Fabrication, proposed model and simulation

Composites Part A 107 (2018) 570–578

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Composites Part A journal homepage: www.elsevier.com/locate/compositesa

Fabrication, proposed model and simulation predictions on thermally conductive hybrid cyanate ester composites with boron nitride fillers

T

Yang Lia,1, Genjiu Xua,1, Yongqiang Guoa, Tengbo Maa, Xiao Zhonga, Qiuyu Zhanga, ⁎ Junwei Gua,b,c, a

MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China c Institute of Intelligence Material and Structure, Institute of Unmanned Systems, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: A. Polymer-matrix composites (PMCs) B. Thermal properties D. Mechanical testing E. Casting

DCPDCE/BADCy hybrid resin and BN fillers were performed to fabricate the thermally conductive BN/DCPDCE/ BADCy composites. When the molar ratio of DCPDCE/BADCy was 0.4/0.6, the dielectric constant (ε) and dielectric loss tangent (tgδ) value of the DCPDCE/BADCy hybrid resin was decreased to 2.92 and 5.08 × 10−3, respectively. Impact and flexural strength was increased to 10.7 kJ/m2 and 100.7 MPa, respectively. And the heat-resistance index (THRI) was 201.6 °C. Furthermore, the thermally conductive coefficient (λ) of the BN/ DCPDCE/BADCy composite with 30 wt% BN fillers was improved to 0.64 W/mK, about 3 times in comparison to that of pristine DCPDCE/BADCy hybrid resin. Compared to that of Maxwell and Russell models, our proposed thermally conductive model could predict the experimental λ values more precisely. THRI value was enhanced from 201.6 °C (Pristine DCPDCE/BADCy hybrid resin) to 206.6 °C. Moreover, the BN/DCPDCE/BADCy composite with 10 wt% BN presented the optimal impact strength (11.7 kJ/m2) and flexural strength (108.4 MPa).

1. Introduction Cyanate ester (CE) resins possess outstanding dielectric constant (ε, 2.6–3.2) [1–3] and dielectric loss tangent (tgδ, 0.002–0.006) [4–6], prominent mechanical strength & thermal stabilities, lower moisture absorption and coefficient of thermal expansion (CTE), as well as excellent solubility, and has been widely applied in the fields of high frequency digital printed circuit boards, high performance wavetransparent materials and aerospace structural materials [7–11]. However, for present ε and tgδ value of pristine CE matrix, gaps still exists in higher performance radomes with ideal broadband (Wavelength of 1–1000 mm, frequency of 0.3–300 GHz) [12,13]. Furthermore, intrinsic low thermally conductive coefficient (λ) value of the pristine CE matrix has also limited its wider application in the fields of thermal diffusion. As matrix for wave-transparent polymeric composites, their dielectric performance is always a chief concern. To our knowledge, two common methods (blend [14,15] and copolymerization [16,17]) are usually performed to modify the dielectric properties of the CE matrix.

Zhang et al. [18] incorporated methyl silsesquioxane into CE matrix by blending method, and the obtained ε value of the modified CE resins was decreased from 3.05 to 2.78, but the flexural strength was decreased by 20%. Yuan et al. [19] adopted poly (urea-formaldehyde) microcapsules to modify CE matrix, and the ε value was decreased from 4.5 to 4.2, but 5% weight loss temperature (T5) was decreased inevitably by 7%. Huang et al. [20] fabricated modified BADCy resins by introducing hyperbranched poly(phenylene oxide) containing epoxy groups, and the ε value was decreased from 2.9 to 2.4, but the glass transition temperature (Tg) value was also decreased by 15%. Zhou et al. [21] also synthesized a serial of hyperbranched polysiloxanes containing epoxy groups to modify CE matrix via copolymerization, the ε value of modified resins was decreased from 2.76 to 2.72, but its Tg value was decreased by 8%. In our previous work [22], novel fluoridecontaining compound of 2-((3-trifluoromethyl)phenoxy)methyl oxirane (TFMPMO) was synthesized to modify bisphenol a dicyanate ester (BADCy) matrix, and the ε and tgδ value was decreased by 8.3% and 17.3%. However, the mechanical properties and thermal stabilities of the above CE modifiers were generally decreased [15]. Meantime, the

