Studies on dielectric properties of GO reinforced bisphenol-Z polybenzoxazine hybrids

Abstract

In the present work, an attempt has been made to reduce the value of the dielectric constant of bisphenol-Z (BPZ) polybenzoxazine (PBz) material by the reinforcement of graphene oxide (GO) into BPZ-PBz matrix by exploiting the concept of polarization to enable them to be utilized as dielectrics in microelectronics applications. GO-BPZ-PBz hybrid materials have been developed using BPZ-Bz and benzoxazine functionalized graphene oxide (GO-Bz) via a facile one step copolymerization technique. The GO-Bz is expected to function as a versatile precursor for polymer grafting through the formation of chemical linkages with the base polymer. The molecular structure of benzoxazine monomers and hybrid polybenzoxazines were confirmed using ¹H and ¹³C NMR, FTIR spectroscopy and XRD patterns. The chemical composition of GO-Bz was characterized by X-ray photoelectron spectroscopy (XPS). Raman spectra were used to ascertain the graphitic nature of the carbon present in the hybrid matrix. The morphological properties of GO-BPZ-PBz have been explained using scanning electron microscope (SEM) and transmission electron microscope (TEM) images. Data obtained from dielectric studies infer that the value of the dielectric constant decreased with increasing the weight percentage of GO-Bz and the lowest value of the dielectric constant (k) of 1.95 was obtained for the 10 wt% GO-BPZ-PBz hybrid composite.

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Introduction

The design and development of low dielectric hybrid polymer composites as the interlayer dielectrics in microelectronic industries have recently led to the great interest in the reduction of resistance–capacitance (RC) time delays, crosstalk, and power dissipation for the next generation high density and high speed integrated circuits.¹ Subsequently, the proposed low k in prospective interlayer dielectrics must also satisfy a variety of requirements such as good thermal stability, chemical inertness, low moisture uptake, good adhesion to semiconductors and metals.² According to the semiconductor industry association (SIA) roadmap, microelectronic devices require low k materials with the value of the dielectric constant being less than 2.0.³ Silica and carbon based materials such as POSS, SBA-15, GO, etc., have been used as a reinforcement for polymeric materials to attain the requirements.^4–7 However, recently many efforts have been made to reduce the value of dielectric constant using polymers such as polyimide, polybenzoxazine, etc., by the introduction of such a porous silica materials.^8–10 It was also noticed that the benzoxazine in combination with suitable organic or inorganic materials have shown remarkable low dielectric behaviour. Hence, the dielectric manufacturers have been focused on the benzoxazine based hybrid materials. In our earlier report we have developed a new class of lamellar structured benzoxazine/POSS hybrid nanocomposites and obtained ultra-low dielectric constant (∼1.7).¹ Leu et al.⁸ reported that the POSS/polyimide nanocomposites with a reduced value of dielectric constant of about 2.3 when compared to that of neat polyimide. The porous PI-PMA-POSS film containing 23.5 mol% MA-POSS copolymer possesses a value of dielectric constant of about 2.2.¹¹ In addition, CNT/polybenzoxazine, CNT/polyimide and reduced graphene oxide (rGO) polymer composites exhibit higher dielectric constant, due to the conducting nature of rGO and CNT.^12–16 Whereas, the GO/polyimide composites display a very low value of dielectric constant of about 2.0, due to the insulating behavior of GO.^6,7

