Flexible-capron toughened epoxy/graphene nanocomposites for high k dielectric and ultraviolet radiation-resistant applications

Abstract

In the present work, flexible hybrid nanocomposites with high k dielectric and UV resistant behaviour have been developed and studied using capron-toughened epoxy resin with different weight percentages (wt%) of graphene oxide (GO). Among the wt% of GO, 0.7 wt% nanocomposite possesses the highest value of dielectric constant. 1 wt% GO nanocomposite has the highest retention of tensile strength (93.35%) with graphene morphology, even after 168 h UV exposure, proving that its 2D graphene network serves as a passive protective layer against UV radiation. High dielectric behaviour and efficient retention of strength properties suggest that the flexible and light-weight hybrid nanocomposite material developed in the present work can be used as potential capacitors in microelectronics for satellites operating at low earth orbit.

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Introduction

Printed circuit boards (PCB) constitute both active and passive components with the active component creating transient voltages as the amount of current drawn changes.¹ Normal operation of the other components present on the board may interfere with the transient noise and induce currents, which results in electromagnetic interference as delta-I noise. Decoupling the capacitors usage avoids such discrete problems; however, high speed digital applications need a higher number of decoupling capacitors, which also occupy a larger area in the PCB; thus, they can reduce the product reliability. Embedded capacitor technology is a good alternative for decoupling capacitors with a higher component density, namely, by eliminating surface mounted components and embedding into the substrate boards with enhanced electrical performance, increased design flexibility and improved reliability.²

Thermosetting polymers find their own place as dielectric materials accordingly as high dielectric and low dielectric polymers with respect to the reinforcing components. Electrical performances of such polymer resins can be improved with fillers which reinforce them and make them promising candidates for use in embedded capacitor technology.³ Metal–polymer composites and ceramic–polymer composites used in embedded capacitors. However, limitation such as low adhesion strength, poor processibility, high density, detrimental mechanical and laminating behaviors restrict their use in light-weight aerospace applications.⁴

In this context, light-weight carbon–polymer composites are expected to possess promising advantages and to be suitable for high dielectric applications. Graphene is a carbon-based monolayer material with sp²-hybridized carbon atoms arranged in a two-dimensional honeycomb-like crystalline structure with hydroxyl and epoxy functionality on its basal planes; moreover, it is found to possess excellent conductivity, inherent mechanical properties and high thermal stability. Compared to other reinforcement materials, graphene oxide is found to exhibit an enhanced physicochemical behavior with light-weight components at much lower loading than that of other metal and ceramic reinforcements.⁵

To date, polyimides, copolyimides and polyimide–carbon nanotube composite have been used as environmentally durable materials for spacecraft applications.^6,7 Polyhedral oligomeric silsesquioxane (POSS) reinforced polybenzoxazine/epoxy (PBZ/EP) nanocomposites can be used as light-weight and high-strength nanocomposites for space and nuclear applications.⁸

To facilitate the light-weight matrix, capron (pk4) was introduced into the epoxy (EP) matrix and was expected to provoke inherent comprehensive mechanical and electrical behaviors with inherent flexibility. Capron (pk4) reacts with the oxirane ring functionality of both epoxy and graphene oxide, and forms cross-linked polyamide linkage segments. Thus, the polyamide network obtained from the capron offers a flexible and resilient cross-linking to the epoxy matrix. In the present study, capron-toughened epoxy resin reinforced with GO was developed. To the best of our knowledge, no reports are available with regards to GO-reinforced epoxy/capron (EP/pk4) hybrid nanocomposites for application in embedded capacitors under space environments with high dielectric behavior and UV radiation resistant behaviour.

Experimental section

Materials

Diglycidyl ether of bisphenol A (DGEBA epoxy resin) (LY556) was utilized as the matrix resin in the present study, and it was received from Ciba-Geigy Ltd. Capron pk4 (ε-caprolactam) was received as a complimentary sample from SRF Limited India. Natural graphite was purchased from Active Carbon India Pvt. Ltd., India. Sulfuric acid (H₂SO₄, 95–98%) was purchased from Ranbaxy Chemicals, India. Hydrogen peroxide (H₂O₂), was purchased from SD Fine Chemicals, India. Potassium permanganate (KMnO₄) was purchased from Merck. Sodium nitrate (NaNO₃) and diphenyldiaminomethane (DDM) were purchased from SRL chemicals.