⁎ Corresponding author at: MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China. E-mail addresses: [email protected], [email protected] (J. Gu). 1 The authors Yang Li and Genjiu Xu contributed equally to this work and should be considered co-first authors.

https://doi.org/10.1016/j.compositesa.2018.02.006 Received 12 January 2018; Received in revised form 2 February 2018; Accepted 5 February 2018 Available online 07 February 2018 1359-835X/ © 2018 Elsevier Ltd. All rights reserved.

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Scheme 1. Possible reaction mechanism of the DCPDCE and BADCy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

DCPDCE/BADCy composites were also discussed and analyzed.

above complex fabrication technology and rigor conditions were always needed. Dicyclopentadiene has attracted more and more attentions owing to its lowly polar alicyclic structure, good moisture absorption and low cost, etc. [23]. Incorporating bulky nonplanar dicyclopentadiene structure into the backbone of the polymers could result in the decreasing water absorption and ε value of the polymeric composites, as well as hardly influence on their heat resistance [24–27]. Hwang et al. [28] synthesized dicyclopentadiene-containing cyanate ester to copolymerize with BADCy matrix, the ε value of the modified BADCy was ranged from 2.58 to 2.95, but never reported their mechanical properties. Compared with that of pristine BADCy matrix, dicyclopentadiene bisphenol cyanate ester (DCPDCE) presents relatively lower ε and tgδ value owing to its lowly polar aliphatic cyclopentadienyl groups [29,30]. Recently, introducing thermally conductive fillers into polymeric matrix is identified as one of the most effective and convenient method to improve the λ values of the polymers [31–36]. Certainly, several thermally conductive CE composites were also fabricated and investigated by adding thermally conductive fillers into CE matrix. Wooster et al. [37] fabricated the thermally conductive SiO2/CE composites, and the corresponding λ value was increased by 74% than that of pristine CE matrix. Mi et al. [38] adopted multiwalled carbon nanotubes (MWCNTs) to improve the thermal conductivities of CE matrix, and the maximum λ value of the MWCNTs/CE composites was improved to 0.65 W/mK. Ling et al. [39] introduced aluminium nitride (AlN) fillers into CE matrix, and the corresponding λ value of the AlN/ CE composites was improved to 2.60 W/mK, 7 times higher than that of pristine CE matrix. Compared to that of other ceramic particles (Al2O3 [40,41], Si3N4 [42,43] and AlN [44–46], etc.), boron nitride (BN) fillers possesses the lowest ε and tgδ value among ceramics fillers [47,48], extremely high λ value [49–51] and excellent thermochemical stability [52–54], etc. Several thermally conductive polymeric composites (BN/ VMQ [55], BN/PPS [56], and BN/EP [57]) filled with BN fillers had been successfully fabricated in our previous works. In our present work, thermally conductive boron nitride/dicyclopentadiene bisphenol cyanate ester/bisphenol A dicyanate ester (BN/ DCPDCE/BADCy) composites were fabricated via blending followed by casting method, herein, hybrid resins of DCPDCE/BADCy as matrix and BN as thermally conductive fillers. The mass fraction of DCPDCE influencing on the dielectric & mechanical properties and thermal stabilities of the DCPDCE/BADCy modified resins were investigated. Furthermore, the mass fraction of BN fillers affecting on the thermal conductivities, mechanical properties and thermal stabilities of the BN/

