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DOI: 10.1039/C6RA10397A (Paper) RSC Adv., 2016, 6, 57357-57362

Fabrication of modified bismaleimide resins by hyperbranched phenyl polysiloxane and improvement of their thermal conductivities

Junwei Gu*, Chaobo Liang† , Jing Dang, Xudong Meng, Lin Tang, Yang Li and Qiuyu Zhang*
Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P. R. China. E-mail: nwpugjw@163.com; qyzhang1803@gmail.com; Tel: +86-29-88431621

Received 22nd April 2016 , Accepted 5th June 2016

First published on 9th June 2016

Abstract

Synthetic terminal amine group hyperbranched phenyl polysiloxane (NH2-HBPSi) is introduced into a bismaleimide/diallylbisphenol A (BMI/DABA) prepolymer to fabricate NH2-HBPSi/BMI/DABA resins. Furthermore, functionalized silicon carbide particle/silicon carbide whisker (fSiCp/fSiCw) hybrid fillers are also used to fabricate fSiCp/fSiCw/NH2-HBPSi/BMI/DABA thermal conductivity composites. A NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is an ideal dielectric material with excellent impact strength and outstanding thermal stability, the corresponding dielectric constant (ε) is 3.12, dielectric loss (tan[thin space (1/6-em)]δ) is 0.0098, impact strength value is 18.7 kJ m−2, glass transition temperature (Tg) value is 281 °C and the 5 wt% thermal weight loss temperature (T5) value is 424 °C. The thermal conductivities of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are both increased with the increasing mass fraction of fSiCp/fSiCw hybrid fillers.

Introduction

Bismaleimide (BMI) resins possess outstanding thermal stabilities, excellent dielectric properties, good flame resistance, wonderful mechanical properties, an attractive cost/performance ratio, etc., and have been widely applied in the fields of electronics, insulation adhesives, etc.1–6 However, the two main disadvantages of the BMI matrix are its inherent brittleness7 and intrinsic low thermally conductive coefficient8 and have limited its broader applications, especially in the field of copper-clad laminate materials in printed circuit boards.9

At present, many methods have been proposed to improve the toughness of the BMI matrix, such as a diamine extending chain,10 O,O-diallyl bisphenol A,11,12 rubbers,13 engineering plastics,14,15 thermosets,16,17 nanofillers,18–20 whiskers,21 etc., but the corresponding thermal resistance and dielectric properties of the BMI modifiers are often inevitably decreased.22

Polysiloxane possesses good toughness, excellent dielectric properties, outstanding thermal stability and flame retardancy. However, the poor interfacial compatibility between polysiloxane and the BMI matrix has limited its toughening effect.23 Herein, hyperbranched polymers (HBPs) present high solubility, low viscosity and good chemical reactivity, and are expected to be excellent toughening agents.24,25 However, related research on HBPs/BMI compounds is seldom reported.

Previous research has revealed that incorporating single thermally conductive fillers, such as hexagonal boron nitride (hBN),5,26,27 silicon carbide whiskers (SiCws),8 and carbon nanotubes (CNTs),2 can effectively improve the thermal conductivities of the BMI matrix. In fact, the addition of thermally conductive hybrid fillers in the BMI matrix makes it easier to form more thermally conductive channels, which effectively increases the thermal conductivities of the BMI composites.

In our present work, the terminal amine group hyperbranched phenyl polysiloxane (NH2-HBPSi) is firstly synthesized by the reaction of phenylsiloxane and γ-aminopropyltriethoxysilane (KH-550). Herein, phenylsiloxane is synthesized via the hydrolysis and subsequent polycondensation of phenyltrimethoxysilane (PhTMs). And synthetic NH2-HBPSi is used to modify the BMI/DABA prepolymer via a copolymerization reaction. How the mass fraction of NH2-HBPSi affects the mechanical, dielectric properties and the thermal stabilities of the NH2-HBPSi/BMI/DABA resins is investigated in detail. Meanwhile, the surface of the SiCp/SiCw hybrid fillers is functionalized by γ-glycidoxy propyl trimethoxy silane (KH-560), and the how the mass fraction of the fSiCp/fSiCw hybrid fillers affects the thermal conductivities of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites is also discussed.

