Preparation of hyperbranched aromatic polyamide grafted nanoparticles for thermal properties reinforcement of epoxy composites

Abstract

Epoxy nanocomposites with hyperbranched aromatic polyamide grafted alumina (Al₂O₃) nanoparticles as inclusions were prepared and their thermal properties were studied. The Al₂O₃ nanoparticles were firstly treated with a silane coupling agent to introduce amine groups, then grafting of the hyperbranched aromatic polyamide started from the modified surface. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) analysis proved hyperbranched aromatic polyamide grafted Al₂O₃ nanoparticles were successfully prepared by solution polymerization. Transmission electron microscopy (TEM) showed that there was a thin polymer layer on the Al₂O₃ nanoparticles surface, which contributes to the uniform dispersion of Al₂O₃ nanoparticles in epoxy matrix and the improvement of the interfacial interaction between Al₂O₃ nanoparticles and epoxy matrix. Thus the glass transition temperature, thermal stability, thermal conductivity and thermomechanical properties of nanocomposites were enhanced.

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Introduction

In the past decades, polymer/inorganic nanocomposites have emerged as a promising research area in the development of new materials for high added-value applications.^1–6 These materials gain benefits by combining the merits of both organic (toughness, processability, etc.) and inorganic (rigidity, thermal stability, etc.) phases. The nature of the interface has been used to grossly divide these materials into two distinct classes.^7,8 Firstly, only weak bonds (hydrogen, van der Waals or π–π bonds) between the matrix and filler are present. Secondly, the two phases are linked together through strong chemical bonds (covalent and ionic bonds). When polymer chains are tethered to the nanoparticle surface by covalent bond, a strong adhesion between nanoparticle and polymer matrix can be resulted. Generally, covalent attachment can be achieved by the so-called grafted procedure. Nanofillers, also at low concentrations, give nanocomposites an improvement of relevant properties such as increased modulus and strength, improved scratch, abrasion, solvent and heat resistance and decreased gas permeability. In particular, thermal properties are one of the most striking features of the nanocomposites. However, the claimed benefits of nanocomposites rely on a good dispersion of the nanoparticles. Previous studies demonstrated that hyperbranched polymer grafted nanoparticles could interfere with agglomeration of the nanomaterials and increase their surface affinities for organic solvent and polymer matrices.^9–12 This is attributed to hyperbranched polymers with a highly branched structure and multiplicity of reactive end groups.¹³

This article describes the preparation of epoxy nanocomposites with hyperbranched aromatic polyamide grafted Al₂O₃ nanoparticles. Epoxy resins have long been applied in a variety of industrial applications, such as paints, coatings, inks, reactive diluents, vacuum pressure impregnation of coils, encapsulation of electronic circuit elements and printed circuit board coatings because of their good heat and chemical resistance, superior mechanical and electrical properties, as well as excellent processability.^14–16 Despite many advantages of epoxy resin, certain properties, such as thermal and mechanical properties, electrical insulating properties as well as low thermal conductivity, should be improved to satisfy the demand of advanced microelectronic packaging.¹⁷ On the other hand, Al₂O₃ nanoparticles have good physical properties, such as abrasion resistance, corrosion resistance, thermal stability, electrical insulation, and relatively high thermal conductivity (about 30 W m⁻¹ K⁻¹), etc.¹⁸ Therefore, the inclusion of inorganic Al₂O₃ nanoparticles into an epoxy matrix for the formation of nanocomposites has proved to be an efficient way to overcome its drawbacks.^19–24

By exploring the role of interfacial bonding in improving the performances of nanocomposites, this paper focuses on how compatibilization between the nanoparticles and matrix affects thermal properties. In order to generate strong interfacial interactions, the Al₂O₃ nanoparticles were grafted hyperbranched aromatic polyamide by two-step route with solution polymerization. Then the hyperbranched aromatic polyamide grafted Al₂O₃ nanoparticles were used to add in epoxy resin to afford the thermosets. The formation, structure and properties of nanoparticles and nanocomposites were addressed on the basis of Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermal conductivity and dynamic mechanical analysis (DMA), etc. We probed their structure characteristics and thermal behavior of nanocomposites as a function of the hyperbranched aromatic polyamide grafted Al₂O₃ content with the aim of exploring the high performance materials, hoping this may help in the design not only of epoxy/Al₂O₃ nanocomposites but also of a wider range of polymer/inorganic nanocomposites.

