Innovative Hyperbranched Polybenzoxazine-Based Graphene Oxide—Poly(Amidoamines) Nanomaterials

polymers

Article Innovative Hyperbranched Polybenzoxazine-Based Graphene Oxide—Poly(amidoamines) Nanomaterials

Elena Iuliana Bîru 1, Sorina Alexandra Gârea 1 and Horia Iovu 1,2,*

1 Advanced Polymer Materials Group, University Politehnica of Bucharest, Gh. Polizu Street, 011061 Bucharest, Romania; [email protected] (E.I.B.); [email protected] (S.A.G.) 2 Academy of Romanian Scientists, 54 Splaiul Independentei Street, 050094 Bucharest, Romania * Correspondence: [email protected]; Tel.: +40-21-402-3922

Received: 6 October 2020; Accepted: 18 October 2020; Published: 21 October 2020

Abstract: The covalent functionalization of graphene oxide (GO) surface with hyperbranched benzoxazine (BZ) structures has been achieved using poly(amidoamine) dendrimers (PAMAM) of different generations. By increasing the PAMAM generation, multiple benzoxazine rings were synthesized decorating the GO layers. The polymerization process and the exfoliation behavior were investigated. The novel BZ-functionalized GO hybrid materials were characterized by a combination of techniques such as FT-IR, XPS, and 1H-NMR for the confirmation of benzoxazine formation onto the GO layer surfaces. Raman and XRD investigation showed that the GO stacking layers are highly disintegrated upon functionalization with hyperbranched benzoxazine monomers, the exfoliation being more probably to occur when lower PAMAM generation (G) is involved for the synthesis of hybrid GO-BZ nanocomposites. The polymerization of BZ rings may occur either between the BZ units from the same dendrimer molecule or between BZ units from different dendrimer molecules, thus influencing the intercalation/exfoliation of GO. DSC data showed that the polymerization temperature strongly depends on the PAMAMgeneration and a significant decrease of this value occurred for PAMAM of higher generation, the polymerization temperature being reduced with ~10 ◦C in case of GO-PAMAM(G2)-BZ. Moreover, the nanoindentation measurements showed significant mechanical properties improvement in case of GO-PAMAM(G2)-BZ comparing to GO-PAMAM(G0)-BZ in terms of Young modulus (from 0.536 GPa to 1.418 GPa) and stiffness (from 3617 N/m to 9621 N/m).

Keywords: benzoxazine functionalization; dendrimers; polymerization

1. Introduction Nowadays the synthesis of versatile polymers represents an emerging demand for industrial sectors that require easy to handle, yet high-performance polymers for construction materials, electronic devices, or aerospace industry. Polybenzoxazines (PBZs) are a unique class of thermoset resins that in many cases surpass the phenolic and epoxy resin properties in terms of thermal resistance, UV and chemical stability, water absorption, void formation, and volumetric shrinkage during the curing process [1–3]. These remarkable features of PBZs are generated by the rich molecular ﬂexibility of the benzoxazine monomers that allows the synthesis of complex polymerizable structures for tailored properties [4]. Benzoxazine (BZ) chemistry is based on the Mannich condensation reaction between phenols, formaldehyde, and primary amines with the formation of a six-membered oxazine ring attached to benzene structure [5]. Therefore, distinctive BZ monomers may be obtained for diﬀerent purposes by solely using suitable functionalized reagents. PBZ resins are produced via further thermal ring-opening polymerization (ROP) forming numerous –OH groups that are inter- and intramolecularly hydrogen

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bonded [6,7]. Although PBZs exhibit high glass temperature (Tg 170–340 ◦C), high char yield and better flammability resistance compared to conventional thermoset resins [1], the curing process takes place at relatively high temperatures (160–220 ◦C) restraining the usage of these polymers at industrial scale [8]. Thus, several strategies were developed in order to reduce the polymerization temperature by designing monomers with self-catalytic effect on the ROP as the –COOH and –OH groups showed effective contribution to accelerate the process [5,9–12]. However, the most significant results were obtained when extra-catalysts were employed such as cyanuric chloride [13], amines [14], sulfonates [15], lithium [16], and amine salts [17,18] that not only lower the ROP temperature but contribute to prolongation of the product life storage also. Recently, studies showed that incorporation of graphene oxide (GO) into polymer-based composite formulations significantly contribute to reduction of polymerization temperature [19]. GO represents a monolayer of carbon atoms covalently bonded in sp2/sp3 hybridization decorated with oxidized organic species. Being a graphene derivative, GO exhibits large surface area, great mechanical properties [20], thermal conductivity and good optical transmittance. Due to its many reactive oxygen functionalities such as epoxy, hydroxyl, and carboxyl [21], GO represents a fruitful platform for polymer composites with applications for sensors [22–24], electronics [25,26], corrosion protection [27], or environmental issues [28]. Zeng and co-workers studied the curing behavior of bisphenol A-based benzoxazine in the presence of GO [29] showing that the polymerization temperature is decreased with ca. 25 ◦C when 3 wt% GO is incorporated. Moreover, the authors investigated the effect of carboxylated GO (GO-COOH) on the curing process of the BZ monomer demonstrating not only the reduction of the polymerization temperature, but also the crosslinking density is significantly enhanced, improving the Tg and thermal properties of the final nanocomposite [30]. Nevertheless, the mechanical properties are increased reducing the brittleness of the pure PBZ. However, it is well known that graphene derivatives exhibit tendency to agglomerate due to π-π * stacking interactions and van der Waals attractions between layers, hindering the inclusion of molecules [31]. Thus, the exfoliation degree of GO layers highly influences the final properties of the materials. For better dispersion of GO into PBZ matrix, high degree of functionalization of its surface with BZ groups is required. In this regard, Ishida and his group proposed the introduction of BZ functionalities onto the GO surface by covalently attaching the BZ monomer separately synthetized through click chemistry route [32]. The synthesis of the BZ rings grafted from the surface of the GO layers is sparsely reported in the literature. We have previously reported a new approach for producing the PBZ/GO nanocomposites employing GO [33] and aminated–GO [34] as starting reagents for BZ synthesis. By this method, the novel hybrid BZ/GO monomers were polymerized showing that the ROP may take place either between the BZ groups located on the same GO layer (in-graphene polymerization) or between the BZ groups located on different GO layers (out-graphene polymerization) and that the ratio between these two types of polymerization strongly influence the exfoliation of GO layers within the PBZ matrix. Herein the synthesis of new GO–hyperbranched BZ structures is reported in order to produce higher content of BZ rings directly onto the GO surface leading to improved thermal and mechanical behavior. Recently, Wang and coworkers managed to prepare hyperbranched polyether epoxy structures grafted to the GO layers in order to improve the exfoliation of GO sheets after the introduction into the benzoxazine matrix [35]. Although recent studies have approached the synthesis of dendrimer-based benzoxazine monomers [36–38], the covalent growth of benzoxazine monomers starting from dendrimer-functionalized GO has not been previously approached. In this study, starting from the idea that hyperbranched benzoxazine may be synthesized by using poly(amidoamine) dendrimers (PAMAM) as primary amine component in the Mannich condensation, GO surface was chemically modified with PAMAM of different generations in order to directly synthesize numerous BZ rings. Three different generations of PAMAM dendrimers (G0, G1, G2) were employed in order to decorate the surface of GO with numerous benzoxazine structures and their polymerization behavior is investigated. Therefore, this study proposes an original route for the decoration of GO layers with Polymers 2020, 12, 2424 3 of 18 Polymers 2020, 12, x FOR PEER REVIEW 3 of 18 multiple benzoxazine rings and critically describes the exfoliation process that depends on the PAMAM 2.generation Materials and and influences Methods the thermal and mechanical properties of the final nanocomposites.

