JOURNAL OF POLYMER SCIENCE
WWW.POLYMERPHYSICS.ORG
FULL PAPER
Cooling Rate Controlled Microstructure Evolution through Flash DSC and Enhanced Energy Density in P(VDF–CTFE) for Capacitor Application Ying-Xin Chen,1 Hong-Wei Lu,1 Zhong-Wang Shen,1 Zhao-Lei Li,2 Qun-Dong Shen3 1
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China 3 Department of Polymer Science and Engineering, Key Laboratory of High, Performance Polymer Materials and Technology of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China Correspondence to: H.-W. Lu (E-mail:
[email protected]) 2
Received 17 February 2017; accepted 22 May 2017; published online in Wiley Online Library DOI: 10.1002/polb.24382
ABSTRACT: Poly(vinylidene fluoride) (PVDF) based polymers are attracting tremendous interest because of their potential applications in advanced energy storage devices. Fundamental understanding of their crystal structure evolution has been proved elusive due to the nature of rapid crystallization rate. Fortunately, flash differential scanning calorimeter (Flash DSC) with a precise control of cooling rate helps to investigate an understanding of structure–property relationships. For the first time, a bimodal distribution of the crystallization rate of P(VDFchlorotrifluoroethylene) (CTFE) in the whole temperature range, and a 3D profile of melting point and enthalpy dependence of annealing temperature and time, which is the corresponding crystal structure evolution and the mechanism of crystal nucleation and growth, are revealed by flash DSC.
Based on the above conclusions, fast cooling or annealing at low temperature regulates the crystallization behavior, favors a tiny ferroelectric b-phases, drastically reduces paraelectric spherulite sizes, and leads to greatly enhanced energy storage capacity, but reduction in discharged efficiency. For instance, compared with other processing methods, P(VDF-CTFE) quenched by liquid nitrogen achieves the highest discharged energy of 10.6 J cm23 at the maximum electric field of 270 C 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: MV m21. V Polym. Phys. 2017, 55, 1245–1253
INTRODUCTION Electrical energy storage plays an essential role
First, taking some chemical modification methods as examples, PVDF exhibits a rather small energy density owing to large and coherent ferroelectric crystals. Randomly incorporating some structural irregularities, such as comonomers of chlorotrifluoroethylene (CTFE) or hexafluoropropene (HFP), into main chains of PVDF gives us a good choice to scale down the paraelectric crystals and increase interfacial area, and thus these PVDF copolymers can achieve higher energy density.13,14 Moreover, poly(methacrylic ester)10,15 and polystyrene16,17 introduced into the side chains of PVDF copolymers could not only dramatically weaken the dipole interactions in oriented polar crystals but also accelerate the reversal switching of polar crystals along the applied electric field, which leads to high charged–discharged efficiency. Unfortunately, their charged and discharged energy densities are lower than that of the pristine PVDF copolymers, which is adverse to high-capacitance applications. More recently, chemical defects that are introduced by photo-induced crosslinking18,19 and hot-pressed cross-linking method20,21 can effectively scale down the paraelectric crystals, and produce more interfaces between crystalline and amorphous phases,