Hence, the reduction of dielectric constant of polybenzoxazine and GO hybrid materials has received enormous research interest because of their unique structural properties.¹² Benzoxazine precursors have been synthesized by the condensation of phenols with primary amine and formaldehyde using Mannich mechanism and subsequently polymerized through ring opening and addition polymerization by thermal treatment in the absence of any catalyst with release of no byproduct.^1,12 In recent years, the development of polybenzoxazine for various applications have been focused, due to its unique properties such as high dimensional stability, high char yield, low moisture uptake, good mechanical and thermal properties, low surface free energy and excellent dielectric behavior.⁴ With a view to improve the performance of polybenzoxazine, polymerizable alkenyl and methoxy silane groups have been introduced into the skeleton of benzoxazine monomers. These functionalized benzoxazines can be coupled with hydroxyl terminated fillers such as POSS, SBA-15, GO, CNT, etc.,^1,5,7,12 to obtain hybrid materials with improved properties. The benzoxazine terminated reinforcements are incorporated in to polybenzoxazine matrix by copolymerization techniques which forming cross linked networks that exhibit high thermal and mechanical stability, good moisture resistant and excellent dielectric properties. Among those nano-fillers GO has been considered as one of the most potential material for the development of hybrid composites especially for use in microelectronics as an insulator in aerospace applications.^7,17 GO has a range of reactive oxygen functional groups on its surface such as hydroxyl, epoxy and carbonyl groups, which renders the insulating behaviour, but the incremental reduction of oxygen can transform the materials (containing sp³ hybridized carbon) into semiconducting materials (containing sp² hybridized carbon).^6,18,19 Thus, the GO and rGO are considered as low-k and high-k dielectrics respectively and these are used to incorporate into polymers to prepare interlayer dielectrics for electronic devices. Moreover, the natural graphite flakes possess sp²-hybridized carbon atoms and are arranged in a honeycomb lattice in one layer and are stacked by the strong van der Waal's forces.²⁰ After the oxidation of graphite into GO, abundant functional groups (e.g., hydroxyl, carboxyl, epoxy, ketone, etc.) were introduced onto the GO layers, and simultaneously part of sp²-carbons were converted into sp³ ones, due to the cleavage of π bonds and the formation of σ bonds with different functional groups.²¹ The reinforcement of GO into the polymer matrix imparts an improved polymer compatibility, despite the previous reports infer the insufficient interactions between GO and polymer significantly reduces the actual performances of GO in the polymer matrix.⁷ Consequently, it requires the development of covalently bonded GO–polymer composites to enhance their thermo-mechanical properties of hybrid polymer composites. In addition, the presence of hydroxyl groups significantly enhances the moisture uptake, which can be reduced by the preparation of ether linked GO-benzoxazine (GO-Bz) through the condensation reaction of GO with triethoxysilane functionalized benzoxazine. Further the hydroxyl free ether linked GO-Bz considerably reduces the polarization throughout the matrix and also reduces the moisture uptake by decreasing intermolecular hydrogen bonding between GO and water molecules. Hence, an investigation of incorporation of GO into the polymer matrix concerning the dielectric properties and applications are warranted.

In the present work, an attempt has been made to develop a class of GO-bisphenol-Z polybenzoxazine (GO-BPZ-PBz) matrix by the copolymerization of different weight percentages of GO-Bz with bisphenol-Z benzoxazine (BPZ-Bz) with a view to reducing the value of dielectric constant. GO-Bz was synthesized through the formation C–O–Si bond by thermal condensation of hydroxyl group of GO with triethoxysilane functionalized benzoxazine monomer. The hybrid composites obtained are characterized by different analytical techniques and the data resulted are discussed and reported.

Experimental

Materials

Analytical grades of phenol, cyclohexanone, concentrated hydrochloric acid, concentrated sulfuric acid, acetic acid, aniline, paraformaldehyde, chloroform and toluene, were purchased from SRL, India and 3-aminopropyltriethoxysilane was purchased from Sigma-Aldrich and were used as received without any further purification. Potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), dimethylacetamide (DMAc), and sulfuric acid (H₂SO₄) were received from Spectrochem, India. Bisphenol-Z was synthesized from cyclohexanone and phenol as per our previous report.¹

Synthesis of bisphenol-Z benzoxazine (BPZ-Bz) (Scheme 1)

To a stirred solution of aniline (6.8 ml, 0.075 mol) in toluene, the paraformaldehyde (5 g, 0.167 mol) was added and stirred for 30 minutes at 0 °C. Subsequently, 10 g of bisphenol-Z (0.037 mol) was added slowly to the reaction mixture and stirred overnight at 80 °C. After the completion of reaction (monitored by TLC), the reaction mixture was extracted with ethyl acetate and washed with 2 N NaOH, water, brine and concentrated the organic layer to yield 95% brownish semi solid.

