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Fluoride ion encapsulated polyhedral oligomeric silsesquioxane: A novel filler for polymer nanocomposites with enhanced dielectric constant and reduced dielectric loss
Journal Pre-proof Fluoride ion encapsulated polyhedral oligomeric silsesquioxane: A novel filler for polymer nanocomposites with enhanced dielectric constant and reduced dielectric loss Yi-Yi Deng, Dai-Lin Zhou, Di Han, Qin Zhang, Feng Chen, Qiang Fu PII:
S0266-3538(19)32479-0
DOI:
https://doi.org/10.1016/j.compscitech.2020.108035
Reference:
CSTE 108035
To appear in:
Composites Science and Technology
Received Date: 3 September 2019 Revised Date:
1 January 2020
Accepted Date: 22 January 2020
Please cite this article as: Deng Y-Y, Zhou D-L, Han D, Zhang Q, Chen F, Fu Q, Fluoride ion encapsulated polyhedral oligomeric silsesquioxane: A novel filler for polymer nanocomposites with enhanced dielectric constant and reduced dielectric loss, Composites Science and Technology (2020), doi: https://doi.org/10.1016/j.compscitech.2020.108035. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Di Han, Feng Chen, and Qiang Fu designed research; Yi-Yi Deng, Dai-Lin Zhou, Di Han, and Qin Zhang performed research; Yi-Yi Deng, Dai-Lin Zhou, Di Han, Qin Zhang, Feng Chen, and Qiang Fu analyzed data; and Yi-Yi Deng, Di Han, and Qiang Fu wrote the paper.
* To whom correspondence may be addressed. Email: [email protected] (Di Han), [email protected] (Qiang Fu).
Fluoride Ion Encapsulated Polyhedral Oligomeric Silsesquioxane: A Novel Filler for Polymer Nanocomposites with Enhanced Dielectric Constant and Reduced Dielectric Loss Yi-Yi Deng, Dai-Lin Zhou, Di Han*, Qin Zhang, Feng Chen, Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
Corresponding authors: E-mail: [email protected] (D. Han); [email protected] (Q. Fu)
KEYWORDS: POSS; PPO; nanocomposites; polymer dielectrics; high temperature capability.
ABSTRACT. Polymer dielectrics have been considered as an ideal choice for preparation of advanced power and electronic systems as their good comprehensive performances. However, it remains a challenge to obtain polymer dielectrics with high dielectric constant (εr ), low dielectric loss (tan δ) and high temperature capability. Although adding high permittivity inorganic nanofillers has become a promising approach to improve the εr of polymers, it usually causes the adverse increment tan δ. Herein, we demonstrate that fluoride ion encapsulated polyhedral oligomeric silsesquioxane (POSS@F-) can be regarded as a novel filler to enhance the εr and reduce the tan δ of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) simultaneously. Compared with the pristine PPO, the εr of PPO/POSS@F- composite can be enhanced from 2.81 to 5.31, while its tan δ is still relatively low (0.0011 at 1 kHz). Meanwhile, the breakdown strength of PPO can also be slightly reinforced from 334.9 KV/mm to 394.0 KV/mm. Owing to the good thermal properties of PPO and POSS@F-, these composites exhibit good dielectric properties and improved dimension stability at
1
high temperature. We believe that these results are particularly important for the preparation of advanced dielectric materials for desirable applications.
1 Introduction Polymer dielectrics with high energy density and high temperature capability are ideal candidates for fabrication of advanced power and electronic systems because of their higher breakdown strength, low dielectric loss, lightweight, facile processability as well as low cost [1-4]. Currently, the best commercially available polymer dielectric is biaxially oriented polypropylene (BOPP), which has the high breakdown strength (~700 MV m-1) and low dielectric loss (~10-4) [5]. However, BOPP only has the energy density around 4 J cm-3 [6], which inevitably results in the cumbrous size and cost of device in practical applications. In addition, BOPP can only operate at the temperature below 105 oC [7], above which, cooling system has to be applied to decrease the environment temperature. This deficiency also brings extra energy consumption, weight, volume and cost of the device. Therefore, it still a constant demand for development of high energy density and high temperature resistance polymer dielectrics (above 150 oC) with low dielectric loss (below 0.01). In general, the energy density (Ue ) of linear dielectrics can be calculated by Ue = 1⁄2ε0 εr E2b (Eq. 1), where ε0 is the permittivity in vacuum, εr and Eb represent the dielectric constant and breakdown strength of dielectrics, respectively. It is clearly that both εr and Eb are essential for achieving high Ue [8-11]. Considering the
of polymers is already as high as 700 MV m-1,
which is relatively closed to that value of diamond (2.1 GV m-1) [12]. Thus, increasing the εr becomes a more practical approach for achieving this goal. A variety of methods have been employed to enhance the εr of polymers, among them, introducing high permittivity inorganic nanofillers into polymer matrix to fabric nanocomposites has become a facile and promising approach [13]. So far, various nanofillers have been proposed for achieving this goal, such as 2
BaTiO3 [14-16], TiO2 [17], SiO2 [18], SiC [19], Ni(OH)2 [20], Mxene [21], and many others. Indeed, they can enhance the εr of polymers. However, as the high surface energy of nanofillers and poor compatibilization between nanofillers and polymers, aggregation tends to occur, which brings inevitable defects and results in the elevated dielectric loss and the reduced Eb [2]. Although grafting polymer brushes onto the surface of nanofillers can enable avoid nanofiller agglomeration, it remains a great challenge for scalable production because of their tedious preparation procedures [13]. Besides, high εr can only be obtained at high filler content, which results in the increased viscosity and processing difficulty of nanocomposites. Polyhedral oligomeric silsesquioxane (POSS) is a class of organic-inorganic hybrid nanoparticles, which consists of an inorganic cage-like siloxane core and multiple pendant organic substitutes [22]. The rigid siloxane cage provides mechanical strength while the organic substitutes can afford the reactivity and compatibility with polymers [23-25]. To date, POSS nanoparticles have been widely used as fillers to enhance the thermal, mechanical and rheological properties of polymers [26-31]. Due to the POSS cage trapped plenty of air, it has been considered as an ideal candidate for reducing the εr of polymers [32-37]. Recently, there emerges a class of novel POSS derivate, named fluoride ion encapsulated POSS (POSS@F-), where a fluoride ion is encapsulated within the siloxane cage and a cation is attached outside of the siloxane cage. It was found that by hydrolysis of siloxanes which contains electron-withdrawing groups (such as phenyl, vinyl, p-tolyl, styrenyl, triflfluoropropyl, etc.) with tetraalkylammonium fluoride (TBAF) or tetramethylammonium fluoride, POSS@F- with different substituent groups could be prepared [38-40]. However, their properties are scarcely to be investigated. It is anticipated that the structural dissimilarity between POSS@F- and POSS probably leads to the distinct properties of them. We envision that the segregated fluoride anion and tetraalkylammonium cation constitute a giant ion pair, which can 3
create large dipole, making POSS@F- probably possesses high permittivity. Meanwhile, fluoride ion is encapsulated within POSS cage, which may hinder the movement of free ions under electric field and endow POSS@F- with low dielectric loss. Meanwhile, the inherent properties of POSS, such as good thermal and mechanical properties, are likely to be reserved in POSS@F-. For these considerations, we envision that POSS@F- maybe a good filler for the preparation of advanced polymer dielectrics. Scheme 1. Chemical structures and illustration of the PPO/Ph-POSS@F- and PPO/Ph-POSS composites.