Synthesis of graphene oxide reinforced capron-toughened epoxy composite

The graphene oxide was synthesised in accordance with a previous report.⁹ The varying weight percentages of graphene oxide (0.1%, 0.3%, 0.5%, 0.7%, and 1.0%) and the epoxy and capron blend of 50/50 wt% were mechanically agitated for 24 h at 60 °C to achieve the required homogeneity. Furthermore, the stoichiometric quantities of diaminodiphenylmethane (DDM) required to cure the epoxy resin were added and transferred to the preheated mould (120 °C). Curing was initiated at 120 °C for 2 h and post-cured at 180 °C for 3 h to obtain the defect-free nanocomposites.

Characterization

Fourier transform infrared (FT-IR) spectra for the samples were recorded on a Perkin Elmer 6X FT-IR spectrometer. The glass transition temperature (T_g) of the samples was determined using a DSC 200 PC differential scanning calorimeter (DSC) (Netzsch Gerateban GmbH). A thermo gravimetric analysis (TGA) was carried out using the DSTA 409 PC analyzer (Netzsch Gerateban GmbH). Raman spectra were measured on a Lab RAM HR UV-VIS-NIR Raman Microscope from HORIBA Jobin-Yvon (633 nm laser source). The dielectric studies of the neat epoxy and GO-epoxy/capron nanocomposites were determined with the help of an impedance analyser, namely, the Solartron impedance/gain phase analyzer (1260) at RT using platinum (Pt) electrode at 30 °C at a frequency range of 1 MHz. This experiment was repeated four times under the same conditions. X-ray diffraction patterns were recorded at room temperature by monitoring the diffraction angle 2θ from 0° to 10° as the standard, on a Rich Seifert (Model 3000) X-ray powder diffractometer. A JEOL JEM-3010 analytical transmission electron microscope, operating at 80 kV with a measured point-to-point resolution of 0.23 nm, was used to characterize the phase morphology of the developed nanocomposites. TEM samples were prepared by dispersing the composites in ethanol mounted on carbon-coated Cu TEM grids and dried for 1 h at 70 °C to form a film of <100 nm. The radiation resistant behaviour was studied using a HML compact-LC-MC-812, Heber multi lamp (12 nos) type photo reactor at room temperature. The power was 220/230 V, AC 1/4 50 Hz and the wavelength was 365 nm. The tensile strength was studied as per the ASTM-D3039 using an Instron testing machine (Model 6025, UK) at 10 mm per minute cross-head speed, using a specimen with a width of 25 mm, a length of 150 mm, and a thickness of 2 mm. A distance of 100 mm was maintained in between the grips. Five specimens were tested for each sample.

Results and discussion

GO was prepared from graphite by the Hummers method and confirmed using XRD and FT-IR and Raman spectra, as illustrated in Fig. 1a and b. In the diffractogram (Fig. 1a) of the pristine graphite sample, the peak appears at 26.5°, indicating the (002) plane with an interlayer spacing of 0.34 nm, whereas the (002) plane of the graphene appears at 11.7° with an increase in interlayer spacing of 0.75 nm. The shifting of the (002) plane of graphite is assigned to the formation of typical GO and confirms the exfoliation of graphite during the oxidation chemical process. Fig. 1b presents the FT-IR spectrum of GO and illustrates the presence of O–H stretching at 3300–3500 cm⁻¹ as a broad peak, a C=O stretching at 1717 cm⁻¹ for the oxygen functionalities and C–O stretching at 1045 and 1204 cm⁻¹. Similarly, the Raman spectra (Fig. 1c) illustrate the first-order scattering of the E_2g mode G band at 1585 cm⁻¹, and the D band at 1393 cm⁻¹ indicates the size reduction of the sp² domain, which is due to the extensive oxidation that occurred during Hummers method.

Fig. 1 (a) XRD diffraction (b) FT-IR spectra and (c) Raman spectra of graphite and graphene oxide.

Fig. 2a and b illustrate the FT-IR spectra of the EP/pk4 matrix and the graphene oxide (GO) reinforced EP/pk4 nanocomposites. The peaks appearing at 1644 and 1541 cm⁻¹ are attributed to amide (–CO–NH–) formation between the acid group of the GO and the amide group of the capron. The absence of a peak at 914 cm⁻¹ confirms the ring-opening polymerization of the epoxy resin with capron and GO. After thermal curing, a new peak appeared at 1724 cm⁻¹, attributed to the formation of the amide group and indicates the curing reaction between the epoxy and capron.^10,11

Fig. 2 FTIR spectra of (a) neat EP/pk4 matrix (b) GO-EP/pk4 nanocomposites.