in advanced electronic devices and electrical power systems, such as hybrid electric vehicles, portable electronics, mobile electronic devices, and grid-scale energy storage.1–5 Among various dielectric polymer materials, poly(vinylidene fluoride)-based (PVDF-based) polymers have generated interest for electrical energy storage devices owing mostly to their high storage capacity of electrical energy and high dielectric constant.6–8 Dielectric materials with high capacities are favorable to reduce the volume, weight, and cost of the electric power system. PVDF-based polymers are semicrystalline, where electric energy storage originates from highly polar CAF bonds and uniform chain packing in the crystalline phase, and are also determined by both the microstructure (crystalline unit cell and chain conformation) and macrostructure (semicrystalline morphology and interface between crystalline and amorphous phases).9,10 These complicated structure and morphologies are varied by the copolymerization with different monomers and/or using various processing methods and treatments.11,12
KEYWORDS: crystal structure; fast cooling; flash differential
scanning calorimeter; high-energy-density capacitor; poly(vinylidene fluoride)-based polymers
C 2017 Wiley Periodicals, Inc. V
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
1245
FULL PAPER
WWW.POLYMERPHYSICS.ORG
leading to both increased energy density and improved charged–discharged efficiency relative to the pristine copolymer. Second, the microstructure of PVDF copolymers can be changed by various processing methods and treatments. Crystallites in P(VDF-trifluoroethylene-chlorofluoroethylene)22–24 formed during rapid cooling exhibit a higher fraction of the all trans chain conformation and stronger polar ordering, while these crystallites have smaller lamellar thickness in comparison with those formed at high temperature. Besides, Bao et al.25 found that P(VDF-TrFE-CFE) annealed at high temperature exhibits the nonpolar phase, while the polar crystal phase with all-trans conformation appears or even becomes dominant when it grows at lower temperature. Above all, the crystal structure evolution in PVDF copolymers is complicated, so there is a need for an understanding of structure–property relationships to create polymers with excellent energy storage capabilities. Recently, fast cooling chip-calorimeter, that is, flash differential scanning calorimeter (Flash DSC), with heating/cooling rates up to one million Kelvin per second has become an important tool in the study of nonisothermal and isothermal crystallization behaviors of polymers.26–34 Fast heating and cooling have been achieved by minimizing the thermal resistance between the chip sensor and the nanogram sample. For flash DSC measurements, the polymers can be cooling either rapidly, leaving no time for homogeneous nucleation and crystallization processes, or slow enough to ensure both processes. Therefore, the transformation of crystal structure by flash DSC takes the advantage of a precise control of the processing condition.23,35 For instance, fast cooling at 5000 K s21 induces ferroelectric crystalline phases in P(VDF-TrFECFE) by cold crystallization when heating back to room temperature, while fast cooling scales down the size of ferroelectric crystal in P(VDF-TrFE).23 In this article, to shed light on the mechanisms of energy storage behavior, a P(VDF-CTFE) copolymer subjected to a variety of processing methods, annealed by different cooling rate and different annealing temperature, has been fully characterized by flash DSC, X-ray diffraction (XRD), FTIR, dielectric, and ferroelectric measurements. Hence, by varying the processing condition through flash DSC, the crystalline structure can be converted between the paraelectric and ferroelectric phases in the same polymer, which provides an opportunity to study especially in the detailed crystal structure evolution, and their influence on dielectric and energy storages. What’s more, direct measurement of the crystallization rate of P(VDF-CTFE) and 3D profiles of enthalpy and melting point in the whole temperature range is an extremely significant work on crystalline structure evolution, which will help to design and tailor this unique class of high-energy-density dielectric materials for capacitor application. EXPERIMENTAL
Materials P(VDF-CTFE) (91/9 mol%) was purchased from Solvay.
