Sustainable in Situ Approach to Covalently Functionalize Graphene

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A Sustainable In-situ Approach to Covalently Functionalize Graphene Oxide with POSS Molecules Possessing Excellent Low Dielectric Behavior Angel Mary Joseph, Baku Nagendra, Kuzhichalil Peethambharan Surendran, and E. Bhoje Gowd Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00028 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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A Sustainable In-situ Approach to Covalently Functionalize Graphene Oxide with POSS Molecules Possessing Excellent Low Dielectric Behavior Angel Mary Josepha,b, Baku Nagendraa,b, K. P. Surendrana,b*, and E. Bhoje Gowda,b* aMaterials

Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum - 695 019, Kerala, India bAcademy

of Scientific and Innovative Research (AcSIR), New Delhi - 110 001, India

Abstract Incorporation of multifunctional inorganic additives to the commercial polymers still stands as the most captivating and effective way to realize the new generation electronic components. Here, we introduce a simple, cost-effective and environmentally benign method, to covalently functionalize graphene oxide (GO) with vinyl and aminopropyl functionalized hybrid silica spheres with a POSS siloxane composition. The reaction has been carried out in a mixture of ethanol and water medium at ambient conditions with the silane precursors. Later, the synthesized hybrid material has been tested for its dielectric properties after blending with syndiotactic polystyrene (sPS), a commercially available insulating semicrystalline polymer. It was observed that dielectric constant decreases with the addition of GOPOSS up to 1.85 with a dielectric loss of 0.02 at 5 GHz. Significant improvements in the thermal properties of the composites were verified with minimal filler loading. Keywords: GOPOSS, nanocomposites, syndiotactic polystyrene, hydrophobicity, dielectric properties

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Introduction Graphene and its derivatives attained ubiquitous attention ever since Geim and Novoselov isolated the monolayer graphene by mechanical exfoliation.1-2 Among its derivatives, graphite oxide stands out as the most important one for a whole host of reasons. In contrast to parent graphene a well-known electrical conductor, graphite oxide is an electrically insulating two-dimensional material.3-4 Structurally this oxide is similar to graphene, except that the edges and basal planes of the sheets are often functionalized with different oxygenated groups such as carboxyl, hydroxyl, epoxy, etc., thereby causing an increased layer to layer distance.5 The stacked layers of graphite oxide are further converted to few-layer graphene oxide (GO) via many methods including simple sonication in polar media.6 Thus, the principal distinction between graphite oxide and graphene oxide is in the higher inter-planar spacing of the latter. During oxidation of graphene to GO, the sp2 carbons are being replaced by sp3 carbons having oxygen functionalities, creating a band gap by widening the bands apart. The unique electrical, mechanical and thermal properties make GO a deserving candidate for a myriad of applications like polymer composites, electrical energy storage systems, nanomechanical devices, etc.7-9 Graphene oxide has been attempted for functionalization both covalently and noncovalently, to enhance various properties including the improved compatibilization with the commercial low-cost hydrophobic polymers and also to prevent the restacking occurred by the ππ interaction.10-12 For example, end-functionalized polymers were reported to form noncovalent bonds with reduced GO in aqueous media which provided stable dispersions in various organic mediums.11 GO has also been functionalized with biocompatible hydrophilic groups such as polyethylene glycol (PEG) molecules, to explore its drug delivery capacity by exploiting the aqueous miscibility and stability in physiological conditions.13 Moreover, the metal nanoparticles decorated GO has also been synthesized for various applications like bioimaging, drug delivery, 2 ACS Paragon Plus Environment

