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1 This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

2 Page of 0 Interactions, Morphology and Thermal Stability of Graphene-xide Reinforced Polymer Aerogel Derived from Star-Like Telechelic Aldehyde- Terminal Benzoxazine Resin 0 0 Almahdi A. Alhwaige,,*,, Saeed M. Alhassan, Marios S. Katsiotis, Hatsuo Ishida,*, Syed Qutubuddin,,* Department of Chemical and Biochemical Engineering, Case Western Reserve University, Cleveland, hio 0-, USA Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, H 0-0, USA Department of Chemical Engineering, The Petroleum Institute, United Arab Emiratis * Corresponding authors: aaa@case.edu (Almahdi Alhwaige), sxq@case.edu (Syed Qutubuddin), and hxi@case.edu (Hatsuo Ishida). n leave from Al-Mergib University, Libya (aaalhwaige@elmergib.edu.ly). Accepted Manuscript

3 Page of 0 Table of Contents Entry 0 0 Accepted Manuscript

4 Page of ABSTRACT Graphene oxide (G)-reinforced nanocomposite aerogels of polybenzoxazine prepared via freeze-drying of G suspensions in benzoxazine precursor solutions has been studied. The synthesis of G is confirmed using Raman and Fourier transform infrared (FT-IR) spectroscopy. Benzoxazine monomer (SLTB(HBA-t0)) has been synthesized using -hydroxybenzaldehyde as a phenolic component, paraformaldehyde, and tri-functional polyetheramine (Jeffamine T- 0) as an amine source. The chemical structure of the benzoxazine monomer is confirmed by nuclear magnetic resonance ( H-MR) spectroscopy and FT-IR. The interactions of G and SLTB(HBA-t0) have been investigated using FT-IR. The morphological and thermal stability of nanocomposite aerogels are examined and compared with the neat polybenzoxazine aerogel. The structures of the aerogels and the effect of G on the morphology of the aerogels are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of G on the ring-opening polymerization of benzoxazine is also evaluated using differential scanning calorimetry (DSC) whereas the thermal stability of the nanocomposite aerogels is characterized by thermogravimetric analysis (TGA). KEYWRDS: Graphene oxide, polybenzoxazines, nanocomposite aerogels, morphology, and thermal properties. Accepted Manuscript

5 Page of ITRDUCTI Aerogels are open-cell porous three-dimensional materials with unusual properties such as ultralow density ( g/cm ), high surface area, and low dielectric permittivity. - Since Kistler developed the aerogels in, sol-gel process has attracted much attention for the fabrication of porous materials. However, active research in this area did not start until about 0 years later. The nano-porous open structure of aerogels, which are formed by replacing the liquid in a gel with gas during their preparation and persists in the dry state.,, The efficiency of cross-linking polymers is expected to produce aerogels with much better properties. Among the cross-linking polymers, polybenzoxazines have gained much attention as a versatile class of attractive polymeric materials. - Although the first synthesis of small molecular weight benzoxazine was reported in 0s, the synthesis and properties of polybenzoxazine derived from monomeric type benzoxazine resins was reported in. More recently, main-chain type cross-linkable polybenzoxazines where the oxazine ring is placed in the main chain have been developed. The development of polybenzoxazine aerogels was first reported in 00. Meanwhile, several other studies on polybenzoxazine-based polymeric and carbon aerogels containing different monomers have been reported.,,-0 The advantages of benzoxazine-based nanostructured materials include near-zero shrinkage upon polymerization, excellent thermal stability, and high char yields., anocomposite materials are regarded one of the fastest growing fields in the polymer industry. Specific properties of polymeric materials can be enhanced by a uniform dispersion of reinforcement in the polymer matrix, in particular using a small quantity of nanofiller. - Many different types of fillers, including clay, carbon black, graphene, carbon nanotubes, carbon fibers, and glass fibers have been investigated for producing materials with improved properties. Accepted Manuscript

