Thermal-induced dielectric response in mechanically durable polyvinylidene uoride/kapok encapsulated polyethylene glycol composite lms

Dielectric materials with thermally responsive property are being pursued in elds such as next-generation sensors, smart switches, and novel actuators. These applications require that the dielectric materials have mechanical durability and stable serviceability besides thermally responsive dielectric behavior. Herein, we report a novel thermally responsive, mechanically durable, and low-cost dielectric composite simply fabricated by vacuum impregnating polyethylene glycol (PEG) into Kapok ber (KF) and compounding them with polyvinylidene uoride (PVDF). A remarkable dielectric susceptibility, controlled dielectric transition temperature and obvious thermal hysteresis of the composite lms induced by the solid-liquid phase transition of PEG are demonstrated. The effect of molecular weights of PEG on the dielectric response behaviors is evaluated. Such thermally responsive dielectric materials with satisfactory mechanical durability will offer a chance toward constructing thermally responsive systems for safe and stable operation.


Introduction
Thermally responsive dielectric materials have received increasing attention in the last decade because of their ability to undergo dielectric transitions in response to temperature stimuli. [1][2][3][4] These materials can undergo transitions from high to low dielectric states or vice versa in a narrow temperature interval, [5][6][7][8][9] and exhibit a distinct bistability, [10][11][12] having a wide range of applications in new smart devices, sensors, switches, actuators, etc. Among these materials, porous molecular materials containing guest small molecules or ions have become a research hotspot. [13] When the temperature reaches a certain temperature, the ordered-disordered transition produced by the guest molecules within the system allows the appearance of two or more signi cantly different dielectric states. [12,14] Despite the excellent dielectric properties of such materials, the complicated design and preparation process, as well as the intrinsic defects in the mechanical properties of the materials, make them a major obstacle in their applications. Moreover, the mechanical properties of thermo-responsive dielectric materials have rarely been reported. It actually is challenging to develop a simple and low-cost approach to fabrication of exible and mechanically durable thermally responsive dielectric materials. Recently, a thermal-induced dielectric susceptibility has been observed in PEG-contained composites. [15,16] The use of PEG as a thermally responsive dielectric material is very promising. The switching between high and low dielectric states can be triggered by the crystalline-amorphous phase transition of PEG induced by heat. [17] However, ensuring the satisfactory mechanical properties of the materials, shape stability as well as enhanced dielectric polarization during the phase transition is a major challenge in their service processes. In recent years, Kapok ber (KF), a cellulose material, has received tremendous attention as a renewable adsorbent and template material [18][19][20] for its natural microtubule structure and high void content of more than 80%, besides its other desirable properties such as low cost, biodegradability and high speci c properties compared with conventional inorganic llers. The usage of KF as a PEG carrier is expected to prevent PEG leakage, ensure the shape stability and achieve low density.
Herein, authors report the simple fabrication of a novel thermally responsive, mechanically durable, and low-cost dielectric composite using natural tubular KF as the PEG carrier, PVDF as the matrix, and PEG as the phase change material, which responds to temperature. The PVDF is utilized to enhance the dielectric polarization and strengthen mechanical property. The structure, thermal, dielectric and tensile properties of the PVDF/KF/PEG composites were investigated, and the effect of the molecular weights of PEG on the dielectric response and thermal hysteresis were evaluated. The results presented in this study contribute to both basic and applied research as well as exhibit high potential for novel electronic device applications.

Materials and preparation
Kapok bers used were common products that can be purchased in the market. PEG (800, 2k, 10k) was purchased from Sinopharm Group Chemical Reagent Co. PVDF was purchased from Shanghai San Aifu New Material Co. All other drugs were of analytical purity and purchased from Sinopharm Chemical Reagent Co. PEG was encapsulated into natural Kapok bers by vacuum impregnation. The Kapok bers were crushed into short bers of about 0.3-0.5 mm in length and placed into a container, then an appropriate amount of dissolved PEG was added to completely submerge the Kapok bers and placed in a vacuum for 1 hour. After the samples were dried, the samples were washed twice with DMF to remove the PEG attached to the surface. Next, a certain amount of PVDF and DMF were added to the Kapok bers, stirred until well dispersed, and the mixture was placed in an oven and left to stand until the solvent evaporated completely. Finally, the resulting sample was hot pressed to obtain PVDF/KF/PEG composite lm. In this study, the mass fractions of the PEG with various molecular weights in KF/PEG depend on their saturated adsorption capacities in KF without leakage at room temperature and atmospheric pressure, and the feed ratios of PVDF to KF/PEG bers in the PVDF/KF/PEG are 7:3.

Characterization
The dielectric properties of the sample lms, including conductivity, dielectric constant and dielectric loss, were tested using a dielectric spectrometer (KYKYSBC-12 LCR Co., Ltd., Japan). The scanning frequency was 100 Hz to 1 MHz and the temperature range was 0-80°C. The surface morphology of the material was characterized using eld emission electron microscopy. The differential scanning calorimetry analysis was performed using a DSC3500 (NETZSCH Group, Germany) under a nitrogen atmosphere with a double-cycle test at a temperature variation rate of 10°C·min -1 from 0-80°C. The chemical structure of the composite phase change energy storage material was tested using Fourier transform infrared spectroscopy with a scan interval of 4000-400 cm -1 . X-ray tests were performed on an X-ray generator with a scan range of 2θ = 5-60°. The microscopic morphology of the composite lms was characterized by means of a eld emission electron microscope (JSM-6360LV, JEOL). Thermogravimetric analysis was performed by an STA2500 (NETZSCH, Germany), where the samples were elevated from 20°C to 800°C at a rate of 10 k·min -1 under a nitrogen atmosphere. The tensile properties of KF/PVDF composites were obtained by stretching dumbbell samples of the composite lms at a stretching rate of 20 mm·min -1 using an Instron stretching machine.