2.1.2. Materials Materials and Methods

2.1. MaterialsThe graphene oxide powder (GO) was purchased from NanoInnova Technologies (Toledo, Spain). Poly(amidoamine) dendrimer solutions (ethylenediamine (EDA) core, 20 wt. % in methanol) The graphene oxide powder (GO) was purchased from NanoInnova Technologies (Toledo, Spain). of generation 0, 1, and 2, namely PAMAM(G0), PAMAM(G1), PAMAM(G2), phenol, Poly(amidoamine) dendrimer solutions (ethylenediamine (EDA) core, 20 wt. % in methanol) of paraformaldehyde, sodium hydroxide (NaOH), anhydrous magnesium sulphate (MgSO4), generation 0, 1, and 2, namely PAMAM(G0), PAMAM(G1), PAMAM(G2), phenol, paraformaldehyde, chloroacetic acid (ClCH2COOH), hydrochloric acid (HCl), sodium hydroxide (NaOH), anhydrous magnesium sulphate (MgSO ), chloroacetic acid (ClCH COOH), ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and chloroform4 were supplied2 by hydrochloric acid (HCl), ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and chloroform were Sigma-Aldrich (Taufkirchen, Germany). N-Hydroxysuccinimide (NHS) was received from Fluka, supplied by Sigma-Aldrich (Taufkirchen, Germany). N-Hydroxysuccinimide (NHS) was received from China. Fluka, China. 2.2. Synthesis of Carboxylated GO (GO-COOH) 2.2. Synthesis of Carboxylated GO (GO-COOH) For the carboxylation of GO, Park’s protocol was adopted [39]. Thus 100 mg of GO were For the carboxylation of GO, Park’s protocol was adopted [39]. Thus 100 mg of GO were dispersed dispersed in 450 mL of distilled water and ultrasonicated for 1 h (5–10 °C, pulse 20, amplitude 60%) in 450 mL of distilled water and ultrasonicated for 1 h (5–10 ◦C, pulse 20, amplitude 60%) to fully to fully exfoliate the graphene layers. Twelve grams of NaOH were slowly added into the exfoliate the graphene layers. Twelve grams of NaOH were slowly added into the yellow-brown yellow-brown suspension maintaining low temperature and ultrasonication conditions for 1 h. suspension maintaining low temperature and ultrasonication conditions for 1 h. Thereafter 12 g Thereafter 12 g ClCH2COOH were introduced in order to transform the –OH groups from GO ClCH2COOH were introduced in order to transform the –OH groups from GO surface into ether bonds surface into ether bonds with carboxyl end-functionalities (C–O–CH2–COOH) as shown in Figure 1. with carboxyl end-functionalities (C–O–CH2–COOH) as shown in Figure1. The GO-COOH suspension The GO-COOH suspension was neutralized with 37% HCl, filtered, repeatedly washed with was neutralized with 37% HCl, ﬁltered, repeatedly washed with distilled water and vacuum dried. distilled water and vacuum dried.

Figure 1. Synthesis of carboxylated graphene oxide. Figure 1. Synthesis of carboxylated graphene oxide. 2.3. Synthesis of GO-COOH Functionalized with Poly(amidoamine) Dendrimer (GO-PAMAM) 2.3. Synthesis of GO-COOH Functionalized with Poly(amidoamine) Dendrimer (GO-PAMAM) Initially, the PAMAM dendrimers, with EDA core branched from G0 to G2 generation number, wereInitially, separated the from PAMAM the methanol dendrimers, solution with via EDA rotary core evaporation branched from and G0 stored to G2 as generation 20 mg/mL number, aqueous were separated from the methanol solution via rotary evaporation and stored as 20 mg/mL aqueous solutions at 2–8 ◦C. Separately, 3 GO-COOH aqueous suspensions (2 mg/mL) were prepared solutions at 2–8 °C. Separately, 3×GO-COOH× aqueous suspensions (2 mg/mL) were prepared and and ultrasonicated for 30 min (5–10 ◦C). Thereafter, 300 mg EDC and 200 mg NHS were added ultrasonicatedin each flask. Thefor 30 role min of (5–10 the EDC °C)./NHS Thereafter, system 300 in thismg EDC case isand to 200 react mg with NHS the were carboxylic added in groups each flask.from GOThe surfacerole of the and EDC/NHS activate the system reaction in this between case is –COOHto react with and primarythe carboxylic amines groups from from PAMAM GO surfacestructures. and Thus, activate the the dendrimer reaction aqueous between solution –COOH of and diff erentprimary generation amines wasfrom added PAMAM to the structures. activated Thus,GO-COOH the dendrimer suspension aqueous and the solution pH of the of mixture different was generation adjusted was to 6–7 added using to 37% theHCl. activated The reaction GO-COOH was suspension and the pH of the mixture was adjusted to 6–7 using 37% HCl. The reaction was maintained at 25 ◦C for 48 h for the conversion of –COOH groups from GO surface into amide bonds (Figuremaintained2). Each at 25 GO-PAMAM(G0,°C for 48 h for the G1,conversion G2) product of –COOH was filtered, groups thoroughlyfrom GO surface washed into with amide distilled bonds water,(Figure introduced 2). Each GO-PAMAM(G0, into dialysis sacks G1, (MWCO G2) product= 12,000) was for filtered, 48 h in thoroughly order to remove washed any with intermediary distilled water,by-products introduced and freeze-dried. into dialysis sacks (MWCO = 12,000) for 48 h in order to remove any intermediary by-products and freeze-dried.

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Figure 2. The functionalization route of GO-COOH with multiple benzoxazine rings.rings.

