Crystal Structure Regulation of Ferroelectric Poly(vinylidene fluoride

Mar 29, 2017 - This is of great importance to fabricate high-quality thin films of γ-PVDF crystals .... photographic film development process with bi...
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Crystal Structure Regulation of Ferroelectric Poly(vinylidene fluoride) via Controlled Melt−Recrystallization Yiran Zheng, Jie Zhang, Xiaoli Sun, Huihui Li, Zhongjie Ren, and Shouke Yan* State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Polyvinylidene fluoride (PVDF) is a known polymer with pronounced polymorphs. Among these polymorphs, the β and γ form crystals with strongest piezo- and pyro-electric activities of polymeric materials have attracted much attention. We report here a simple melt−recrystallization approach for producing γ-phase-rich PVDF thin films through selective melting and subsequent recrystallization. The proposed approach comprises the selective melting of both the nonpolar α-PVDF and polar γ-PVDF crystals but not the stable γ′-PVDF ones and subsequent isothermal recrystallization at 160 °C. During the recrystallization process, the nonmelting stable γ′-PVDF crystals can induce the direct γ-PVDF crystallization of the melt. In contrast, locally ordered domains survived in the molten α-PVDF crystal regions also result in a direct formation of γ-PVDF crystals. This is of great importance to fabricate high-quality thin films of γ-PVDF crystals for advanced applications. to the thermodynamically more stable γ-PVDF crystals while keep the original morphology and outline of the original αphase. These crystals are frequently referred to as γ′-phase since they exhibit a higher melting temperature than the γ-PVDF crystals produced directly from melt crystallization.19 It should be pointed out that the phase transition of the α-PVDF crystals to the γ-crystals proceeds also very slowly.21 However, at relatively low crystallization temperatures, e.g., below ∼155 °C, the transition from α-phase to the γ-phase is of very limited extent, even after prolonged annealing at higher temperatures.19 Therefore, it is difficult to obtain γ-phase-rich films by isothermal melt crystallization. This leads to the development of a simple and highly efficient approach for preparing γ-phaserich PVDF thin film highly expected. To control the crystal structure, several skillful techniques have been frequently used. Surface-induced epitaxial crystallization is confirmed to be very efficient for regulating the crystal structure, molecular chain orientation, and special arrangement simultaneously.22−24 Self-nucleation, or selfseeding, provides another way for controlling the crystal structures and morphology of polymers.25−29 For example, Reiter et al.30 have successfully generated arrays of orientationcorrelated polymer crystals of uniform size and shape through a self-seeding approach. Very recently, a new crystallization pathway concerning the heterogeneous nucleation of one polymorph on another, referred to as cross-nucleation, has attracted much attention.31−40 Taking the slightly higher melting temperature of the γ-PVDF crystals compared to that of their α-counterparts, selective melting of the α-PVDF crystals and subsequent melt−recrystallization may provide an

1. INTRODUCTION Polyvinylidene fluoride (PVDF) is a known polymer with strongest piezo- and pyro-electric activities. It has been therefore the most widely investigated ferroelectric polymers due to good polarization, solvent compatibility, and easy process ability into thin, light, tough, and flexible films.1−4 It is now well-known that the piezo- and pyroelectricities of PVDF can be achieved only in its certain crystal structure, such as β-, γ-, and δ-phases.5−8 Among these, the β- and γ-phases are the electrically most active phases, and their promotion within the material is an ongoing pursuit due to the strong interest in application of many related areas.9 The PVDF can form, however, at least five different crystalline phases, indicated as α, β, γ, δ, and ε, under different conditions.10 Its nonpolar α-phase with two TGTG′ chains arranged in a monoclinic unit cell is the most common one when crystallized from melt at relatively low temperature.11,12 The polar β- and γ-phases can be fabricated only under some special crystallization processes and/or after post thermal or mechanical treatments.13−17 For example, the most popular polar β-phase with best piezoelectric, pyroelectric, and ferroelectric properties can be obtained by mechanical stretching of the nonpolar α-phase or crystallization from melt under specific conditions, such as using nucleation agents or external electric field.9,10 For the γphase, it can be obtained through melt−crystallization at extremely high temperatures and/or through solid−solid phase transition by annealing at temperatures close to the melting temperature of the α-PVDF crystals.17−19 It was reported that the growth rate of the γ-PVDF crystallites is many times lower than that of the α-PVDF ones at around 155 °C.19,20 Even though the difference in growth rates is progressively reduced with increasing temperature and can be ultimately reversed at very high temperature, the growth rates of both α- and γcrystals dropped significantly with temperature. The solid phase transition goes from the kinetically favored α-form spherulites © XXXX American Chemical Society

