Honghong Gonga,
Bei Miaoa,
Xiao Zhangb,
Junyong Lu*b and
Zhicheng Zhang*a
aDepartment of Applied Chemistry, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi'an Jiaotong University, Xi'an, P. R. China 710049. E-mail: zhichengzhang@mail.xjtu.edu.cn
bNational Key Laboratory of Science and Technology on Vessel Integrated Power System, Naval University of Engineering, Wuhan, P. R. China 430034. E-mail: jylu@xinhuanet.com
First published on 18th December 2015
In an effort to achieve antiferroelectric-like behavior and high Ue under a high electric field, a series of poly(methyl methacrylate) (PMMA) grafted P(VDF-TrFE-CTFE) with increased PMMA content were synthesized via an atom transfer radical polymerization process. All the grafted copolymer films were prepared by a solution-casting process followed by uniaxially stretching with varied extension ratios. The dependence of crystalline, ferroelectric, dielectric and energy storage properties on both the orientation ratio and PMMA content was systematically discussed. Besides the orientation of the polymer chains in the amorphous phase together with the alignment of crystalline and ferroelectric grains, uniaxially drawing the films leads to an improvement in both the content and size of the crystalline and ferroelectric phases. Thanks to the strong confinement of rigid PMMA and the alignment induced high breakdown strength, the antiferroelectric-like behavior could be retained up to 675 MV m−1 with a discharged energy density of 23.3 J cm−3 in the copolymer bearing 24 wt% PMMA with an extension ratio of 300%. These results may help to deeply understand the ferroelectric transition mechanism of PVDF based ferroelectric polymers and provide a promising strategy for the design of polymers with high energy storage capability.
Among the known commercial polymers, poly(vinylidene fluoride) (PVDF) based fluoropolymers have attracted continuously increasing interests as dielectric materials for their relatively high εr of 10–12 besides of their well-known excellent ferro- and piezo-electric properties.11–16 However, PVDF based normal ferroelectric polymers render a rather small Ue owing to the large remnant polarization (Pr) and low saturation electric field (Es). To release the energy stored, many efforts have been devoted to modify PVDF and its copolymers physically and chemically, and they have been successfully turned from normal ferroelectrics into relaxed ferroelectrics.13,17–28 However, due to their ferroelectric relaxation nature, the charge and discharge curves on displacement–electric field (D–E) hysteresis loops do not match very well and relatively large hysteresis together with high Ul (about 40%) are observed.
To confine the ferroelectric relaxation by weakening the coupling among adjacent ferroelectric domains, low polar polystyrene (PS) has been introduced in P(VDF-CTFE) and P(VDF-TrFE-CTFE) as side chains to form micro-insulator layers surrounding the ferroelectric domains. Due to the low polarity of the PS-rich layer at the amorphous–crystalline interface, the compensation polarization is substantially decreased resulting in a novel confined ferroelectric behavior in the grafted copolymers. Rather low dielectric loss (tanδ of 0.006@1 kHz) and Ul of 17.5%@550 MV m−1 are detected in P(VDF-CTFE)-g-PS (34 wt%) copolymers, although Ue is reduced from 9.2 to 4.8 J cm−3 at 400 MV m−1 as well.29 Most interestingly, drawing P(VDF-TrFE-CTFE)-g-PS bearing 14 wt% of PS segment uniaxially results into the ferroelectric transition from linear dielectrics to ferroelectrics with a double hysteresis loops (DHLs), which is closely dependent on the extension ratio. Comparing with the DHLs observed in PVDF based normal ferroelectrics under low electric field in a very narrow range and within limited polarizing circles, DHLs are found to be well maintained under relatively high electric field in the stretched P(VDF-TrFE-CTFE)-g-PS films. However, when the extension ratio is 450%, the Ue is obtained as only 2.5 J cm−3@150 MV m−1 together with Ul approximate 30% in the grafted polymer films mainly due to the quickly vanishing of DHL behavior under elevated electric field.30,31 Therefore, fabricating polymeric films with antiferroelectric-like behavior under high electric field together with high breakdown strength should be a desirable way to achieve high Ue in metalized film capacitors.