2.4. Synthesis of Benzoxazine-Functionalized GO-PAMAM 2.4. Synthesis of Benzoxazine-Functionalized GO-PAMAM For the formation of BZ rings onto the GO surface,surface, each GO-PAMAM of didifferentfferent generation previously synthesizedsynthesized was was considered considered as theas aminethe amine component component in the Mannichin the Mannich condensation condensation reaction withreaction paraformaldehyde with paraformaldehyde and phenol. and Therefore,phenol. Theref dependingore, depending on the number on the number of amino of surface amino surface groups fromgroups each from PAMAM each PAMAM dendrimer dendrimer used for GO-COOHused for GO functionalization,-COOH functionalization, the necessary the amountnecessary of reagentsamount wasof reagents determined. was determined. The amine, The paraformaldehyde amine, paraformaldehyde and phenol and components phenol components were reacted were in 1:2:1 reacted molar in ratio1:2:1 molar using ratio chloroform using chloroform as a solvent as and a solvent the reaction and the was reaction maintained was maintained under reflux under for reflux 6 h at for 65 6◦ C.h The GO-PAMAM(G0 2)-BZ products were purified using 1 M NaOH aqueous solution in order at 65 °C. The GO-PAMAM(G0÷ ÷ 2)-BZ products were purified using 1 M NaOH aqueous solution in toorder remove to remove the unreacted the unreacted phenol andphenol the solventand the was solv eliminatedent was eliminated through rotary through evaporation. rotary evaporation. Figure2 is depictingFigure 2 is the depicting synthesis the route synthesis for the functionalizationroute for the functionalization of GO-COOH withof GO-COOH benzoxazine with structures benzoxazine using PAMAM(G0).structures using Similar PAMAM(G0). products were Similar obtained prod whenucts PAMAM(G1) were obtained and PAMAM(G2) when PAMAM(G1) were employed and inPAMAM(G2) the hybrid BZ were monomers employed synthesis in the exhibiting hybrid BZ hyperbranched monomers synthesis structures exhibiting with benzoxazine hyperbranched moieties. structuresThe molecular with benzoxazine structure moieties. of the intermediary and final products was investigated through FT-IR 1 and TheH-NMR molecular to confirm structure the of formationthe intermediary of the and benzoxazine final products rings. was The investigated results for through synthesized FT-IR hybrid materials were compared to BZ monomers separately prepared using PAMAM(G0 2), and 1H-NMR to confirm the formation of the benzoxazine rings. The results for synthesized hybrid÷ paraformaldehydematerials were compared and phenol to accordingBZ monomers to Lu et separately al. protocol prepared [36] and their using final PAMAM(G0 chemical structure ÷ 2), isparaformaldehyde presented in Figure and3 .phenol The solvent according was to evaporated Lu et al. protocol under [36] vacuum and their to obtain final achemical yellow product.structure 1 FT-IRis presented (KBr, cm in− Figure): 1220 3. and The 1035 solvent (asymmetric was evapor and asymmetricated under C–O–Cvacuum stretching), to obtain a 925 yellow (oxazine product. ring), 1 δ 756FT-IR (ortho-disubstituted (KBr, cm−1): 1220 and benzene). 1035 (asymmetricH-NMR (600 and MHz, asymmetric DMSO-d C–O–C6): = 7.07–6.68 stretching), (m, 16H,925 (oxazine aromatic ring), CH), δ = 4.8 (s, 4H, O–CH –N), δ = 3.9 (s, 4H, Ar–CH –N). 756 (ortho-disubstituted2 benzene). 1H-NMR (6002 MHz, DMSO-d6): δ = 7.07–6.68 (m, 16H, aromatic CH), δ = 4.8 (s, 4H, O–CH2–N), δ = 3.9 (s, 4H, Ar–CH2–N).

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Figure 3.3. TheThe final final structures of benzoxazinebenzoxazine (BZ) monomers obtained from poly(amidoamine)poly(amidoamine) dendrimers (PAMAM) dendrimers.dendrimers. 2.5. Measurements 2.5. Measurements Fourier transformed infrared (FT-IR) spectra were registered on Bruker Vertex70 spectrometer Fourier transformed infrared (FT-IR) spectra were registered on Bruker Vertex70 spectrometer (Billerica, MA, USA). The samples were prepared in KBr pellets and 32 scans were recorded at −1 (Billerica,1 MA, USA). The samples were prepared in KBr pellets and 32 scans were recorded at 4 cm 4 cm− cm resolution. cm resolution. The NMR spectra were recorded on a Bruker Advance III HD spectrometer (Bruker, Rheinstetten, The NMR spectra were recorded on a Bruker Advance III HD spectrometer (Bruker, Germany) operating at 600.12 MHz for 1H. For the NMR analysis, a 5 mm multinuclear inverse detection Rheinstetten, Germany) operating at 600.12 MHz for 1H. For the NMR analysis, a 5 mm multinuclear z-gradient probe was used. The analyses were performed using deuterated dimethylsulfoxide as solvent inverse detection z-gradient probe was used. The analyses were performed using deuterated and tetramethylsilane as internal standard. 1H–NMR spectra were registered using a standard pulse dimethylsulfoxide as solvent and tetramethylsilane as internal standard. 1H–NMR spectra were sequence, as delivered by Bruker, with TopSpin 3.5.6 spectrometer control and processing software. registered using a standard pulse sequence, as delivered by Bruker, with TopSpin 3.5.6 spectrometer X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Scientific K-Alpha control and processing software. equipment (Ho Chi Minh, Vietnam), using a monochromatic Al Ka source (1486.6 eV), at a pressure X-ray 9photoelectron spectroscopy (XPS) analyses were performed on a Thermo Scientific of 2 10− mbar. The binding energy was calibrated by placing the C1s peak at 284.8 eV as K-Alpha× equipment (Ho Chi Minh, Vietnam), using a monochromatic Al Ka source (1486.6 eV), at a internal standard. pressure of 2 × 10−9 mbar. The binding energy was calibrated by placing the C1s peak at 284.8 eV as Raman investigation was done using a Renishaw inVia Raman microscope system 473 nm laser internal standard. excitation, 100 objective and 0.4 mW incident power. Raman investigation× was done using a Renishaw inVia Raman microscope system 473 nm laser The X-ray diffraction analyses (XRD) were performed on a Panalytical X’PERT MPD X-ray excitation, 100× objective and 0.4 mW incident power. Diffractometer (Malvern Panalytical, Royston, UK), in the range 2θ = 2–50◦. An X-ray beam characteristic The X-ray diffraction analyses (XRD) were performed on a Panalytical X’PERT MPD X-ray to Cu Kα radiation was used (λ = 1.5418 Å). Diffractometer (Malvern Panalytical, Royston, UK), in the range 2θ = 2–50°. An X-ray beam Thermogravimetric analyses (TGA) were performed with a Q500 TA instrument from 30 to 800 ◦C characteristic to Cu Kα radiation was used (λ = 1.5418 Å). using nitrogen (gas flow rate of 90 mL/min and 10 ◦C/min as heating rate). Thermogravimetric analyses (TGA) were performed with a Q500 TA instrument from 30 to 800 Differential scanning calorimetry (DSC) curves were recorded on DSC 402 F 1 equipment from °C using nitrogen (gas flow rate of 90 mL/min and 10 °C/min as heating rate). Netzsch (Selb, Germany). The non-isothermal method was used to scan the samples under nitrogen, Differential scanning calorimetry (DSC) curves were recorded on DSC 402 F 1 equipment from from 30 to 300 ◦C using a 5 ◦C/min heating rate. Netzsch (Selb, Germany). The non-isothermal method was used to scan the samples under nitrogen, Hardness (H) and Young’s modulus (E) were determined using a Nanoindenter G200, from 30 to 300 °C using a 5 °C/min heating rate. Agilent Technologies (Santa Clara, CA, USA). The samples were fixed on sample holder for Hardness (H) and Young’s modulus (E) were determined using a Nanoindenter G200, Agilent the NanoVision stage. All indents were performed using a Berkovich diamond tip with a 20-nm Technologies (Santa Clara, CA, USA). The samples were fixed on sample holder for the NanoVision radius. The indentations were performed using the Express Test to a Displacement method from stage. All indents were performed using a Berkovich diamond tip with a 20-nm radius. The the NanoSuit software; for each sample 100 indents at a 50 µm distance from each other at a displacement indentations were performed using the Express Test to a Displacement method from the NanoSuit into the surface of 1000 nm and a poisson ratio of 0.17 were done. software; for each sample 100 indents at a 50 μm distance from each other at a displacement into the surface of 1000 nm and a poisson ratio of 0.17 were done.

