Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion

Journal Pre-proofs Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion Xianghui Ou, Xuemin Lu, Shuangshuang Chen, Qinghua Lu PII: DOI: Reference:

S0014-3057(19)32175-5 https://doi.org/10.1016/j.eurpolymj.2019.109368 EPJ 109368

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European Polymer Journal

Received Date: Revised Date: Accepted Date:

23 October 2019 8 November 2019 11 November 2019

Please cite this article as: Ou, X., Lu, X., Chen, S., Lu, Q., Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion, European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj.2019.109368

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Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion Xianghui Oua, Xuemin Lua, Shuangshuang Chenb, Qinghua Lua,* aSchool

of Chemistry and Chemical Engineering, Shanghai Key Laboratory of

Electrical Insulation and Thermal Aging, Shanghai JiaoTong University, 800 Dongchuan Road, Shanghai, 200240, China bSchool

of Chemical Science and Engineering, Tongji University, Siping Road 1239,

Shanghai, 200092, China *Corresponding

Author. E-mail: [email protected]

Abstract

It is a great challenge to fabricate flexible substrate materials with an ultrahigh heat resistance, low coefficient of thermal expansion (CTE) and high thermal conductivity, which is required to process low temperature poly-silicon thin film transistors in active matrix organic light-emitting-diode devices. Despite the development of new monomers is also possible to simultaneously improve the heat resistance and dimensional stability of polyimides, the space for improvement is limited and the cost is greatly increased. Traditional polymer nanocomposites can increase the glass transition temperature (Tg) and reduce the CTE, but they cause a significant drop in mechanical properties. Herein, an amino-functionalized boron nitride nanosheets (BNNS_APS) hybrid polyimide (PI) was prepared by in situ polymerization. The

hybrid PI exhibited ultrahigh heat resistance, excellent thermal conductivity, low CTE and good mechanical properties with 1 wt% BNNS_APS doping. The Tg of the hybrid PI was improved up to 473 °C and the thermal conductivity was increased by 100% compared with pure PI. The CTE was less than 7 ppm/K, and the tensile strength and elongation at break increased to 336 MPa and 16.6%, respectively. These results are attributed to the good dispersion of BNNS_APS in the PI matrix and the strong interactions between the BNNS_APS and PI chains. The mechanism was proposed and discussed in detail. Key Words: polyimide; hybrid; boron nitride nanosheet; heat resistance; thermal conductivity.

1. Introduction

With its great advantages of threshold voltage stability and high carrier mobility, flexible active-matrix organic light-emitting-diode (AMOLED) devices that are based on low temperature poly-silicon thin film transistors (LTPS TFTs) have attracted intensive attention as an optimal candidate in next-generation displays[1-4]. High-stable flexible substrate is one of the most critical technologies for developing flexible AMOLED devices[2]. According to conventional LTPS technologies, flexible substrates must withstand a high temperature above 450 °C [1, 2]. The high thermal and dimensional stability of the flexible substrate is also required to avoid poor registration accuracy and layer-to-layer stripping. Hence, in general, the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg) of flexible

substrate materials should approximate the glass’s CTE and exceed 450 °C, respectively. Polymer-based materials are the most popular candidates for flexible substrate because of their advantages of high flexibility, low cost and easy processability[5]. Among them, aromatic polyimides (PIs) are the most promising flexible substrate materials due to their high heat resistance, excellent comprehensive performance and mature application in electronics[5-8]. However, most traditional PIs, including the commercial PIs cannot satisfy the harsh thermal property requirements. Therefore, the fabrication of PIs with higher Tg, lower CTE and higher thermal stability is required urgently in science and engineering. It is often difficult to meet the simultaneous requirements for the heat resistance, thermal and dimensional stability and mechanical properties by designing new monomers based intrinsic high heat-resistant PIs. For example, biphenyl polyimide is a kind of PI film with the best thermal stability among the commercial polyimides, but the Tg cannot meet the requirements of flexible substrate materials. Inorganic nanofiller addition to form inorganic-organic hybrid nanocomposites is a popular, simple, low cost and effective approach to improve the comprehensive properties of polymers[9-12]. Three main types of nanofillers were investigated to enhance the thermal and/or mechanical properties of polyimides: (1) zero-dimensional (0D) nanoparticles, such as SiO2[13-16], TiO2[17-19], Al2O3[20], POSS[21] and carbon black[22]; (2) one-dimensional (1D) nanotubes or nanorods, such as carbon nanotubes[23, 24] and sepiolite[25]; and (3) two-dimensional (2D) nanosheets, such as graphene[26-28], graphene oxide[29-31], montmorillonite[32] and MoS2[33].