The DSC analyses of neat and reinforced composites are illustrated in Fig. 3a. Among them, the neat pk4 toughened EP matrix exhibits the glass transition temperature only at 129 °C, whereas with the incorporation of graphene oxide, T_g value begins to increase (Table 1).

Fig. 3 (a) DSC analysis and (b) TGA profile of neat EP/pk4 and GO-EP/pk4 nanocomposites.

Table 1 Thermal properties of neat EP/pk4 and GO-reinforced EP/pk4 nanocomposites

Sample	Tg (°C)	Char yield at 700 °C (%)
Neat EP/pk4	135	0.00
0.1% GO-EP/pk4	140	0.78
0.3% GO-EP/pk4	154	12.97
0.5% GO-EP/pk4	161	14.60
0.7% GO-EP/pk4	165	16.63
1.0% GO-EP/pk4	166	18.26

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This phenomenon was due to the increased cross-linking density occurring between the reinforced GO and the epoxy matrix as well as with toughened polyamide. The restricted chain mobility of the matrix is due to the inter-flake interaction and π–πi stacking between the GO with EP and pk4. Grafting a pk4 yields polyamide chain (PA6) over the surface are factors responsible for the rise in the values of T_g. The neat capron-toughened epoxy matrix was found to exhibit a poor thermal stability whose char yield became zero at 410 °C.

This is due to the presence of a higher aliphatic content of capron in the resultant matrix. However, due to the incorporation of GO into the matrix, the thermal stability improves significantly. For example, 1.0 wt% GO-reinforced composites shows 30% of the char yield at 700 °C. This may be explained by the initial stage of degradation occurring below 200 °C; moreover, the onset of degradation may be attributed to the decomposition of water molecules, unreacted hydroxyl groups, and carbonyl groups of acid from the composites. However, the second stage degradation below 420 °C is primarily due to the degradation of flexible amide segments of polyamide (PA6). The degradation behavior of capron-toughened graphene-reinforced epoxy composite is close to that of neat PA6 (430 °C), which indicates and confirms that the weight loss of this stage is due to the decomposition of grafted PA6 chains.

The dielectric constants of the GO-EP/pk4 capron nanocomposites are illustrated in Fig. 4a. With increase in the GO reinforcements, the values of the dielectric constant of neat EP/pk4 3.91 increases to 4.94 for 0.1 wt% material at 1 MHz and increases subsequently to 5.24, 6.82 and 11.01 with increasing concentration of GO as 0.3, 0.5 and 0.7 wt%, respectively. However for 1.0 wt% GO-EP/pk4, the value of dielectric constant decreased to 6.10. An improvement in the average electric field of the polymer matrix, attained with the reinforcement of GO contributes to the enhanced value of the dielectric constant. Beyond 0.7 wt% of GO, the decrease in the value of the dielectric constant may be due to the poor dispersion of the reinforcement. Homogenously dispersing GO into the EP/pk4 matrix creates a local amplified electric field and thus contributes to an enhanced capacitance value for the nanocomposites.^12–16 However, the relative increase in dielectric constant with the increase in GO content is found to decrease with the increase in frequency, which is attributed to be caused by the leakage current from the long-range transport of charge carrier in the low frequency range.¹⁶

Fig. 4 (a) Dielectric constant, (b) dielectric loss, and (c) conductivity behavior of neat EP/pk4 and GO-reinforced EP/pk4 nanocomposites.

The dielectric loss tangent of the neat EP/pk4 and GO-EP/pk4 capron nanocomposites are illustrated in Fig. 4b. At 1 MHz, the tan δ value for neat EP/pk4 is about 0.035, whereas in the case of 0.7 wt% of GO-EP/pk4, the tan δ value is 0.002 at 1 MHz. It is well known that there are three factors responsible for the dielectric loss of materials, namely, direct current (DC) conduction, space charge migration (interfacial polarization contribution), and the movement of molecular dipoles (dipole loss). Therefore, the lowering of the dielectric loss tangent of a GO reinforced EP/pk4 can be explained by the formation of an insulating outer layer to the dielectric centre, which thus results in a restricted migration and accumulation of special charges within the nanocomposites. Moreover, the restricted movement of the molecular dipoles due to the surface functionality of GO, permits better bonding sites with the EP/pk4 matrix, which then improves the dispersion and enhances the interfacial adhesion of the nanocomposites.^17,18 Thus, the GO-EP/pk4 finds good dielectric application with a high k dielectric value.