2. Experimental 2.1. Main materials Bisphenol A dicyanate ester (BADCy) and dicyclopentadiene bisphenol cyanate ester (DCPDCE), with a relative molecular weight of 278.32 g/mol and 841.52 g/mol, respectively, were both purchased from Wuqiao Resin of Jiangsu Factory Co., Ltd., (Jiangsu, China); Dibutyltin dilaurate was supplied by Alfa Aesar Chemical Co., Ltd., (Tianjin, China); Boron nitride (BN), with a average diameter of 2–3 um, was received from Xuzhou Hongwu Nanomaterial Co., Ltd., (Jiangsu, China); Acetone (analytical pure) was obtained from Tianjin Fuyu Fine Chemical Co., Ltd., (Tianjin, China); Deionized water was prepared by our own laboratory. 2.2. Fabrication of DCPDCE/BADCy hybrid resins BADCy was placed into a beaker at 120 °C equipped with magnetic stirrer, and a certain proportion of DCPDCE was then added into the above beaker and stirred to gain uniform liquid. After degasification in a vacuum oven at 120 °C, the mixture of DCPDCE/BADCy was then poured into a preheating mold at 120 °C for curing according to the procedure [58] of 150 °C/1 h + 180 °C/2 h + 200 °C/6 h, followed by post-curing at 220 °C for another 2 h, finally to obtain the hybrid resins of DCPDCE/BADCy. The possible reaction mechanism was shown in Scheme 1. 2.3. Fabrication of BN/DCPDCE/BADCy composites Hybrid resins of DCPDCE/BADCy were firstly heated to 120 °C and stirred uniformly. Then BN fillers were then added and stirred uniformly. After degasification in a vacuum oven, the above mixtures were poured into a preheating mold at 120 °C for curing according to the following procedure of 150 °C/1 h + 180 °C/2 h + 200 °C/6 h, followed by post-curing at 220 °C for another 2 h, finally to obtain the BN/ DCPDCE/BADCy composites. And the corresponding schematic diagram of the fabrication for the BN/DCPDCE/BADCy composites was presented in Fig. 1. 2.4. Characterizations Thermal gravimetric (TG) analyses of the samples were performed 571

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Fig. 1. Schematic diagram of the fabrication for the BN/DCPDCE/BADCy composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

introduction of dicyclopentadiene structure with the non-planar lowpolar groups could effectively increase the free volume of DCPDCE/ DCPDCE hybrid resins, which could effectively decrease the electronegativity of benzene rings and dilute the dipole concentration of hybrid systems, resulting in the decrease of ε and tanδ value of the DCPDCE/DCPDCE hybrid resins. For a given DCPDCE loading, the ε value of the DCPDCE/BADCy hybrid resins was decreased with the increasing testing frequency. However, the tgδ value was increased firstly, but decreased with the increasing testing frequency. The reason was that all the electronic, atomic and orientation polarization could keep up with the electric field changes under lower testing frequency (< 104 Hz). During the moderate testing frequency range (104–105 Hz), the dipoles could catch up with the changes of electric field. To overcome the friction resistance of the dipole orientation, and the produced heat could result in dielectric loss. However, only electronic polarization could follow the changes of the electric field under higher testing frequency (106–107 Hz), and the dielectric properties were mainly resulted from the atomic and electronic polarization, resulting in the relatively lower tgδ value [59,60].

with a heating rate of 10 °C/min (argon atmosphere), over the whole range of temperature (40–800 °C) by STA 449F3 (NETZSCH C Corp., Germany); Dynamic mechanical analyses (DMA) of the samples were measured at 5 °C/min at 1 Hz over the whole range of temperature (25–300 °C) by Q800 (TA Instruments Corp., America); Dielectric constant (ε) and dielectric loss tangent (tgδ) values of the samples were measured using a Novocontrol Technologies Alpha-A high-resolution dielectric analyzer (Novocontrol Corp., Germany) at room temperature, and the corresponding specimen dimension was 100 mm × 100 mm × 4 mm; Thermally conductive coefficient (λ) and thermal diffusivity (α) values of the samples were measured using TPS2200 Hot Disk instrument (AB Corp., Sweden) by a transient plane source method according to standard ISO 22007-2: 2008, and the corresponding specimen dimension was 20 mm × 20 mm × 1.3 mm; Scanning electron microscope (SEM) morphologies of the samples were analyzed by VEGA3-LMH (TESCAN Corp., Czech Republic); Flexural strength of the samples were measured using an Electron Omnipotence Experiment Machine SANS2CMT5105 (Shenzhen New Sansi Corp., China) according to the standard ISO 178-1993; Impact strength values of the samples were measured with a XCJ-40 impact testing machine (Chengde Materials Testing Corp., China) according to the standard ISO 179-1993.

