Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels

Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels

CARBON x x x ( 2 0 1 4 ) x x x –x x x Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon Rayleigh s...

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CARBON

x x x ( 2 0 1 4 ) x x x –x x x

Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels Simona Longo a, Marco Mauro a, Christophe Daniel a, Pellegrino Musto b, Gaetano Guerra a,* a

Department of Chemistry and Biology and INSTM Research Units, Universita` degli Studi di Salerno, via Ponte Don Melillo, 84084 Fisciano, SA, Italy b Institute of Chemistry and Technology of Polymers, National Research Council of Italy, Pozzuoli, NA, Italy

A R T I C L E I N F O

A B S T R A C T

Article history:

Robust monolithic aerogels based on syndiotactic polystyrene (s-PS) and graphene oxide

Received 17 March 2014

(GO), and exhibiting high surface areas (240–290 m2/g) and low density (0.02–0.2 g/cm3),

Accepted 2 June 2014

are presented. These aerogels are obtained by supercritical carbon dioxide extraction of

Available online xxxx

s-PS/GO gels, which were prepared by s-PS dissolution in GO dispersions in suitable organic solvents. In contrast to other GO-based aerogels, the s-PS/GO aerogels are blue, not black. This blue color is due to Rayleigh scattering by isolated particles of reduced graphene oxide, whose thickness is much smaller than the wavelength of the light. The s-PS/GO aerogels exhibit the functionalities of the nanoporous-crystalline d form of the polymer and of reduced and structurally uncorrelated GO layers and maintain the ductility of pure polymer aerogels. Uses can be anticipated as supported GO-based catalyst or ‘‘masterbatches’’ for GO rich nanocomposites.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

The development of three-dimensional (3D) structures with graphene layers is expected to further expand its relevance [1–4] both in the number of applications and in manufacturability [5–12]. In many reports, 3D physically crosslinked graphene aerogels were obtained from GO suspensions, generally by drying followed by GO (thermal or chemical) reduction [5–9]. Monolithic aerogels exhibiting covalent carbon bonding between the graphene sheets, rather than simple physical cross-links, have been prepared by using organic sol–gel chemistry [10– 12]. In particular, thermosetting (e.g., phenolic) resins are used to produce organogels, which are supercritically dried and thermally reduced to yield crosslinked polymer/GO

aerogels. Pyrolysis of these aerogels, by carbonization of the organic cross-links, leads to graphene aerogels [10,11]. Monolithic polymeric aerogels can be easily obtained not only by drying of gels based on crosslinked thermoset polymers [13–19], but also of physically crosslinked gels formed by thermoplastic semicrystalline polymers [20–28]. Robust monolithic aerogels have been also obtained for thermoplastic polymers exhibiting nanoporous crystalline phases [29–38], like syndiotactic polystyrene (s-PS) [39–45] and poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) [46–49]. These nanoporous-crystalline polymeric aerogels present, beside disordered amorphous micropores (typical of all aerogels), all identical nanopores of the crystalline phases, which are able to absorb low molecular-mass molecules also when

* Corresponding author: Fax: +39 089 969603. E-mail address: [email protected] (G. Guerra). http://dx.doi.org/10.1016/j.carbon.2014.06.003 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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present in traces and hence suitable for molecular separation [50–54], sensor [55–60] and catalysis [61,62] applications. In this paper, we show that s-PS/GO high surface area and robust aerogels can be easily obtained. These aerogels show a blue color, unprecedented for GO based materials, due to Rayleigh scattering by GO uncorrelated layers.

2.

Experimental section

2.1.

Materials

High surface area graphite (HSAG), with Synthetic Graphite 8427 as trademark, was purchased from Asbury Graphite Mills Inc. HSAG is a milled graphite with a minimum carbon wt% of 99.8 and a surface area of 308 m2/g. Sulfuric acid, sodium nitrate, potassium permanganate and 1,2-dichlorobenzene (DCB) were purchased from Sigma–Aldrich and used without any further purification. The syndiotactic polystyrene (s-PS) used in this study was manufactured by Dow Chemicals under the trademark Questra 101. 13C nuclear magnetic resonance characterization showed that the content of syndiotactic triads was over 98%. The mass average molar mass obtained by gel permeation chromatography (GPC) in trichlorobenzene at 135 C was found to be Mw = 3.2 · 105 g mol1 with a polydispersity index Mw/Mn = 3.9.

2.2.

Preparation methods

2.2.1.