Results And Discussion
Structural analysis of KF/PEG and PVDF/KF/PEG composite lms Figure 1 and S1 shows the XRD patterns of KF/PEG, PVDF/KF/PEG composite lms, pristine KF and pure PEG, respectively. It can be seen from the gures that the pristine KF is semi-crystalline in crystal 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)  The TG curves of KF, KF/PEG, and PVDF/KF/PEG composite lms 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 ber 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 signi cant 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 were obtained by calculating the residual carbon rates of different composite lms, where PVDF/KF/PEG800 adsorbed the most mass of PEG with a mass ratio of 15.3%, compared to 14.9% and 10.7% for PVDF/KF/PEG2k and PVDF/KF/PEG10k, respectively. SEM images (Figure 3) show the cross sections of the PVDF/KF/PEG composite lms. The KF displays a high hollow ratio of ~80%, which provides su cient 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 e ciency of the composites. Figure 3 shows that the KF/PEG bers are randomly dispersed and disoriented in the PVDF matrix, and the tubular structure of the KF bers remains intact after the solution casting and hot pressing. After compounding the KF/PEG ber 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 lms is an essential prerequisite for triggering dielectric response. Figure 4 and Figure S2 show the freezing and melting DSC curves of the KF/PEG and PVDF/KF/PEG. Figure S2(d) 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 S1).

Tensile properties of PVDF/KF/PEG composite lms
The PEG leakage during the phase transition can be effectively prevented by using the hollow tubular KF. However, the hollow structure of KF inevitably leads to unfavorable 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 di cult 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 4 shows the tensile strengths, Young's modulus and elongation at break of the PVDF/KF/PEG lms as functions of KF content. The stress-strain curves of the composite lms are displayed in Figure  S3. The drop in the tensile strength has been caused by the increase in void content with increasing ber loading. But even in the case of high ber content, relatively high speci c strength is attained. The Young's modulus decreases with increasing KF mass fraction to a minimum at 6 wt% KF and then increases with further increasing KF content up to a maximum at 12 wt% KF giving more than 130% the value of the neat PVDF. The elongation at break increased with increasing KF content, attained maximum values at 3 wt%, and then decreased. With KF concentration increasing, more and more KF are subject to the external force, causing the composites become stiff and hard. Therefore, the KF/PEG content should be controlled within a reasonable range according to the requirements of practical applications for the strength and exibility of the composite lms. As we know, the macroscopic mechanical properties of such composites are determined predominantly by the properties of the ber-matrix interface. The mechanical properties are expected to further improve via strengthening the ber-matrix interface in consideration of the inadequate bond strength in this composite system.

Dielectric properties of PVDF/KF/PEG composite lm
The effects of frequency and PEG molecular weight on the dielectric properties of the PVDF/KF/PEG composite lms is shown in Figure 4. The relative dielectric constant (ε r ) of nearly all specimens goes down as frequency goes up (Figure 4a). The dielectric constants of the composite lm at the same frequency are signi cantly higher than that of the pristine PVDF, especially at low frequency range. This indicates that the polarization ability of the composite lms is signi cantly enhanced due to the introduction of polar PEG of relatively small molecular weights. [21] 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 eld. It can be seen from Fig. 5b that the loss tangents of the PVDF and the composites decrease with frequency increase and then begin to increase at frequency ~10 5 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 bers contribute to the signi cant 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 signi cantly increase with temperature in the measured temperature range. The PVDF/KF/PEG800 undergo a noteworthy ε r 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 signi cant 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. [22][23][24][25] 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 18.1°C (Table S2). 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.

Conclusion
A novel thermally responsive, mechanically durable, and low-cost dielectric composite simply fabricated by vacuum impregnating PEG into KF and compounding them with PVDF has been reported. The structure, thermal, dielectric and tensile properties of the PVDF/KF/PEG composites were investigated, and the effect of the molecular weights of PEG on the dielectric response and thermal hysteresis were evaluated. The results reveal a remarkable dielectric susceptibility, controlled dielectric transition temperature and obvious thermal hysteresis of the composite lm induced by the solid-liquid phase transition of PEG. The change in polarity before and after the phase transition induced by heat can result in signi cant difference in dielectric state, which facilitates the dielectric bistability of the composites. An 18.1°C wide hysteresis loop in the thermally-responsive dielectric composites at temperatures near room temperature is attained. Such thermally responsive dielectric materials, based on a non-ferroelectric phase transition, with satisfactory mechanical durability will offer a chance toward constructing low-cost thermally responsive systems for safe and stable operation.       Dielectric constant (a) and dielectric loss tangent (b) versus frequency for the PVDF and PVDF/KF/PEG containing PEG with various molecular weights.

Figure 7
Temperature dependence of the dielectric constants of the pure PVDF and PVDF/KF/PEG composite lms at 100 Hz.