3.2. Mechanical properties of DCPDCE/BADCy hybrid resins Molar fraction of DCPDCE affecting on the mechanical properties of the DCPDCE/BADCy hybrid resins was shown in Fig. 3. Both the impact and flexural strength of the DCPDCE/BADCy hybrid resins were gradually decreased with the increasing addition of DCPDCE. When the molar ratio of DCPDCE to BADCy was 0.4:0.6, the corresponding impact and flexural strength of DCPDCE/BADCy hybrid resin was 10.7 kJ/ m2 and 100.7 MPa, slightly lower than that of pristine BADCy matrix (Impact strength of 11.6 kJ/m2 and flexural strength of 112.3 MPa), respectively. It could be attributed that the introduction of the dicyclopentadiene structure could increase the molecular chain rigidity of the DCPDCE/BADCy hybrid resins and restrict the movement of the chain segments to a certain extent.

3. Results and discussion 3.1. Dielectric properties of DCPDCE/BADCy hybrid resins Fig. 2 presented the molar fraction of DCPDCE affecting on the ε and tgδ values at different frequency of the DCPDCE/BADCy hybrid resins. The ε value of DCPDCE/BADCy hybrid resins was gradually decreased with the increasing addition of DCPDCE, and the obtained tgδ (4 × 10−3–6.4 × 10−3) value was also lower than that of pristine BADCy matrix (tgδ of 6.73 × 10−3). When the molar ratio of DCPDCE to BADCy was 0.4:0.6, the obtained DCPDCE/BADCy hybrid resin presented the optimal dielectric properties, and the corresponding ε and tanδ value at 10 MHz was 2.92 and 5.08 × 10−3, decreased by 5.5% and 24.5% in comparison to that of pristine BADCy matrix (ε of 3.09 and tanδ of 6.73 × 10−3), respectively. The reason was that the

3.3. Thermal properties of DCPDCE/BADCy hybrid resins Fig. 4 presented TGA and DMA curves of the DCPDCE/BADCy 572

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Fig. 2. Contents of DCPDCE affecting on the ε and tgδ values at different frequency of the DCPDCE/BADCy hybrid resins. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

activity of the chain segments for DCPDCE/BADCy hybrid systems, resulting in the decrease of Tg value. In our study, the influence of decreased crosslinking density on Tg value was greater than that of increased molecular chain rigidity.

hybrid resins, and the corresponding characteristic thermal data were collected in Table 1. Compared to that of pristine BADCy matrix, with the increasing addition of DCPDCE, the corresponding weight loss temperatures (T5, T30 and T50) and the THeat resistance index (THRI) [61] values of the DCPDCE/BADCy hybrid resins were all decreased. When the molar ratio of DCPDCE to BADCy was 0.4:0.6, the corresponding THRI value of the obtained DCPDCE/BADCy hybrid resin was 201.6 °C, and still possessed relatively excellent thermal stability. The reason was that intrinsic thermal stability of aliphatic dicyclopentadiene moiety was relatively poor. Moreover, the addition of DCPDCE might probably decrease the crosslinking density of the DCPDCE/BADCy hybrid resins, as well as decrease the content of triazine ring. As also seen from Fig. 4 and Table 1, the Tg value of the DCPDCE/ BADCy hybrid resins was also decreased with the increasing addition of DCPDCE. When the molar ratio of DCPDCE to BADCy was 0.4:0.6, the corresponding Tg value of the obtained DCPDCE/BADCy hybrid resin was 226.7 °C. One hand, the introduction of double cyclopentadienyl ring structure would increase the molecular chain rigidity of DCPDCE/ BADCy hybrid systems, in favor of improving the Tg value. On the other hand, the introduction of bulky dicyclopentadiene moiety in the backbone could also decrease the crosslinking density and increase the

