Surface Modification of Nano-Al₂O₃ with Silane Coupling Agent and Its Effect on the Compressive Strength of PI/Al₂O₃ Composites

Abstract

In this study, a series of nano-Al₂O₃ reinforced polyimide (PI) composites were fabricated using the hot compression method. A silane coupling agent was used to modify the surface of nano-Al₂O₃ powder to enhance the PI/Al₂O₃ interface compatibility. The effects of Al₂O₃ contents on the microstructures, thermal behavior and mechanical properties of PI/Al₂O₃ composites were investigated. The results suggested that chemical reactions occurred during in situ modification. The nano-Al₂O₃ was cross-linked to the PI matrix, which reduced the mobility of PI chains and improved the elastic modulus of the composites. When the Al₂O₃ content was 30 wt.%, the compressive strength of composites reached the maximum value of 313 MPa and increased by 26.5% than that of the PI matrix.

1. Introduction

Polyimide (PI) has great potential in the field of flexible composite materials or microelectronic devices due to its excellent structural flexibility, biocompatibility, chemical stability and high dielectric constant [1,2,3]. In particular, aromatic polyimide, containing rigid imide and aromatic ring structures, is classified as a high-performance polymer owing to its excellent thermal and mechanical properties. Currently, PI is one of the best temperature-resistant materials in resin matrix composites and is considered one of the most potential materials to replace metal as structural components in the field of aerospace [4,5]. But, compared to metals and ceramics, the weak mechanical performance and low-temperature resistance greatly limit the application of PI. Therefore, it is of great importance to enhance its strength, modulus and thermal stability to broaden its development potential.

The results of a growing number of studies suggest the use of nanoparticles, such as boron nitride [6], alumina [7,8], carbon nanotubes [9], graphene oxide [10,11], silica [12,13] and layered silicate [14], can improve the mechanical properties, temperature resistance, flame retardancy, optics, wear resistance and dielectric properties of polymers [15,16,17,18,19]. Compared to the microparticles, the nanoparticles can achieve better performance with less addition owing to their large specific surface area [20]. Among these nanoparticles, nano-Al₂O₃ has a series of unique properties, such as good thermal conductivity, high strength, mechanical strength, wear resistance, thermal stability and electrical insulation. Nevertheless, nano-Al₂O₃ has high surface activity and tends to form agglomerates or clusters in the polymer matrix. In addition, differences in hydrophobicity usually lead to poor interfacial interaction between Al₂O₃ and polymers, which consequently results in the degradation of mechanical properties. Therefore, it is necessary and important to modify the surface of nanoparticles to overcome their tendency to aggregate and improve their dispersion in the polymer matrix.

It is well known that grafting synthetic polymers is a useful approach for modifying the surface of nanoparticles. For instance, Belgacemi et al. [21] used the 3-aminopropyltrimethoxysilane (APS) coupling agent to modify the surface of nano-Al₂O₃ and prepared epoxy/oxidized ultrahigh molecular weight polyethylene (UHMWPE)/Al₂O₃ composites. The flexural modulus increased from 6.9 to 13.2 GPa, while the flexural strength increased by 25% to 159 MPa after adding 5 wt.% Al₂O₃. The hydrolysis of silane coupling agents produces silicon–oxygen bonds and hydroxide ions [22]. Duan et al. [23] used three coupling agents to surface modify nano-Al₂O₃, including 3-aminopropyltriethoxysilane (KH550), γ-methacryloyloxypropyltrimethoxysilane (KH570) and tetraisopropyl di(dioctylphosphate) titanate (NDZ401). The results on the DC electrical properties of Al₂O₃/low-density polyethylene (LDPE) composites revealed that surface modification significantly affected their properties. However, there are only a few studies on the compressive properties of PI-based composites reinforced using nano-Al₂O₃. In addition, due to the high surface energy of nanoparticles, the content of nanoparticles was usually less than 10%.

