Mechanical properties of graphene films enhanced by homo-telechelic functionalized polymer fillers via π–π stacking interactions

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Abstract

Most researches on graphene/polymer composites are focusing on improving the mechanical and electrical properties of polymers at low graphene content instead of paying attention to constructing graphene’s macroscopic structures. In current study the homo-telechelic functionalized polyethylene glycols (FPEGs) were tailored with π-orbital-rich groups (namely phenyl, pyrene and di-pyrene) via esterification reactions, which enhanced the interaction between polyethylene glycol (PEG) molecules and chemical reduced graphene oxide (RGO) sheets. The π–π stacking interactions between graphene sheets and π-orbital-rich groups endowed the composite films with enhanced tensile strength and tunable electrical conductivity. The formation of graphene network structure mediated by the FPEGs fillers via π–π stacking non-covalent interactions should account for the experimental results. The experimental investigations were also complemented with theoretical calculation using a density functional theory. Atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), UV–vis and fluorescence spectroscopy were used to monitor the step-wise preparation of graphene composite films.

Introduction

Graphene has drawn extensive attention from both the experimental and theoretical communities. Graphene’s highly conjugated 2D structure contributes to its excellent mechanical strength, such as the high breaking strength of 42 N m−1 and the Young’s modulus of 1.0 TPa [1], [2]. The incorporation of individual graphene sheets into polymer matrices brings about a new class of polymer composite materials, which have received increasing attention in recent years due to the enhanced properties [3], [4]. Graphene has been extensively utilized as filler to improve polymers’ mechanical properties via various methodologies, such as solution blending [5], in-situ polymerization [6], latex blending [7], [8], and melt compounding [9]. Besides, Kim and co-workers [10] developed a new fabrication process involving infiltration of epoxy into the porous graphene foam which formed via chemical vapor deposition on Ni surface.

Many researchers are now focusing on improving the mechanical properties of polymer at low graphene content [11], [12], [13], instead of paying attention to constructing graphene’s macroscopic structures [14]. In order to improve the properties of macroscopic graphene, various methodologies including divalent ion (Mg2+, Ca2+) modification [15], hydrogen bonding [16], [17], covalent functionalization [18], [19], [20], π–π stacking interactions [21], [22], [23] and other treatments [24], [25] have been explored to functionalize graphene or graphene oxide (GO) sheets. The main purpose of those treatments is to achieve uniformly aligned graphene sheets along the plane direction, in order to improve the mechanical and electrical properties of grapheme/polymer composites [26], [27]. Although strength and stiffness can be significantly enhanced via the above treatments, it still remains a great challenge to obtain ultra-tough materials based on the graphene sheets [28]. As those ultra-tough and conductive artificial materials have great potential in aerospace, flexible supercapacitor electrodes, electromechanical actuator, and tissue engineering [29], [30], [31].

Hydrogen bonding, electrostatic interaction and coordination, π–π stacking interaction have been extensively applied as non-covalent approaches to modify graphene [21], [32]. As we all know, PEG macromolecules can insert into the interior of GO sheets’ skeleton through the hydrogen-bonding interactions between the GO nanosheet surface and PEG. However, the RGO and PEG has limited interaction because the removing of Cdouble bondO and Osingle bondH groups during reduction. Therefore, pyrene, a π-orbital rich group, is introduced to increase interactions between PEG and RGO, as it has strong π–π stacking interactions with other polyaromatic materials [21], [33], [34], [35]. Although π–π stacking modification increases the doping density and the density of electron–hole puddles, it does not disrupt the conjugation of graphene sheets [36], [37]. Accordingly, it remains the sp2 structure of graphene sheets. Pyrene functionalized amphiphilic block copolymers have also been designed to directly exfoliate graphene from graphite in our previous report [38], where the pyrene pendent copolymer might work like a polymer ‘‘glue’’ to bind the adjacent graphene sheets to form stable composites.

Herein, we prepared three homo-telechelic FPEGs that were modified with phenyl, pyrene and di-pyrene groups on both side of PEG chains, respectively. The strong π–π interaction between graphene and FPEG greatly improved the tensile strength of composite film. In addition, it was also found that the increasing distance between graphene sheets not only weakened the tensile strength of composite film but also damaged the interaction among graphene sheets, leading to compromised electrical conductivity of composite films.

Section snippets

Materials

Natural graphite powders with an average diameter of 3 mm were supplied from Xiamen Knano Graphite Technology Corporation. Sulfuric acid (98%, AR), phosphorus (V) oxide (98%, AR), potassium permanganate (AR), hydrogen peroxide (30%, AR) were purchased from Shanghai Jinlu Chemical. Hydrazine hydrate (AR) was purchased from Sinopharm Chemical Reagent. Ultrapure water was made by the Flom ultrapure water system. Succinic anhydride (SA, 99%), 2-amino-2-methyl-1,3-propanediol (AMPD, 98%) and

Preparation and characterization of RGO and homo-telechelic functionalized PEG precursors

During the reduction of GO in the presence of hydrazine, the color of GO suspension changed from yellow-brown to black. The structural change of GO during the reduction was also investigated by UV–vis absorption spectra (Fig. S1 in supporting information) and Raman spectroscopy (Fig. S2). The red-shift of the UV absorption should be resulted from the increasing π-electron density [42]. The Raman spectrum of GO has two prominent peaks at 1343 and 1598 cm−1 which should be attributed to the D and

Conclusion

In summary, three kinds of FPEG functionalized with phenyl, pyrene and di-pyrene moieties on both sides were successfully synthesized and utilized as fillers to prepare graphene/FPEG composite films using facile solution filtration method. The as-prepared graphene/FPEG composites exhibited enhanced mechanical properties and tunable electrical conductivities, depending on the feed ratios of the FPEG fillers. When the FPEG feed ratio was controlled properly the maximal tensile strength could be

Acknowledgements

This work was supported by the Natural Science Foundation of China with Grant Nos. 51173087 and 21305133, and NSF of Shandong (ZR2011EMM001) and Taishan Scholars Program and Jilin Province Science and Technology Development Plan Project (No. 201201006) for financial support.

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