Unraveling the role of surface molecular structure on vacuum flashover for fluorinated copolymers

Journal Pre-proofs Full Length Article Unraveling the Role of Surface Molecular Structure on Vacuum Flashover for Fluorinated Copolymers Chao Wang, Wen-Dong Li, Jia Guo, Xi Chen, Zhi-Hui Jiang, Xiao-Ran Li, Bao-Hong Guo, Guan-Jun Zhang PII: DOI: Reference:

S0169-4332(19)33248-9 https://doi.org/10.1016/j.apsusc.2019.144432 APSUSC 144432

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

2 July 2019 12 October 2019 17 October 2019

Please cite this article as: C. Wang, W-D. Li, J. Guo, X. Chen, Z-H. Jiang, X-R. Li, B-H. Guo, G-J. Zhang, Unraveling the Role of Surface Molecular Structure on Vacuum Flashover for Fluorinated Copolymers, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144432

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Unraveling the Role of Surface Molecular Structure on Vacuum Flashover for Fluorinated Copolymers Chao Wang, Wen-Dong Li, Jia Guo, Xi Chen, Zhi-Hui Jiang, Xiao-Ran Li, Bao-Hong Guo, and Guan-Jun Zhang* State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China * Corresponding author, Tel: +86-029-82668172 E-mail: [email protected] ABSTRACT: To investigate the influence of surface molecular structure on the electrical performance of fluorinated copolymers, Ultraviolet (UV) curable resin containing different types of fluorinated monomers (ethers and alkyls) were prepared by photopolymerization. The X-ray photoelectron spectrum indicated the presence of fluorinated monomers on the surface after UV radiation. Furthermore, the surface morphology measurement revealed that these monomers cause a slight change to the surface roughness. Subsequently, the electrical properties were examined through a pulsed flashover test in vacuum. The results showed an improvement in the electrical strength of the specimen doped with dodecafluoroheptyl methacrylate. In contrast, the sample doped with perfluoropolyether chains exhibited no alleviation in terms of flashover performance, despite the fact that its surface F 1s / C 1s atomic ratio is 10 times higher than that of the former. A coarsegrained model based on quantum chemical calculations reveals that traps are introduced at the surface because of the grafted fluorinated chains. Further explanation basing on the free volume theory indicates that the higher gas absorption arising from the amorphous structure during the electron-induced degassing process accounts for the deterioration of the insulation system, which 1

was verified by surface charge behavior before and after the flashover. KEYWORDS: molecular structure; electrical strength; vacuum flashover; dielectric; fluorinated polymer 1. Introduction The flashover phenomenon across polymer surfaces severely perturbs the operation of many electro-vacuum devices owing to its much lower breakdown threshold than the bulk or vacuum gap [1-4]. Previous studies related to the flashover mechanism revealed that a secondary electron emission avalanche (SEEA) was a crucial contribution to the flashover [5]. An initial electron bombardment on a dielectric can cause secondary electron emission and ionization of neutral molecules from the desorption gases [4]. Generally, the penetration depth of the primary incident electrons can be approximated by the WittryKyser formula, where the maximum depth Rw-k (nm) is equal to 78.9E01.7/ [6]. That is, effective surface layer thickness is determined by the initial electron energy (E0, keV) and the density of the target material (g cm-3). For polymer materials, 𝜌 is usually considered as 1 g cm-3. Many simulation works have indicated that the incident electron energy in the self-sustained stage is less than 50 eV [7, 8]. That is, most of the interactions between the energized electrons and solid materials occur in the surface layer at <5 nm depth. Hence, the interfacial properties (i.e., elemental composition and outgassing probability) is a key factor for the subsequent development of the flashover. Direct or plasma-assisted fluorination of polymers has been validated as an effective method to enhance their surface electrical strength [9-14]. The former approach usually operates in a stainless reactor with different proportions of the F2/N2 mixture [15]. The advantages of the direct fluorination are its ease of implementation and large handling capacity [16]. Nevertheless, its 2

