**3. Raman investigation of graphene and graphene oxide nanocomposites**

Polymeric composites are biphasic materials consisting essentially of a continuous phase, commonly referred to as polymer matrix, and a reinforcing or filler agent, which is the discrete phase. The purpose of this association is to obtain materials with enhanced properties, superior to those of individual components, capable of replacing natural materials (wood, rocks, etc.), aluminum and its alloys, and other metallic materials. Polymeric composites are obtained from a wide range of matrices (epoxy resins, polyester resins, phenol-formaldehyde resins, vinyl polymers, elastomers, polyimides, etc.) with reinforcing materials (boron fibers, glass fibers, or filler materials (wood flour, starch, silica, talc, asbestos, etc.).

Nanocomposites represent a new class of composites, characterized by the coexistence of two distinct phases (an organic one which is the polymer as the continuous phase and an inorganic phase dispersed in the continuous phase, the latter exhibiting nanometric dimensions). The advantages of these structures consist in global properties superior to the individual components such as improved optical clarity, high mechanical resistance, better conductivity, leading to important uses in electronics, optics, constructions, etc. In order to obtain nanocomposites, two important aspects should be considered: firtsly, the nanoparticle must be compatible with the polymer and to show satisfactory interfacial interaction; secondly, the most convenient way to uniformly disperse the nanoparticles in the polymer matrix should be chosen. In most cases polymeric nanomaterials exhibit multifunctionality by combining more than one properties.

When the polymer is unable to intercalate between the graphene layers, a phase separation (two distinct phases) is obtained, the properties of which resemble the microcompounds. In addition to this class, two other types of composites can be prepared: intercalated structures where most of the time a single polymer chain is interposed between layers of graphene, resulting in a multilayered structure in which the polymer-graphene layers alternate and exfoliated structures in which the graphene layers are completely dispersed in the continuous polymer matrix (**Figure 9**).

graphene in other matrices and surfaces to enhance its applicability, that which would other-

Being a derivative of graphene, the graphene oxide structure and functionalized graphene oxide can be also successfully characterized by Raman spectroscopy. For this study different structures of commercial graphene oxide (**Figure 7**) were investigated using a Raman spectrometer

wise be more difficult using pristine graphene.

**Figure 7.** Chemical structures of graphene oxide investigated by Raman spectroscopy.

190 Raman Spectroscopy

**Figure 8.** Raman spectra of various graphene oxides.

equipped with confocal microscope.

**Figure 9.** Schematic structure of graphene – based polymer composites.

Graphene nanosheets–polystyrene nanocomposites (GNS-PS) were prepared by in situ emulsion polymerization and reduction of graphene oxide using hydrazine hydrate [62]. The nanocomposites displayed high electrical conductivity, and a considerable increase in glasstransition temperature and good thermal stability of PS are also achieved. Raman spectroscopy was employed as an efficient tool to probe the structural characteristics and properties of graphene and graphene-based materials.

**Figure 10** shows a schematic diagram for the formation of the GNS-PS nanocomposites. TEM images revealed polystyrene microspheres with diameters ranging from 90 to 150 nm attached to the graphene surface, particularly along the edges of the stacked nanosheets with a thickness of several nm. This suggests that the compatibility between PS microspheres and GNS is sufficient to obtain nanosized dispersion without an additional surface treatment.

Regarding the Raman spectrum of GNS, two intense features are assigned to the D band at 1331 cm−1 and the G band at 1594 cm−1. The G peak was assigned to vibrations of sp<sup>2</sup> carbon atoms. The peak intensity ratio Id/Ig of GNS nanocomposites was calculated as 1.156. This fact demonstrated the presence of localized sp3 defects within the sp2 carbon network, which shows the chemical grafting of polymers to the GNS surface.

it more attractive compared to graphene for the manufacture of electronic components or in use for nanocomposites synthesis. Polymer reinforced with graphene oxide has been reported