Instead of low polar PS, different poly(methacrylic ester)s (PXMAs) were grafted onto P(VDF-TrFE-CTFE) and the introduction of PXMA with excellent compatibility with the main chains could effectively impede the crystallization of P(VDF-TrFE-CTFE). The D–E hysteresis behaviors of P(VDF-TrFE-CTFE) were finely tuned from typical ferroelectric to either anti-ferroelectric or linear dielectric by facilely grafting the required types and amount of XMA monomers onto it. The outstanding confining and decoupling effects of PXMA side chains on the disorientation of the high polar crystal domains and the transition from normal ferroelectric to anti-ferroelectric or linear dielectric are responsible for its significantly decreased Ul, well maintained Ue and the greatly improved Es. A low Ul of 16.6% and relatively high Ue of 19.3 J cm−3 under electric field of 675 MV m−1 was achieved in P(VDF-TrFE-CTFE)-g-PEMA copolymer.32 Among all the investigated three types of PXMA, poly(methyl methacrylate) (PMMA) exhibits the strongest confinement onto the ferroelectric performance of P(VDF-TrFE) for its highest polarity, modulus at ambient temperature and glass transition temperature (Tg).
In an effort to improve the breakdown strength and retain the anti-ferroelectric-like behavior under high electric field, in this work, uniaxial stretching has been conducted onto a family of solution cast P(VDF-TrFE-CTFE)-g-PMMA films bearing different PMMA content. The dependence of the crystalline, dielectric and ferroelectric properties of the films with varied PMMA content on the extension ratio has been carefully investigated. Thanks to the strong confinement of PMMA side chains and orientation induced ferroelectric phase transition, DHL behavior of P(VDF-TrFE-CTFE)-g-PMMA with 24 wt% of PMMA and an extension ratio of 300% could be well maintained up to 675 MV m−1 and the highest Ue is obtained as 23.3 J cm−3. Meanwhile, the uniaxial stretching is also found be responsible for the significantly improved Eb comparing with the as-casted films, thus the significantly increased Ue at Eb.
Scheme 1 Synthesis of P(VDF-TrFE-CTFE)-g-PMMA graft copolymer and 1H NMR spectra of P(VDF-CTFE), P(VDF-TrFE-CTFE) and P(VDF-TrFE-CTFE)-g-PMMA. |
Successful grafting of PMMA side chains from Cl sites in P(VDF-TrFE-CTFE) was characterized with 1H NMR as shown in Scheme 1. Comparing with P(VDF-CTFE), new signals emerging at 5.3–5.8 ppm were assigned to the protons on TrFE (–CF2CFH–) converted from CTFE in P(VDF-CTFE). After grafted reaction, new signal at 3.5–3.7 ppm is assigned to the protons on –OCH3 of PMMA. The proton signals on VDF adjacent to CTFE (–CF2–CH2–CFCl–CF2–) at 3.4–3.6 ppm vanish gradually as PMMA units are grafted. The chemical composition and the weight content of PMMA of the graft copolymer could be facilely calculated from 1H NMR and the method had been reported in our previous work.32
Films of the graft copolymers with different PMMA contents and extension ratios were named as follows. The film “10-100” denoted the nonstretched P(VDF-TrFE-CTFE)-g-PMMA(10 wt%) (100%), where number 10 refers to the weight content of PMMA (10 wt%) and number 100 indicates the extension ratio is 100% (nonstretched). Thus, “24-200” is for the sample containing 24 wt% PMMA with a stretching ratio of 200% as listed in Table 1.