disadvantages are manifold: the hazardous fluorine gas or its mixture can cause severe damage to the workers’ health and significant air pollution. In contrast, non-thermal plasma-assisted fluorination avoids the abovementioned deficiencies because of its unique advantages such as high efficiency, low cost, and non-pollution [11]. However, the detailed process of the reaction and stability require further verification. Notably, all these methods inevitably change the surface morphology and elemental compositions simultaneously; however, the role of the surface molecular structure and the corresponding fluorination mechanism in flashover enhancement are not well understood. Fluorinated macromolecules have always attracted much attention owing to their remarkable properties such as high thermostability, chemical inertness, good water and oil repellency, and low friction coefficient [17-19]. Photopolymerization is one of the recent advancements in the field of fluoropolymers, which has comprehensively been well reviewed in [20]. Compared with the conventional thermal curing technique, photopolymerization is efficient (higher curing speed), ecofriendly (no volatile organic compound), energy-saving (can operate at room temperature), and economical (low investment on equipment) [21]. Therefore, many researchers have been attempted to apply ultraviolet (UV) curable resin to the electrical insulation field [22-24]. In addition, owing to the surface segregation phenomenon [25], doping a very low amount of fluorinated oligomers in a typical photosensitive resin can result in exceptional surface performance [20, 26, 27], again making the UV-curing technique a natural choice. In our previous works, the feasibility of applying UV-cured resin to the real power system has been validated [24] and the effectiveness of UV-assisted fluorination for electrical strength enhancement using a fluorinated alkene monomer has been confirmed [28]. However, because the structure and chemistry at the interfaces is important and can 3

sometimes dominate the overall properties as component sizes or dimensions shrink to the atomic or nanoscale [29, 30], the relationship between the molecular structures of fluorinated polymers and the resulting insulating properties remains unclear. Herein, we attempt to provide an in-depth explanation for the interaction between the molecular structure and electron dynamics at the surface through a comparative analysis. Fluorinated polymers with different molecular structures at the interface were prepared by photopolymerization. Thereafter, the influence of each structure on the flashover was further investigated. To the best of our knowledge, this is the first study that investigates the effects of a vacuum/dielectric interface in the flashover process from the perspective of molecular structure engineering, offering an opportunity for a fundamental understanding of the surface electrical strength enhancement of fluorinated polymers. 2. Experimental section 2.1 Materials Acrylate resin (High Temp, Formlabs Inc., USA), photoinitiator (Omnirad 819, Curease Chemical, China), dodecafluoroheptyl methacrylate (DFHMA, Aladdin Reagent (Shanghai) Co., Ltd, China), and acrylic modified perfluoropolyether (PFPE 1605, Sinochem Environmental Protection Chemicals (Taicang) Co., Ltd., China) are commercially available. The molecular structures of DFHMA and PFPE are described in Fig. 1.

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Fig. 1. Molecular structures of the fluorinated monomers 2.2 Preparation of modified UV-cured polymer Fig. 2 describes the preparation of the UV-cured specimen. The fluorinated monomer (i.e., DFHMA or PFPE) was doped in the matrix with different proportions. Based on the previous research [26, 27], the specific doping concentrations vary from 0 to 2 wt% at intervals of 0.5 wt%. Ultrasonic dispersion and stirring in vacuum were adopted in sequence for better uniformity. Then, the uncured resin was poured into a die, which was then cured and heated using 405 nm UV light and resistive heater, respectively.

Fig. 2. Preparation of photopolymerized fluorinated specimens It is pertinent to note that heating time and temperature are the two key factors for the segregation of the fluorine-containing groups as no solvent is involved here [31-34]. To ensure that the fluorine groups successfully segregate to the surface, the uncured resins containing DFHMA (UV-F-1) and PFPE (UV-F-2)were heated at 5 min / 60 °C and 30 min / 80 °C, respectively, prior to the UV curing process. Note that the values about the curing time and temperature were optimized by preliminary experiments. Subsequently, these thermally treated resins were cured in a customized 405 nm UVcuring chamber to obtain the testing specimens. Finally, all specimens were cleaned by anhydrous ethanol and dried for 2 h in a vacuum chamber at an ambient temperature of 60 °C. 2.3 Characterization 5

