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Article

Preparation and Characterization of Fluorine-Containing Polyimide Films with Enhanced Output Performance for Potential Applications as Negative Friction Layers for Triboelectric Nanogenerators

by
Zhen Pan
1,
Shunqi Yuan
2,
Xi Ren
1,
Zhibin He
3,
Zhenzhong Wang
1,
Shujun Han
1,
Yuexin Qi
1,
Haifeng Yu
3 and
Jingang Liu
1,*
1
Engineering Research Center of Ministry of Education for Geological Carbon Storage and Low Carbon Utilization of Resources, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
2
RAYITEK Hi-Tech Film Co., Ltd., Shenzhen 518105, China
3
Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, School of Material Science and Engineering, Peking University, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Technologies 2023, 11(5), 136; https://doi.org/10.3390/technologies11050136
Submission received: 26 August 2023 / Revised: 20 September 2023 / Accepted: 28 September 2023 / Published: 3 October 2023
(This article belongs to the Section Assistive Technologies)
Browse Figures
Versions Notes

Abstract

:
Nanotechnologies are being increasingly widely used in advanced energy fields. Triboelectric nanogenerators (TENGs) represent a class of new-type flexible energy-harvesting devices with promising application prospects in future human societies. As one of the most important parts of TENG devices, triboelectric materials play key roles in the achievement of high-efficiency power generation. Conventional polymer tribo-negative materials, such as polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), and the standard polyimide (PI) film with the Kapton® trademark based on pyromellitic anhydride (PMDA) and 4,4′-oxydianiline (ODA), usually suffer from low output performance. In addition, the relationship between molecular structure and triboelectric properties remains a challenge in the search for novel triboelectric materials. In the current work, by incorporating functional groups of trifluoromethyl (–CF3) with strong electron withdrawal into the backbone, a series of fluorine-containing polyimide (FPI) negative friction layers have been designed and prepared. The derived FPI-1 (6FDA-6FODA), FPI-2 (6FDA-TFMB), and FPI-3 (6FDA-TFMDA) resins possessed good solubility in polar aprotic solvents, such as the N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP). The PI films obtained via the solution-casting procedure showed glass transition temperatures (Tg) higher than 280 °C with differential scanning calorimetry (DSC) analyses. The TENG prototypes were successfully fabricated using the developed PI films as the tribo-negative layers. The electron-withdrawing trifluoromethyl (–CF3) units in the molecular backbones of the PI layers provided the devices with an apparently enhanced output performance. The FPI-3 (6FDA-TFMDA) layer-based TENG devices showcased an especially impressive open-circuit voltage and short-circuit current, measuring 277.8 V and 9.54 μA, respectively. These values were 4~5 times greater when compared to the TENGs manufactured using the readily accessible Kapton® film.

1. Introduction

Energy, as the fundamental material basis for human society’s progress, is not only crucial for improving individuals’ quality of life and living standards, but is also essential for sustaining the stability and development of society. As the Internet of Things (IoT) era approaches, the increasing use of intelligent electronics brings with it a growing demand for a lightweight, sustainable, long-lasting, and miniaturized energy source [1,2,3,4,5]. Therefore, researchers are gradually becoming interested in, and motivated by, the mechanical energy generated in natural environments [6,7,8,9]. One notable energy-harvesting method applying triboelectric nanogenerators (TENGs) was proposed by Z. L. Wang to convert mechanical energy into electricity [10]. These well-designed TENGs are capable of efficiently generating electricity by collecting distributed high-entropy energies, such as wind energy, human motion energy, vibration energy, and other low-frequency energies. That is to say, TENGs make it possible to utilize any available energy surrounding people in an environment [11,12,13,14,15,16]. As an emerging energy-harvesting technique, TENGs have demonstrated immense potential, owing to their remarkable advantages in terms of energy conversion efficiency, power generation, dependability, and eco-friendliness [17,18,19].
Typically, a TENG device comprises a combination of triboelectric materials possessing distinct surface electrical potentials, which function as a tribo-positive layer and a tribo-negative layer, respectively, to realize the generation and transfer of an electric charge when they come into contact with each other. Therefore, the performance and application areas of the TENG technique are highly related to the designed triboelectric materials, with a high charge density and diversified functions [20,21,22,23,24]. A series of well-known polymers containing extensive amounts of electron-withdrawing groups are widely utilized as a negative friction layer to assemble TENG devices, such as poly(vinylidene fluoride) (PVDF), polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), and polyimide (PI) [25,26,27]. Among these materials, PI films enjoy great popularity due to their extraordinary heat-resistant, mechanical, insulated, and dielectric properties, which are critical for TENG applications in various areas of electricity and energy, such as wearable electronics and flexible displays [28,29,30].
However, the most frequently used and commercialized PI films, such as the Kapton® (DuPont, Wilmington, DE, USA) film based on pyromellitic anhydride (PMDA) and 4,4′-oxydianiline (ODA), usually exhibit a relatively limited electron-withdrawing ability. This does not help in achieving a high energy-harvesting ability. Thus, in order to fulfill the urgent requirement for high-performance TENGs, it is imperative to develop PI materials with a higher electron affinity. For instance, different ionically charged groups, such as sodium sulfate and quaternary ammonium chlorides, which can realize the development of multifunctional triboelectric materials with different polarities, are conjugated to PI layers through chemical modification [31]. In addition, functionalities, via sulfone (–SO2–) and trifluoromethyl (–CF3) groups, were introduced into the PI molecular backbones to develop triboelectric materials with high thermal charge stability, which can support the effective generation of charge under high-temperature working conditions [32]. These studies highlight the potential to improve the performance of triboelectric generators through adjustments in the chemical composition of the polymer. Moreover, the incorporation of functional fillers, such as graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotube, and MXene, has also been regarded as a promising way to prepare triboelectric materials with a high charge density [33,34,35]. rGO fillers have been embedded in the PI friction layer as electron traps to improve the TENG’s output performance. The maximum output power density of a TENG consisting of PI-containing rGO sheets reached 6.3 W/m2, which is 30 times greater than that of the pristine device [36]. Therefore, exploring the potential link between the molecular structure of an object and its macroscopic electrification properties could inspire innovative approaches to the development of triboelectric matrix polymers with enhanced charge densities to improve TENG devices’ performance and broaden their application area.
The current work aims to investigate the relationship between the molecular structure of PI layers and the triboelectric properties via molecular simulations and experiments. In order to develop new PI materials with superior triboelectric properties, three solution-processable PIs containing trifluoromethyl (–CF3) with a strong electron-withdrawing ability, including FPI-1 (6FDA-6FODA), FPI-2 (6FDA-TFMB), and FPI-3 (6FDA-TFMDA), were synthesized. The mechanism by which the triboelectric properties of TENG devices can be enhanced via the incorporation of –CF3 groups was researched in greater detail.

