulus, as shown in the insets of Figure 5a,b. Then, the corresponding peaks move to the position at
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◦ C,
45 with values of 0.143 and 0.833 GPa, respectively. Moreover, we attempted to test the membranes the position at 45 °C, with values of 0.143 and 0.833 GPa, respectively. Moreover, we attempted to at higher 80 ◦temperatures C), but the disparity of but the the nanoindentation data became too wide, testtemperatures the membranes(over at higher (over 80 °C), disparity of the nanoindentation data ◦ especially neartoo 100wide, C. This is likely a result glass since Nafion 117 membranes are known became especially near 100 °C.ofThis is transition, likely a result of glass transition, since Nafion 117 ◦ C. to havemembranes a glass transition temperature range of 120–140 are known to have a glass transition temperature range of 120–140 °C.
4. Variation (a) hardness (b) reduced modulus membranesalong along the the contact contact depth Figure Figure 4. Variation of (a) of hardness and and (b) reduced modulus of of membranes depth at at different temperatures. different temperatures.
One possible reason for the transition (at 45 °C) is that the measurements presents a combined
One possible reason dehydration for the transition (at by 45 a◦ C) is that effect the measurements a combined effect of membrane followed softening with increasingpresents temperature. The response of Nafion membranes are astrongly influenced bothincreasing relative humidity and effect mechanical of membrane dehydration followed by softening effect by with temperature. temperature, but during nanoindentation it is hardinfluenced to control by theboth relative humidity whenand The mechanical response of Nafion membranestests, are strongly relative humidity varying the temperature. The heating chamber has an initial relative humidity (RH) of 58% temperature, but during nanoindentation tests, it is hard to control the relative humidity when varying (environment humidity), and was isolated from environmental condition during heating. Therefore, the temperature. The heating chamber has an initial relative humidity (RH) of 58% (environment the membrane water content is not constant, and the influences of moisture and temperature could humidity), and was isolated from environmental condition during heating. Therefore, the membrane be added together. To get a better application of nanoindentation in PFSA membrane tests, water content is not constant, and the influences of moisture and temperature could be added together. developing a combined temperature and humidity controlling system would be a technical challenge. To get a better application of nanoindentation PFSAtomembrane tests, developing a combined Although the nanoindentation technique isinproven be suitable for determining the mechanical temperature andofhumidity system would be adifficulties. technical challenge. properties polymers,controlling it encounters certain unsolved The applicability of the OliverAlthough the nanoindentation technique is proven to be for determining the mechanical Pharr method has been discussed and modified by Cheng et suitable al. and Nagn et al. for viscoelastic solids [32–35]. In addition, Díez-Pascual indicates that difficulties. creep influences the conditionofofthe pure elastic properties of polymers, it encounters certain unsolved The applicability Oliver-Pharr in theand casemodified of long holding timesetand highNagn unloading and is still solids a matter of methodunloading, has been even discussed by Cheng al. and et al. rates, for viscoelastic [32–35]. concern; choosing which criterion to fit the unloading data to is also deserving of attention [21]. It is In addition, Díez-Pascual indicates that creep influences the condition of pure elastic unloading, well known that long holding times and high unloading rates can be used to minimize the creep effect even in the case of long holding times and high unloading rates, and is still a matter of concern; during nanoindentation tests, though the appropriate experimental conditions may be different for choosing which criterion to fit the unloading data to is also deserving of attention [21]. It is well known different material properties. Therefore, it is necessary to perform future research on the optimization that long holding times and high unloading rates can be used to minimize the creep effect during criteria of loading/unloading rates and holding periods for Nafion membranes. Furthermore, due to nanoindentation tests, though thesolids, appropriate experimental may be different measures for different the creep effect of viscoelastic it is not ideal to describeconditions the instrumented-indentation material properties. Therefore, it is necessary to perform future research on the optimization criteria of time-dependent modulus in terms of “elastic modulus” and “reduced modulus”. Rather, it should of loading/unloading rates and holding periods for Nafion membranes. Furthermore, due to the be the “instantaneous” modulus of viscoelastic solids (E0), which was suggested by Cheng et al. [36] and also used by Oyensolids, [37]. Future work willtofocus on these creep effect of viscoelastic it is not ideal describe theissues. instrumented-indentation measures of time-dependent modulus in terms of “elastic modulus” and “reduced modulus”. Rather, it should be the “instantaneous” modulus of viscoelastic solids (E0 ), which was suggested by Cheng et al. [36] and also used by Oyen [37]. Future work will focus on these issues.
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Figure 5. Effect of elevated temperatures on (a) average hardness and (b) reduced modulus of
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Figure 5. Effect of elevated temperatures on (a) average hardness and (b) reduced modulus of membranes (insets are the added measurements at 35, 45 and 55 °C), and the typical distribution of membranes (insets are the added measurements at 35, 45 and 55 ◦ C), and the typical distribution measured (c) hardness and (d) reduced modulus at different temperatures. Different colors in (c) and of measured (c) hardness and (d) reduced modulus at different temperatures. Different colors in (c) and (d) correspond to different measure points. (d) correspond to different measure points.
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4. Conclusions In this work, the mechanical properties of Nafion® 117 membranes were examined for a range of temperature conditions using a nanoindenter equipped with a hot-stage heating system. The load-depth curves of the membranes were obtained at various temperatures, all of which suggest that they may exhibit temperature-dependent shape recovery ratios and a noticeable shape memory effect, even at 10 ◦ C. The hardness and reduced modulus of the membranes do not vary monotonously with increasing temperature, and, instead, present the tendency of increasing first and then decreasing, when the temperature increases from 10 to 70 ◦ C, reaching their maximum values of 0.143 and 0.833 GPa at 45 ◦ C. Acknowledgments: This work was supported by National Natural Science Foundation of China (Grant Nos. 11102139 and 11472195), and the State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics (Grant No. MCMS-0414Y01). Author Contributions: Re Xia conceived and designed the experiments; Wen-Ping Wu performed the experiments; Hongjian Zhou and Runni Wu analyzed the data; Re Xia and Wen-Ping Wu contributed reagents, materials and analysis tools; Re Xia and Runni Wu wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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