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February 9, 2017 March 24, 2017 March 29, 2017 March 29, 2017 DOI: 10.1021/acs.iecr.7b00543 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research efficient way for producing high-quality γ-phase-rich PVDF thin films owing to the aforementioned reasons. For example, after melting of the α-PVDF crystals, the surviving solid γ-PVDF crystals may induce crystallization of the PVDF melt in its γphase through a homoepitaxy or another phase through a crossnucleation pathway. Moreover, locally ordered domains previously included in the original α-PVDF crystal lattice may survive due to insufficient melting. These ordered domains may transform into the γ-form nuclei for minimizing the potential energy of the chains arising from internal steric and electrostatic interactions. This may also induce the crystallization of PVDF in its γ-form, known as a self-nucleation process. The purpose of this work is to check whether these can be employed to fabricate γ-phase-rich PVDF thin films. Here in this paper, the experimental details as well as some interesting results will be presented. The corresponding mechanisms are discussed accordingly.

Figure 1. Optical microscopic images of a PVDF thin film isothermally crystallized from melt at 155 °C for 90 h and then cooled to room temperature on air. (b) Enlarged part of a α-PVDF spherulite shown in (a) to show the banded feature.

spherulites), which differ mainly in birefringence. The spherulites with strong birefringence exhibit concentric extinction bands, which can be well-recognized in the enlarged micrograph shown in Figure 1b. The periodic light extinction band is caused by twisted lamellae appearing flat-on and edgeon alternatively as illustrated by the AFM image shown in Figure 2a, i.e., the occurrence of lamellar twisting along radial

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The PVDF used in our experiments was bought from Apollo Scientific Limited. The molecular weight was measured by GPC to be Mw = 2.3 × 105, with a dispersity of 1.2. Samples for optical microscopy observation were prepared by casting a drop of 20 mg/mL solution of PVDF in N,N-dimethylformamide on clean glass glides. Samples with relatively flat and smooth surface for atomic force microscopy (AFM) observation were prepared by spin-coating few drops of solution onto Si wafer at 100 °C to regulate the film thickness. The Si wafers were washed before use first by a mixture of concentrated sulfuric acid and hydrogen peroxide with volume ratio of 7:3 at 120 °C for 1 h, then by distilled water in an ultrasound bath several times, and finally dried under N2 atmosphere. The film thicknesses used both for optical and atomic force microscopy studies were estimated by ellipsometer to be around 60 μm. The samples have been heat-treated at 200 °C for 15 min to eliminate previous thermal history and subsequently quenched to desired temperatures (indicated in the corresponding text and figure captions) for isothermal crystallization. 2.2. Characterization. For optical microscopy observations, an Axioskop 40A Pol optical microscope (Carl Zeiss) equipped with a Linkam THMS600 hot stage was used in this study. The accuracy of the temperature control is ±0.1 °C. All of the pictures shown in this paper were taken under crossed polarizers. For AFM observations, an Agilent Technologies 5500 atomic force microscope (Agilent Technologies Co. Ltd., USA) was used. Tapping mode was used during AFM measurement. For FTIR analysis, a Spectrum 100 FT-FTIR spectrometer (PerkinElmer) was used. FTIR spectra in the wavenumber range from 450 to 4000 cm−1 were obtained by averaging 16 scans at a 2 cm−1 resolution.