Zero-dimensional nanoparticles could increase the Tg and thermal decomposition temperature (Td) and decrease the CTE of the polyimides to some extent. However, a high nanoparticle loading, even though above 30% is generally required to achieve these goals, which is detrimental to the mechanical properties, especially the elongation at break[15, 21, 34-36]. The incorporation of 1D nanotubes or nanorods with a relatively lower loading than nanoparticles into the polyimides could enhance the young modulus and tensile strength. In some cases, the CTE of the composites also decreased[25, 37]. Unfortunately, these methods also damaged the elongation at break and showed no apparent effect on the Tg. Several 2D nanosheets that endowed the composites with a higher Tg, better mechanical properties and/or a lower CTE have been reported [30, 31]. However, the enhancement degree of the properties (particularly Tg) via traditional 2D nanofiller introduction is also modest. To date, to the best of our knowledge, hybrid polymers with a small amount of nanofillers that can improve the overall performance and meet the needs of flexible substrates have not yet been reported. With miniaturization in high-power microelectronic devices and new approaches in nanoelectronics, such as 3D chip stack architectures and flexible electronics, there are needs and opportunities for novel materials to help address some of these pressing heat-dissipation challenges[38-46]. Hence, the thermal conductivity (TC) of flexible substrates that are used in AMOLED has become increasingly important because of the significant role of substrates in thermal management[38]. However, most polymers, including PIs, have a low thermal conductivity (< 0.3 W/mK). Thus, high

TC inorganic fillers, such as metal oxides (e.g., MgO and Al2O3) and ceramics (e.g., boron nitride (BN) and their nanosheets (BNNS)), were used to enhance the TC of polymers[47-50]. Among them, BNNS that possess an ultrahigh theoretical TC are one kind of 2D nanofiller with potential to enhance the heat-resistant and mechanical properties. BNNS may be the ideal filler to improve all PI properties that are required for flexible-display substrates. In this work, amino-functionalized boron nitride nanosheets (BNNS_APS) were synthesized and used to fabricate hybrid polyimide (PI-BNNS_APS) by in situ polymerization of biphenyl tetracarboxylic dianhydride (BPDA) and p-phenylene diamine (PDA) in the presence of BNNS_APS. For comparison, a hybrid material (PI-BNNS) of PI and BNNS without any modification was also synthesized. Compared with pure PI and PI-BNNS counterparts, the PI-BNNS_APS hybrid film exhibited better heat-resistance, dimensional stability and mechanical and thermal conductivity properties. The relationships between properties and structures are also discussed. The hybrid method offers a new strategy for the fabrication of PI films with excellent comprehensive properties.

2. Experimental section

2.1 Materials

Boron nitride (BN) powder (average lateral size: 0.5–2 μm, 99.9% purity) was purchased from Shanghai Aladdin Chemistry Co. Sodium hydroxide (NaOH), urea, toluene, 3-aminopropyltriethoxysilane (APS) and N, N'-dimethylacetamide (DMAc,

anhydrous) were obtained from J&K Reagent Co., Ltd (Beijing, China). Biphenyl tetracarboxylic dianhydride (BPDA, 99.5% purity) and p-phenylene diamine (PDA, 99.9% purity) were supplied by Chinatech Chemical Co., Ltd (Tianjin, China). All reagents were used as received without further treatment.