1246
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
JOURNAL OF POLYMER SCIENCE
Preparation of P(VDF-CTFE) Films at Different Conditions The samples were first hot-pressed at 220 8C, and then were cooled down to room temperature at slow cooling rate of 0.1 K s21 and 1 K s21 by hot and cold stages (Instec HCS302), or with liquid nitrogen (LN) instead of the fast cooling rate, named as Q0.1, Q1, and QLN, respectively. Besides, the samples first were quenched by LN, and then annealed at different annealing temperature (110, 60, and 30 8C) for 1000 s, named as S1, S2, and S3, respectively. Finally, the thickness of the films was about 30–40 lm. Dielectric and Charging/Discharging Properties Dielectric spectra were acquired over a broad frequency (103–106 Hz) using TH2828 LCR meter (Tonghui Electronics). The unipolar D-E loops were acquired by Radiant Technologies Precision Premier II equipped with 10,000V amplifiers at the frequency of 10 Hz. Gold electrodes of 60 nm thick were sputtered on both surfaces of the polymer films for electric measurements. Flash DSC Measurements Fast-scan measurements were performed using commercially available chip-calorimeter (Flash DSC1, Mettler-Toledo Co.) with mechanical-intercooler and nitrogen purge gas. The calorimetric chip-sensor was conditioned five times and corrected one time according to the standard procedure before use. The bulk samples were cut into small pieces (about 100 nanogram) and then were transferred to the sensor center within an area of 0.2–0.5 mm2. Temperature Programs We start our observations from isothermal annealing of fastcooled PVDF at various temperatures and the half crystallization time which is calculated. The sample was first melted at 240 8C for 60 s to erase its thermal history and then was cooled to a specific temperature for a certain period up to 6400 s before quenching and heating scans at the fixed cooling and heating rates of 5000 and 1000 K s21, respectively. The temperature programs of our isothermal measurements are depicted in Figure 1(a,b). Next, we observed the cooling curves of apparent heat capacities of P(VDF-CTFE) under various cooling rates, followed with their heating curves under fixed high heating rates. In details, PVDF sample was first melted at 240 8C for 60 s to erase its thermal history and then was cooled with various cooling rates (–5 to 25000 K s21) down to 280 8C; after 10 s, it was heated back from 280 to 240 8C under a rate of 1000 K s21. The temperature programs of our cooling measurements are depicted in Figure 1(c). Structure Charaterization The wide-angle XRD measurements were carried out by a D8 Advance X-ray generator with a copper target. The wavelength used was 1.5406 3 10210 m. The FTIR spectra at room temperature and temperature-dependent FTIR spectra were recorded with the films using a Bruker FTIR spectrometer with the model Vertex 70v.
JOURNAL OF POLYMER SCIENCE
WWW.POLYMERPHYSICS.ORG
FULL PAPER
FIGURE 1 Temperature–time profiles of fast-scan measurements. The samples were prepared (a) by quenching at a rate of 5000 K s21, followed by isothermal crystallization at various temperature for 1000 s, then by heating and melting at constant heating rates; (b) at the cooling rate 25000 K s21 at the selected temperature for various time durations up to 6400 s; (c) by crystallization at different cooling rates, followed by melting at high-rate heating. [Color figure can be viewed at wileyonlinelibrary.com] RESULTS AND DISCUSSION
Energy Storage Characteristics under Different Processing Conditions P(VDF-CTFE) copolymers have also been shown to generate large electric polarization, high energy density, and rapid polarization switching, which is used as an energy storage capacitor. Especially, in terms of understanding the fundamental mechanisms of crystal nucleation and growth and how to tune crystal structure will achieve excellent energy storage capabilities. Various processing methods, such as different processing temperature, uniaxial stretching, and electric poling field, are used to alter the crystalline structure. Here, P(VDF-CTFE) samples, which are first quenched by LN and then annealed at three typical annealing temperature 110, 60, and 30 8C, were chosen to compare the crystal structure evolution and their related energy storage. The effects of annealing temperature on the dielectric properties of P(VDF-CTFE) are presented in Figure 2(a). It is obvious that the annealing temperature has a great effect on the polymer with the dielectric constant at 1 kHz increasing from 12.9 to 14.9, with the decrease in annealing temperature from 110 to 30 8C. At low electric fields, relative dielectric constant mostly reflected the mobility of the dipole moments in the amorphous phase and small crystal domains. To some extent, the increasing dielectric constant observed in P(VDF-CTFE) was attributed to its relatively lower crystallinity and smaller crystal domain. Among these three
