Preparation and Characterization of Graphene/RTV Silicone Rubber

Composites

Jian Dong a*, Penghui Wang b, Daobao Sun c, Yuliang Xu d and Kepeng Li e

School of Chemical Engineering, Taishan Medical University, Taian, Shandong, 271016, China

a dongjian@tsmc.edu.cn,

b 824859828@qq.com,

c 534546928@qq.com,

d xyliang0720@163.com,

e likepeng06@163.com

Keywords: graphene; RTV silicone rubber; composites; morphology; mechanical properties

Abstract. In this article we report the preparation of a graphene/room temperature vulcanized (RTV)

silicone rubber composite. Both the morphology and the properties of the composite were

investigated in detail. SEM study shows that the composite has a microphase-separated structure.

PDMS is the continuous phase, and the randomly distributed graphene nanosheets and a few

aggregates are the dispersed phase. However, DSC curves of the composites have only one glass

transition temperature (Tg). With the increases of the graphene content, Tg increases and Tm decreases.

Mechanical properties tests show that the addition of graphene has a significant reinforcement effect

on silicone rubber. The tensile strength is 0.37MPa with graphene mass fraction at 1.0%, which

increases 76% compared with that of pure silicone rubber parallel sample.

Introduction

Graphene is an atomically thick, two-dimensional sheet composed of sp2 carbon atoms arranged in a

honeycomb structure. Since its discovery by K. S. Novoselov and A. K. Geim in 2004 [1]

, it has

attracted tremendous attention from researchers in recent years owing to its large specific surface area

and unique electrical, mechanical and thermal properties. With the development of wide-scale

applicability including facile synthesis and high yield, this exciting material is ready for its practical

application in the preparation of polymer nanocomposites [2]

. In past studies, different polymer

matrixes have been employed, such as polystyrene (PS), polyurethane (PU), poly(methyl

methacrylate) (PMMA) and poly(vinyl alcohol) (PVA), etal. Electrical and thermal conductivity and

mechanical properties have been improved significantly when a suitable amount of graphene was

dispersed into the different polymer hosts [3-5]

. Thus far, little has been done to explore the composite

based on graphene nanosheets and silicone rubber [6]

.

The present contribution reports on the preparation, morphology, and properties of the

graphene/room temperature vulcanized (RTV) silicone rubber composites. The obtained composites

exhibited a significant enhancement of mechanical property at a low loading of graphene.

Experimental

Materials. Graphite oxide (GO) was synthesized by oxidizing graphite using a modified

Hummer’s method [7]

. Hydrazine hydrate was supplied by Tianjin BASF Chemical Industry. PDMS

(weight-average molecular weight 50,000) were supplied by Jinan Guobang Chemical Company.

PDMS was kept at 80℃ under vacuum for 24h to eliminate the cyclic compounds and oligomers.

Toluene and tetraethyl orthosilicate (TEOS) were supplied by Sinopharm Chemical Reagent

Company. Dibutyl tin dilaurate was supplied by Tianjin Reagent Company.

Preparation of graphene. The GO was suspended in water and exfoliated through ultrasonication

for 1h. The exfoliated GO was reduced to graphaene nanoplatelets by refluxing the GO solution with

hydrazine hydrate at 100℃ for 24h. The final products were then centrifuged, washed, and finally

vacuum-dried.

Advanced Materials Research Vols. 652-654 (2013) pp 11-14Online available since 2013/Jan/25 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.652-654.11

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Preparation of Graphene/RTV Silicone Rubber Composite. Graphene suspension was

obtained by dispersing graphene powder in toluene with the aid of sonication at room temperature.

The suspension was then combined with a solution of PDMS in toluene. Ultrasonication was then

applied to the mixture for 1h. After removing the solvent under vacuum, 5wt%TEOS was added to the

graphene-PDMS system and the mixture was stirred vigorously for 5 min. 1.0 wt% Dibutyl tin

dilaurate was then added as catalyst. After stirring for another 2 min, the resulting mixture was

transferred into a polytetrafluoroethylene mold. The cross-linking reaction was allowed to proceed at

room temperature for 7 days. The composite was then removed and stored at room temperature under

vacuum for at least 24h prior to investigation.

Characterization. Transmission Electron Microscopy (TEM) was performed on graphene by

using a JEM-2100 transmission electron microscope (JEOL Ltd., Japan). TEM samples were

prepared by dispersing graphene in absolute ethanol with the aid of sonication to form a homogeneous

suspension. Then, the suspension was dropped on a TEM copper grid and the solvent was evaporated

overnight at room temperature. Dimensions of graphene on a freshly cleaved mica substrate were

estimated with contact-mode atomic force microscopy (Agilent 5500 model, USA). Scanning

electron microscope (SEM) images of graphene/RTV silicone rubber composites were acquired using

a S-4800 scanning electron microscope (Hitachi, Japan). The surfaces and fracture section of the

sample were coated with gold just before examination. Differential scanning calorimeter (DSC)

measurements were conducted in a Q10 DSC apparatus (TA Instruments, USA) at a heating rate of

20℃/min. The tensile strength and elongation at break of all the composites were measured using a

AGS-H tensile testing machine (Shimadzu, Japan). The experiments were carried out at room

temperature at a crosshead speed of 100mm/min.