Figure 2. AFM amplitude images present the structure of (a) α- and (b) γ-PVDF spherulites, respectively. The sample was prepared by isothermal crystallization from melt at 150 °C for 250 min.

direction of the spherulites.41−43 This is a typical feature of αPVDF spherulites, and these spherulites are confirmed to be the α-PVDF spherulites by selective melting at relative lower temperature, e.g., 171 °C. The other type of spherulites display weak birefringence. They are demonstrated by selective melting to be the γ-phase PVDF spherulites which melt at about 177 °C. The weak birefringence of the γ-PVDF spherulites is related to the fact that they are predominantly composed of disordered flat-on lamellae as revealed in the AFM image shown in Figure 2b.44 Figure 3a shows an optical micrograph of a PVDF thin film isothermally crystallized from melt at 160 °C for 100 h and then cooled naturally to room temperature. The morphology shown in Figure 3a is somewhat different from that shown in Figure 1a. Now we see only one big weak-birefringent spherulite in the middle of the picture but several strongbirefringent spherulites randomly dispersed in a matrix with microcrystallites. The big spherulites resulted from isothermal crystallization, while the microcrystallites were generated during cooling. In situ melting of the sample by heating at a rate of 2 °C/min helped to determine the crystal phases of PVDF crystallized isothermally at 160 °C and during cooling. As can be seen from Figure 3b, all of the microcrystallites and the strong-birefringent spherulites except for the five marked by the arrows in Figure 3a disappeared when the sample was heated to 171 °C. This indicates that all of the microcrystallites and most of the strong-birefringent spherulites are composed of α-PVDF

3. RESULTS AND DISCUSSION Figure 1 shows the optical micrographs of a PVDF thin film heat-treated at 200 °C for 15 min, crystallized isothermally at 155 °C for 90 h, and finally cooled naturally to room temperature. From Figure 1a, we see the big spherulites produced by isothermal crystallization and microcrystallites between the big spherulites crystallized during cooling process. This indicates that the crystallization of PVDF at 155 °C for 90 h is not complete. It is clear that there are two kinds of spherulites grown isothermally at 155 °C (the bigger B

DOI: 10.1021/acs.iecr.7b00543 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Optical micrographs taken during the melting process of a PVDF thin film, which was crystallized from the melt isothermally at 160 °C for 100 h and then cooled to room temperature on air. The heating rate was 2 °C/min. The pictures were taken at (a) room temperature, (b) 171 °C, (c) 177 °C, and (d) 187 °C.

Figure 4. Optical micrographs taken during the isothermal melt− recrystallization process of the PVDF sample shown in Figure 3d by cooling from 187 to 160 °C for (a) 8, (b) 50, (c) 80, (d) 150, (e) 200, (f) 230, (g) 300, (h) 405, and (i) 450 min. The arrows and ellipses mark the transformed and nontransformed original α-PVDF crystals, respectively, while the circles indicate the newly formed PVDF crystals at 160 °C.

crystals. After heating the sample to 177 °C, the big weakbirefringent spherulite melted; see Figure 3c. This means that the big weakly birefringent spherulite is composed of γ-PVDF crystals. It should be noted that the arrow-labeled strongbirefringent spherulites remain unchanged at 177 °C and melt until 187 °C (Figure 3d), indicating the occurrence of solidstate phase transition from α-phase to high-melting-point γ′phase. One can find that only the spherulites connected with the γ spherulite transformed into their γ′-counterparts which illustrates the importance of the adjacent γ spherulites on the solid phase transition.19 From Figures 1a and 3a, we can clearly see that the γ-PVDF spherulite grown at 160 °C exhibits an irregular morphology compared to those formed at 155 °C, which was also observed by Gregorio et al.17 Moreover, the γPVDF spherulite formed at 160 °C is much larger than the αPVDF spherulites grown under the same condition. It is also much larger than the γ-PVDF spherulite formed at 155 °C. This is related to the increased growth rate of γ-PVDF compared to its α-counterpart.20 It was well-documented that a polymer melt can preserve a strong memory of the pristine crystalline state, especially when the material is molten in the proximity of the tail of its melting endotherm. The memory effect exists noticeable effects on the subsequent crystallization process of the insufficiently relaxed melt. This phenomenon is often addressed as “melt memory” or “self-nucleation”.25−30,44−46 In the present case, the end of melting peak for γ′-PVDF crystals is about 187 °C. If there exists the self-nucleation effect of γ′-PVDF phase, then it may be of great significance for producing high content γ′-PVDF crystals. Therefore, the recrystallization process of the sample shown in Figure 3d was performed by quickly cooling from 187 to 160 °C soon after the disappearance of the birefringence of the γ′-phase. As presented in Figure 4a, the spherulites originally in the γ′phase appear again only 8 min after the temperature reached 160 °C. The spherulites form in a way similar to the photographic film development process with birefringence in the whole original γ′-phase region appearing again simultaneously and getting strongest within 50 min; see Figure 4b. At