Based on the above background, aromatic polyimide and nano-Al₂O₃ powders were used to prepare PI/Al₂O₃ composites. The silane coupling agent was used to modify the surface of nano-Al₂O₃ with the aim of enhancing the PI/Al₂O₃ interface compatibility in the present work. The PI-based composites with different Al₂O₃ contents were fabricated using the hot-pressing method and post-heat treatment. The effects of Al₂O₃ content and surface modification on the compressive strength of PI were elaborated. In addition, the thermal behaviors of PI were investigated using differential scanning calorimetry.

2. Materials and Experimental Procedures

2.1. Materials and Surface Modification of Nano Al₂O₃

The PI and Al₂O₃ powder (Beijing Sunspraying New Material Co., Ltd., Beijing, China) were used in the preparation of composite materials. The molecular weight and intrinsic viscosity of polyimide were 30,000 and 0.5–0.7 dL/g, respectively. The specific surface area and particle size distribution of Al₂O₃ were 230 m²/g and 20–50 nm, respectively. Figure 1 shows the synthesis of polyimide as well as the crosslinked products. The schematic diagram of the surface modification of Al₂O₃ with 3-aminopropyltriethoxysilane (KH550) silane coupling agent (≥99.0%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) is shown in Figure 2. As shown in Figure 2, the Al₂O₃ particle was dried at 100 °C for 12 h to remove surface adsorbents, and then ultrasonically dispersed for 30 min by adding Al₂O₃ to an ethanol solution. The silane coupling agent was added to the mixed solution of ethanol and water. The mass ratio of the coupling agent, ethanol and water was 20:72:8. Furthermore, acetic acid (Sinopharm Chemical Reagent Co., Ltd. Shanghai, China) was added to adjust the pH to 4. The acidic media improved the hydrolysis reaction, and the resultant silanol was very stable. And, the solution was hydrolyzed with ultrasound for 30 min. Then, the mixture of Al₂O₃ powder and silane coupling agent was stirred for 4 h at 80 °C. The mass ratio of the silane coupling agent and Al₂O₃ was 1 to 20. The degree of silane grafting was characterized through differential scanning calorimetry (DSC). The DSC method easily reflected the weight loss, especially the loss of organic polymer at high temperatures. In this paper, the silane coupling agent belonged to the organic polymer. The surface grafting-modified Al₂O₃ using a silane coupling agent will decompose and experience weight loss after high-temperature treatment. The loss value meant a decrease in silane grafting. Therefore, the difference in weight loss between the modified Al₂O₃ and the unmodified Al₂O₃ was the grafting specific gravity of the coupling agent. Figure 3 shows the TGA results of the original and modified Al₂O₃. The weight loss of nanoparticles <100 °C was mainly due to water desorption. The weight loss between 100 and 800 °C was mainly caused by the loss of organic groups grafted on the nanoparticle surface [23]. For the original Al₂O₃ powder, the total loss was 4.1%, and this is due to the loss of absorbed moisture. And the weight loss was up to 8.3% for modified Al₂O₃ powder. Thus, the grafting rate of Al₂O₃ after modification was 4.2 wt.%.

2.2. Preparation of PI/Al₂O₃ Composites

The PI and surface-modified Al₂O₃ powders were stirred and mixed in a high-speed mechanical mixer at 2000 rpm for 30 min. The compositions of Al₂O₃/PI composites are listed in

Table 1. Samples and compositions of PI/Al₂O₃ composites.

Samples	PI/wt.%	Al2O3/wt.%	Modified through KH550 Silane Coupling Agent
PI	100	0	No
10 A	90	10	No
20 A	80	20	No
30 A	70	30	No
40 A	60	40	No
50 A	50	50	No
10 A-K	90	10	Yes
20 A-K	80	20	Yes
30 A-K	70	30	Yes
40 A-K	60	40	Yes
50 A-K	50	50	Yes

. The preparation process was divided into two stages: The first stage was hot pressing, and then the mixture was dispersed and pressed for 30 min under a pressure of 100 MPa at 220 °C. The second stage was post-treatment, conducted in three steps: first at 180 °C for 1 h, then at 200 °C for 1 h, and finally at 230 °C for 4 h.

