Stretching-Induced Relaxor Ferroelectric Behavior in a Poly

Article pubs.acs.org/Macromolecules

Stretching-Induced Relaxor Ferroelectric Behavior in a Poly(vinylidene fluoride-co-trifluoroethylene-cohexafluoropropylene) Random Terpolymer Yue Li,†,‡ Thibaut Soulestin,§,∥ Vincent Ladmiral,§ Bruno Ameduri,§ Thierry Lannuzel,∥ Fabrice Domingues Dos Santos,∥ Zhong-Ming Li,† Gan-Ji Zhong,*,† and Lei Zhu*,‡ †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, Sichuan, P. R. China ‡ Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United States § Ingénierie et Architectures Macromoléculaires (IAM), UMR 5253 CNRS, ENSCM, UM, Institut Charles Gerhardt de Montpellier, 8, rue de l’Ecole Normale, 34296 Cedex 5 Montpellier, France ∥ Piezotech S.A.S., Arkema-CRRA, rue Henri-Moissan, 69493 Cedex Pierre-Bénite, France S Supporting Information *

ABSTRACT: Relaxor ferroelectric (RFE) polymers exhibiting narrow hysteresis loops are attractive for a broad range of potential applications such as electric energy storage, artificial muscles, electrocaloric cooling, and printable electronics. However, current state-of-the-art RFE polymers are primarily poly(vinylidene fluoride-co-trifluoroethylene-co-X) [P(VDF-TrFE-X)] random terpolymers with X being 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE). Potential dehydrochlorination at elevated temperatures can prevent the melt-processing of these Cl-containing terpolymers. It is desirable to achieve the RFE behavior for Cl-free terpolymers such as P(VDF-TrFE-HFP), where HFP stands for hexafluoropropylene. Nonetheless, HFP units were mostly excluded from the crystalline structure because of their large size, and thus no RFE behavior was observed when crystallized from the quiescent melt. Intriguingly, mechanical stretching could effectively pull the HFP units into the P(VDF-TrFE) crystals, forming nanosized ferroelectric (FE) domains with a strong physical pinning effect. Consequently, the RFE behavior was observed for the uniaxially stretched P(VDF-TrFE-HFP) film. Thermal annealing above the Curie temperature (ca. 50 °C) without tension led to the return of the normal FE behavior with broad hysteresis loops. However, thermal annealing above Curie temperature under tension prevented the exclusion of HFP units from the crystalline structure, and thus relatively stable RFE behavior was achieved. Various characterization techniques were utilized to unravel the structure−property relationships for these P(VDF-TrFE-HFP) films. In addition, the RFE behavior of P(VDF-TrFE-HFP) was compared to those of other terpolymers. This study provides a unique and simple strategy solely based on film processing to achieve the RFE behavior for P(VDF-TrFE)-based terpolymers.

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INTRODUCTION Dielectric constants of most insulating polymers are limited to 2−5 because of the covalent nature of chemical bonds in organic compounds.1,2 To enhance the dielectric constants of polymers, an effective strategy is to utilize orientational (or dipolar) polarization of dipolar functional groups.3−5 For example, liquid water exhibits a high dielectric constant of 80 and a relaxation peak at 20 GHz at room temperature.6,7 To achieve the high dielectric constant and low loss properties as those of water in the power frequency range (3), and 1287 cm−1 (T>4).8,39 Compared with other terpolymers,8,22,40 Tn conformations were dominant for all the P(VDF-TrFE-HFP) terpolymer films, whereas only a weak peak was observed at 614 cm−1 for the TG conformation. In addition, no absorption band was noted at 526 cm−1 for the T3) and short (e.g., T3) Tn sequences were noted. These can be quantified by the ratio of the 845 and 470 cm −1 absorption bands after peak deconvolution, i.e., A845/A470, as seen in Figure 7B. Supposedly, the higher the A845/A470 ratio, the longer the Tn sequence in the crystals. The TerP-Q film exhibited the highest A845/A470 ratio, suggesting a long Tn sequence. The TerP-S had the lowest