the same time, the spherulites at places where the original isothermally grown α-PVDF spherulites were located (indicated by the ellipses) begin to crystallize; compare Figures 4b−d and 3a. These spherulites grew via normal melt− crystallization, i.e., started from the center nuclei and propagated radially outward gradually leading to the increase of spherulite size with time. After 200 min of isothermal crystallization at 160 °C (Figure 4e), some new crystalline nuclei (indicated by the circles) formed at places of original αPVDF microcrystallites crystallized during cooling. Moreover, a further growing thin layer surrounding the original γ′-PVDF spherulites was produced (Figure 4e), which can be more clearly observed with increasing crystallization time (Figure 4f− i). It should be pointed out that the big γ-PVDF spherulite observed in Figure 3a,b does not appear again. The above melt−recrystallization behavior can be understood in the following way. Owing to the difference in their melting point, the relaxation status of the PVDF molecular chains in spherulites of different crystalline forms is different. For the high melting point transformed γ′-PVDF crystals, some locally ordered molecular chains previously included in the crystal lattice can be preserved and retain memory of the original chain alignment. These locally ordered clusters act as self-nuclei and lead to the crystallization of the molten γ′-PVDF crystals in a manner similar to the photographic film development process. This has also been observed in a previous study on the melting and recrystallization of isotactic polypropylene.47,48 For the low melting temperature α- and γ-PVDF crystals, the recrystallization of the molten crystals represents a more or less normal melt−crystallization process due to a higher relaxation extent of the molecular chains. During the melt−crystallization process, the appearance of some original α-PVDF spherulites again at the same places implies that they are caused either by selfnucleation or by heterogeneous nucleation. Moreover, the further growth of the spherulites generated by the molten γ′PVDF crystals is approximately at the same time as the spherulites grown at the places of original α-PVDF spherulites. C

DOI: 10.1021/acs.iecr.7b00543 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research This demonstrates that the early recrystallized molten γ′-PVDF crystals do not propagate continuously toward the PVDF melt but rather exhibit nucleation ability toward the PVDF melt. As a consequence, a uniform thin layer surrounding these spherulites was created. To check the crystal form of the spherulites grown at different stages shown in Figure 4i, in situ melting of them was monitored via optical microscope. Figure 5 shows the melting process of the crystals shown in Figure 4i. When heating the sample to 173 °C, some of the