2. Materials and Methods

2.1. Materials

The aromatic dianhydride 6FDA with a purity higher than 99.5% was purchased from ChinaTech (Tianjin) Chem. Co., Ltd. (Tianjin, China) and vacuum-dried at 180 °C for 24 h before use. The fluorine-containing diamines, including 2,2′-bis(trifluoromethyl)-4,4′-diaminophenyl ether (6FODA, purity: 99.6%), 2,2′-bis(trifluoromethyl)benzidine (TFMB, purity: 99.8%), and 3,5-diaminobenzotrifluoride (TFMDA, purity: 99.6%), were supplied by Changzhou Sunlight Pharmaceutical Co., Ltd. (Changzhou, Jiangsu, China) and used directly. Polytetrafluoroethylene (PTFE) film (thickness: 30 μm) was purchased from Sigma-Aldrich, Shanghai, China, and used as received. Electronic-grade N,N-dimethylacetamide (DMAc, purity: 99.9%, water content < 50 ppm), spectroscopically pure N-methyl-2-pyrrolidone (NMP, purity: 99.9%, water content < 50 ppm) and other reagents such as anhydrous ethanol were purchased from Beijing Innochem Science & Technology Co., Ltd. (Beijing, China) and used as received.

2.2. Characterization Methods

To investigate the solubility of the PI resins in the tested solvents (10 wt.% solid content), the resins were mixed with the solvents and mechanically stirred for 24 h at room temperature. The solubility was visually determined and graded as completely soluble (++), partially soluble (+), and insoluble (−). The number average molecular mass (Mn) and weight average molecular mass (Mw) of the PI resins were determined through gel permeation chromatography (GPC) analysis using a Shimadzu high-performance liquid chromatography (HPLC) system (Kyoto, Japan). The mobile phase was NMP. To measure the hydrogen nuclear magnetic resonance (1H-NMR) of the PI resins, an Avance III HD 600 MHz nuclear magnetic resonance spectrometer (Bruker, Fällanden, Switzerland) was utilized. The physical and chemical features of the PI films were analyzed using different techniques. The Fourier transform infrared (FTIR) spectra were scanned using a Shimadzu Iraffinity-1S FT-IR spectrometer (Kyoto, Japan) over a range of 4500 cm−1 to 400 cm−1. The wide-angle X-ray diffraction (XRD) analysis was performed using a Rigaku D/max-2500 X-ray diffractometer (Tokyo, Japan). The differential scanning calorimetry (DSC) analysis was carried out using a TA-Q 100 thermal analysis system (New Castle, DE, USA) under nitrogen atmosphere with a heating rate of 10 °C/min. The thermal decomposition temperature of the PI films was determined using the STA-8000 thermogravimetric analyzer (Perkin-Elmer, Waltham, MA, USA). The testing took place inside a nitrogen environment at a heating speed of 20 °C per minute. To assess the thermal behavior, the TGA test spanned from 30 to 750 °C. A TMA402F3 thermal analysis system (NETZSCH, Selb, Germany) was utilized for performing thermo-mechanical analysis (TMA) within a nitrogen atmosphere. The heating rate for this analysis was set at 5 °C per minute over a temperature range of 50 to 450 °C. The evaluation encompassed measurements of the composite films’ coefficients of linear thermal expansion (CTE) in the temperature range of 50 to 250 °C.
The UV-Vis spectra of the PI films were obtained by utilizing a spectrophotometer model U-3210 manufactured by Hitachi (Tokyo, Japan) at room temperature. The yellow indices of the PI films were determined by employing a color i7 spectrophotometer released by X-rite (Grand Rapids, MI, USA). The thickness of the PI samples used for measurement was 30 μm. The color parameters were assessed using the CIE (International Commission on Illumination) Lab equation. The lightness factor, denoted by L*, ranges from 0 (representing black) to 100 (signifying white). An a* value above zero represents a red color, while a negative a* value suggests a green color. The electrical properties of the TENGs were measured using a 6517B electrometer (Keithley Instruments, Inc., Cleveland, OH, USA), including open-circuit voltage (Voc), short-circuit current (Isc), and transferred short-circuit charge (Qsc). The geometry of the PI models was optimized using Gauss density functional theory (DFT) and the frontier molecular orbitals were calculated to determine the electrostatic potentials, the highest energy occupied molecular orbital (HOMO) energies, the lowest energy unoccupied molecular orbital (LUMO) energies, and the corresponding energy gap values (εHOMO − εLUMO). The Gaussian 09 code with B3LYP exchange-correlation and a basis of 6−31G(d) was utilized for the simulation and calculations.