**Figure 10.** a) Synthesis of GNS-PS nanocomposites; (b) TEM image of CNS-PS nanocomposites; (c) Raman spectra of the

Recently, graphene oxide with numerous carboxylic groups (GO-COOH) was modified with benzoxazine rings in order to produce exfoliated graphene oxide – polybenzoxazine [63]. The carboxylic groups from GO surface were treated with tyramine (TYR) in order to synthesize a lot of phenolic groups using the activation of the carboxylic groups from GO surface by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide/N-Hydroxysuccinimide

previously obtained further reacted 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 (GO-Bz). The study demonstrated that GO-COOH is a good candidate for the preparation of benzoxazine-based nanocomposites due to the abundance of oxidized functional groups on its surface. Raman spectroscopy was successfully employed to demonstrate the efoliation of the graphene

respectively. GO-TYR

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system (EDC/NHS) and the chlorination method employing SOCl<sup>2</sup>

pristine GNS and GNS-PS nanocomposites (Hu H et al. (2010) copyrights).

sheets through the polybenzoxazine matrix.

in the literature [62].

The prominent D peak revealed some structural defects are created during the reduction process of the oxidized functional groups, while GNS synthesized through the exfoliation method is considered a more effective route for graphene sheets production. The G band from the GNS Raman spectrum was noticed at 1594 cm−1 and upshifted by 5 cm−1 in the composites of GNS–PS. By combining the XRD and Raman spectroscopy, Hu and co-workers proved that the substantial structure of the GNS has been maintained after PS microspheres were linked to the edges of the stacked graphene nanoplatelets, which is advantageous for improving the electrical and thermal properties of polymer.

Interest in graphene oxide has increased significantly due to its epoxy, hydroxyl, carbonyl and carboxyl functional groups, allowing its functionalization and the formation of various monomers on its surface. Dispersibility of graphene oxide in water and other solvents makes

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Graphene nanosheets–polystyrene nanocomposites (GNS-PS) were prepared by in situ emulsion polymerization and reduction of graphene oxide using hydrazine hydrate [62]. The nanocomposites displayed high electrical conductivity, and a considerable increase in glasstransition temperature and good thermal stability of PS are also achieved. Raman spectroscopy was employed as an efficient tool to probe the structural characteristics and properties

**Figure 10** shows a schematic diagram for the formation of the GNS-PS nanocomposites. TEM images revealed polystyrene microspheres with diameters ranging from 90 to 150 nm attached to the graphene surface, particularly along the edges of the stacked nanosheets with a thickness of several nm. This suggests that the compatibility between PS microspheres and GNS is sufficient to obtain nanosized dispersion without an additional surface treatment.

Regarding the Raman spectrum of GNS, two intense features are assigned to the D band at

atoms. The peak intensity ratio Id/Ig of GNS nanocomposites was calculated as 1.156. This

The prominent D peak revealed some structural defects are created during the reduction process of the oxidized functional groups, while GNS synthesized through the exfoliation method is considered a more effective route for graphene sheets production. The G band from the GNS Raman spectrum was noticed at 1594 cm−1 and upshifted by 5 cm−1 in the composites of GNS–PS. By combining the XRD and Raman spectroscopy, Hu and co-workers proved that the substantial structure of the GNS has been maintained after PS microspheres were linked to the edges of the stacked graphene nanoplatelets, which is advantageous for improving the

Interest in graphene oxide has increased significantly due to its epoxy, hydroxyl, carbonyl and carboxyl functional groups, allowing its functionalization and the formation of various monomers on its surface. Dispersibility of graphene oxide in water and other solvents makes

defects within the sp2

carbon

carbon network, which

1331 cm−1 and the G band at 1594 cm−1. The G peak was assigned to vibrations of sp<sup>2</sup>

of graphene and graphene-based materials.

192 Raman Spectroscopy

**Figure 9.** Schematic structure of graphene – based polymer composites.

fact demonstrated the presence of localized sp3

electrical and thermal properties of polymer.

shows the chemical grafting of polymers to the GNS surface.

**Figure 10.** a) Synthesis of GNS-PS nanocomposites; (b) TEM image of CNS-PS nanocomposites; (c) Raman spectra of the pristine GNS and GNS-PS nanocomposites (Hu H et al. (2010) copyrights).

it more attractive compared to graphene for the manufacture of electronic components or in use for nanocomposites synthesis. Polymer reinforced with graphene oxide has been reported in the literature [62].