Experimental

Materials

Al₂O₃ nanoparticles with an average diameter of 30 nm were obtained from Kaier Nano (Hefei, China). A cycloaliphatic epoxy resin (6105, DOW Chemicals, USA) along with hardener of methyl-hexahydrophthalic anhydride (MHHPA, Shanghai Li Yi Science & Technology Development Co. Ltd., China) was used in the present study. Neodymium(III) acetyulacetonate trihydrate (Nd(III)acac) purchased from Aldrich Chemicals was used as latent catalyst. γ-Aminopropyl-triethoxysilane (γ-APS) purchased from GE silicones was used as coupling agent. The grafting monomer, 3,5-diaminobenzoic acid (DABA, Tokyo Chemical Industry Co., Ltd., Japan) was purified by recrystallization from water and dried in a vacuum at 80 °C for 12 h. Lithium chloride (LiCl) was dried at 230 °C overnight before use. Other solvents, like triphenyl phosphate (TPP), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethyform amide (DMF), methanol, acetone and pyridine were analytical grade, provided by Sinopharm Chemical Reagent co., Ltd, China, and were used without further purification.

Silanization of Al₂O₃ nanoparticles

Prior to silane modification, as received Al₂O₃ nanoparticles were dried in a vacuum oven at 180 °C for 24 h to remove moisture absorbed at the surface. 1 g of as received Al₂O₃ nanoparticles and an appropriate amount of the silane (0.5–1 wt% based on the weight of nanoparticles) were added into a 500 ml three-necked flask, equipped with a mechanical stirrer and a reflux condenser, and mixed in high purity dimethylbenzene by stirring at 110–120 °C for at least 4 h. After filtration, the γ-APS treated Al₂O₃ nanoparticles (named Al₂O₃-APS) were washed several times with dimethylbenzene to remove the unreacted siloxane moieties and dried in a vacuum oven at 120 °C for 2 h to remove the residual solvent.

Grafting of hyperbranched aromatic polyamide from Al₂O₃ nanoparticles surface

In a flask, 0.76 g Al₂O₃-APS and 0.76 g DABA were charged in 10 mL NMP and stirred until the DABA was dissolved completely. The 2.5 mL of pyridine and 2.6 mL of TPP were charged into the flask. The solution was heating to 100 °C and stirred under nitrogen for 3 h. After temperature was decrease to room temperature, the solution was poured into 50 mL of methanol to precipitate the polymer. The polymer was collected by filtration and purified by reprecipitaion from DMF solution into methanol containing 0.1% LiCl. The product was finally filtered and washed with cold methanol and dried in a vacuum at 90 °C to a constant weight. The Al₂O₃-APS grafted hyperbranched aromatic polyamide was denoted as Al₂O₃-HBP in the following text.

Preparation of epoxy/Al₂O₃-HBP nanocomposites

The epoxy/Al₂O₃-HBP nanocomposite was prepared as follows. Firstly, the required quantity of Nd(III)acac was added to the epoxy resin, stirred and degassed at 80 °C in a three-necked flask. The homogeneous solution was then cooling down to ambient temperature. Secondly, a desired amount of Al₂O₃-HBP was ultrasonically dispersed in acetone for 1 h at room temperature. The Al₂O₃-HBP/acetone suspension was then added to the pretreated epoxy resin step by step under stirring. Afterwards, the temperature was increased to 60 °C and the mixture was sonicated for 1 h, followed by intensive mechanical stirring for 1 h to ensure good homogeneity and remove the acetone entirely. Thirdly, MHHPA was added under vigorous mechanical stirring. Subsequently, the solution was degassed for 30 min. Finally, the mixture was poured onto preheated stainless steel molds, pre-cured in an oven at 135 °C for 2 h, followed by a post-cure at 165 °C for 14 h. The molds were left in the oven and allowed to cool gradually to room temperature. The experimental details of the process of formation of epoxy/Al₂O₃-HBP nanocomposites are shown in Scheme 1. Nanocomposites containing different weight fractions (5%, 10%, 15% and 20%) of the as received Al₂O₃ and Al₂O₃-HBP were prepared.