Preparation of graphite oxide

Graphite oxide (GO) samples were prepared by Hummers’ method [63], from graphite samples. 120 mL of sulfuric acid and 2.5 g of sodium nitrate were introduced into a 2000 mL three-neck round bottomed flask immersed into an ice bath and 5 g of graphite were added, under nitrogen, with a magnetic stirring. After obtaining an uniform dispersion of graphite powders, 15 g of potassium permanganate were added very slowly to minimize the risk of explosion. The reaction mixture was thus heated to 35 C and stirred for 24 h. The resulting dark green slurry was firstly poured into a copious amount of deionized water, and then centrifuged at 10,000 rpm for 15 min with a Hermle Z 323 K centrifuge. The isolated GO powder was washed twice with 100 mL of a 5 wt% HCl aqueous solution and subsequently with deionized water. Finally, it was dried at 60 C for 12 h.

2.2.2.

Solvothermal reduction of GO in DCB

Reduced GO dispersions were obtained by adding the appropriate GO amount in 5 mL of DCB, following by solvothermal reduction of GO in a 5000 mL batch bath ultrasound (Badelin Sonorex RK 1028 H) at 100 C for 2 h [64].

2.2.3.

Preparation of s-PS/GO gels

s-PS/GO gels were prepared, in hermetically sealed test tubes, by heating the reduced GO dispersions above the boiling point of the solvent until complete dissolution of the polymer and the appearance of a homogeneous solution had occurred. The hot solution was then cooled to room temperature, where gelation occurred. For instance, 520 mg of s-PS and 5 mL of

20 wt% of GO dispersion were mixed to obtain s-PS/GO gels. The overall amount of polymer and GO in the gels was generally fixed to 10 wt%.

2.2.4.

Preparation of s-PS/GO aerogels

Aerogels were obtained by treating s-PS/GO gels with a SFX 200 supercritical carbon dioxide (scCO2) extractor (ISCO Inc.) using the following conditions: T = 40 C, P = 200 bar, extraction time t = 6 h. The prepared s-PS/GO aerogels present a weight composition ranging between 95/5 and 50/50. The aerogels, as prepared from gels with an overall polymer-GO content of 10 wt%, present a porosity close to 90%.

2.3.

Characterization

2.3.1.

Wide angle X-ray diffraction

Wide-angle X-ray diffraction (WAXD) patterns with nickel filtered Cu-Ka radiation were obtained, with an automatic Bruker D8 Advance diffractometer, in reflection. Patterns were recorded for the diffraction angle range 2 < 2h < 80. The intensities of the WAXD patterns were not corrected for polarization and Lorentz factors, to allow an easier comparison with most literature data. The Dhkl correlation length of crystals was determined applying the Scherrer equation Dhkl ¼ Kk=ðbhkl cos hhkl Þ

ð1Þ

where: K is the Scherrer constant, k is the wavelength of the ˚ , CuKa), bhkl is the width at half irradiating beam (1.5419 A height, and hhkl is the diffraction angle. The instrumental broadening, b, was determined by obtaining a WAXD pattern of a standard silicon powder 325 mesh (purity >99%), under the same experimental conditions. For each observed reflection with bhkl < 1, the width at half height was evaluated by subtracting the unavoidable instrumental broadening of the closest silicon reflection from the experimental width at half height, Bhkl: 2

b2hkl ¼ ðB2hkl  b Þ

2.3.2.

ð2Þ

Scanning electron microscopy

The internal morphology of the aerogels was characterized by means of a scanning electron microscope (SEM, Zeiss Evo50 equipped with an Oxford energy dispersive X-ray detector). Samples were prepared by fracturing small pieces of the monoliths in order to gain access to the internal part of the specimen. In fact, the external lateral surfaces of most samples were found to be flat and free of porosity. Low energy was used (5 keV) to obtain the highest possible surface resolution. Before imaging, all specimens were coated with gold using a VCR high resolution indirect ion-beam sputtering system. The samples were coated depositing approximately 20 nm of gold. The coating procedure was necessary to prevent surface charging during measurement and to increase the image resolution.

2.3.3.

Raman spectroscopy

The Raman spectra were collected by a confocal Raman spectrometer (Horiba-Jobin Yvon Mod. Aramis) operating with a diode laser excitation source emitting at 532 nm. The 180 back-scattered radiation was collected by an Olympus

Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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metallurgical objective (MPlan 50·, NA = 0.75) with confocal and slit apertures both set to 200 lm. A grating with 600 grooves/mm was used throughout. The radiation was focused onto a Peltier-cooled CCD detector (Synapse Mod. 354308) operating in the Raman-shift range 3200–800 cm1.