At present, the established key criteria for evaluating the high temperature capability of polymer dielectrics are the glass transition temperature (Tg) and thermal stability. To achieve this, a variety of high-performance engineering plastics with high-temperature resistance have been used for achieving this goal [1, 41-44]. Herein, we report an approach to fabricate advanced polymer dielectrics through blending a fluoride encapsulated octaphenyl POSS (Ph-POSS@F-, as shown in Scheme 1) with poly (2,6-dimethyl-1,4-phenylene oxide) (PPO). One reason for the selection of PPO as matrix is due to its high Tg (> 200 oC). Very recently, it has also been applied for preparation of 4
polymer dielectrics with high temperature capability via post functionalization of sulfonyl side groups [45]. The other reason is due to that both PPO and Ph-POSS@F- have lots of benzene rings, which is benefit for achieveing good filler dispersion. For comparison, a commercially available octaphenyl POSS (Ph-POSS, Scheme 1) was also incorporated into PPO matrix as the control group. The dielectric properties, including dielectric constant, dielectric loss, and breakdown strength of PPO/Ph-POSS@F- composites were evaluated in both room and elevated temperatures. Besides, their thermal properties, including thermal stability, Tgs and coefficient of thermal expansions (CTEs), were also investigated. 2 Experimental section
2.1 Materials. The following chemicals were used as received: poly (2, 6-dimethyl-1, 4-phenyl-ene oxide) (PPO, Aldrich, Mw = 40000 Da), phenyltriethoxysilane (Adamas, >99%), tetra-n-butylammonium fluoride trihydrate (TBAF·3H2O, Aldrich, 95%), octaphenyl POSS (Ph-POSS, Hybrid Plastics, MS0840), (CD3)2SO (99.8%D, Aldrich, 99.9%), chloroform (Chengdu Kelong Chemical Co., Ltd., A.R.), methanol (Chengdu Kelong Chemical Co., Ltd., A.R.), acetone (Chengdu Kelong Chemical Co., Ltd., A.R.). It should be noted that tetrahydrofuran (THF, Chengdu Kelong Chemical Co., Ltd., A.R.) used for reaction was dried over a solvent purification system (SPS-5, Etelux, Beijing, China). 2.2 Instrumentation and Characterizations. All of the 1H,
13
C ,29Si and
19
F NMR spectra
were performed in (CD3)2SO by using a Bruker 400 MHz NMR spectrometer at room temperature. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum were recorded on a MALDI TOF-TOF Mass Spectrometer (AXIMA Performance). Thermogravimetric analysis (TGA) was performed on a thermo-analyzer instrument (TA Instruments Inc., USA) with a
5
scan rate of 10 oC/min under nitrogen atmosphere. The differential scanning calorimetry (DSC) was carried out by using a TA Instruments under nitrogen atmosphere with a heating rate of 10 oC/min to acquire their glass transition temperatures (Tgs). Wide angle X-ray diffraction (WAXD) experiments were recorded on an X’Pert pro MPD X-ray diffractometer (PANalytical B.V., Holland) with a Cu Kα radiation (40 kV, 40 mA). Samples were scanned in the diffraction angle (2θ) range of 2-60o. Scanning Electron Microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) experiments were carried out on a field emission scanning electron microscopy (SEM, JSM-7500F/X-MAX50, Japan) with accelerating voltage of 10 kV, and the fracture surface of samples was coated with a thin gold layer before observation. Transmission electron microscopy (TEM) images of the thin-slice samples were recorded on a Tecnai F20 TEM with an accelerating voltage of 200 kV on a digital CCD camera. The dielectric constant and dielectric loss of the samples were measured by a broad frequency dielectric spectrometer Novocontrol (Concept 50, Germany) with a frequency ranging from 10 Hz to 1 MHz at different temperatures. The breakdown strength of the samples was carried out from dielectric withstand voltage test (DDJ-50 kV, China). In order to obtain the extract Eb, each sample was measured at over 15 times. The coefficient of thermal expansion (CET) of samples was characterized by a thermo-mechanical analysis (TMA Q400, TA Instrument Inc., USA) with a force of 0.1 N. Samples were heated from 50 oC to 225 oC with a heating rate of 5 oC/min and each sample was tested for three times.