2.3. Microstructural Characterization, Thermal Stability and Mechanical Properties

The interface of composites was observed using scanning electron microscopy (SEM, SU3500, Timei Scientific Instrument Co., Ltd., Shanghai, China) and transmission electron microscopy (TEM, JEM-2100, JEOL Japan Electronics Co., Ltd., Tokyo, Japan). Differential scanning calorimetry (DSC) was performed on METTLER TOLEDO TGA/DSC3+ (METTLER TOLEDO, Ltd., Shanghai, China) under a nitrogen atmosphere. The composites were heated up to 800 °C at a rate of 10 °C/min. Crystalline structures of both PI and Al₂O₃ powders were characterized using X-ray diffraction (XRD, D8 DVANCE, Bruker Technology Co. Ltd., Karlsruhe, Germany) with Cu Ka radiation (1.5406 nm) using a 0.008° step size, 8°/min scanning speed, 45 kV tube voltage, 40 mA tube current, and diffraction angle from 10 to 90°. The peak fitting of XRD patterns was carried out using MDI Jade 6.0 software. Fourier transform infrared spectroscopy (FTIR) was performed using a Micro-Infrared Spectroscopy (Bruker VERTEX70) covering the range from 4000 to 400 cm⁻¹ to evaluate the structure of surface-functionalized nano-Al₂O₃. X-ray photoelectron spectrometry (XPS, Thermo Fisher ESCALAB Xi+, Thermo Fisher Scientific, Waltham, MA, USA) was used to analyze the chemical composition of samples. The compressive strength of all composites was measured using a universal tensile testing machine (WDW-100KN, Ningbo Dexun Testing Equipment Co., Ltd., Ningbo, China) with a constant load rate of 1 mm/min. Three samples of each composition were tested to ensure repeatability and an average value was recorded. The roughness of the fracture surface was measured using a surface roughness measuring instrument (TR200, Shanghai Gaozhi Precision Instrument Co., Ltd., Shanghai, China). The porosity was estimated using Archimedes’ principle (water displacement). In the Archimedean method, samples used for compression testing were used to measure porosity. The samples were cleaned and dried. The mass of corresponding samples was recorded as M₃. Then, the samples can be operated by pumping in distilled water for 5–7 h to facilitate pore penetration. This mass was recorded as M₂. Subsequently, the floating mass was measured in distilled water and recorded as M₁. Based on the equal volumes of the Archimedean principle (${\rho }_{0}gV=(M_2-M_1)g$), the P was calculated using the following equation:

$$P=(M_2-M_3)/(M_2-M_1).$$

(1)

3. Results

3.1. Thermal Stability of PI/Al₂O₃ Composites

DSC analysis was conducted to investigate the thermal behavior of PI/Al₂O₃ composites. The heat flow density curves of PI powder and hot-pressed PI and the weight loss curves of PI/Al₂O₃ composites are demonstrated in Figure 4. It can be seen from Figure 4a that there is an exothermic peak on the heat flow density curve of the PI powder. The appearance of a peak at 230 °C indicates the occurrence of cross-linking reactions [24,25]. As seen from the heat flow density curve of the hot-pressed PI, the T_g of the hot-pressed PI material was 356.5 °C. In the weight loss curves shown in Figure 4b, it can be found that the thermal stability of the PI/Al₂O₃ composites improved after the addition of Al₂O₃, which hindered the transfer of heat. The temperature of 5% weight loss (T_5%) of hot-pressed PI was 450.74 °C. And the thermal deposition temperature (T_5%) of composites with 10 wt.%, 30 wt.% and 50 wt.% Al₂O₃ were 451 °C, 457.47 °C and 455.79 °C, respectively.