X-ray photoelectron spectra (XPS; ThermoFisher Scientific ESCALAB Xi+, USA) were recorded to confirm the segregation of the fluorinated monomers. A laser scanning confocal microscope (LSCM; Olympus LEXT OLS4000, Japan) and atomic force microscope (AFM; Bruker INNOVA, USA) were employed to characterize the morphology of the specimens. The measurement of the average roughness (Ra) was performed on a contact type instrument (MarSurf M 300 C, Germany). Pulse flashover in vacuum was implemented at a self-designed platform. A schematic of the surface breakdown measurement and the corresponding characterization strategy are shown in Fig. 3. The voltage waveform generated by the circuit in Fig. 3(a) is demonstrated in the blue dashed box in Fig. 3(c). The sizes of the electrode and prepared specimens used for the study are shown in Fig. 3(b). In most previous experiments, three voltage indexes (i.e., the first breakdown voltage (Ufb), conditioned voltage (Uco), and hold-off voltage (Uho)) were conventionally used to evaluate the surface flashover characteristics of the testing specimens at different stages. As shown in Fig. 3(c), at each voltage level, the impulse voltage is applied five times to the specimen; Ufb represents the voltage at which the flashover first occurs, while Uco and Uho, respectively represent the voltages at which the flashover occurs and then disappears during each of the five times.

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Fig. 3. Experimental setup for flashover strength characterization. (a) Schematic of vacuum flashover circuit (b) electrode structure (c) characterization strategy The measurements of the surface charge decay in air and charge accumulation in vacuum were performed at the self-designed platforms using the prescribed testing strategy in references [35, 36] and the experimental setup is demonstrated in Fig. 4. The principles of the surface potential measurement in atmosphere and vacuum can be briefly described as follows: A strong E field causes gas ionization near the needle electrode in Fig. 4(a); then, the electrons migrate to the dielectric surface under the force of the electrical field between the mesh and the ground electrode. The negative charges deposited on the insulator’s surface decreases the surface potential leading to a decrease in the potential difference between the mesh and dielectric surface, which terminates the migration process. Under this condition, the surface charge can also decay through other processes such as the neutralization caused by the ambient humidity or migration to

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the bulk or ground electrode. Finally, a dynamic equilibrium between the charge generation and transportation is achieved and the surface potential becomes stable. The field emission at the cathode in Fig. 4(b) is the source of the electrons in vacuum. Thereafter, the initial electrons are accelerated by the applied E field. The electrons in vacuum have much higher energy than those in air because no impact occurs until they reach the dielectric surface. The high energy electrons (primary electrons) consecutively bombard the insulator’s surface, contributing to the emission of Auger, secondary, and backscattered electrons. As many electrons are lost, the originally neutral dielectric surface becomes positively charged.

Fig. 4. Experimental setup for (a) charge decay rate measurement in air and (b) surface potential distribution measurement in vacuum 3. Results and discussion 3.1 Characterization of surface morphology and chemical composition Typically, multifarious fluorination routes can significantly alter the macro/microscopic properties of polymers (e.g., surface smoothness and elemental composition) [11, 37, 38]. Many literatures have reported that a proper surface roughness and fluorine element concentration level are conducive to the enhancement of the surface electrical performance in vacuum [36, 37, 39, 40]. 8

While, a rougher surface increases the diffuse reflection of the electrons, resulting in the loss of energy and quantity of secondary electrons, the strong electron affinity of the F atoms inhibits the secondary electron emission from the solid dielectrics, due to the absorption of the free electrons. However, there is no established in-depth explanation to support this argument because it is difficult to separately determine the degree of influence of these factors in practice. In the present study, we have considered surface roughness and chemical composition as independent factors. Fig. 5 shows the characterization of the surface roughness of UV-F-1 and UV-F-2 (1 wt%, prepared by the method described in Fig. 2) using an LSCM and AFM. The LSCM measurements indicate that the overall roughness is below 2 m and we observe a slight difference between the different specimens. A more precise characterization performed by AFM shows that UV-F-2 has a relatively smoother surface while the roughness remains at the same level. The variance in the nm-scale (see Fig. 5(c) and (d)) is ascribed to the high mobility of the PFPE chains. The amorphous fluorinated monomers (PFPE) tend to fill the holes, peaks, and valleys present on the surface, thereby reducing the surface roughness remarkably [41].

