Graphene Oxide-Polymer Composite Langmuir Films Constructed by Interfacial Thiol-Ene Photopolymerization
- Xiaona Luo†1, 2,
- Kai Ma†2,
- Tifeng Jiao1, 2, 3Email author,
- Ruirui Xing2, 3,
- Lexin Zhang2,
- Jingxin Zhou2 and
- Bingbing Li4Email author
© The Author(s). 2017
Received: 27 September 2016
Accepted: 25 January 2017
Published: 8 February 2017
The effective synthesis and self-assembly of graphene oxide (GO) nanocomposites are of key importance for a broad range of nanomaterial applications. In this work, a one-step chemical strategy is presented to synthesize stable GO-polymer Langmuir composite films by interfacial thiol-ene photopolymerization at room temperature, without use of any crosslinking agents and stabilizing agents. It is discovered that photopolymerization reaction between thiol groups modified GO sheets and ene in polymer molecules is critically responsible for the formation of the composite Langmuir films. The film formed by Langmuir assembly of such GO-polymer composite films shows potential to improve the mechanical and chemical properties and promotes the design of various GO-based nanocomposites. Thus, the GO-polymer composite Langmuir films synthesized by interfacial thiol-ene photopolymerization with such a straightforward and clean manner, provide new alternatives for developing chemically modified GO-based hybrid self-assembled films and nanomaterials towards a range of soft matter and graphene applications.
Graphene oxide (GO)-based composites have received great attention over the past decade due to abundant oxygen-containing functional groups, which demonstrate GO moderate water dispersibility and reactive sites for further modification [1–4]. For examples, in recent years, Kim’s group has achieved excellent research works on the investigation of chemical modification of carbon-based materials and graphene nanocomposites as well as the applications for supercapacitors and liquid crystals [5–10]. In recent years, a lot of efforts have been devoted to rational design and controlled synthesis of various GO-based organic/inorganic nanocomposites, which promises better processability with electronic, optical, and electrochemical properties as well as great developments for nanomaterial applications [11–13]. The chemical modification and functionalization of GO materials with organic compounds are needed in order to make them appropriate for good dispersion and various applications [14–16]. For example, various small molecules including long-chain alkylamine, isocyanate derivatives, porphyrin, dopamine, or tetrathiafulvalene can be utilized to modify GO to obtain good dispersed in solvent and useful optical properties [17–21]. In comparison with small molecules, polymers can be also employed to modify GO to improve the properties of GO-based composites in distinct domains. For example, poly(2-(dimethylamino) ethyl methacrylate) and poly(vinyl alcohol) have been utilized to functionalized GO with different chemical reactions [22, 23]. However, this strategy more or less suffers from GO sheet aggregation, incomplete adsorption of organic molecules, and undesired side effects, which is unfavorable for subsequent desired applications. Alternatively, click chemistry has attracted great development in recent years due to its modular nature, high selectivity, and yields [24–27]. In contrast to click chemistry for Cu(I) system , no catalyst is required as the reaction is initiated thermally or photochemically. It is reported that the click chemistry has been displayed to functionalize GO with the thiol-ene/thiol-yne reactions or the azide-alkyne reactions [29–32]. Hence, it remains a formidable challenge to directly synthesize GO-based composites with eco-friendly and condition-gentle process in an effective organized self-assembly manner. On the other hand, Langmuir and Langmuir-Blodgett (LB) techniques are well known as a sophisticated and effective way in organizing molecules or building blocks in a two-dimensional confined environment to obtain interfacial organized films [33–35].
Since the initial reports about GO self-assembly in Langmuir films from Huang’s group [36, 37], some recent studies present the successful preparation of GO monolayer or GO-based composite films using the LB assembly method [38–46]. It can be expected that a combination of GO-based composites involved in click chemistry and self-assembly films by Langmuir technique should be particularly advantageous owing to their excellent biocompatibility, moderate nanostructures, and enhanced mechanical and chemical properties. To the best of our knowledge, GO-polymer composite films, particularly Langmuir films by interfacial thiol-ene photopolymerization, have not yet been reported and obtained. Thus, aqueous soluble poly(ethylene glycol) diacrylate (abbreviated as DA) containing a number of ethylene glycol as molecular skeleton and ene residues as headgroups capable of donating photopolymerization has been chosen for the formation of composite films.