3.2. Surface Modification of Al₂O₃ Powder

Figure 5 depicts the morphologies and X-ray diffraction patterns of PI and Al₂O₃ powders. As shown in Figure 5a,b, the particle size of PI powder was approximately 2~10 μm and exhibited an irregular shape, while the size of Al₂O₃ powder ranged from 20 to 50 nm (see Figure 5d). Nano-Al₂O₃ particles are susceptible to agglomeration due to their high surface activity. The Al₂O₃ particles used are micron-scale particles formed through the agglomerate of nanoparticles in the SEM image (Figure 5c). And the particle size of undispersed particles was 2 to 10 μm. As shown in Figure 5e,f, the XRD pattern of PI powder exhibited only one broad peak and no sharp diffraction peaks corresponding to crystalline phases. In addition, XRD results of Al₂O₃ powder revealed that the phase structure consisted of the corundum and spinel phases (α-Al₂O₃ and γ-Al₂O₃).

Figure 6 shows the statistical results of the porosity of PI/Al₂O₃ composites with or without modified Al₂O₃ using the silane coupling agent, respectively. It can be seen from Figure 5 that the porosity of the composites slightly decreased after adding the modified Al₂O₃. The increased porosity had a close relationship with the agglomeration of nano-Al₂O₃. When the content of Al₂O₃ reached 50%, the porosity of the composites dramatically increased due to particle agglomeration. Figure 7 provides the fracture morphology of PI/Al₂O₃ composites. As shown in Figure 7, many Al₂O₃ particles accumulated in the PI matrix and formed a large number of pores, especially for the sample with 50% original or modified Al₂O₃, which was harmful to the performance of the composites. It can be observed from Figure 7a–d that the agglomeration of Al₂O₃ was weakened to some extent after modification.

The hydrolyzed silane coupling agent reacted with the hydroxyl group on the surface of Al₂O₃ to form bonds, which can reduce the agglomeration of nano-Al₂O₃ and improve the interfacial compatibility with the polymer. The original and modified Al₂O₃ powders were characterized using FTIR and XPS to examine whether chemical reactions occurred during the in situ modification, and the results are shown in Figure 8. The Fourier transform infrared spectra of nano-Al₂O₃ are shown in Figure 8a,b. For the original Al₂O₃ powder, the peak at 1645 cm⁻¹ corresponded to the bending vibration of O-H, and the peaks at 1075 cm⁻¹, 816 cm⁻¹ and 746 cm⁻¹ were the vibration of Al-O. After modification, some new peaks appeared, which corresponded to the bending vibration of -NH₂ and -CH₂- at 1570 and 1470 cm⁻¹, while 1431 cm⁻¹ and 1053 cm⁻¹ were the vibration peaks of Si-C-H and Al-O-Si, respectively. The vibration peak of -OH disappeared, and the vibration intensity of Al-O was significantly reduced. The emergence of new peaks of -NH₂, -CH₂-, Si-C-H and Al-O-Si suggested that the silane coupling agent was successfully coated on the surface of Al₂O₃ nanoparticles. The XPS results of the original and modified Al₂O₃ powders are shown in Figure 8c–f. It can be seen that the new peaks of N and Si appeared on the modified Al₂O₃, while C, Al and O were only present in the original one. Figure 9 exhibits the XRD patterns and the FTIR spectra of PI/Al₂O₃ composites with and without modified nano-Al₂O₃. After modification, the bonding of Al₂O₃ and PI was enhanced, and the amorphous peak strength of the polymer increased in XRD patterns. In the FTIR spectra, the peaks at 1380 cm⁻¹, 1730 cm⁻¹ and 1780 cm⁻¹ correspond to the stretching vibration peaks of C-N and C=O, respectively. The Si-O peak at 1100 cm⁻¹ appeared in the composites with modified Al₂O₃. Additionally, the O-H peak decreased significantly, and the Al-O peaks at 815 cm⁻¹ and 663 cm⁻¹ decreased.