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Fig. 5. Microscopic morphologies of UV-F-1 and UV-F-2. (a) (b) LSCM (scanning area 128 × 128m) (c) (d) AFM results (scanning area 10 ×10m) The influence of the doping concentration on the surface roughness is summarized in Table 1. We observe that as the doping concentration increases, the density of the surface fluorinated monomers also increases, leading to a decrease in the surface roughness of both materials. These results satisfactorily represent the expected behavior of the fluorinated monomers [41]. Some previous studies reported that only when the surface is sufficiently rough (i.e., Ra is larger than at least 1 m) it can suppress the charging, inhibit flashover development, and thereby improve the insulation property [36, 42]. Hence, in our experiments, the influence of the surface roughness can be ignored because the Ra value of all specimens is below 0.1 m. Table 1 Surface roughness (Ra, m) of specimens with different doping concentration Concentration (wt%)

0

0.5

1

1.5

2

10

UV-F-1

0.061

0.079

0.071

0.061

0.055

UV-F-2

0.061

0.070

0.065

0.037

0.032

The appearance of the F 1s peak at 688 eV and the X-ray induced Auger peaks (between 800 and 900 eV in Fig. 6 indicate that fluorinated monomers are segregated at the surface of the resulting products. The increase in the N 1s peak of UV-F-2 is further evidence for the segregation of the PFPE. As expected, the curve-fitting analysis shown in Fig. 7 is also in good agreement with the molecular structure illustrated in Fig. 1. In detail, compared with the pristine specimen (shown in Fig. 7(a)), the fluorine peaks in Fig. 7(b1) and Fig. 7(c1) are attributed to the C-F bonds of the doped monomers. The differences between these two monomers are reflected in the O 1s and C 1s scans, as attested by the peaks formed by O-CF2 (534.8 eV, from the segment of the PFPE), C-F2 (291.7 eV), and C-F3 (293.6 eV) groups in Fig. 7(c2), Fig. 7(b3), and Fig. 7(c3), respectively.

Fig. 6. XPS spectra of three specimens

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Fig. 7. F 1s, O 1s and C 1s spectra of the three specimens The quantitative descriptions in Table 2 indicate that an increase in the amount of fluorinated monomers employed in the initial mixture produces surfaces with higher F/C atomic ratio of the copolymer: particularly, while the ratio for the specimen modified by DFHMA can increase to 0.083 at 2 wt% concentration, UV-F-2 exhibits a much higher F/C ratio, i.e., up to 0.898 at 2 wt% concentration, which is more than 10 times that of UV-F-1 at the same loading concentration. Table 2 F 1s / C 1s atomic ratio from XPS analyzes for the UV-cured polymer modified by UV-F1 and UV-F-2 Concentration (wt%)

0

0.5

1

1.5

2

UV-F-1

0

0.071

0.079

0.080

0.083

UV-F-2

0

0.654

0.747

0.798

0.898

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Thus, we successfully decoupled the influence of the two critical factors (namely, surface roughness and elemental composition) for a vacuum flashover through the preparation technique described above. 3.2 Surface flashover voltage measurement Typically, although a single flashover does not cause permanent damage to the dielectric, it can produce small carbonized channels, or tracks, branchs, and residual charges. Therefore, after a series of pulses, the accumulated defects would cause the failure of the insulator [43]. As mentioned above, Ufb, Uco, and Uho are defined according to the flashover occurrence time described in Fig. 3(c). Accordingly, Ufb represents the defect tolerance of the insulating material and Uho represents its actual performance under the operating conditions [11, 36]. The correlations between these indexes and doping concentration are shown in Fig. 8. Interestingly, although UV-F-2 has a much higher F/C ratio, it exhibits a weaker electrical strength compared even with the pristine specimens. Moreover, as a direct consequence of the increase in the doping concentration of UV-F-2, there is a correlated decrease in the hold-off voltage. On the contrary, the doping concentration has a positive influence on the surface electrical properties of UV-F-1: more than 20% increase in Uco and Uho is observed in the specimen doped with 2 wt% DFHMA. Note that the flashover voltage such as Uho or Uco in Fig. 8(a) is not a completely monotonic growth, which should account for the randomness and complexity of the discharge process. Hence, based on these facts, it can be conjectured that the differences in the chemical structure at the interfaces have a significant influence on the surface electrical properties.