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etc.14-15 Functionalized GO was studied for its electronic applications as well. Covalently bonded BaTiO3 @ GO was found to increase the dielectric constant of the polyimide composites as high as 285 at 100 Hz.16 GO based composite paper with high dielectric constant and low dielectric loss has also been realized by the hydrogen bonded GO at triblock copolymer by a vacuumassisted self-assembly method.17 The covalent functionalization of GO with silsesquioxanes derivatives render a good platform to synthesize a new hybrid material with multiple functionalities. Polyhedral oligomeric silsesquioxanes (POSS), itself is an organic-inorganic hybrid material with Si-O-Si core and organic periphery.18 When prepared the nanocomposites with POSS, quite often the peripheral organic moieties helps to augment the interaction with the organic polymer matrix, whereas the inherent nanoporosity of the hybrid filler assists to attain a lower dielectric constant. Literature is enriched with the POSS incorporated polymer nanocomposites with low dielectric constant, where the decrease in dielectric constant is achieved by the orientation of the filler in the matrix, by the introduction of nanoporosity, etc.19-21 POSS modified GO was reported to possess excellent solubility in organic solvents with a superhydrophobic behavior.22 It is also found to increase the glass transition temperature and thermal stability of the host polymer matrix. Recently, Lu et al. utilized POSS @ GO as a potential nanofiller to reduce the dielectric constant of fluoropoly(ether ether ketone) and found that the dielectric constant reduced to 2.01 at 1 MHz, less than the host polymer.23 Despite the numerous applications of POSS @ GO hybrid, reports on chemical grafting of POSS at GO is scarce in the literature. The reported synthesis of this material is often involved with rather difficult steps with the utilization of harsh and toxic chemicals mainly via the amidation reaction or click reactions.22,

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Unlike the previous reports, herein, we introduce a

simple, and cost-effective method to covalently functionalize GO. For the first time, the 3 ACS Paragon Plus Environment

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aminopropyl and vinyl functionalized hybrid silica spheres with a POSS siloxane composition was introduced into the GO layers via a facile, in-situ hydrolytic co-condensation reaction. The reaction was carried out in a mixture of environmentally benign ethanol and water medium at ambient conditions with the easily available silane precursors. Later, the synthesized hybrid material was blended with syndiotactic polystyrene (sPS), a commercially available insulating semicrystalline polymer, to yield an impressive low-κ material, opening up a myriad of applications in microelectronics. Experimental Section Materials. Graphite flakes with 99.8 % purity (~325 nm mesh size) were purchased from Alfa Aesar. (3-aminopropyl)triethoxysilane (99%), triethoxyvinylsilane (97%), and xylene solvent were procured from Aldrich Chemicals Co. Potassium permanganate, sulfuric acid, hydrogen peroxide and absolute ethanol were received from Merck chemicals. Syndiotactic polystyrene (sPS) pellets (Mw~272000, Đ~ 2.28) used in the present study were kindly provided by Idemitsu Petrochemical Co., Ltd. Japan. All the above-mentioned chemicals and polymer were utilized as received. For the synthesis of POSS, Millipore grade water was used. Preparation of Graphene Oxide. Graphene oxide was synthesized by the modified Hummers’ method reported elsewhere with slight changes.25 Initially, 1 g of graphite powder and 1g of sodium nitrate were added to 70 mL of sulfuric acid (98%). This mixture was then cooled by keeping it in ice bath and stirred for 2 hours. After that, 3 g of KMnO4 was cautiously added to the reaction vessel while keeping the reaction temperature below 20 °C. After the complete addition of KMnO4, the temperature was elevated to 35 °C and kept under continuous stirring for a day. This was followed by the addition of 120 mL of deionized water at a higher temperature of 90 °C for 15 min. Thereafter, an additional 200 mL of deionized water was added to the 4 ACS Paragon Plus Environment

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reaction mixture, and finally15 mL of 30% H2O2 was added to quench the reaction completely. The product was centrifuged at 8000 rpm, repeatedly washed with 1 M HCl and deionized water and then freeze-dried to obtain GO. Preparation of GO-POSS Hybrid. Preparation of GOPOSS was done through an in-situ approach by utilizing the already established synthesis method for the hybrid silica spheres with a POSS – siloxane composition.26 Typically, 0.5 g of GO was added to a mixture of aminopropyltriethoxysilane and vinyltriethoxysilane (1:3 molar ratio) dissolved in a mixture of ethanol and water (15: 1 V/V ratio). The mixture was then sonicated for thirty minutes to yield homogeneous dispersion of the ingredients and kept for aging in a sealed container for three days under ambient conditions. After aging, solvents were allowed to evaporate at atmospheric temperature and pressure. Preparation of GOPOSS- sPS Composite. The composites of sPS containing different weight percentages of GOPOSS were prepared by following a solution blending in xylene. Firstly, the sPS pellets were dissolved in xylene at 170 °C. After ensuring the complete dissolution of sPS, the necessary amount of GOPOSS was added to the polymer solution under constant stirring. The mixture was then refluxed for seven hours to result in homogeneous dispersion of the GOPOSS in sPS matrix. The final hot solution was precipitated in 100 mL of ethanol. The filtered precipitate was washed with ethanol several times and then finally dried in a vacuum oven at 120 °C. The samples for the dielectric measurements were prepared by a hot pressing technique in a laminating press using suitable steel moulds at a temperature of 200 °C and pressure of 2 MPa for 90 min.