Entry | Mole ratioa | PMMA/wt%b | D/nmc | TC/°Cd | ΔHC/J g−1d | Tm/°Cd | χc/%d |
---|---|---|---|---|---|---|---|
a The mole ratio of VDF/TrFE/CTFE/MMA determined by 1H NMR.b The weight percentage of PMMA in P(VDF-TrFE-CTFE)-g-PMMA graft copolymers.c The apparent crystallite size along (110/200)[D(110/200)β] calculated using the Scherrer equation.d Determined by DSC during the second heating process. ΔHC is the heat enthalpy of the Curie transition for the copolymer. | |||||||
10-100 | 80/18/2/7.6 | 10 | 6.5 | 95.2 | 2.4 | 128.7 | 9.6 |
10-300 | 80/18/2/7.6 | 10 | 6.7 | 98.1 | 5.0 | 130.6 | 11.3 |
16-100 | 80/18/2/12.7 | 16 | 5.0 | 98.5 | 2.1 | 127.3 | 8.3 |
16-300 | 80/18/2/12.7 | 16 | 5.6 | 98.8 | 4.1 | 129.5 | 8.5 |
21-100 | 80/18/2/18.6 | 21 | 3.9 | 97.0 | 1.9 | 122.9 | 5.0 |
21-300 | 80/18/2/18.6 | 21 | 5.9 | 97.7 | 3.4 | 125.6 | 5.5 |
24-100 | 80/18/2/21.7 | 24 | 3.0 | 95.0 | 1.1 | 119.4 | 3.0 |
24-200 | 80/18/2/21.7 | 24 | 5.9 | 96.0 | 2.8 | 121.0 | 3.5 |
24-300 | 80/18/2/21.7 | 24 | 5.6 | 98.6 | 2.5 | 121.5 | 3.3 |
24-400 | 80/18/2/21.7 | 24 | 5.6 | 101.8 | 2.0 | 122.0 | 3.7 |
Fig. 1 XRD spectra of as-casted and after uniaxial stretched P(VDF-TrFE-CTFE)-g-PMMA films containing various content of PMMA. |
Uniaxial stretching below the melting temperature of PVDF is widely utilized to transfer the low polar α or γ crystals into high polar β-PVDF for high ferro- and piezo-electricity.34–38 Only β-phase is detected in present PMMA-grafted polymers and the uniaxial stretching is not necessary to realize the crystal phase transition. However, the stretching does lead to the significantly improved crystallinity. Comparing with the as-casted films, the stretched samples possess significantly sharpened reflection peak at 2θ = 19.4° in XRD spectra and reduced dispersive peak at about 17° attributed to the amorphous structure. That means the orientation may lead to the crystallization of P(VDF-TrFE) main chain in amorphous phase along the drawing direction, which may be confirmed by the sharpened absorption peaks corresponding to β-PVDF in FTIR results as well. To quantificationally characterize the phase transition happened during the stretching, the apparent crystallite size along (110/200) [D(110/200)β] was calculated using the Scherrer equation: Dhkl = (Kλ)/(Bhklcosθ), where Dhkl is the average crystallite size along the (hkl) direction, K is the shape factor (a value of 0.94 is used in this case), λ is the wavelength, and Bhkl is the full width at half-maximum for the (hkl) diffraction. As shown in Table 1, after uniaxially stretched at 100 °C, the crystalline size of lamellae is significantly increased, especially for the samples containing more PMMA side chains. For example, Dhkl of 21-100 is improved from 3.9 nm to 5.9 nm of 21-300, and Dhkl of 24-100 is improved from 3.0 nm to 5.6 nm of 24-300. In addition, the crystallinity of all the samples was evaluated with DSC by χc = Hf/(Hf0ω), where Hf0 = 104.7 J g−1 is the enthalpy of PVDF with 100% crystallinity, ω is the weight percent of the P(VDF-TrFE-CTFE) in the graft copolymer. As listed in Table 1, both the crystallinity and melting temperature of samples is more or less increased after stretched. That means the uniaxial orientation is favorable of the increasing of crystallinity and crystal grain size. The reduction in amorphous phase and increase in crystal phase observed in XRD results could clearly demonstrate that the improved crystalline properties are mostly originating from the orientation induced recrystallization of polymer chains in amorphous phase.