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Fig. 8. Results of vacuum flashover testing for (a) UV-F-1 (b) UV-F-2 4. Theoretical analysis and experimental support According to the theory based on the SEEA model, the flashover process can be divided into four stages: (1) field electron emission from the cathode triple junction (the point where the electrode, insulator, and vacuum meet) forming the multipactor that extends from the cathode to the anode. (2) The saturated secondary electron emission stage where the multipactor reaches the anode, which is supposed to be in dynamic equilibrium with the average secondary electron yield that equals 1. (3) The outgassing process caused by the persistent electron collisions with the insulator surface, which is accompanied by an increase in the local pressure. (4) The gas breakdown occurring when the electron-triggered consecutive ionization transforms into an avalanche. Many literatures have reported that the fluorination of a polymer can accelerate surface charge dissipation thereby affecting the multipactor process [9, 44]. Shallow traps can be introduced by the surface fluorination process. However, as both surface morphology and chemical composition are altered during the conventional surface treatment process, it is not easy to determine the dominant factor for charge transportation. Based on the unique characteristics of the method described in the experimental section, we can separate these two factors by excluding the influence of the surface roughness. In

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the following sections, the influence of the fluorinated molecular structures on the two key factors (namely surface electron traps and desorbed gas) on flashover development are analyzed. 4.1 Determination of surface electron traps The variation of the surface potential with time recorded by the electrostatic voltmeter in Fig. 4 are shown in Fig. 9. Generally, very similar results were obtained with respect to the maximum potential (around 7.8 kV) and charge decay rate (less than 4% after 100 min) for the three specimens. Particularly, the specimen with 1 wt% DFHMA exhibits a faster decay rate (U = 258 V), while the specimen doped with 1% PFPE exhibits the lowest initial potential: the initial potentials of UV-P, UV-F-1, and UV-F-2 are 7843.62, -7803.42, and -7672.36 V, respectively. As described in Section 2.3, the value of the initial potential is a result of the dynamic equilibrium between the charge generation and transportation. According to the surface electron trap information calculated by the isothermal surface potential decay method (Fig. 10), the maximum peaks shift from 1.17 to 1.23 eV and 1.08 eV, respectively for UV-F-1 and UV-F-2. More number of shallow traps around 1.0~1.1 eV are observed at the surface than the pristine sample, leading to a faster charge dissipation rate. The improvement in the charge transporting capability is readjusted to the equilibrium point, resulting in a lower initial surface potential (i.e., absolute value, at the beginning of the tests).

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Potential (V)

UV-P UV-F-1 UV-F-2

U=228V

Ustart=-7672.36V U=258V

Ustart=-7803.42V U=206V

Ustart=-7843.62V 0

1000

2000

3000 4000 Time (s)

5000

6000

Fig. 9. Surface potential decay characteristics for the three specimens The integral results about the distribution profile indicate that the area ratio of UV-P, UV-F-1, and UV-F-2 is 1:1.46:1.19. In other words, the total energy of the surface traps increases dramatically, which may be attributed to the high electron affinity of the fluorinated groups.

Surface electron trap density (a.u.)

UV-P UV-F-1 UV-F-2

0.8

0.9 1.0 1.1 1.2 1.3 Surface electron trap energy (eV)

Fig. 10. Surface electron trap distribution for the three specimens To provide corroborative evidence and a fundamental understanding of the experimental results, quantum chemistry calculations of bisphenol A epoxy acrylate grafted with the fluorinated monomers were performed using the density functional theory. All the calculations were performed 16

based on the B3LYP/def2-TZVP level of theory implemented in the quantum chemistry program package ORCA [45] and the visualization was performed using visual molecular dynamics (VMD) [46]. The calculated results of the optimized structures and density of states (DOS) are shown in Fig. 11. Invariably, the highest occupied molecular orbitals (HOMOs; shown in Fig. 11(b) and Fig. 11(e)) of the two types of structures appear at the benzene ring in bisphenol A epoxy acrylate. In contrast, the lowest unoccupied molecular orbitals (LUMOs; shown in Fig. 11(a) and Fig. 11(d)) tend to be occupied by the bulk/fluorinated chain interface sites. The DOS shown in Fig. 11(c) and Fig. 11(f) provide an intuitive explanation from the perspective of the energy band structure. The white region in Fig. 11(c) and Fig. 11(f) represents the forbidden band while its left and right regions indicate the valence and conduction bands, respectively. Compared with the total DOS distribution, two types of grafted fluorinated chains supply the DOS near the conduction band. Generally, the electron traps near the conduction band are considered shallow [30]. In other words, a small number of shallow traps are introduced at the surface after grafting fluorinated monomers, which is in good agreement with the experimental results shown in Fig. 9.