The experimental used materials, poly(ethylene glycol) diacrylate (M w = 1000 g mol−1, abbreviated as DA), cysteamine (95%, abbreviated as CA), and chloroacetic acid were purchased from Aladdin Reagent (Shanghai, China). Graphite powder (325 mesh, 99%) was purchased from Alfa Aesar Chemicals (Shanghai, China). N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC-HCl) and N-hydroxysuccinimide (NHS) were purchased from Sigma-Aldrich and were used without purification. Sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), potassium nitrate (KNO3), hydrogen peroxide (H2O2, 30%, w/w), and hydrochloric acid (HCl) were purchased from Beijing Chemicals or Tianjin KaiTong Chemicals and used without further purification. All aqueous solutions were prepared with water purified in a double-stage Milipore Milli-Q Plus purification system. Firstly, graphene oxide (GO) was prepared from graphite powder by a modified Hummers method . Then, the carboxyl-functionalized GO (named as GO–COOH) was prepared according to the reported literature  and freeze dried in low temperature −50 °C). Thiol functionalization was done following analogous procedure described in the literatures [49, 50] and modified as described in the following. The obtained dispersion of exfoliated GO–COOH (100.0 mL of 1.5 mg/mL) was mixed with 2.55 g of NHS and 4.28 g of EDC-HCl into a round flask, under ice bath. After stirring vigorously for 2 h, 3.0 g of CA was added to the mixture and left stirring overnight under ice bath and next 3 days at room temperature. The resulting thiol-modified GO solid (named as GO-SH) was separated by filtration by using an amide membrane filter (0.2 μm), repeatedly washed with deionized Milli-Q water. Then, the solid was dispersed with water and dialyzed in water for 4–5 days with dialysis tubing (MWCO 12400). After dialysis, GO-SH powder was obtained by freeze-drying.
The interfacial characterization and LB film transfer were carried out in KSV-NIMA Minitrough LB system. The trough was carefully cleaned with chloroform and ethanol and then filled with DI water or aqueous DA solution. After a sonication of 10 min, some volumes of GO-SH spreading solution (0.504 mg/mL, different mixed solvent ratios and volumes) were dropwise spread onto pure water surface or DA subphase solution (0.500 g/L) using a glass syringe. Surface pressure was monitored using a tensiometer attached to a Wilhelmy plate. The film was compressed by barriers at a speed of 2 mm/min. For the Langmuir film of GO-SH on DA subphase by photoreaction, UV light of 365 nm was irradiated on interfacial films at 5 min before compression begin and kept through whole compress process. The GO-SH monolayer was transferred to substrates at various points during the compression by vertically dipping the substrate into the trough and slowly pulling it up (2 mm/min). Mica, quartz, and, CaF2, glass plates were used as the substrates to transfer monolayer or multilayer for the next morphological and spectral characterizations. Quartz and glass plates were treated with 1:1:5 NH4OH:H2O2:H2O (by volume) and washed repeatedly with deionized water before use.
The morphology of composite films were characterized by using a field-emission scanning electron microscopy (FE-SEM, S-4800II, Hitachi, Japan) with the accelerating voltage of 5–15 kV. The chemical composition of the samples was characterized by energy-dispersive X-ray spectroscopy (EDXS). EDXS analysis was typically performed at an accelerating voltage of 200 kV, using an Oxford Link-ISIS X-ray EDXS microanalysis system attached to SEM. Atomic force microscopy (AFM) images were measured by using a Nanoscope model MultiMode 8 Scanning Probe Microscope (Veeco Instrument, USA) with silicon cantilever probes. Raman spectroscopy was performed using a HORIBA Jobin Yvon XploRA PLUS confocal Raman microscope equipped with a motorized sample stage. The wavelength of the excitation laser was 532 nm, and the power of the laser was kept below 1 mW without noticeable sample heating. The intensity of a Raman peak was extracted from the maximum value after baseline subtraction over corresponding spectral range. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically, the hydrocarbon C1s line at 284.8 eV from adventitious carbon is used for energy referencing. Both survey scans and individual high-resolution scans for S(2p), O(1s), and C(1s) peaks were recorded. FTIR spectra were recorded on a Fourier infrared spectroscopy (Thermo Nicolet Corporation) by the conventional KBr disk tablet method or composite films on CaF2 plates. X-ray diffraction study was carried out by using an X-ray diffractometer (SmartLab, Rigaku) equipped with a conventional Cu Kα X-ray radiation (λ = 1.54 Å) source and a Bragg diffraction setup. Elemental analysis was carried out with a FlashEA Carlo-Erba-1106 Thermo-Quest. A UV lamp (20 mW/cm2, λ = 365 nm; LUYOR-3405; LUYOR Corporation) was used to irradiate the Langmuir films to perform the photochemical reactions.