3 Results and discussion Ph-POSS@F- was prepared through hydrolysis and condensation of phenyltriethoxysilane with the presence of TBAF according to previous reported [38] (Scheme S1). The successful synthesis of Ph-POSS@F- was verified by 1H,
13
C,
19
F and
29
Si NMR spectra as well as MALDI-TOF mass
spectra. Specifically, two multiple peaks appeared at 7.70-7.29 ppm in the 1H NMR spectrum were 6
characteristic of the proton on the benzene ring, the peaks at ~3.14 ppm were attributed to the methylene resonances near the nitrogen atom (Figure S1A). From
13
C NMR spectrum, these
characteristic peaks were also detected (Figure S1B). A single peak was observed at ~80.8 ppm in the 29Si NMR spectrum, indicating the formed well-defined cage structure (Figure S1C). Meanwhile, a single peak at ~26.8 ppm in the
19
F NMR spectrum was detected, which assigned to the
encapsulated fluoride ion in POSS cage (Figure S1D). In addition, MALDI-TOF mass spectrum of Ph-POSS@F- (Figure S2) showed identical molecular weight, which matched well with the calculated monoisotopic values. According to the above analysis, we conclude that Ph-POSS@F- has been successfully synthesized. The PPO/Ph-POSS@F- composites were prepared through a solution blend and co-precipitation between PPO and Ph-POSS@F- with varied ratios under hot-press process. And PPO/Ph-POSS composites were also prepared in a similar way. The composites containing 1, 5, 10, 15 and 20 wt% Ph-POSS@F- and Ph-POSS are referred to as PF1-20 and P1-20, respectively. The cross-sectional morphology of these composites was observed by scanning electron microscopy (SEM). At lower filler content, all of the samples show homogeneous morphology and contain no discernible agglomeration for PF1, PF10 and PF20. (Figure S3A-D). Wide angle X-ray diffraction (WAXD) results show that these aggregations are the small crystal of POSS@F- (Figure S4). SEM-energy dispersive X-ray spectrometry (EDS) mapping was also performed for analysis the distribution of POSS@F-. Tanking PF10 as an example, C, O, Si, F and N elements were uniformly contained in the polymer matrix (Figure S5). The dispersion of POSS@F- in composite samples were also characterized by TEM (Figure S6). We found that POSS@F- (black dots in TEM images) is uniform dispersed into the PPO matrix, even for the sample with high POSS@F- content. It should be noted
7
that both PF1-20 and P1-20 have extremely low electrical conductivities, indicating the excellent insulativity of all samples (Figure S7).
Figure 1. The effect of filler content on εr and tan δ PPO at ambient temperature. (A) PPO/Ph-POSS@F-, (B) PPO/Ph-POSS, (C) the summarized εr and tan δ at 103 Hz. The εr and dielectric loss (tan δ) of PF1-20 were investigated using broad frequency dielectric spectrometer at ambient temperature from 10 to 106 Hz. As shown in Figure 1A, Ph-POSS@F- can enhance the εr without increasing the tan δ of PPO, both εr and tan δ of PF1-20 are independent on frequency changes. Compared with the εr of pristine PPO at 103 Hz (εr = 2.81), the εr for PF1, PF5, PF10, PF15 and PF20 are 3.04, 3.94. 4.63, 4.96 and 5.31, respectively. The tan δ of pristine
8
PPO at 103 Hz is 2.4×10-3, as for PF1, PF5, PF10, PF15 and PF20, their tan δ are 3.4×10-4, 6×10-4, 7.4×10-4, 9.1×10-4 and 1.1×10-3, respectively (Figure 1C). By contrast, Ph-POSS leads to the reduced εr of PPO. As shown in Figure 1B and C, the εr for P1, P5 and P10 are reduced compared with the pristine PPO, and P10 sample has the lowest εr of 2.40 at 103 Hz. Further increasing Ph-POSS content, the εr of P20 rises back to 2.80 at 103 Hz, which is probably due to the agglomeration of Ph-POSS in polymer matrix leads to the enhanced possibility of polarization and moisture absorption [32]. Usually, the introduction of free ions (cations and anions) inevitably induces the increased loss, whereas the dielectric loss of PPO has not raised. The reason is probably due to the fluorine ion is locked within the silsesquioxane cage, which hinders the movement of free ions under electric field. Based on these analyze, we conclude that Ph-POSS@F- has the unique properties, which can not only enhance the εr , but also can mitigate the tan δ of PPO at the same time.
Considering the demand of high temperature application of polymer dielectrics, we investigated the dielectric properties of PPO/Ph-POSS@F- composites at varied temperature. Before that, thermal properties of PPO/Ph-POSS@F- composites were examined by thermalgravimetric analysis (TGA) and differential scanning calorimetry (DSC). All of the composite samples exhibit good thermal stabilities and are similar to the pristine PPO (Figure 2A). Besides, the heat-resistance index has been calculated to further illustrate the thermal property of PPO/Ph-POSS@F- composites [46, 47], and the results were summarized in Table 1. As can be seen, the addition of POSS@F- results in the slight decline of heat-resistance index (from 220.7 oC to 216.5 oC), indicating the introduction of POSS@F- cannot remarkably deteriorate the thermal stability of the composites. Meanwhile, Ph-POSS@F- has minute effect on the glass transition temperature (Tg) of PPO (Figure 2B). Compared with the pristine PPO, the Tg of PF20 drops from 213.4 oC to 208.1 oC, and the variation is negligible. To evaluate the dimensional stability of the composites at various temperature, the 9
coefficient of thermal expansion (CTE) of these composites was measured. As shown in Figure 2C, Ph-POSS@F- can enable reduce the CTE of PPO. CTEs in the range of 50 to 185 oC for pristine PPO, PF1, PF5, PF10, PF15 and PF20 are 118.6, 105.6, 92.8, 89.7 86.7 and 81.4 ppm/oC, respectively. The reason of declined CETs is mainly attributed to the introduced large amount of inorganic silsesquioxane cage. As the good thermal properties and high temperature dimension stability, PPO/Ph-POSS@F- composites are attractive candidates for high temperature applications.