2.2 Exfoliation of BN

The method for BN exfoliation has been reported elsewhere[51] with modified postprocess. In a typical synthesis: 2 g BN and 40 g urea (mass ratio of 1:20) were mixed together inside a milling container using a planetary ball mill at 600 rpm for 10 h at ambient temperature. After ball milling, the obtained powder was dispersed in deionized water at an initial concentration of 4 mg/mL and sonicated for 30 min. The BN dispersion was centrifuged at 3000 rpm for 10 min to remove the aggregated material and thick flakes. The supernatant was filtered and washed by 300 mL deionized water 3 times to remove the impurity. The final obtained filtered cake (boron nitride nanosheets, BNNS) was dried at 60°C for 24 h.

2.3 Preparation of BNNS_APS

To achieve more hydroxyl groups at the edge of BNNS, the alkaline solution was used to treat BNNS powder[52]. BNNS (1 g) was dispersed in 200 mL of 5 M aqueous NaOH solution in a 250 mL round flask and then transferred into an oil bath to stir at 120 °C for 24 h under reflux. After cooling to room temperature, the product was filtered and rinsed repeatedly with deionized water until the pH was close to

neutral. Finally, the sample, which was denoted by BNNS_OH, was dried at 60 °C for 24 h. APS grafting on the edge of BNNS was as follow: 1 g BNNS_OH and appropriate APS (3 wt%–5 wt% of BNNS_OH) were dispersed into 200 mL toluene. The mixture was sonicated for 60 min and refluxed at 110 °C under nitrogen for 8 h with magnetic stirring. After cooling to room temperature, the dispersion was filtered and washed quickly with 200 mL anhydrous ethanol three times to remove the residual APS. The obtained sample, which was denoted by BNNS_APS, was dried at 60 °C for 24 h.

2.4 Preparation of PI-BNNS and PI-BNNS_APS hybrid films

Different mass fraction of BNNS or BNNS_APS (0.1 wt%, 0.5 wt% and 1 wt%) were dispersed in DMAc and sonicated for 1 h. The PDA was placed into the dispersion. After the PDA had dissolved completely, equal molar quantities of BPDA were added gradually into the mixture solution under mechanical stirring and kept stirring for 24 h to make a viscous PAA solution (solid content 10 wt%). The PAA solution was cast onto a clean glass plate and dried at 80 °C for 2 h, 100 °C for 2 h and 120 °C for 2 h to evaporate the free solvent. Thermal imidization of the PAA films was performed at 120 °C for 1 h, 200 °C for 1 h, 250 °C for 2 h, 300 °C for 2 h, 350 °C for 2 h and 400 °C for 1 h. The final hybrid film thickness was maintained at ~25 μm.

2.5 Characterization

Fourier-transform infrared (FTIR) and attenuated total reflection-Fourier-transform infrared (ATR-FTIR) spectra were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrophotometer (Perkin-Elmer Corp. Connecticut, USA). X-ray diffraction (XRD, Germany) studies were acquired using a PANalytical diffractometer with Cu Ka radiation as the X-ray source. The ultraviolet-visible (UV-Vis) spectra were obtained from a UV-1800 (Shimadzu Corp., Japan) spectrophotometer. The BN morphology and cross-sectional images of the hybrid films were characterized by field emission scanning electron microscopy (FE-SEM) using a NanoSEM450 (USA). The BNNS thickness was characterized by Multimode Nanoscope IIIa atomic force microscopy (AFM). Thermo-gravimetric analysis (TGA) was recorded on a Pyris 1 TGA (Perkin-Elmer Corp., Connecticut, USA) from room temperature to 800 °C under nitrogen at 10 °C/min. Dynamic thermo-mechanical analysis (DMA) was measured by Q800 (TA Instruments, USA) at 5 °C/min under nitrogen. Thermal mechanical analysis (TMA) was used to study the coefficient of thermal expansion (CTE) of the hybrid films from 40 to 400 °C at a 5 °C/min heating rate by TMA Q400 instrument (TA Instruments, USA). Tensile tests were carried out on Instron 4456 test machine (USA) with dumbbell-shaped samples. The thermal diffusivity (D, mm2/s) and specific heat [Cp, J/(g·K)] were determined by the LFA467 light flash system (Netzsch, Germany) at 25 °C, and the density (ρ, g/cm) was assessed by a FK-120S multifunctional densitometer (Furbs Corp., China) using anhydrous alcohol as a medium. The thermal conductivity λ [W/(mK)] was calculated as λ = ρ × Cp × D.