Experimental

Materials

N,N′-4,4′-Bismaleimide diphenylmethane (BMI) is purchased from Hubei FengHua Chemical Factory (Hubei, China); diallylbisphenol A (DABA) is supplied by Qinyang Tianyi Chemical Industrial Co. Ltd. (Henan, China); SiC particles (SiCps), with a grain diameter of 1.5 μm, density of 3.2 g cm−3, and specific surface area of 2.5 m2 g−1, and SiC whiskers (SiCws), with a density of 3.2 g cm−3 and an aspect ratio of more than 20, are both purchased from Xuzhou Hongwu Nanomaterial Co. Ltd. (Jiangsu, China); phenyltrimethoxysilane (PhTMs) and γ-aminopropyltriethoxysilane (KH-550) are both received from Hangzhou Dadi Chemical Co., Ltd (Zhejiang, China); γ-glycidoxypropyltrimethoxysilane (KH-560) is supplied by Nanjing Shuguang Chemical Group Co., Ltd (Jiangsu, China); dibutyltin dilaurate (DBTDL) is purchased from Tianjin Gangji Industrial and Trading Co., Ltd. (Tianjin, China); absolute ethanol, acetone and diluted hydrochloric acid are all supplied by Tianjin Ganglong Chemical Group Co., Ltd (Tianjin, China).

Preparation of NH2-HBPSi/BMI/DABA resins

Distilled water, absolute ethanol and diluted hydrochloric acid are stirred together, followed by the addition of PhTMs. The mixture is reacted at 70 °C for 12 h, cooled to room temperature and stood for 24 h, followed by removal of the top layer. And then the bottom layer is washed with excess distilled water, followed by being dissolved in acetone. Then excess distilled water is added to precipitate the final target product phenylsiloxane (PhSi), followed by drying in a vacuum oven at 80 °C for another 24 h.

PhSi and KH-550 are stirred together, followed by the addition of DBTDL. The mixture is then reacted at 100 °C for 1 h, cooled to room temperature and stood for 24 h, followed by drying in a vacuum oven at 80 °C for another 24 h to obtain NH2-HBPSi. Fig. 1 shows a schematic diagram of the synthesis of PhSi and hyperbranched polysiloxane containing terminal amine groups (NH2-HBPSi).


image file: c6ra10397a-f1.tif
Fig. 1 Schematic diagram of the synthesis of PhSi and hyperbranched polysiloxane containing terminal amine groups (NH2-HBPSi).

BMI and DABA are mixed well at 50 °C, heated up to 140 °C and reacted for 40 minutes, to obtain the BMI/DABA prepolymer. Then NH2-HBPSi is added to the BMI/DABA prepolymer with mechanical stirring. And the NH2-HBPSi/BMI/DABA prepolymer is degassed in a vacuum oven at 140 °C, then poured into a preheated mold. Finally the corresponding NH2-HBPSi/BMI/DABA prepolymer is cured according to the following: 150 °C/2 h + 170 °C/1 h + 190 °C/2 h + 210 °C/2 h, followed by post-curing at 240 °C for 4 h. Fig. 2 presents a schematic diagram of copolymerization between NH2-HBPSi and the BMI/DABA prepolymer.


image file: c6ra10397a-f2.tif
Fig. 2 Schematic diagram of copolymerization between NH2-HBPSi and the BMI/DABA prepolymer.

Preparation of the fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites

Surface functionalization of the fSiCp/fSiCw hybrid fillers is performed according to our previous work.28 BMI/DABA and NH2-HBPSi are heated up to 140 °C and stirred for 40 minutes, to obtain the NH2-HBPSi/BMI/DABA prepolymer. Then the fSiCp/fSiCw hybrid fillers are added to the prepolymer and stirred. The mixture of fSiCp/fSiCw/NH2-HBPSi/BMI/DABA is degassed in a vacuum oven at 140 °C, then poured into a preheated mold. Finally the fSiCp/fSiCw/NH2-HBPSi/BMI/DABA thermal conductivity composites are cured according to the following curing process: 150 °C/2 h + 170 °C/1 h + 190 °C/2 h + 210 °C/2 h, followed by post-curing at 240 °C for 4 h.