Recently, graphene oxide with numerous carboxylic groups (GO-COOH) was modified with benzoxazine rings in order to produce exfoliated graphene oxide – polybenzoxazine [63]. The carboxylic groups from GO surface were treated with tyramine (TYR) in order to synthesize a lot of phenolic groups using the activation of the carboxylic groups from GO surface by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide/N-Hydroxysuccinimide system (EDC/NHS) and the chlorination method employing SOCl<sup>2</sup> respectively. GO-TYR previously obtained further reacted 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 (GO-Bz). The study demonstrated that GO-COOH is a good candidate for the preparation of benzoxazine-based nanocomposites due to the abundance of oxidized functional groups on its surface. Raman spectroscopy was successfully employed to demonstrate the efoliation of the graphene sheets through the polybenzoxazine matrix.

In the Raman spectra for the raw material (GO-COOH), GO-TYR and final monomers GO-Bz obtained by both methods, the characteristics of the graphene structure are noticed, namely the intense signals D and G, which proves the presence of the graphene structure in the final compound (**Figure 11**). At the same time, it is worth mentioning the appearance of the 2D band that characterizes the arrangement and the number of graphene plans. Graphene, the twodimensional form of graphite, consisting of sp2 hybridized carbon atoms has attracted the attention of researchers in recent years due to its excellent thermal, mechanical, electrical and barrier. All these excellent properties have been shown to the monolayer graphene, the increase of the number of layers leading to the decrease of its properties. For this reason graphene structure has been extensively studied. Raman spectroscopy allows the investigation and determination of the number of layers of the graphene, this information being extracted from the 2D spectrum band. Thus, for products obtained by the EDC/NHS method, the band is wider even in the final benzoxazine product, indicating aggregation in the form of multiple layers of the graphene plans, provided that a part of the benzoxazine monomer did not polymerize and therefore, there was no driving force needed to move the graphene aggregates into independent layers.

In the case of thionyl chloride products, the 2D band is sharper, which proves that most of the graphene aggregates have disintegrated due to the polymerization of the benzoxazine rings,

**Figure 12.** The nanostructure synthesized by polymerization of benzoxazine - functionalized graphene oxide (Biru I

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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*"

Raman spectroscopy is a powerful instrument for investigating carbon nanomaterials. As a highly sensitive technique Raman spectroscopy is recommended for detection of small changes in structural morphology of various carbon nanomaterials playing an important role as a direct or complementary tool in any laboratory working with carbon allotropes. Raman spectrum shows specific signals for each carbon allotrope when the monochromatic radiation interacts with the sample. Diamond and graphite exhibit significant differences in the Raman spectrum even if both are entirely made of C-C bonds. The Raman spectrum of pure diamond exhibits an extremely sharp signal at ~1332 cm−1 which arises from the stretching of the C-C bond. Instead, in the Raman spectrum of graphite two distinguishable peaks are revealed at ~1350 cm−1 (D band) and ~1580 cm−1 (G band) revealing that the graphite is not as uniform in structure as diamond. Also more complex structures can be investigated by Raman spectroscopy. The Raman spectrum of C60 fullerene exhibits strong signals at 1467 cm−1 and 1567cm−1 revealing that C60 is composed

which has led to the cancellation of the attractions between the graphene plans.

polymerization will lead to more exfoliated structures of GO-Bz (**Figure 12**).

**4. Conclusions and outlook**

et al. (2016) copyrights).

**Figure 11.** Raman spectra of GO-COOH, GO-TYR, GO-Bz obtained by: a) EDC/NHS activation method; b) chlorination with SOCl2 (Biru I et al. (2016) copyrights).

**Figure 12.** The nanostructure synthesized by polymerization of benzoxazine - functionalized graphene oxide (Biru I et al. (2016) copyrights).

In the case of thionyl chloride products, the 2D band is sharper, which proves that most of the graphene aggregates have disintegrated due to the polymerization of the benzoxazine rings, which has led to the cancellation of the attractions between the graphene plans.

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 (**Figure 12**).