2.3.4.

BET measurements

Nitrogen adsorption at liquid nitrogen temperature (77 K) was used to measure surface areas of carbon powders and polymeric aerogels with a Nova Quantachrome 4200e instrument. Before the adsorption measurement, powders were degassed at 100 C under vacuum for 24 h, while polymeric aerogels were degassed at 30 C, in the same conditions. The surface area values were determined by using 11-point BET analysis [65].

2.3.5.

Colorimetric measurements

Colorimetric measurement have been carried out using a KONICA MINOLTA Spectrophotometer (CM-2500d).

2.3.6.

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the oxidation procedure described in the Section 2, are shown in Fig. 1A and B, respectively. The graphite oxidation leads to an increase of the distance between the layers from 0.399 nm up to 0.84 nm while the in-plane periodicities (d100 and d110) remain unaltered [66]. Sonication at 100 C of the GO sample of Fig. 1B in an organic solvent (DCB) leads to stable dispersions (Fig. 2A and B). The X-ray diffraction of the powder as recovered by filtration after solvo-thermal reduction and, subsequently, extracted with scCO2 is shown in Fig. 1C. The pattern shows the maintenance of the in-layer graphitic order (1 0 0 and 1 1 0 reflections) and the replacement of the peak at d = 0.84 nm with a peak at d = 0.35 nm clearly indicating the formation of reduced GO by the used solvothermal procedure. s-PS/GO black gels can be easily prepared by dissolution at 180 C of s-PS in reduced GO dispersions, followed by cooling to room temperature of the hot solution (Fig. 2C). In particular, we have verified that stable gels can be obtained, at least for an overall amount of polymer and GO in the gels being in the range 2–20wt%.

Mechanical tests

The compressive properties of the aerogels were determined by the general compression test-SI method, using cylindrical specimens and Instron 3365 testing instrument equipped with a load cell of 10 kN. Cylindrical specimens of diameter 8,0 mm and height 2,0 cm were used. The top and bottom surface of the aerogels specimens were polished to ensure a good contact with the compression fixture. A crosshead speed of 1,00 mm/min was used and data for 3 specimens were collected to obtain average values of compressive modulus.

3.

Results and discussion

The X-ray diffraction patterns of the starting high surface area graphite (HSAG) and of the derived GO, as obtained by

Fig. 1 – X-ray diffraction (CuKa) patterns in the 2h range 2– 80 of: the starting graphite (A), of the derived GO (B) and of the reduced GO, as coagulated from dispersions like those used for aerogel preparations (C). Miller indexes and d spacings in nm are indicated close to the main diffraction peaks.

Fig. 2 – Preparation procedure for s-PS/GO aerogels: (A and B) dispersion of GO in the organic solvent, before (A) and after (B) sonication at 100 C for 2 h; (C) black organogel as obtained after dissolution at 180 C of s-PS in the GO dispersion (s-PS/GO, 80/20 by wt; overall solvent content in the gel: 90 wt%); (D) blue aerogel with porosity of 90%, as obtained by solvent extraction by scCO2. (A color version of this figure can be viewed online.)

Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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By extracting with supercritical carbon dioxide (scCO2) these s-PS/GO black organogels, monolithic blue aerogels are obtained, which essentially present the same size and shape of the precursor gels (Fig. 2D) and hence a porosity in the range 98–80% and a density in the range 0.02–0.2 g/cm3. A photograph and an optical micrograph of a monolithic sPS/GO, 80/20 aerogel, with porosity P = 90%, are shown in Fig. 3A and B. The micrograph shows the presence of GO particles, having average size of roughly 1–2 lm, evenly dispersed in an uniform polymer-rich matrix. For the sake of comparison, monolithic s-PS/graphite aerogels were also prepared from graphite (HSAG) dispersions in DCB, by a preparation procedure strictly analogous to that one used for s-PS/rGO aerogels. Aerogels with HSAG present larger graphite aggregates, even for low content of nanofiller (4 wt%), as shown by the photograph and the micrograph of Fig. 3B and B 0 , respectively. The large color difference between the obtained aerogels has been quantitatively compared by colorimetric measurements, as shown in Fig. 4, for aerogels having a porosity of 90%. The white aerogels constituted only by s-PS are highly reflective while the black aerogels with HSAG are of course highly absorbent. Aerogels with 20 wt% of GO present a higher reflectance with respect to the analogous aerogel with