3.3. Mechanical Properties of PI/Al₂O₃ Composite Materials

The compression test was used to explore the effect of Al₂O₃ content and surface modification on the compressive strength of PI/Al₂O₃ composites. Figure 10 provides the compressive strength and modulus of PI/Al₂O₃ composites. As shown in Figure 10a, the compressive strength of the composites gradually increased with the increase of Al₂O₃ content. When the Al₂O₃ content was higher than 30 wt.%, the strength of the composites decreased due to the excessive filler content and uneven distribution of Al₂O₃ in the PI matrix. The compressive strength of composites with modified Al₂O₃ was higher than that of the original one. In addition, the composites with 30 wt.% Al₂O₃ exhibited the highest compressive strength for both of composites with or without modified Al₂O₃. The compressive strength of pure PI was 247.5 MPa. The compressive strength of composite with 30 wt.% Al₂O₃ was 313.0 MPa, and its compressive strength was 26.5% higher than pure PI. Similar to the results reported in previous studies, Sezavar et al. [26] found that the addition of 15 wt.% Al₂O₃ to PMMA caused an increase of approximately 25% in elastic modulus. From the compressive modulus shown in Figure 10b, it was found that the compressive modulus increased gradually with increasing the content of Al₂O₃. The compressive modulus increased after modification, while the composites with 40 wt.% Al₂O₃ exhibited the highest compressive modulus for both samples with modified or original Al₂O₃ powders.

From the stress–strain curve shown in Figure 10c,d, it can be revealed that when the content of Al₂O₃ was below 20 wt.%, the strength of the composites increased slightly while maintaining good plasticity. When the content of Al₂O₃ reached 30 wt.%, the strength of the composite material was significantly improved, whereas the plasticity was reduced. The characteristic morphology of the fracture surface of PI/Al₂O₃ composites is demonstrated in Figure 11. As shown in Figure 11a, the fracture surface of pure PI was smooth and glassy; similar results can also be found in some previous studies [27,28,29,30]. The SEM images for 10 A-K and 30 A-K are shown in Figure 11b,c, respectively. It can be seen that the number of river-like cracks in pure polyimide reduced, and the roughness of the fracture surface increased with the increase of Al₂O₃ content.

Table 2. Roughness values of the PI/Al₂O₃ nanocomposites.

Samples	Roughness/μm
PI	0.995
10 A	1.176
20 A	1.596
30 A	2.111
40 A	2.424
50 A	3.058
10 A-K	1.680
20 A-K	1.986
30 A-K	2.492
40 A-K	2.542
50 A-K	3.207

shows the roughness values of the PI/Al₂O₃ nanocomposites and proves the increasing tread. For the sample of 10 A-K, the cracks propagated in a parabolic shape. In the case of 30 A-K, the roughness continued to increase, and some Al₂O₃ particles were well-dispersed throughout the polyimide matrix. However, after further increasing the content of Al₂O₃ powder, many pores or cracks, as well as agglomerates of nano-Al₂O₃, were clearly observed in the composite with 50 wt.% Al₂O₃, as shown in Figure 11d. The corresponding roughness also increased with an increase in the content of Al₂O₃.

4. Discussion

The thermosetting resin has weak crack propagation resistance, and the addition of inorganic nanoparticles is expected to improve the toughness of the polymer. An appropriate number of nano-sized powders can improve strength and modulus while ensuring good ductility. The cracks propagated at 45° in the pure PI sample, indicating the occurrence of shear deformation. However, it can only absorb a small amount of energy, while the excess energy will cause cracks to spread rapidly. When the content of Al₂O₃ was 10 wt.%, the cracks propagated in a parabolic shape, suggesting that the propagation path of the cracks was deflected. Additional energy input was required for the propagation of cracks. The propagation occurred locally by means of tensile and shear stresses, which induced an increase in the surface area of the fracture, leading to an increase in fracture toughness [31,32]. When the content of Al₂O₃ was less than 30 wt.%, the plasticity of the composite was maintained while the strength was increased.

