Beyond Carbon Nanotubes: Functionalized Graphene … · T. Ramanathan,§ A. A. Abdala,# S. Stankovich,§ D. A. Dikin,§ M. Herrera-Alonso,# R. D. Piner,

Beyond Carbon Nanotubes 3/27/2008 1

Supplementary Information (SI) to accompany

Beyond Carbon Nanotubes: Functionalized Graphene Sheets for Polymer Nanocomposites

T. Ramanathan,§ A. A. Abdala,# S. Stankovich,§ D. A. Dikin,§ M. Herrera-Alonso,# R. D. Piner,§ D. H. Adamson,# H. C. Schniepp,# X. Chen,§ R. S. Ruoff,§ S. T. Nguyen,§ I. A. Aksay,#

R. K. Prud’homme,# L. C. Brinson§

§Northwestern University, Evanston IL 60208 #Princeton University, Princeton NJ 08544

SI-1. Preparation of Graphite Oxide and FGS:

Graphite oxide was prepared using a modification of the Staudenmaier1 method as described

previously.2 The FGS was also prepared as described previously.2 Note that the oxidation step

can be hazardous. The reaction results in the formation of chlorine dioxide gas, which is

explosive at high concentrations. To minimize risk, it is recommended that the addition of

potassium chlorate be done slowly while monitoring the temperature.

SI-2. Preparation of expanded graphite (EG):

Graphite (1 g) was treated with 4:1 v/v mixture of concentrated sulfuric and nitric acid

(50 ml) for 24 h at room temperature. Upon completion, the suspension was diluted with de-

ionized water (150 ml) and filtered over a Buchner funnel. The remaining solid residue was

washed with copious amounts of water until the filtrate was no longer acidic and then dried in an

oven at 100ºC overnight. This dried material was placed in a 50-ml quartz tube and heated

rapidly with a propane blow torch (Model TX9, BernzOmatic, Medina, NY) set at medium

1 Staudenmaier, L. Ber. Dtsch. Chem. Ges. 31, 1481 (1898). 2 Schniepp, H.C., Li, J.-L., McAllister, M. J. et al., J. .Phys. Chem. B 110 (17), 8535 (2006).

© 2008 Nature Publishing Group


intensity while under dynamic vacuum to produce the expanded graphite (EG). SEM images of

the resulting EG is shown in Fig. SI-1.

SI-3. Dispersion of FGS in organic solvents:

Dispersions of FGS were made at a 0.25 mg/ml concentration by sonicating FGS (5 mg) in a

given solvent (20 ml) for 5 h in a Fisher Scientific FS6 ultrasonic bath cleaner (40 watt power).

The dispersions were then left standing under ambient conditions.

The following was observed: FGS dispersions in methylene chloride, dioxane, DMSO, and

propylene carbonate precipitated within 8 h after sonication. The dispersion in nitrobenzene was

more stable, but FGS still precipitated out completely after 24 h. In THF, a moderately stable

dispersion was observed accompanied by fairly substantial precipitation after 24 h. However,

the THF dispersion still remained blackish after a week. More stable dispersions can be obtained

in DMF, NMP, 1,2-dichlorobenzene, and nitromethane: they were still quite black after one

week albeit with a small amount of sedimentation.

SI-4. Chemical Analysis of Nanofillers:

High-resolution XPS (Omicron, ESCA Probe, Taunusstein, Germany) spectra for

SWCNT, EG and FGS are shown in Figure SI-2. Samples were de-gassed overnight within the

XPS intro-chamber (~10-8 mbar) prior to analysis of the sample. Data were collect using 15kV

and 20mA power at 10-9 mbar vacuum. The raw XPS data for each sample were analyzed to

determine peak locations and areas in relation to specific binding energies which best fit the

experimental data. Aside from the main C-C peak at 284.6 eV, additional photoemission present

at higher binding energy at ~286eV peak for the SWCNT, EG, and FGS indicates the presence of

surface oxides. The peak areas and use of atomic sensitivity factors (for C1s is 1 and O1s is



2.85) provides the atomic concentration of oxygen element, providing a quantitative measure of

the extent of functionalization. Surface oxygen concentrations for SWCNT, EG, and FGS is 1.3

atomic %, 1.8 atomic % and 7.6 atomic % respectively. The low atomic oxygen concentration

for SWCNT and EG indicates that the surface oxides on these nanofillers are mainly attributed to

atmospheric oxidation or residual oxides resulting from the SWCNT purification process and

acid intercalation process to make EG.