Analysis and characterization

Fourier transform infrared (FTIR) spectra of the samples are obtained on Bruker Tensor 27 equipment (Bruker Corp., Germany); thermal gravimetric (TG) analyses of the samples are carried out at 10 °C min−1 (argon atmosphere), over the whole range of temperature (25–800 °C) by an STA 449F3 (NETZSCH C Corp., Germany); dynamic mechanical analysis (DMA) of the samples is performed using a DMA/SDTA861e (Mettler-Toledo Co., Switzerland), at a heating rate of 5 °C min−1 at 1 Hz; the flexural strength of the samples is measured by an Electron Omnipotence Experiment Machine SANS-CMT5105 (Shenzhen New Sansi Corp., China) according to standard ISO 178-1993; the impact strength of the samples is measured with a ZBC-4B impact testing machine (Shenzhen New Sansi Corp., China) according to standard ISO 179-1993; the dielectric constant (ε) and dielectric loss (tan[thin space (1/6-em)]δ) values of the samples are measured using a high frequency Q instrument, QBG-3D, and a dielectric constant detector, S914 (Shanghai Aiyi Electronic Equipment Co. Ltd., China); thermally conductive coefficients of the samples are measured using a Hot Disk instrument (AB Corporation, Sweden) by standard ISO 22007-2:2008. The measurements are performed on two parallel samples (20 mm × 20 mm × 4 mm) by putting the sensor between two slab shaped samples (z-direction).

Results and discussion

FTIR analyses of PhTMs, PhSi and NH2-HBPSi

Fig. 3 presents the FTIR spectra of PhTMs, PhSi and NH2-HBPSi. The bands at 810 cm−1 and 2840 cm−1 can be assigned to the stretching absorption peaks of Si–O–CH3 and –CH3 in PhTMs, respectively. However, the peaks of PhSi at 810 cm−1 and 2840 cm−1 both disappear, owing to the hydrolysis and subsequent polycondensation of PhTMs. And for NH2-HBPSi, the absorption peak near 1650 cm−1 can be assigned to the Si–C stretching vibration. The absorption peak at 1000–1200 cm−1 can be attributed to the Si–O–Si bending vibration, revealing that the alkoxy group has been hydrolyzed into an oxygen group. The absorption peaks at 1590 cm−1 and 1650 cm−1 can be assigned to the benzene skeleton vibration. The absorption peak at 2930 cm−1 can be attributed to the alkyl group stretching vibration. The stretching absorption peak at 3350–3450 cm−1 is generated by the combination of –NH2 and Si–OH groups.
image file: c6ra10397a-f3.tif
Fig. 3 FTIR spectra of PhTMs, PhSi and NH2-HBPSi.

Mechanical properties of the NH2-HBPSi/BMI/DABA resins

Fig. 4 shows how the mass fraction of NH2-HBPSi affects the impact strength and flexural strength of the NH2-HBPSi/BMI/DABA resins.
image file: c6ra10397a-f4.tif
Fig. 4 How the mass fraction of NH2-HBPSi affects the impact strength and flexural strength values of the NH2-HBPSi/BMI/DABA resins.

With the increasing mass fraction of NH2-HBPSi, the impact strength values of the NH2-HBPSi/BMI/DABA resins show a general upward trend. And the maximum impact strength value of the NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is improved to 18.7 kJ m−2, increased by 150.8% compared to that of pure BMI/DABA. The reason is that the Si–O–Si group in NH2-HBPSi can increase the toughness of BMI/DABA. And the introduction of NH2-HBPSi can also enlarge the chain length between cross-linking points of BMI/DABA, to effectively prevent the extending of cracks.

With the increasing mass fraction of NH2-HBPSi, the flexural strength values of the NH2-HBPSi/BMI/DABA resins are increased firstly, but decreased with excessive mass fractions of NH2-HBPSi. And the maximum flexural strength value of the NH2-HBPSi/BMI/DABA resin with 10 wt% NH2-HBPSi is improved to 159.9 MPa, increased by 129.2% compared to that of pure BMI/DABA. The reason is that an appropriate mass fraction of NH2-HBPSi can promote the curing activity of the BMI/DABA system, increase the cross-linking density and effectively decrease the internal defects of the BMI/DABA system, finally improving the flexural strength values of the NH2-HBPSi/BMI/DABA resins.