As presented in Fig. 3, comparing to the as-casted samples, the stretched films possess about 2–4 °C elevated F–P temperature and the sharpened peak. The content of ferroelectric phase could be estimated with the enthalpy (ΔHC) of F–P transition peak on DSC. As listed in Table 1, the stretched samples show improved ΔHC as well as the Curie temperature (TC), which means both the size and content of the ferroelectric phase are improved induced by the orientation. It has been known that the P(VDF-TrFE) ferroelectrics possess the typical order-disorder character change associated with ferro- to para-electric transition. The ferroelectric phase basically consists of regular all-trans polymer chains packed in parallel or β-form crystals. Meanwhile, a certain amount of disorder comprising some defects would be generated for the quenching process from high temperature.15 As temperature approaches TC, such disorder grows rapidly to divide all-trans regions into sub-domains, which may undergo large fluctuations as a result of the competition between the cooperative and entropy forces. In the paraelectric phase, both the size and the volume fraction of such sub-domains would be diminished with the increasing of temperature. The introduction of rigid amorphous PMMA chains may improve TC by strengthening ferroelectric domain wall and reduced ΔHC by decreasing the ferroelectric domain contents. Apparently, both the improved TC and ΔHC in all the stretched films mean the orientation facilitates more and large order structures and thus the ferroelectric domains. With the assistance of stretching uniaxially, both the size and the content of the ferroelectric domains are improved as indicated in Table 1. Apparently, the impedance effect of PMMA onto the formation of crystalline and ferroelectric phase in large scale is weakened by the stretching.
In general, uniaxially stretching the P(VDF-TrFE-CTFE)-g-PMMA films results in the improved crystallinity as well as the thickness of crystal grains but invisible change of crystal forms. Meanwhile, the F–P phase transition performance of the grafted copolymers has been enhanced as well characterized with both increased ΔHC and TC. Besides, stretching would lead to part of the pre-existing flat-on β crystals in as-casted films be pulled apart and orientated with their c-axes parallel (or the crystalline lamellae transverse) to the drawing direction, which has been well reported in literature.27,43
Most importantly, stretching would cause the orientation of polymer chain in amorphous phases as well as the ferroelectric domains. From the SEM images and the cartoon as show in the Fig. 4, the well-grown lamellae are usually aligned along the thickness direction of the films.
Fig. 4 SEM images of sections of P(VDF-TrFE-CTFE)-g-PMMA films (A, 21-100; B, 21-300) and scheme of lamellae orientation in the film before and after uniaxially stretched. |
Comparing with the as-casted films, uniaxial stretched samples exhibit significantly increased εr as indicated in Fig. S4.† It has been well reported that the dielectric properties of PVDF and its copolymers are anisotropic with respect to different crystal orientations.34–36 In the as-casted films, the ferroelectric domains are dispersed randomly in all the directions although the dipole moments of CF2 in these domains are well aligned and densely packed together. Under low electric field, the contribution of these isotopically distributed ferroelectric domains to the dielectric constant shows no priority along the dielectric field than the other directions. However, the polymer chains both in crystal and amorphous phase of the stretched samples would be aligned along the drawing direction and perpendicular to the external field. As a result, the dipoles exhibit a priority in a plane parallel to the electric field depending on the stretching ratio. The orientation polarizability of transverse crystals and aligned amorphous phase is enhanced, which contributes a lot to εr. Apparently, the alignment of dipole moments plays a crucial role in the improvement of εr besides of their polarity. The continuously improved dielectric constant from the sample 24-100 to 24-400 could further confirm the significant contribution of dipole orientation onto εr. That could be explained by the long range interactions among ferroelectric domains composed of aligned P(VDF-TrFE-CTFE) dipoles. These interactions are realized through the compensation polarization outside the ferroelectric domain and are anisotropic in nature, when an external electric field is applied.29 The relaxation properties of films with different crystal orientations were studied with the frequency spectra of tanδ, as shown in Fig. S4.† All samples show only one relaxation peak at high frequencies (∼1 MHz). The absence of relaxation peak at low frequency (∼10 Hz) (α-relaxation) confirms the pure β crystals in all the films. After stretched, all films exhibit slightly reduced tanδ in the testing frequency range. Neither micro-Brownian motion of non-crystalline chain segments nor the molecular motion on the amorphous/crystalline interfaces in the stretched films is significantly altered comparing with the as-casted ones since the crystallinity is only slightly improved by the stretching as discussed above. In general, orienting the ferroelectric domains as well as the dipoles in amorphous phase by stretching the films uniaxially could effectively improve their εr but reduce their tanδ.
