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Benzoxazine-functionalized graphene oxide for synthesis of new nanocomposites
Accepted Manuscript Benzoxazine - functionalized graphene oxide for synthesis of new nanocomposites I. Biru, C.M. Damian, S.A. Gârea, H. Iovu PII: DOI: Reference:
S0014-3057(16)30432-3 http://dx.doi.org/10.1016/j.eurpolymj.2016.08.024 EPJ 7460
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
16 May 2016 12 August 2016 25 August 2016
Please cite this article as: Biru, I., Damian, C.M., Gârea, S.A., Iovu, H., Benzoxazine - functionalized graphene oxide for synthesis of new nanocomposites, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/ j.eurpolymj.2016.08.024
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Benzoxazine - functionalized graphene oxide for synthesis of new nanocomposites I. Biru, C.M. Damian, S.A. Gârea, H. Iovu* Advanced Polymer Materials Group, University Politehnica of Bucharest, Romania [email protected]
Abstract The new approach was to grow benzoxazine rings on graphene oxide chemically modified. This paper describes the preparation of benzoxazine – functionalized graphene oxide using the activation of the carboxylic groups from graphene oxide (GO) surface by 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide/N-Hydroxysuccinimide system (EDC/NHS) and the chlorination method employing SOCl2 respectively. The carboxylic groups from GO surface were treated with tyramine for synthesizing the hydroxyl groups that will further react with benzylamine and formaldehyde in order to form the benzoxazine rings. Finally a nano structure with strong covalent bonds between the graphene sheets and the polybenzoxazine chains was achieved. The formation of multi-benzoxazine functionalized graphene oxide was checked by FT-IR, 1H-NMR, TGA, Raman spectrometry, XRD, HR-TEM and XPS analysis. Keywords: graphene polymerization 1.
oxide,
benzoxazine,
covalent
functionalization,
ring
opening
Introduction
Due to their great potential, the interest in carbon materials has noticeably increased in the last years thus leading to a significant development for scientific and industrial research. As a two-dimensional monolayer of sp2-bonded carbon atoms arranged in a hexagonal lattice [1], graphene has attracted important attention due to its unique combination of superior properties. With high specific surface area, intrinsic mobility, Young`s modulus and good thermal and electrical properties [2] graphene and its derivatives have determined the scientists to use them as nanofiller incorporated in polymer matrix leading to a new class of polymer nanocomposites with enhanced properties [3]. By chemical oxidation of graphite using strong oxidizing agents and ultrasonic cleavage [4] graphene oxide (GO) is obtained, as monolayer carbon nanomaterial. In order to improve the compatibility and performance of GO and enlarge its application, especially in the area of polymer nanocomposites, a better interaction between graphene sheets and polymer matrix by functionalization of GO surface is required. Since a lot of oxygen functional groups such as hydroxyl, epoxy, carbonyl and carboxyl attached to the basal planes and sheet edges of graphene oxide are given by oxidative treatment [5], functionalization of graphene sheets is much easier to perform and allows the transfer of the excellent properties of graphene oxide to polymer-based nanocomposites. There are many functional groups that can be chemically anchored onto graphene oxide surface like amines [6] or alkyl amines of varying chain length [7], trimethoxy silanes [8], benzazoles [9], polyhedral oligomeric silsesquioxane (POSS) [10, 11], and others. A
lot of papers are related to formation of new covalent bonds between GO and various amines. Y. Hu and coworkers have successfully functionalized graphene sheets with different amine groups that showed good dispersion in the used solvents and increased thermal properties of composites [12]. Y. Jian-lin et. al have also functionalized graphene oxide with ethylenediamine (EA) and 1, 6 – hexanediamine (HA) showing that one carbon atom in nine to ten of the carbon atoms in GO was functionalized by an amine group and that the thermal stability of the GO functionalized by HA was much higher than the one functionalized by EA [13]. In particular, carboxylic groups of the graphene oxide surface have proven to be good candidates for chemical modification. Therefore, in order to realize a covalent bond between carboxylic acid functionalized graphene oxide (GO-COOH) and amines, some coupling reactions are required for the activation of these groups. Most activation reactions are done using thionyl chloride (SOCl2) [14-16], 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide (EDC) [17] or N, N`-dicyclohexylcarbodiimide (DCC) [18]. The benzoxazine resins have lately generated much interest due to their many advantages against the conventional phenolic thermosets [19]. As a novel type of phenolic resins, polybenzoxazines have many unique characteristics including low water absorption, near-zero volumetric shrinkage upon polymerization and molecular design flexibility to tailor the properties [20]. Another attractive advantage of these monomers is that benzoxazines may be thermally polymerized without using any catalyst [19]. However, pure benzoxazine-based polymers exhibit also some drawbacks, such as high curing temperature and poor mechanical properties like brittleness of the material in the polymerized and crosslinked form [21]. To improve these shortcomings, the addition of nanofillers to polybenzoxazine matrix is an important method to produce nanomaterials with increased stiffness, toughness and higher dimensional stability compared with the neat polymer. Although several types of polybenzoxazine – based composites have been obtained [2226], no study referring to the growth of the benzoxazine monomers on the graphene oxide surface was reported. A study about the introduction of benzoxazine rings onto graphene oxide surface was reported by F. Meng and coworkers [27] using the click chemistry route involving the benzoxazine monomer already prepared, not synthesizing it. Our study aims to develop a new route to produce benzoxazine-modified graphene oxide by synthesizing the benzoxazine rings directly onto the graphene oxide surface through adequate chemically modification of the surface. Furthermore a new concept is proposed for the exfoliation of graphene sheets which considers as a key factor the balance between polymerization -in and -out of the graphene layers. Therefore we proposed for the first time a simpler alternative to produce polybenzoxazine – graphene oxide nanocomposites with exfoliated structure which are formed by polymerization of benzoxazine rings attached to the same graphene layer. Thus true covalent bonds between polybenzoxazine matrix and graphene oxide as nanofiller are to be established. The final nanocomposite structure will be an exfoliated one with very high stiffness and also less brittle than usually produced polybenzoxazines. 2. Materials and methods 2.1. Materials Graphene oxide with carboxylic groups (GO-COOH) was received from NanoInnova Technologies (Spain). The amount of COOH groups in GO-COOH was 0.7 mmol of COOH/g. Tyramine, paraformaldehyde, tetrahydrofuran (THF), 1,4-dioxane, 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide (EDC), sodium phosphate dibasic (Na2HPO4) were 2