6 Page of Growing literature about the still-developing field of polybenzoxazine nanocomposites presents major advances that are worthy of consideration. Polybenzoxazine based nanocomposites have attracted the interest of researchers in film form; however, research in development of polybenzoxazine-based nanocomposite aerogels is limited. Very recently, graphene-reinforced benzoxazine films have been investigated. -0 The current study, involves the incorporation of G into aldehyde-terminal benzoxazine matrix in the form of nanocomposite aerogel, which has not yet been reported. (a) (b) H C HC H C CH Y CH X CH G SLTB(HBA-t0) Scheme Chemical structures of: (a) graphene oxide (G) and (b) the star-like telechelic benzoxazine monomer (SLTB(HBA-t0)) used in this work. Graphene nanosheets show unusual enhancement on the properties of nanocomposite materials. 0- G (Scheme a) is a layered material produced by the treatment of natural graphite using strong mineral acids and effective oxidizing agents, which introduce a variety of oxygen-based functional groups to the basal planes and edges of graphene layers. - Graphite is the inexpensive natural source available for production of graphene and G sheets. 0-, Polymer composites with exfoliated graphite have been reported since. The forces of interaction between the polymers and G are primarily dipole-dipole interaction and/or Z CH Accepted Manuscript

7 Page of hydrogen bonding arising from the polar groups in the polymer and covalent bond formation with the functional groups of hydroxyl (-H), epoxide (-CC-), and carboxyl (-CH) on the surfaces of the G. For example, graphene-based nanocomposite studies were reported using polymers, such as polybenzoxazine,,0 polyaniline, polystyrene-polyacrylamide copolymer, poly(allylamine), poly(vinyl alcohol), 0 and epoxy. The development of carbon-based aerogels from graphene and G have been reported., Combination of benzoxazine and G is a promising approach to develop the field of benzoxazine nanocomposite aerogels with properties not provided by neat polybenzoxazines. In the current study, a star-like telechelic benzoxazine (Scheme b) was used with various G contents for synthesizing novel nanocomposite aerogels. The morphological and thermal stability of these aerogels have been studied in detail. The novelty of the current study involves our report on the first proposed mechanism of interaction between aldehydes-terminal benzoxazine and G nanosheets.. EXPERIMETAL SECTI.. Reagents Graphene oxide (G) was synthesized from natural graphite powder by modified Hummer s method as previously described., Synthesis of G is described in the Supporting Information, S.. Graphite powder (micro 0) was obtained from Asbury Graphite Mills, Inc. Concentrated sulfuric acid (H S, %), potassium permanganate (KMn crystal), hydrogen peroxide (H, 0% aqueous solution), concentrated hydrochloric acid (HCl) were purchased from Fisher Scientific (USA). Accepted Manuscript

8 Page of Star-like telechelic benzoxazine was synthesized from -hydroxybenzaldehyde (HBA, %, Aldrich Chemicals, USA), polyetheramine (Star-like jeffamine T-0, Mn = 0 g/mol, kindly supplied by Huntsman) and paraformaldehyde (% w/w, Sigma-Aldrich Chemicals, USA) using,-dioxane as solvent. The telechelic is hereinafter abbreviated as SLTB(HBAt0). Glacial acetic acid (CH CH),,-dioxane, chloroform, and dimethylsulfoxide (DMS) were purchased from Fisher Scientific (USA). All chemicals were used without further purification... Preparation of SLTB(HBA-t0)/G nanocomposite aerogels ew nanocomposite aerogels were prepared via freeze-drying of various G suspensions in SLTB(HBA-t0) solutions (Fig. ). Samples are abbreviated as SLTB(HBA-t0)/G-x where x is the various G content of 0,,, and 0 wt%. The starting precursor of benzoxazine (SLTB(HBA-t0)) was prepared by our previously reported method and can be found in the Supporting Information, S.. The preparation of the aerogels will be presented using SLTB(HBA-t0)/G- as an example of nanocomposite aerogel containing wt% G. Specifically, 0.0 g of G were dispersed in 0 ml of DMS, while at the same time, 0. g of SLTB(HBA-t0) were dissolved in 0 ml of DMS using a magnetic stirrer at ambient conditions. After completely dissolving SLTB(HBA-t0), the solution was added slowly to the G colloidal suspension under vigorous agitation using a mechanical stirrer at 000 rpm at room temperature. The mixing lasted for h, after which the mixture was sonicated for 0 min. Then, the collected suspension was transferred into glass vials, sealed, and then heated slowly to o C in an oven for h. The attained nanocomposite product was partially cross-linked due to ring-opening polymerization of benzoxazine precursor. The collected colloidal suspension was frozen using ethanol and solid carbon dioxide at -0 o C and ambient pressure. Finally, the frozen Accepted Manuscript