3.

Results and discussion

3.1 BNNS and BNNS_APS fabrication

Fig. 1 Fabrication process and characterization of BNNS and BNNS_APS. (a) Schematic diagram of BNNS and BNNS_APS fabrication. (b) AFM image and corresponding line-scan profile of BNNS. (c) XRD patterns of pristine BN and BNNS. (d) TGA curves of BN, BNNS and BNNS_APS. (e) FTIR spectra of BNNS and BNNS_APS.

BNNS_APS was obtained from the BN according to a modified urea-assisted exfoliation and further functionalization as shown in Fig. 1(a). As shown in the SEM

images of Fig. S1, the lateral size and thickness of the initial BN powder were ~0.5–2 μm and dozens of nanometers, respectively. After 10 h of ball milling and washing, the obtained BNNS with a yield of approximately 50% was obtained and characterized by AFM, XRD and TGA. The typical AFM image and corresponding line-scan profile of the BNNS in Fig. 1(b) showed that the BNNS thickness was ~4 nm. As shown in Fig. 1(c), two main XRD peaks could be observed at 26.8° and 41.7°, which correspond to (002) and (100) planes of BN, respectively. Compared with the pristine BN, the intensities of the (002) and (100) peaks of BNNS decreased and the half-width broadened, which indicated the presence of thin nanosheets and lesser extended/ordered stacking in the vertical direction of the boron nitride plane. The TGA curves of BN and BNNS are shown in Fig. 1(d). The weight loss of BNNS indicated that functional groups were created and bonded to the edges of BNNS[51]. The FTIR spectrum of BNNS in Fig. 1(e) showed a broad peak at 3426 cm-1, which could be attributed to N–H and O–H stretching vibrations. Therefore, groups of –NH2 and –OH existed at the edges of the BNNS. The strong absorption at 1372 cm-1 results from the B–N stretching and the peak at 817 cm-1 is attributed to B–N bending. In order to strengthen the interfacial interactions between BNNS and polyimide matrix, coupling agent of APS was used to further modify BNNS. The TGA curve and FTIR spectrum of the obtained BNNS_APS are shown in Fig. 1(d) and 1(e), respectively. The TGA of BNNS_APS exhibited a rapid decrease between 300-500 °C and the increased mass loss of BNNS_APS than that of BNNS indicated that the APS groups were successfully grafted to the BNNS. The new peaks of

BNNS_APS compared with BNNS at 2922 cm-1, 2850 cm-1 are ascribed to –CH2–, and the band from 900 cm-1 to 1100 cm-1 is assigned to Si–O vibration[53], which also proved that BNNS_APS has been fabricated successfully, in agreement with the TGA results.

3.2 Dispersion of nanosheets in hybrid PIs

To investigate the dispersion state of BNNS and BNNS_APS in the polyimide matrix, SEM analysis was used to investigate the cross-sections of the hybrid polyimide films. As shown in Fig. 2, the cross-sectional morphology of the pure PI (Fig. 2a) presented plastic deformation, corresponding to a typical tough fracture behavior. For the hybrid polyimide films, increased vein patterns formed on the fractured surfaces with an increase in filler content because of the good interfacial adhesion between the polyimide matrix and the BNNS or BNNS_APS, and thus the films exhibited remarkably higher plastic deformations (Fig. 2b–e and Fig. S2). When the BNNS content exceeded 0.5 wt%, some voids were observed in the PI-BNNS hybrid polyimide (highlighted with red dotted ovals in Fig. 2b and d), suggesting partial aggregation of BNNS in the PI matrix. Compared with PI-BNNS, few voids were observed in the PI-BNNS_APS, indicating the excellent dispersion of BNNS_APS in the PI matrix and the stronger interfacial interactions between nanosheets and matrix. UV–Vis spectrum was used to further demonstrate the dispersion differences between BNNS and BNNS_APS in the PI matrix. Fig. 2(f) shows the UV–Vis