purchased from Sigma Aldrich (Germany). Thionyl chloride (SOCl2) and potassium phosphate monobasic (KH2PO4) were received from Riedel - de Haen. Benzylamine and NHydroxysuccinimide (NHS) were supplied by Fluka (Germany). All reagents and solvents were used as received without further treatment. 2.2. Instruments Fourier Transform Infrared Spectroscopy (FTIR) spectra were registered on a BRUKER VERTEX 70 spectrometer in 400–4000 cm−1 region. The samples were mixed with KBr powder and pressed in a pellet. The resolution was 4 cm−1 and 32 scans were performed. Proton nuclear magnetic resonance (1H-NMR) spectra were used to check the formation of benzoxazine rings. The spectra were registered on a BRUKER equipment operating at 400 MHz. Deutherated chloroform was used as a solvent and tetramethylsilane as an internal standard. Thermogravimetric analyses (TGA) were done on a Q500 TA instrument from 30 to 800°C using nitrogen with a heating rate of 10°C/min and a nitrogen flow rate of 90 ml/min. The Raman spectra were recorded on a DXR Raman microscope, from Thermo Fisher Scientific (Wisconsin, USA). The excitation laser wavelength was 532 nm using a laser power of 14 mW. The Raman spectra were collected in the range of 3200–200 cm–1. The X-Ray Diffraction Analysis (XRD) was done on a XRD 6000 SHIMADZU diffractometer. A test was carried out using the CuKα radiation source filtered with Ni. The patterns were automatically recorded with scan step of 0.02° and counting time of 0.6 s/step for diffraction angles 2 theta ranged between 3 and 12°, at room temperature. The HR-TEM analysis was performed on Tecnai G2 F30 S-TWIN equipment provided with 200 kV emission gun. XPS analysis was done on a K-Alpha instrument from Thermo Scientific, using a monochromated Al Kα source (1486.6 eV), at a pressure of 2×10-9 mbar. Charging effects were compensated by a flood gun and binding energy was calibrated by placing the C 1s peak at 284.8 eV as internal standard. Deconvolution of C 1s peaks was done after substraction of Shirley background. 2.3. Reaction between carboxylic groups of GO and tyramine (TYR) The carboxylic groups from graphene oxide will be treated with a hydroxyamine in order to gain a lot of hydroxyl groups on graphene oxide. These groups will react further on with amine and formaldehyde to give the benzoxazine rings which will be subsequently polymerized to produce the polybenzoxazine structure including the graphene oxide sheets exfoliated within the polymer. In order to react the –COOH groups of graphene oxide with tyramine two methods were employed. The first method consists in activation of the carboxylic groups from GO surface using EDC : NHS = 1:3 as activators (Fig. 1 – a). The activation process was done using two phosphate buffer solutions (PBS) with pH=5.5 and pH=7.2, respectively. To activate the GO and the carboxylic groups on the surface with tyramine, 5.42 mg of EDC were dissolved in 10 mL of PBS with pH=5.5. Then 50 mg of GO-COOH and 12.07 mg of NHS were added. The suspension was sonicated for 30 min on ice bath. The pH of solution was increased to 7.2 using PBS and further 4.8 mg of tyramine were added and the suspension was 3
sonicated for another 90 min on ice bath. Finally the suspension was filtered, washed with PBS 5.5 and dried at vacuum oven for 48 h. The second method consists in the reaction between GO-COOH and thionyl chloride (SOCl2). Furthermore, the chloroacyl groups from the edges of the graphene sheets will react with the amine groups from tyramine (Fig. 1 - b) and therefore hydroxyl groups are obtained on the graphene oxide surface.
Fig. 1 Synthesis of OH-functionalized graphene oxide obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
100 mg GO-COOH were refluxed with SOCl2 (60 ml) in the presence of DMF (0.5 ml) at 76°C for 24 h under nitrogen atmosphere to form acyl chloride functionalized GO (GOCl). The excess of solvent and SOCl2 were removed by distillation under reduced pressure. 9.6 mg of tyramine (TYR) dissolved in THF were added. The obtained solid (GO-TYR) was washed with tetrahydrofuran (THF) six times and dried at 40°C in a vacuum oven overnight.
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2.4. Synthesis of benzoxazine - functionalized graphene oxide (GO-Bz) In order to synthesize the benzoxazine - functionalized graphene oxide monomers (Fig. 2) the general procedure of benzoxazines synthesis was followed [19]. 1 mg of formaldehyde (37% in water) and 5 mL dioxane were introduced into a 50 ml three neck flask equipped with a magnetic stirrer, condenser, and dropping funnel. The mixture was cooled by an ice bath. Then, the right amount of benzylamine (BZA) in 2 ml dioxane was added dropwise, keeping the temperature below 10°C. The mixture was stirred for 30 min and then a solution of 30 mg GO-TYR in 10 ml dioxane was added. The reaction temperature was increased under reflux for 6 h. To purify the benzoxazine – functionalized graphene oxide, the synthesized product was washed five times with 3 N NaOH aqueous solution and finally with water and dried at vacuum oven for 48 h.
Fig. 2 Synthesis of multi-benzoxazine functionalized graphene oxide
3. Results and discussion The formation of multi-benzoxazine functionalized graphene oxide was checked by FT-IR, 1HNMR, TGA, Raman spectrometry, XRD, HR-TEM and XPS analysis. Theoretically, only the OH from phenol groups will react to form benzoxazine rings, but this should be further checked for -OH groups from graphene oxide considering the "sp2 environmental". The detailed chemical structure of the benzoxazine - functionalized graphene oxide and its precursors was revealed by FT-IR (Fig. 3). The FT-IR spectra interpretation of raw materials and final product (GO-Bz) is difficult since some important signals from benzoxazine rings are overlapped with signals from GO-COOH and GO-TYR structures. An important signal at 1234 cm-1 from GO-COOH structure was observed, corresponding to asymmetric stretching vibration of C-O bond (Fig. 3 - a). This signal is shifted to 1245 cm-1 for 5
GO-TYR corresponding to unreacted groups from GO-TYR structure. For GO-Bz this signal is replaced by a strong one at 1218 cm-1 corresponding to asymmetric stretching vibration of C-O bond from benzoxazine ring. The GO-TYR and GO-Bz spectra also differs by the signal from ~3400 cm-1 corresponding to the OH group form tyramine unit of GO-TYR. This band completely dissapears in case of GO-Bz, wich proves that the phenolic groups were consumed to form benzoxazine rings.