There are three possible reinforcement mechanisms for the PI/Al₂O₃ composite. In one of the mechanisms, the reinforcement was due to the hydrolysis of silane coupling agents, which produce silicon–oxygen bonds and hydroxide ions. Silicon–oxygen bonds are very stable bonds in silane coupling agents, which can strongly adhere to the surface of inorganic materials. Aminopropyl groups at the other end can form hydrogen bonds with polymers when cured [22,33,34]. Furthermore, the second reason is attributed to the transfer of shear stresses in the PI/Al₂O₃ interface, and the nanoparticle-matrix interaction is the major factor for its occurrence [35]. After the modification of nano-Al₂O₃, the bonding of the adsorption layer was enhanced. The appearance of -NH₂, -CH₂-, Si-C-H and Al-O-Si peaks in the FTIR spectrum and N and Si peaks in photoelectron spectra indicated that a modified layer was successfully prepared on the surface of nano-Al₂O₃, resulting in the improvement of strength and modulus. Another reinforcement effect was promoted through the formation of the three-dimensional network structure. During the crosslinking reactions, the PI chains were adsorbed to the surface of the nano-Al₂O₃ to form a local hardening zone, which reduced the mobility of the PI chains and enhanced the elastic modulus [36,37]. Figure 12 shows the schematic diagram of crack propagation in PI/Al₂O₃ composites. Under the action of external force, micro-cracks were generated in PI, molecular chains were broken, and the cracks propagated rapidly (see Figure 12a). It can be seen from Figure 12b that many Al₂O₃ particles were dispersed throughout the PI matrix. These Al₂O₃ particles can effectively hinder the propagation of the cracks, but parts of the particles were agglomerated in 30 A. After modification, the nano-Al₂O₃ particles were well-dispersed throughout the PI matrix (see Figure 12c); thus, the strength and modulus were further improved.

The strength of composites with a suitable content of nanoparticles was higher than that of pure PI matrix. For instance, Liu et al. [38] found that the PI/reduced graphene oxide (RGO)/Fe₂O₃ composites showed a compressive strength of 130.97 MPa and was 35.9% higher than pure PI. Zhao et al. [39] fabricated PI/nano-SiO₂ composites via an in situ polymerization method, in which the compressive strength of the composite with 5 wt.% nano-SiO₂ was 125.51 MPa, which increased by 42.6% compared to the pure PI with a compressive strength of 88.02 MPa. Compared to the above previous results, the compressive strength of PI/Al₂O₃ composites in this study was significantly improved. The compressive strength of 30 wt.% PI/Al₂O₃ can reach 313 MPa. However, as depicted in Figure 12d, when the content of nano-Al₂O₃ was too high, PI could not fill the gaps of agglomerates due to the agglomeration of excessive filler, leading to an increase in porosity and a decrease in strength. Additionally, the agglomeration of nano-Al₂O₃ easily caused the interfacial debonding of Al₂O₃ from the PI matrix, resulting in the initiation of cracks and stress concentration [40]. Therefore, the PI/Al₂O₃ composites with 30 wt.% modified Al₂O₃ exhibited the highest compressive strength.

5. Conclusions

In this study, a series of nano-Al₂O₃-reinforced polyimide (PI) composites were fabricated using the hot compression method. The thermal stability of the PI/Al₂O₃ composites was improved after the addition of Al₂O₃. The silane coupling agent was used to modify the surface of nano-Al₂O₃ to enhance the interface compatibility between Al₂O₃ and PI. The effects of Al₂O₃ contents on the microstructures and compressive properties of PI/Al₂O₃ composites were explored. Some main conclusions are as follows:

• Chemical reactions occurred during the surface modification of nano-Al₂O₃. The appearance of -NH₂, -CH₂-, Si-C-H and Al-O-Si peaks in infrared spectroscopy and N and Si peaks in photoelectron spectra suggested that a modified layer was successfully prepared on the surface of Al₂O₃ nanoparticles.
• Surface modification enhanced the interfacial compatibility between the Al₂O₃ and PI matrix. The compressive strength and compressive modulus of composites with modified Al₂O₃ were higher than those with original Al₂O₃ to some extent.
• The compressive strength and compressive modulus of the composites initially increased and then decreased with the increase of Al₂O₃ content. The improvement of compressive properties of composites was due to the transfer of shear stresses in the nanoparticle–matrix interface and the formation of the three-dimensional network structure. When the Al₂O₃ content was 30 wt.%, the compressive strength of composites reached the maximum value of 313 MPa and increased by 26.5% compared to that of the PI matrix.