transmittance spectra of pure PI and its hybrid films with various BNNS or BNNS_APS contents, and the schematic diagram (Fig. 2g) depict the transmittance difference between PI-BNNS and PI-BNNS_APS hybrid films. The light scattered and reflected in the BNNS aggregation regions because of the large thickness. The transmittance of PI-BNNS_APS is higher than that of the PI-BNNS control, indicating the better dispersity and uniformity of BNNS_APS in the PI matrix film, in consistence with the SEM analysis.

Fig. 2 Dispersion state characterization of nanosheets in the hybrid films. Cross-sectional SEM images of the hybrid PIs: (a) Pure PI, (b) PI-BNNS-0.5%, (c) PI-BNNS_APS-0.5%, (d) PI-BNNS-1%, (e) PI-BNNS_APS-1%. (f) UV–Vis spectra of hybrid PIs with different filler contents. (g) Schematic diagram of nanosheets dispersion state and light transmission in PI-BNNS and PI-BNNS_APS hybrid films.

3.3 Effect of nanosheets on matrix structure and Tg of hybrid materials

Fig. 3 (a) Schematic diagrams of different formed interphase in the two series of hybrid PIs under various interfacial interactions. (b) XRD patterns and (c) crystallinity of pure PI and hybrid PIs with various filler contents. (d) Glass transition temperature of PI-BNNS and PI-BNNS_APS.

FTIR spectroscopy was used to investigate the effect of nanosheets on the chemical structure of the PI matrix (Fig. S3). There were no distinctive differences between the pure PI, PI-BNNS and PI-BNNS_APS, suggesting that the nanosheets had no effect on the polycondensation and chemical structure of the PI matrix. The interfacial interactions between the filler surface and local polymer chain segments can change

the polymer segment mobility. Such perturbations in polymer molecular mobility extend several radii of gyration and create regions of polymer, the interphase, with properties and response that is different from that of the host bulk polymer[11]. For one thing, the greater interfacial areas between hybrid constituents contain, the more regions of whole interphase could form. For another, the stronger interfacial interactions between filler and polymer matrix can more effectively affect the surrounding polymer chain and thus form a larger volume of interphase for each unit of filler[10]. Fig. 3(a) shows schematic diagrams of the interfacial interactions between nanosheet and PI matrix and the interphase in the two series of hybrids for different interfacial interactions. First, the π–π interactions between the BNNS basal plane and benzene ring of the PI chain induced the PI chain to accumulate orderly on the BNNS surface[54]. Thus, the nanosheet plane could act as a nucleation site to facilitate PI chain crystallization. Fig. 3(b) shows the X-ray diffraction patterns of hybrid PIs with various filler contents. The diffraction peaks of pure PI at ~18.2°, 20.9° and 25.3° could be assigned to (110), (200) and (210) lattice planes, respectively, which is consistent with previous reports[55, 56]. For the two series of hybrid PIs, a new diffraction peak appears at 26.8°, corresponds to the (002) lattice plane of BN. Although the positions of the three diffraction peaks of the PI matrix do not change, their intensities changed to some extent, demonstrating their different crystallinities. Therefore, their crystallinities were calculated by peak fitting, and the results are shown in Fig. 3(c). The crystallinity of pure PI is 25.8% and that of the PI matrix in