WWW.MATERIALSVIEWS.COM
samples, the P(VDF-CTFE) annealed at 30 8C had the lowest crystallinity and smallest crystal domain. Therefore, the response of the dipole moment was the highest. On the other hand, though higher dielectric constant were obtained in the P(VDF-CTFE) with decreasing processing temperature, much higher dielectric loss tangent (tan d) was simultaneously were observed at the full range of frequency (1000 Hz to 1 MHz), as shown in Figure 2(a). For example, the tan d of S1 at 1 kHz is 0.053, and 0.056 for S2. Furthermore, the temperature dependence on the D-E loops for fast cooled P(VDFCTFE) at different electric field is presented in Figure 2(b). At the same electric field of 200 MV m21, fast cooled P(VDFCTFE) annealed at 30 8C (S3) exhibits a high level of electric displacement about 5.1 lC cm22, while that of the sample annealed at 110 8C (S1) is only 3.8 lC cm22. However, when the electric field is reduced to zero, the polymer annealed at 30 8C exhibits higher remnant polarization than that annealed at 110 8C. This observation may be closely related to the formation of the tiny b-phase in low annealing temperature, as proved by the following XRD results. For capacitor applications, the charged and discharged energy density can be directly calculated from unipolar D-E loops and discharged efficiency is defined as 100Ue (discharged)/Ue (charged), as shown in Figure 2(c,d). With a decrease in the annealing temperature, fast cooled samples show enhancement in discharged energy density, but reduction in discharged efficiency. For instance, the discharged energy densities of S1 is about 8.0 J cm23 at the maximum electric field of 260
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
1247
FULL PAPER
WWW.POLYMERPHYSICS.ORG
JOURNAL OF POLYMER SCIENCE
FIGURE 2 P(VDF-CTFE) annealed at 110, 60, and 30 8C for 1000 s (S1, S2, and S3). (a) Dielectric spectra and loss tangent, (b) unipolar D-E hysteresis loops, (c) discharged energy density, and (d) charged–discharged efficiency as a function of the field strength. [Color figure can be viewed at wileyonlinelibrary.com]
MV m21, and those of S3 is 9.9 J cm23 at the maximum electric field of 270 MV m21. However, the efficiency of the samples S1 is 80.8%, while that of the samples S3 is only 77%. Based on the above discussions, it is necessary to further investigate into an understanding of structure–property relationships, to create ferroelectric polymer systems with excellent energy storage capabilities. Isothermal Crystallization at Various Temperatures and Time Durations The enhancement of energy density is closely related to the semicrystalline structure of P(VDF-CTFE). In general, very slow cooling and heating rates in the conventional DSC cannot effectively track crystallization behavior and microstructure evolution due to the fast crystallization ability of PVDFbased polymers. Flash DSC with rapid cooling and heating rate overcomes this shortcoming, and offers more detailed and useful way to study the microstructure evolution of P(VDF-CTFE), particularly at low temperature. Temperature programs of our isothermal measurements are depicted in Figure 1(a,b). In Figure 3(a), the heating curves of apparent heat capacities of P(VDF-CTFE) prepared at various isothermal temperatures for 1000 s are summarized. One can see a small peak, which may be ascribed to the melting of secondary crystals formed during annealing at the preset temperatures, shifts from 32 8C to higher temperature until it merges with the dominant melting peaks at about 135 8C with an increase in isothermal temperatures. Meanwhile, the high-temperature
1248
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
melting peaks around 131 8C, which is ascribed to the melting of primary PVDF a-phase crystals formed during the first cooling at 25000 K s21, are insensitive to the isothermal temperatures and keep constant. Even more interestingly, further details of the second melting peak at low temperature 30 8C can be observed by flash DSC, as shown in Figure 3(b). When the annealing time is below 0.04 s, a broad coldcrystallization peak appears. With the increase in annealing time at 30 8C, the peak at 45–75 8C became increasingly stronger, suggesting that the low temperature peaks must be secondary crystals formed during annealing at 30 8C. Furthermore, 3D profiles of enthalpy, which is closely related to the second crystal size, is dependence on the annealing temperature and time, as shown in Figure 3(d). Furthermore, its crystallization half-time (t1/2) can be obtained in the time window of our observation, as demonstrated in Figure 3(c). The t1/2, which is an expression of the overall crystallization rate, is plotted as a function of isothermal crystallization temperatures, as shown in Figure 3(d), and X–Y projection of t1/2 is depicted clearly in Figure 3(e). It is observed that P(VDF-CTFE) shows two inversely bell-shaped curve of overall t1/2 (inversely proportional to the overall crystallization rate) as explained by the classical theory of nucleation and growth, as was functionalized with the well-known Turnbull–Fisher equation.36 Two minima were observed, which are related to that the high temperature minimum is the fast growth rate of the primary crystallization and the low temperature minimum is the fast growth rate of the secondary crystallization. The sample annealed at low temperature forms second crystal between neighboring primary crystals