Figure 6. 2D XRD patterns of (A) TerP-S, (B) TerP-SAR, and (C) TerP-SA. The vertical direction is the drawing direction. Corresponding 1D XRD profiles along (D) the equator and (E) the meridian directions are obtained from the 2D XRD patterns in (A−C). The 1D XRD profiles of the CoP-S film along the equator and meridian directions are shown for comparison.

alone.34 It is likely that a significant portion of the HFP units were pulled/dragged into the crystalline region under strong mechanical stretching because of plastic deformation.38 Currently, the direct proof of the HFP inclusion in the P(VDF-TrFE) crystals is still lacking. In the future, we will pursue solid-state 19F NMR characterization to quantify the percentage of included HFP units. These internal HFP defects could cause more gauche defects in the original CLFE crystals, resulting in an even more disordered crystalline structure. In addition, the enlarged interchain distance could enable easier dipole or domain switching in the defective crystals under an alternating electric field. Nonetheless, this stretching-induced defective crystalline structure was not stable because of the inclusion of large HFP units. Upon annealing at 65 °C with free ends (note that

Figure 7. (A) FTIR spectra for different P(VDF-TrFE-HFP) terpolymer films: (a) TerP-Q, (b) TerP-S, (c) TerP-SAR, and (d) TerP-SA. (B) Peak intensity ratios of A503/A470 and A845/A470 for different P(VDF-TrFE-HFP) terpolymer films. G

DOI: 10.1021/acs.macromol.7b01205 Macromolecules XXXX, XXX, XXX−XXX

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Figure 8. (A) First and (B) second heating DSC thermograms for different terpolymer films. (C) Heats of transition (ΔHf,C) for the Curie peaks and (D) crystallinities for different terpolymer films.

Scheme 1. Schematic Representation of Stretching-Induced Nanodomain Formation in P(VDF-TrFE-HFP) Terpolymer Films and Subsequent Annealing Effects with Fixed and Free Ends, Respectivelya

a

The HFP units are represented by the red dots.

8C. The TerP-Q film had the highest ΔHf,C whereas the TerP-S film had the lowest ΔHf,C, suggesting that the TerP-Q film was the most ferroelectric and the TerP-S film was the least ferroelectric (i.e., RFE). After annealing with free ends, the TerP-SA film showed a ΔHf,C close to that of the TerP-Q film. For the TerP-SAR film, the ΔHf,C was close to that of the TerPS film. After melt-recrystallization (the cooling rate was 10 °C/ min), the ΔHf,C values were similar for all films during the second heating. This was consistent with the expectation that the large HFP units were excluded from the crystalline region after quiescent melt crystallization. Using the ΔH0f of 30.7 J/g for the terpolymer, crystallinities were calculated from ΔHf in DSC of the terpolymer films (Figure 8D). For the first heating, the TerP-Q film exhibited the lowest crystallinity (30 wt %) whereas the TerP-S film showed the highest crystallinity (43 wt %). This is consistent with the expectation that HFP units were included into the crystalline regions because of stretching and the crystallinity increased. After annealing with free ends, the crystallinity decreased to 36 wt % for the TerP-SA film, while that of the TerP-SAR film was 39 wt %, being intermediate between those for the TerP-S and TerP-SA films. After melt-recrystallization,

A845/A470 ratio, suggesting the highest amount of gauche defects induced by the inclusion of large HFP units in the crystals. After annealing above the TC with free ends, the TerPSA displayed an increased A845/A470 ratio compared to that of the TerP-S film. This was consistent with the explanation that HFP defects were largely excluded from the P(VDF-TrFE) crystals for the TerP-SA film. The A845/A470 ratio for the TerPSAR film was between the values for the TerP-S and TerP-SA films, consistent with the explanation that only a fraction of the HFP units was excluded from the crystals. Not only the A845/ A470 ratio showed this trend, the A503/A470 peak ratio also demonstrated a similar trend (Figure 7B), confirming that the long Tn sequence should decrease in the order of TerP-Q ∼ TerP-SA > TerP-SAR > TerP-S. Since it was difficult to use XRD peak fitting to calculate the crystallinity for the TerP-S, TerP-SAR, and TerP-SA films (see Figure 6), DSC was used to determine the crystallinities. Figures 8A and 8B show the first and second heating DSC thermograms for the TerP-Q, TerP-S, TerP-SAR, and TerP-SA films. The Curie peaks were observed around 50 °C, and the crystal melting peaks were ca. 135−137 °C. The heat of transitions for the Curie peaks (ΔHf,C) are displayed in Figure H