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Fig. 11. Quantum chemistry calculation results. (a), (b) and (d), (e) respectively represent the LUMO, HOMO for UV-F-1 and UV-F-2. (c) and (f) show the DOS of UV-F-1 and UV-F-2, respectively. It is also pertinent to note that the bond energy of C-O (358 kJ/mol) is much lower than that of C-F (485 kJ/mol). Hence, after many times of flashover, small cracks may occur at the surface, resulting in a decrease in the hold-off voltage, as shown in Fig. 8. 4.2 Free volume theory for understanding the role of outgassing In theory, a small difference in the surface traps cannot contribute to such variance in flashover strength. Therefore, to further disclose the discrepancy in the fluorinated molecular structure in the flashover process, their influence on the absorbed gases is discussed in this section: The absorbed gases play a significant role in the flashover process, which is especially pronounced for polymers, as they tend to absorb more gases than the inorganic materials [47]. Sun et al. [8] revealed a complete flashover development over the surface by considering the plasmasurface interaction, and the equation to estimate the flashover voltage (Vf) is expressed as follows: 18

𝑑

1 𝐴

3(𝑃crit −𝑘𝑇𝑛0 )√𝐷𝑙 𝜋𝜀0

𝑉f = 2𝜀 √2 (𝐴1 − 1) ∙ √ 0

0

1−exp⁡(−3)

𝑘𝑇𝛾

√2𝐴0 𝑚𝑒 ,

(1)

where Pcrit is the critical breakdown pressure for the ionization avalanche, d is the distance between anode and cathode, A1 is the maximum value in the secondary electron yield profile, 0 is the permittivity of vacuum (8.85 × 10-12 F/m), k is the Boltzmann constant (1.38 × 10-23 J/K), me is the mass of electron, A0 is the initial energy of the secondary electrons emitted from the dielectric surface, and γ is the desorption probability, which depends on the amount of absorbed gas. Specifically, the absorbed gas is a key factor for the flashover development, while the other parameters remain unaltered. Under the free volume model, the absorption of the molecules in the polymers greatly depends on the available free volume (this is an intrinsic property of the polymer matrix and is created by the gaps between the entangled polymer chains) [48]. Generally, for most polymer/gas systems, the solubility coefficients can be expressed as follows [49]: Ssc = Sa∙(1  c),

(2)

where Ssc is the solubility of a semi-crystalline polymer, c is the degree of crystallinity of the polymer, and Sa is the solubility of the amorphous structure. In other words, as the crystalline regions are more intensive than the amorphous ordered regions, the free volume is lower in these regions. Interestingly, many literatures have reported that long fluoroalkyl side chains such as UV-F-1 can be smartly used as a barrier against gas diffusion, owing to their high crystallinity [20]. Additionally, chains such as UV-F-2 can move freely because of their sufficiently low Tg values (as low as −100 °C), resulting in an enhanced surface reorganization and higher free volume [20, 50]. Fig. 12 demonstrates the influence of molecular structures on the gas absorption. Apparently, the increased free volume caused by the side chains can absorb more gas. In addition, it is observed that the main desorbed gas components of the polymers during electron irradiation are similar, i.e., H2, H2O, CO2, 19

etc. [51].