only 4 wt% of HSAG. Moreover, the reflectance markedly decreases going from blue to red, with a dependence on wavelength being roughly proportional to 1/k. The occurrence of uniform light blue color for s-PS/GO aerogels, even for high GO content (Fig. 3A) is rather unique, because known composites as well as foams containing carbonaceous fillers are generally black. We have also verified that analogous aerogels constituted by GO and other thermoplastic polymers, like for instance isotactic poly-4-methylpentene-1 [26], PPO [34] and polyethylene [38] are grey or black depending on GO concentration. The X-ray diffraction (CuKa) patterns of the s-PS/GO aerogels, with porosity of 90% and presenting different polymer/ GO weight ratios are shown in Fig. 5A–E. The patterns show the presence of the diffraction peaks of the nanoporous-crystalline d form of s-PS [39] (Miller indexes indicated close to the curve A of Fig. 5), for the entire composition range. In this respect, it is worth adding that the degree of crystallinity of the polymer increases, going from the pure s-PS aerogel to the 95/5 aerogel (from nearly 45% up to 55%), suggesting a nucleating effect of graphitic layers on the crystalline d form. The reduced GO diffraction peaks are clearly detected only for the s-PS/GO 50/50 aerogel, for which well defined in-plane 1 0 0 and 1 1 0 peaks and a broad 0 0 2 peak, whose broadness

Fig. 3 – Photographs (A and B) and optical micrographs (A 0 and B 0 ) of monolithic aerogels, with porosity P = 90%, as prepared with cylindrical molds (diameter of 7 mm): (A and A 0 ) s-PS/GO, 80/20 by wt; (B and B 0 ) s-PS/HSAG 96/4 by wt (units on the rulers are in cm and 1 cm equals to 20 lm). Both aerogels present the same size and shape of the precursor gel. (A color version of this figure can be viewed online.) Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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Fig. 4 – Reflectance of s-PS-based aerogels with porosity of 90%: (A) pure s-PS; (B) s-PS/GO, 80/20 aerogel; (C) s-PS/HSAG, 96/4 aerogel; (D) s-PS/HSAG, 80/20 aerogel. Photographs of the cylindrical aerogels are shown close to the right end of the curves. (A color version of this figure can be viewed online.)

Fig. 5 – X-ray diffraction (CuKa) patterns in the 2h range 5– 50 of s-PS-based aerogels with porosity of 90 wt% and presenting different polymer/filler weight ratios: (A) 100/0; (B) 95/5 with GO; (C) 80/20 with GO; (D) 70/30 with GO; (E) 50/ 50 with GO; (F) 96/4 with HSAG and (G) 80/20 with HSAG. The Miller indexes of the main reflections of the nanoporouscrystalline d form of s-PS are indicated in A. The symbol g indicates reflections relative to the graphitic component.