9 Page of 0 poly(hba-t0/g-x) suspension was transferred to a VirTis AdVantage@ EL- freeze-drier for five days. Aerogels were obtained after drying the frozen solvent by sublimation under high vacuum conditions. Fig. Illustration of preparation process for novel polybenzoxazine/g nanocomposite aerogels. HC HC HC Y X CH CH CH Z CH H CH HC n HC Y H X CH n CH CH HC Z n H Accepted Manuscript SLTB(HBA-t0) Poly(SLTB(HBA-t0)) Scheme Thermally accelerated synthesis of poly(sltb(hba-t0)) from its precursor.

10 Page of Cross-linking and carbonization of the aerogels The resulting aerogels were thermally treated at 0 o C, o C, and 0 o C for h to obtain the fully polymerized and cross-linked polybenzoxazine nanocomposite aerogels (Scheme ). The polymerized aerogels were carbonized under nitrogen flow of 0 ml/min and a temperature ramp rate of 0 o C/min from ambient temperature to 00 o C... Characterization Micro-Raman scattering studies were carried out at room temperature using a Horiba Jobin-Yvon LabRam HR00 spectrometer which is equipped with a charge coupled detector and two grating systems (00 and 00 lines/mm). A Hee laser (λ =. nm), with an optical power of mw and a spot size of µm, was focused on the sample with an lympus microscope. Raman shifts were calibrated with a Si (00) wafer using the 0 cm - peak. X-ray diffraction (XRD) was used to determine the increment of the d-spacing in the G sheets through intercalation of the cationic organic. X-ray diffractograms of the aerogels were obtained by Cu-Kα radiation (λ= 0. nm) with scanning rate of 0. degrees/min at room temperature. XRD is performed on dried thin film of aerogels. The aerogel samples were cut into thin disks prior to X-ray measurements. Bragg s equation, λ d sinθ ( ) = where d is the layer spacing and θ is the angle of diffraction, was used to compute the d-spacing for G nano-sheets. Fourier-transform infrared (FT-IR) spectroscopy was used to investigate the oxidation of graphene, polymerization of benzoxazine and the interactions between benzoxazine and G. FT- IR spectra were obtained using a Bomem Michelson MB-00 FT-IR spectrometer with a dry air purging unit and a deuterated triglycine sulfate detector at a resolution of cm -. Textural morphology of the aerogels was observed with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). SEM analysis was performed with a FEI Accepted Manuscript

11 Page 0 of Quanta 0 (FEG), coupled with Energy Dispersion X-Ray Spectroscopy (EDS). Regarding sample preparation, a thin slice (~. mm) cut on the vertical axis of each of the cylindrically shaped specimens was mounted on standard aluminum SEM stabs using conductive silver glue. TEM analysis was performed with a FEI Tecnai G0, coupled with EDS and a Gatan GIF energy filtered camera. Samples were crushed using a mortar and pestle and then dispersed in high purity ethanol (. %) in an ultrasonic bath for ~0 s. A drop of the dispersed phase would be deposited on Cu grid covered with a thin layer of amorphous carbon (lacey carbon), following which the grid would be immediately transferred to the TEM holder to avoid contamination. The apparent bulk density ( b ) of the aerogel was obtained from the aerogel mass (m) and its volume (V) as described in Eqn. () =.... () The values of skeletal density ( S ) were obtained by using helium in gas-displacement pycnometer. The apparent porosity percentage of the aerogels was calculated according to the following relation. %= 00.. () The thermal stability of the aerogels were evaluated by thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA) using a TA Instruments High Resolution 0. A sample (-0 mg) from each obtained aerogel was placed in a platinum pan and heated from room temperature to 00 o C at a temperature ramp rate of 0 o C min - under nitrogen atmosphere with a gas flow rate of 0 ml min -. Accepted Manuscript 0

