Structural analysis of KF/PEG and PVDF/KF/PEG composite films
Figure 1 shows the XRD patterns of KF, KF/PEG, and PVDF/KF/PEG composite films, respectively. It can be seen from Fig.1 that the natural KF is amorphous in structure, and all the PVDF/KF/PEG show distinct characteristic peaks at 2θ = 19.20° and 2θ = 23.35°, corresponding to the (120) and (112) crystal planes of PEG (Fig.S1), respectively. Moreover, no other new characteristic peaks are observed in Fig.1a, indicating the absence of chemical reactions during the impregnation of the KF with PEG. The XRD patterns of both KF/PVDF and PVDF/KF/PEG composite films (Fig.1b) display the characteristic peaks of PVDF at 2θ = 18.24°, 20.16°, 26.56°, corresponding to (020), (110), (021) crystal plane of PVDF. And the characteristic peaks of PEG also can be found at 2θ=19.1° and 23.6°. The above results show that both the compounded PEG and PVDF maintain their crystallization ability, and there is no significant change in the crystal structure.
The TG curves of KF, KF/PEG, and PVDF/KF/PEG composite films are exhibited in Figure 2. The major thermal weight loss of pure PVDF starts at 450°C and ends at 480°C. The thermal weight loss of the samples reaches 60% at this stage and the total thermal weight loss up to 800°C is about 68%. The main weight loss of PEG800, PEG2k and PEG10k start at 350°C-380°C, and end at 420°C-440°C, respectively. An initial mass loss of the KF is found due to the water adsorbed on the fiber surface, and the weight loss of KF mainly occurs in the range of 250-350°C, which can be attributed to the degradation of a large amount of hemicellulose in KF. There is almost no weight loss of the PVDF/KF/PEG before 270°C, and as the temperature increased from 270°C to 480°C, TG curves of all the composites showed significant weight loss in function of temperature increase. The mass loss in the range of 270-350°C, 350-450°C and 450-480°C is primarily attributed to the thermal pyrolysis of the KF, PEG and PVDF, respectively. The PEG mass ratios (Table S1) were obtained by calculating the residual carbon rates of different composite films, where PVDF/KF/PEG800 adsorbed the most mass of PEG with a mass ratio of 17.3%, compared to 16.6% and 10.9% for PVDF/KF/PEG2k and PVDF/KF/PEG10k, respectively.
SEM images (Figure 3) show the cross sections of the PVDF/KF/PEG composite films. The KF displays a high hollow ratio of ~80%, which provides sufficient space for PEG impregnation. Meanwhile, the unique hollow structure that possesses a wall thickness of around 1 μm facilitates not only the reduction in the mass proportion of non-phase change components but also the improvement in the dielectric response efficiency of the composites. Figure 3 shows that the KF/PEG fibers are randomly dispersed and disoriented in the PVDF matrix, and the tubular structure of the KF fibers remains intact after the solution casting and hot pressing. After compounding the KF/PEG fiber with PVDF matrix, the solid PEG can still be clearly observed from the hollow cavity of the KF, and there is obvious interface debonding in the PVDF matrix and KF. This is mainly due to the PEG crystallization-induced shrinkage and the weak bond between PEG and the KF inner surface. It can also be found that the KF as the PEG carrier does not inhibit or promote the phase change of PEG.
Thermal-induced phase transition in the PVDF/KF/PEG
The phase change of the PEG in the composite films is an essential prerequisite for triggering dielectric response. Figure 4 and Figure S2 show the melting and freezing DSC curves of the KF/PEG and PVDF/KF/PEG. Figure S2d shows both the melting and crystallization temperatures of the KF/PEG increase as the molecular weight of the PEG increases. The melting transitions of the KF/PEG containing PEG of various molecular weights occur within the range of 14.0-24.6 °C, 49.9-64.5°C and 51.8-65.8°C, with peak temperatures of 19.6°C, 53.8°C and 61.9°C, respectively. The molecular weight dependence of the melting and crystallization temperatures of the PVDF/KF/PEG is similar to those of the KF/PEG. In addition, the latent heats of the KF/PEG and PVDF/KF/PEG are obviously lower than that of the pure PEG due to the incorporation of the KF and/or PVDF that does not undergo phase change within the measured temperature range (Table S2).
Mechanical properties of PVDF/KF/PEG composite films
The PEG leakage during the phase transition can be effectively prevented by using the hollow tubular KF. However, the hollow structure of KF and its high moisture absorption like other cellulose fibers inevitably lead to poor mechanical properties, and ubiquitous micro-biological attack could result in further degradation in the mechanical properties and the reduction in its service life. This makes the KF/PEG difficult to address the mechanical performance requirements of electronic device applications. Therefore, the incorporation of PVDF is not only expected to enhance the dielectric polarization of the composites but also increase their mechanical strength and life span. Figure 5 shows the specific strength of the PVDF/KF films as functions of KF content. The stress-strain curves of the composite films are displayed in Figure S3. The drop in the specific strength has been caused by the increase in void content with increasing fiber loading. But even in the case of high fiber content, relatively high specific strength is attained. As we know, the macroscopic mechanical properties of such composites are determined predominantly by the properties of the fiber-matrix interface. The mechanical properties are expected to further improve via strengthening the fiber-matrix interface in consideration of the inadequate bond strength in this composite system.