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Characterization The surface morphological characterization of the prepared GOPOSS was carried out using transmission electron microscopy (TEM) analysis (JEOL 2010 transmission electron microscope), after drop casting the sonicated sample on a carbon-coated copper grid. Atomic force microscopy (AFM) analysis in the tapping mode (Bruker Multimode, Germany) was done to inspect the surface morphology and thickness of the GOPOSS. For this, the samples were drop casted and dried on a thin mica sheet. The chemical states of GO and GOPOSS were studied by X-ray photoelectron spectroscopy (XPS) using PHI 5000 Versa Probe II (ULVAC-PHI Inc., USA) equipped with microfocused (200 μm, 15 KV) monochromatic Al-Kα X-Ray source (hν = 1486.6 eV). Firstly, the survey scans were acquired on the samples with X-ray source power of 23.7W and pass energy of 187.85 eV and then high-resolution spectra for the major detected elements were recorded at 46.95 eV pass energy. The obtained spectra were analyzed with the curve fitting software MultiPak for the chemical state assignments. The infrared spectra were collected using a Perkin-Elmer series FT-IR spectrometer Two, over the wavenumber range of 4000 – 400 cm-1 to confirm the covalent functionalization of GO with POSS. Here, the samples in its powder form were mixed with KBr and pressed to form of pellets. The FTIR spectra were collected with 32 scans at a resolution of 1 cm-1. Raman spectra of various samples were collected with the WITec Raman microscope (Witec Inc. Germany, alpha 300R), with a laser beam directed to the sample. Samples were excited with a 632.8 nm excitation wavelength laser, and Stokes-shifted Raman spectra were collected in the range of 0− 3000 cm−1 with 1 cm−1 resolution. Wide-angle X-ray diffraction (WAXD) measurements were used to confirm the intercalation of POSS moiety inside the GO layers with the aid of a Xeuss SAXS/WAXS system from Xenocs 6 ACS Paragon Plus Environment

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using a Genix microsource where the generator was operated at 50 kV and 0.6 mA. A FOX2D mirror and two pairs of scatterless slits from Xenocs were employed to collimate the Cu Ka radiation (l = 1.54 Å). The 2D-patterns recorded on a Mar345 image plate were processed using the Fit2D software. All measurements were done in the transmission mode. The thermal stability of various samples was measured using thermogravimetric analyzer TA Q50 (TA Instruments, New Castle, DE, USA) under nitrogen gas atmosphere at a heating rate of 10 °C/min. Laser flash thermal property analyzer (FlashLine 2000, Anter Corporation, Pittsburgh, USA) was used to measure the thermal conductivity of the samples, by following the relation TC = α × ρ × Cp

(1)

where α corresponds to the thermal diffusivity, ρ to the density, and Cp is the specific heat capacity of the sample. Alumina has been used as the reference material in order to measure the Cp values of all of the composites. The density of the samples was measured using Archimedes technique. TMA analyzer (TMA/SS7300, SII NanoTechnology Inc.Tokyo, Japan) was employed to measure the coefficient of thermal expansion (CTE) in the temperature range 30 °C to 150 °C by applying a pressure of 0.1N. Moisture absorption by the sPS-GOPOSS composites were estimated using a chemical balance of accuracy of ±0.1 mg by measuring the weight gained by the samples after dipping in distilled water for 24 h. Morphological analysis and elemental mapping of composites were carried out using an EDS-coupled scanning electron microscope (Zeiss EVO 18 Cryo-SEM, Jena, Germany). The radio frequency (300 Hz to 3 MHz) dielectric properties of the samples were measured with the aid of an LCR meter (LCR HiTESTER, Hioki 3532-50) by following the well known parallel plate capacitor method, where both sides of the circular disc-shaped samples were sputtered with silver electrodes. The measurements were carried out at room temperature and the dielectric data 7 ACS Paragon Plus Environment