¹H NMR (400 MHz, CDCl₃) δ (ppm). 7.28–6.7 (m, 16H, Ar), 5.31 (s, 4H, O–CH₂–N), 4.58 (s, 4H, Ar–CH₂–N), 2.15–2.14 (m, 4H, cyclohexyl) and 1.51–1.49 (m, 6H, cyclohexyl).

¹³C NMR (400 MHz, CDCl₃) δ (ppm). 151.96–116.55 (aromatic carbons), 79.07 (–O–CH₂–Ar), 50.63 (–N–CH₂–Ar), 45.10–22.86 (cyclohexyl carbons).

Synthesis of triethoxysilane terminated benzoxazine (APTES-Bz) (Scheme 1)

In a typical experiment, to a stirred solution of paraformaldehyde (0.06 mol) in chloroform (30 ml), calcium hydride (CaH₂) was added under nitrogen atmosphere and the temperature was raised to 65 °C and then 3-aminopropyltriethoxysilane (0.03 mol) was added to the reaction mixture with vigorous stirring, followed by the addition of phenol (0.03 mol) and stirred for 3 h at the same temperature. After that the solid residues were filtered out and the filtrate was concentrated to yield 90% of transparent viscous liquid.

¹H NMR (400 MHz, CDCl₃) δ (ppm). 7.51–6.82 (m, 4H, Ar), 4.82 (s, 2H, O–CH₂–N), 3.98 (s, 2H, Ar–CH₂–N), 3.78–3.58 (m, 6H, O–CH₂–CH₃), 2.78–2.71 (t, 2H, N–CH₂), 1.81–1.69 (m, 2H, N–CH₂–CH₂), 1.19 (m, 9H, O–CH₂–CH₃), 0.68–0.61 (m, 2H, Si–CH₂).

Preparation of graphene oxide (GO) (Scheme 1)

GO was prepared from natural graphite by the process of Hummers method⁷ using a mixture of sodium nitrate, sulfuric acid, and potassium permanganate. To a suspended solution of natural graphite (10 g) in concentrated sulfuric acid in an ice bath at about 0 °C, sodium nitrate (5 g) was added gradually and stirred for 10 minutes. Followed by, potassium permanganate was added slowly to the reaction mixture at the same temperature. Then the solution was oxidized by thermal process, and the temperature was maintained at about 40 °C for 24 h. After that, distilled water was added slowly to the reaction mixture under the controlled temperature of about below 100 °C. Subsequently, 30% hydrogen peroxide and excess amount of distilled water were added for the termination of reaction and obtained precipitate was centrifuged and washed several times with water until the pH of about 7 and dried in a vacuum oven at 70 °C.

Preparation of benzoxazine functionalized graphene oxide (GO-Bz) (Scheme 1)

GO-Bz was obtained by refluxing 1.0 g of GO and 4.0 g of triethoxysilane functionalized benzoxazine in ethanol (100 ml) for 48 h. After that the product was cooled and filtered, subsequently washed with ethanol and dried overnight at 70 °C under vacuum.

Preparation of neat polybenzoxazine (PBz) matrix

In a glass mold, solution of BPZ-Bz in tetrahydrofuran (THF) was heated at 100 °C overnight to evaporate the solvent and then cured stepwise at 120, 140, 160, 180, 200 and 220 °C for 1 h each and obtained dark brown film.

Preparation of graphene oxide-bisphenol Z polybenzoxazine (GO-BPZ-PBz) composites (Scheme 2)

The varying weight percentages of GO-Bz (1, 3, 5, 7, 10 and 15 wt%) were added to a solution of BPZ-Bz (2 g) in 10 ml THF and stirred for 30 min at 30 °C. The solutions were poured into respective glass mold and heated at 100 °C for 3 h and then cured stepwise at 120, 140, 160, 180, 200 and 220 °C for 1 h each, to obtain hybrid composite material.