Dielectric properties of PVDF/KF/PEG composite film
The effects of frequency and PEG molecular weight on the dielectric properties of the PVDF/KF/PEG composite films is shown in Figure 6. The relative dielectric constant (εr) of nearly all specimens goes down as frequency goes up (Figure 6a). The dielectric constants of the composite film at the same frequency are significantly higher than that of the pristine PVDF, especially at low frequency range. This indicates that the polarization ability of the composite films is significantly enhanced due to the introduction of polar PEG of relatively small molecular weights (Yamamoto, Minami, Hui, Morikawa, & Kimizuka, 2020). Besides, the PEG with lower molecular weight has a higher relative proportion of polar sections, and thus results in higher electric polarization under the electric field. It can be seen from Fig. 6b that the loss tangents of the PVDF and the composites decrease with frequency increase and then begin to increase at frequency ~105 Hz, which is typical feature of conduction electron and interfacial polarization relaxation. It is worth noting that the loss tangent values of all the composites are higher than that of PVDF. and the smaller the molecular weight of PEG, the higher the loss tangent value of the composites at the same frequency, especially in the middle and low frequency range, which is related to the introduction of polar PEG molecules. Additionally, the increased defects and interfaces inside the composites caused by the introduction of fibers contribute to the significant increase in interfacial loss.
The dielectric constants of the PVDF, PVDF/KF/PEG800, PVDF/KF/PEG2k, and PVDF/KF/PEG10k at 1k Hz are plotted against temperature in Fig. 7. The pure PVDF shows a gradual rise in dielectric constant with temperature. While the dielectric constants of all the composites significantly increase with temperature in the measured temperature range. The PVDF/KF/PEG800 undergo a noteworthy εr increase around 25°C, which matches the melting temperature span of PEG800 in the composite. Both the PVDF/KF/PEG2k and PVDF/KF/PEG10k also exhibit a similar trend of the temperature dependence of dielectric constant to the PVDF/KF/PEG800. The dielectric susceptibility in the composites is temperature dependent, and the temperature intervals of the dielectric transition are roughly consistent with the melting temperature ranges of the relevant molecular weight of the PEG in the composites. The melting phase transition of PEG upon heating increases considerably the effective dipole moment that is mainly associated with orientational polarization, and thus contributes to the dramatic rise of the dielectric constant. In addition, the conductivity of the composites also showed a similar transition pattern with temperature to the dielectric constant (Figure S4). Although the temperature range of the dielectric transition does not completely consistent with that of the conductivity susceptibility, it can be inferred that conduction electron polarization partially contributes to the dielectric response. With the increase of PEG molecular weight, the temperature width of the dielectric transition of the composites gradually decreases. Among them, the dielectric transition temperature range of PVDF/KF/PEG10k is the narrowest, which may be related to the relatively high perfection degree of crystal structure of the PEG10k.
The change in polarity before and after the phase transition can result in significant difference in dielectric state, which provides the precondition for the dielectric bistability of the composites. For thermally-responsive dielectric materials, a relatively wide hysteresis loop at temperatures near room temperature is also important for their practical application besides bistability (Cao et al., 2021; Liu, Haibibu, Xu, Han, & Wang, 2020; Tripathi et al., 2021; Yamaguchi, 2021). The structural phase transition of PEG, depending on the molecular weight, typically occurs at or near room temperature. The thermal hysteresis behavior of the PVDF/KF/PEG composites containing PEG of different molecular weights during the heating and cooling cycles is illustrated in Figure 8. The three composites exhibited different dielectric transition characteristics during heating and cooling, and the dielectric thermal hysteresis loop gradually widens as the molecular weight of PEG in the composites increases, among which the hysteresis loop width of PVDF/KF/PEG10k achieves 16.5°C (Table S3). Unlike other types of dielectric responsive materials based on inorganics or organic small molecules, PEG, as a polymer, possesses relatively long relaxation time because of the high molecular weight. The long-chain structure of PEG plays an important role in the melting behavior and crystallization kinetics caused by intermolecular friction within the PEG. The supercooling effect affected by the dynamics or relaxation of polymer chains leads to the differences in the temperature shifts of crystallization and melting, accordingly generating a dielectric hysteresis loop. The molecular chain length of the PEG plays critical role on the intermolecular actions. As the molecular weight increases, the intermolecular interactions between the PEG chains increases, leading to enhanced supercooling effect and thus the increased relaxation time.