shown in Figure 4 were obtained from the in situ optical micrographs. It was found that the α-PVDF spherulites grow at a rate of 0.078 μm/min, which is slightly higher than that reported in ref 49 (0.078 vs 0.064 μm/min). This should be caused by incomplete relaxation of the molecular chains due to the low melting temperature. The relatively extended molecular chains can be more easily packed into the crystal lattice. Moreover, the γ-PVDF spherulites grown at the original αPVDF spherulites show a growth rate of 0.079 μm/min. It is slightly higher than the α-PVDF spherulites (0.079 vs 0.078 μm). This implies that the regrowth of those α-PVDF spherulites into γ-PVDF spherulites is also a kinetically controlled process. It should be pointed out that the growth rate of the γ-PVDF spherulites in the present condition is much higher than those grown from the isotropic melt (0.079 vs 0.014 μm).49 This also reflects the great influence of oriented molecular chain segments on the crystallization kinetics of long chain polymers.50 Another well-known phenomenon is that the γ-PVDF spherulites grown from melt exhibit much weaker birefringence compared to that of their α-counterparts. The γ-PVDF spherulites shown in Figure 4 actually exhibit the same birefringence as the α-ones. This is related the different crystal orientation. Figure 6 shows the AFM images of the γ-PVDF spherulites grown from the original α-PVDF spherulites. It can be seen that these γ-PVDF spherulites share more or less the αPVDF spherulites (compare Figures 6 and 2a). The existence of abundant edge-on lamellae is the reason for the strong birefringence of regrown γ-PVDF spherulites. According to the above results, two points should be addressed here. First, we discuss about the recrystallization of γ′-PVDF into γ-PVDF crystals. It was well-characterized that the γ′- and γ-PVDF crystals have the same crystal structure with the same molecular chain conformation. Therefore, the surviving locally ordered molecular chains can transform easily into γ-PVDF crystalline nuclei and initiate the growth of the γPVDF crystals. Second, for the γ-PVDF spherulites formed at places of the original α-PVDF spherulites, it was welldocumented that α-PVDF crystals can transform into γ′PVDF crystals through solid phase transition.17−19 The solid phase transition involves chain movement of localized sevenbond rotation.51 This should be more easily achieved for the surviving locally ordered molecular chains in α-PVDF crystal lattice owing to the high mobility of the molecular chains. As a consequence, γ-PVDF crystallization from molten α-PVDF crystals takes place. It is now clear that insufficient melting of both γ′- and αPVDF crystals is in favor of γ-PVDF recrystallization. The question arises about whether or not the solid state γ′-PVDF crystals can induce the crystallization of PVDF melt and produce similar γ′-PVDF crystals. To answer this question, another experiment was carried out as shown in Figure 7. The sample was first crystallized isothermally at 160 °C for 100 h, ensuring the formation of some transformed γ′-PVDF spherulites, then heated to 181 °C for melting all other crystals except for the transformed γ′-PVDF spherulites (see Figure 7a), and finally cooled to 160 °C for isothermal crystallization. As presented in Figure 7b, during the cooling process from 181 to 160 °C, the γ′-PVDF spherulites become a little bit bigger, and the birefringence of them gets stronger. This indicates the occurrence of secondary crystallization and further growth of the existing spherulites. The further growth of the γ′-PVDF spherulites is more evident after isothermal growth for 12 h at

Figure 5. Optical micrographs of the PVDF sample shown in Figure 4i taken during the melting process at (a) 160 °C, (b) 173 °C, and (c) 177 °C.

Figure 6. AFM height (left) and phase (right) images present the structure of recrystallized γ-PVDF spherulites in place of the original α-PVDF spherulites. The sample was prepared exactly the same way as the sample shown in Figure 4.

spherulites disappeared completely; see Figure 6b. These molten spherulites belong to the α-form PVDF. It should be pointed out that the remaining spherulites are mainly those recrystallized from the originally transformed γ′-PVDF spherulites, including their induced surrounding parts. A few spherulites, as indicated by the arrows in Figure 5b, melt partially at 173 °C. One may associate these nonmelting parts to the transformed α-PVDF spherulites. This is not the case since all crystals melt completely at 177 °C (Figure 5c), indicating that the absence of α−γ transition. From the above experimental results, following conclusions can be made: (i) Melting the γ′-PVDF crystals at 187 °C and recrystallization at 160 °C ensures the formation of γ-PVDF crystals, which can further induce γ-PVDF crystallization of the melt. (ii) Some new γ-PVDF spherulites have been created at places of the original α-PVDF spherulites, as indicated by the arrows in Figure 5b. (iii) Unlike the γ-PVDF spherulite grown from isotropic melt, as shown in Figure 3a, the γ-PVDF spherulites formed via insufficient melting and subsequent recrystallization exhibit a morphology similar to that of their α-counterparts; compare the spherulites in Figures 4i and 5a with those in Figure 3a. (iv) The solid phase α−γ transition does not occur in the melt−recrystallization process for 450 min. It was well-documented that the selective crystallization of different phases is generally a kinetically controlled process. Therefore, the crystallization kinetics of those PVDF crystals D