3.4. Thermal conductivities of BN/DCPDCE/BADCy composites For a determined DCPDCE/BADCy hybrid resin (DCPDCE/ BADCy = 0.4/0.6, mol/mol), the mass fraction of BN fillers affecting on the thermally conductivities of the BN/DCPDCE/BADCy composites was shown in Fig. 5. Obvious improvement of the λ and α value for the BN/DCPDCE/BADCy composites was observed with the addition of BN fillers. And obtained λ and α value of the BN/DCPDCE/BADCy composite with 30 wt% BN fillers was increased to 0.64 W/mK and 0.57 mm2/s, about 3 times in comparison to that of pristine DCPDCE/ BADCy hybrid resin (λ of 0.25 W/mK and α of 0.20 mm2/s). The reason was that BN fillers possessed relatively higher λ and α value than that of DCPDCE/BADCy hybrid resin. Meantime, BN fillers with lower content dispersed randomly inner the DCPDCE/BADCy hybrid resin and hardly presented interaction between each other, resulting in a slight improvement for the λ and α value. With the increasing addition of BN

Fig. 3. Effects of DCPDCE contents on the mechanical properties of DCPDCE/BADCy hybrid resins. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. TGA(a) and DMA(b) curves of DCPDCE/BADCy hybrid resins. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 Characteristic thermal data of DCPDCE/BADCy hybrid resins and the BN/DCPDCE/BADCy composites. Samples

BADCy DCPDCE/BADCy = 0.2/0.8(mol/mol) DCPDCE/BADCy = 0.4/0.6(mol/mol) DCPDCE/BADCy = 0.6/0.4(mol/mol) DCPDCE/BADCy = 0.8/0.2(mol/mol) DCPDCE 5 wt% BN/DCPDCE/BADCy(0.4 mol/0.6 mol) 10 wt% BN/DCPDCE/BADCy(0.4 mol/0.6 mol) 20 wt% BN/DCPDCE/BADCy(0.4 mol/0.6 mol) 30 wt% BN/DCPDCE/BADCy(0.4 mol/0.6 mol)

Weight loss temperature/°C

THeat-resistance

T5

T30

T50

416.7 406.9 395.9 400.3 383.9 377.6 397.8 408.2 410.3 410.5

439.4 424.2 421.9 420.7 423.3 425.3 427.0 425.0 428.9 429.1

517.4 456.8 450.5 446.9 446.3 446.1 464.7 480.6 529.7 646.8

210.8 204.5 201.6 202.1 199.7 190.0 203.5 205.0 206.5 206.6

* ° index / C

Tg

267.9 258.1 226.7 213.5 207.4 189.8 – – – –

* The sample’s heat-resistance index is calculated by Eq. (6)

(6)

THeat − resistance index = 0.49∗ [T5 + 0.6∗ (T30−T5)] where T5 and T30 is the corresponding decomposition temperature of 5% and 30% weight loss, respectively.

and α value. Herein, a suitable model for the thermally conductive BN/DCPDCE/ BADCy composites can be proposed by analyzing series and parallel connection [62] among BN fillers and DCPDCE/BADCy hybrid resin. For series connection from the Fourier’s law, the corresponding λ value of the BN/DCPDCE/BADCy composites is shown in the Eq. (1):

λs−1 = φf λ f−1 + (1−φf ) λm−1

(1)

For parallel connection, the corresponding λ value of the BN/ DCPDCE/BADCy composites is also shown in the Eq. (2):

λp = φf λ f + (1−φf ) λm

(2)

In fact, the series connection and parallel connection are simultaneously existed in the BN/DCPDCE/BADCy composites. Therefore, it is necessary to adjust two formulae (Eqs. (1) and (2)) above, finally to obtain a suitable model for the thermally conductive BN/DCPDCE/ BADCy composites. For series connection model, it is assumed that BN fillers and the DCPDCE/BADCy hybrid resin are in series connection, and the parallel connection part is ignored. Therefore, we can adjust the series connection model as Eq. (3):