Fig. 3 The FT-IR spectra of GO-COOH, GO-TYR, GO-Bz obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
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Another important signal was noticed at 1058 cm-1 corresponding to symmetric stretching vibration of C-O bond from GO-COOH structure (Fig. 3 - b). This signal was replaced for GOBz with the signal at 1097 cm-1 corresponding to symmetric stretching vibration of C-O bond from benzoxazine ring. The GO-COOH used as raw material presents a broad signal at 1600 cm1 assigned to C=C bond from graphene structure. This signal was not observed for GO-Bz spectra. Instead, a significant signal was noticed at 1566 cm-1 corresponding to the outside plane vibration of C-H bond from benzoxazine ring in case of GO-Bz. Also, GO-TYR and GO-Bz spectra show a significant difference against GO-COOH due to the appereance of new bands at 803 cm-1 and 829 cm-1 respectively, wich may be assigned to symmetric stretching vibration of C-N bond. However this bond can not be used to make a difference between the structures of GO-TYR and GO-Bz since it appears in both spectra. The yield of the carboxylic groups transformation into amidic groups was calculated as a ratio between the intensity of 1700 cm-1 signal corresponding to C=O bond from –COOH and the intensity of 1600 cm-1 signal corresponding to C=C bond from graphene structure wich remains unmodified along the process. It was achieved 12% amidation yield for activation reaction with EDC/NHS and 70% amidation yield using SOCl2 method. The benzoxazine – functionalized graphene oxide products were characterized by 1H-NMR in order to prove the benzoxazine rings formation (Fig. 4). The 1H-NMR spectra of synthesized products showed the presence of proton signals from benzoxazine rings. The NMR spectrum of GO-Bz (EDC/NHS) indicates an important signal at 3.9 ppm (G) corresponding to protons from the methylene group of Ar – CH2 – N–. Also, an important signal at 4.85 ppm (H) was noticed, corresponding to protons from the methylene group of –N – CH2 – O– structure of benzoxazine rings. Aromatic protons are noticed in the range 6.5–7.5 ppm. The signal at 3.7 ppm (C) from 1 H-NMR spectrum of GO-Bz (SOCl2) is assigned to the methylene groups from the oligomers produced by opening of some benzoxazine rings during synthesis.
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a)
b)
Fig. 4 1H-NMR spectra of benzoxazine - functionalized graphene oxide monomers obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
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Fig. 5A shows the thermogravimetric analysis (TGA) of the raw material GO-COOH, GO-TYR and the final compounds GO-Bz obtained in both methods. It is clearly observed that the thermostability of the intermediary and final products increased significantly against raw material GO-COOH.
Fig. 5A The TGA curves of GO-COOH, GO-TYR and GO-Bz obtained by EDC/NHS activation method and chlorination method with SOCl2, respectively
Thus, in case of activation method with EDC/NHS, the temperature corresponding to loss mass of 3% (Td3%) is increased only with 14°C against GO-COOH. This fact is in good correlation with the low amidation yield obtained from FT-IR data which showed that only 12% of carboxylic groups were transformed into amidic groups. Further, the benzoxazine formation reaction increased Td3% with 17°C against GO-TYR. The increase of thermal stability is much consistent for using the SOCl2 method. In this case the Td3% is increased with 113°C for GOTYR. This fact is in good correlation with the 70 % amidation yield obtained from FT-IR data which means that an important amount of –COOH groups were transformed into amidic chains. These chains include aromatic rings from GO-TYR structure which definitively increase the overall thermal stability of GO-TYR. In case of GO-Bz obtained by SOCl2 method a low decrease of Td3% with 14°C was observed, probably due to presence of some opened benzoxazine rings. Table 1. The TGA data for GO-COOH, GO-TYR and GO-Bz Material (EDC/NHS) GO-COOH GO-TYR GO-Bz
Td3% (°C) 82.9 96 113.5
Material (SOCl2) GO-COOH GO-TYR GO-Bz
Td3% (°C) 82.9 195.8 181.2 9
Fig. 5B The dTG curves of GO-COOH, GO-TYR and GO-Bz obtained by EDC/NHS activation method and chlorination method with SOCl2, respectively
Also from Fig. 5B the derivative curves of weight loss may be observed which show the main processes of mass loss for GO-COOH, GO-TYR and GO-B z. Thus a low peak below 100°C is noticed for all the compounds assigned to the loss of adsorbed moisture [28]. The main peak for GO-COOH is located at 206°C, which was attributed to the degradation of functional groups of GO-COOH, according to the literature [29]. Moreover for GO-TYR and GO-Bz synthesized by EDC/NHS method, this peak is shifted to lower values of temperature, showing that only a lower percentage of carboxylic groups from GO-COOH were succeeded to be transformed into amidic and benzoxazine groups, respectively. However this peak is not anymore noticed for GO-TYR and GO-Bz synthesized by SOCl2 method, which is a supplementary proof that most of the carboxylic groups were successfully transformed into amidic and further to benzoxazine groups. Another significant peak was remarked around 400°C for GO-TYR and GO-Bz synthetized by EDC/NHS method which may be explained by the decomposition of the excess of tyramine (TYR) physically adsorbed onto modified GO surface. This is due to the low amidation yield which caused a significant unreacted TYR quantity to adhere to the surface. On the contrary, for GO-TYR and GO-Bz obtained by SOCl2 method, no decomposition peaks are noticed around 400°C meaning that the most of TYR added was successfully reacted with GO-COOH and furthermore benzoxazine groups are formed.
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In the Raman spectra for the raw material (GO-COOH), GO-TYR and final compounds GO-Bz obtained by both methods, the characteristics of the graphene structure, namely the intense signals D and G, which prove the presence of the graphene structure in the final product (Fig. 6) are observed.
Fig. 6 Raman spectra of GO-COOH, GO-TYR, GO-Bz obtained by: a) EDC/NHS activation method; b) chloronation with SOCl2
It is also important to notice the presence of the 2D band that characterizes the arrangement and number of graphene layers. Therefore, for the compounds obtained using EDC/NHS activation, the 2D band is broader even for the final GO-Bz which means that the aggregation in several 11
layers of graphene occurs. This is the case of the monomer for which the benzoxazine rings do not polymerize and consequently the driving force required for the disentanglement of the graphene sheets into independent layers does not exist. In the case of the compounds obtained by SOCl2 method, the 2D band is sharper which proves that the majority of the graphene aggregations were disintegrated due to the polymerization of the benzoxazine rings, this process leading to partially cancel the attraction between the graphene sheets (Fig. 7).