Characterization

¹H NMR spectra were recorded on a Brucker-300 NMR spectrometer. Fourier-transform infrared (FTIR) spectra of KBr disks were obtained using a Bruker Tensor 27 FT-IR spectrophotometer. The X-ray diffraction analysis of the samples was carried out using a Rigaku, miniflux II-C X-ray diffractometer (30 kV, 20 mA) with a copper target (1.54 Å) at a scan rate of 4° min⁻¹. A high resolution X-ray photoelectron spectrometer (XPS) (ESCA pHI 1600, Physical Electronics, Lake Drive East, Chanhassen, MN, USA) was used to detect the presence of surface elements. Thermogravimetric and differential scanning calorimetric analyses of polybenzoxazine films were carried out with a Exstar 6300 at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. The surface overview of the composites was identified from FEI QUANTA 200F high resolution scanning electron microscope (HRSEM). Samples required for the high resolution transmission electron microscopy (HRTEM) analysis were first dispersed in ethanol and sonicated for 15 minutes. After that, the dispersion was dropped over the mesh of 200 copper nets. HRTEM images were captured using TECNAI G₂ S-Twin transmission electron microscope, with an acceleration voltage of 250 kV. Raman spectra were measured on a Lab RAM HR UV-Vis-NIR Raman Microscope from HORIBA Jobin-Yvon (785 nm laser source). Dielectric constant was determined by Broad band Dielectric Spectrometer (BDS), NOVOCONTROL Technologies GmbH & Co. (model concept 80) at 30 °C.

Results and discussion

To investigate the dielectric behaviour of GO reinforced BPZ polybenzoxazine matrix, the varying weight percentages of GO-Bz were copolymerized with BPZ-Bz and further the value of dielectric constant and dielectric loss of resulting polymer composites were ascertained. To prepare GO-BPZ-PBz, the precursors were synthesized and confirmed by the FTIR and NMR spectral analysis. The BPZ-Bz monomer was synthesized as per our earlier report¹ as shown in Scheme 1. Fig. 1a shows FTIR spectra of BPZ, a broad band at 3400 cm⁻¹ represents hydroxyl group of BPZ. The appearance of bands at 2931 cm⁻¹, 2853 cm⁻¹ and 819 cm⁻¹ are assigned to the symmetric and asymmetric stretching C–H bonds, and para-substituted benzene rings, respectively. The BPZ-Bz shows (Fig. 1a) the disappearance of band at 3400 cm⁻¹ indicates the absence of hydroxyl group and the bands related to N–C–O and C–O–C of benzoxazine ring can be seen at 948 cm⁻¹ and 1232 cm⁻¹, respectively. Fig. 2 shows the ¹H and ¹³C NMR spectra of BPZ-Bz, the presence of respective proton and carbon peaks as described in the experimental part confirms the successful formation of BPZ-Bz.¹

Scheme 1 Synthesis of BPZ-Bz and APTES-Bz and preparation of GO, GO-Bz.

Fig. 1 FTIR spectra of BPZ, BPZ-Bz (a) and GO, GO-Bz (b).

Fig. 2 ¹H and ¹³C NMR spectra of BPZ-Bz.

The GO containing high density oxygen was prepared from graphite by Hummers method as per the earlier report.⁷ Fig. 1b shows the FTIR bands at 3400, 1715, and 1045 cm⁻¹ represents the hydroxyl, carbonyl, and C–O stretching frequencies of GO, respectively. To prepare GO-Bz, APTES-Bz was synthesized and confirmed by ¹H NMR spectra. The appearance of peaks at 4.82 and 3.98 ppm are associated to O–CH₂–N and Ar–CH₂–N respectively, which indicate the formation of benzoxazine ring. Subsequently, the GO-Bz was characterized by FTIR spectra and is shown in Fig. 1b. The bands related to N–C–O, and C–O–C can be seen at 932 cm⁻¹ and 1233 cm⁻¹, respectively. The appearance of bands at 1112 and 1021 cm⁻¹ associated to the stretching and bending vibrations of Si–O–C bonds which indicate the successful functionalization of benzoxazine with GO. Scheme 2 shows the schematic representation of formation of GO-BPZ-PBz matrix and it was characterized by FTIR spectra. Fig. 3 shows the FTIR spectra of GO-BPZ-PBz, the disappearance of bands related to N–C–O bond at 948 cm⁻¹ indicate the cleavage of benzoxazine ring and the formation of polybenzoxazine matrices. The cured polybenzoxazines were obtained as a film and the photograph of a film was presented in Scheme 2.