Scheme 1 Preparation process of the epoxy/Al₂O₃-HBP nanocomposites.

Characterization

Fourier-transform infrared (FT-IR) was conducted with a Perkin-Elmer Paragon 1000 instrument over the range of 4000–400 cm⁻¹. Nuclear magnetic resonance (¹H-NMR) measurement was conducted on a Varian mercury plus-400 spectrometer with DMSO-d₆ as the solvent. The average size and size distribution of the nanoparticles were determined by dynamic light scattering method using a Nano-ZS90 ZetaSizer instrument (Malvern Instruments Ltd., UK). Transmission electron microscopy (TEM, JEM-2100, Japan) was used to observe the dispersion of the nanoparticles in the ethanol and in the composite. The specimens (about 70 nm in thickness) for TEM observation were trimmed using a microtome machine with a diamond knife. Field emission scanning electron microscope (FE-SEM, JEOL JEM-7401, Japan) was used to observe the dispersion of the nanoparticles in the composite samples. Samples were sputtered with thin layers of gold to avoid the accumulation of charge. Thermal gravimetric analyses (TGA) were performed with a Netzsch TG-209 F3 instrument (NETZSCH, Germany) at a heating rate of 20 °C min⁻¹ in nitrogen atmosphere. Differential scanning calorimetry (DSC, 200 F3, NETZSCH, Germany) was performed at temperatures from 20 to 250 °C at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. Thermal conductivities of the nanocomposites were measured with LFA 447 Nanoflash (NETZSCH, Germany) according to ASTM E1461, using the measured heat capacity and thermal diffusivity, with separately entered density data. Samples were prepared in cylindrical shape of 12.7 mm in diameter and 1.0–2.0 mm in thickness. Dynamic mechanical analysis (DMA) was performed on a DMA Q800 dynamic mechanical analyzer (TA Instruments, USA), operating in the single-cantilever mode at an oscillation frequency of 1 Hz. Testing temperature was set as from room temperature to 250 °C at a heating rate of 3 °C min⁻¹.

Results and discussion

Characteristics of the surface modified Al₂O₃ nanoparticles

In present work, the amino groups were firstly introduced onto the Al₂O₃ particles surface as an initiator site by the treatment of Al₂O₃ with γ-APS, and then hyperbranched polyamide was grafted from these amino groups. The FTIR spectra of as received Al₂O₃, Al₂O₃-APS and Al₂O₃-HBP nanoparticles are shown in Fig. 1. In as received Al₂O₃ nanoparticles, the strong absorption at 3468 cm⁻¹ is attributed to hydroxyl group (–OH) peak stretching and peak at 1637 cm⁻¹ is due to –OH bending. The spectra of silane-modified Al₂O₃ nanoparticles are very similar to those of as received Al₂O₃ nanoparticles, but some differences can still be detected. The peak appears at 2925 and 2854 cm⁻¹ which are characteristics of asymmetric and symmetric –CH₂ stretching vibrations respectively. The appearance of a band at 1490 cm⁻¹ is due to –CH₂ bending (scissoring) vibration. Bands at 1567 and 1120 cm⁻¹ are due to N–H bending (scissoring) and C–N stretching respectively. For the spectrum of Al₂O₃-HBP nanoparticles, the peak at 1651 and 1549 cm⁻¹ can be assigned to amide C=O stretching. The peak at 1603 and 1446 cm⁻¹ which are characteristics of aromatic structure. These changes of characteristic peaks indicate that silane and hyperbranched aromatic polyamide have already been grafted successfully on the surface of Al₂O₃ nanoparticles.