PI-BNNS increases with an increase in BNNS content because of the role of the nucleation site, which reaches 26.4% and 32.8% for 0.5 wt% and 1 wt% BNNS, respectively. For the PI matrix in PI-BNNS_APS-0.5% and PI-BNNS_APS-1%, the crystallinity is 30.2% and 37.0%, respectively, which is higher than PI-BNNS counterpart. This result was attributed to the better dispersion of BNNS_APS, where more planes of nanosheet could promote crystallization of the PI matrix. Second, the initial groups of –NH2 and –OH at the edges of BNNS could form hydrogen bonds with the C=O of the PI chain, and the relative weak interfacial interactions created an interphase that surrounded the BNNS. Due to the partial aggregation of BNNS, the entire volume of interphase in the PI-BNNS hybrid system is small. Moreover, for BNNS_APS, stronger covalent interfacial interactions exist, except for the two aforementioned weak interactions between hybrid components because of the coupling effect of APS[57]. Synergistically, all interactions created a larger interphase region that surrounded the BNNS_APS. Meanwhile, the uniform dispersion of BNNS_APS in the PI matrix PI resulted in an increase of total volume of the interphase region in the PI-BNNS_APS. The increased volume of created interphase allowed for better hybrid system properties to be achieved, especially for the Tg (initial temperature of chain segmental motion)[10]. The dynamic thermo-mechanical analysis technique was used to investigate the Tg of the pure PI and its hybrid PIs. The DMA curves (tan δ) of PI-BNNS and PI-BNNS_APS are shown in Fig. S4. A comparison of Tg between PI-BNNS and PI-BNNS_APS as a function of nanosheet content is shown in Fig. 3(d)

and Table 1. Increasing the BNNS content, the Tg of the PI-BNNS hybrids increased gradually and reached 427 °C at a 1 wt% loading of BNNS because of the formation of a small volume of interphase. As expected, the Tg of the PI-BNNS_APS improved significantly with an increase in BNNS_APS loading due to the formation of a larger volume of interphase. With only 0.5 wt% BNNS_APS, the Tg of the hybrid PI reached 467 °C, an improvement of 63°C. The Tg can be increased further to 473°C when 1wt% of BNNS_APS was added.

Fig. 4 Comparisons of Tg enhancement between PI-BNNS_APS hybrid and other literature works on polyimide nanocomposites, such as PI/SiO2[36], PI/TiO2[35], PI/Al2O3[20], PI/B2O3[58], PI/POSS[21], PI/MoS2[33] and PI/D400-GO[30].

A comparison of the enhancement of Tg among PI-BNNS_APS and other literature of PI nanocomposites with different filler contents is shown in Fig. 4. In previous reports[10], it is challenging to improve the glass transition temperature of nanocomposites at a very low filler loading. In this work, a remarkbly improved Tg at a low loading of BNNS_APS (< 1 wt%) was achieved in PI-BNNS_APS, which

benefited from covalent inorganic-organic hybrid interactions and better nanosheet dispersion.

Table 1 Thermal properties of hybrid PIsa Composite films

Tg CTE (°C) (ppm/K) Pure PI 404 5.57 PI-BNNS-0.1% 404 5.45 PI-BNNS-0.5% 407 5.47 PI-BNNS-1% 428 6.89 PI-BNNS_APS-0.1% 424 5.96 PI-BNNS_APS-0.5% 467 6.04 PI-BNNS_APS-1% 473 6.57

Td5 (°C) 595 595 596 595 596 596 598

Thermal properties THRI Char yield TC (°C) (wt%) (W/mK) 314.2 63.36 0.22 314.5 63.31 0.22 315.3 64.27 0.24 317.1 64.99 0.28 314.7 63.33 0.25 317.0 65.02 0.37 319.5 65.53 0.44

a: Tg, glass transition temperature; CTE, coefficient of thermal expansion from 50°C to 400 °C; Td5, decomposition temperature at 5% weight loss; THRI, heat-resistance index; Char yield, residual wt% at 800 °C in nitrogen; TC, thermal conductivity.

3.4 Thermal conductivity and stability of hybrid PIs

The thermal conductivities of the PI-BNNS and PI-BNNS_APS hybrids are shown in Fig. 5(a) and Table 1. The thermal conductivities of PI-BNNS and PI-BNNS_APS showed a slight and sharp enhancement, respectively, with continuous increasing filler content. For PI-BNNS, the thermal conductivity improved by only 27% at a 1 wt% loading, whereas the thermal conductivity for the PI-BNNS_APS increased by 100% compared with pure PI. At low content of conductive filler, the filler–filler networks could not form and the filler surface was surrounded by polymer matrix. Therefore, the filler–matrix interface has a huge effect on the TC of the hybrid PIs.