The AC conductivity measured for neat EP/pk4 and GO-EP/pk4 nanocomposites at room temperature are illustrated in Fig. 4c. The neat EP/pk4 matrix behaves as an electrical insulator with an electrical conductivity of about 2.29 × 10⁻⁶ S μm⁻¹ at 1 MHz. However, the incorporation of 0.1 wt% of GO into capron-toughened epoxy significantly increases the electrical conductivity to 2.41 × 10⁻⁵ S μm⁻¹ at 1 MHz. With 0.7 wt% of GO reinforcement, the highest value of electrical conductivity was observed, i.e., 5.14 × 10⁻⁴ S μm⁻¹. With further loading of 1.0 wt% GO, a gradual increase was noticed, and the conductivity was 6.08 × 10⁻⁴ S μm⁻¹.

An incorporation of GO into the polymer matrix causes an in situ reduction and forms a conductive network, and also π–π stacking between the graphene layers. Hence, an improved conductive behaviour was achieved in the resultant nanocomposites.¹⁵

In order to ascertain the UV radiation resistant behavior of the composites, the neat epoxy/capron, and different wt% GO reinforced nanocomposites were placed in the multi lamp 365 nm with an 8 W power UV photo reactor and subjected to ultra-violet radiation for a period of one week (168 h) at room temperature. It is well established that graphene is a promising material against UV radiation. After UV exposure, the GO-EP/pk4 nanocomposites were subjected to tensile strength analysis, and the results compared with that of UV unexposed nanocomposites (Table S2†).

From Table S2,† it can be observed that graphene oxide reinforced nanocomposites retain almost the same value of tensile strength without any significant change after UV exposure. The increase in GO concentration enhances the strength retaining behavior of the nanocomposites. This may be explained by the graphene protecting the polymer matrix from the UV exposure and substantiating its stability toward ultraviolet rays, forming a stable layer.^19,20 The 1.0% GO reinforced nanocomposites show only 6.65% strength loss after UV exposure, whereas the neat sample lost the value of tensile strength of about 48.31%. Thus, the strength retaining behaviour of GO reinforced composites validate the excellent performance and offer effective protection against UV radiation attack and can be used as a potential material to protect microelectronic devices in satellites operating in low earth orbit.

The neat EP/pk4 matrix has a tensile strength of 29.97 MPa with flexible behavior. However, with the incorporation of GO, the tensile strength significantly increases with the increase in the wt% of the reinforcement. The values of tensile strength for 0.1, 0.3, 0.5, 0.7 and 1.0 wt% GO-reinforced EP/pk4 composites are 39.02, 54.36, 63.42, 76.67 and 40.56 MPa, respectively. The 0.7 wt% reinforced EP/pk4 composites shows higher values of tensile strength than that of other GO-EP/pk4 composites, which indicates that the highest interfacial interaction resulted between the EP/pk4 and graphene layers.

The enhancement in the value of tensile strength is not only attributed to the interfacial attraction but is also explained by the formation of a long chain polar polyamide (PA6) skeleton in the epoxy matrix, and it seems to possess significant load-bearing properties for the resultant composites. However, above 0.7 wt% loading of graphene oxide, the reduction in strength is mainly because the load transfer from the polymer to the sheets is too large for the polymer sheets and they break under the load. It is also due to the uneven distribution of graphene oxide in the matrix.^21,22 In addition, the UV exposed and nonexposed composites were analyzed using Raman spectroscopy, and the results are presented in Fig. S1(a–d).†

The I_G/I_D ratio obtained from the Raman spectra of both UV-exposed and UV-non-exposed composites samples suggest that there is no significant disturbance in the graphene network; moreover, it confirms that the graphene acts as a shielding layer against UV radiation. From the Raman spectra and tensile strength analysis, it was inferred that the sustainability of the nanocomposites was extended with physical properties against radiation.²⁰