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indicates a correlation length perpendicular to the graphitic planes D? = 3.5 nm, are observed (Fig. 5E). For GO content lower than or equal to 30 wt% (Fig. 5B–D), the absence of the narrow 0 0 2 peak of the reduced GO (Fig. 1C) clearly indicates that most of GO is constituted by structural layers exhibiting negligible order in the direction perpendicular to the graphitic plane. The X-ray diffraction patterns of the s-PS/HSAG aerogels, with porosity of 90% and presenting different polymer/filler weight ratios are shown in Fig. 5F and G. Differently from the case of s-PS/GO aerogels, s-PS/HSAG aerogels exhibit an intense 0 0 2 diffraction peak, already clearly apparent for a HSAG content as low as 4 wt% (Fig. 5F), indicating a correlation length perpendicular to the graphitic planes higher than 10 nm. Additional information relative to the polymer and GO distribution in the aerogels has been achieved by the RAMAN imaging technique, whereby the spectroscopic contrast is supplied by the GO peak at 1345 cm1. The Raman image collected on a 16 · 16 lm2 area is reported in Fig. 6. There are regions where the GO spectrum is dominant (see spectrum A), and others where it is barely detectable (see spectrum B). The analysis reveals that GO segregates in the form of lenticular domains having a size from 1 to 3 lm. The observation that, even when focusing on a GO domain, the s-PS spectrum is still comparable in intensity to the GO spectrum (Fig. 6, spectrum B) is an indication that the thickness of the GO domains is smaller than the spatial resolution along the z axis (2.5 lm). In the polymer-rich areas (the blue zones in Fig. 6) the GO peak at 1345 cm1 shows a consistent intensity, which indicates that GO is not completely phase-separated, but is also present and uniformly distributed within the polymeric continuous phase. Scanning electron micrographs (SEM) of polymer-rich regions of s-PS-based aerogels are shown in Fig. 7. In particular, SEM images of aerogels with porosity of 90%, for the pure polymer and for composite aerogels with GO and HSAG (polymer/filler ratio of 80/20 wt%) are shown in Fig. 7A–C, respectively. SEM images of s-PS/GO aerogels (Fig. 7B) are dominated by the typical fibrillar morphology (with diameters of roughly 50–100 nm) of d form s-PS aerogels (Fig. 7A). Moreover, the fibrillar polymer matrix presents uniformly distributed GO platelets (white spots with diameters of roughly 80–200 nm in Fig. 7B). The SEM of the analogous s-PS/HSAG aerogels, present completely different morphologies with a strongly reduced density of fibrils exhibiting increased diameters (300–500 nm, Fig. 7C). Relevant information relative to the s-PS/GO aerogels also comes from surface area evaluations, as conducted by the BET method [65]. The data of Table 1 show that, although the starting graphite exhibits a high surface area, the derived GO and reduced GO samples present low surface areas. The surface area of the composite s-PS/GO aerogels is, however, much closer to that one of the pure s-PS aerogels (312 m2/g), i.e. much higher than the weight-average calculated values. It is worth noting that although the starting graphite as well as the graphite when subjected to the same procedures used for aerogel preparation exhibits high surface areas (308 and 233 m2/g, respectively; Table 1), the s-PS/HSAG aerogels present surface areas much smaller than those observed for

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Fig. 6 – RAMAN microscopy map (16 · 16 lm) of a s-PS/GO aerogel with weight ratio 80/20 and P = 90%. Image reconstruction is based on the intensity of the GO peak at 1345 cm1. The spectra denoted A and B are collected at the corresponding positions in the map. (A color version of this figure can be viewed online.)

Fig. 7 – SEM images of s-PS-based aerogels with P = 90%: pure s-PS (A); s-PS/GO, 80/20 by wt (B); s-PS/HSAG, 96/4 by wt (C).

Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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Table 1 – Total surface area (SBET) of the starting graphite and of the derived GO, of both samples treated as in the aerogel preparation procedure, and of aerogels with P = 90% and exhibiting the nanoporous-crystalline d form, for different polymer/filler weight ratios. Sample

SBETa (m2 g1)

HSAG HSAG treated as in aerogel preparation s-PS/HSAG, 80/20 s-PS/HSAG, 96/4 GO GO treated as in aerogel preparation (reduced GO) s-PS/rGO, 50/50 aerogel s-PS/rGO, 70/30 aerogel s-PS/rGO 80/20 aerogel s-PS aerogel

308 233 173 277 0.8 2.0 238 254 289 312

a Total area evaluated following the BET model in the standard 0.05 < P/P0 < 0.3 pressure range.

s-PS/GO aerogels (Table 1). For instance, the s-PS/HSAG,80/20 aerogel presents SBET (173 m2/g) much smaller than for the s-PS/GO,80/20 aerogel (289 m2/g). The high surface area values observed for s-PS/GO aerogels clearly supports the previous conclusion, mainly based on X-ray diffraction analyses, that GO in the aerogels is mostly constituted by uncorrelated structural layers, i.e. graphene layers or stacks of few graphene layers with negligible order in the direction perpendicular to the graphitic plane. The optical reflectance analysis of Fig. 4 and the presence of uncorrelated GO layers (as mainly established by X-ray diffraction of Fig. 5 and BET analyses of Table 1) clearly suggest that the unusual blue color of the s-PS/GO aerogels is due to a Rayleigh scattering from isolated particles of reduced GO. In fact, analogous blue colors due to Rayleigh scattering from nanoparticles in different media have been observed in many different fields of science [67–71]. The occurrence of isolated and uncorrelated GO layers in the s-PS aerogel is possibly due to the good nanofiller dispersion in the polymer gels, which is maintained in the aerogels constituted by polymer fibrils of nanometric diameter (Fig. 7A and B). For the prepared aerogels, mechanical properties have also been evaluated. In particular, compression stress–strain tests are compared for s-PS and s-PS/GO, 80/20 wt% aerogels (both with porosity of 90%) in Fig. 8. The presence of GO slightly reduces the elastic modulus (from 1.9 to 1.6 MPa) while a good ductility is maintained, as also shown by the photographs of the starting and compressed s-PS/GO aerogels (insets in Fig. 8). Most monolithic solids with high porosity (e.g., activated carbons) are instead brittle and easily collapse under compression. Therefore, ultralight materials with good mechanical properties (mainly toughness) are in strong demand. The present robust high-surface area s-PS/GO aerogels, exhibiting the nanoporous crystalline d form and isolated GO layers, are hence expected to find several applications. For instance, these composite aerogels can be used as monolithic