Fig. 1 FTIR spectra of as received Al₂O₃, Al₂O₃-APS and Al₂O₃-HBP.

¹H-NMR spectra provide more detailed evidence to prove that hyperbranched aromatic polyamide is successfully grafted onto the surface of Al₂O₃ nanoparticles. As can be seen from Fig. 2, the characteristic peaks from the aromatic protons are clearly detected in the ¹H-NMR spectra of Al₂O₃-HBP nanoparticles.^12,25 The hyperbranched aromatic polyamide shows three of aromatic proton peaks, which can be attribute to linear, dendritic and terminal units. The peaks at 8.3 and 7.9 ppm are attributed to the aromatic protons of the dendritic units. Meanwhile the peaks at 7.3 and 6.8 ppm which are characteristics of the aromatic protons of linear units and those at 6.3 and 6.0 ppm can be assigned to the aromatic protons of terminal units. In addition, the proton peaks of the amide group (amide C=O) at 10.6–9.8 ppm are also observed clearly, but the signals of terminal amine groups at 5.15 ppm are very weak.

Fig. 2 ¹H-NMR spectra of Al₂O₃-HBP.

The TGA data of as received Al₂O₃, Al₂O₃-APS and Al₂O₃-HBP are listed in Fig. 3. It can be seen from Fig. 3 that the weight loss of the nanoparticles show the order of as received Al₂O₃ < Al₂O₃-APS < Al₂O₃-HBP at 800 °C. The weight grafting ratios (G_r) can be calculated by using the following equation.^26,27

Where W_Al2O3-HBP,T and W_Al2O3,T are the percent weight loss of Al₂O₃-HBP and as received Al₂O₃ particles between 50 °C and temperature T. On this basis, the value of Gr(%) for the Al₂O₃-HBP particles was 38.8% according to eqn (1). Meanwhile the initial decomposition temperature of Al₂O₃-HBP particles was estimated to be 407 °C. These TGA data present further evidence to prove the hyperbranched aromatic polyamide have been grafted successfully on the surface of Al₂O₃ nanoparticles.

Fig. 3 TGA curves of as received Al₂O₃, Al₂O₃-APS and Al₂O₃-HBP measured in N₂.

Microscopy is very useful for the characterization of generated filler structures. The TEM images obtained for the as received Al₂O₃ and Al₂O₃-HBP particles dispersed in ethanol are compared in Fig. 4. The inset in Fig. 4(a) and 4(b) illustrates the identical selected area electron diffraction (SAED) patterns for both Al₂O₃ powders, which indicates that only the surface of the Al₂O₃ particles was modified by hyperbranched polyamide leaving the particle core structure intact. Comparison of TEM imaged Fig. 4(a) and 4(b) shows that the surface modification dramatically affects the aggregate particle size, which after grafted hyperbranched polyamide is reduced by more than 10 times from the micrometre to tens of nanometres scale. Fig. 4(c) depicts the HRTEM image of Al₂O₃-HBP particles. It can be obviously seen that a thin polymer layer was observed on the surface of Al₂O₃ particles. The success in preparing hyperbranched aromatic polyamide grafted on the surface of Al₂O₃ nanoparticles was further confirmed by the elemental signatures of C, Si, Al, and O in the EDS as shown in inset of Fig. 4(c). The peaks of Cu are from the copper grid. The aggregate particle size reduction by surface modification is further supported by the results of dynamic light scattering measurement. Fig. 5 shows that a peak with large particle size is observed for as received Al₂O₃ nanoparticle, indicating that the Al₂O₃-HBP nanoparticles have better dispersibility in ethanol.

Fig. 4 TEM images of particles. (a) As received Al₂O₃ particle, inset: corresponding SAED patterns. (b) Al₂O₃-HBP, inset: corresponding SAED patterns. (c) High-resolution TEM image of Al₂O₃-HBP, inset: corresponding EDS patterns.