Figure 2. (A) TGA and (B) DSC curves of PF0-20. (C) The average CTEs of PF0-20 in the range of 50 to 185 oC.
Table 1. Thermal properties of PPO/POSS@F-. Samples
Tg (℃)
Weight loss temperature T5 a (℃)
T30 a (℃)
THRI b (℃)
CTE (ppm/oC)
PPO
213.4
440.6
456.9
220.7
118.6
PF1
211.8
439.4
454.9
219.9
105.6
PF5
211.1
435.4
451.9
218.2
92.8
PF10
210.1
431.1
452.0
217.4
89.7
PF15
208.7
431.9
452.3
217.6
86.7
PF20
208.1
426.6
452.1
216.5
81.4
a
T5 and T30 is the decomposition temperature of 5 wt% and 30 wt% PPO/POSS@F- weight loss.
b
Heat-resistance index THRI is calculated using the formula: THRI = 0.49*[T5 + 0.6*(T30-T5)].
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Then, temperature dependent dielectric spectra of PPO, PF5, PF10 and PF20 were performed from 25 to 175 oC. As displayed in Figure 3A and Figure S8, both εr and tan δ for all of the samples show frequency independence at lower temperature. While at higher temperature, the εr and tan δ show slightly enhancement at relatively low frequency, which is mainly because of the impurity ions in these samples. The εr and tan δ of PPO, PF5, PF10 and PF20 at 103 Hz are summarized in Figure 3B, it is noteworthy that the temperature variation has limited effect on their εr . Taking PF20 as an example, the measured εr at 103 Hz is 5.31 at 25 oC and 5.26 at 175 oC, respectively. Meanwhile, the tan δ of PF20 at high temperature are relatively low, and the value at 103 Hz is only 0.0039 at 175
o
C. These results suggest the excellent high temperature dielectric properties of
PPO/Ph-POSS@F- composites.
Figure 3. (A) The
and tan δ of PF20 at different temperatures, (B) effect of temperature on εr
and tan δ of pristine PPO, PF5, PF10 and PF20 at 103 Hz.
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The breakdown strength of PPO/Ph-POSS@F- composites was analyzed using a two-parameter Weibull distribution function as follows:
P = 1- exp -
Eb β α
where P is the cumulative probability of electrical failure, α is the characteristic breakdown strength when P is 63.2%, Eb is the breakdown strength of the sample, and β is the slope parameter that evaluates the scatter of data. As depicted in Figure 4A, PF5 possesses the highest Eb of 394.0 KV/mm at 25 oC, which is increased 17.8% compared with pristine PPO (334.9 KV/mm), while the β slightly decreases from 18.8 to 16.8. Further increasing the Ph-POSS@F- content, the Eb value has a slight decrease, probably owing to the increased defects of samples. The high temperature breakdown strength was also measured (Figure 4B and Figure S9), the breakdown strength of all the samples exhibit a slight reduction upon heating. The merely reduced Eb also suggested the good electrochemical stability of the composites at high temperature. To probe the effect of introducing POSS@F- on the energy density of the composites, the breakdown strength can be calculated following the given formula (Eq. 1) since the PPO is a linear polymer. As depicted in Figure S10, the breakdown strength of composites initially increases when increasing the POSS@F- loading due to the improved εr and breakdown strength, and the significantly improved energy density can be obtained with 5wt% POSS@F- loading (improving from 1.39 of pure PPO to 2.71). While the energy density tends to drop with further increment of POSS@F- because of the reducing breakdown strength.
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Figure 4. The Weibull plots of breakdown strength of pristine PPO, PF1, PF5, PF10, PF15 and PF20 at (A) 25 oC, (B) 150 oC, (C) Weibull breakdown strength of the composites as a function of temperature. Table 2 lists some literatures about PPO based dielectrics for comparison with our work. Both post-functionalization and adding fillers methods have been carried out. As expected, the incorporation of high
filler (e.g., CCTO, CNT/GE, CNT/GP) promotes the significant increase of
, while the tan δ rises simultaneously by order of magnitude. Although the modified PPO displays higher
, higher Tg, and outstanding storage density by introducing highly polar groups, the tan δ
still shifts to higher value. By contrast, the tan δ of PPO/Ph-POSS@F- composites has no obvious change, despite the increment of
. Besides, their breakdown strength is improved with optimum
loading of Ph-POSS@F-. More importantly, at relatively high temperature up to 175 oC, the highest tan δ still reaches an extremely low value (ca. 0.0039), which represents the excellent stability of PPO/POSS@F- for high-temperature applications.
Table 2. The DSC data and dielectric properties of several PPO-based dielectrics. Method
tan δ
Tg/oC
Ref.
5.31 (25 ℃)
0.0011 (1 kHz)
208.1
This
5.26 (175 ℃)
0.0039 (1 kHz)
CCTO
7.08
0.073 (100 Hz)
CCTO-g-GO
8.60
~0.076 (100 Hz)
Polymer
Filler
PPO
Ph-POSS@F-
(1 kHz)
work
PPO Mixed filler
-
[48]
13
PPO
Post-polymer
SO2-PPO25
CNT/GP
~180
1.69 (1 kHz)
-
[49]
CNT/GE
283
1.40 (1 kHz)
-
5.9 (100 ℃)
0.003 (1 kHz)
211
[45]
8.2 (50 ℃)
0.007 (1 kHz)
228
functionalization SO2-PPO52
4 Conclusions In summary, we demonstrate that Ph-POSS@F- can be regarded as a novel filler to enhance the εr and reduced the tan δ of PPO simultaneously. The εr of PPO is positive correlation with the content of Ph-POSS@F-. Meanwhile, the Eb of PPO can be improved by adding the low content of Ph-POSS@F-. In addition, the PPO/Ph-POSS@F- composites exhibit good thermal stability and dimension stability, making them have good dielectric properties at high temperature. Considering the vast category of POSS nanoparticles, great opportunities exist for the preparation of polymers or polymer composites with good dielectric properties and high temperature capability through rational structure design. It calls for the future research endeavours into the study of advanced polymer dielectrics based on POSS@F-.