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3. Results and Discussion 3. Results and Discussion

3.1.3.1. XPS XPS Investigation Investigation Firstly,Firstly, GO-COOH GO-COOH surface surface loading loading with with dendrime dendrimerr structures structures was was investigated investigated through through XPS XPS to probeto probe the thechemical chemical attachment attachment of hyperbranched of hyperbranched poly(amidoamines). poly(amidoamines). As observed As observed in Figure in Figure 4 the4 XPSthe XPSSurvey Survey spectrum spectrum of the of thecarboxylated carboxylated GO GO show showss binding binding energies energies at atonly only 284.8 284.8 eV eV and and 532 532 eV, eV, correspondingcorresponding toto C1s C1s and and O1s O1s peaks peaks [40]. New[40]. signalNew occurssignal inoccurs the XPS in Surveythe XPS spectra Survey of GO-PAMAM spectra of GO-PAMAMstructures corresponding structures corresponding to N1s peak from to N1s the nitrogen peak from containing the nitrogen dendrimers. containing As adendrimers. result, the atomic As a result,composition the atomic for each composition material isfor modiﬁed each material with increasing is modified the with PAMAM increasi generationng the PAMAM (Table1). generation Moreover, (Tablethe attachment 1). Moreover, of primary the attachment amine functionalities of primary amine was revealed functionalities by N1s XPSwas deconvolutionrevealed by N1s spectra XPS deconvolution spectra showing the presence of –NH2 (~399.7 eV) and amide bonds (~401 eV) showing the presence of –NH2 (~399.7 eV) and amide bonds (~401 eV) indicating that the amino indicatingfunctionalization that the has amino indeed functionalization taken place at thehas surfaceindeed oftaken graphene place oxideat the [surface41]. of graphene oxide [41].

Figure 4. The XPS Survey spectra of GO-COOH before (a) and after functionalization (b) with Figure 4. The XPS Survey spectra of GO-COOH before (a) and after functionalization (b) with PAMAM(G0) and N1s deconvolution spectra of GO-PAMAM(G0) (c); GO-PAMAM(G1) (d) structures. PAMAM(G0) and N1s deconvolution spectra of GO-PAMAM(G0-c); G1-d)) structures. Table 1. Atomic composition of GO-COOH before and after PAMAM functionalization. Table 1. Atomic composition of GO-COOH before and after PAMAM functionalization. Sample C (%) O (%) N (%) Sample C (%) O (%) N (%) GO-COOH 67 33 0 GO-COOH 67 33 0 GO-PAMAM(G0)GO-PAMAM(G0) 76 76 20 20 4 4 GO-PAMAM(G1)GO-PAMAM(G1) 72 72 20 20 8 8 GO-PAMAM(G2)GO-PAMAM(G2) 71 71 20 20 9 9

3.2.3.2. FT-IR FT-IR Analyses Analyses TheThe reaction reaction progress progress from from GO-COOH GO-COOH to to BZ-functionalized BZ-functionalized GO-PAMAM GO-PAMAM structures structures was was investigatedinvestigated throughthrough FT-IR FT-IR spectrometry spectrometry (Figure (Fig5).ure Similar 5). spectraSimilar werespectra registered were forregistered PAMAM(G1, for PAMAM(G1, G2)-based structures. The FT-IR spectrum for the raw material GO-COOH showed the presence of the carboxyl groups (1728 cm−1) [42], broad -OH signal (~3300 cm−1), C=C bonds from the

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G2)-basedPolymers 2020 structures., 12, x FOR PEER The REVIEW FT-IR spectrum for the raw material GO-COOH showed the presence7 of 18 of 1 1 the carboxyl groups (1728 cm− )[42], broad -OH signal (~3300 cm− ), C=C bonds from the planar 1 1 structureplanar structure of graphene of graphene (1620 cm (1620− ) and cm− some1) and residualsome residual C–O–C C–O–C groups groups (1048 (1048 cm− )cm characteristic−1) characteristic to GO structureto GO structure [43,44]. The[43,44]. interaction The interaction of GO-COOH of GO with-COOH PAMAM with PAMAM dendrimers dendrimers led to new led signals to new appearance signals 1 −1 1 inappearance the FT-IR spectra in the FT-IR of each spectra GO-PAMAM of each synthesized.GO-PAMAM The synthesized. bands located The atbands ~1630 located cm− andat ~1630 ~1540 cm cm − areand assigned ~1540 cm to the−1 are O= C–NHassigned stretching to the vibrationsO=C–NH fromstretchi PAMAMng vibrations structures. from Additionally, PAMAM structures. the presence 1 1 −1 −1 ofAdditionally, the signals from the presence ~2925 cm of− theand signals ~2850 cmfrom− corresponding~2925 cm and to ~2850 –CH2 cm– groups corresponding from the ethylene to –CH core2– ofgroups the dendrimers from the isethylene conﬁrming core thatof the amidoamines dendrimers groups is confirming were covalently that amidoamines grafted to GO-COOHgroups were [45 ]. Thecovalently formation grafted of the to GO-COOH hybrid GO-PAMAM-BZ [45]. The format monomersion of the hybrid was proven GO-PAMAM-BZ through FT-IR monomers investigation was forproven each through dendrimer FT-IR generation investigation employed for each in the dendrimer synthesis generation by comparing employed the ﬁnal in the products synthesis with by BZ monomerscomparing obtained the final from products PAMAM with dendrimers BZ monome as aminers obtained source from for BZ PAMAM synthesis. dendrimers Therefore, asas observedamine fromsource the for FT-IR BZ spectra,synthesis. the Therefore, characteristic as observed signals of fr theom benzoxazinethe FT-IR spectra, structure the werecharacteristic pointed outsignals through of the benzoxazine structure were pointed1 out through 1the absorbance signals from ~1030 cm−1 and the absorbance signals from ~1030 cm− and ~1220 cm− corresponding to ether bond from the oxazine ~1220 cm−1 corresponding to ether bond from the oxazine ring (C–O–C symmetric and1 asymmetric 1 ring (C–O–C symmetric and asymmetric stretching mode) [36]. In addition, the ~920 cm− and ~750 cm− −1 −1 peaksstretching assigned mode) to the[36]. oxazine In addition, cycle the and ~920 ortho-disubstituted cm and ~750 cm benzene peaks were assigned revealed. to the The oxazine formation cycle of and ortho-disubstituted benzene were revealed. The formation of the oxazine ring grafted from the oxazine ring grafted from GO-COOH was also demonstrated through 1H-NMR analyses. GO-COOH was also demonstrated through 1H-NMR analyses.

Figure 5. FT-IRFT-IR spectra spectra of of hybrid hybrid monomers monomers GO-PAMAM(G0)-BZ GO-PAMAM(G0)-BZ.. 3.3. 1H-NMR Results 3.3. 1H-NMR Results Figure6 shows the 1H-NMR spectra of the PAMAM dendrimers and their corresponding Figure 6 shows the 1H-NMR spectra of the PAMAM dendrimers and their corresponding benzoxazinebenzoxazine monomers monomers and and final final GO-PAMAM-BZ GO-PAMAM-BZ structures structures for thefor casethe case of PAMAM(G0). of PAMAM(G0). Similar Similar spectra werespectra registered were registered for the structures for the st basedructures on based PAMAM(G1). on PAMAM(G1). The presence The ofpresence the protons of the from protons –N–CH from2–Ar– – (3.9 ppm) (b) and O–CH –N– (4.8 ppm) (a) units in the 1H-NMR spectra of each GO-PAMAM-BZ N–CH2–Ar– (3.9 ppm) (b)2 and O–CH2–N– (4.8 ppm) (a) units in the 1H-NMR spectra of each compoundGO-PAMAM-BZ demonstrates compound that demonstrates pendant benzoxazine that pendant structures benzoxazine are formed structures onto are GO-PAMAM formed onto [36 ]. TheGO-PAMAM presence of[36]. the The aromatic presence rings of is the noticed aromatic at 7.1–6.6 rings ppm.is noticed In case at of7.1–6.6 GO-PAMAM(G2)-BZ ppm. In case of an additionalGO-PAMAM(G2)-BZ signal was observedan additional at 3.7 si ppmgnal was (c) corresponding observed at 3.7 to ppm Mannich (c) corresponding type structures, to Mannich indicating type that ringstructures, opening indicating polymerization that ring of opening BZ rings polymerizat has occurredion duringof BZ rings the synthesis has occurred (Figure during7) [46 the]. synthesis (Figure 7) [46].