JOURNAL OF POLYMER SCIENCE
WWW.POLYMERPHYSICS.ORG
FULL PAPER
FIGURE 3 Heating curves of heat capacities of fast-cooled P(VDF-CTFE) after annealed (a) at various temperatures for 1000 s, and (b) at various time durations of the selected temperature Tx 5 30 8C, following the temperature program shown in Figure 1(a,b). (c) 3D profiles of enthalpy curve dependence of various annealing temperature and time. Among them, the curves of t1/2 as function of temperature and X–Y projection point with the red mark is labeled out. (d) Logarithmic time-enthalpy curve during isothermal crystallization at 30 8C. (e) Isothermal t1/2 as a function of temperatures is extracted from Figure 3(d). (f). 3D profiles of the maximum melting point dependence of various annealing temperature and time. [Color figure can be viewed at wileyonlinelibrary. com]
due to homogeneous nucleation, while the sample annealed at high temperature forms a-phase crystal due to the heterogeneous nucleation.37 Besides, 3D profiles of the maximum melting point, which may be ascribed to the second crystal size, is dependence on temperature and time, as shown in Figure 3(f). Based on 3D profiles of enthalpy and melting point, the details of crystal structure evolution are clearly revealed by flash DSC. At low temperature, the crystallites have smaller lamellar thickness and more ferroelectric phase than that at high temperature, which affords high energy density, but reduction in discharged efficiency. Our observations contribute to further understanding and modification of energy storage behavior in P(VDF-CTFE).
WWW.MATERIALSVIEWS.COM
Crystal Structure Evolution under Different Cooling Rate Based on the above discussions, the small crystal and optimization of a/b-phase ration is the best choice to achieve high energy density. Flash DSC with fast cooling rate possesses excellent advantage to tune crystal size and the transformation of crystal structure. The cooling and subsequent heating curves of P(VDF-CTFE) under various cooling rates are summarized in Figure 4. At the cooling rate of 10 K s21, the exothermic peaks around 96 8C are ascribed to the crystallization of a phase. One can see that when the cooling rates increase from 210 to 25000 K s21, the crystallization peaks shift down to lower temperatures about 16 8C with less latent heat, implying the scaling down of lamellae thickness as well as the lowered crystallinity.
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
1249
FULL PAPER
WWW.POLYMERPHYSICS.ORG
JOURNAL OF POLYMER SCIENCE
FIGURE 4 (a) Cooling and (b) subsequent heating curves of apparent heat capacities of P(VDF-CTFE) under the cooling rates varied from 21 to 25000 K s21 as labeled near the corresponding curves and the fixed heating rates 1000 K s21, following the temperature program shown in Figure 1(c). Curves are shifted off vertically to benefit parallel comparison. [Color figure can be viewed at wileyonlinelibrary.com]
Therefore, the melting peak around 148 8C were found in the subsequent heating curves, which shifts down toward low temperature with the increase in cooling rates; in particular, it decreases from 149 to 143 8C for an increase in cooling rate from 1 to 100 K s21. For cooling rates higher than 100 K s21, this exothermal peak is less sensitive to the cooling rate and remains constant, as shown in Figure 2(b). During fast cooling, the degree of crystallinity is lower, and thus smaller crystallites are formed in the quenched or fast cooling P(VDF-CTFE). Energy Storage Characteristics under Different Cooling Rate The room temperature dielectric constant and the corresponding dielectric loss tangent as a function of frequency
for P(VDF-CTFE) quenched at the cooling rate of 0.1 and 1, and LN are shown in Figure 5(a). It can be found that there is an increase in dielectric constant and loss tangent over all frequencies as the samples are quenched by the increasing cooling rate. For instance, relative dielectric constant at 1 kHz of P(VDF-CTFE) quenched at the slow cooling rate of 0.1 and 1 K s21 is 9.9 and 8.4, and while that of P(VDFCTFE) quenched by LN is 14.5. With the increasing cooling rate, the increasing dielectric constant observed in P(VDFCTFE) was attributed to its relatively lower crystallinity and smaller crystal domain, and the increasing dielectric loss may be closely related to the generation of b phase. Among these three samples, the P(VDF-CTFE) quenched by LN had the lowest crystallinity and smallest crystal domain.