DOI: 10.1021/acs.iecr.7b00543 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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According to these results, two aspects should be discussed here. First, the further growth of the γ′-PVDF spherulites in the γ-form can be associated with homoepitaxy since they exhibit exactly the same crystal structure. It can, however, also be induced by cross-nucleation. Cavallo et al.51 has recently confirmed that the form I poly(1-butlene) crystals can serve as seeds for the growth of its form II crystals at their periphery through cross-nucleation. This has also been frequently observed for some other polymers.27−29 Unfortunately, we cannot distinguish whether the homoepitaxy or crossnucleation plays a role at moment. Second, the fact that both the molten γ- and α-PVDF crystals at 181 °C grow into γPVDF crystals at 160 °C may be understood in the following way. By melting PVDF γ-crystals at 181 °C, which is quite close to its nominal peak melting temperature (around 177 °C), the relaxation of molecular chain packed in the crystal lattice is limited. As a result, abundant locally ordered molecular chains with the same conformation as in the γ-PVDF crystal lattice survived and served as nuclei for initiating the growth of PVDF crystals directly in its γ-form. This is similar to the melt recrystallization of form I isotactic poly(1-butene) as reported by Li et al.52 For the α-PVDF crystals, more locally ordered domains will survive at 181 °C than at 187 °C. Therefore, as already discussed above, the molten α-PVDF recrystallizes into γ-PVDF through self-nucleation coupled with a conformational adjustment in the ordered domains. Moreover, the nucleation ability of α-PVDF is expected to decrease significantly at 160 °C, which is close to the melting temperature of α-PVDF crystals. This is also much beneficial for the γ-PVDF recrystallization. According the above experimental results, it is clear that melt−recrystallization of γ′-PVDF produces γ-PVDF crystals, which can further induce the growth of γ-PVDF into the melt (Figure 5). Solid γ′-PVDF spherulites can also induce the growth of γ-PVDF into the melt (Figure 7). By comparing Figures 8a and 5, we further found that melting of the α-PVDF crystals at a relatively low temperature, e.g., 181 °C, is more effective for a quick recrystallization with enhanced nucleation density of γ-PVDF. This clearly indicates the remarkable influence of melting status of α-PVDF crystals on the subsequent crystallization process. At a relatively low melting temperature, there are domains consisting of locally ordered molecular chain segments previously included in crystal lattice. These ordered segments promote the crystallization rate and influence the selection of polymorph due to the change of free energy for nucleation. A similar effect has been reported for some other polymers with polymorphisms.27−29 A recent study by Men and co-workers on the melt−recrystallization of random butene-1/ethylene copolymer shows that the selection of different polymorphs depends strongly on an interplay between the size of ordered domain as well as the size and energy barrier of the critical nucleus corresponding to different crystal forms.53,54 In our case, when the sample was melted at 187 °C, a higher extent of relaxation of the molecular chains in α-PVDF crystals is reached, or at most, only some small locally ordered domains exist in the melt of the original α-PVDF. These ordered domains may be not big enough to transform into γ-PVDF nuclei and trigger the crystallization of PVDF in γform. However, as schematically presented in Figure 10, when the sample was melted at a relatively low temperature, e.g., 181 °C, abundant locally ordered domains with sufficient size may exist in the melt of the original α-PVDF. Normally, these locally ordered domains will induce crystallization upon cooling in the

Figure 7. Optical micrographs of a PVDF sample first crystallized isothermally at 160 °C for 100 h, then heated to 181 °C for complete melting of all α and γ crystals except for the transformed γ′-PVDF spherulites (a) and finally cooled to 160 °C (b) and isothermally crystallized for 12 h at 160 °C (c). The circled parts indicate the PVDF spherulites grown in its α-form.

160 °C (Figure 7c), which leads to the formation of a weakly birefringent surrounding layer of the γ′-PVDF spherulites. Moreover, many new spherulites were generated. During the melting process (Figure 8), it was found that only a few of the

Figure 8. Optical micrographs of the PVDF sample shown in Figure 6c after heating to (a) 169 °C and (b) 177 °C. The α-PVDF spherulites in the circled parts have been melted away at 169 °C, while the others except for the γ′-PVDF spherulites melted out at 177 °C.

newly formed spherulites melted away when the sample was heated to 169 °C (compare the circled parts of Figures 7c and 8a, which correspond to the α-PVDF crystals). Most of the recrystallized spherulites including the further grown surrounding layer of the γ′-PVDF spherulites melt at 177 °C, reflecting the formation of γ-form PVDF crystals. This has also been confirmed by the infrared results presented in Figure 9. It is clear that the intensities of the absorption bands corresponding to γ-PVDF crystals increase remarkably after recrystallization, while those for the α-PVDF crystals decrease accordingly. This confirms the occurrence of recrystallization of γ-PVDF spherulites from the original α-PVDF spherulites.