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was derived from the complex impedance measurements. The microwave dielectric properties at 5 GHz were measured with a split post dielectric resonator (SPDR QWED, Poland) set-up resonating in the TE01δ resonant mode which was connected to a vector network analyzer (Model No. E5071C ENA series; Agilent Technologies, Santa Clara, USA). Here, a rectangular block sample with dimensions 40mm x 40mm x 1mm was used. Results and Discussion In a typical synthesis pathway, we first prepared GO by the modified Hummers’ method and identified its formation by various spectroscopic and other analytical techniques which will be discussed in detail in the coming sections. The highly polar GO readily disperses in ethanol/water mixture, which is the solvent medium for the formation of POSS molecules. When the POSS molecules are compelled to form in-situ, the formation occurs via an inherent covalent bonding with the carboxyl and hydroxyl groups of the GO. This reaction probably occurs via a two-step mechanism. In the initial step, the silanization of GO takes place by the hydrogen bonding that occurs between the hydroxyl groups present on the basal planes of GO and the selected silanols. This is followed by the condensation of the silanols leading to the formation of the caged POSS molecules.27-28 It is anticipated that the POSS molecules will intercalate in between the layers of GO as shown in the schematic representation in Figure 1a, which results in the increase of the layer to layer distance considerably. In the present case, the in-situ formed POSS molecules possessed vinyl and aminopropyl groups and we tapped the advantage of aminopropylsilane since this holds a self-catalytic activity to promote the hydrolysiscondensation reaction leading to the formation of POSS molecules.26, 29 Visually it can be realized that the highly hydrophilic GO was getting converted to hydrophobic as it was functionalized to GOPOSS (Figure 1b). This can be due to the reduction in 8 ACS Paragon Plus Environment

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the number of free hydroxyl and carboxyl groups, in conjunction with the fact that the POSS molecules are extensively functionalized with hydrophobic vinyl groups. The hybrid material was found to be completely rejected from the water phase and was well dispersed in the nonpolar medium, on the contrary to the highly stable water dispersions of GO. The hydrophobicity of the hybrid material was identified with the water contact angle experiment as well. As observed from Figure 1c-1e, the contact angle increased from 51° for GO to 137° for GOPOSS. Pure POSS exhibited a water contact angle of 128°. The developed hybrid material showed reasonable dispersibility in common organic solvents like acetone, THF, xylene, etc., but tends to settle down after some time (see supporting information Figure S1). This image further reiterates the point that xylene is the better solvent with comparatively lower settling rate of the material. The dispersibility of GOPOSS in organic solvents might arise from the reduction in the number of the hydroxyl and carboxyl groups of GO and the occurrence of vinyl functional groups on the periphery of the intercalated POSS molecules.30

Figure 1. (a) Schematic representation of the in-situ formation of POSS molecules in between the layers of GO and (b) the photographic images of the hydrophilic GO and hydrophobic GOPOSS. (c), (d) and (e) are the water contact angles of GO, POSS and GOPOSS respectively.


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XPS analysis can be used as a potential experimental technique to understand the surface functionalization of GO and GOPOSS. From the survey spectra given in Figure 2, it is clear that GO consist mainly of C1s and O1s peaks. Whereas, when it is functionalized with POSS to form the hybrid, in addition to C1s and O1s peaks, Si2p, Si2s and N1s peaks are clearly visible. These are peaks arising from the POSS molecules of Si-O-Si core and aminopropyl and vinyl functionalized periphery, clearly indicating the grafting POSS to the GO layers. Also, from the C 1s spectrum given in Figure 2c and 2d, it is well understood that the intensity of C-OH and C-OC peak intensity reduces considerably and the peaks corresponding to C=O and O-C=O are almost disappearing after the functionalization of GO with POSS.

Figure 2. (a) and (b) XPS survey spectra of GO and GOPOSS, (c) and (d) are the C 1s spectra of GO and GOPOSSS

FTIR spectra provide the most important evidence for the formation of the GOPOSS hybrid. The spectra of various materials are given in Figure 3a. As expected, natural graphite does not contain any functional groups. On the other hand, when it is oxidized, the characteristic