2.3. PI Resins Synthesis and Films Preparation

A series of fluorine-containing PI resins with designed molecular structures, including FPI-1 (6FDA-6FODA), FPI-2 (6FDA-TFMB), and FPI-3 (6FDA-TFMDA), were synthesized via a two-stage chemical imidization procedure according to the formula shown in Table 1. For illustration, the synthesis procedure has been depicted in detail for the case of FPI-3. Into a three-necked 500 mL flask equipped with a mechanical stirrer, a nitrogen inlet, and a cold bath, was added TFMDA (8.8070 g, 0.05 mol) and ultra-dry DMAc (80.0 g). The clear diamine solution was obtained after stirring at room temperature for 10 min under nitrogen. Then, 6FDA (22.2120 g, 0.05 mol) was rapidly added and an additional DMAc (13.1 g) was added as well. A solid content of 25 wt.% was achieved for the polymerization mixture. After 3 h, the cold bath was removed, and the temperature rose to room temperature (25 °C). Then, the reaction was continued until the total polymerization time reached 24 h. The pale-yellow and viscous solution of poly(amic acid) (PAA) was subjected to the addition of acetic anhydride (51.0 g, 0.5 mol) as the dehydrating agent, followed by the catalyst pyridine (31.6 g, 0.4 mol), with vigorous stirring. The chemical procedure for imidization was conducted at room temperature, lasting for another 24 h. Subsequently, the reaction mixture was poured into an excessive amount of aqueous ethanol solution (75% volume). As a result, FPI-3 resin was precipitated from the ethanol solution, presenting a silky appearance due to its pale-yellow color. The resin was thoroughly immersed in ethanol solution and subsequently filtered to separate it from the solution. The collected resin was first dried in air environment and then in vacuo at 120 °C for 24 h. The pale-yellow fibrous FPI-1 resin was finally obtained. Yield: 28.51 g (97.6%); Mn: 5.37 × 104 g/mol; Mw: 1.06 × 105 g/mol; PDI: 1.65. 1H-NMR (DMSO-d6, ppm): 8.24–8.22 (d, 2H), 8.01–7.98 (m, 4H), 7.83–7.76 (m, 4H), and 7.46–7.44 (d, 2H).
The other PI resins, including FPI-1 (6FDA-6FODA) and FPI-2 (6FDA-TFMB), were prepared according to a similar procedure as mentioned above, except that TFMDA was replaced with 6FODA for FPI-1, and with TFMB for FPI-2.
FPI-1 (6FDA-6FODA): Mn: 2.52 × 105 g/mol; Mw: 3.22 × 105 g/mol; PDI: 1.26. 1H-NMR (DMSO-d6, ppm): 8.28–8.26 (d, 2H), 8.05–8.01 (m, 4H), 7.87–7.81 (m, 4H), and 7.72–7.69 (d, 2H).
FPI-2 (6FDA-TFMB): Mn: 1.09 × 105 g/mol; Mw: 1.82 × 105 g/mol; PDI: 1.68. 1H-NMR (DMSO-d6, ppm): 8.23–8.19 (d, 2H), 8.00–7.93 (m, 5H), and 7.76 (s, 2H).
The fully dried FPI-3 resin was dissolved in DMAc at room temperature, with a solid content of 15 wt.%. The clear and transparent PI solution obtained was purified via filtration through a 1.0 μm Teflon syringe filter. The PI varnish was de-foamed in vacuo and then cast onto a clean glass with a doctor knife. The wet film was then thermally baked in a clean oven following the regimen of 80 °C for 2 h, 150 °C for 1 h, 180 °C for 1 h, 200 °C for 1 h, and 250 °C for 1 h. After that, the resulting glass carrier was cooled to room temperature and immersed in deionized water. The FPI-3 film was peeled off of the substrate, and the free-standing film was dried under vacuum at 120 °C for 24 h. FTIR (cm−1): 1786, 1724, 1489, 1435, 1377, 1315, 1244, 1132, 1051, and 719.
The other PI films, including FPI-1 (6FDA-6FODA) and FPI-2 (6FDA-TFMB), were prepared by following a similar procedure as that mentioned above.
FPI-1 (6FDA-6FODA)—FTIR (cm−1): 1786, 1724, 1491, 1425, 1364, 1310, 1254, 1120, 1072, and 719.
FPI-2 (6FDA-TFMB)—FTIR (cm−1): 1786, 1722, 1470, 1402, 1350, 1313, 1240, 1192, 1134, and 718.