Fig. 7 The nanostructure synthesized by polymerization of benzoxazine - functionalized graphene oxide
The benzoxazine polymerization may take place either between the rings of the same GO layer (“in-graphene” polymerization) or between the rings of two neighbors of GO layers (“outgraphene” polymerization). The final balance between these two types of structure will give the ratio between intercalated and exfoliated structures. Consequently the “in-graphene” polymerization will lead to more exfoliated structures of GO-Bz. The XRD curves (Fig. 8) show a significant peak at 2θ=12.5⁰ assigned to the GO-COOH stacking conformation consisting in more layers. For GO-Bz the XRD curves show a significant difference between the benzonxazine – functionalized graphene oxide obtained by EDC/NHS (Fig. 8 - a) and SOCl2 (Fig. 8 - b) respectively. Thus GO-Bz synthetized by EDC/NHS presents a small peak at 2θ=13⁰ which means that the benzoxazine – functionalized graphene oxide layers are still forming an order structure. On the contrary, GO-Bz synthetized through SOCl2 method shows no peak in the range 2θ=020⁰ which is a supplementary proof that the exfoliation of benzoxazine – functionalized graphene oxide layers occurs. This is in good agreement with the Raman spectra and 1H-NMR data previously discussed.
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Fig. 8 XRD curves of GO-COOH and GO-Bz obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
The typical morphology of GO-Bz was characterized by HR-TEM. The images from Fig. 9 (a, b) show non-transparent, non-corrugated, and non-curled morphology. However some differences may be distinguished between Go-Bz synthesized on two different procedures, through SOCl2 (Fig. 9 - a) and EDC/NHS (Fig. 9 - b) method, respectively. Thus for GO-Bz obtained with SOCl2 the structure seems to be exfoliated with graphene sheets dispersed in the polybenzoxazine mass. On the opposite, GO-Bz synthetized with EDC/NHS shows a compact display of GO sheets, with no exfoliation, the benzoxazine monomer being formed at the surface of these sheets.
a)
b)
Fig. 9 High-magnification TEM images of GO-Bz obtained by: a) chlorination with SOCl2 b) EDC/NHS activation method 13
The significant XPS peaks assigned to GO structure are noticed from C1s deconvoluted spectrum peak at 284.4 eV (C=C) and at 292.2 eV (π-π* satellite peak). Three other peaks may be deconvoluted indicating the functionalities containing oxygen attached to the graphene surface (spectrum not shown). Thus the peaks at 285.7 eV (C-O from hydroxilic groups), 287.7 eV (C=O from carbonylic groups) and 289.5 eV (-O-C=O from carboxylic groups) are evidenced. For the benzoxazine structures (both GO-Bz obtained by EDC/NHS method (Fig. 10a) and GO-Bz synthetized by SOCl2 method (Fig. 10-b) new peaks are revealed in the C1s deconvoluted spectra among which the most significant is that located at 288.1 eV and 287.9 eV respectively which is assigned to the new –N-C-O- group from the benzoxazine ring. Also, a new peak at 285.2 eV and 285.1 eV respectively is noticed being attributed to the C-N groups from both the benzoxazine rings and tyramine structure. The presence of phenolic OH groups are pointed out through the signal at 286.4 eV and 286.3 eV respectively assigned to the C-OH units from both the unreacted phenol groups (tyramine structure) and phenol groups from the opened polymerized benzoxazine rings.
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Fig. 10 XPS spectra of a) GO-Bz obtained by EDC/NHS activation method; b) GO-Bz obtained by chlorination with SOCl2
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4. Conclusions
For the first time the structures of benzoxazine-functionalized graphene oxide were successfully synthetized by chemically-growing of the benzoxazine ring onto the GO surface. The yield of the whole process strongly depends on the activation method employed for obtaining of chloroacyl groups on GO, thus chlorination with SOCl2 gives higher yields. Consequently the chlorinated GO treated with tyramine and then the obtained OH groups being reacted with formaldehyde and benzylamine finally gives benzoxazine groups on the GO surface which are partially opened, thus forming polybenzoxazine chains either in the same layer of GO or between different layers of GO. The balance between these two structures is the major factor, which influences the ratio between intercalated and exfoliated structures. However, considering the hypothetic structure of exfoliated GO-Bz, it seems that the “ingraphene” polybenzoxazine chains are not bended one to another, acting individually in the nanocomposite mass. If the “out-graphene” polymerization is predominant a more ordered structure should be considered, the GO layers being assembled at less or higher interlayer distance, which depends on the polymerization degree of benzoxazine units.
5. Acknowledgements This research was financially supported by Sectoral Operational Programme Human Resources Development, financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/156/1.2/G/135764, contract PN II 59/2012 no. CH 39-12-09 and contract 105/10.10.2011 no. CH 39-15-01. The work of C. M. Damian has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132397.
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[19] N.N. Ghosh, B. Kiskan, Y. Yagci, Polybenzoxazines—New high performance thermosetting resins: Synthesis and properties, Prog. Polym. Sci. 32 (2007) 1344–1391. [20] B. Kiskan, N. N. Ghosh,Y. Yagci, Polybenzoxazine-based composites as high-performance materials, Polym Int, 60 (2011), 167–177. [21] R. Huang, S. O. Carson, J. Silva, T. Agag, H. Ishida, J. M. Maia, Interplay between rheological and structural evolution of benzoxazine resins during polymerization, Polymer, 54 (2013), 1880-1886. [22] S. M. Alhassan, S. Qutubuddin, D. A. Schiraldi, T. Agag, H. Ishida, Preparation and thermal properties of graphene oxide/main chain benzoxazine polymer, European Polymer Journal, 49 (2013), 3825–3833. [23] C. R. Arza, H. Ishida, F.H. J. Maurer, Quantifying Dispersion in Graphene Oxide/Reactive Benzoxazine Monomer Nanocomposites, Macromolecules, 47 (2014), 3685−3692. [24] M. Zeng, J. Wang, R. Li, J. Liu, W. Chen, Q. Xu, Y. Gub, The curing behavior and thermal property of graphene oxide/benzoxazine nanocomposites, Polymer, 54 (2013), 3107-3116. [25] Guo-mei Xu, Tiejun Shi, Jianhua Liu, Qidong Wang, Preparation of a Liquid Benzoxazine Based on Cardanol and the Thermal Stability of its Graphene Oxide Composites, . Appl. Polym. Sci., 2014, 1-8. [26] G. Xu, T. Shi, Q. Wang, J. Liu, Y. Yi, Facile way to prepare two novel DOPO-containing liquid benzoxazines and their graphene oxide composites, J. Appl. Polym. Sci., 132 (2015), 1-11. [27] F. Meng, H. Ishidab, X. Liu, Introduction of benzoxazine onto the graphene oxide surface by click chemistry and the properties of graphene oxide reinforced polybenzoxazine nanohybrids, RSC Adv., 4 (2014), 9471-9475. [28] Y. Zhao, H. Ding, Q. Zhong, Preparation and characterization of aminated graphite oxide for CO2 capture, Appl. Surf. Sci., 258 (2012), 4301-4307. [29] A. A. Alhwaige, S. M. Alhassan, M. S. Katsiotis, H. Ishida, S. Qutubuddin, Interactions, morphology and thermal stability of graphene-oxide reinforced polymer aerogels derived from star-like telechelic aldehyde-terminal benzoxazine resin, RSC Adv., 5 (2015), 92719-92731.