Dielectric properties of NH2-HBPSi/BMI/DABA resins

How the mass fraction of NH2-HBPSi affects the dielectric constant and dielectric loss values of the NH2-HBPSi/BMI/DABA resins is shown in Fig. 5.
image file: c6ra10397a-f5.tif
Fig. 5 How the mass fraction of NH2-HBPSi affects the dielectric constant and dielectric loss values of the NH2-HBPSi/BMI/DABA resins.

With the increasing mass fraction of NH2-HBPSi, the dielectric constant (ε) values of the NH2-HBPSi/BMI/DABA resins are increased firstly, but decreased with an excessive mass fraction of NH2-HBPSi. However, the dielectric loss (tan[thin space (1/6-em)]δ) values are gradually decreased. The ε and tan[thin space (1/6-em)]δ value of the NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is 3.12 and 0.0098, respectively.

NH2-HBPSi possesses a similar ε value to that of BMI/DABA. The introduction of NH2-HBPSi in the BMI/DABA system can induce the improvement of interfacial polarization between NH2-HBPSi and BMI/DABA. However, the reaction between NH2-HBPSi and the BMI matrix can effectively decrease the interfacial charge accumulation, in favor of reducing the interfacial polarization effect. Therefore, the ε value of the NH2-HBPSi/BMI/DABA resins shows little change. Meanwhile, the tan[thin space (1/6-em)]δ value of NH2-HBPSi is far below that of BMI/DABA. The Si–O–Si group in NH2-HBPSi can enhance the orientation and relaxation of the BMI/DABA system, and the introduction of NH2-HBPSi can also increase the cross-linking density and effectively decrease the internal defects of the BMI/DABA system, finally decreasing the tan[thin space (1/6-em)]δ value of the NH2-HBPSi/BMI/DABA resins.

Thermal properties of BMI/DABA and the NH2-HBPSi/BMI/DABA resins

Fig. 6 shows the DMA curves of BMI/DABA and the NH2-HBPSi/BMI/DABA resins.
image file: c6ra10397a-f6.tif
Fig. 6 DMA curves of BMI/DABA and the NH2-HBPSi/BMI/DABA resins.

The glass transition temperature (Tg) value of pure BMI/DABA is 256 °C. And the corresponding Tg value of the NH2-HBPSi/BMI/DABA resin is improved to 265 °C (5 wt% NH2-HBPSi) and 281 °C (20 wt% NH2-HBPSi). This can be attributed to the addition of NH2-HBPSi that may occupy the space of the BMI molecular chain, to form relatively larger cross-linking networks of the BMI/DABA system, finally hindering the rotation and motion of the BMI molecular chain. Meanwhile, the reaction of NH2-HBPSi and the BMI matrix can further increase the Tg value.

TGA curves of BMI/DABA and the NH2-HBPSi/BMI/DABA resins are presented in Fig. 7. And the corresponding characteristic thermal data are listed in Table 1.

 
Theat-resistance index = 0.49 × [T5 + 0.6 × (T30 − T5)] (1)
T5 and T30 is the corresponding decomposition temperature of 5% and 30% weight loss.


image file: c6ra10397a-f7.tif
Fig. 7 TGA curves of BMI/DABA and the NH2-HBPSi/BMI/DABA resins.
Table 1 Characteristic thermal data of BMI/DABA and the NH2-HBPSi/BMI/DABA resinsa
Samples Temperatures of weight loss/°C Theat-resistance index/°C Residues at 800 °C/%
5 wt% 10 wt% 30 wt%
a The sample’s heat-resistance index was calculated by eqn (1).29,30
BMI/DABA 407 423 455 213.5 36.3
NH2HBPSi/BMI/DABA (5 wt% NH2-HBPSi) 418 432 465 218.7 41.9
NH2HBPSi/BMI/DABA (20 wt% NH2-HBPSi) 424 440 496 228.9 53.9