Fig. 5. Contents of BN fillers affecting on the thermally conductivities of the BN/ DCPDCE/BADCy composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fillers, the touching and connection probabilities of BN-BN inner composites were increased, further to form more thermally conductive channels of BN-BN, resulting in the significant improvement for the λ

λ c,s = λs + αp φf (λ f −λm) 574

(3)

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where λ c,s denotes the adjusted λ value of series connection condition. αp is the parallel connection factor (0 ⩽ αp ⩽ 1), and αp φf is the part of parallel BN fillers ignored in series model. Thus, αp φf (λ f −λm) is the underestimated value. For parallel connection model, it’s assumed that BN fillers and the DCPDCE/BADCy hybrid resin are in parallel connection, and the series connection part is also ignored. Therefore, we can adjust the series connection model as Eq. (4):

λ c,p = λp−α s φf (λ f −λm)

(4)

Herein, λ c,p is the adjusted λ value of parallel connection condition. αs is the parallel connection factor (0 ⩽ αs ⩽ 1), and αs φf is the part of series BN fillers ignored in parallel model. The sum of the parallel connection factor and parallel connection factor of the BN/DCPDCE/BADCy composites is 1 (Eq. (5)):

αs + α p = 1

(5)

Based on the experimental λ value of the BN/DCPDCE/BADCy composite with 2.5 vol% BN, the αp and corresponding λ c value can be calculated according to Eqs. (3)–(5). Experimentally and theoretical λ values of the BN/DCPDCE/BADCy composites from Maxwell model [63], Russell model [64] and our proposed model were also shown in Fig. 6. It could be seen that the theoretical λ value fromm Maxwell model and the Russell model was obviously lower than that of experimental λ value. Compared to that of Maxwell model and Russell model, our proposed thermally conductive model could predict the experimental λ values more precisely.

Fig. 7. Effects of BN contents on the mechanical properties of the BN/DCPDCE/BADCy composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of BN fillers, more interfacial defects and stress concentration points were probably introduced into the DCPDCE/BADCy hybrid resin, mainly ascribed to an incomplete dispersion of BN fillers inner the DCPDCE/BADCy hybrid resin, as well as relatively worse interfacial compatibility between BN fillers and DCPDCE/BADCy hybrid resin. Fig. 8 showed the SEM images of impact fractures for the BN/ DCPDCE/BADCy composites. The surface morphology of the impact fracture for pristine DCPDCE/BADCy hybrid resin was observed to be smooth, representing a typical brittle fracture (Fig. 8a). With lower addition of BN fillers, the surface morphology of the impact fracture of the BN/DCPDCE/BADCy composites presented bifurcation (Cracks in multiple directions, Fig. 8b-c). With the excessive addition of BN fillers, some BN particles were easily to be aggregated and the defects were also produced (Fig. 8e). The above phenomenon was consistent with the changes of the mechanical properties in Fig. 7.

3.5. Mechanical properties of BN/DCPDCE/BADCy composites For a determined DCPDCE/BADCy hybrid resin (DCPDCE/ BADCy = 0.4/0.6, mol/mol), the mass fraction of BN fillers affecting on the mechanical properties of the BN/DCPDCE/BADCy composites was presented in Fig. 7. Both the impact and flexural strength of the BN/ DCPDCE/BADCy composites were increased firstly, but decreased with excessive addition of BN fillers. And the impact and flexural strength of the BN/DCPDCE/BADCy composite with 10 wt% BN fillers was enhanced to 11.7 kJ/m2 and 108.4 MPa, increased by 7.6% and 9.3% in comparison to that of pristine DCPDCE/BADCy hybrid resin (Impact strength of 10.7 kJ/m2 and flexural strength of 100.7 MPa), respectively. The reason was that relatively lower addition of BN fillers could be in favor of transferring stress. However, with the excessive addition