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New route to prepare nanocomposites based on benzoxazine and graphene oxide is proposed. The original idea was to grow benzoxazine rings directly onto the graphene oxide surface. Two functionalization methods of graphene oxide were employed. Polymerization of benzoxazine rings from the graphene oxide surface leads to exfoliated nanocomposites.
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Graphical abstract
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S0014-3057(16)30432-3 http://dx.doi.org/10.1016/j.eurpolymj.2016.08.024 EPJ 7460
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
16 May 2016 12 August 2016 25 August 2016
Please cite this article as: Biru, I., Damian, C.M., Gârea, S.A., Iovu, H., Benzoxazine - functionalized graphene oxide for synthesis of new nanocomposites, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/ j.eurpolymj.2016.08.024
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Benzoxazine - functionalized graphene oxide for synthesis of new nanocomposites I. Biru, C.M. Damian, S.A. Gârea, H. Iovu* Advanced Polymer Materials Group, University Politehnica of Bucharest, Romania [email protected]
Abstract The new approach was to grow benzoxazine rings on graphene oxide chemically modified. This paper describes the preparation of benzoxazine – functionalized graphene oxide using the activation of the carboxylic groups from graphene oxide (GO) surface by 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide/N-Hydroxysuccinimide system (EDC/NHS) and the chlorination method employing SOCl2 respectively. The carboxylic groups from GO surface were treated with tyramine for synthesizing the hydroxyl groups that will further react with benzylamine and formaldehyde in order to form the benzoxazine rings. Finally a nano structure with strong covalent bonds between the graphene sheets and the polybenzoxazine chains was achieved. The formation of multi-benzoxazine functionalized graphene oxide was checked by FT-IR, 1H-NMR, TGA, Raman spectrometry, XRD, HR-TEM and XPS analysis. Keywords: graphene polymerization 1.
oxide,
benzoxazine,
covalent
functionalization,
ring
opening
Introduction
Due to their great potential, the interest in carbon materials has noticeably increased in the last years thus leading to a significant development for scientific and industrial research. As a two-dimensional monolayer of sp2-bonded carbon atoms arranged in a hexagonal lattice [1], graphene has attracted important attention due to its unique combination of superior properties. With high specific surface area, intrinsic mobility, Young`s modulus and good thermal and electrical properties [2] graphene and its derivatives have determined the scientists to use them as nanofiller incorporated in polymer matrix leading to a new class of polymer nanocomposites with enhanced properties [3]. By chemical oxidation of graphite using strong oxidizing agents and ultrasonic cleavage [4] graphene oxide (GO) is obtained, as monolayer carbon nanomaterial. In order to improve the compatibility and performance of GO and enlarge its application, especially in the area of polymer nanocomposites, a better interaction between graphene sheets and polymer matrix by functionalization of GO surface is required. Since a lot of oxygen functional groups such as hydroxyl, epoxy, carbonyl and carboxyl attached to the basal planes and sheet edges of graphene oxide are given by oxidative treatment [5], functionalization of graphene sheets is much easier to perform and allows the transfer of the excellent properties of graphene oxide to polymer-based nanocomposites. There are many functional groups that can be chemically anchored onto graphene oxide surface like amines [6] or alkyl amines of varying chain length [7], trimethoxy silanes [8], benzazoles [9], polyhedral oligomeric silsesquioxane (POSS) [10, 11], and others. A
lot of papers are related to formation of new covalent bonds between GO and various amines. Y. Hu and coworkers have successfully functionalized graphene sheets with different amine groups that showed good dispersion in the used solvents and increased thermal properties of composites [12]. Y. Jian-lin et. al have also functionalized graphene oxide with ethylenediamine (EA) and 1, 6 – hexanediamine (HA) showing that one carbon atom in nine to ten of the carbon atoms in GO was functionalized by an amine group and that the thermal stability of the GO functionalized by HA was much higher than the one functionalized by EA [13]. In particular, carboxylic groups of the graphene oxide surface have proven to be good candidates for chemical modification. Therefore, in order to realize a covalent bond between carboxylic acid functionalized graphene oxide (GO-COOH) and amines, some coupling reactions are required for the activation of these groups. Most activation reactions are done using thionyl chloride (SOCl2) [14-16], 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide (EDC) [17] or N, N`-dicyclohexylcarbodiimide (DCC) [18]. The benzoxazine resins have lately generated much interest due to their many advantages against the conventional phenolic thermosets [19]. As a novel type of phenolic resins, polybenzoxazines have many unique characteristics including low water absorption, near-zero volumetric shrinkage upon polymerization and molecular design flexibility to tailor the properties [20]. Another attractive advantage of these monomers is that benzoxazines may be thermally polymerized without using any catalyst [19]. However, pure benzoxazine-based polymers exhibit also some drawbacks, such as high curing temperature and poor mechanical properties like brittleness of the material in the polymerized and crosslinked form [21]. To improve these shortcomings, the addition of nanofillers to polybenzoxazine matrix is an important method to produce nanomaterials with increased stiffness, toughness and higher dimensional stability compared with the neat polymer. Although several types of polybenzoxazine – based composites have been obtained [2226], no study referring to the growth of the benzoxazine monomers on the graphene oxide surface was reported. A study about the introduction of benzoxazine rings onto graphene oxide surface was reported by F. Meng and coworkers [27] using the click chemistry route involving the benzoxazine monomer already prepared, not synthesizing it. Our study aims to develop a new route to produce benzoxazine-modified graphene oxide by synthesizing the benzoxazine rings directly onto the graphene oxide surface through adequate chemically modification of the surface. Furthermore a new concept is proposed for the exfoliation of graphene sheets which considers as a key factor the balance between polymerization -in and -out of the graphene layers. Therefore we proposed for the first time a simpler alternative to produce polybenzoxazine – graphene oxide nanocomposites with exfoliated structure which are formed by polymerization of benzoxazine rings attached to the same graphene layer. Thus true covalent bonds between polybenzoxazine matrix and graphene oxide as nanofiller are to be established. The final nanocomposite structure will be an exfoliated one with very high stiffness and also less brittle than usually produced polybenzoxazines. 