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Figure 6. 1H-NMR spectra for PAMAM(G0), PAMAM(G0)-BZ and GO-PAMAM(G0)-BZ. Figure 6. 1H-NMR spectra for PAMAM(G0), PAMAM(G0)-BZ and GO-PAMAM(G0)-BZ. Figure 6. 1H-NMR spectra for PAMAM(G0), PAMAM(G0)-BZ and GO-PAMAM(G0)-BZ.

Figure 7. 1H-NMR spectra for PAMAM(G2), PAMAM(G2)-BZ and GO-PAMAM(G2)-BZ. Figure 7. 1H-NMR spectra for PAMAM(G2), PAMAM(G2)-BZ and GO-PAMAM(G2)-BZ. 3.4. Raman SpectrometryFigure 7. 1H-NMR spectra for PAMAM(G2), PAMAM(G2)-BZ and GO-PAMAM(G2)-BZ. 3.4. Raman Spectrometry 3.4.The Raman arrangement Spectrometry of the GO layers during the functionalization with hyperbranched dendrimers The arrangement of the GO layers during the functionalization with hyperbranched and benzoxazine structures was evaluated through Raman spectrometry.This non-destructive technique dendrimersThe arrangement and benzoxazine of the structuresGO layers was during evaluated the functionalizationthrough Raman withspectrometry. hyperbranched This provides useful information regarding the extent of functionalization, the formation of defects non-destructivedendrimers and technique benzoxazine provides structures useful informatio was evaluatedn regarding through the extent Raman of functionalization, spectrometry. Thisthe andformationnon-destructive the number of defects of technique GO and layers. the provides number As shown ofuseful GO in layers.informatio Figure As8 ,shownn the regarding Raman in Figure the spectrum extent8, the Ramanof of functionalization, the spectrum investigated of the the GO 1 1 derivativesinvestigatedformation exhibits of GO defects derivatives two and distinguishable the exhibitsnumber twoof peaks GO distinguishable layers. at ~1360 As cmshown− peaks(D in band) Figureat ~1360 and 8, the ~1590cm −Raman1 (D cm band)− spectrum(G and band). ~1590 of Thethe D 3 2 bandcminvestigated−1 is (G caused band). byGO The the derivatives D existenceband is caused exhibits of sp byhybridized twothe existencedistinguishable C atoms of sp3 consideredhybridized peaks at ~1360 asC atoms defects cm −considered1 in(D the band) sp asandplanar defects ~1590 sheet 2 ofin graphenecm the−1 (Gsp 2band). planar subsequently The sheet D band of graphene functionalized. is caused subsequently by the The existence G functi band of onalized.sp is3 characteristic hybridized The G C band atoms to all is spconsidered characteristichybridized as defects to carbon all nanomaterialsspin2 hybridizedthe sp2 planar and carbon sheet arises nanomaterials of as graphene a consequence subsequentlyand arises of the as stretchingafuncti consequenceonalized. vibrations of The the G stretching ofband C-C is bond characteristic vibrations from the of toC-C planar all 1 structurebondsp2 hybridized from of graphene.the planar carbon Anotherstructure nanomaterials important of graphene. and Raman arises Another as signal a consequenceimportant is the 2D Raman peak of the (~2700signal stretching is cm the− vibrations )2D that peak characterizes (~2700 of C-C thecmbond arrangement−1) that from characterizes the planar and the structurethe number arrangement of of graphene. layers and for the Another the numb graphene-like erimportant of layers Raman materials. for the signalgraphene-like is the 2D materials. peak (~2700 cm−1) that characterizes the arrangement and the number of layers for the graphene-like materials.

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Figure 8. The Raman spectra of the raw material GO-COOH and hybrid monomers GO-PAMAM-BZ. Figure 8. The Raman spectra of the raw material GO-COOH and hybrid monomers TheGO-PAMAM-BZ. broadening of D and G bands after functionalization and the increase of the ratio between the intensity of D and G band (I /I ) are strong indicators of structural changes in the graphene The broadening of D and GD bandsG after functionalization and the increase of the ratio between layers during the functionalization suggesting important information regarding the content of the sp2 the intensity of D and G band (ID/IG) are strong indicators of structural changes in the graphene hybridizedlayers during carbon the atoms functionalization within the materials suggesting [47 important,48] (Table information2). In case of regarding GO-PAMAM(G0 the content 2)of itthe was 3 ÷ clearlysp2 hybridized observed thatcarbon the atoms ID/IG within ratio has the increased materials as[47, a48] result (Table of more2). In spcasehybridized of GO-PAMAM(G0 C atoms ÷ located 2) it withinwas theclearly GO observed structure, that due the to theID/IG reaction ratio has conditions increased with as PAMAM.a result of Further, more sp when3 hybridized GO-PAMAM(G0) C atoms reactslocated to form within BZ ringsthe GO bonded structure, to the PAMAMdue to the moieties, reaction a higherconditions increase with of PAMAM. ID/IG ratio Further, occurs probably when 3 becauseGO-PAMAM(G0) of more defects reacts of to spformC BZ atoms rings appeared bonded to within the PAMAM the GO moieties, structure. a higher However, increase this of I isD/I notG validratio for occurs GO-PAMAM(G1, probably because G2)-BZ of structures, more defects for whichof sp3 C the atoms ID/IG appeared ratio decreases within againthe GO since structure. the high numberHowever, of BZ this rings is not formed valid for may GO-PAMAM(G1, interact with the G2)-B grapheneZ structures, lattice for through which the ID/IG rearrangement ratio decreases of π- π stacking,again since thus the contributing high number to of a smallerBZ rings number formed ofma defects.y interact Moreover, with the grap thehene shape lattice of 2D through band the from therearrangement Raman spectra of (Figureπ-π stacking,8) may thus explain contributing a lot about to a thesmalle processesr number developed of defects. duringMoreover, the the formation shape of GO-PAMAM(G0-2)-BZof 2D band from the Raman structures. spectra (Figure Thus, 8) the may sharpness explain a of lot 2D about band the increased processes as developed the reaction during goes onthe from formation GO-COOH of GO-PAMAM(G0-2)-BZ to GO-PAMAM and finallystructures. to GO-PAMAM-BZ. Thus, the sharpness This of means 2D band that increased the assembly as the of graphenereaction oxide goes on layers from is GO-COOH affected through to GO-PAMAM the formation and finally of intercalated to GO-PAMAM-BZ. and even This exfoliated means structuresthat the becauseassembly of PAMAM of graphene chains oxide and mostlylayers is due affected to BZ andthrough PBZ structuresthe formation finally of achieved.intercalated and even exfoliated structures because of PAMAM chains and mostly due to BZ and PBZ structures finally achieved. Table 2. The ID/IG ratio resulted from the Raman spectra of investigated materials.

Table 2. The ID/IG ratio resultedSample from the Raman ID /spIGectra(473nm) of investigated materials.