Fig. 5 The particle size and its distribution of Al₂O₃ nanoparticles in ethanol, (a) as received Al₂O₃ particle and (b) Al₂O₃-HBP particle.

Dispersion of Al₂O₃-HBP particles in epoxy matrix

The morphology of the fractured surfaces for the neat epoxy and 15 wt% epoxy/Al₂O₃-HBP nanocomposite are showed in Fig. 6, respectively. In the case of the neat epoxy, the fractured paths of river line patterns can be observed, indicating brittle fracture. The nanocomposite fail in brittle manner due to a crack deflection mechanism caused by the Al₂O₃-HBP particles. This crack deflection will increase the roughness of the fracture surfaces due to the tilting and twisting of the crack front. The inset in Fig. 6(b) shows that no obvious naked particles and inorganic clusters are present, suggesting that covalent bonding exists between epoxide groups in the epoxy matrix and amine groups on the Al₂O₃-HBP particles. The covalent bonding provided strong interfacial interaction between Al₂O₃-HBP particles and epoxy matrix. Consequently, the hyperbranched aromatic polyamide grafted Al₂O₃ nanoparticles can significantly improve the dispersion and interfacial bonding in composites.

Fig. 6 SEM images of fractured surface of (a) neat epoxy and (b) 15 wt% epoxy/Al₂O₃-HBP nanocomposite. The inset in (b) is a magnification of the area indicated.

The good dispersion in the epoxy matrix is further supported by the TEM results. TEM can be used to observe the morphology of the nanocomposite, which indicates the dispersion abilities of 10 wt% as received Al₂O₃ particles and 10 wt% Al₂O₃-HBP particles in the epoxy matrix, as shown in Fig. 7. Fig. 7(a) shows the TEM images of the epoxy composites with 10 wt% as received Al₂O₃ particles, which show the form of large conglobations and agglomerates. The Al₂O₃-HBP particles exhibit better dispersion than as received Al₂O₃ particles in the epoxy matrix due to the existence of the amine groups on the surface of Al₂O₃ particles which results in reduced surface free energy and facilitates particle dispersion, as shown in Fig. 7(b). Though the Al₂O₃-HBP particles are not dispersed individually, it presents a high degree dispersion and the size of particle agglomerates is less or around 100 nm. No serious agglomerations, such as microsize agglomerates, are observed for the epoxy/Al₂O₃-HBP nanocomposite. As reported previously, the hyperbranched aromatic polyamide had a good solubility in organic solvents, such as DMF, DMAc, m-cresol, and methoxyethanol.^25,28 The good solubility of the hyperbranched aromatic polyamide in organic solvents can possess good dispersion in epoxy matrix, and the better interfacial adhesion of Al₂O₃-HBP particles in epoxy monomer exhibits the higher viscosity than neat epoxy resin, which can prevent Al₂O₃-HBP particle reaggregation during the curing process. In summary, the results of SEM and TEM indicate that Al₂O₃-HBP particles are well dispersed and possess good compatibility with the epoxy matrix.

Fig. 7 TEM images of (a) 10 wt% epoxy/Al₂O₃ nanocomposite and (b) 10 wt% epoxy/Al₂O₃-HBP nanocomposite. The insets in (a) and (b) are magnifications of the areas indicated.