2. Materials and methods 2.1. Materials Graphene oxide with carboxylic groups (GO-COOH) was received from NanoInnova Technologies (Spain). The amount of COOH groups in GO-COOH was 0.7 mmol of COOH/g. Tyramine, paraformaldehyde, tetrahydrofuran (THF), 1,4-dioxane, 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide (EDC), sodium phosphate dibasic (Na2HPO4) were 2
purchased from Sigma Aldrich (Germany). Thionyl chloride (SOCl2) and potassium phosphate monobasic (KH2PO4) were received from Riedel - de Haen. Benzylamine and NHydroxysuccinimide (NHS) were supplied by Fluka (Germany). All reagents and solvents were used as received without further treatment. 2.2. Instruments Fourier Transform Infrared Spectroscopy (FTIR) spectra were registered on a BRUKER VERTEX 70 spectrometer in 400–4000 cm−1 region. The samples were mixed with KBr powder and pressed in a pellet. The resolution was 4 cm−1 and 32 scans were performed. Proton nuclear magnetic resonance (1H-NMR) spectra were used to check the formation of benzoxazine rings. The spectra were registered on a BRUKER equipment operating at 400 MHz. Deutherated chloroform was used as a solvent and tetramethylsilane as an internal standard. Thermogravimetric analyses (TGA) were done on a Q500 TA instrument from 30 to 800°C using nitrogen with a heating rate of 10°C/min and a nitrogen flow rate of 90 ml/min. The Raman spectra were recorded on a DXR Raman microscope, from Thermo Fisher Scientific (Wisconsin, USA). The excitation laser wavelength was 532 nm using a laser power of 14 mW. The Raman spectra were collected in the range of 3200–200 cm–1. The X-Ray Diffraction Analysis (XRD) was done on a XRD 6000 SHIMADZU diffractometer. A test was carried out using the CuKα radiation source filtered with Ni. The patterns were automatically recorded with scan step of 0.02° and counting time of 0.6 s/step for diffraction angles 2 theta ranged between 3 and 12°, at room temperature. The HR-TEM analysis was performed on Tecnai G2 F30 S-TWIN equipment provided with 200 kV emission gun. XPS analysis was done on a K-Alpha instrument from Thermo Scientific, using a monochromated Al Kα source (1486.6 eV), at a pressure of 2×10-9 mbar. Charging effects were compensated by a flood gun and binding energy was calibrated by placing the C 1s peak at 284.8 eV as internal standard. Deconvolution of C 1s peaks was done after substraction of Shirley background. 2.3. Reaction between carboxylic groups of GO and tyramine (TYR) The carboxylic groups from graphene oxide will be treated with a hydroxyamine in order to gain a lot of hydroxyl groups on graphene oxide. These groups will react further on with amine and formaldehyde to give the benzoxazine rings which will be subsequently polymerized to produce the polybenzoxazine structure including the graphene oxide sheets exfoliated within the polymer. In order to react the –COOH groups of graphene oxide with tyramine two methods were employed. The first method consists in activation of the carboxylic groups from GO surface using EDC : NHS = 1:3 as activators (Fig. 1 – a). The activation process was done using two phosphate buffer solutions (PBS) with pH=5.5 and pH=7.2, respectively. To activate the GO and the carboxylic groups on the surface with tyramine, 5.42 mg of EDC were dissolved in 10 mL of PBS with pH=5.5. Then 50 mg of GO-COOH and 12.07 mg of NHS were added. The suspension was sonicated for 30 min on ice bath. The pH of solution was increased to 7.2 using PBS and further 4.8 mg of tyramine were added and the suspension was 3
sonicated for another 90 min on ice bath. Finally the suspension was filtered, washed with PBS 5.5 and dried at vacuum oven for 48 h. The second method consists in the reaction between GO-COOH and thionyl chloride (SOCl2). Furthermore, the chloroacyl groups from the edges of the graphene sheets will react with the amine groups from tyramine (Fig. 1 - b) and therefore hydroxyl groups are obtained on the graphene oxide surface.
Fig. 1 Synthesis of OH-functionalized graphene oxide obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
100 mg GO-COOH were refluxed with SOCl2 (60 ml) in the presence of DMF (0.5 ml) at 76°C for 24 h under nitrogen atmosphere to form acyl chloride functionalized GO (GOCl). The excess of solvent and SOCl2 were removed by distillation under reduced pressure. 9.6 mg of tyramine (TYR) dissolved in THF were added. The obtained solid (GO-TYR) was washed with tetrahydrofuran (THF) six times and dried at 40°C in a vacuum oven overnight.
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2.4. Synthesis of benzoxazine - functionalized graphene oxide (GO-Bz) In order to synthesize the benzoxazine - functionalized graphene oxide monomers (Fig. 2) the general procedure of benzoxazines synthesis was followed [19]. 1 mg of formaldehyde (37% in water) and 5 mL dioxane were introduced into a 50 ml three neck flask equipped with a magnetic stirrer, condenser, and dropping funnel. The mixture was cooled by an ice bath. Then, the right amount of benzylamine (BZA) in 2 ml dioxane was added dropwise, keeping the temperature below 10°C. The mixture was stirred for 30 min and then a solution of 30 mg GO-TYR in 10 ml dioxane was added. The reaction temperature was increased under reflux for 6 h. To purify the benzoxazine – functionalized graphene oxide, the synthesized product was washed five times with 3 N NaOH aqueous solution and finally with water and dried at vacuum oven for 48 h.