GO-COOHSample I 0.78D/IG (473nm) GO-PAMAM(G0)GO-COOH 0.85 0.78 GO-PAMAM(G0)GO-PAMAM(G1) 0.86 0.85 GO-PAMAM(G1) 0.86 GO-PAMAM(G2) 0.86 GO-PAMAM(G2) 0.86 GO-PAMAM(G0)-BZ 1.02 GO-PAMAM(G0)-BZ 1.02 GO-PAMAM(G1)-BZGO-PAMAM(G1)-BZ 0.88 0.88 GO-PAMAM(G2)-BZGO-PAMAM(G2)-BZ 0.83 0.83

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It can be noticed that the 2D band is sharper for PAMAM(G0)-based structures which means It can be noticed that the 2D band is sharper for PAMAM(G0)-based structures which means that in this case only a few layers of GO are still forming the stacking layers, the other layers being that in this case only a few layers of GO are still forming the stacking layers, the other layers being penetrated by the PAMAM and BZ voluminous structures. For PAMAM(G1, G2)-based structures, penetrated by the PAMAM and BZ voluminous structures. For PAMAM(G1, G2)-based structures, this is slightly different since the same tendency of GO layers disintegrations occurs, but this is this is slightly different since the same tendency of GO layers disintegrations occurs, but this is coupled coupled with a partially process of polymerization of BZ rings. with a partially process of polymerization of BZ rings. The propagation of the polybenzoxazine formation is more likely to take place between The propagation of the polybenzoxazine formation is more likely to take place between benzoxazine rings located closer one to another on the same dendrimer branch (in-dendrimer benzoxazine rings located closer one to another on the same dendrimer branch (in-dendrimer polymerization) producing intercalated structures due to high density of intra- and intermolecular polymerization) producing intercalated structures due to high density of intra- and intermolecular hydrogen bonding and forming a rigid network structure. Out-dendrimer polymerization occurring hydrogen bonding and forming a rigid network structure. Out-dendrimer polymerization occurring between benzoxazine rings from different dendrimer branches leads to GO interlayer between benzoxazine rings from different dendrimer branches leads to GO interlayer disaggregation disaggregation and the final exfoliated or intercalated structure of the hybrid nanocomposite is and the final exfoliated or intercalated structure of the hybrid nanocomposite is determined by determined by the polymerization occurrence (Figure 9). The results are in good agreement with the polymerization occurrence (Figure9). The results are in good agreement with 1H-NMR analyses 1H-NMR analyses which showed that polybenzoxazine structures are formed as well apart from which showed that polybenzoxazine structures are formed as well apart from GO-PAMAM(G2)-BZ. GO-PAMAM(G2)-BZ.

Figure 9. Polymerization routes for hybrid GO-PAMAM functionalized with benzoxazine rings. Figure 9. Polymerization routes for hybrid GO-PAMAM functionalized with benzoxazine rings. 3.5. XRD Tests 3.5. XRDFigure Tests 10 shows the XRD curves for the investigated materials. The XRD pattern of GO-COOH showsFigure a sharp 10 peakshows at 2theθ = XRD11.85 curves◦ corresponding for the investig to (001)ated graphene materials. oxide The and XRD another pattern less of intense GO-COOH peak atshows 2θ = a42.1 sharp◦ common peak at to 2 grapheneθ = 11.85° materials corresponding [49]. After to (001) the functionalizationgraphene oxide and with another PAMAM, less the intense (001) peakpeak positionat 2θ = 42.1° is shifted common to lower to graphene degrees materials and the intensity [49]. After of thethe signalsfunctionalization is significantly with decreased PAMAM, as the a result(001) ofpeak GO position layers disaggregation is shifted to causedlower degrees by the chemical and the introduction intensity of ofthe hyperbranched signals is significantly structures betweendecreased GO as sheets.a result After of synthesisGO layers of disaggrega GO-PAMAM(G0),tion caused the intensityby the ofchemical 2θ signal introduction from 9.19◦ ofis significantlyhyperbranched decreased structures which between means GO that sheets. intercalated After structuressynthesis of are GO-PAMAM(G0), formed through the the dispersion intensity ofof GO2θ signal layers. from Athigher 9.19° is degree significantly of benzoxazine decreased functionalization, which means that the intercalated interplanar structures spacing is are increased formed duethrough to separation the dispersion of GO layers.of GO Intercalated layers. At structureshigher degree are obtained of benzoxazine in case of functionalization, GO-PAMAM(G1)-BZ the andinterplanar clearly more spacing exfoliated is increased structures due into case separation of GO-PAMAM(G2)-BZ of GO layers. Intercalated (Table3), probably structures due are to theobtained e ffect 1 ofin BZcase polymerization of GO-PAMAM(G1)-BZ showed by H-NMR and data clearly and confirmed more exfoliated from Raman structures spectra. in case of GO-PAMAM(G2)-BZ (Table 3), probably due to the effect of BZ polymerization showed by 1H-NMR data and confirmed from Raman spectra.

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Figure 10. a b FigureXRD 10. curves XRD forcurves the investigated for the materials:investigated ( ) GO-PAMAM(G0)-BZ,materials: (a) GO-PAMAM(G0)-BZ, ( ) GO-PAMAM(G1)-BZ, (b) (c) GO-PAMAM(G2)-BZ.GO-PAMAM(G1)-BZ, (c) GO-PAMAM(G2)-BZ. Table 3. Structural parameters obtained from XRD patterns. Table 3. Structural parameters obtained from XRD patterns.

SampleSample 2θ2Positionθ Position (◦) (°) d-spacingd-spacing (Å) (Å) GO-COOHGO-COOH 11.8511.85 7.46 7.46 GO-PAMAM(G0)GO-PAMAM(G0) 9.249.24 9.61 9.61 GO-PAMAM(G1) 8.22 10.74 GO-PAMAM(G1) 8.22 10.74 GO-PAMAM(G2) 7.72 11.44 GO-PAMAM(G0)-BZGO-PAMAM(G2) 7.729.19 11.44 9.65 GO-PAMAM(G1)-BZGO-PAMAM(G0)-BZ 9.197.7 9.65 11.46 GO-PAMAM(G2)-BZ 7.01 12.61 GO-PAMAM(G1)-BZ 7.7 11.46

3.6. TGA Data GO-PAMAM(G2)-BZ 7.01 12.61 TGA investigation was employed in order to evaluate the thermal stability of the hybrid 3.6. TGAGO-based Data materials. As displayed in Figure 11, the GO-COOH thermogram indicates two stages of TGAdecomposition: investigation Firstly, was the employed thermal inevaporation order to evaluate of the theadsorbed thermal moisture stability from of the the hybrid hydrophilic GO-based GO-COOH surface takes place below 100 °C and secondly, the thermal degradation of the oxidized materials. As displayed in Figure 11, the GO-COOH thermogram indicates two stages of decomposition: functionalities by releasing CO and CO2 that occurs below 200 °C [30]. A supplementary Firstly, the thermal evaporation of the adsorbed moisture from the hydrophilic GO-COOH surface degradation stage at ~240 °C is observed in case of GO-PAMAM(G0 ÷ 2)-BZ attributed to the thermal takesdecomposition place below of 100 the◦ poly(amidoamine)C and secondly, chains the thermal [50]. The degradation thermal stability of theof GO-PAMAM(G0)-BZ oxidized functionalities is by releasingnoticeably CO increased and CO compared2 that occurs to the belowraw and 200 inte◦rmediaryC[30]. Amaterials supplementary (Table 4) due degradation to formation stage of at ~240 aromaticC is observed oxazine inrings case onto of GO-PAMAM(G0the GO-PAMAM surface.2)-BZ As attributed expected, in to case the of thermal GO-PAMAM(G2)-BZ decomposition of ◦ ÷ the poly(amidoamine)the thermostability chains is much [50 higher]. The caused thermal by stability the higher of GO-PAMAM(G0)-BZdensity of H-bonding resulted is noticeably during increased the comparedpolymerization to the raw of the and benzoxazine intermediary structures, materials the polybenzoxazine (Table4) due to chains formation being already of aromatic confirmed oxazine ringsby onto 1H-NMR the GO-PAMAM spectra. However, surface. Asa slight expected, thermal in case stability of GO-PAMAM(G2)-BZ decrease is observed the thermostabilityin case of is muchGO-PAMAM(G1)-BZ higher caused by compared the higher to densityGO-PAMAM(G0)-BZ of H-bonding probably resulted due during to theformation polymerization of less of benzoxazine rings and the existence of some free poly(amidoamine) functionalities. the benzoxazine structures, the polybenzoxazine chains being already conﬁrmed by 1H-NMR spectra. However, a slight thermal stability decrease is observed in case of GO-PAMAM(G1)-BZ compared to GO-PAMAM(G0)-BZ probably due to formation of less benzoxazine rings and the existence of some free poly(amidoamine) functionalities. Polymers 2020, 12, 2424 12 of 18 Polymers 2020, 12, x FOR PEER REVIEW 12 of 18