2.4. TENG Devices Fabrication

The TENG devices were fabricated with copper conductive tape (thickness: ~40 μm) as electrodes, PI films (thickness: ~30 μm) as the tribo-negative layers, and natural rubber (NR) film (thickness: ~1 mm) with 15% carbon black as the tribo-positive material [37,38,39]. The active contact area was 16 cm2 (40 mm × 40 mm). Polyethylene terephthalate (PET) film was used as a substrate for protection. Furthermore, in order to achieve an effective contact/separation process, a linear motor has been used in this paper to ensure regular motion frequency. A TENG device with PTFE (thickness: ~30 μm) as the tribo-negative layer was also fabricated for reference.

3. Results and Discussion

3.1. PI Resins Synthesis and Films Preparation

Triboelectric materials with PI layers have been widely studied and applied in the field of TENGs. Based on the diversity of molecular structures, functional groups with strong electron-withdrawing abilities, such as –CF3 and sulfonyl groups, have been proven to be efficient for enhancing the output performance of the devices. Thus, in the current work, three fluorine-containing PI layers were designed and synthesized according to the procedure shown in Figure 1. First, the poly(amic acid) (PAA) precursors with equal stoichiometry of the 6FDA dianhydride and corresponding diamines were synthesized. Then, the PAA solutions were chemically cyclized via the dehydration reagents of acetic anhydride (Ac2O) and pyridine (molar ratio of Ac2O/pyridine = 5/4). During the dehydration courses, all three polymerization systems maintained their homogeneous states, and no gelling or precipitating phenomena were observed in the resins. The good solubility of the PI resins in the polymerization medium was demonstrated. Finally, the PI filaments were obtained by precipitating the PI solution in an inadequate solvent of an aqueous ethanol solution. The PI resins derived exhibited flexible and tough characteristics, indicating their high molecular weights. This could be proven by the intrinsic viscosities ([η]inh) and molecular mass data summarized in Table 2. All the PI resins exhibited [η]inh values exceeding 0.70 dL/g and number average molecular mass (Mn) values higher than 5.0 × 104 g/mol. These values increased with the order of FPI-3 < FPI-2 < FPI-1, reflecting the increasing polymerization reactivities of TFMDA < TFMB < 6FODA. This could be mainly ascribed to the electronic effects in the diamine monomers. In the case of TFMDA, the two amino groups and the –CF3 substituent attached to the same phenyl ring. Thus, the electron densities in the amino units were apparently decreased due to the highly electron-withdrawing –CF3 groups, resulting in a decreased reactivity during the nucleophilic substitution reaction when the amine attacked the anhydride carbonyl carbons to form the PAA chains. Similarly, the TFMB diamine also showed inferior reactivity due to the electron mobility along the conjugated biphenylene units in the compound. By contrast, the diamine of 6FODA exhibited the highest reactivity due to the electron-donating feature of the oxygen bridges. Thus, FPI-1 showed the highest [η]inh and Mn values in the series of polymers.
The confirmation of the chemical structures of the PI resins was accomplished through the utilization of 1H-NMR measurements, with the results being displayed in Figure 2. The chemical shifts of the hydrogen protons are highly affected by the chemical environments in the polymers. Protons residing within the 6FDA moiety and those adjacent to the electron-withdrawing imide carbonyl groups (designated as Hb and Ha, respectively) exhibited absorptions located at the most upfield positions in the spectra. Moreover, the protons adjacent to the –CF3 groups with electron withdrawal in the diamine moiety (He for FPI-1 and FPI-2) also showed absorption at the second farthest downfield point in the spectra. For FPI-3, the He proton showed a single absorption peak at the farthest point upfield in the spectra, although it was ortho-substituted to the –CF3 group. These discerned structural characteristics in the spectra agree well with the anticipated molecular structures of the targeted polymers.
The PI resins were soluble, not only in polar aprotic solvents with high boiling points, such as NMP, DMAc, and γ-butyrolactone (GBL), but also in some solvents with moderate or low boiling points, such as cyclopentanone (CPA) and tetrahydrofuran (THF) at room temperature, as shown in Table 2. The loose molecular chain packing in the current PI resins has a significant impact on their good solubility. This loose packing is mainly due to the presence of bulky hexafluoroisopropylene units in the dianhydride moiety and the –CF3 substituents in the diamine moiety. It is these structural characteristics that give the PI resins their amorphous nature, as demonstrated by the XRD results presented in Figure 3. No sharp crystalline absorption peaks were observed in the scattering angle (2 theta) range of 10–80° for all of the samples.
The good organo-solubility of the current PI resins makes it possible to fabricate the PI films via solution processing procedures using the pre-imidized PI resins as the starting materials instead of the conventional PAA precursors. Thus, the PI films could be obtained at relatively low processing temperatures (80~250 °C). This is important for the practical applications of the PI functional layers in some temperature-sensitive fields, including the fabrication of TENG devices. Figure 4 shows the FTIR spectra of the PI film, from which the chemical structural information of the polymers can be clearly inferred. First, the presence of absorption peaks around 1786 cm−1 and 1724 cm−1 indicates the asymmetric and symmetric stretching vibrations of the carbonyl group (C=O) in the imides, respectively, while the peaks at 1377 cm−1 and 719 cm−1 represent the stretching vibration of imide C–N bonds and the imide carbonyl bending vibration, respectively. The structural information implies the successful formation of imide rings in the polymers. Additionally, two types of C–F absorptions were detected in the spectra. Those located around 1132 cm−1 can be ascribed to the C–F stretching vibrations of the aliphatic fluoro chains in the 6FDA moiety, whereas the ones around 1315 cm−1 correspond to asymmetric stretching vibrations of the C–F bonds on the benzene rings of the diamine moiety. Thus, the PI films with the desired chemical structures were successfully prepared.

