Effects of the alkylamine functionalization of graphene oxide on the properties of polystyrene nanocomposites
© Jang et al.; licensee Springer. 2014
Received: 7 March 2014
Accepted: 19 May 2014
Published: 28 May 2014
Alkylamine-functionalized graphene oxides (FGOs) have superior dispersibility in low-polar solvents and, as a result, they interact with low-polar polymers such as polystyrene. In this work, the functionalization of graphene oxide using three types of alkylamines, octylamine (OA), dodecylamine (DDA), and hexadecylamine (HDA), was performed, and nanocomposites of polystyrene (PS) and FGOs were prepared via solution blending. Different dispersions of FGOs over PS were obtained for the three alkylamines, and the properties of the PS composites were influenced by the length of the alkylamine. A better thermal stability was observed with a longer chain length of the alkylamine. On the other hand, functionalization with the shortest chain length alkylamine resulted in the highest increase in the storage modulus (3,640 MPa, 140%) at a 10 wt.% loading of FGO.
Graphene has attracted intensive interest due to its extraordinary electrical, thermal, and mechanical properties [1, 2]. Among its wide range of applications, recent studies have demonstrated that polymer nanocomposites based on graphene have resulted in dramatic improvements in the mechanical, electrical, and gas barrier properties of pristine polymers [3–6]. Homogeneous dispersion of graphene in the polymer matrix is an essential requirement to obtain the desired properties. Graphene oxide (GO) with abundant oxygen-containing groups, such as epoxy, hydroxyl, and carboxyl, can be well dispersed in a polymer matrix due to its good interaction with polymer chains [7–9]. However, the agglomeration of graphene sheets due to van der Waals forces only allows for a successful colloidal suspension within a narrow range of organic solvents. Park et al. reported that highly reduced graphene oxide was dispersed in organic solvents with a sum of solubility parameters (δp and δH) in the range of 13 to 29 . Recently, it was reported that alkylamine-functionalized graphene oxide (FGO) exhibited good dispersion in solvents and a strong interfacial interaction with low-polar organic solvents and polymers [11–17]. GO modified with HDA showed superior dispersion up to 7 mg/mL in organic solvents with low Hansen solubility parameters, such as xylene and toluene . Thus, they could be effectively used as a nanofiller even in low-polar polymers such as polyethylene [19, 20].
In this work, three alkylamines, OA, DDA, and HDA, with different alkyl chain lengths were utilized to examine the effect of alkylamine functionalization of GO on the properties of FGO/PS composites. When the FGO/PS nanocomposites were prepared by solution blending, the FGOs were homogeneously dispersed over the PS matrix even at a high concentration in chloroform.
Preparation of FGO and FGO/PS nanocomposites
GO was prepared by a modified Hummers method using expanded graphite (Grade 1721, Asbury Carbons, Asbury, NJ, USA) which was heated for 10 s in a microwave oven. The ratio of GO to alkylamines (CH3(CH2)7NH2, CH3(CH2)11NH2, CH3(CH2)15NH2, Sigma Aldrich, St. Louis, MO, USA) was fixed at 1.0 g of GO to 0.01 mol of alkylamine. The alkylamine solutions were prepared by dissolving 0.010 mol of OA, DDA, or HDA in 30 mL of ethanol (SK Chemicals, Gyeonggi-do, Korea). The FGOs were produced by gradually adding the alkylamine solution into the GO solution (1.0 mg/mL) followed by stirring at 60°C for 12 h. During the alkylamine functionalization, the color of the GO solution gradually changed from yellow to black. This change was accompanied by an aggregation of graphene particles due to the hydrophobicity of the alkylamine-functionalized GO, indicating the simultaneous functionalization and slight reduction of GO [14, 19]. The suspensions were filtered and washed three times with methanol. The obtained products were denoted as FGO-OA, FGO-DDA, and FGO-HDA, respectively.
For solution blending of the FGOs and PS, we selected chloroform (OCI Chemical, Seoul, Korea), which is an effective media for both FGOs and PS. Based on the amount of PS (Mw approximately 192,000 g mol−1, Sigma Aldrich, St. Louis, MO, USA), the FGO loadings relative to PS were fixed at 0.5, 1.0, 2.0, 3.0, 5.0, and 10.0 wt.%. Solution blending was easily performed by adding 5 g of PS into the FGO in chloroform. The resulting FGO/PS solution was stirred for 2 h followed by sonication for 30 min. After that, the FGO/PS suspension was coaggregated by pouring the solution into 1.5 L of methanol (SK Chemicals, Gyeonggi-do, Korea) under vigorous stirring for 1 h. The products were filtered and washed three times with methanol and dried at 60°C for 12 h.
The compositions of the FGO/PSs were analyzed using an elemental analyzer (EA; Flash 2000, Thermo Scientific, Hudson, NH, USA). Fourier transform infrared (FT-IR) spectra were analyzed using an FT-IR spectrometer (Nicolet 380, Thermo Scientific, Madison, WI, USA). The morphologies of the freshly fractured surface of the neat PS and FGO/PS composites film were observed by scanning electron microscopy (SEM; JSM-6500FE, JEOL, Tokyo, Japan). A small amount of the FGO/PS nanocomposites was dispersed in ethanol in order to obtain meticulous field emission transmission electron microscope (FETEM; JEM-2100 F, JEOL, Tokyo, Japan) images. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10°C/min (Q50, TA Instruments, New Castle, DE, USA). The dynamic mechanical properties of the FGO/PS composites were measured using a dynamic mechanical analyzer (DMA-Q800, TA Instruments, New Castle, DE, USA) in the single cantilever deformation mode at a frequency of 1 Hz from 0°C to 180°C at a heating rate of 3°C/min.
Results and discussion
The elemental analysis was further used to confirm the covalent functionalization of GO with DDA. The N contents were determined to be 3.07, 3.17, 3.21, and 3.21 wt.% for reaction times of 6, 12, 18, and 24 h, respectively, while the Cgraphene/O ratios were in the range of 2.01 to 2.43. After 12 h of reaction, the Cgraphene/N ratio tended to saturate around 12.5, corresponding to one DDA molecule per six aromatic rings on the GO sheet.
Glass transition temperatures obtained from the tan δ curves
FGO loading (wt.%)
Three types of FGO/PS composites were successfully prepared by solution blending. FGOs in the form of grafted alkylamines showed excellent dispersion over PS even at 10 wt.% loading. The dispersed FGOs formed different morphologies over the PS matrix due to the steric effects resulting from the different chain lengths of the alkylamines. All of the FGO/PS composites possessed improved thermal properties and storage moduli with FGO loading. FGO-HDA/PS, which has the longest chain length, showed the best thermal stability compared to other alkylamines. On the other hand, the storage modulus of the FGO-OA/PS composite achieved a maximum value of 3,640 MPa at 10 wt.% FGO-OA loading, which corresponded to 140% of the pristine PS. The functionalization of GO with alkylamines is thought to improve the compatibility of GO with various low-polar polymers due to their good interfacial interaction.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2011–0022485).
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