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peaks of different functional groups such as C-OH stretching, C=O stretching and the broad O-H peaks are observed at 1624 cm-1,1730 cm-1, and 3400 cm-1in the spectra.31-32 The as-synthesised POSS molecules contain the characteristic peaks arising from the symmetric stretching vibration of Si-O-Si groups and Si-OH groups of the completely condensed siloxanes at 1050 cm-1 and 1149 cm−1 along with the peak for some incompletely condensed siloxanes at 950 cm-1.18, 33 In the hybrid system, the peaks corresponding to the carbonyl groups of GO have almost disappeared, and the hydroxyl peak intensity is incontrovertibly decreased, whereas the peak corresponding to Si-O-Si groups of the POSS molecules at 1050 cm-1 are clearly observed which indicates the covalent grafting of the POSS to the GO sheets.34 Raman spectra is another powerful tool to identify various functionalizations of graphene derivatives. Figure 3b gives the Raman spectra for various samples. In the case of graphene oxide, the peaks at 1340 cm-1 and 1573 cm-1 represents the well-known D band, originating from the structural defects due to the extensive functionalization and G bands, corresponding to the first order scattering.35 When graphite was functionalized to form graphene oxide, there occurs a considerable change in the peak intensities of D and G bands and shows an increased ID/IG ratio of 1.23, when compared to that of pristine graphite (ID/IG is 0.09). This indicates the formation of sp3 hybridized carbon atoms due to the disorder occurred by the functionalization on the graphene layers. When GO was further functionalized to form the GOPOSS hybrid, the ratio is found to be slightly decreased to 1.02. This might have been due to the increase in the ordering of the GO layers when POSS molecules are forming covalent bonds with the functional groups on it. Additionally, the X-ray diffraction pattern will show substantial changes with the functionalization GO layers, since it can cause the changes in the interlayer distances. Figure 3c


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shows the WAXD patterns of graphite, GO, POSS and GOPOSS. The 002 peak of graphite appeared at 26° with a d-spacing of 0.34 nm was found to be repositioned to a much lower 2θ value of 10° consequent to the modified Hummers’ method to oxidize graphite to graphene oxide. The d-spacing between the GO layers was near to 0.90 nm, suggesting the possible exfoliation of the layered structure.36 Further shift of the 002 peak to 2θ~4.5° in the case of GOPOSS is a clear manifestation of the increased interlayer distance between the GO layer resulted from the intercalation of the POSS molecules by the covalent bonding with the surface functional groups of GO.22 In other words, there occurs a considerable increase in d-spacing, ~ 2.02 nm after the functionalization, which assures that the functionalization happened on the surface GO, and the spherical POSS molecules occupied a space in between the layers. It is further observed that the characteristic X-ray diffraction peak of POSS appearing near 2θ~8° originating from the long-range order of the caged structure, remains in the same position after the grafting with the GO layers.26, 37 Thermal stability measured for various samples is given in Figure 3d. The thermal stability of GOPOSS shows a clear hybrid behavior, in contrast to that of GO and POSS. The weight loss below 100 °C observed is due to the release of the absorbed water on the hydrophilic surface of the GO layers. The weight loss in this region is minimized for GOPOSS than pristine GO, indirectly indicating the hydrophobicity of the synthesized GOPOSS. The considerable weight loss in the region 100-560 °C can be ascribed to the thermal degradation of the oxygencontaining functional groups on the GO layers and the degradation of the organic functional moieties on the periphery of POSS cages.38 This intermediate weight loss is reduced to around 32 % in GOPOSS, suggesting the decreased availability of the free functional groups. Also, it is observed that the thermal stability is quite high for the hybrid material than the GO, possibly arising from the covalent grafting of the thermally stable POSS molecules to the GO layers. 12 ACS Paragon Plus Environment

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Figure 3. (a) FTIR spectra of graphite, GO, POSS and GOPOSS (b) Raman spectra of graphite, GO and GOPOSS (c) WAXD patterns of graphite, GO, POSS and GOPOSS and (d) TGA of graphite, GO, POSS and GOPOSS.

The surface morphology of the prepared GO and GOPOSS was checked by transmission electron microscopy which is given in Figure 4. The TEM image of GO (Figure 4a) revealed the existence of very thin and transparent sheets, indicating the formation of exfoliated sheets, generated by the modified Hummers’ treatment. Even though the surface of the GO showed some wrinkled nature possibly due to the extensive functionalization, it retained the crystalline behavior as indicated by the HRTEM image given in Figure 4b. The FFT pattern given in Figure 4c shows the typical six-fold symmetry as that of single layer graphene. After the functionalization by the in-situ method, it is seen that the spherical POSS molecules were 13 ACS Paragon Plus Environment

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attached to the GO sheets which were designated as dark spots in Figures 4d-4e. None of the POSS molecules were seen as detached molecule standing independently in the entire region of the analysis, even after sonicating for a pretty long duration, implying its effective bonding with the GO. From Figure 4e, it can be clearly understood that the POSS molecules were intercalated in between the layers of GO, thereby effectively increases the layer-to-layer distance as it is very clear from the marked area, which was corroborated by the XRD experiments.