3.2. Thermal Properties

PIs are well known for excellent heat resistance, which is quite beneficial to the development of TENGs that will be operated in harsh environments [40,41,42]. In order to investigate the effects of functional groups on the high-temperature resistance of the PI layers, TGA, DSC, and TMA tests were conducted, and the corresponding thermal data are summarized in Table 3. Figure 5 depicts the TGA plots of the PI films measured under a nitrogen atmosphere. It could be deduced from the chart that all the PI films maintained their initial weights below 420 °C, after which they began to decompose, and they showed 5% weight loss temperatures (T5%) in the range of 534~541 °C. The most rapid thermal decomposition occurred in the temperature range of 606~619 °C, and the films left 50.9~53.5 wt.% of their original weights at 750 °C (Rw750). Thus, the herein-developed fluorine-containing PI films exhibited excellent thermal stability, indicating that the incorporation of the –CF3 groups apparently improved the polarity of the intra-molecular covalent bonds. This, in turn, enhanced the heat resistance of the polymers. It is worth noting that FPI-2 (6FDA-TFMB) exhibited the highest thermal stability, with T5% and Rw750 values of 541 °C and 53.5 wt.%, respectively. This could be attributed to the rigid-rod biphenylene units in the diamine moiety, besides the –CF3 groups.
The glass transition temperatures (Tg) of FPI films were determined via DSC and TMA tests, respectively. Figure 6 shows that the PI films displayed distinct glass transition properties within the temperature range of 282.4–304.5 °C. As expected, the FPI-1 film with flexible –O– linkages showed the lowest Tg value of 282.4 °C, while the FPI-2 film with rigid-rod biphenylene units showed the highest value of 304.5 °C. A similar trend was observed in the TMA measurements shown in Figure 7. With the increase in the test temperature, all the films showed expanding behaviors until the temperature reached the Tg values of the polymers. All the films began to shrink around the Tg, which persisted until the finish of the molecular chain rearrangement. After that, the films expanded again until fracturing or breaking. The revealed Tg values were in the range of 300.4 °C to 336.5 °C, which are about 10 °C higher than those yielded by using DSC measurements. In addition, the linear expansion data determined prior to Tg reveal the linear coefficients of thermal expansion (CTE) for the PI films. The CTE values for FPI-1, FPI-2, and FPI-3 in the temperature range of 50–250 °C were found to be 51.3 × 10−6/K, 25.7 × 10−6/K, and 42.5 × 10−6/K, respectively. As expected, the FPI-2 with a rigid-rod molecular skeleton exhibited the lowest CTE value. The low-CTE feature of the PI films is beneficial to their application in TENGs, especially in high-temperature environments.