12 Page of 0 0. RESULTS AD DISCUSSI.. Preparation of graphene oxide (G) The oxidation of graphite has been confirmed by Raman spectroscopy and FT-IR. As shown in Fig., the Raman spectrum of natural graphite exhibits strong lines at cm - and cm -, which are assigned to the G band and D band, respectively, while the D band is shown as a weak band at cm -.,, The two most intense lines appearing at and 0 cm - for G correspond to the G and D bands, respectively. As shown in the G spectra, the D band shifts to higher frequency and becomes prominent. The ratio of D to G band intensity (I D /I G ) equals to., indicating the reduction in the average size of sp domains due to extensive oxidation., Fig. Raman spectra of graphite (lower spectrum) and G (upper spectrum). FT-IR results confirm the introduction of oxygen-based functional groups on the basal plane and edge of graphene (Supporting Information, Fig. S). Expectedly, there is no significant peak found in the natural graphite spectrum, while the spectrum of G shows characteristic band at Accepted Manuscript

13 Page of 0 0 cm - (the H stretching mode), 0 cm - (the C= stretching mode of carboxylic acid groups), 0 cm - (=C-H stretching vibrations) and 0 cm - (C- stretching vibrations). These results are in agreement with reported literature Interactions of G and polybenzoxazine Recently, much attention has focused on the development of benzoxazines-based nanocomposite materials.,0 However, interaction of G with benzoxazine containing functional groups have not yet been reported. G contains several reactive moieties on the surface of its nanosheets including epoxide, hydroxyl, and carboxyl groups.,, The interactions between benzoxazine and epoxy during the polymerization have been investigated.,0 Jubsilp et al. 0 reported that DSC thermograms of benzoxazine/epoxy blends showed at least two possible reactions: (i) the exothermic peak due to polymerization of benzoxazine monomers. Phenolic structure is an initiator and catalyst for polymerization of both benzoxazine - and epoxy. (ii) The reaction between the epoxide groups on the epoxy with the phenolic hydroxyl groups on polybenzoxazine. Accordingly, we suggest that epoxy groups on the surface of G sheets attach the polybenzoxazine structure during the polymerization step. Furthermore, hydrogen bonding between the polar groups on both polybenzoxazine and G may take place. The proposed interactions between SLTB(HBA-t0) as aldehyde-terminal benzoxazine and G are shown in Schemes and, while FT-IR was used to confirm these interactions. The structure of SLTB(HBA-t0) benzoxazine monomer was confirmed with H-MR and FT-IR as shown in the Supporting Information Fig. S and S, respectively; detailed discussion can be found in S.. FT-IR was used to study the polymerization behaviour of SLTB(HBA-t0)/G nanocomposite aerogels. A typical FT-IR profiles of neat SLTB(HBAt0) at o C and SLTB(HBA-t0)/G-% aerogel at various temperatures (0, 0, 00 and Accepted Manuscript

14 Page of o C) are shown in the Supporting Information, Fig. S. FT-IR spectrum of pristine G (Supporting Information, Fig. S) indicates that the hydroxyl functional groups on the surface of graphene sheets have characteristic band at 0 cm - (the H stretching mode). FT-IR results of SLTB(HBA-t0) (Supporting Information, Fig. Sa) reveals that there is no absorption in the range cm -. After dispersion of G in SLTB(HBA-t0) matrix, a broad band appearing at cm - indicates the presence of hydroxyl funtional groups ( H) of G. The frequency shift from 0 cm - to cm - is likely due to the weakening of H hydrogen bonds caused by the more hydrophobic environment of G surrounded by SLTB(HBA-t0), which is consistent with the exfoliated G. The samples heated at higher temperatures show more absorption intensities of this peak due to the newly created phenolic H groups in the polybenzoxazine chains. The phenolic hydroxyl group is also confirmed by the appearance of new absorption peak around 00 cm. From the Supporting Information Fig. S, it is obvious that the characteristic bands of the G functional groups heavily overlap with this newly generated H band and it makes difficult to study the interaction of G and benzoxazine by FT-IR. The problem is compounded by the low concentration of G compared to the amount of polybenzoxazine matrix in all samples studied. Therefore, further experiments were performed to confirm SLTB(HBA-t0)/G interactions. The investigation of interfacial interactions between SLTB(HBA-t0) and G were evaluated by FT-IR using sample with high concentration of G (SLTB(HBA-t0)/G is 0/0 weight percent), which forms a thin-layer of SLTB(HBA-t0) on G nanosheets. The sample was prepared by blend of a concentration of benzoxazine (0 wt %) and G (0 wt %) and thoroughly mixed for one min. The solution was casted on KBr pellet and dried at Accepted Manuscript