The dielectric properties of the P(VDF-TrFE-CTFE)-g-PMMA graft copolymer films with different degree of crystal orientation at high electric fields were studied with D–E hysteresis loop measurement as presented in Fig. 5. For the as-casted films, D–E loops of grafted polymers could be finely tuned by varying the PMMA content depending on the polarizing electric field. At lower PMMA content (ca. 10 wt%), the grafted polymers exhibits D–E loops gradual evolution from linear dielectrics to anti-ferroelectric-like and relaxor ferroelectrics with the increasing of external electric field. As PMMA content increases to over 16 wt%, only evolution from linear dielectric to anti-ferroelectric-like performance would be observed even the poling field goes up to 400 MV m−1. It has been well discussed that the D–E loops evolution is dominated by the confinement of rigid PMMA segments on the ferroelectric domains orientation and relaxation along the external field switching.33 The confinement is in proportion to the content of PMMA, and the ferroelectric domains orientation is balanced by the confine force and the switching force constructed between the ferroelectric domains and the poling electric field.
Fig. 5 Bipolar D–E loops of P(VDF-TrFE-CTFE)-g-PMMA films with varied PMMA contents before and after uniaxial stretching. |
For the sample 10-100 with lower content of PMMA, there were propeller-shaped D–E loops with two-step dipole switching depending on the external field. When the polarization field was less than 150 MV m−1, it showed the linear dielectric performance with a narrow loop. Under electric field over 150 MV m−1, relaxor ferroelectric or typical ferroelectric loops would be detected. Under electric field over 200 MV m−1, relaxor and normal ferroelectric loops were obtained. Comparing with the relatively low critical electric field of 50–75 MV m−1 reported in P(VDF-TrFE) (VDF/TrFE = 80/20 mol%),44 the introduction of rigid PMMA segments leads to improved critical field for the orientation of ferroelectric domains. However, the confinement contributed by the relatively low PMMA content is so poor that the DHL behavior of sample 10-100 could only be retained up to 200 MV m−1. As PMMA content increases, the linear dielectric loops could be retained up to 300 MV m−1 in 24-100 sample. Over these critical fields, only antiferroelectric-like behavior with DHLs and no transition from antiferroelectric-like to relaxor or normal ferroelectric could be detected. That could be attributed to the continuously increased confinement effect of PMMA side chains and the gradually decreased ferroelectric domains.
After stretched for 300%, the linear dielectric loops could only be maintained below 100 MV m−1 in all the samples. As external electric field increases, sample 10-300 is turned into relaxor and normal ferroelectric at about 200 MV m−1, and the other stretched samples could retain the DHLs up to their breakdown (from 500 to 675 MV m−1). No visible transition from antiferroelectriclike to relaxor or normal ferroelectric is observed, which is different from the previous report about the P(VDF-TrFE-CTFE)-g-PS copolymer.30
That means the balance between the confinement of the amorphous phase on the ferroelectric domains and the alignment force has been altered. Based on the DSC and XRD results discussed above, the improved ferroelectric domains and the crystallinity would lead to the elevated alignment force under the consistent external field. On the other hand, the uniaxial stretching induced molecular chain c-axe aligning along the direction is favorable for the flipping of the dipole moment along the electric field, which is responsible for the increased displacement under consistent electric field as well. These factors are responsible for the fattened loops and lowered critical field for retaining the linear dielectric behavior.