Scheme 2 Preparation of GO-BPZ-PBz and photograph of GO-BPZ-PBz film.

Fig. 3 FTIR spectra of BPZ-PBz and GO-BPZ-PBz composites.

X-ray photoelectron spectroscopy (XPS) was used to elucidate the surface composition of GO and GO-Bz. The survey scans of GO (Fig. 4a) exhibits two peaks related to O 1s (534.1 eV) and C 1s (285 eV). Fig. 4b shows the survey scans of GO-Bz which exhibits the additional elemental peaks such as N 1s (401.4 eV), Si 2s (154 eV) and Si 2p (104.2 eV) including C 1s (284.7 eV) and O 1s (534.5 eV), which were attributed to the composition of APTES-Bz grafted GO, confirming the successful functionalization of benzoxazine on the surface of GO. Detailed analysis of deconvolated XPS spectra (Fig. 5) of GO-Bz provides clear evidence that the GO was chemically modified with benzoxazine. Fig. 5 shows the binding energy of different chemical bonding exist in the GO-Bz. The deconvolution of C 1s spectra (Fig. 5a) indicates that the concentration of sp³ hybridized C–C bond is higher than that of sp² hybridized C=C bond. Hence, the higher concentration of less polarizable C–C bonds significantly reduces the value of dielectric constant in the hybrid GO-BPZ-PBz composites. In addition the existing chemical bonds such as C–N, C–Si, C–O and Si–O evidently support that the successful grafting of APTES-Bz to GO.

Fig. 4 XPS survey scans of GO (a) and GO-Bz (b).

Fig. 5 C 1s (a), O 1s (b), N 1s (c) and Si 2p (d) deconvolution XPS spectra of GO-Bz.

The structural modifications of GO-BPZ-PBz hybrids were investigated by XRD analysis and are shown in Fig. 7. Fig. 6 shows the XRD pattern of GO, the appearance of diffraction peak at 11.6° and the disappearance of peak at 26.5° illustrates the complete conversion of graphite into GO.^7,17 In addition, the increasing interlayer distance from graphite (d ∼ 0.33 nm) to GO (d ∼ 0.76 nm) indicates the successful formation of typical oxidized GO with distinct distance.²² The d-spacing values were calculated from the characteristic diffraction angle using Bragg's equation (λ = 2d sin θ).²³ The graphene oxide exhibits a broad diffraction peak at 10.46° with an interlayer distance of 0.85 nm. This broad interlayer distance infers that the graphite is totally transformed into GO, and that the existence of hydroxyl and epoxy groups were significantly enlarged the distance between the graphene layers which is higher than that of graphite (0.32 nm).²²

Fig. 6 XRD pattern of GO and GO-Bz.

The XRD pattern (Fig. 6) of GO-Bz shows two broad peaks in the ranges from 8.2° to 13.3° and 14.1 to 25° corresponds to the GO and benzoxazine respectively, whereas the neat GO shows the sharp peak at 10.46°. Consequently, the GO-Bz was slightly shifted to lower angle region and exhibits little higher interlayer space than that of GO indicates the successful incorporation of benzoxazine into the GO. In addition, the crystallinity inherently decreased from graphite to GO and to GO-Bz, it might be due to the incorporation of oxygen and benzoxazine group. Further, the varying weight percentages of GO-Bz were copolymerized with BPZ-Bz to form GO-BPZ-PBz which shows a significant distinct diffraction pattern and can be seen in Fig. 7. The benzoxazine monomer shows a predominant amorphous phase. Whereas, the hybrids with the concentration of GO-Bz significantly enhanced their peak intensity, in particular, 10 wt% of GO-Bz hybridized polymer composites exhibits higher peak intensity. Besides, the small gaps existing between the layers which were filled by air/vacuum that reduces the value of dielectric constant to a significant extent.