FIGURE 5 (a) Dielectric spectra and loss tangent (tan d) of P(VDF-CTFE) quenched at the cooling rate of 0.1, 1 K s21, and LN (Q0.1, Q1, and QLN). (b) Unipolar polarization hysteresis loops, (c) The charged and discharged energy densities of Q1 and QLN as a function of the field strength. (d) Charged–discharged efficiency as a function of the field strength. [Color figure can be viewed at wileyonlinelibrary.com]
1250
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
JOURNAL OF POLYMER SCIENCE
WWW.POLYMERPHYSICS.ORG
FULL PAPER
FIGURE 6 (a) Room temperature XRD patterns, and (b) the corresponding FTIR spectrum of P(VDF-CTFE) quenched separately at the cooling rates of 20.1, 21, and LN (Q1, Q2 and QLN), respectively. (c) Room temperature XRD patterns of S1, S2, and S3. (d) The temperature-dependent FTIR spectrum of the P(VDF-CTFE) film in the temperature range 25–120 8C, and the film first quenched by LN. [Color figure can be viewed at wileyonlinelibrary.com]
Therefore, the response of the dipole moment was the highest to test the hypothesis that fast cooling can scale down the crystalline domains, and then improve charged energy storage behavior, the unipolar polarization loops were measured for P(VDF-CTFE) quenched at different cooling rate as presented in Figure 5(b). With the gradual increase in field strength, electric displacement of P(VDF-CTFE) reaches maximum, indicating storage of electric energy in the copolymer, while steadily reduction in filed strength leads to electric energy discharging from the copolymer. Interestingly, under the relatively high field strength of 200 MV m21, the film quenched at cooling rate of 1 K s21 is 3.2 lC cm22, while that of P(VDF-CTFE) quenched by LN has high polarization level of 5.2 lC cm22. For dielectrics, the energy density is Ð equal to the integral U 5 EdD. The dependence of the charged and discharged energy densities on the field strength is summarized in Figure 5(c). Under the same electric fields, with respect to P(VDF-CTFE) quenched at slow cooling rate, P(VDF-CTFE) quenched at rapid cooling rate exhibits the enhancement in charged and discharged energy density. For instance, the charged and discharged energy densities of P(VDF-CTFE) quenched at slow cooling rate of 1 K s21 is about 10.8 and 8.5 J cm23 at the maximum electric field of 280 MV m21, while those of the sample quenched by LN is 13.3 and 10.6 J cm23 at the maximum electric field of 270 MV m21. Besides, discharged energy density of the quenched sample is also higher than the sample annealed at 30 8C (i.e., 9.9 J cm23 at the maximum electric field of 270 MV m21). Unfortunately, the efficiency of the sample S1 is
WWW.MATERIALSVIEWS.COM
82.7%, while that of the sample S3 is only 78.7% at the field of 200 MV m21. It indicates that fast cooling can scale down the thickness of the paraelectric crystals, and leads the dipoles crystals to switch easier than the pristine P(VDFCTFE) during charging, and thus enhances the electric saturated polarization, charged–discharged efficiency and improved energy density at the same electric field. Crystalline Phase Characterization We performed further characterization of polymorphic crystalline phases of P(VDF-CTFE) under various cooling rates, using wide-angle X-ray diffraction (WAXD) and FTIR, as summarized in Figure 6(a,b), respectively. Room temperature WAXD patterns of P(VDF-CTFE) crystals