With the increasing mass fraction of NH2-HBPSi, at the same degree of weight loss, the thermal loss temperatures of BMI/DABA and NH2-HBPSi/BMI/DABA resins are both increased. When the weight loss is 30 wt%, the weight loss temperature is 455 °C (pure BMI/DABA), 465 °C (5 wt% NH2-HBPSi), and 496 °C (20 wt% NH2-HBPSi). And the corresponding heat-resistance index is 213.5 °C, 218.7 °C and 228.9 °C, respectively. This reveals that the thermal stabilities of the NH2-HBPSi/BMI/DABA resins are improved with the increasing mass fraction of NH2-HBPSi. The reason is that the reaction of NH2-HBPSi and the BMI matrix can increase the cross-linking density of the BMI/DABA system, finally improving the thermal stability. Moreover, the hyperbranched structure of NH2-HBPSi can also restrain the extending of thermal cracking, enhancing the thermal stabilities of the NH2-HBPSi/BMI/DABA resins. In addition, the residuals of BMI/DABA and the NH2-HBPSi/BMI/DABA resins are both beyond 36%, revealing that the BMI/DABA and NH2-HBPSi/BMI/DABA resins possess excellent thermal stabilities.

Thermally conductivities of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites

Fig. 8 presents the thermal conductivities of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites.
image file: c6ra10397a-f8.tif
Fig. 8 Thermal conductive coefficient values of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites.

The thermally conductive coefficient values of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are gradually increased with the increasing mass fraction of the fSiCp/fSiCw hybrid fillers. The reason is that more thermally conductive channels of fSiCp–fSiCp, fSiCp–fSiCw and/or fSiCw–fSiCw are probably formed with the increasing mass fraction of the fSiCp/fSiCw hybrid fillers, finally increasing the thermal conductivity coefficient values of the composites.

For a given fSiCp/fSiCw hybrid filler loading, the thermal conductivities of the fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are gradually decreased with the increasing mass fraction of NH2-HBPSi. And the thermal conductivities of the fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are all lower than those of the fSiCp/fSiCw/BMI/DABA composites. The reason is that the intrinsic thermally conductive coefficient value of the NH2-HBPSi is far below that of the fSiCp/fSiCw hybrid fillers. Furthermore, NH2-HBPSi may prevent the interconnection between the fSiCp/fSiCw hybrid fillers, against the formation of thermally conductive channels of fSiCp–fSiCp, fSiCp–fSiCw and/or fSiCw–fSiCw.

Conclusions

The maximum impact strength value of the NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is improved to 18.7 kJ m−2, increased by 150.8% compared to that of pure BMI/DABA. The maximum flexural strength value of the NH2-HBPSi/BMI/DABA resin with 10 wt% NH2-HBPSi is improved to 159.9 MPa, increased by 129.2% compared to that of pure BMI/DABA. The ε values of the NH2-HBPSi/BMI/DABA resins are increased firstly, but decreased with an excessive mass fraction of NH2-HBPSi. However, the tan[thin space (1/6-em)]δ values are gradually decreased. The corresponding ε and tan[thin space (1/6-em)]δ value of the NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is 3.12 and 0.0098. The Tg values and thermal stabilities of the NH2-HBPSi/BMI/DABA resins are gradually increased with the increasing mass fraction of NH2-HBPSi. The thermal conductivities of the fSiCp/fSiCw/BMI/DABA and fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are both increased with the increasing mass fraction of the fSiCp/fSiCw hybrid fillers. For a given fSiCp/fSiCw hybrid filler loading, the thermal conductivities of the fSiCp/fSiCw/NH2-HBPSi/BMI/DABA composites are all lower than those of the fSiCp/fSiCw/BMI/DABA composites.

The NH2-HBPSi/BMI/DABA resin with 20 wt% NH2-HBPSi is an ideal dielectric material with excellent impact strength and outstanding thermal stability, ε is 3.12, tan[thin space (1/6-em)]δ is 0.0098, impact strength is 18.7 kJ m−2, Tg is 281 °C and T5 reaches up to 424 °C.

Acknowledgements

The authors are grateful for the support and funding from the Foundation of National Natural Science Foundation of China (No. 51403175), the Shaanxi Natural Science Foundation of Shaanxi Province (No. 2015JM5153), the Fundamental Research Funds for the Central Universities (No. 3102015ZY066 and 3102015BJ(II)JGZ020), and the Undergraduate Innovation Program (No. 201610699275 and 201610699371).

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Footnote

† Chaobo Liang contributed equally to this work and should be considered co-first author.
This journal is © The Royal Society of Chemistry 2016
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