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Figure 9. Structure−property relationships for (a) HTPE P(VDF-TrFE)/terpolymers, (b−d) stretched RFE terpolymers (X = CFE, CTFE, and HFP), and (e) CLFE P(VDF-TrFE)/unstretched P(VDF-TrFE-HFP). D−E loops in (a, e): Reproduced with permission from ref 41. Copyright 2012 Elsevier. D−E loops in (b, c): Reproduced with permission from ref 22.

at 22 °C), the P(VDF-TrFE-CFE) film exhibits fairly weak FTIR absorption bands at 848 and 508 cm−1 for the long Tn (n ≥ 3) sequence, but obvious 614 and 530 cm−1 bands for the TG conformation (Figure S8D). This FTIR spectrum is similar to that of the HTPE phase in P(VDF-TrFE) (Figure S8B). Meanwhile, its 2D XRD pattern (Figure S8C) is also similar to that of the HTPE phase in P(VDF-TrFE) (Figure S8A) with the absence of the (001) reflection for the long Tn sequence. These results suggest that the incorporation of the CFE units in the P(VDF-TrFE-CFE) terpolymer is effective to preserve (or freeze) the paraelectric phase below the TC. Supposedly, the D−E loops for the P(VDF-TrFE-CFE) film should be similar to that of the HTPE P(VDF-TrFE) (Figure 9a). Nonetheless, DHLs instead of narrow SHLs are observed for P(VDF-TrFECFE) (Figure 9b). This is attributed to the weak physical pinning effect of the CFE units in the crystals. At high enough poling electric field, CFE units are switched and FE domains with a long Tn1 sequence are formed. These FE domains are not stable. Upon removing the poling field, the frozen PE structure is obtained and DHLs are observed. Second, at 0 °C (i.e., below the TC at 28 °C), the P(VDFTrFE-CTFE) terpolymer exhibits stronger absorption bands at 848 and 508 cm−1 than those at 614 and 530 cm−1 in the FTIR spectrum (Figure S8F), whereas the 2D XRD pattern shows a weak (001) reflection on the meridian (Figure S8E). Both results indicate that a longer Tn2 sequence exists in P(VDFTrFE-CTFE). Because of the stronger physical pinning effect exerted by CTFE units in the crystals, narrow SHLs are noted (Figure 9c). Finally, the P(VDF-TrFE-HFP) terpolymer behaves differently. Because of the large size, HFP units are mostly excluded from the crystals after the melt crystallization, forming the CLFE phase (Figure 9e). It is noted that HFP units are excluded from the crystals for all P(VDF-HFP) random copolymers, no matter whether the films are stretched or not.42,43 Only upon uniaxial stretching, the HFP units are pulled into the crystalline P(VDF-TrFE) phase, resulting in the

the crystallinities for the second heating were around 35 wt % for all films. On the basis of the above XRD and FTIR structural studies, the main conclusions can be drawn for various terpolymer films in Scheme 1. Upon quenching from the melt, the CLFE phase is obtained in the TerP-Q film with large HFP units mostly excluded from the crystalline region. The FE domains are relatively large and the interchain distance is l1. After strong mechanical stretching, some HFP units are pulled into the crystalline regions in the TerP-S film, expanding the interchain distance (l2 > l1) and dividing the original large FE domains into nanodomains. Meanwhile, large HFP units apply a strong physical pinning effect on the neighboring dipoles and prohibit their rotation in the crystals. As a result, the RFE behavior prevails and the FE response is the weakest for the TerP-S film. After annealing above the TC (e.g., 65 °C) with free ends, the HFP units are excluded from the crystalline regions in the TerP-SA film, returning to a FE state close to that of the CLFE phase in the quenched film. However, when annealing at 65 °C with fixed ends, only part of the HFP units are excluded from the crystalline regions and slightly enlarged nanodomains are preserved in the TerP-SAR films. The interchain distance, l3, is in between l2 and l1, and the RFE behavior can become metastable. Discussion on Structure−Property Relationships in Various P(VDF-TrFE)-Based Terpolymers. The dielectric and ferroelectric properties of various stretched P(VDF-TrFEHFP) films are summarized in sections II and III of the Supporting Information. From this study, the P(VDF-TrFEHFP) terpolymer is quite different from other P(VDF-TrFE)based terpolymers previously reported.8,22 Figure 9 summarizes the structure−property relationships for the P(VDF-TrFE-X) (X = CFE, CTFE, and HFP) terpolymers, in comparison with the HTPE and CLFE phases of the P(VDF-TrFE) copolymers. The XRD and FTIR structural characterization data for these films are supplied in Figure S8, and the FE behavior is seen in the bottom panel of Figure 9. First, at 0 °C (i.e., below the TC I