Fig. 8 – Stress–strain curves in compression for aerogels, both with porosity of 90%: s-PS (red curve) and s-PS/GO, 80/ 20 wt% (blue curve). The insets show photographs (parallel and perpendicular to the cylinder axis) of the s-PS/GO aerogel before and after the compression test. (A color version of this figure can be viewed online.)

supported catalysts, which can exploit the molecular sorption ability and diffusivity [29–38] of the nanoporous-crystalline polymer phase and the catalytic activity [72–78] of the GO nanoplatelets. In fact, substantial improvements of catalytic activities have been already observed for Au-based [61] and TiO2 based [62] nanoparticles, when dispersed in s-PS aerogels. s-PS/GO aerogels, due to their thermoplastic nature, can also facilitate the GO dispersion in polymer melts, without GO re-aggregation, i.e. could be possibly used as GO ‘‘masterbatches’’ for melt polymer processing, which should allow obtaining GO-rich nanocomposites.

4.

Conclusions

Stable s-PS/GO organogels can be prepared by dissolution of sPS in GO dispersions in organic solvents, at least for an overall amount of polymer and GO in the gels being in the range 2– 20wt%. By extracting these s-PS/GO black organogels with supercritical carbon dioxide, monolithic blue aerogels, which essentially present the same size and shape of the precursor gels and hence roughly porosity in the range 98–80% and density in the range 0.02–0.2 g/cm3, are obtained. This blue color is observed only for s-PS/GO aerogels while analogous s-PS/ graphite aerogels or other polymer/GO aerogels are as usual black. The phenomenon is due to the occurrence in the nanoporous aerogels of Rayleigh scattering by the uncorrelated GO nanoplatelets that, as usual, decreases going from blue to red light. X-ray diffraction patterns show that the 0 0 2 graphitic reflection appears only as a broad peak and only for GO content higher than 30 wt%, thus confirming that (at least for GO content lower or equal to 20 wt%) most of GO is constituted by structural layers that exhibit negligible order in the direction perpendicular to the graphitic plane. The X-ray diffraction patterns also show that all the prepared aerogels exhibit the nanoporous-crystalline d form of s-PS. The dispersion of the

Please cite this article in press as: Longo S et al. Rayleigh scattering by graphene-oxide in syndiotactic polystyrene aerogels. Carbon (2014), http:// dx.doi.org/10.1016/j.carbon.2014.06.003

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GO platelets in the aerogels has been also studied by optical and electron microscopy (SEM) as well as by RAMAN imaging. Although GO presents a low surface area (nearly 2 m2/g), the surface area of the composite s-PS/GO aerogels is high (240–290 m2/g) and close to those of pure s-PS d form aerogels. This again supports the conclusion that reduced GO is mostly present in the aerogels as uncorrelated graphitic layers. s-PS/GO composite aerogels present mechanical properties, and in particular high ductility, close to those of analogous aerogels based on pure s-PS. These robust high-surface area s-PS/GO aerogels, exhibiting the nanoporous crystalline d form and mostly uncorrelated graphene oxide layers, are expected to have several applications, like e.g., as monolithic supported catalysts or as ‘‘masterbatches’’ for polymer composite processing.

Acknowledgements We thank Dr. Luca Giannini of Pirelli Tyre Research Center, Prof. Maurizio Galimberti and Dr. Valeria Cipolletti of Polytechnic of Milan, Prof. Roberto Pantani, Dr. Felice De Santis, Prof. Ernesto Reverchon and Prof. Vincenzo Venditto of the University of Salerno for useful discussions. Dr. Pietro La Manna of the Institute of Chemistry and Technology of Polymers of CNR is acknowledged for technical support in RAMAN measurements. Financial support of ‘‘Ministero dell’ Istruzione, dell’ Universita` e della Ricerca’’ (PRIN) is also gratefully acknowledged.

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