Results and discussion

Graphene Characterization. The obtained graphene nanosheets were fully analyzed by TEM and

AFM observations. Fig.1 shows a TEM image of graphene nanosheets. Large graphene nanosheets (a

few hundred square nanometers) were observed to be situated on the top of the copper grid, where

they resemble crumpled silk veil waves. Graphene nanosheets were rippled and entangled with each

other. They are transparent and exhibit a very stable nature under the electron beam. The most

transparent and featureless regions in Fig.1 are likely to be monolayer graphene nanosheet. Fig.2

shows an AFM image of graphene nanosheets, which reveals that the typical lateral size of graphene

is on the order of micrometers. Ripples and folds of the graphene nanosheets also can be seen in the

AFM image. The measured height of the flat graphene nanosheets is between 1-3 nm. While the

pristine graphene sheet is flat with a well-known van der Waals thickness of ~0.34nm, the as-prepared

graphene sheets are expected to be 1-9 graphene layers thick. Furthermore, the vertical distance

between the crest and the trough in the wrinkled sheets may contribute to the higher apparent

thickness.

Fig.1. TEM image of graphene. Fig.2. AFM image of graphene.

Composite Morphology. Fig.3 shows the SEM images of graphene/RTV silicone rubber

composite with a loading of 1.0 wt % graphene nanosheets. Fig.3 (a, b) show its surfaces, which are in

contact with air and with the polytetrafluoroethylene mold, respectively. Both surfaces exhibit a

12 Advances in Materials and Materials Processing

corrugated morphology, which is due to (1) the material shrinkage occurring during the crosslinking

or (2) the corrugation and scrolling of graphene sheets in the PDMS matrix. Fig.3 (c) shows the

fracture section of the composite. The image demonstrates that the graphene nanosheets randomly

disperse in the PDMS matrix and some graphene nanosheets protrude from the fracture surface. It

indicates that the graphene nanosheets can achieve relatively good dispersion in RTV silicone rubber

by solution-blending method. The image also reveals that there are a few graphene aggregates in the

PDMS matrix, which due to the inherent curl, folding function of graphene sheet [8]

, and the weak

interaction between graphene and PDMS matrix.

(a) (b) (c)

Fig.3. SEM images of graphene/RTV Silicone Rubber Composite.

DSC analysis. The transition behaviors of the composites were investigated by DSC. The observed

thermograms are illustrated in Fig.4. Glass transition and melting temperatures are given in Table 1.

As can be seen from Fig.4, the two composites have a crystalline melting point (Tm) around -44.7℃

and-45.2℃, respectively. This observation shows that even the presence of rigid domains of graphene

in the RTV silicone rubber does not prevent the formation of an organized structure of PDMS chains

in the network. Apart from the melting peak, both composites exhibit one glass transition temperature

(Tg). With the increases of the content of graphene, Tg increases. The reason may be attributed to the

reducing of the molecular chain flexibility of PDMS, which due to the interactions between graphene

and PDMS.

Fig.4. DSC thermograms of the composites.

Table 1 DSC analysis of the composites with different graphene mass fractions

Graphene mass fraction

[%]

Tg

[℃]

Tm

[℃]

0.1 -126.3 -44.7

1.0 -123.4 -45.2

Advanced Materials Research Vols. 652-654 13

Mechanical properties. Table 2 shows the mechanical properties of the composites with different

graphene mass fration. It is obvious that the addition of graphene has a significant reinforcement

effect on silicone rubber. Upon the graphene nanosheets loading, the tensile strength of the

composites apparently exhibits an enhancing trend. The tensile strength of the composite containing

1.0 wt % graphene is up to 0.37 MPa, while that of the pure silicone rubber parallel sample is 0.21

MPa; i.e. the tensile strength increased by 76%. The trend of the elongation at break with the graphene

mass fraction is similar to that of the tensile strength. The elongation at break of the composite

containing 1.0 wt % graphene is 224%, which increases 62% compared with that of pristine silicone

rubber.

Table 2 Mechanical properties of the composites with different graphene mass fractions

Sample

code

Graphene mass fraction

[%]

Tensile strength

[MPa]

Elongation at break

[%]

1 0 0.21 138

2 0.1 0.25 165

3 0.5 0.34 201

4 1.0 0.37 224

Conclusion

In this paper, the graphene/RTV silicone rubber composites were prepared by incorporating

graphene nanosheets into PDMS matrix by solution blending and ultrasonic dispersion method. The

composites thus formed exhibit superior mechanical properties with respect to the pristine RTV

silicone rubber parallel sample. However, more work should be continued. Further improvement of

mechanical properties via enhancing the interfacial interaction between the graphene nanosheets and

the PDMS matrix is the key challenge.

Acknowledgements

This work was supported by the Research Award Fund for Outstanding middle-aged and young

scientists of Shandong Province, China (Grant No. BS2010CL045) and the Science and Technology

Innovation Action Plan for College Students of Taian City (Grant No. 2011D1029).

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