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RFE phase with a relatively long Tn3 sequence. Therefore, TrFE units are important for the inclusion of HFP units in the crystalline region upon stretching. This is evidenced by the FTIR absorption bands at 848 and 508 cm−1 and the (001) reflection in the 2D XRD pattern (Figure S8G,H). Because of the strong pinning effect of the HFP units in the crystals, narrow SHLs with Dmax values lower than those of P(VDFTrFE-CTFE) are observed for P(VDF-TrFE-HFP) than those for P(VDF-TrFE-CTFE) (Figure 9d). Judging from the high apparent dielectric constant and the low hysteresis loop loss, P(VDF-TrFE-CTFE) should be the best choice for the RFE behavior.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation, Division of Materials Research, Polymers Program (DMR1402733). Y.L. acknowledges financial support from China Scholarship Council. G.J.Z. and Z.M.L. thank National Natural Science Foundation of China for partial financial support (Grant No. 51528302). B.A., V.L., and T.S. thank CNRS, French National Agency of Research and Technology (ANRT), and Arkema for the financial support (CIFRE convention no. 2013/0540). The authors thank Ms. Yanfei Huang for the measurement of tensile stress−strain curve for the TerP-Q film.

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CONCLUSIONS From this study, the RFE behavior could be successfully achieved in a uniaxially stretched P(VDF-TrFE-HFP) 58.4/ 38.0/3.6 terpolymer, which otherwise (i.e., unstretched) exhibited the normal FE behavior with broad hysteresis loops. From XRD and FTIR studies, the structure−property relationships were understood. After appropriate stretching, the large HFP units were dragged into the P(VDF-TrFE) crystals because of plastic deformation, resulting in the expansion of the interchain distance and a decreased content of long Tn sequences. Because of the inclusion of HFP units into the P(VDF-TrFE) crystals, the FE domains are divided into nanodomains, and the included HFP units had a strong pinning effect on the dipole/domain switching under an alternating electric field. However, the RFE behavior was not stable in the TerP-S film. Upon thermal annealing with free ends above the TC (∼50 °C), the HFP units were excluded again from the crystalline regions, and the normal FE behavior returned for the TerP-SA film. However, if the TerP-S film was annealed at 60 °C with fixed ends, a significant amount of the HFP units could remain in the TerP-SAR film. As a result, the RFE behavior of the terpolymer film became metastable.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01205. Tensile stress−strain curve for the TerP-Q film, bipolar D−E loops for P(VDF-TrFE-HFP) films stretched under different stretching rates, frequency-scan BDS results for the TerP-Q and TerP-S films at various temperatures, dielectric and ferroelectric properties of various stretched terpolymer films, and 2D XRD patterns and FTIR spectra for various P(VDF-TrFE) copolymer and P(VDF-TrFE-X) terpolymer films (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (L.Z.). *E-mail [email protected] (G.-J.Z.). ORCID

Thibaut Soulestin: 0000-0002-1534-0230 Vincent Ladmiral: 0000-0002-7590-4800 Bruno Ameduri: 0000-0003-4217-6664 Zhong-Ming Li: 0000-0001-7203-1453 Gan-Ji Zhong: 0000-0002-8540-7293 Lei Zhu: 0000-0001-6570-9123 J

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DOI: 10.1021/acs.macromol.7b01205 Macromolecules XXXX, XXX, XXX−XXX