Fig. 7 XRD pattern of BPZ-PBz and GO-BPZ-PBz composites.

To assess the surface morphological properties of neat BPZ-PBz, GO and GO-BPZ-PBz composites, the SEM images were carried out using scanning electron microscope. The microstructure of BPZ-PBz (Fig. 8a) shows the dense morphology with a large number of voids, whereas, the GO (Fig. 8b) exhibits as a sheets with some surface wrinkle. 10% GO-BPZ-PBz (Fig. 8c) shows a smooth surface with some crumbled morphology, which indicates the existence of GO sheets in the polybenzoxazine system.

Fig. 8 SEM images of BPZ-PBz (a), GO (b) and 10% GO-BPZ-PBz composite (c).

The internal microstructure of hybrid composites has been examined using HRTEM images. Fig. 9 shows the TEM images of 10 wt% GO-BPZ-PBz hybrids which shows the homogeneous distribution of GO in the polybenzoxazine hybrids which infer the successful incorporation of GO into the BPZ-PBz system. It is possible that the observed dark regions are associated with GO and the less intense regions correspond to BPZ-PBz. Thus the GO sheets have been arranged with distinct distance to form desired microstructure during the copolymerization between GO-Bz and BPZ-Bz.

Fig. 9 TEM images of 10% GO-BPZ-PBz composite (a & b).

The thermal properties of GO-PBz hybrids were studied by thermogravimetric analysis and the results are shown in Fig. 10. The thermal stability of polybenzoxazine matrix is also one of the important factors to be considered in the device fabrication. TGA is a powerful tool to determine the thermal stability of materials, the quantity of removal of organic substances can be determined by the measurement of weight loss, because the covalent bonds exist in the links between the graphene sheet and its substituents are thermally stripped off in the higher temperature.⁷ As per the earlier report, GO has significantly lower thermal stability, due to the decomposition of functional groups (epoxy, hydroxyl, carboxyl) of GO occurs at below 200 °C.²⁴ Typically, the neat PBz exhibit higher thermal stability and lower char yield. Although, the GO can decomposed easily and may serve as catalyst¹⁷ for the degradation of PBz and thus the GO-PBz composites exhibits slightly lower decomposition temperature and higher char yield than that of neat BPZ-PBz and are listed in Table 1. In detail, the initial weight loss below 250 °C is probably due to the removal of adsorbed moisture and residual solvents. The major weight loss above 300 °C is attributed to the removal of oxygen functionalities and further degradation was assigned to the degradation of hybrid polymer network which is consistent with our earlier investigation.^1,17 However, the incorporation of GO into the PBz matrix is a facile method to fabricate the high performance composites. In order to understand the thermal polymerization of benzoxazine the typical DSC analysis was performed and the results are shown in Fig. 11 and 12. The BPZ-Bz and GO-Bz monomers shows (Fig. 11) a broad exothermic peak maximum at 215 °C and 220 °C respectively indicates the presence of an exothermic reaction which is probably due to the occurrence of ring opening and addition polymerization of benzoxazine monomers. As per the earlier reports, the presence of hydroxyl and carboxylic acid groups in GO could not only decrease the cure temperature, but also increase the cure rate of Bz.²⁵ Despite, during the surface modification of GO with benzoxazine functionalized triethoxy silane, the hydroxyl and carboxyl groups of GO were co-condensed with triethoxy silane to form ether (C–O–Si–) and ester (COO–Si–) groups. These ether and ester functionalities may not affect in the process of curing of benzoxazine. However, it is very difficult to understand the high curing temperature of GO-Bz. The previous reports states that the benzoxazine functionalized POSS shows higher curing temperature (210 °C and 224 °C).²⁶ Consequently, the formation of silica layer may increase the curing temperature of benzoxazine. Hence, the surface modification of GO with triethoxy silane based compounds might be contribute to the increase of cure temperature of GO-Bz. In addition, thermally cured neat BPZ-PBz and GO-BPZ-PBz hybrids exhibit (Fig. 12) an absence of exothermic peak indicates that the complete polymerization of benzoxazine monomers.