3.3. Optical Properties

The optical transparency of the PI layers is usually not one of the biggest concerns in relation to TENG applications. However, the molecular design and production methodologies of the PI tribo-negative layers with the best output performance are somewhat consistent with those for the development of colorless and highly transparent PI films. Thus, it is necessary to investigate the structure–property relationships of the herein-developed PI films. The optical properties of the PI films, including the cutoff wavelength (λcut), the transmittances at the wavelengths of 400 nm (T400) and 450 nm (T450), the CIE color parameters of L*, a* and b*, and the haze, are tabulated in Table 4.
Due to the formation of intramolecular and intermolecular charge transfer complexes (CTC) in the conventional PI structure, visible light is significantly absorbed, resulting in the deep coloring of the film, which limits the use of TENGs based on PI films in the optical field. To investigate the impact of the introduction of –CF3 on the optical transparency of these FPI films, the UV-Vis spectra and CIE Lab color parameters were tested, and the results are presented in Table 4 and Figure 8, respectively. Figure 8 shows the remarkable optical transparency of the FPI films, characterized by a cutoff wavelength (λcut) below 350 nm. At a specific wavelength of 450 nm, they showed transmittance (T450) values of 83.0% for FPI-1, 81.0% for FPI-2, and 75.7% for FPI-3, respectively. In addition, the FPI film exhibited yellow indices (b*) in the range of 1.37–4.37, and haze values from 1.93% to 2.10%. Basically, FPI-3 showed inferior optical properties compared to those of the other two counterparts. The optical properties of the derived FPI-3 were negatively affected by the coloration of the starting TFMDA diamine, which could be attributed to the oxidation-sensitive characteristic of the meta-substituted TFMDA diamine.

3.4. Triboelectric Properties

In this study, all TENGs were fabricated using the traditional vertical contact-separation mode, as illustrated in Figure 9a. To assess the efficacy of triboelectric nanogenerators (TENGs), a range of PI films were subjected to cyclic compressive forces of around 30 N at a frequency of 2Hz. The ensuing measurements focused on open-circuit voltage (Voc), short-circuit current (Isc), transferred short-circuit charge (Qsc), and output power as essential indicators of TENG performance. This standard evaluation of PI films allows for the meaningful analysis of and comparisons among different TENG models. The resulting values are given in Table 5. The values of Voc and Isc are shown in Figure 9b,c. Initially, the TENG based on commercial PI (Kapton®) showed a lower electrical output performance of less than 63.4 V and 2.05 μA. In comparison, the TENGs made of FPI films exhibited superior triboelectric output with the introduction of the –CF3 group. The output performance of the TENG was enhanced, achieving a Voc of 204.6 V and Isc of 5.45 μA, by utilizing the FPI-1 as a negative layer, and it was further improved to 258.9 V and 6.56 μA by applying the FPI-2 film. It is important to highlight that the TENG using an FPI-3 film achieved a Voc of 277.8 V and Isc of 9.54 μA, surpassing these values of the TENG based on Kapton® by 4 and 5 times, respectively. A similar pattern was observed for the transferred charge. The Qsc of all TENGs was measured with identical mechanical force and frequency. According to Figure 10a, the TENG with the Kapton® film demonstrated a Qsc of 36 nC, and the Qsc increased to 61 nC and 76 nC for the FPI-1 and FPI-2 films, respectively. The highest transferred charge of 98 nC was observed when FPI-3 film was utilized, which is approximately triple that of the initial TENG. Furthermore, the Qsc values of the PTFE film-based TENGs were also acquired for comparison, as shown in Figure 10b. It can clearly be seen that the transferred charge of the TENGs consisting of FPI-3 exceeds that of those with the PTFE film (60 nC) [43]. Additionally, the study also examined the variations in output voltage and output power with increasing loads (ranging from 2 MΩ to 1000 MΩ), as depicted in Figure 10c. The voltage exhibited a direct correlation with the load resistance, ultimately reaching saturation at approximately 252 V. The instantaneous power density under loading reached a maximum of 510.05 mW/m2 at a load resistance of 50 MΩ. In order to assess the potential applications in the future, confirmatory experiments were conducted. The test conditions outlined in this study demonstrated the ability of the TENG to illuminate a total of 15 commercial LEDs, as shown in Figure 10d.
In order to clarify the mechanism by which TENGs are improved by using FPIs, Gaussian 09 package was employed to conduct density functional theory (DFT) calculations. The calculations were completed using the nonlocal hybrid Becke three-parameter Lee–Yang–Parr (B3LYP) function and the 6–31 G* basis set. At first, the electrostatic potential distribution (ESP) of FPIs was calculated, because the distribution of charge traps of a dielectric polymer is directly related to the electrostatic potential [44,45]. The potential maps of Figure 11 show both positive (red part) and negative ESP (blue part) regions. Among them, most of the negative ESP regions are distributed around the oxygen atoms in the imide ring and the fluorine atoms in the diamine moiety, which can provide corresponding electron capture sites. Moreover, the formation of charge trap and trapping characteristics is also reflected by the frontier orbital energy levels. It is well-known that electron acceptors in amorphous polymers consistently correspond to the LUMO, regardless of the contact configurations and interface distance. For this reason, we further calculated the energy levels to explore the relationship between the molecular structure and triboelectric properties [46,47,48]. In Figure 12, the overall distribution of the levels of LUMO up to LUMO+3 for FPI is noticeably lower than that of a standard PI film using Kapton®, which is advantageous for electron injection, allowing triboelectric materials to gain electrons. Furthermore, energy gap (Δε, |εHOMO – εLUMO|) values are also presented in Figure 12, providing a clearer understanding of the effects of the introduction of strong electron-withdrawing groups for energy level variation. This enables an exploration of the impact of structural changes on triboelectric performance. The molecular orbital levels indicate that the Δε values of FPI after fluorination modification are significantly higher than those of Kapton® (2.80 eV). Focusing on FPI films, it can be observed that the increase in Δε values follows the sequence FPI-1 (3.77 eV) < FPI-2 (4.03 eV) < FPI-3 (4.12 eV), implying that FPI possesses a greater ability to retain localized states, making it more efficient in electron capture. Consequently, in terms of electrical output performance, FPI-3 outperforms the two alternative polymers. Thus, both experimental and theoretical investigations of the PI films prove that the molecular orbital of polymers plays an important role in determining the triboelectric performance.