Acknowledgments We are grateful for the National Natural Science Foundation of China (51721091).
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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.
S0266-3538(19)32479-0
DOI:
https://doi.org/10.1016/j.compscitech.2020.108035
Reference:
CSTE 108035
To appear in:
Composites Science and Technology
Received Date: 3 September 2019 Revised Date:
1 January 2020
Accepted Date: 22 January 2020
Please cite this article as: Deng Y-Y, Zhou D-L, Han D, Zhang Q, Chen F, Fu Q, Fluoride ion encapsulated polyhedral oligomeric silsesquioxane: A novel filler for polymer nanocomposites with enhanced dielectric constant and reduced dielectric loss, Composites Science and Technology (2020), doi: https://doi.org/10.1016/j.compscitech.2020.108035. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Di Han, Feng Chen, and Qiang Fu designed research; Yi-Yi Deng, Dai-Lin Zhou, Di Han, and Qin Zhang performed research; Yi-Yi Deng, Dai-Lin Zhou, Di Han, Qin Zhang, Feng Chen, and Qiang Fu analyzed data; and Yi-Yi Deng, Di Han, and Qiang Fu wrote the paper.
* To whom correspondence may be addressed. Email: [email protected] (Di Han), [email protected] (Qiang Fu).
Fluoride Ion Encapsulated Polyhedral Oligomeric Silsesquioxane: A Novel Filler for Polymer Nanocomposites with Enhanced Dielectric Constant and Reduced Dielectric Loss Yi-Yi Deng, Dai-Lin Zhou, Di Han*, Qin Zhang, Feng Chen, Qiang Fu* College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
Corresponding authors: E-mail: [email protected] (D. Han); [email protected] (Q. Fu)
KEYWORDS: POSS; PPO; nanocomposites; polymer dielectrics; high temperature capability.
ABSTRACT. Polymer dielectrics have been considered as an ideal choice for preparation of advanced power and electronic systems as their good comprehensive performances. However, it remains a challenge to obtain polymer dielectrics with high dielectric constant (εr ), low dielectric loss (tan δ) and high temperature capability. Although adding high permittivity inorganic nanofillers has become a promising approach to improve the εr of polymers, it usually causes the adverse increment tan δ. Herein, we demonstrate that fluoride ion encapsulated polyhedral oligomeric silsesquioxane (POSS@F-) can be regarded as a novel filler to enhance the εr and reduce the tan δ of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) simultaneously. Compared with the pristine PPO, the εr of PPO/POSS@F- composite can be enhanced from 2.81 to 5.31, while its tan δ is still relatively low (0.0011 at 1 kHz). Meanwhile, the breakdown strength of PPO can also be slightly reinforced from 334.9 KV/mm to 394.0 KV/mm. Owing to the good thermal properties of PPO and POSS@F-, these composites exhibit good dielectric properties and improved dimension stability at
1
high temperature. We believe that these results are particularly important for the preparation of advanced dielectric materials for desirable applications.
1 Introduction Polymer dielectrics with high energy density and high temperature capability are ideal candidates for fabrication of advanced power and electronic systems because of their higher breakdown strength, low dielectric loss, lightweight, facile processability as well as low cost [1-4]. Currently, the best commercially available polymer dielectric is biaxially oriented polypropylene (BOPP), which has the high breakdown strength (~700 MV m-1) and low dielectric loss (~10-4) [5]. However, BOPP only has the energy density around 4 J cm-3 [6], which inevitably results in the cumbrous size and cost of device in practical applications. In addition, BOPP can only operate at the temperature below 105 oC [7], above which, cooling system has to be applied to decrease the environment temperature. This deficiency also brings extra energy consumption, weight, volume and cost of the device. Therefore, it still a constant demand for development of high energy density and high temperature resistance polymer dielectrics (above 150 oC) with low dielectric loss (below 0.01). In general, the energy density (Ue ) of linear dielectrics can be calculated by Ue = 1⁄2ε0 εr E2b (Eq. 1), where ε0 is the permittivity in vacuum, εr and Eb represent the dielectric constant and breakdown strength of dielectrics, respectively. It is clearly that both εr and Eb are essential for achieving high Ue [8-11]. Considering the
of polymers is already as high as 700 MV m-1,
which is relatively closed to that value of diamond (2.1 GV m-1) [12]. Thus, increasing the εr becomes a more practical approach for achieving this goal. A variety of methods have been employed to enhance the εr of polymers, among them, introducing high permittivity inorganic nanofillers into polymer matrix to fabric nanocomposites has become a facile and promising approach [13]. So far, various nanofillers have been proposed for achieving this goal, such as 2
BaTiO3 [14-16], TiO2 [17], SiO2 [18], SiC [19], Ni(OH)2 [20], Mxene [21], and many others. Indeed, they can enhance the εr of polymers. However, as the high surface energy of nanofillers and poor compatibilization between nanofillers and polymers, aggregation tends to occur, which brings inevitable defects and results in the elevated dielectric loss and the reduced Eb [2]. Although grafting polymer brushes onto the surface of nanofillers can enable avoid nanofiller agglomeration, it remains a great challenge for scalable production because of their tedious preparation procedures [13]. Besides, high εr can only be obtained at high filler content, which results in the increased viscosity and processing difficulty of nanocomposites. Polyhedral oligomeric silsesquioxane (POSS) is a class of organic-inorganic hybrid nanoparticles, which consists of an inorganic cage-like siloxane core and multiple pendant organic substitutes [22]. The rigid siloxane cage provides mechanical strength while the organic substitutes can afford the reactivity and compatibility with polymers [23-25]. To date, POSS nanoparticles have been widely used as fillers to enhance the thermal, mechanical and rheological properties of polymers [26-31]. Due to the POSS cage trapped plenty of air, it has been considered as an ideal candidate for reducing the εr of polymers [32-37]. Recently, there emerges a class of novel POSS derivate, named fluoride ion encapsulated POSS (POSS@F-), where a fluoride ion is encapsulated within the siloxane cage and a cation is attached outside of the siloxane cage. It was found that by hydrolysis of siloxanes which contains electron-withdrawing groups (such as phenyl, vinyl, p-tolyl, styrenyl, triflfluoropropyl, etc.) with tetraalkylammonium fluoride (TBAF) or tetramethylammonium fluoride, POSS@F- with different substituent groups could be prepared [38-40]. However, their properties are scarcely to be investigated. It is anticipated that the structural dissimilarity between POSS@F- and POSS probably leads to the distinct properties of them. We envision that the segregated fluoride anion and tetraalkylammonium cation constitute a giant ion pair, which can 3
create large dipole, making POSS@F- probably possesses high permittivity. Meanwhile, fluoride ion is encapsulated within POSS cage, which may hinder the movement of free ions under electric field and endow POSS@F- with low dielectric loss. Meanwhile, the inherent properties of POSS, such as good thermal and mechanical properties, are likely to be reserved in POSS@F-. For these considerations, we envision that POSS@F- maybe a good filler for the preparation of advanced polymer dielectrics. Scheme 1. Chemical structures and illustration of the PPO/Ph-POSS@F- and PPO/Ph-POSS composites.