Fig. 2 Synthesis of multi-benzoxazine functionalized graphene oxide
3. Results and discussion The formation of multi-benzoxazine functionalized graphene oxide was checked by FT-IR, 1HNMR, TGA, Raman spectrometry, XRD, HR-TEM and XPS analysis. Theoretically, only the OH from phenol groups will react to form benzoxazine rings, but this should be further checked for -OH groups from graphene oxide considering the "sp2 environmental". The detailed chemical structure of the benzoxazine - functionalized graphene oxide and its precursors was revealed by FT-IR (Fig. 3). The FT-IR spectra interpretation of raw materials and final product (GO-Bz) is difficult since some important signals from benzoxazine rings are overlapped with signals from GO-COOH and GO-TYR structures. An important signal at 1234 cm-1 from GO-COOH structure was observed, corresponding to asymmetric stretching vibration of C-O bond (Fig. 3 - a). This signal is shifted to 1245 cm-1 for 5
GO-TYR corresponding to unreacted groups from GO-TYR structure. For GO-Bz this signal is replaced by a strong one at 1218 cm-1 corresponding to asymmetric stretching vibration of C-O bond from benzoxazine ring. The GO-TYR and GO-Bz spectra also differs by the signal from ~3400 cm-1 corresponding to the OH group form tyramine unit of GO-TYR. This band completely dissapears in case of GO-Bz, wich proves that the phenolic groups were consumed to form benzoxazine rings.
Fig. 3 The FT-IR spectra of GO-COOH, GO-TYR, GO-Bz obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
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Another important signal was noticed at 1058 cm-1 corresponding to symmetric stretching vibration of C-O bond from GO-COOH structure (Fig. 3 - b). This signal was replaced for GOBz with the signal at 1097 cm-1 corresponding to symmetric stretching vibration of C-O bond from benzoxazine ring. The GO-COOH used as raw material presents a broad signal at 1600 cm1 assigned to C=C bond from graphene structure. This signal was not observed for GO-Bz spectra. Instead, a significant signal was noticed at 1566 cm-1 corresponding to the outside plane vibration of C-H bond from benzoxazine ring in case of GO-Bz. Also, GO-TYR and GO-Bz spectra show a significant difference against GO-COOH due to the appereance of new bands at 803 cm-1 and 829 cm-1 respectively, wich may be assigned to symmetric stretching vibration of C-N bond. However this bond can not be used to make a difference between the structures of GO-TYR and GO-Bz since it appears in both spectra. The yield of the carboxylic groups transformation into amidic groups was calculated as a ratio between the intensity of 1700 cm-1 signal corresponding to C=O bond from –COOH and the intensity of 1600 cm-1 signal corresponding to C=C bond from graphene structure wich remains unmodified along the process. It was achieved 12% amidation yield for activation reaction with EDC/NHS and 70% amidation yield using SOCl2 method. The benzoxazine – functionalized graphene oxide products were characterized by 1H-NMR in order to prove the benzoxazine rings formation (Fig. 4). The 1H-NMR spectra of synthesized products showed the presence of proton signals from benzoxazine rings. The NMR spectrum of GO-Bz (EDC/NHS) indicates an important signal at 3.9 ppm (G) corresponding to protons from the methylene group of Ar – CH2 – N–. Also, an important signal at 4.85 ppm (H) was noticed, corresponding to protons from the methylene group of –N – CH2 – O– structure of benzoxazine rings. Aromatic protons are noticed in the range 6.5–7.5 ppm. The signal at 3.7 ppm (C) from 1 H-NMR spectrum of GO-Bz (SOCl2) is assigned to the methylene groups from the oligomers produced by opening of some benzoxazine rings during synthesis.
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a)
b)
Fig. 4 1H-NMR spectra of benzoxazine - functionalized graphene oxide monomers obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
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Fig. 5A shows the thermogravimetric analysis (TGA) of the raw material GO-COOH, GO-TYR and the final compounds GO-Bz obtained in both methods. It is clearly observed that the thermostability of the intermediary and final products increased significantly against raw material GO-COOH.
Fig. 5A The TGA curves of GO-COOH, GO-TYR and GO-Bz obtained by EDC/NHS activation method and chlorination method with SOCl2, respectively
Thus, in case of activation method with EDC/NHS, the temperature corresponding to loss mass of 3% (Td3%) is increased only with 14°C against GO-COOH. This fact is in good correlation with the low amidation yield obtained from FT-IR data which showed that only 12% of carboxylic groups were transformed into amidic groups. Further, the benzoxazine formation reaction increased Td3% with 17°C against GO-TYR. The increase of thermal stability is much consistent for using the SOCl2 method. In this case the Td3% is increased with 113°C for GOTYR. This fact is in good correlation with the 70 % amidation yield obtained from FT-IR data which means that an important amount of –COOH groups were transformed into amidic chains. These chains include aromatic rings from GO-TYR structure which definitively increase the overall thermal stability of GO-TYR. In case of GO-Bz obtained by SOCl2 method a low decrease of Td3% with 14°C was observed, probably due to presence of some opened benzoxazine rings. Table 1. The TGA data for GO-COOH, GO-TYR and GO-Bz Material (EDC/NHS) GO-COOH GO-TYR GO-Bz
Td3% (°C) 82.9 96 113.5
Material (SOCl2) GO-COOH GO-TYR GO-Bz
Td3% (°C) 82.9 195.8 181.2 9
Fig. 5B The dTG curves of GO-COOH, GO-TYR and GO-Bz obtained by EDC/NHS activation method and chlorination method with SOCl2, respectively
Also from Fig. 5B the derivative curves of weight loss may be observed which show the main processes of mass loss for GO-COOH, GO-TYR and GO-B z. Thus a low peak below 100°C is noticed for all the compounds assigned to the loss of adsorbed moisture [28]. The main peak for GO-COOH is located at 206°C, which was attributed to the degradation of functional groups of GO-COOH, according to the literature [29]. Moreover for GO-TYR and GO-Bz synthesized by EDC/NHS method, this peak is shifted to lower values of temperature, showing that only a lower percentage of carboxylic groups from GO-COOH were succeeded to be transformed into amidic and benzoxazine groups, respectively. However this peak is not anymore noticed for GO-TYR and GO-Bz synthesized by SOCl2 method, which is a supplementary proof that most of the carboxylic groups were successfully transformed into amidic and further to benzoxazine groups. Another significant peak was remarked around 400°C for GO-TYR and GO-Bz synthetized by EDC/NHS method which may be explained by the decomposition of the excess of tyramine (TYR) physically adsorbed onto modified GO surface. This is due to the low amidation yield which caused a significant unreacted TYR quantity to adhere to the surface. On the contrary, for GO-TYR and GO-Bz obtained by SOCl2 method, no decomposition peaks are noticed around 400°C meaning that the most of TYR added was successfully reacted with GO-COOH and furthermore benzoxazine groups are formed.