Results and Discussion
The thiol-functionalized GO-SH material can serve as building blocks for preparation of Langmuir films and subsequent in situ interfacial thiol-ene photopolymerization under UV light irradiation by standard Langmuir self-assembly technique. Figure 1 demonstrates the illustration of GO-SH composite Langmuir films constructed by in situ thiol-ene photopolymerization. For this, the precursor GO, the intermediate GO–COOH, and the final building block GO-SH were synthesized and characterized by many methods. XRD curves of as-prepared materials show the disappearance of Bragg peaks at 2θ value of 11.2° assigned to the (001) diffraction peak and presence of new broad peak at 2θ value of 22.1° for GO-SH (Additional file 1: Figure S1), which indicate successful thiolation in GO sheet and 3D structural composite formation due to thiol-functionalization . The elemental analysis data (Additional file 1: Table S1) of as-prepared GO-based materials show the obvious increment of carbon composition and appearance of sulfur component with value of 10.10 ± 0.07 wt.% for obtained GO-SH. The comparison of XPS data and elemental analysis data show similar calculated N/C ratios and S/C ratios (Additional file 1: Table S2), which is almost in agreement with the above obtained results. Thus, we inspected the interfacial phase behaviors and characterized the Langmuir and the transferred LB films of GO-SH by means of morphological and spectral methods.
Analysis of XPS data of as-prepared multilayer GO-SH LB films from different subphasea
GO-SH LB Films
N/C ratio [%]
S/C ratio [%]
DA subphase with UV light
In order to evaluate the successful functionalization in GO sheet and characterize the prepared composite films, FT-IR spectra were measured. The spectra of GO and GO–COOH in Additional file 1: Figure S4 show typical bands due to skeletal vibrations of graphene domains at 1632 cm−1 and characteristic OH and C=O stretching at 3432 and 1737 cm−1, respectively [61–64]. As for spectrum of GO-SH, some new characteristic bands appeared, such as amide C=O stretching at 1640 cm−1, N-H bending at 3312 cm−1 and N-H stretching at 1548 cm−1, which indicated the effective functionalization of thiol groups. In addition, obvious changes were observed for the IR spectra of transferred multilayer LB films of GO-SH before and after thiol-ene photopolymerization with DA subphase (Additional file 1: Figure S5). The spectrum of GO-SH composite films after thiol-ene photopolymerization show obvious intensity increment for bands at 2921 and 2850 cm−1 (C–H modes of methylene moieties), 1727 cm−1 (C=O stretching), and 1640 cm−1 (amide C=O stretching), indicating the addition of cysteamine molecules in films and effective thiol-functionalization. Moreover, the measured XRD curves of multilayer LB films of GO-SH before and after thiol-ene photopolymerization reaction show similar broad Bragg peaks centered at 2θ value of 22.7° (Additional file 1: Figure S6), which suggest the retainment of 3D structures in composite films. Taken together, the supramolecular graphene oxide-polymer composite Langmuir films have been constructed by in situ thiol-ene photopolymerization, providing a potential for further application in GO functionalization and soft matter engineering.
In summary, we have presented a one-step chemical preparation of graphene oxide-polymer composite Langmuir films by in situ thiol-ene photopolymerization reaction in a facile and effective manner. Thiol-functionalized graphene oxide alone serves as either a crosslinking agent or an amphiphile for the formation of composite Langmuir films. The obtained GO-SH-DA composite films are demonstrated by the presence of a 3D nanostructure with embedded GO sheets in crosslinked films. The mechanism for the formation of the stable composite films involves a chemical thiol-ene photopolymerization reaction of thiol groups modified on GO sheet with ene components in soluble polymer chains. The resulting GO-polymer composite films can be easily transferred onto a supported substrate with assistance of the LB assembly method. Owing to the specific mechanical and chemical properties of graphene oxide and polymer composition, the present prepared composite films will have great potentials for application of soft matter engineering and graphene self-assembled nanomaterials.
This work was financially supported by the National Natural Science Foundation of China (No. 21473153), the Support Program for the Top Young Talents of Hebei Province, the China Postdoctoral Science Foundation (No. 2015 M580214), the Scientific and Technological Research and Development Program of Qinhuangdao City (No. 201502A006), and the Open Funding Project of the National Key Laboratory of Biochemical Engineering (No. 2013KF-02).
XL and KM contributed equally to this work. XL and KM participated in the analysis and the testing of the Langmuir films. RX and LZ carried out the synthesis and characterization of GO materials. TJ and BL supervised this work, helped in the analysis and interpretation of data, and, together with LZ, worked on the drafting and revisions of the manuscript. JZ conceived of the study and participated in its design and characterization. LZ and JZ participated in the design of the study and provided analysis instruments. All authors read and approved the final manuscript.
XL, KM, and RX are PhD students. JZ is a research assistant. TJ, LZ, and BL are professors.
The authors declare that they have no competing interests.
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