3.6. Thermal properties of BN/DCPDCE/BADCy composites For a determined DCPDCE/BADCy hybrid resin (DCPDCE/ BADCy = 0.4/0.6, mol/mol), TGA curves of the BN/DCPDCE/BADCy composites were shown in Fig. 9, and the corresponding characteristic thermal data were also presented in Table 1. The corresponding T5, T30, T50 and THRI value of the pristine DCPDCE/BADCy hybrid resin and BN/ DCPDCE/BADCy composites was higher than 395.9 °C, 421.9 °C, 450.5 °C and 201.6 °C, respectively. It revealed that the pristine DCPDCE/BADCy hybrid resin and BN/DCPDCE/BADCy composites all presented relatively excellent thermal stabilities. Moreover, the THRI value of the BN/DCPDCE/BADCy composites was gradually enhanced with the increasing addition of BN fillers. And the corresponding THRI value of the BN/DCPDCE/BADCy composite with 30 wt% BN was enhanced to 206.6 °C, higher than that of pristine DCPDCE/BADCy hybrid resin (201.6 °C). The reason was that BN fillers possessed relatively higher λ value than that of pristine DCPDCE/BADCy hybrid resin, in favor of absorbing outside heat. And BN fillers could effectively hinder the effusion of pyrolysis volatiles. In addition, good combination between BN fillers and DCPDCE/BADCy hybrid resin was also beneficial for increasing the interfacial bonding strength. Finally, the final thermal stabilities of the BN/DCPDCE/BADCy composites were enhanced accordingly. 4. Conclusions

Fig. 6. Experimentally and theoretical λ values of the BN/DCPDCE/BADCy composites from Maxwell model, Russell model and our proposed model. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

When the molar ratio of DCPDCE to BADCy was 0.4:0.6, the DCPDCE/BADCy hybrid resin presented the optimal comprehensive properties. The corresponding ε and tanδ value at 10 MHz was 575

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0 wt% BN

5 wt% BN

20 wt% BN

10 wt% BN

30 wt% BN

Fig. 8. SEM morphologies of impact fractures for the BN/DCPDCE/BADCy composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

composite was improved to 0.64 W/mK and 0.57 mm2/s, about 3 times in comparison to that of pristine DCPDCE/BADCy hybrid resin. Compared to that of Maxwell model and Russell model, our proposed thermally conductive model could predict the experimental λ values more precisely. And the corresponding THRI value was also enhanced from 201.6 °C (Pristine DCPDCE/BADCy hybrid resin) to 206.6 °C. In addition, the BN/DCPDCE/BADCy composite with 10 wt% BN presented the maximum impact strength (11.7 kJ/m2) and flexural strength (108.4 MPa), increased by 9.3% and 7.6%, compared to that of pristine DCPDCE/BADCy hybrid resin, respectively. Acknowledgements The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (Nos. 51773169 and 51403175); Space Supporting Fund from China Aerospace Science and Industry Corporation (No. 2017-HT-XG); Aeronautics Science Fund (Nos. 2016ZF03010 and 2015ZF53074); Open Fund from State Key Laboratory of Solid Lubrication of Lanzhou Institute of Chemical Physics (LSL-1715); Fundamental Research Funds for the Central Universities (No. 3102017jg02003); Y. Li thanks for the Seed Foundation of Innovation and Creation for Graduate Students in NPU; X. Zhong thanks for the Undergraduate Innovation & Business Program in NPU (Nos. 201710699268 and 201710699382).

Fig. 9. TGA curves of the BN/DCPDCE/BADCy composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

decreased to 2.92 and 5.08 × 10−3, decreased by 5.5% and 24.5% in comparison to that of pristine BADCy matrix (ε of 3.09 and tanδ of 6.73 × 10−3), respectively. The impact and flexural strength of the DCPDCE/BADCy hybrid resin was 10.7 kJ/m2 and 100.7 MPa, slightly lower than that of pristine BADCy matrix (Impact strength of 11.6 kJ/ m2 and flexural strength of 112.3 MPa), respectively. The corresponding THRI and Tg value was 201.6 °C and 226.7 °C, respectively, representing relatively excellent thermal stability. With the addition of 30 wt% BN fillers, the λ and a value of the BN/DCPDCE/BADCy

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