Glass transition temperatures of nanocomposites

The DSC thermograms of cured neat epoxy and epoxy/Al₂O₃-HBP nanocomposites with different Al₂O₃-HBP contents are shown in Fig. 8. It is known that the glass transition temperature increases, keeping a good correlation with the crosslinking density.²⁹ Since the degree of crosslinking in the epoxy matrix is not uniform, the glass temperature transition occurs in a wide temperature range. Therefore, three characteristic temperatures, namely the onset temperature, the intermediate temperature, and the end temperature, are observed from the DSC spectra. Here the intermediate temperature is denoted as the glass transition temperature. As compared to the neat epoxy, an obvious increase of T_g is observed for epoxy/Al₂O₃-HBP nanocomposites, indicating that the improvement in crosslinking density in the epoxy matrix caused by the amino groups on the surface of Al₂O₃-HBP nanoparticles involving in a ring-opening etherification reaction. This effect can be understood in terms of decreasing the free volume.³⁰ Meanwhile, the T_gs of the epoxy/Al₂O₃-HBP nanocomposites were decreased with increasing the content of Al₂O₃-HBP nanoparticles. There could be the main reason that Al₂O₃-HBP nanoparticles agglomerate in the epoxy matrix. Once nanoparticles agglomerate, the interactions are more between Al₂O₃-HBP nanoparticles rather than Al₂O₃-HBP nanoparticles and epoxy matrix. The agglomerated Al₂O₃-HBP nanoparticles cannot impose any restrictions on the mobility of epoxy, and therefor higher loading of Al₂O₃-HBP nanoparticles results in the lower T_g.

Fig. 8 DSC curves of neat epoxy and epoxy/Al₂O₃-HBP nanocomposites with different Al₂O₃-HBP contents.

Thermal stability of nanocomposites

Thermal stability is very important for polymeric materials. In the present study, the thermal stability of the epoxy/Al₂O₃-HBP nanocomposites has been investigated by TGA by using heating rate of 20 °C min⁻¹. Fig. 9 shows the TG and DTG thermograms of the epoxy/Al₂O₃-HBP nanocomposites. The weight loss of the nanocomposites due to degradation is monitored as a function of temperature. The characteristic thermal parameters selected were the temperature for 5% weight loss and 50% weight loss and maximum degradation temperature, which is the highest thermal degradation rate temperature. The results are summarized in the Table 1. When the temperature was raised to above 250 °C, the neat epoxy matrix began to decompose and the decomposition temperature became higher when the Al₂O₃-HBP nanoparticles were added into the epoxy matrix. At same temperature, the TGA curves of the nanocomposites indicated that the weight loss of the nanocomposites was less than that of the neat of epoxy matrix. In other words, the epoxy/Al₂O₃-HBP nanocomposites have a higher decomposition temperature when the neat epoxy matrix loses the same weight. For instance, incorporation of 5 wt% Al₂O₃-HBP nanoparticles into the epoxy matrix significantly improves the thermal stability because it increases the temperature at 5% weight loss by 23.3 °C. The maximum degradation temperature (T_max) was also increased by addition of the Al₂O₃-HBP nanoparticles. It is worth pointing out that the thermal stability of epoxy/Al₂O₃-HBP nanocomposites was decreased with the increasing Al₂O₃-HBP contents. The epoxy/Al₂O₃-HBP nanocomposites exhibit step weight loss and, as expected, the thermal degradation of epoxy/Al₂O₃-HBP nanocomposites is two stages between 250 and 550 °C. The significant weight loss occurring in the first stage, between 250 and 400 °C, is attributed to the decomposition reaction of epoxy network.³¹ In the second stage, from 400 to 550 °C, is attributed to the degradation of hyperbranched aromatic polyamide. In addition, the amount of Al₂O₃ nanoparticles in the nanocomposites could be caculated accurately in the TGA plots. The experimental results show that the residual weight percent at 700 °C of the epoxy/Al₂O₃-HBP nanocomposites increases upon addition of increasing amount of Al₂O₃-HBP fillers.

Fig. 9 (a) TGA and (b) DTG curves of the epoxy/Al₂O₃-HBP nanocomposites.