Figure 4. (a-c)TEM, HRTEM and FFT images GO and (d-e) TEM images of GOPOSS and (f) is the EDAX spectra of GOPOSS

AFM height images captured in the tapping mode visualizes the flat sheet-like morphology of the synthesized GOPOSS (Figure 5a). As seen from the cross-section of the height image given in Figure 5b, the sheets exhibited lateral dimensions of 5-7 µm, whereas the thickness was approximately 10 nm. Being atomistically thin, the single layer graphene and


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graphene oxide should ideally show thickness less than 1 nm.39-40 So, the increased thickness of the GOPOSS hybrid reiterates the stacked nature of the material. No POSS molecules were seen separated from the layered structure, quite similar to that of TEM images, indicating that the smaller spherical POSS molecules are rather intercalated between the layers of GO, which lies in line with the other analyses. The increased surface area calculated for GOPOSS than the bare POSS provides an additional hint towards the intercalated structure of the hybrid material with increased inter layer distance. The surface areas were found to be 40.2 m2/g and 29.8 m2/g for GOPOSS and POSS respectively, as it was calculated from the BET adsorption isotherm, given in Supporting Information Figure S2.

Figure 5. (a) AFM image of GOPOSS and (b) the corresponding cross-sectional height image.

The elemental composition of the synthesized GOPOSS has been checked with the aid of elemental mapping associated with the SEM and EDAX techniques and is given in Supporting information Figure S3. Carbon, oxygen and silicon can be clearly mapped from GOPOSS, and is visible throughout the selected area, authenticating the chemical homogeneity of the prepared material. EDAX measurement carried out on the sample (given in Figure 4f) also verified the presence of elements like oxygen and silicon of the POSS molecules in addition to the carbon atoms present in graphene oxide. 15 ACS Paragon Plus Environment

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We utilized the developed GOPOSS as a multifunctional nanofiller for syndiotactic polystyrene and investigated the effect of these on various physical properties of the matrix polymer. The surface morphology of the neat polymer and the composites with the hybrid filler has been examined using SEM and is given in Figures 6a and 6b, respectively. The crosssectional image shown in Figure 6b visualizes the uniform dispersion of the sheet-like GOPOSS in sPS matrix. This uniform dispersion plays an influential role in deciding the properties of sPSGOPOSS composites like its exciting dielectric properties. The homogeneous dispersion of the hybrid filler through the host polymer was further visualized with the help of elemental mapping analysis. The observed increment in the amount of C atoms in the composite than the GOPOSS is from the hydrocarbon-based polymer matrix. The elements C, Si and O from the hybrid filler were found to be uniformly dispersed in the entire scanned area (see the panels for Figure 6c). The introduction of the hydrophobic GOPOSS hybrids into the polymer matrix has also been found to enhance the overall hydrophobicity of the sPS-GOPOSS composites. The water contact angle has been increased from 91° for sPS to 104° in the case of sPS-4GOPOSS (Images are given in the inset of Figures 6a and 6b). This enhanced hydrophobicity will help to maintain a high moisture resistance for a long period.


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Figure 6. SEM images of (a) sPS and (b) sPS-4GOPOSS. The contact angle measured for each sample is given in the insets (c) shows the surface morphology of sPS-4GOPOSS and the elemental mapping of various elements in sPS-4GOPOSS (d) moisture resistance measured for various samples.

Declination to moisture absorption is an important criterion to be satisfied, for qualifying a material as a suitable interlayer dielectric. We have checked the moisture absorption property of the developed composites and is given in Figure 6d. It was observed that the composites are highly impervious to moisture and the rate of moisture absorption decreases with an increase in the filler loading. This is due to the high hydrophobicity of the incorporated hybrid fillers. It is 17 ACS Paragon Plus Environment