Figure 11.FigureTGA 11. curves TGA curves for GO-COOHfor GO-COOH and and final ﬁnal be benzoxazinenzoxazine function functionalizedalized products. products.

TableTable 4. The 4. The TGA TGA data data for for GO-COOHGO-COOH and and final ﬁnal products. products.

Weight Loss, % Sample Weight Loss, % Td3%, °C Td5%, °C Sample (25–800 °C) Td3%, ◦C Td5%, ◦C (25–800 ◦C) GO-COOH 90 58.8 76.7 GO-PAMAM(G0)-BZGO-COOH 9059.7 58.884.9 76.7118.6 GO-PAMAM(G1)-BZGO-PAMAM(G0)-BZ 59.761.4 84.980.8 118.6109.3 GO-PAMAM(G2)-BZ 54.9 113.1 136.8 GO-PAMAM(G1)-BZ 61.4 80.8 109.3 3.7. DSC ResultsGO-PAMAM(G2)-BZ 54.9 113.1 136.8 Polymers 2020, 12, x FOR PEER REVIEW 13 of 18 Figure 12 shows the DSC curves of GO-COOH and the final GO-PAMAM-BZ materials. The 3.7. DSC ResultsDSC Polymerizationcurve of GO-COOH of BZ presents rings aachieved sharp peak onto at 216the °CGO-PAMAM assigned to thesurface release also of COreveals2 and aCO strong from dependencethe decomposition of the polymerization of oxygen containing heat on thefunctional PAMAM groups, generation. in good This correlation was obviously with expectedthe results as Figurethereported 12 highest shows by generation Qiu the et DSC al. of [51]. curvesPAMAM This of employedprocess GO-COOH is (G2) also wi andevidencedll allow the the ﬁnal in formation TGA GO-PAMAM-BZ curves of the (Figure highest 11) materials. number since ofa The DSC BZsignificantly units and mass therefore loss occurreda higher betweenvalue of 190polymerizati °C and 220on °C. heat (434.2 J/g) is obtained. However, this curve of GO-COOH presents a sharp peak at 216 ◦C assigned to the release of CO2 and CO from valueThe is not DSC considerably peak at 216 °Chigh is noand longer may beobserved proper on removed the DSC during curves an for industrial GO-PAMAM-BZ process productswhich is the decompositionworthywhich is to a ofconsiderstrong oxygen proof if the containing thatgeneral the propertiesfunctionalization functional and es groups, peciallywith PAMAM inthe good mechanical molecules correlation ones really will with occurred be higher the resultsat than the reported by Qiu et al.forcarboxylic [classical51]. This groupsbenzoxazines. process from is Thethe also valueGO evidenced surface.of the polymerization Mo inreover, TGA curvesa heat sharp for (Figure GO-PAMAM(G2)-BZpeak is 11 noticed) since afor signiﬁcantlyis alllimited the mass since a partial polymerization process of BZ units formed through synthesis reaction already loss occurredGO-PAMAM-BZ between 190 samples◦C and at 220a temperature◦C. value depending on the generation of PAMAM occurredemployed, as being it was attributed revealed byto 1theH-NMR polymerization spectra. of the BZ rings synthesized on the GO-PAMAM surface. Thus, for GO-PAMAM(G0)-BZ, this peak is located at 161.9 °C and it is shifted to lower temperature values as the generation increased. This may be explained by the auto-catalytic effect of BZ rings which being located at lower distance between them as the generation of PAMAM increases will be easier available for polymerization, either through in-dendrimer polymerization or out-dendrimer polymerization (Figure 9). Therefore, in the case of GO-PAMAM(G2)-BZ, the polymerization peak is located at 152.4 °C which is a very good improvement in comparison with the high temperature usually required for benzoxazine polymerization (180–220 °C) [52] and PAMAM-BZ monomers obtained by Lu et al. [36]. This performance may open new fields for use of polybenzoxazines as the BZ polymerization temperature is not a barrier anymore due to its high value.

Figure 12.FigureDSC 12. curves DSC curves for GO-COOHfor GO-COOH andand final ﬁnal be benzoxazinenzoxazine functionalized functionalized products. products.

3.8. Nanoindentation The nanomechanical properties of GO-PAMAM(G0 ÷ 2)-BZ were evaluated by nanoindentation technique and the average values of Young’s modulus, stiffness, and hardness are presented in Figure 13. The degree of GO dispersion and the homogeneity of the analyzed hybrid samples highly influence the nanomechanical properties of the final nanocomposites [53]. The raw material GO-COOH shows the elasticity modulus (E) of 0.614 GPa, hardness (H) of 0.057 GPa and stiffness (S) of 4416 N/m (Table 5). It is noticed that the mechanical properties of hybrid materials are directly influenced by the generation of dendrimers used in the GO functionalization step. Therefore, a minor decrease of mechanical properties compared to GO-COOH was observed for GO-PAMAM(G0)-BZ in terms of Young’s modulus and stiffness as a consequence of introduction of organic functionalities which makes the structure of the material less compact, thus contributing to more voids within the material. By increasing the PAMAM generation, the number of benzoxazine rings is increased and minor mechanical properties improvement may be observed in case of GO-PAMAM(G1)-BZ. However, the most relevant results are obtained in case of GO-PAMAM(G2)-BZ showing a significant increase of all the mechanical properties studied probably due to the highly crosslinked network of polybenzoxazine, formed through the numerous BZ rings which are suitable for polymerization being located at less distance.

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The DSC peak at 216 ◦C is no longer observed on the DSC curves for GO-PAMAM-BZ products which is a strong proof that the functionalization with PAMAM molecules really occurred at the carboxylic groups from the GO surface. Moreover, a sharp peak is noticed for all the GO-PAMAM-BZ samples at a temperature value depending on the generation of PAMAM employed, being attributed to the polymerization of the BZ rings synthesized on the GO-PAMAM surface. Thus, for GO-PAMAM(G0)-BZ, this peak is located at 161.9 ◦C and it is shifted to lower temperature values as the generation increased. This may be explained by the auto-catalytic effect of BZ rings which being located at lower distance between them as the generation of PAMAM increases will be easier available for polymerization, either through in-dendrimer polymerization or out-dendrimer polymerization (Figure9). Therefore, in the case of GO-PAMAM(G2)-BZ, the polymerization peak is located at 152.4 ◦C which is a very good improvement in comparison with the high temperature usually required for benzoxazine polymerization (180–220 ◦C) [52] and PAMAM-BZ monomers obtained by Lu et al. [36]. This performance may open new fields for use of polybenzoxazines as the BZ polymerization temperature is not a barrier anymore due to its high value. Polymerization of BZ rings achieved onto the GO-PAMAMsurface also reveals a strong dependence of the polymerization heat on the PAMAM generation. This was obviously expected as the highest generation of PAMAM employed (G2) will allow the formation of the highest number of BZ units and therefore a higher value of polymerization heat (434.2 J/g) is obtained. However, this value is not considerably high and may be proper removed during an industrial process which is worthy to consider if the general properties and especially the mechanical ones will be higher than for classical benzoxazines. The value of the polymerization heat for GO-PAMAM(G2)-BZ is limited since a partial polymerization process of BZ units formed through synthesis reaction already occurred as it was revealed by 1H-NMR spectra.