4. Conclusions

In this work, an effective approach to improving the triboelectric properties of PI tribo-negative layers was proposed. The approach involves the incorporation of –CF3 groups with electron-withdrawing characteristics into the dianhydride and diamine structures. The output performance of the TENGs made using the derived FPIs was analyzed via molecular simulations to demonstrate the effects of the LUMO and energy gap. The incorporation of –CF3 groups into the molecular structure of the FPI film manifested lower LUMO values and larger energy gap values than those of the traditional PI layers, resulting in a substantial improvement in the triboelectric performance. Meanwhile, good thermal resistance and optical transparency were obtained by the FPI layers. This structural modification strategy, aiming at enhancing the triboelectric properties of PI layers, made it possible to derive a high output performance from the TENGs, which could be applied in energy-harvesting operations in high-temperature or optically transparent environments.

Author Contributions

Conceptualization, J.L.; methodology, S.Y., H.Y. and J.L.; investigation, Z.P., X.R., Z.W. and Z.H.; data curation, Z.P., X.R., Y.Q. and S.H.; writing—original draft preparation, Z.P.; writing—review and editing, J.L.; supervision, S.Y., H.Y. and J.L.; funding acquisition, S.Y. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Shenzhen Science and Technology Program (No. JSGG20210629144539012).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Author Shunqi Yuan was employed by the company of RAYITEK Hi-Tech Film Company, Co., Ltd., Shenzhen, China. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be constructed as potential conflict of interest.