15 Page of room temperature for FTIR measurements. Then, the casted film was subjected to a ring-opening polymerization of benzoxazine monomer schedule of h at each 00,, o C. Fig. shows FT-IR spectra of neat G, neat SLTB(HBA-t0), and treated SLTB(HBAt0)/G nanocomposite at various temperatures. The FT-IR characteristics of G are separately discussed in detail (Supporting Information, Fig. S). The important characteristic infrared absorptions of the neat SLTB(HBA-t0) structure are clearly observed as discussed (Supporting Information, S. and Fig. S). The FT-IR spectrum of adsorbed layer of SLTB(HBA-t0) on G sheets (Fig. c) shows a characteristic absorption band at cm, which is attributed to the C= stretching band of carboxylic acid-functional groups of G., This peak slightly changed with heat treatment due to functional groups attached to the carbonyl with a benzene ring or double bond in conjugation with carbonyl. The results in Fig. c reveal that mixing of G to SLTB(HBA-t0) significantly shifts the absorption band at cm - of polyether chain to lower wavenumber due to hydrogen bonding with the adsorbed water on the G-functional groups. After thermal treatment from 00 o C up to o C, this band returned to the same position (Fig. b) due to the evaporation of moisture. Moreover, the ester-linkage formation in benzoxazine/epoxy resin blend system has been reported. The characteristic FT-IR bands of epoxy band (around 0-00 cm - ) overlap with the absorption bands of benzoxazine, making it difficult to follow the disappearance of epoxy bands upon polymerization. However, the observation of the relative increase in the band at cm - is due to ester group formation. More recently, chemical interaction of G with phenolic resin has been reported. Zhou et al. documented that esterification reaction occurred between hydroxyl groups ( H) on resole and carboxyl groups ( CH) on G, as confirmed by the absorption band at cm -. Therefore, herein, the absorption band at cm - is attributed to Accepted Manuscript

16 Page of 0 the esterification reaction between phenolic H and carboxylic groups of G after heat treatment. In addition, the absorption bands at and cm - are assigned to C as a result of the oxidation of aldehyde groups. In aldehyde-functional benzoxazines, oxidation of aldehyde groups to form carboxylic groups has been reported. Therefore, ester formation 0 resulting from carboxylic groups-epoxy reaction possibly occurred as seen in Scheme a. In this case of oxidation of aldehyde groups, the released C would present a great advantage to partially contribute to the porous structure of the aerogels, further investigation will be discussed in details elsewhere. Fig. FT-IR spectra of interfacial interaction of G and SLTB(HBA-t0): (a) neat G, (b) neat SLTB(HBA-t0), (c), (d), (e), and (f) SLTB(HBA-t0)/G (0/0 weight percent) at, 00,, and o C, respectively. Accepted Manuscript

17 Page of 0 0 (a) (b) H H HC H HC H C R R C H H HC H H H HC H R R HC HC Scheme Proposed chemical interaction between SLTB(HBA-t0) and G: (a) Esterification-based on carboxylic acid of oxidized aldehyde-functional benzoxazine and (b) Etherification-based on the residual aldehyde-functional benzoxazine. Furthermore, during heat treatment of ring-opening polymerization of benzoxazine, the produced phenolic hydroxyl is proposed to react with epoxy groups of G (etherification). The polyether band at cm - became a broader peak and overlapped with cm -, which attributed to the chemical attachment of polybenzoxazine with G via phenolic hydroxyl-epoxy reaction as proposed in Scheme b. The resulting absorption band increased with increasing heat treatment due to production of phenolic hydroxyl H. The absorption bands in the range 0- cm - in the FT-IR spectra of SLTB(HBA-t0)/G nanocomposites become overlapped bands due to newly appeared C=C band from thermal treatment of G. 0 Heat treatment of G partially decomposes the functional groups resulting in aromatic conjugated bonds of graphene. Accepted Manuscript

18 Page of 0 CH H C Y HC CH X CH CH Z HBA-t0 CH (a) HC HC H H R R HC HC CH HC Y HC X HC Z CH (b) C H Scheme Proposed hydrogen bonding network formed between oxygen-functionality on G and SLTB(HBA-t0): (a) Hydrogen bonding in polymerized aldehyde-functional benzoxazine [Adapted from ref, ] and (b) Hydrogen bonding in polymerized SLTB(HBA-t0)/G. HC HC HC H H C H H H H C H R H R R H C H H H H H H H H H H H H H C C H H HC H C H C H H H H HC R R Accepted Manuscript Hydrogen bonding is possible between the aldehyde-functional benzoxazines and G formed. Polybenzoxazines contain extensive intermolecular and intramolecular hydrogen