After the tensile study of the composites, the morphology of the fractured surfaces was examined using SEM analysis with the SEM images presented in Fig. 5. It is inferred that the graphene layers are well dispersed in the EP/pk4 matrix without any significant aggregation. The smooth surface of the neat EP/pk4 matrix indicates the low ductile behavior of the epoxy and capron blend. The homogeneous phase bearing no micro cracks shows that the oxirane ring of the epoxy was cured with amide linkage of the capron. However, the incorporation of GO and the in situ polymerization of capron and epoxy induces a fractured surface with a higher amount of rough surface. Thus, it is clearly ascertained that with an increase in the GO wt%, small clusters of polyamide (PA6) particles are found in the epoxy matrix at lowers level of reinforcement. On the other hand, with a higher percentage loading of GO, the capron reacts with the carbonyl and the oxirane ring functional groups present on the surface of GO to form the polyamide (PA6) fibrous linkage, which can be observed from Fig. 5e. Thus, the nanocomposites hold two phases such as a nanofibril phase and a normal matrix phase, which is consistent with similar reports.²³

Fig. 5 SEM image of the neat EP/pk4, 0.1% GO-EP/pk4, 0.3% GO-EP/pk4, 0.5% GO-EP/pk4, 0.7% GO-EP/pk4 and 1.0% GO-EP/pk4 nanocomposites (a–e) UV nonexposed and (f–h) UV exposed.

The morphological behavior of both UV-nonexposed and UV-exposed samples suggest that the polyamide (PA6) network suffers from UV irradiation because of their long aliphatic skeleton. Fig. 3e and j clearly illustrate the effect of irradiation on the polyamide fiber skeletons and shows the resultant non-uniform fibril morphology. However, the graphene incorporation and highly aromatic epoxy rings protect the skeletons and shield them from UV radiation.

Fig. 6 illustrates the TEM images of 0.7 wt% GO-EP/pk4 nanocomposites. The existence of a PA6 fibril network over the graphene oxide is obviously seen in Fig. 5. The morphology of GO is seen in the epoxy/capron matrix, which seems to possess laminar, waves and folds due to the formation of polyamide fibres between the surface functional groups of GO and the amide groups of capron. This illustrates the occurrence of strong interfacial adhesion and covalent linkages between the reinforcement and matrix material.²⁴ However, good dispersion in the matrix is achieved by the exfoliation of graphene oxide only at lower loading.

Fig. 6 TEM images of the 0.7% GO-EP/pk4 nanocomposites.

The hybrid network structure of neat epoxy/capron and different wt% GO-reinforced EP/pk4 nanocomposites was characterized with XRD and are presented in Fig. 7. The XRD patterns exhibit a broad amorphous peak at 2θ = 18.5°, which indicates that the graphene oxide molecules are homogeneously dispersed in the EP/pk4 networks through covalent bonding to form highly stable nanocomposites.

Fig. 7 XRD profiles of neat EP/pk4 and GO-EP/pk4 nanocomposites.

Conclusion

In the present study, reinforcing graphene oxide into an epoxy/capron matrix leads to an in situ reduction and formation of flexible amide toughening nanocomposites. From the developed nanocomposites, 0.7 wt% GO-reinforced nanocomposites were found to possess the highest value of dielectric constant compared to other nanocomposites. The materials with high conductivity and flexibility are more suitable for printed circuit board application with high durability against deformation. Moreover, the nanocomposites can be used as high dielectric materials in low earth orbit applications with improved radiation resistant behaviour with prolonged longevity.

Acknowledgements

The authors thank BRNS, G. no.: 2012/37C/9/BRNS, Mumbai, Govt. of India., for the financial support and also thank Dr Manmohan Kumar, Senior Scientific Officer, BARC, Mumbai. The authors thank Dr M. R. Vengatesan, Sungkyunkwan University, Suwon, South Korea and Dr S. Devaraju, Pusan National University, Busan, South Korea.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 Raman Spectra of (a) 0.3% GO-EP/pk4, (b) 0.5% GO-EP/pk4, (c) 0.7% GO-EP/pk4 and (d) 1.0% GO-EP/pk4 nanocomposites. Table S2 dielectric constant, tensile strength and I_G/I_D of neat EP/pk4 and GO reinforced EP/pk4 nanocomposites. See DOI: 10.1039/c4ra04213a

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