3.8. Nanoindentation The nanomechanical properties of GO-PAMAM(G0 2)-BZ were evaluated by nanoindentation ÷ technique and the average values of Young’s modulus, stiffness, and hardness are presented in Figure 13. The degree of GO dispersion and the homogeneity of the analyzed hybrid samples highly influence the nanomechanical properties of the final nanocomposites [53]. The raw material GO-COOH shows the elasticity modulus (E) of 0.614 GPa, hardness (H) of 0.057 GPa and stiffness (S) of 4416 N/m (Table5). It is noticed that the mechanical properties of hybrid materials are directly influenced by the generation of dendrimers used in the GO functionalization step. Therefore, a minor decrease of mechanical properties compared to GO-COOH was observed for GO-PAMAM(G0)-BZ in terms of Young’s modulus and stiffness as a consequence of introduction of organic functionalities which makes the structure of the material less compact, thus contributing to more voids within the material. By increasing the PAMAM generation, the number of benzoxazine rings is increased and minor mechanical properties improvement may be observed in case of GO-PAMAM(G1)-BZ. However, the most relevant results are obtained in case of GO-PAMAM(G2)-BZ showing a significant increase of all the mechanical properties studied probably due to the highly crosslinked network of polybenzoxazine, formed through the numerous BZ rings which are suitable for polymerization being located at less distance. Polymers 2020, 12, 2424 14 of 18 Polymers 2020, 12, x FOR PEER REVIEW 15 of 19

Figure 13. Nanomechanical properties: (a) Young’s modulus and (b) stiffness and hardness of the raw Table 5. Mechanical properties of GO-COOH and GO-PAMAM(G0÷2)-BZ materials obtained by material (GO-COOH) and hybrid materials. nanoindentation. Table 5. Mechanical properties of GO-COOH and GO-PAMAM(G0 2)-BZ materials obtained Sample E (GPa) Hardness (GPa) ÷ Stiffness (N/m) by nanoindentation.GO-COOH 0.614 0.057 4416 GO-PAMAM(G0)-BZ 0.536 0.036 3617 Sample E (GPa) Hardness (GPa) Stiffness (N/m) GO-PAMAM(G1)-BZ 0.763 0.078 5117 GO-PAMAM(G2)GO-COOH-BZ 1.418 0.614 0.157 0.057 44169621 GO-PAMAM(G0)-BZ 0.536 0.036 3617 4. ConclusionsGO-PAMAM(G1)-BZ 0.763 0.078 5117 New nanomaterialsGO-PAMAM(G2)-BZ based on benzoxazine 1.418 (BZ)- 0.157functionalized graphene 9621 oxide (GO) -poly(amidoamines) (PAMAM) structures were synthesized to gain a lot of BZ rings on the materials surface, capable to form a crosslinked network by polymerization. 4. Conclusions The XPS curves, FTIR and 1H-NMR spectra confirmed BZ rings formation at the amine Newfunctional nanomaterials groups of PAMAM. based However, on benzoxazine this process (BZ)-functionalizeddepends on the number graphene of amino functional oxide (GO) -poly(amidoamines)groups from PAMAM, (PAMAM) meaning structures that the were generation synthesized of PAMAM to gain has a a lot significant of BZ rings influe onnce the on materials the surface,final capable morphology to form of GO a crosslinked-PAMAM-BZ network structures. by polymerization.Thus, by employing PAMAM(G2) with a higher number of amino groups, more BZ rings are formed and these will further react to form The XPS curves, FTIR and 1H-NMR spectra confirmed BZ rings formation at the amine functional polybenzoxazine chains which are directly detected from 1H-NMR spectra and indirectly from XRD, groupsTGA of PAMAM. and nanoindentation. However, this Therefore, process thedepends mechanical on the properties number of of amino GO-PAMAM(G2) functional groups-BZ are from PAMAM,significantly meaning higher that than the generationfor the other of two PAMAM PAMAM has generations a significant (G0, influence G1)-based on materials. the final morphology of GO-PAMAM-BZThe formation structures. of BZ units Thus, at the by surface employing of PAMAM PAMAM(G2) amino functionalities with a higher exhibits number also of an amino groups,important more influence BZ rings to are the formedintercalation and process these willof GO further layers since react the to XRD form data polybenzoxazine showed an increase chains whichof are the directly basal detecteddistance betweenfrom 1H-NMR the GO spectra layers and probably indirectly due from to the XRD, partial TGA polymerization and nanoindentation. of Therefore, the mechanical properties of GO-PAMAM(G2)-BZ are significantly higher than for the other two PAMAM generations (G0, G1)-based materials. Polymers 2020, 12, 2424 15 of 18

The formation of BZ units at the surface of PAMAM amino functionalities exhibits also an important influence to the intercalation process of GO layers since the XRD data showed an increase of the basal distance between the GO layers probably due to the partial polymerization of BZ-functionalized PAMAM chains within the GO layers. This fact is more obvious in the case of GO-PAMAM(G0)-BZ by appearance of sharper 2D band in the Raman spectrum which means a few layers of graphene oxide which are still stacked in assemblies. This effect is less noticeably for GO-PAMAM(G2)-BZ from Raman spectra, but XRD data show in this case the highest degree of intercalation among all the generations of PAMAM-based materials. The polymerization process of BZ units achieved by synthesis onto GO-PAMAM surface occurs at a temperature which strongly depends on the generation of PAMAM employed. As the generation of PAMAM increased, a less distance between the BZ units is obtained, that will favor the polymerization process and therefore the polymerization occurs at lower temperatures. As a consequence, for all the GO-PAMAM-BZ products, the polymerization temperature is significantly less than for classical benzoxazines which may open industrial applications for these structures considering that for most of the BZ used in different applications, the high polymerization temperature is a significant disadvantage. Due to the promising results obtained in this study, the functionalization of GO layers with benzoxazine monomers using dendrimers of higher generations needs to be performed for further investigation of the thermal and mechanical properties of the GO-PAAMAM-BZ nanocomposites.

Author Contributions: E.I.B.: Investigation, Validation, Data curation, Formal analysis, Writing—original draft, Writing—review and editing. S.A.G.: Methodology, Investigation, Writing—review and editing. H.I.: Supervision, Writing—original draft, Writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by a grant of the Ministry of Research and Innovation, CNCS UEFISCDI, project number PN-III-P4-ID-PCE-2016-0818, within PNCDI III. E. I. Bîru acknowledges the support of Operational Programme Human Capital of the Ministry of European Funds through the Financial Agreement 51668/09.07.2019, SMIS code 124705. The nanoindentation measurements were possible due to European Regional Development Fund through Project No. P_36_611, MySMIS code 107066, INOVABIOMED. The authors acknowledge the support of PubArt Programme from University Politehnica of Bucharest. Acknowledgments: We thank Eugeniu Vasile for the XRD investigation of the materials. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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