Fig. 10 TGA curve of BPZ-PBz and GO-BPZ-PBz composites.

Table 1 Weight loss, char yield (Yc), dielectric constant and dielectric loss of neat BPZ-PBz and GO-BPZ-PBz composites

Sample	T10 (°C)	Yc (%)	Dielectric constant (ε′)	Dielectric loss (ε′′)
BPZ-PBz	316.5	12.2	3.49 ± 0.01	0.029 ± 0.001
1% GO-BPZ-PBz	301.3	21.6	3.24 ± 0.01	0.025 ± 0.001
3% GO-BPZ-PBz	293.5	28.5	2.91 ± 0.01	0.022 ± 0.001
5% GO-BPZ-PBz	291.8	29.6	2.45 ± 0.01	0.016 ± 0.001
7% GO-BPZ-PBz	287.2	35.2	2.23 ± 0.01	0.011 ± 0.001
10% GO-BPZ-PBz	282.0	39.6	1.95 ± 0.01	0.007 ± 0.001
12% GO-BPZ-PBz	—	—	2.11 ± 0.01	0.013 ± 0.001

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Fig. 11 DSC profile of BPZ-Bz and GO-Bz monomer.

Fig. 12 DSC profile of BPZ-PBz and 10% GO-BPZ-PBz composites.

The role of GO in the reduction of dielectric constant was investigated by the Raman spectra and the results obtained are shown in Fig. 13 and 14. Generally, the less polarization of the matrix reduces the capacitance between the electrodes.⁷ GO is an insulating material which can be used to make the composites with polymers with a view to reduce the value of dielectric constant.^6,7,27 The earlier reports suggested that the perfect graphite has a single Raman active mode at 1575 cm⁻¹ named as G band arises from E_2g mode of sp² carbons, whereas the nanocrystalline diamond has a single band at 1180 cm⁻¹ arises from T_2g mode of sp³ carbons, but the GO exhibits both D and G band at 1308 and 1610 cm⁻¹ respectively.^24,28–30 Fig. 13 shows the Raman spectra of GO, the appearance of peak at 1610 cm⁻¹ (G band) and 1308 cm⁻¹ (D band) concomitant to sp² sites, because the excitation resonates with π states. According to Ferrari and Robertson, the G peak is due to bond stretching of all pairs of sp² atoms in both rings and chains, whereas the D peak is due to the breathing modes of A_1g symmetry which is forbidden in perfect graphite and only becomes active in the presence of disorder.²⁹ The disordered sites were produced during the oxidation of graphite into GO, and simultaneously part of sp²-carbons were converted into sp³ ones. These sp² to sp³ bond modifications significantly reduces the polarization of the material and the dielectric constant as well because of those amendments reduces the conjugation and confines π-electrons.

Fig. 13 Raman spectra of GO and GO-Bz.

Fig. 14 Raman spectra of GO-BPZ-PBz composites.

The Raman spectra (Fig. 13) of GO-Bz shows the peaks at 1313 cm⁻¹ and 1592 cm⁻¹ corresponds to D and G bands which were slightly shifted from their corresponding GO peaks at 1308 (D band) and 1610 cm⁻¹ (G band) respectively. The intensity of D band for GO and GO-Bz (Fig. 13) are higher than that of G band due to the formation of disordered sites.^31,32 Moreover, Fig. 14 shows the Raman spectra of various weight percentages of GO reinforced BPZ-PBz hybrids, the retaining peak positions of D and G bands infer the presence of GO in their hybrids without any disruption. In addition, GO-BPZ-PBz hybrids have the lower intensity of G band than that of D band indicates the presence of higher amount of disordered sites and smaller the sp² hybridization which could contribute to the reduction of value of dielectric constant by the way of reducing the polarization throughout the hybrid matrix, due to the existence of more circuitous route or completely constrained in narrow spaces inside the GO network.