References

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Technologies 11 00136 g001
Figure 1. Synthesis pathway for PI resins.
Figure 1. Synthesis pathway for PI resins.
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Figure 2. 1H-NMR spectra of FPI resins.
Figure 2. 1H-NMR spectra of FPI resins.
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Figure 3. XRD spectra of FPI polymers.
Figure 3. XRD spectra of FPI polymers.
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Figure 4. FTIR spectra of FPI films. (*) Absorption peaks located around 1132 cm−1. (★) Absorption peaks located around 1315 cm−1.
Figure 4. FTIR spectra of FPI films. (*) Absorption peaks located around 1132 cm−1. (★) Absorption peaks located around 1315 cm−1.
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Figure 5. TGA and DTG curves of FPI films in nitrogen.
Figure 5. TGA and DTG curves of FPI films in nitrogen.
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Figure 6. DSC curves of the FPI films.
Figure 6. DSC curves of the FPI films.
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Figure 7. TMA curves of the FPI films.
Figure 7. TMA curves of the FPI films.
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Figure 8. UV-Vis spectra of the FPI films.
Figure 8. UV-Vis spectra of the FPI films.
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Figure 9. (a) Schematic diagram of the structure for TENGs. (b) Open-circuit voltage of the PI-based TENGs. (c) Short-circuit current of the PI-based TENGs.
Figure 9. (a) Schematic diagram of the structure for TENGs. (b) Open-circuit voltage of the PI-based TENGs. (c) Short-circuit current of the PI-based TENGs.
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Figure 10. (a) Transferred short-circuit charge of the PI-based TENGs. (b) Transferred charge generated by the TENGs based on PTFE and 6FDA-TFMDA films, respectively. (c) 6FDA-TFMDA TENG’s output voltage and output power density at different loads. (d) Fifteen LEDs can be lit directly using the 6FDA-TFMDA TENG.
Figure 10. (a) Transferred short-circuit charge of the PI-based TENGs. (b) Transferred charge generated by the TENGs based on PTFE and 6FDA-TFMDA films, respectively. (c) 6FDA-TFMDA TENG’s output voltage and output power density at different loads. (d) Fifteen LEDs can be lit directly using the 6FDA-TFMDA TENG.
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Figure 11. Molecular orbit energy levels of the PI films.
Figure 11. Molecular orbit energy levels of the PI films.
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Figure 12. Energy level distribution diagram of the PI films.
Figure 12. Energy level distribution diagram of the PI films.
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Table 1. Formula for the PI synthesis.
Table 1. Formula for the PI synthesis.
PI6FDA (g, mol)6FODA (g, mol)TFMB (g, mol)TFMDA (g, mol)DMAc (g)
FPI-122.2120, 0.0516.8115, 0.05NA aNA117.8
FPI-222.2120, 0.05NA16.0115, 0.05NA152.9
FPI-322.2120, 0.05NANA8.8070, 0.0572.4
a Not applicable.
Table 2. Inherent viscosities, molecular weights, and solubility of CPI resins.
Table 2. Inherent viscosities, molecular weights, and solubility of CPI resins.
FPI[η]inh a
(dL/g)		Molecular Weight bSolubility c
Mn (×104
g/mol)		Mw (×104
g/mol)		PDINMPDMAcGBLCPATHF
FPI-11.2225.2132.211.26++++++++++
FPI-20.9610.8718.221.68++++++++++
FPI-30.735.3710.561.65++++++++++
a Inherent viscosities measured with a 0.5 g/dL PI solution in NMP at 25 °C. b Mn: number average molecular mass. Mw: weight average molecular mass. PDI: polydispersity index, PDI = Mw/Mn. c ++: soluble at room temperature. GBL: γ-butyrolactone. CPA: cyclopentanone. THF: tetrahydrofuran.
Table 3. Thermal properties of FPI films.
Table 3. Thermal properties of FPI films.
SamplesT5% a
(°C)		Tmax a
(°C)		Rw750 a
(%)		Tg,DSC a
(°C)		Tg,TMA a
(°C)		CTE a
(×10−6/K)
FPI-154060850.9282.4300.451.3
FPI-254161953.5304.5336.554.0
FPI-353460652.3292.6302.342.5
a T5%: Temperatures at 5% weight loss. Tmax: Temperatures at the most rapid thermal decomposition rate. Rw750: Residual weight ratio at 750 °C in nitrogen. Tg,DSC: Glass transition temperatures according to the DSC measurements. Tg,TMA: Glass transition temperatures according to the TMA measurements (peaks of tanδ plots). CTE: linear coefficient of thermal expansion in the range of 50–250 °C.
Table 4. Optical properties of the FPI films.
Table 4. Optical properties of the FPI films.
Samplesλcut a
(nm)		T400 b
(%)		T450 b
(%)		L* ca* cb* cHaze
(%)
FPI-134341.383.096.04−0.551.761.93
FPI-234764.581.095.86−0.201.372.08
FPI-333264.875.794.87−0.744.372.10
a λcut: Cutoff wavelength. b T400, T450: Transmittance at the wavelength of 400 nm and 450 nm, respectively. c L*, a*, b*: color parameters, see Section 2.2.
Table 5. Triboelectric properties of the FPI films.
Table 5. Triboelectric properties of the FPI films.
SamplesVoc (V) aIsc (μA) bQsc (nc) c
Kapton®63.42.0536
FPI-1204.65.4561
FPI-2258.96.5676
FPI-3277.89.5498
a Voc: open-circuit voltage. b Isc: short-circuit current. c Qsc: transferred short-circuit charge.
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Pan, Z.; Yuan, S.; Ren, X.; He, Z.; Wang, Z.; Han, S.; Qi, Y.; Yu, H.; Liu, J. Preparation and Characterization of Fluorine-Containing Polyimide Films with Enhanced Output Performance for Potential Applications as Negative Friction Layers for Triboelectric Nanogenerators. Technologies 2023, 11, 136. https://doi.org/10.3390/technologies11050136

AMA Style

Pan Z, Yuan S, Ren X, He Z, Wang Z, Han S, Qi Y, Yu H, Liu J. Preparation and Characterization of Fluorine-Containing Polyimide Films with Enhanced Output Performance for Potential Applications as Negative Friction Layers for Triboelectric Nanogenerators. Technologies. 2023; 11(5):136. https://doi.org/10.3390/technologies11050136

Chicago/Turabian Style

Pan, Zhen, Shunqi Yuan, Xi Ren, Zhibin He, Zhenzhong Wang, Shujun Han, Yuexin Qi, Haifeng Yu, and Jingang Liu. 2023. "Preparation and Characterization of Fluorine-Containing Polyimide Films with Enhanced Output Performance for Potential Applications as Negative Friction Layers for Triboelectric Nanogenerators" Technologies 11, no. 5: 136. https://doi.org/10.3390/technologies11050136

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