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already well-proven that the water contact angle of the polymer increased when suitable moisture repellent filler was added. The very low moisture absorption tendency is advantageous for the use of these materials as the next generation low-k dielectrics. The thermal properties measured for various samples are given in Figure 7. From the thermogravimetric analysis (Figure 7a), it is well evident that a small amount of the hybrid filler has a substantial ability to improve the thermal stability of the composites. The 10 % weight loss observed for different sPS-GOPOSS composites are summarized in Table 1. It is clear from the results that, there is almost 40 °C increase in thermal stability achieved by the incorporation of 4 wt % GOPOSS to sPS. The enhanced thermal stability in the presence of GOPOSS can be due to the inherent heat resistance of the POSS molecules along with the tortuous path effect of the layered material in the polymer matrix.34, 41 The homogeneous arrangement of the layered filler in the sPS matrix as shown in the schematic representation in the inset of Figure 7a, will help to prevent the easy permeation of heat through the matrix. The thermal conductivity measured for sPS-GOPOSS is given in Figure 7b. Thermal conductivity has been slightly increased with the increase in filler loading, even though the enhancement is not so prominent as in the case of graphene.42 The observed increment can be due to the probable filler to filler interconnect in the composite, whereas the decreased value when compared to the graphene-based composites can be due to the expected phonon scattering occurring at the defected surface of the GOPOSS.34 Thermal conductivity enhancement (TCE) calculated as per the formula given in Eq. 2 is tabulated in Table 1. A maximum of 50 % increment in the thermal conductivity is observed for the sPS-8GOPOSS sample. 𝑇𝐶𝐸 =

%&'() *%)'+ %)'+

(2)


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Figure 7. (a) TGA thermograms and (b) Thermal conductivity measured for various samples. Table 1: 10 % weight loss calculated from TGA, and the thermal conductivity enhancement calculated for various sPS-GOPOSS composites.

Sample sPS sPS - 2% GOPOSS sPS - 4% GOPOSS sPS - 6% GOPOSS sPS - 8% GOPOSS

10% weight loss temperature (°C) (±1)

TCE (%)

326

-

365

38.4

367

42.3

371

46.1

372

50

Since the developed GOPOSS exhibits remarkable properties like enhanced hydrophobicity and increased porosity as evidenced from the high surface area, it is expected to act as an insulating nanofiller to decrease the dielectric constant of the polymeric matrices. Also, in GOPOSS the carbon atoms are in the sp3 hybridized state, unlike in graphene where carbon is sp2 hybridized. This mechanism will reduce the conjugation of π electrons and thus increase the insulating behavior or decrease the dielectric constant. In order to prove the excellent dielectric behavior of the newly synthesized GOPOSS, these were employed as the reinforcing additives for the well known dielectric polymer sPS. Even though sPS is a low-κ material (κ=2.53 at 1 19 ACS Paragon Plus Environment

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MHz) due to the arrangement of the phenyl side groups on the polymer backbone which results in a net decreased dipole moment,18 it was expected that the inherent nanoporosity of the GOPOSS hybrid filler assists to attain a yet lower dielectric constant in sPS. With this objective, up to 8 wt% of filler was added to the polymer and the dielectric properties were checked from 100 kHz to 1 MHz by following the well established parallel plate capacitor theory. Here the dielectric properties were measured as a function of frequency from 100 kHz to 1 MHz. At the lower end of the selected frequency region, all the four polarizations namely electronic, dipolar, ionic and interfacial polarizations are believed to contribute to the observed dielectric constant.43 The dielectric constant and dielectric loss measured for various samples in the RF region of the electromagnetic spectra is given in Figure 8. There is a gradual decrease in dielectric constant as the filler loading increases and after an optimum level, the dielectric constant increases. The lowest dielectric constant that we could achieve was 1.92 at 1MHz at an optimum loading of 4 wt% of GOPOSS. This lowering of dielectric constant can be explained using various reasons. Firstly, the density of the composites follows the same trend as that of the dielectric constant (see supporting information Figure S4). Also, POSS molecules are inherently microporous due to the subtractive porosity developed during the hydrolytic co-condensation reaction. Obviously, these hydrophobic nanoporous structures are of low dielectric constant in nature.19, 44 In addition, the proven increase in the layer-to-layer distance between the GO layers arising due to the intercalation of POSS cage leads to the second level of porosity. Since porosity represents nothing but the presence of air with k = 1, these two levels of porosity might be playing the central role in reducing the dielectric constant of sPS-POSS composites up to an optimum lower value. The restricted interfacial polarization, a reason for the reduced dielectric constant in many of the polymer composite systems, is also supposed to be active in the present case.18, 44 Here, the layered-like structure of GOPOSS with the insulating oxygen-containing functional groups on it 20 ACS Paragon Plus Environment

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is supposed to prevent the polarization and thus dielectric confinement occur effectively upto an optimum concentration. After the optimum level loading, there will be a possible aggregation of the hybrid filler, which will enhance the interfacial polarization, leading to an increased dielectric constant (see Figure 8c). The dielectric loss is yet another important parameter to be considered while the material is anticipated for the real-time low-κ applications. In principle, dielectric loss is created by the dissipation of the electrical energy by various means such as electrical conduction, dielectric relaxation, etc.45 An ideal dielectric material should possess minimum dielectric loss (tan δ), to be able to qualify for microelectronic as well as telecommunication applications. The tan δ measured for various composites at 1 MHz is given in Figure 8b. It is seen that the dielectric loss is very low in all the composites.