Schematic diagrams of the heat-transfer processes of the two series of hybrid PI systems are shown in Fig. 5(a). The relative weak interactions between PI-BNNS hybrid constituents resulted in high interface thermal resistance and phonon scattering, corresponding to moderate TC improvement. In contrast, the excess strong covalent inorganic-organic interactions in the PI-BNNS_APS hybrid decreased the interface thermal resistance and facilitated the phonon transfer, contributing to the remarkable TC increase. Meanwhile, the better dispersion of BNNS_APS in the PI matrix created better filler–filler heat transfer networks than that of BNNS, resulting in the higher TC. The higher crystallinity of the PI matrix also favored the TC enhancement because the ordered region was more favorable to thermal vibration than the amorphous region[59]. Fig. 5(b) and (c) show the TGA curves of the hybrid PIs under nitrogen and the magnification image between 650 °C and 800 °C, respectively. The pure PI exhibited an excellent decomposition temperature at 5% weight loss (Td5), reaching to 595 °C. Introducing BNNS had no obvious effect on the Td5 because of the intrinsic high thermal stability of pure PI. The Td5 of the PI-BNNS_APS was improved slightly relative to the pure PI. The heat-resistance index (THRI), calculated by THRI=0.49*[Td5+0.6*(Td30-Td5)] (Td30 is the decomposition temperature at 30% weight loss), was also usually used to assess the thermal stability of materials[45, 60]. The THRI of the hybrid PIs (shown in Table 1) increased with increasing the filler content, and that of BNNS_APS based hybrid PIs was higher than BNNS based hybrid PIs counterpart. There was also a small difference in the char yield at 800 °C. The residue

of the hybrid PIs increased with the amount of nanosheets, whereas that of the PI-BNNS_APS was better than the corresponding PI-BNNS. These results suggested that BNNS_APS was more advantageous to improve the thermal stability of the hybrids. This is because, on one side, nanosheet can act as a barrier and is capable of altering the microstructure of the material (i.e., crystallinity), where the more barriers, due to better nanosheet dispersion and higher crystallinity in the PI-BNNS_APS than PI-BNNS, slowed down the decomposition rate; on other side, strong covalent interactions between nanosheet and matrix cause lower mobility of polymer chains, resulting in the decrease of decomposition rate.

Fig. 5 (a) Thermal conductivities of the hybrid films with different filler contents and schematic diagrams of heat transfer processes of two series of hybrid PIs. (b) TGA curves of the hybrid PIs in nitrogen and (c) magnification images between 650 °C and 800 °C. (d) TMA curves of the hybrid PIs.

The CTEs of the two series of hybrid PIs were evaluated by TMA, shown in Fig.

5(d). Because of the rod-like PI chains, the CTE of pure PI was ultra-low, at only 5.57 ppm/K. Compared with pure PI, the CTEs of the PI-BNNS were almost maintained constant except the small increase of the PI-BNNS-1% due to partial aggregation of BNNS in the PI matrix. For the PI-BNNS_APS, the CTEs increased slightly with increasing the BNNS_APS content. The slight increase in CTE was attributed to the trifling decrease of PI chains in-plane orientation. Because the BNNS_APS can act as cross-linking points and prevent the PI chains orienting to the in-plane direction to a certain extent. Nevertheless, the CTEs of all composite films still remained less than 7 ppm/K (near the CTE of the glass), which could satisfy the requirements for flexible display substrate applications.