Table 1 thermal properties of neat epoxy and epoxy/Al₂O₃-HBP nanocomposites

Al2O3-HBP (wt%)	Weight loss temperature/°C			Glass transition temperature/°C
	T 5%	T 50%	T max	T g (DSC)	ΔTg (DSC)	T g (DMA)	ΔTg (DMA)
0	323.1	374.2	379.8	191.7	—	204.9	—
5	346.4	384.3	388.1	205.6	13.9	218.1	13.2
10	338.3	385.2	385.2	200.6	8.9	216.0	11.1
15	341.1	383.8	384.7	198.3	6.6	208.7	3.8
20	326.2	383.7	370.7	197.2	5.5	208.1	3.2

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Thermal conductivity of nanocomposites

The thermal conductivity of epoxy nanocomposites was measured by the laser flash method, as shown in Fig. 10. Fig. 10(a) shows the variation of thermal conductivity of epoxy/Al₂O₃-HBP nanocomposite as a function of temperature.

Fig. 10 (a) Thermal conductivity of epoxy/Al₂O₃-HBP nanocomposite as a function of test temperature. (b) Thermal conductivity and thermal conductivity enhancement of nanocomposites with various contents at 25 °C.

The effective thermal conductivity for neat epoxy was found to be 0.236 W m⁻¹ K⁻¹ at 25 °C and increases with temperature over the temperature range investigated. The thermal conductivity of epoxy/Al₂O₃-HBP nanocomposites exhibits temperature dependences similar to the neat epoxy, although the values of 5 wt% epoxy/Al₂O₃-HBP nanocomposite and 20 wt% epoxy/Al₂O₃-HBP nanocomposite deviates from this trend at 100 °C. Meanwhile, the thermal conductivity of epoxy/Al₂O₃-HBP nanocomposites increases with increasing Al₂O₃-HBP content for all samples. The results are consistent with the general trend for highly disordered dielectric materials.³²

Fig. 10(b) compares the thermal conductivity and thermal conductivity enhancement of epoxy-based nanocomposites prepared with the as received Al₂O₃ particles and Al₂O₃-HBP particles at 25 °C. Here, λ enhancement (%) = (λ₁ − λ₀)/λ₀, λ₀ is the thermal conductivity of the neat epoxy matrix (0.236 W m⁻¹ K⁻¹) and λ₁ is the thermal conductivity of the nanocomposites due to the addition of fillers at 25 °C. The results show that there are a 69% enhancement in thermal conductivity for epoxy/Al₂O₃-HBP nanocomposite and only a 21% enhancement in thermal conductivity for epoxy/Al₂O₃ nanocomposite by adding the same quantity (20 wt%). Clearly, the Al₂O₃-HBP fillers provide substantially greater thermal conductivity enhancement when embedded into epoxy as compared to as received Al₂O₃ fillers. The experimental measurements of thermal conductivity for epoxy/Al₂O₃-HBP nanocomposites show higher thermal conductivity than epoxy/Al₂O₃ nanocomposites, which may be attributed to two reasons. First, the rigid linkage between Al₂O₃ and epoxy matrix which provides good interface compatibility that may reduce interfacial thermal resistance. Second, the good interface compatibility allows Al₂O₃-HBP particles to disperse well in the epoxy matrix. The Al₂O₃-HBP particles are well dispersed and possess good compatibility in epoxy matrix confirmed by the above SEM and TEM results. A model was proposed to present the efficient network for heat flow in the nanocomposite is shown in Fig. 11. The model of heat flow for nanocomposites shows the three phenomena as follows: (1) the hyperbranched aromatic polyamide layer on the surface of Al₂O₃ particles; (2) good dispersion leading to the higher contact area between Al₂O₃-HBP particles and epoxy; (3) the strong covalent bonding formation between Al₂O₃-HBP particles and epoxy matrix during the process of the epoxy curing reaction. As reported previously, the polymer layer on the surface of particles will decrease the scattering for the phonon scattering.³³ Thus, this can explain that better Al₂O₃-HBP particle dispersion causes greater enhancement of thermal conductivity in the epoxy nanocomposites. Moreover, the strong covalent bonding between Al₂O₃-HBP particles and epoxy matrix forms a heat flow network that can reduce the large number of interfacial thermal resistance throughout the epoxy/Al₂O₃-HBP nanocomposites. As a result, the epoxy/Al₂O₃-HBP nanocomposites exhibit higher thermal conductivities than the epoxy/Al₂O₃ nanocomposites.

