One-side non-covalent modification of CVD graphene sheet using pyrene-terminated PNIPAAm generated via RAFT polymerization for the fabrication of thermo-responsive actuators
Introduction
Since its debut in 2004 [1], graphene has been under limelight due to its outstanding performance in electrical, optical, thermal, mechanical application, storage materials and sensors [2], [3], [4], [5], [6], [7], [8]. In previous work, a number of protocols have been employed for the synthesis of graphene, for example, chemical reduction of graphene oxide [9], [10], [11], mechanical exfoliation from graphite [1], [12], ball milling of graphite [13], [14] and chemical vapor deposition (CVD) [15], [16], [17], [18]. Chemical vapor deposition (CVD) has recently been considered as the most reliable method for the preparation of high-quality graphene films with large-area, good flexibility, high transparency, and superb electrical conductivity. However, graphene’s special properties such as zero band gap prohibit it to be directly utilized for some specific applications [19]. Therefore, graphene is usually applied in a form of composite with some other materials. Graphene-polymer nanocomposites have been prepared for varied applications, e. g. electronics, conductive and heat dissipation films [20], [21], [22]. Many methodologies have been reported for the generation of graphene-polymer composites. Graphene-polymer composites are usually prepared by solution mixing [23], high-speed shearing combined with ice-cooling [24], melt compounding [25] and in situ polymerization [26]. Kim et al. [27] prepared chemically modified graphene (CMG)/polyvinylidene fluoride-co-hexafluoropropylene (PVdF) graphene-polymer composites films for the fabrication of all-solid state flexible supercapacitors. Huang et al. [28] reported the preparation of reduced graphene oxide (RGO)/poly(vinyl pyrrolidone) (PVP) composite films for the fabrication of high-performance and flexible electrochemical capacitors.
Motivated by the great demand for stimuli-responsive actuators and sensors, significant efforts have been devoted toward the exploration of stimuli-responsive materials. Chemi- or physisorption of small molecules on graphene can lead to its deformation depending on coverage and nature of the interactions. Li et al. [29] demonstrated that the curvature of graphene can be achieved using thin films of thermoset polymers deposited over graphene ribbons. The interaction of graphene with small liquid droplets can also result in significant deformation due to surface tension effects [30].
Graphene sheets have been covalently modified by various polymers. Dai’s [31] group has successfully PEGylated nanographene oxide for the delivery of water-insoluble cancer drugs using π-π stacking interactions. Likewise, Chen and coworkers modified a few graphene-based materials including GO (graphene oxide) film, RGO (reduced graphene oxide), epitaxial graphene on SiC and CVD (chemical vapor deposition) graphene using self-initiated photografting and photopolymerizations (SIPGP) to achieve specific polymer functionalized graphene-based materials for fabricating patterned polymer brushes [32]. Similar approaches were also described for the functionalization of carbon nanotubes [33], [34].
RAFT polymerization has been established as a universal method for making controlled architectures since it was invented by CSIRO [35], [36], [37]. In addition, the RAFT approach has been widely used for the synthesis of end-group functionalized polymers either by modifying the extant polymers or via in situ polymerizations using pre-functionalized RAFT agents [38], [39], [40], [41], [42], [43]. Pyrene-terminated polymers have been made using RAFT polymerization in a few works before [44], [45], [46], [47]. However, to our knowledge, the modification of CVD-graphene using pyrene-terminated polymers via π-π stacking has not yet been reported. In this work, we reported, for the first time, the modification of the one-side of CVD graphene with well-defined pyrene-terminated PNIPAAm via π-π stacking interactions for the fabrication of thermo-responsitive flexible actuators. A thermo-responsive intelligent switch was also fabricated using these thermo-responsive composite films.
Section snippets
Materials
Ethanethiol (>99%), carbon disulfide (>99%), dioxane (>99%) and tosyl chloride (>99.9%) were purchased from Tianjin Guangfu Fine Chemical Research Institute. Dichloromethane (>99%), n-hexane (>97%), propyl acetate (>99.5%) and tetrahydrofuran (THF > 99%) were purchased from Tianjin Fuyu. 2,2-Azobis(2-methylpropionitrile) (AIBN) and sodium hydrogen carbonate were purchased from Tianjin Rgent chemicals company. N, N-dicyclohexylcarbodiimide (DCC, 99%) was purchased from Aladdin.
Synthesis of pyrene-terminated poly(N-isopropylacrylamide) (PNIPAAm) using a pyrene-terminal RAFT agent and the subsequent attachment of the PNIPAAm onto the CVD graphene via π-π interaction, as well as the deformation test
The synthesis of pyrene-functional RAFT agent was carried out via a condensation reaction between the hydroxyl groups from pyrene methanol and the carboxylic acid group from the RAFT agent in the presence of DCC and DMAP. The pyrene-functional RAFT agent was then used to synthesize poly(N-isopropylacrylamide) (PNIPAAm) with pyrene-terminal groups (Scheme 1). As shown in Scheme 2 the graphene sheet was grown on copper foil via chemical vapor deposition (CVD) of methane in the presence of
Conclusions
We have successfully synthesized graphene-PNIPAAm composite films that can be utilized as thermo-responsive actuators. The composite films were facilely prepared by non-covalent π-π stacking interactions between CVD graphene and pyrene terminated PNIPAAm, and the latter was synthesized via RAFT polymerization uisng a pyrene terminated RAFT agent. The lower critical solution temperature (LCST) of PNIPAAm endowed the CVD graphene-PNIPAAm with controllable thermo-responsibility. When the
Acknowledgements
This work was supported by the Natural Science Foundation of China (51173087) and Qingdao (12-1-4-2-2-jch), Qingdao Innovation Leading Expert Program.
Degang Jiang received the bachelor’s degree in chemistry from Qingdao University, China, in 2014. Now, he is a postgraduate student of Qingdao University. His research interests focus on the graphene and its nanomaterials.
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Cited by (0)
Degang Jiang received the bachelor’s degree in chemistry from Qingdao University, China, in 2014. Now, he is a postgraduate student of Qingdao University. His research interests focus on the graphene and its nanomaterials.
Huihui Zhu received the bachelor’s degree in microelectronics from Qingdao University, China, in 2014. Now, she is a postgraduate student of Qingdao University. Her research interests focus on the nanomaterials and electrochemistry.
Wenrong Yang received his Ph.D degree in chemistry in 2002 from the University of New South Wales. He currently is Deakin University, working on biological and biomedical applications of CNTs and graphene. He also is exploring single-molecule conductivity by scanning probe microscopy.
Liang Cui received the bachelor’s degree in material science from Qingdao University, China, in 2011. And then, he got his postgraduate degree in Qingdao University. Now, he is a doctor in Qingdao University. His research interests focus on graphene materials.
Jingquan Liu received his bachelor’s degree from Shandong University in 1989. His master’s and Ph.D degrees were obtained from the University of New South Wales (UNSW) in 1999 and 2004, respectively. His research interests focus on the synthesis of various bio- and nano-hybrids of versatile polymeric architectures.