3.5 Mechanical properties of hybrid PIs

The influence of BNNS and BNNS_APS on the mechanical properties of hybrid PIs was evaluated in Fig. 6 and Table 2. With increasing the BNNS content, the tensile strength and elongation at break of the PI-BNNS increased gradually, whereas the modulus decreased slightly compared with pure PI. For PI-BNNS-1%, the tensile strength and elongation at break reached 281 MPa and 12%, an improvement of 6.8% and 37.9% respectively, compared with pure PI. Along with the continuous increase of BNNS_APS, the tensile strength and elongation at break of PI-BNNS_APS improved rapidly, meanwhile the modulus maintained unchanged. With only 1 wt% doping of BNNS_APS, the tensile strength and elongation at break of the hybrid PIs improved up to 336 MPa and 16.6%, which was a 27.7% and 90.8% enhancement,

respectively, compared with pure PI. The better improvement of BNNS_APS relative to the BNNS in mechanical properties could be ascribed to: (Ⅰ) the better dispersion of nanosheets in the PI matrix has larger interface area for the same filler content, leading to a larger portion of stress transfer regions; (Ⅱ) covalent bonds between the hybrid constituents can effectively enhance the load transfer; (Ⅲ) the more crystalline phase of the PI matrix acts as an additional stiff constituent in the hybrid system to bear load.

Fig. 6 Tensile strength and elongation at break of PI-BNNS and PI-BNNS_APS as a function of filler content.

Table 2 Mechanical properties of hybrid PIs a Mechanical properties Composite films σ (MPa) ε (%) Eb (GPa) Pure PI 263±5 8.7±1.5 5.8±0.3 PI-BNNS-0.1% 268±8 9.3±1.3 5.5±0.3 PI-BNNS-0.5% 271±12 10.3±2.2 5.6±0.2 PI-BNNS-1% 281±8 12.0±1.6 5.6±0.3 PI-BNNS_APS-0.1% 287±12 10.3±0.9 5.8±0.1 PI-BNNS_APS-0.5% 295±8 12.8±1.8 5.8±0.2 PI-BNNS_APS-1% 336±14 16.6±1.1 5.9±0.2 a: σ, tensile strength; ε, elongation at break; Eb, elastic modulus.

4.

Conclusions

To avoid complex synthesis, a simple and efficient strategy to improve the comprehensive performance of polyimide and meet the needs of flexible substrate was proposed. A small number of amino-functionalized BNNS with a lamellar structure could improve the heat resistance and mechanical properties of the PI, and provide the hybrid PI with an excellent thermal conductivity. By using only 1% BNNS_APS, the Tg of the hybrid PI was improved by 69 °C and up to 473 °C, and the tensile strength and elongation at break of the hybrid PI reached 336 MPa and 16.6%, which was enhanced 27.7% and 90.8%, respectively, compared with pure PI. The CTE was near that of glass and most importantly, this approach endowed the hybrid PI film with an excellent thermal conductivity; the TC value was double to the pure PI. These results could be attributed to the better dispersion of nanofiller and stronger inorganic-organic interactions between the BNNS_APS and PI molecular chains (including π–π stacking, hydrogen bonds and covalent bonds), which promoted larger interface area and volume of interphase region. The excellent comprehensive properties of the PI-BNNS_APS hybrid films could be suitable candidates for flexible AMOLED substrate applications.

Research data for this article

Data not available/Data will be made available on request.

Acknowledgments

This work was supported by National Natural Science and Foundation of China (Key Program 51733007) and Shanghai Key Projects of Basic Research (16JC1403900).

Appendix A. Supplementary material

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Graphic abstract Thermal conductive hybrid polyimide with ultrahigh heat resistance, excellent mechanical properties and low coefficient of thermal expansion Xianghui Oua, Xuemin Lua, Shuangshuang Chenb, Qinghua Lua,*

Highlights 1, Simple and efficient strategy to improve polyimide overall performances was proposed 2, Thermal conductivity was increased by 100% at only 1 wt% BNNS_APS loading 3, Glass transition temperature was improved by 69 °C and up to 473 °C 4, Tensile strength and elongation at break reached 336 MPa and 16.6%, respectively 5, The hybrid PI film could be a suitable candidate for flexible AMOLED substrates

Notes The authors declare no competing financial interest.