Elemental analyses for SWCNT, EG, and FGS materials were performed by Atlantic

Microlab, Inc. (Norcross, GA) and the results are shown in Table 1. The higher oxygen content

of FGS compared to EG and SWCNT confirms that FGS has more surface functional groups

(oxides). The specific surface area was measured by gas absorption method (BET) by

Quantachrome Instruments, (Boynton Beach, FL) and the results are shown in Table 1. The

thermal exfoliation of GO leads to the high specific surface area of the product FGS material.

SI-5. Processing of nanocomposites:

We noted in the main text that ultrasonication, rather than shear mixing, is used for

processing the SWCNT nanocomposites; shear mixing can fragment the tubes and thus degrade

their properties. For verification, a control composite sample with SWCNTs was fabricated

using the shear-mixing procedure and yielded a lower composite modulus; thus, this method was

not pursued further. The ultrasonication method has been shown to work well to separate

SWCNTs from the bundles and results in a well-dispersed sample for both the as-received

SWCNT and functionalized SWCNT considered in a separate paper.3 Note that, although the as-

received SWCNTs are well-distributed throughout the sample, there is localized clustering at the

3 Ramanathan, T., Liu, H., Brinson, L.C., J. Polymer Sci.: Part B Polym. Phys. 43, 2269 (2005).



micron-scale level that leads to nanotube-rich and nano-tube poor regions (Fig. SI-3). This

localized aggregation results in a composite where the interphase of altered polymer near the

nanoparticles is not percolated, and hence the composite retains the bulk Tg signature of the host

matrix material.

SI-6. Percolation of Interphase:

Given the extraordinarily small fraction (0.05 wt%) of FGS in PMMA required for

rheological percolation (glass transition shift of 30°C), basic calculations were performed to

determine the likely extent of the influence of this nanofiller on polymer chain dynamics with

respect to the polymer radius of gyration, Rg. For the PMMA used with molecular weight of

350,000, the Rg is on the order of 40nm. Assuming individually dispersed FGS 0.34nm x

1000nm x 1000nm in dimension; and further assuming flat sheets, arranged in a perfect periodic

array; a spacing of 10 Rg between sheets yields a volume fraction of FGS of 0.05%. (Volume

fraction and weight fraction are similar, given the similar densities of the materials.) Thus, for a

composite with uniform dispersion of 0.05% FGS volume fraction, an influence domain of

around 5 Rg from the nanoparticle surface into the neighboring polymer can result in a percolated

(continuous) interphase. Such a magnitude for the region of altered dynamics is reasonable in

comparison with the literature on polymer mobility in the vicinity of surfaces (see references in

the main text). Naturally, many other complications exist in the real system, including random

orientation, wrinkled sheets, and non-periodic dispersion; nevertheless, this simplified

calculation provides a rough, but useful, approximation of interphase extent.



SI-7. X-ray Diffraction:

To complement microscopy data, XRD was performed on neat PMMA as well as the 1-wt%

FGS/PMMA and EG/PMMA composites (Fig. SI-1(A)). The sample was scanned at 4°/min in a

Rigaku diffractometer (Rigaku Americas, The Woodlands, TX) using CuKα radiation

(λ = 0.15 nm) with a filament voltage of 40 kV and a current of 20 mA. The diffracted X-ray

from the sample was detected in the range between 10° and 80° of 2θ. The XRD patterns for

EG/PMMA composites clearly show the characteristic 0.34-nm interlaminar spacing of native

graphite, even in the composites having low weight percent loading of the nanofiller. In contrast,

the XRD patterns for FGS/PMMA composites completely lack this peak.