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In the Raman spectra for the raw material (GO-COOH), GO-TYR and final compounds GO-Bz obtained by both methods, the characteristics of the graphene structure, namely the intense signals D and G, which prove the presence of the graphene structure in the final product (Fig. 6) are observed.
Fig. 6 Raman spectra of GO-COOH, GO-TYR, GO-Bz obtained by: a) EDC/NHS activation method; b) chloronation with SOCl2
It is also important to notice the presence of the 2D band that characterizes the arrangement and number of graphene layers. Therefore, for the compounds obtained using EDC/NHS activation, the 2D band is broader even for the final GO-Bz which means that the aggregation in several 11
layers of graphene occurs. This is the case of the monomer for which the benzoxazine rings do not polymerize and consequently the driving force required for the disentanglement of the graphene sheets into independent layers does not exist. In the case of the compounds obtained by SOCl2 method, the 2D band is sharper which proves that the majority of the graphene aggregations were disintegrated due to the polymerization of the benzoxazine rings, this process leading to partially cancel the attraction between the graphene sheets (Fig. 7).
Fig. 7 The nanostructure synthesized by polymerization of benzoxazine - functionalized graphene oxide
The benzoxazine polymerization may take place either between the rings of the same GO layer (“in-graphene” polymerization) or between the rings of two neighbors of GO layers (“outgraphene” polymerization). The final balance between these two types of structure will give the ratio between intercalated and exfoliated structures. Consequently the “in-graphene” polymerization will lead to more exfoliated structures of GO-Bz. The XRD curves (Fig. 8) show a significant peak at 2θ=12.5⁰ assigned to the GO-COOH stacking conformation consisting in more layers. For GO-Bz the XRD curves show a significant difference between the benzonxazine – functionalized graphene oxide obtained by EDC/NHS (Fig. 8 - a) and SOCl2 (Fig. 8 - b) respectively. Thus GO-Bz synthetized by EDC/NHS presents a small peak at 2θ=13⁰ which means that the benzoxazine – functionalized graphene oxide layers are still forming an order structure. On the contrary, GO-Bz synthetized through SOCl2 method shows no peak in the range 2θ=020⁰ which is a supplementary proof that the exfoliation of benzoxazine – functionalized graphene oxide layers occurs. This is in good agreement with the Raman spectra and 1H-NMR data previously discussed.
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Fig. 8 XRD curves of GO-COOH and GO-Bz obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2
The typical morphology of GO-Bz was characterized by HR-TEM. The images from Fig. 9 (a, b) show non-transparent, non-corrugated, and non-curled morphology. However some differences may be distinguished between Go-Bz synthesized on two different procedures, through SOCl2 (Fig. 9 - a) and EDC/NHS (Fig. 9 - b) method, respectively. Thus for GO-Bz obtained with SOCl2 the structure seems to be exfoliated with graphene sheets dispersed in the polybenzoxazine mass. On the opposite, GO-Bz synthetized with EDC/NHS shows a compact display of GO sheets, with no exfoliation, the benzoxazine monomer being formed at the surface of these sheets.
a)
b)
Fig. 9 High-magnification TEM images of GO-Bz obtained by: a) chlorination with SOCl2 b) EDC/NHS activation method 13
The significant XPS peaks assigned to GO structure are noticed from C1s deconvoluted spectrum peak at 284.4 eV (C=C) and at 292.2 eV (π-π* satellite peak). Three other peaks may be deconvoluted indicating the functionalities containing oxygen attached to the graphene surface (spectrum not shown). Thus the peaks at 285.7 eV (C-O from hydroxilic groups), 287.7 eV (C=O from carbonylic groups) and 289.5 eV (-O-C=O from carboxylic groups) are evidenced. For the benzoxazine structures (both GO-Bz obtained by EDC/NHS method (Fig. 10a) and GO-Bz synthetized by SOCl2 method (Fig. 10-b) new peaks are revealed in the C1s deconvoluted spectra among which the most significant is that located at 288.1 eV and 287.9 eV respectively which is assigned to the new –N-C-O- group from the benzoxazine ring. Also, a new peak at 285.2 eV and 285.1 eV respectively is noticed being attributed to the C-N groups from both the benzoxazine rings and tyramine structure. The presence of phenolic OH groups are pointed out through the signal at 286.4 eV and 286.3 eV respectively assigned to the C-OH units from both the unreacted phenol groups (tyramine structure) and phenol groups from the opened polymerized benzoxazine rings.
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Fig. 10 XPS spectra of a) GO-Bz obtained by EDC/NHS activation method; b) GO-Bz obtained by chlorination with SOCl2
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4. Conclusions
For the first time the structures of benzoxazine-functionalized graphene oxide were successfully synthetized by chemically-growing of the benzoxazine ring onto the GO surface. The yield of the whole process strongly depends on the activation method employed for obtaining of chloroacyl groups on GO, thus chlorination with SOCl2 gives higher yields. Consequently the chlorinated GO treated with tyramine and then the obtained OH groups being reacted with formaldehyde and benzylamine finally gives benzoxazine groups on the GO surface which are partially opened, thus forming polybenzoxazine chains either in the same layer of GO or between different layers of GO. The balance between these two structures is the major factor, which influences the ratio between intercalated and exfoliated structures. However, considering the hypothetic structure of exfoliated GO-Bz, it seems that the “ingraphene” polybenzoxazine chains are not bended one to another, acting individually in the nanocomposite mass. If the “out-graphene” polymerization is predominant a more ordered structure should be considered, the GO layers being assembled at less or higher interlayer distance, which depends on the polymerization degree of benzoxazine units.
5. Acknowledgements This research was financially supported by Sectoral Operational Programme Human Resources Development, financed from the European Social Fund and by the Romanian Government under the contract number POSDRU/156/1.2/G/135764, contract PN II 59/2012 no. CH 39-12-09 and contract 105/10.10.2011 no. CH 39-15-01. The work of C. M. Damian has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132397.
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New route to prepare nanocomposites based on benzoxazine and graphene oxide is proposed. The original idea was to grow benzoxazine rings directly onto the graphene oxide surface. Two functionalization methods of graphene oxide were employed. Polymerization of benzoxazine rings from the graphene oxide surface leads to exfoliated nanocomposites.
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Graphical abstract
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