19 Page of bonding between phenolic hydroxyl H and electronegative atoms such as and.,,-0 Also, hydrogen bonding interaction between the aldehyde groups and polybenzoxazine has been reported as seen in Scheme a. In the polymerized aldehyde-functional benzoxazine monomer, the aldehyde groups contain electronegative atoms, which can form intermolecular hydrogen bonding with H atoms of phenolic hydroxyl groups. Herein, the H atoms of hydroxyl groups of G may show similar tendency (Scheme b). Fig. S shows the infrared spectra of the C= stretching (aldehyde groups) of SLTB(HBA-t0) and SLTB(HBA-t0)/G samples. The neat SLTB(HBA-t0) gives a carbonyl absorption peak at cm - (Fig. Sb).,, After adding the G into the SLTB(HBA-t0) system (Fig. Sc), the carbonyl stretching frequency is shifted into cm -, corresponding to the hydrogen bonded aldehyde groups of SLTB(HBA-t0) with functional groups on G sheets as described in Scheme b. This new band ( cm - ) gradually shifts into the hydrogen bonded carbonyl ( cm - ) with increasing heat treatment of nanocomposite sample (SLTB(HBA-t0)/G). These results suggest that there interaction in the residual aldehyde groups of polybenzoxazine resins. These results are in agreement with literature. As seen in Fig. S, the existence of hydrogen bonding makes the frequency of the C= band ( cm - ) decrease in the FT-IR spectrum. The hydrogen bonding results from the interaction of electronegative atoms in the residual aldehyde groups with H atoms of hydroxyl groups of both G and polybenzoxazine. Also, the phenolic hydroxyl groups of benzoxazine may form hydrogen bonding with hydroxyl groups of G. Further hydrogen bonding may occur between carboxylic groups of G and H atoms of polybenzoxazine ( H and CH). According to these results, one can conclude that polymerized SLTB(HBAt0)/G contains a large amount of intermolecular hydrogen bonding formed by residual aldehyde and phenolic hydroxyl groups with oxygen-functional groups of G (Scheme ). Accepted Manuscript

20 Page of 0 Alhikari et al. reported that functional groups (-H, -CH) are able to form hydrogen bonds with other molecules under appropriate conditions. As result of hydrogen bonds, polymers can be assembled with G to form a hydrogel matrix., To conclude the FT-IR results, 0 incorporation of neighboring group of aldehyde moieties in benzoxazine monomers play a role for providing more interaction sites during hybridization of polybenzoxazine with G. These interactions contribute to the enhanced performance of the aerogels as a new class of nanocomposite aerogels. The strong interaction of benzoxazine chains with G nanosheets leads to high performance nanocomposites-based materials... X-ray diffraction analysis (XRD) Fig. displays the X-ray diffractograms of graphite, G, and the nanocomposite aerogels. In the case of oxidation of the neat graphite, the d 00 diffraction peak in the natural graphite shifted from around θ = o to lower θ values, indicating the G formation.,,, Accepted Manuscript Fig. XRD diffractograms of profiles of: (a) G, (b) graphite, (c), (d), (e), and (f) of SLTB(HBA-t0)/G nanocomposites with various G contents of 0 wt% wt% wt%, and 0 wt% G, respectively.