At present, the established key criteria for evaluating the high temperature capability of polymer dielectrics are the glass transition temperature (Tg) and thermal stability. To achieve this, a variety of high-performance engineering plastics with high-temperature resistance have been used for achieving this goal [1, 41-44]. Herein, we report an approach to fabricate advanced polymer dielectrics through blending a fluoride encapsulated octaphenyl POSS (Ph-POSS@F-, as shown in Scheme 1) with poly (2,6-dimethyl-1,4-phenylene oxide) (PPO). One reason for the selection of PPO as matrix is due to its high Tg (> 200 oC). Very recently, it has also been applied for preparation of 4
polymer dielectrics with high temperature capability via post functionalization of sulfonyl side groups [45]. The other reason is due to that both PPO and Ph-POSS@F- have lots of benzene rings, which is benefit for achieveing good filler dispersion. For comparison, a commercially available octaphenyl POSS (Ph-POSS, Scheme 1) was also incorporated into PPO matrix as the control group. The dielectric properties, including dielectric constant, dielectric loss, and breakdown strength of PPO/Ph-POSS@F- composites were evaluated in both room and elevated temperatures. Besides, their thermal properties, including thermal stability, Tgs and coefficient of thermal expansions (CTEs), were also investigated. 2 Experimental section
2.1 Materials. The following chemicals were used as received: poly (2, 6-dimethyl-1, 4-phenyl-ene oxide) (PPO, Aldrich, Mw = 40000 Da), phenyltriethoxysilane (Adamas, >99%), tetra-n-butylammonium fluoride trihydrate (TBAF·3H2O, Aldrich, 95%), octaphenyl POSS (Ph-POSS, Hybrid Plastics, MS0840), (CD3)2SO (99.8%D, Aldrich, 99.9%), chloroform (Chengdu Kelong Chemical Co., Ltd., A.R.), methanol (Chengdu Kelong Chemical Co., Ltd., A.R.), acetone (Chengdu Kelong Chemical Co., Ltd., A.R.). It should be noted that tetrahydrofuran (THF, Chengdu Kelong Chemical Co., Ltd., A.R.) used for reaction was dried over a solvent purification system (SPS-5, Etelux, Beijing, China). 2.2 Instrumentation and Characterizations. All of the 1H,
13
C ,29Si and
19
F NMR spectra
were performed in (CD3)2SO by using a Bruker 400 MHz NMR spectrometer at room temperature. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrum were recorded on a MALDI TOF-TOF Mass Spectrometer (AXIMA Performance). Thermogravimetric analysis (TGA) was performed on a thermo-analyzer instrument (TA Instruments Inc., USA) with a
5
scan rate of 10 oC/min under nitrogen atmosphere. The differential scanning calorimetry (DSC) was carried out by using a TA Instruments under nitrogen atmosphere with a heating rate of 10 oC/min to acquire their glass transition temperatures (Tgs). Wide angle X-ray diffraction (WAXD) experiments were recorded on an X’Pert pro MPD X-ray diffractometer (PANalytical B.V., Holland) with a Cu Kα radiation (40 kV, 40 mA). Samples were scanned in the diffraction angle (2θ) range of 2-60o. Scanning Electron Microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) experiments were carried out on a field emission scanning electron microscopy (SEM, JSM-7500F/X-MAX50, Japan) with accelerating voltage of 10 kV, and the fracture surface of samples was coated with a thin gold layer before observation. Transmission electron microscopy (TEM) images of the thin-slice samples were recorded on a Tecnai F20 TEM with an accelerating voltage of 200 kV on a digital CCD camera. The dielectric constant and dielectric loss of the samples were measured by a broad frequency dielectric spectrometer Novocontrol (Concept 50, Germany) with a frequency ranging from 10 Hz to 1 MHz at different temperatures. The breakdown strength of the samples was carried out from dielectric withstand voltage test (DDJ-50 kV, China). In order to obtain the extract Eb, each sample was measured at over 15 times. The coefficient of thermal expansion (CET) of samples was characterized by a thermo-mechanical analysis (TMA Q400, TA Instrument Inc., USA) with a force of 0.1 N. Samples were heated from 50 oC to 225 oC with a heating rate of 5 oC/min and each sample was tested for three times.