Fig. 11 The model of heat flow for nanocomposites (a) epoxy/Al₂O₃ and (b) epoxy/Al₂O₃-HBP.

Thermomechanical properties of nanocomposites

Dynamic mechanical analysis (DMA) was performed to obtain the temperature dependent properties of materials such as the storage modulus and the tanδ, as shown in Fig. 12. The dynamic properties reflect the amount of the energy in the composite stored as elastic energy and the amount of energy dissipated during the strain process. These properties are highly dependent on the existence of fillers: dispersion within the matrix, volume fraction, geometrical characteristics, and load transfer from the filler to the matrix.³⁴Fig. 12(a) shows the storage modulus of the neat epoxy and its nanocomposites. The addition of Al₂O₃-HBP fillers to epoxy resin showed influence irregularly on storage modulus in the glass region. In contrast, there was a stronger effect of Al₂O₃-HBP fillers in the rubbery region at elevated temperatures where the improvement in the elastic properties of nanocomposites was clearly observed. This phenomenon can be explained in terms of interfacial interactions between the Al₂O₃-HBP fillers and epoxy. The covalent bonding formation between Al₂O₃-HBP particles and epoxy matrix reduced the mobility of the local matrix material around the Al₂O₃-HBP particles, increasing the thermal stability at elevated temperatures.

Fig. 12 (a) Storage modulus and (b) loss factors of epoxy/Al₂O₃-HBP nanocomposite as a function of test temperature.

The glass-transition temperature (defined as the temperature at which maximum tanδ is reached) is an indicator of the degree of crosslinking in the epoxy/Al₂O₃-HBP nanocomposites. The higher glass-transition temperature reflects a higher crosslinking density. As shown in Fig. 12(b), it is interesting to note the maximum value of tanδ for the epoxy/Al₂O₃-HBP nanocomposites is higher than that of the neat epoxy. Moreover, the magnitude of tanδ is lower when Al₂O₃-HBP fillers are incorporated, confirming the interaction between Al₂O₃-HBP particles and epoxy matrix again. The T_g of 5 wt% epoxy/Al₂O₃-HBP nanocomposite is 218.1 °C, about a 13.2 °C increase compared to that of neat epoxy. A comparison of the T_g values from the DMA and DSC data is obtained in Table 1. While the absolute magnitudes of T_g measured by these different methods differ, the trend in terms of changes of T_g from the neat epoxy are quite similar. It should also be noted that the glass transition is a diffuse thermodynamic transition and measurement method and sample preparation always affect the absolute value of T_g obtained.

Conclusions

In order to obtain uniform dispersion of Al₂O₃ nanoparticles in the epoxy matrix and combine the design of Al₂O₃-epoxy interfacial interaction with the demand for the improvement of the thermal properties of the nanocomposites, we carried out hyperbranched aromatic polyamide grafting of Al₂O₃ nanoparticles. In the grafting procedure, the Al₂O₃ nanoparticles were firstly treated with a silane coupling agent to generate amine groups on their surface, and then grafting of the hyperbranched aromatic polyamide started from the modified surface by solution polymerization. The results of FTIR, NMR and TGA showed hyperbranched aromatic polyamide grafted Al₂O₃ nanoparticles were successfully prepared by solution polymerization. TEM showed that there was a thin polymer layer on the Al₂O₃ nanoparticles surface, which contributes to the uniform dispersion of Al₂O₃ nanoparticles in epoxy matrix and the improvement of the interfacial interaction between Al₂O₃ nanoparticles and epoxy matrix. Thus, the T_gs, thermal stability and dynamic mechanical properties of the epoxy/Al₂O₃-HBP nanocomposites showed an obvious increase compared to the neat epoxy. In addition, the epoxy nanocomposites containing Al₂O₃-HBP nanoparticles exhibited higher thermal conductivity than those containing as received Al₂O₃ nanoparticles.

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