Fig. 15 shows the dielectric constant versus frequency spectra of neat BPZ-PBz and varying weight percentages of GO-BPZ-PBz hybrids and the values are listed in Table 1. Interestingly, the neat polymer showed a relatively low k value of 3.49 and this may be due to the presence less polar cyclohexyl group that reduces the polarization which in turn significantly reduces the k-value of BPZ-PBz. In the present work, with an increase in the concentration of GO-Bz up to 10 wt% the corresponding k values are decreased, beyond 10 wt% of GO-Bz incorporation, the reverse trend was noticed. 12 wt% GO-BPZ-PBz possesses the k value of about 2.11, it may be due to the agglomeration of GO.³³ However, the optimum concentration of 10 wt% of GO-Bz exhibits the lowest value of dielectric constant of 1.95 at 1 MHz. Furthermore, the interlayer distance also contributes to the reduction in the value of dielectric constant as described above in the XRD part, where the polybenzoxazine formed with entangled networks between the GO sheets with distinct interlayer distance as represented in Scheme 2. The morphological studies also evidently support that the formation of desired microstructure with homogeneous distribution of GO in the GO-BPZ-PBz hybrids. From the above observation it can be clearly ascertained that the value of dielectric constant of GO-BPZ-PBz hybrid composites is mainly depends on the presence of GO content which significantly contributes to the reduction of polarization throughout the matrix and influence the structural rearrangements. In addition, the less polarization behaviour of GO-BPZ-PBz hybrids might be due to the modifications of sp² to sp³ carbon atoms and the insertion of air/vacuum between the distinct layered microstructure. Dielectric loss is one of the key factors in understanding the utilization of dielectric material in the integrated circuits. Typical frequency versus dielectric loss curves for GO-BPZ-PBz hybrids are shown in Fig. 16. The observed value of dielectric loss is ultimately very low (0.0073) at 1 MHz for 10 wt% of GO-BPZ-PBz composites which contributes to the reduction of cross talk and power consumption.³⁴ From this investigation, it is suggested that the design of GO-BPZ-PBz hybrids could be effectively used as interlayer dielectric material for upcoming microelectronic devices.

Fig. 15 Dielectric constant of BPZ-PBz and GO-BPZ-PBz composites.

Fig. 16 Dielectric loss of BPZ-PBz and GO-BPZ-PBz composites.

Conclusion

The low k material has been developed with a view to reducing the value of dielectric constant, through facile one step copolymerization of GO-Bz and BPZ-Bz. GO-Bz is considered as a versatile starting platform for the fabrication of composite films through the grafting of benzoxazine functionalized trisilanol onto the GO surface. GO possess both sp² and sp³ carbon atoms and the ratio of these carbon atoms were contributes to the reduction of polarization and dielectric constant as well. Data resulted from dielectric studies, it was observed that the incorporation of GO-Bz into BPZ-PBz effectively reduces the value of dielectric constant up to 10 wt% and beyond this concentration the reverse trend was noticed. It was further inferred that the value of dielectric constant of 10 wt% GO-BPZ-PBz hybrid composite reduces the value by 44% when compared to that of neat BPZ-PBz matrix. The low k value was mainly attributed to less polar nature of the composites and the existence of pores between the interlayer of the hybrid materials. It is concluded that this kind of GO filled polybenzoxazine composites can be used as a potential dielectric material for the next generation microelectronics device fabrications.

Acknowledgements

The authors thank Dr K. Gunasekaran and Mr M. Kesavan, Dept. of Crystallography and Biophysics, University of Madras, for providing the NMR facility.

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