3 Results and discussion Ph-POSS@F- was prepared through hydrolysis and condensation of phenyltriethoxysilane with the presence of TBAF according to previous reported [38] (Scheme S1). The successful synthesis of Ph-POSS@F- was verified by 1H,
13
C,
19
F and
29
Si NMR spectra as well as MALDI-TOF mass
spectra. Specifically, two multiple peaks appeared at 7.70-7.29 ppm in the 1H NMR spectrum were 6
characteristic of the proton on the benzene ring, the peaks at ~3.14 ppm were attributed to the methylene resonances near the nitrogen atom (Figure S1A). From
13
C NMR spectrum, these
characteristic peaks were also detected (Figure S1B). A single peak was observed at ~80.8 ppm in the 29Si NMR spectrum, indicating the formed well-defined cage structure (Figure S1C). Meanwhile, a single peak at ~26.8 ppm in the
19
F NMR spectrum was detected, which assigned to the
encapsulated fluoride ion in POSS cage (Figure S1D). In addition, MALDI-TOF mass spectrum of Ph-POSS@F- (Figure S2) showed identical molecular weight, which matched well with the calculated monoisotopic values. According to the above analysis, we conclude that Ph-POSS@F- has been successfully synthesized. The PPO/Ph-POSS@F- composites were prepared through a solution blend and co-precipitation between PPO and Ph-POSS@F- with varied ratios under hot-press process. And PPO/Ph-POSS composites were also prepared in a similar way. The composites containing 1, 5, 10, 15 and 20 wt% Ph-POSS@F- and Ph-POSS are referred to as PF1-20 and P1-20, respectively. The cross-sectional morphology of these composites was observed by scanning electron microscopy (SEM). At lower filler content, all of the samples show homogeneous morphology and contain no discernible agglomeration for PF1, PF10 and PF20. (Figure S3A-D). Wide angle X-ray diffraction (WAXD) results show that these aggregations are the small crystal of POSS@F- (Figure S4). SEM-energy dispersive X-ray spectrometry (EDS) mapping was also performed for analysis the distribution of POSS@F-. Tanking PF10 as an example, C, O, Si, F and N elements were uniformly contained in the polymer matrix (Figure S5). The dispersion of POSS@F- in composite samples were also characterized by TEM (Figure S6). We found that POSS@F- (black dots in TEM images) is uniform dispersed into the PPO matrix, even for the sample with high POSS@F- content. It should be noted
7
that both PF1-20 and P1-20 have extremely low electrical conductivities, indicating the excellent insulativity of all samples (Figure S7).
Figure 1. The effect of filler content on εr and tan δ PPO at ambient temperature. (A) PPO/Ph-POSS@F-, (B) PPO/Ph-POSS, (C) the summarized εr and tan δ at 103 Hz. The εr and dielectric loss (tan δ) of PF1-20 were investigated using broad frequency dielectric spectrometer at ambient temperature from 10 to 106 Hz. As shown in Figure 1A, Ph-POSS@F- can enhance the εr without increasing the tan δ of PPO, both εr and tan δ of PF1-20 are independent on frequency changes. Compared with the εr of pristine PPO at 103 Hz (εr = 2.81), the εr for PF1, PF5, PF10, PF15 and PF20 are 3.04, 3.94. 4.63, 4.96 and 5.31, respectively. The tan δ of pristine
8
PPO at 103 Hz is 2.4×10-3, as for PF1, PF5, PF10, PF15 and PF20, their tan δ are 3.4×10-4, 6×10-4, 7.4×10-4, 9.1×10-4 and 1.1×10-3, respectively (Figure 1C). By contrast, Ph-POSS leads to the reduced εr of PPO. As shown in Figure 1B and C, the εr for P1, P5 and P10 are reduced compared with the pristine PPO, and P10 sample has the lowest εr of 2.40 at 103 Hz. Further increasing Ph-POSS content, the εr of P20 rises back to 2.80 at 103 Hz, which is probably due to the agglomeration of Ph-POSS in polymer matrix leads to the enhanced possibility of polarization and moisture absorption [32]. Usually, the introduction of free ions (cations and anions) inevitably induces the increased loss, whereas the dielectric loss of PPO has not raised. The reason is probably due to the fluorine ion is locked within the silsesquioxane cage, which hinders the movement of free ions under electric field. Based on these analyze, we conclude that Ph-POSS@F- has the unique properties, which can not only enhance the εr , but also can mitigate the tan δ of PPO at the same time.
Considering the demand of high temperature application of polymer dielectrics, we investigated the dielectric properties of PPO/Ph-POSS@F- composites at varied temperature. Before that, thermal properties of PPO/Ph-POSS@F- composites were examined by thermalgravimetric analysis (TGA) and differential scanning calorimetry (DSC). All of the composite samples exhibit good thermal stabilities and are similar to the pristine PPO (Figure 2A). Besides, the heat-resistance index has been calculated to further illustrate the thermal property of PPO/Ph-POSS@F- composites [46, 47], and the results were summarized in Table 1. As can be seen, the addition of POSS@F- results in the slight decline of heat-resistance index (from 220.7 oC to 216.5 oC), indicating the introduction of POSS@F- cannot remarkably deteriorate the thermal stability of the composites. Meanwhile, Ph-POSS@F- has minute effect on the glass transition temperature (Tg) of PPO (Figure 2B). Compared with the pristine PPO, the Tg of PF20 drops from 213.4 oC to 208.1 oC, and the variation is negligible. To evaluate the dimensional stability of the composites at various temperature, the 9
coefficient of thermal expansion (CTE) of these composites was measured. As shown in Figure 2C, Ph-POSS@F- can enable reduce the CTE of PPO. CTEs in the range of 50 to 185 oC for pristine PPO, PF1, PF5, PF10, PF15 and PF20 are 118.6, 105.6, 92.8, 89.7 86.7 and 81.4 ppm/oC, respectively. The reason of declined CETs is mainly attributed to the introduced large amount of inorganic silsesquioxane cage. As the good thermal properties and high temperature dimension stability, PPO/Ph-POSS@F- composites are attractive candidates for high temperature applications.
Figure 2. (A) TGA and (B) DSC curves of PF0-20. (C) The average CTEs of PF0-20 in the range of 50 to 185 oC.