exhibit several peaks in Figure 6(a). P(VDF-CTFE) samples quenched at slow cooling rate of 0.1 and 1 K s21 exhibit four peaks at 17.3, 18.4, 19.9, and 26.8, which can be assigned to (100), (020), (110), and (021) reflections of the nonpolar crystalline a-phase similar to PVDF, respectively. The corresponding interplanar spacings (d) are 5.12, 4.85, 4.45, and 3.34 Å, as shown in Table 1. The corresponding characteristic absorption bands at 975, 796, 764, 534, and 484 cm21 shown in Figure 6(b), which have been ascribed to TG1TG– conformation in the crystalline a-phase, is dominant in slow cooled P(VDF-CTFE) samples.38,39 With the increase in cooling rates, the sample quenched by LN exhibits a new diffraction peak around 20.88 (d 5 4.25 Å), ascribed to the b-phase of VDF sequences, and generated by homogeneous nucleation upon heating back from fast cooling to the room temperature. The
JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2017, 55, 1245–1253
1251
FULL PAPER
WWW.POLYMERPHYSICS.ORG
TABLE 1 Lattice Constant and Coherence Length for the 110 Reflection of P(VDF-CTFE) Copolymers Treated with Different Conditions b-Phase d, A˚
a-Phase d, A˚
L, nm
Q0.1
4.45
10.3
Q1
4.48
12.1
QLN
4.28
L, nm
5.5
S1 S2 S3
4.28
8.6
4.48
5.5
4.47
10.8
4.47
9.8
4.47
7.5
corresponding bands at 509, 841, and 1284 cm21, which belong to Tn>53G and Tn>54G conformations in the crystalline b- or c-phase, gradually increases with increase in the cooling rate. Furthermore, based on calculations from the Scherrer equation and the full width at half maximum (instrument peak broadening was not taken into account), the crystallite size of the P(VDF-CTFE) which quenched at slow cooling rate of 1 K s21 is 12.3 nm, larger than that of the P(VDF-CTFE) (5.5 nm) which quenched by LN. It is reasonable that fast cooling rate can scale down the size of lamellar. Interestingly, there is a new ferroelectric phase that is formed and its size is about 5.5 nm as shown in Figure 6(b) and Table 1. The results are consistent with the observed results of flash DSC. We went further to compare the crystalline phases of two fast-cooled polymer samples after isothermal crystallization for 1000 s at various temperatures, and summarized the room temperature WAXD patterns and temperaturedependent FTIR spectrums in Figure 6(c,d), respectively. Obviously, fast-cooled P(VDF-CTFE) exhibit the reduction size of a-phase from 10.8 to 7.5 nm with the decrease in annealing temperature. Fast cooled P(VDF-CTFE) forms a new ferroelectric phase at low temperatures, as evidenced by the emerging peak around 2h 5 20.88 in Figure 5(c). Moreover, with respect to P(VDF-CTFE) quenched by LN, P(VDF-CTFE) annealed at low temperature exhibit the growth of ferroelectric phase from 5.5 to 8.6 nm shown in Table 1, which is consistent with the result that the melting peaking shifted to high temperature with the increase in annealing time shown in Figure 4(b). Meanwhile, temperature-dependent FTIR spectrum in Figure 6(d) and the inset again show the enhancement in the bands of 509, 841, and 1284 cm21, the slight reduction of the bands at 1276, 902, 843, 762, 534, and 484 cm21 and as function of temperature. On the other hand, the intensity of the band at 904 cm21, which corresponds to the amorphous component, increased significantly from 50 to 120 8C. As a word, the secondary crystals of P(VDF-CTFE) annealed at low temperature