SI-8. Thermal property measurements:

Thermal degradation properties of the composites were examined by thermal gravimetric

analysis (TGA) on a SDT 2960 Simultaneous DTA-TGA instrument (TA Instruments, New

Castle, DE). Pieces of the composites (~10 mg) were loaded into the TGA instrument and

heated from 40 to 800ºC at a rate of 10ºC/min under a flowing N2 atmosphere. Data are shown

in Fig. SI-4(A), where it is seen that the onset of thermal degradation is increased for all

nanocomposites relative to neat PMMA, with the largest increase observed for FGS/PMMA.

As stated in the Methods section, while DMA measurements were performed on all

FGS/PMMA samples to obtain complete temperature dependent modulus curves (e.g., Fig. SI-

4(B)) and Tg values, complementary DSC measurements were performed on selected

FGS/PMMA samples to obtain Tg values. A comparison of the Tg values from the DMA and

DSC data is obtained in Table SI-3. While absolute magnitudes of Tg measured by these different

methods differ, the trends in terms of changes of Tg from the baseline polymer are quite similar

and the conclusions regarding percolation are identical. It should also be noted that the glass



transition is a diffuse thermodynamic transition and measurement method and sample

preparation always affect the absolute values of Tg obtained. For the results in this paper,

processing and characterization procedures were maintained as identical as possible for each

sample and the base polymer was from the same batch from the manufacturer.

SI-9. Transmission Electron Microscopy and Field-Emissions SEM:

To complement the SEM images of the fracture surfaces, TEM and SEM were performed on 100

nm thick microtomed FGS/PMMAand EG/PMMA samples of 5-wt% (Fig. SI-7). TEM analysis

was performed on Hitachi HF2000 at 200 KV; SEM was performed on a Nova NanoSEM 600,

FEI Co.. The SEM of FGS/PMMA EG/PMMA reveals a stark contrast in morphology of the

composites and topology of the nanoplates and their distribution (Fig. SI-7b-e). These

micrographs suggest that FGSs are extremely wrinkled and randomly dispersed within the

matrix, whereas EGs at the same concentration are mostly straight platelets indicating their out-

of-plane rigidity. It is also clear that the effective volume occupied by FGS sheets is significantly

bigger than the volume occupied by EGs in the composite matrix.



Fig. SI-1. (a) XRD pattern of EG/PMMA and FGS/PMMA composites. The peak around 2θ = 15° in both samples is indicative of the intrinsic ordering of the amorphous polymer. Note that no graphite layer structure peak at 26° or graphite oxide peak at 13° is seen, which suggests that the dispersion of FGS in PMMA is probably close to the single-sheet level. (b) An SEM image of EG revealing the thick, layered structure to be contrasted with the thin wrinkled sheet images of FGS in Figure 1.

Fig. SI-2. High-resolution XPS (Omicron, ESCA Probe, Taunusstein, Germany) spectra for (a) SWCNT, (b) EG, and (c) FGS indicating oxygen moieties on the FGS surface. See text and Table SI-1.



Fig. SI-3. An SEM image of a nanocomposite synthesized using as-received SWCNT nanofiller at 1 wt% loading in PMMA (SWCNT/PMMA). The inset illustrates the local clustering of the nanotubes, resulting in a rheologically non-percolated system.



Fig. SI-4. TGA traces (a) and plots of storage modulus vs. temperature (b) for PMMA nanocomposites synthesized with different nanofillers at 1-wt% loading in PMMA. Data for neat PMMA and EG and SWNT based composites taken from references 6 and 12. All nanofillers increase the onset of thermal degradation, with the largest increase observed for the FGS/PMMA composite. The observed composite moduli exceed the modulus observed for neat PMMA at all temperatures, with the largest increases through the highest temperatures observed for the FGS/PMMA composite.