Table 1. Thermal properties of PPO/POSS@F-. Samples
Tg (℃)
Weight loss temperature T5 a (℃)
T30 a (℃)
THRI b (℃)
CTE (ppm/oC)
PPO
213.4
440.6
456.9
220.7
118.6
PF1
211.8
439.4
454.9
219.9
105.6
PF5
211.1
435.4
451.9
218.2
92.8
PF10
210.1
431.1
452.0
217.4
89.7
PF15
208.7
431.9
452.3
217.6
86.7
PF20
208.1
426.6
452.1
216.5
81.4
a
T5 and T30 is the decomposition temperature of 5 wt% and 30 wt% PPO/POSS@F- weight loss.
b
Heat-resistance index THRI is calculated using the formula: THRI = 0.49*[T5 + 0.6*(T30-T5)].
10
Then, temperature dependent dielectric spectra of PPO, PF5, PF10 and PF20 were performed from 25 to 175 oC. As displayed in Figure 3A and Figure S8, both εr and tan δ for all of the samples show frequency independence at lower temperature. While at higher temperature, the εr and tan δ show slightly enhancement at relatively low frequency, which is mainly because of the impurity ions in these samples. The εr and tan δ of PPO, PF5, PF10 and PF20 at 103 Hz are summarized in Figure 3B, it is noteworthy that the temperature variation has limited effect on their εr . Taking PF20 as an example, the measured εr at 103 Hz is 5.31 at 25 oC and 5.26 at 175 oC, respectively. Meanwhile, the tan δ of PF20 at high temperature are relatively low, and the value at 103 Hz is only 0.0039 at 175
o
C. These results suggest the excellent high temperature dielectric properties of
PPO/Ph-POSS@F- composites.
Figure 3. (A) The
and tan δ of PF20 at different temperatures, (B) effect of temperature on εr
and tan δ of pristine PPO, PF5, PF10 and PF20 at 103 Hz.
11
The breakdown strength of PPO/Ph-POSS@F- composites was analyzed using a two-parameter Weibull distribution function as follows:
P = 1- exp -
Eb β α
where P is the cumulative probability of electrical failure, α is the characteristic breakdown strength when P is 63.2%, Eb is the breakdown strength of the sample, and β is the slope parameter that evaluates the scatter of data. As depicted in Figure 4A, PF5 possesses the highest Eb of 394.0 KV/mm at 25 oC, which is increased 17.8% compared with pristine PPO (334.9 KV/mm), while the β slightly decreases from 18.8 to 16.8. Further increasing the Ph-POSS@F- content, the Eb value has a slight decrease, probably owing to the increased defects of samples. The high temperature breakdown strength was also measured (Figure 4B and Figure S9), the breakdown strength of all the samples exhibit a slight reduction upon heating. The merely reduced Eb also suggested the good electrochemical stability of the composites at high temperature. To probe the effect of introducing POSS@F- on the energy density of the composites, the breakdown strength can be calculated following the given formula (Eq. 1) since the PPO is a linear polymer. As depicted in Figure S10, the breakdown strength of composites initially increases when increasing the POSS@F- loading due to the improved εr and breakdown strength, and the significantly improved energy density can be obtained with 5wt% POSS@F- loading (improving from 1.39 of pure PPO to 2.71). While the energy density tends to drop with further increment of POSS@F- because of the reducing breakdown strength.
12
Figure 4. The Weibull plots of breakdown strength of pristine PPO, PF1, PF5, PF10, PF15 and PF20 at (A) 25 oC, (B) 150 oC, (C) Weibull breakdown strength of the composites as a function of temperature. Table 2 lists some literatures about PPO based dielectrics for comparison with our work. Both post-functionalization and adding fillers methods have been carried out. As expected, the incorporation of high
filler (e.g., CCTO, CNT/GE, CNT/GP) promotes the significant increase of
, while the tan δ rises simultaneously by order of magnitude. Although the modified PPO displays higher
, higher Tg, and outstanding storage density by introducing highly polar groups, the tan δ
still shifts to higher value. By contrast, the tan δ of PPO/Ph-POSS@F- composites has no obvious change, despite the increment of
. Besides, their breakdown strength is improved with optimum
loading of Ph-POSS@F-. More importantly, at relatively high temperature up to 175 oC, the highest tan δ still reaches an extremely low value (ca. 0.0039), which represents the excellent stability of PPO/POSS@F- for high-temperature applications.
Table 2. The DSC data and dielectric properties of several PPO-based dielectrics. Method
tan δ
Tg/oC
Ref.
5.31 (25 ℃)
0.0011 (1 kHz)
208.1
This
5.26 (175 ℃)
0.0039 (1 kHz)
CCTO
7.08
0.073 (100 Hz)
CCTO-g-GO
8.60
~0.076 (100 Hz)
Polymer
Filler
PPO
Ph-POSS@F-
(1 kHz)
work
PPO Mixed filler
-
[48]
13
PPO
Post-polymer
SO2-PPO25
CNT/GP
~180
1.69 (1 kHz)
-
[49]
CNT/GE
283
1.40 (1 kHz)
-
5.9 (100 ℃)
0.003 (1 kHz)
211
[45]
8.2 (50 ℃)
0.007 (1 kHz)
228
functionalization SO2-PPO52
4 Conclusions In summary, we demonstrate that Ph-POSS@F- can be regarded as a novel filler to enhance the εr and reduced the tan δ of PPO simultaneously. The εr of PPO is positive correlation with the content of Ph-POSS@F-. Meanwhile, the Eb of PPO can be improved by adding the low content of Ph-POSS@F-. In addition, the PPO/Ph-POSS@F- composites exhibit good thermal stability and dimension stability, making them have good dielectric properties at high temperature. Considering the vast category of POSS nanoparticles, great opportunities exist for the preparation of polymers or polymer composites with good dielectric properties and high temperature capability through rational structure design. It calls for the future research endeavours into the study of advanced polymer dielectrics based on POSS@F-.
Acknowledgments We are grateful for the National Natural Science Foundation of China (51721091).
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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.