Figure 9. FTIR spectra in the region of 1500−500 cm−1 of a PVDF sample crystallized from the melt isothermally at 160 °C for 100 h (in black) and after heating to 181 °C to melt the α and γ crystals and then cooling to 160 °C for isothermal crystallization for 14 h (in red). E

DOI: 10.1021/acs.iecr.7b00543 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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relatively high. Consequently, mainly α-PVDF crystals formed again with only some of the α-PVDF spherulites growing in γform PVDF crystals. These γ-PVDF crystals should also result from the locally ordered molecular chains previously included in the α-PVDF crystal lattice, which undergo conformational adjustment as happened during solid−solid state α−γ′-phase transition. By heating the PVDF sample to 181 °C, which is lower than the melting point of γ′-PVDF crystals but higher than the melting point of other PVDF crystals, the solid γ′-PVDF crystals also induce the crystallization of PVDF melt directly in its γ-form. This is originated either from homoepitaxy when the exact same crystal structure of γ- and γ′-PVDF crystals is considered or from cross-nucleation. For the molten α-PVDF crystals, their recrystallization behavior is quite different from those molten at 187 °C. By melting at 181 °C, more locally ordered domains previously included in crystal lattice of αPVDF remain in the melt. These domains also have a relatively higher order compared with those surviving at 187 °C. As a result, they can more effectively induce crystallization of the PVDF melt in its γ-form through adjustment of the chain conformation. This provides a simple way to produce highcontent γ-PVDF films.

Figure 10. Schematic representation shows the melting of the α-PVDF crystals and subsequent recrystallization in its γ-form triggered by locally ordered domains. The locally ordered α domains with TGTG′ conformation due to insufficient melting can transform more easily to the T3GT3G′ owing to the higher chain mobility and therefore induce the formation of γ-PVDF crystals.

same crystalline modification known as self-nucleation process. The situation for the PVDF is, however, different. It is welldocumented that phase transformation from α to γ can even take place in solid state through molecular chain segmental flipflop and inversion motions at elevated temperature, leading to formation of γ form, i.e., the frequently named γ′-phase.51,55 During the melting process, while most of the molecular chains get fully relaxed, the molecular chains in the locally ordered domains are expected to more easily adjust their conformation and transform into γ-PVDF nuclei than in solid state. It is these ordered domains that trigger the recrystallization of PVDF in γform. This provides an efficient way to produce the films with rich γ-PVDF crystals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0086-10-64455928. Fax: 0086-10-64455928. ORCID

Huihui Li: 0000-0001-5745-4079 Shouke Yan: 0000-0003-1627-341X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundations of China (Nos. 21434002 and 51521062) and the program of Introducing Talents of Discipline to Universities (B08003) is gratefully acknowledged.

4. CONCLUSIONS Thin films of PVDF containing α-form, γ-form, and transformed γ′-form crystals were obtained by long-time isothermal crystallization of the isotropic PVDF melt at 160 °C. With the advantage of diverse melting temperatures of these PVDF crystals in different phases, melting of PVDF crystals at different temperatures and subsequent recrystallization of the molten PVDF was followed by using optical microscope. It was demonstrated that the melting status of PVDF crystals plays an important role on the polymorph selection of the recrystallized PVDF. By heating the PVDF sample to 187 °C, melting of all kinds of PVDF crystals including the transformed high melting point γ′-PVDF crystals was evident. This can be judged from the nonbirefringent feature of the sample under optical microscope with crossed polarizers. The molecular chain relaxation of PVDF originally in different crystal forms is, however, not the same. This results in the different crystallization behavior when cooled down to 160 °C again. The original γ′-PVDF crystals appear quickly in a way similar to the photographic film development process. They grow in γ-PVDF crystals owing to the existence of abundant locally ordered molecular chains previously included in the γ′-PVDF crystal lattice, which produce γ-PVDF crystalline nuclei and therefore trigger the γPVDF crystal growth directly from the melt. These early formed γ-PVDF crystals were found to exhibit the ability for inducing further growth of the γ-PVDF crystals. For the αPVDF crystals, the relaxation of the PVDF molecular chains is

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