Fig. SI-5a. Normalized tan delta peaks (averages of 5 samples each) from the DMA shown for all nanocomposites at 1 wt% loading as well as for FGS/PMMA at three lower wt% loadings. FGS/PMMA composites demonstrate a remarkable ~30°C increase in Tg (peak of the tan delta curve), well beyond those observed for SWCNT/PMMA or EG/PMMA nanocomposites. A peak broadening, but no shift, in the Tg is seen for SWNT/PMMA and only a modest increase in Tg is observed for EG/PMMA. That the dramatic Tg shift of 30°C for FGS/PMMA nanocomposite starts at the lowest loading measured (0.01 wt%) and then remains constant for all loading beyond 0.05 wt% is evidence of rheological percolation of the altered interphase polymer. Data for PMMA and SWNT/PMMA from reference 6.



Fig. SI-5b. Tg plots for PMMA and ARG/PMMA, EG/PMMA, and SWNT/PMMA composites. At 0.1-wt% loading, Tg decreases for EG/PMMA;for SWNT/PMMA and ARG/PMMA, Tg remains same as neat PMMA.

Fig. SI-6. Optical images of FGS/PMMA nanocomposites at 0, 0.01, 0.05, 1 wt% loadings of FGS, indicating optical transparency at the lowest weight fractions.



a b

c d

200 nm 1 µm

200 nm



e

Fig. SI-7. Microscopy of nanocomposites from 5 wt% 100-nm-thick microtomed samples: (a) TEM image and (b-d) STEM and SEM image of FGS/PMMA nanocomposites and (e) SEM of EG/PMMA nanocomposites. The persistent wrinkled nature of the sheets in the nanometer length scale and apparent higher volume fraction of the FGS relative to the EG can be clearly observed. Note that the SEM and TEM images are not able to resolve to the single sheet level.



Table SI-1. Elemental analysis and surface area data for SWCNT, EG, and FGS.

Sample C (at%) O (at%) H (at%) N (at%) Surface area (m2/g)

SWCNT 95.21 0.84 - - 827 EG 93.68 1.59 - 0.11 20 FGS 83.37 6.58 9.75 0.1 1500

Table SI-2. Polymer, nanoparticle, and processing information for the data in Figure 3. Reference 12 provides a more comprehensive tabulation of available literature on exfoliated graphite-polymer nanocomposites.

Reference Polymer Nanoparticle Processing Wt(%) Tg increment °C

Tg method

Celik and Warner (2007)30

Polystyrene EG Solution followed by extrusion to drawn nanofibres

5 7 DSC

Uhl et al. (2005)31

ABS-PS EG Melt blending 5 6 DMA

Cho et al. (2005)10

PETI5 EG Solution 1 3 5

5 8.5 10

DMA

Xiao et al. (2002)32

Polystyrene GIC Solution 3.9 5 DSC

Zheng and Wong (2003)4

PMMA EG Solution 1 8 DMA

Yasmin and Daniel (2004)33

Epoxy Graphite platelet

Direct mixing 2.5 5.0

2 3

DMA

Yuen et al. (2007)34

Epoxy MWNT Solution 0.5 1

22 25

TMA

Putz et al (2004)35

PMMA SWNT In-situ polymerization

0.014 0 DMA

Table SI-3. Comparison of DMA and DSC values for Tg for selected FGS/PMMA composites. While DMA values are averages over 5 samples, DSC data are for 1-2 samples each.

Sample Tg (°C) (DSC)

ΔTg (°C) (DSC)

Tg (°C) (DMA)

ΔTg (°C) (DMA)

PMMA 102 105 0.01 wt% FGS/PMMA 113 11 122 17 0.05 wt% FGS/PMMA 125 23 134 29 1 wt% FGS/PMMA 127 25 134 29


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Beyond Carbon Nanotubes: Functionalized Graphene … · T. Ramanathan,§ A. A. Abdala,# S. Stankovich,§ D. A. Dikin,§ M. Herrera-Alonso,# R. D. Piner,