Both the fine confinement of aligned PMMA on P(VDF-TrFE-CTFE) and the dependence of phase transition evolution on the stretching ratios could be further confirmed by the polymers containing 24 wt% PMMA stretched with increased ratios. As indicated in Fig. 6, as stretching ratio increases, Pm is increasing gradually under the electric field over 100 MV m−1 while Pr shows little improvement. That is rather different from the results observed in P(VDF-TrFE-CTFE)-g-PS copolymers, where the Pr starts to increase significantly at 100 MV m−1 for the non-confinement of PS segments.30 That could further confirm the conclusion that PMMA with fine compatibility with P(VDF-TrFE-CTFE) main chain and exhibit excellent confinement efficiency onto the disorientation of the aligned ferroelectric domains. In general, the desirable confinement of PMMA segments is allowing the orientation of the enlarged ferroelectric domains but forcing them back as soon as the electric field is removed.
Fig. 6 Bipolar D–E loops, maximum polarization (Pm) and remnant polarization (Pr) as a function of electric field for P(VDF-TrFE-CTFE)-g-PMMA (24%) graft copolymer with different drawing ratio. |
The characteristic Eb of the P(VDF-TrFE-CTFE)-g-PMMA copolymers with different contents of PMMA was analyzed with Weibull statistics and the results are presented in Fig. 7. For the as-casted films, Eb is enhanced from 249 MV m−1 (Fig. S3†) for the solution-cast pristine P(VDF-TrFE-CTFE) to 310 MV m−1 for the grafted Discharged energy density (Ue) as a function of electric field copolymer with 24 wt% PMMA (24-100). In addition, the sample 24-100 also exhibited a relative narrow breakdown distribution with higher β of 10.1 than that for P(VDF-TrFE-CTFE) of 6.4 (Fig. S3†) and indicated relatively higher level of dielectric reliability. Compared with Eb and β of P(VDF-TrFE-CTFE)-g-PMMA films with different PMMA content before and after uniaxial stretching, both Eb and β had been enhanced after stretching. With the same extension ratio, the films bearing higher PMMA content possess gradually elevated Eb from 342 MV m−1 in sample 10-300, to 478 MV m−1 of sample 24-300. Meanwhile, Eb is gradually increased with the increasing of stretch ratio from the result of 24-100, 24-200, 24-300 and 24-400 samples.
Fig. 7 Weibull distribution of the breakdown electric field for P(VDF-TrFE-CTFE)-g-PMMA films with varied PMMA contents before and after uniaxial stretching. |
Fundamentally, the enhanced Eb may be originated from the control of thin film morphology (including chain and/or crystal motion) under high applied electric fields. The local chain and/or polar crystal motions in the polymer thin films, which are essential for PVDF based ferroelectrics, may create free volume (or even defects) that are harmful to breakdown strength. As a result, the normal P(VDF-TrFE-CTFE) copolymers with crystals in large scale usually possess a low Eb near the polarization Es, because the amorphous phase is more vulnerable to electrical breakdown after repeated flipping of large polar crystal domains. Introducing bulky commoners (such as CTFE and HFP),4,47 chemically crosslinking PVDF or P(VDF-TrFE) followed by stretching or quenching films at extreme low temperature48,49 could effectively reduce the crystal size and crystallinity, which may achieve high Es as well as Eb in the resultant copolymers. Similarly, the incorporated PMMA would not only significantly decrease the crystal size and the crystallinity but also greatly improve Es for the confinement effect. Therefore, by controlling the chain and crystal motion during the orientation polarization switching, the possible free volume (or even defects) formed would be greatly suppressed, and thus, substantially improved Eb is obtained in P(VDF-TrFE-CTFE)-g-PMMA copolymers. Most importantly, the defect inside the polymer film could reduce and subsequently the quality of film improved through stretching process, so the Eb had been further improved after uniaxial stretching.
Fig. 8 Discharged energy density (Ue) as a function of electric field for the P(VDF-TrFE-CTFE)-g-PMMA graft copolymers with varied PMMA contents. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22617a |
This journal is © The Royal Society of Chemistry 2016 |