Figure 8. (a) and (b) are Dielectric constant and dielectric loss factor of sPS-GOPOSS composites measured for various samples in the low-frequency region (c) variation of dielectric constant and dielectric loss at 1 MHz measured for various samples with respect to the filler loading.


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The microwave dielectric properties give a more clear picture of the hybrid dielectric material developed since the interfacial polarization and orientation polarizations cannot respond to the fast-moving microwave fields. The dielectric properties of sPS added with 4 wt% of GOPOSS were measured at 5 GHz using a split post dielectric resonator. The dielectric constant and loss were determined from the perturbation of the resonant frequency and quality factor of the resonant cavity after inserting the thin dielectric sheet.46 The microwave dielectric constant touched an impressively low value for sPS-4GOP, which is 1.85 with a dielectric loss of 0.02 (at 5GHz). This is in good agreement with the theory that the dielectric constant decreases with the increase in frequency as interfacial polarization and orientation polarizations, that show up at radio frequency range, will not contribute at the higher frequency region.46 This ultra low dielectric constant, even lower than the POSS alone incorporated sPS which exhibited k=1.92 at 5 GHz for sPS-6 vol % POSS,18 with a desirable low loss factor, is a promising alternative to conventional low-κ materials. As such, porous low-κ materials are in high demand for integrated passive device (IPD) packaging applications, where more research is needed to alleviate the plasma damage of porous ILD materials during the Damascene integration processes. Conclusions Here, we could formulate a novel, easy and environmentally benign method to prepare hydrophobic and electrically insulating GOPOSS nanofillers. The POSS intercalated layered structure of GO has been characterized using various analytical methods. When these fillers are added to the organic polymer matrix, it is found to be acting as a potential multifunctional filler. Thermal stability and hydrophobicity of the host sPS are found to be increased with a desirable decrease in the dielectric constant. The lowest possible dielectric constant that we could achieve was 1.85 at 5 GHz with a low dielectric loss of 0.02 for 4 wt % GOPOSS loaded sPS hybrid, which opens up new avenues of applications as both inter- and intra-layer dielectrics in 22 ACS Paragon Plus Environment

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microelectronics. The increased porosity and the increased surface area of the GOPOSS are expected to play influential roles in deciding the overall performance of the composite materials. Further, this kind of low dense porous ultra low-κ dielectric could be ideal for packaging applications when proper integration schemes were used. Associated Content Supporting Information: The Supporting Information is available free of charge on the ACS Publications website. Figure S1. Photographic images of the dispersibility of the GOPOSS in various organic solvents (a) just after sonication and (b) after 30 minutes. Figure S2. BET nitrogen adsorption-desorption analysis of POSS and GOPOSS Figure S3. (a) SEM image and (b-d) elemental mapping of the GOPOSSS hybrid. Figure S4. Variation in density of sPSGOPOSS composites with respect to the filler loading.

Author Information Corresponding Authors *E-mail: [email protected]; tel+91-471-2515258; fax +91-471-2491712 (K.P.S) [email protected]; tel +91-471-2515474; fax +91-471-2491712 (E.B.G.).

ORCID K. P. Surendran: 0000-0001-5269-1489 E. Bhoje Gowd: 0000-0002-2878-5845 Notes: The authors declare no competing financial interest. 23 ACS Paragon Plus Environment

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Acknowledgements: The authors thank Dr. Saju Pillai, Mr. Kiran Mohan, Ms. Soumya, Mr. A. Peer Mohamed and Mr. Vishnu of CSIR-NIIST, for extending the XPS, TEM, SEM, surface area and AFM measurements. A.M.J. thanks the University Grants Commission for the award of a research fellowship. EBG thanks the Department of Science and Technology (Government of India) for the research project vide No.: SB/S3/CE/070/2014. References 1.

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