Fig. 12. Vacuum/block copolymer interface (a) UV-F-1 (b) UV-F-2 In a high vacuum environment (10-3 Pa), the absorbed gases diffuse under the concentration gradient. As shown in Fig. 13, the crystalline regions generated by UV-F-1 function as excluded phases for the sorption process and as impermeable barriers for diffusion and sorption. Furthermore, the crystalline regions present a constraint on the polymer chains in the amorphous region. This chain restriction influences the sorption process in the amorphous phase by limiting the effective path length of the diffusion and reducing the polymer chain mobility, which creates a higher activation energy for diffusion. In contrast, UV-F-2 with its freely dangling chains at the surface absorbs more gases while the absorbed gases in the bulk can easily migrate to the surface. Meanwhile, the electron bombardment causes continuous gas desorption. The high energy electrons near the surface initiate impact ionization, generating more electrons for maintaining the subsequent stronger gas ionization. Similarly, desorption induced by bond breaking can aggravate ionization [51]. In such cases, the higher desorbed gas generates a plasma sheath, contributing to the decrease in the breakdown threshold.

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Fig. 13. Gas migration and electron-induced desorption process in UV-F-1 and UV-F-2 To provide corroborative evidence for the proposed model in Fig. 13, the surface charge behavior before and after flashover were investigated. As shown in Fig. 14(a) and Fig. 14(b), positive charges are accumulated at the surface, as described in Section 2.3, when negative pulsed voltage is applied to the insulator. After 100 pulses, the surface potential in Fig. 14(c) and Fig. 14(d) is saturated and the maximum potential for both is around 5.6 kV. A slight difference is observed between the surface potential of UV-F-1 and UV-F-2 until flashover occurs at 50 kV / 1 N and 25 kV / 2 N, respectively. Furthermore, while the maximum potential in Fig. 14(e) is much higher than that in Fig. 14(f), the potential distribution of UV-F-1 is non-uniform and the positive charges around the breakdown channel are remarkably neutralized. Moreover, as mentioned at the beginning of this section, while the applied voltage increases, flashover occurs when electron-triggered consecutive gas ionization transforms into an avalanche. The charged particles in the plasma migrate under the electric field force, neutralizing the surface charges. In other words, higher gas desorption contributes to stronger ionization and generate innumerous particles for neutralization, resulting in a lower surface potential and more uniform potential distribution. Hence, the difference in the charge accumulation characteristics after the flashover is an indirect evidence for the conjectures in Fig. 12 and Fig. 13. 21

Fig. 14. Surface potential distribution of UV-F-1 (a, c, and e) and UV-F-2 (b, d, and f) at different stages (U and N indicate the applied voltage and the number of pulses, respectively) Conclusions In summary, an efficient and environmentally benign method for the surface fluorination of polymer insulating material has been presented in this study. Fluorinated polymers with different surface structures were prepared by UV photopolymerization, and the interplay between the fluoride groups and surface flashover development process were further investigated. Our results showed that, using a typical acrylate resin and a low amount of the fluorinated monomers (less than 2 wt% can realize surface fluorination while the surface morphology remains almost unchanged. Monomers with different structures (ethers and alkenes) bring substantial differences in surface electric performance. On the one hand, by utilizing the unique advantages conferred by the fluorinated group, the polymer modified by fluorinated alkene remarkably

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enhances the surface breakdown performance: The strong C-F bonds in the fluorinated alkene improve the resistance to electron bombardment. Moreover, higher crystallinity caused by the long molecular chain decreases the free volume of surface monolayer, and is thus considered beneficial for alleviating the electron-induced degassing process. On the other hand, polyether (i.e., PFPE) with C-O-F in the molecular structure has a deleterious effect on the electrical properties: The surface amorphous state of UV-F-2 increases the free volume and thus absorbs more gases. Similarly, the observed lower Ufb and Uho are attributed to the higher desorption rate and relatively lower bond energy of the C-O bond, respectively. In this case, high energy electron bombardment during the flashover process causes bond breaking, which also contributes to gas desorption. We believe that this work unravels the role of surface molecular structure in electrical strength enhancement of the fluorinated polymers and is also conducive to guiding the material design for advanced dielectric material from the perspective of surface molecular structure engineering. Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Grant Nos. U1766218, 11775175, 51521065). We also thank Miss Liu Jiamei at the Instrument Analysis Center of Xi'an Jiaotong University for her assistance with XPS analysis.

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Highlights

Fluorinated monomers are segregated to the surface, causing little change to roughness. Monomer like alkyls is conducive for electrical strength enhancement whereas ethers type has opposite effect. Different outgassing probability mainly accounts for discrepancy of surface flashover strength.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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