21 Page 0 of The results show that the d-spacing of graphene layers was increased from 0. nm to 0. nm after the oxidation process due to the formation of oxygen-functional moieties which are acting as spacers. The challenges are to achieve molecule-level dispersion and maximum interfacial interaction between the nanofiller and the polymer matrix at low loading. The XRD pattern of neat SLTB(HBA-t0) (Fig. c) shows a typical diffraction of an amorphous polymer with a broad peak at θ = o. In the case of SLTB(HBA-t0)/G aerogels, the effect of SLTB(HBA-t0) on exfoliation of G also has been evaluated. Up to wt% of G, no obvious diffraction peak is seen below θ = 0 o. This means that G may be fully exfoliated in polybenzoxazine matrix due to the strong interaction between G and SLTB(HBA-t0), although the concentration of G is so low and verification of complete exfoliation only by XRD is difficult. However, the aerogel containing 0 wt% G shows a broad diffraction peak at θ =. o, indicating that polybenzoxazine has intercalated fraction of G. bservation of intercalated fraction implies the trend to aggregate the nanoparticles starts around this concentration. As a result of polybenzoxazine intercalation, the d-spacing of G increased from 0. nm to. nm... Morphological and microstructure analysis of G reinforced aerogels Effects of hybridization of SLTB(HBA-t0) matrix with G on morphology and structure of benzoxazine aerogel have been investigated. The density and porosity of all obtained aerogels are summarized in Table. Slightly differences were observed in physical structures of neat and nanocomposite polybenzoxazine aerogels. The apparent density ( b ) and porosity of the obtained aerogels are in the range of g cm - and.0. %, respectively. These results Accepted Manuscript confirmed highly porous structure of the hybrid aerogels. The results indicated that bulk density ( b ) of neat polybenzoxazine aerogel is higher than the nanocomposite aerogels, which 0

22 Page of 0 0 can be related to their microstructures behavior as shown in Fig.. By comparing SEM micrographs for the neat polybenzoxazine and nanocomposite aerogel (SLTB(HBA-t0)/G- %), samples containing G have more overlapping layers in the morphology than G-free aerogels. This increase in the number of layers may lead to the observed increase in the porosity with increasing G content. Table Bulk density and porosity of the obtained aerogels. Sample code Bulk density (g cm - ) Porosity (%) SLTB(HBA-t0)/G SLTB(HBA-t0)/G- 0.. SLTB(HBA-t0)/G- 0.. SLTB(HBA-t0)/G- 0.. SLTB(HBA-t0)/G Scanning Electron Microscopy (SEM) The morphology of polybenzoxazine aerogels has been reported earlier., To the best of our knowledge, this is the first time reported in literature where morphology and microstructure of polybenzoxazine/g aerogels is studied with SEM.SEM analysis shows that all samples contain a highly porous structure, a result of solvent removal; however, pore sizes do not exhibit uniform distribution. The cross section of the polybenzoxazine aerogel is shown in Fig. a and b. The cross section of the specimen containing % G (SLTB(HBA-t0)/G-%) is shown in Fig. c and d, while lateral sections are shown in Fig. e, f and g. Accepted Manuscript

23 Page of 0 a Fig. SEM micrographs of polybenzoxazine-based aerogels at various microscope magnifications: (i) Cross-section images of polymeric aerogel without G: (a) and (b). (ii) SLTB(HBA-t0)/G-% aerogel: (c) and (d) cross-section, (e) and (f) lateral-section;(g) inset shows the fractured structure of the layer separating two aerogel pores. Accepted Manuscript

24 Page of It is clear that the pore structure of nanocomposite aerogels is different than that of the organic aerogel as the pore sizes in polymeric aerogel (Fig. b) appear larger than those of the G reinforced aerogel (image d). The structure of pores is dependent on the size of the ice crystals during the fabrication of the aerogels. During the freezing, as the ice crystals grow, micro-ribbons form on the walls of the aerogel., The small size of ice crystal growth leads to narrow pore, resulting in an increase in the surface area. In addition, the G reinforced aerogels exhibit a more layered structure than the polymeric aerogel as shown by SEM images of lateral sections (side view) of the % G reinforced aerogels (Fig. e, f, g). Furthermore, it is clearly seen that the surface of the layers exhibits increased roughness. In addition, the layers extend through the length of the aerogel and connected together via strands to create irregular pores. Fig. g illustrates the morphology of the fractured structure of edge between two aerogel pores at high magnification ( 0000 and scale of µm). The inner surface of the fracture is very different from that of the outer surface due to the presence of the nanofiller. It is interesting to note that the center area shows rough appearance with what appears as a hierarchical structure, while the G is distributed on the interior surface of the layer without any aggregation. Carbon aerogels possess large surface area, which have advantages in catalysis and adsorption applications as well as electrodes for supercapacitors. Morphological behavior of carbon aerogels has been extensively studied using SEM.,, Polybenzoxazine-based carbon aerogels and xerogels have been investigated., Herein, morphology of carbon neat polybenzoxazine and polybenzoxazine/g nanocomposites aerogels studied. Fig. contains the SEM micrographs of carbonized SLTB(HBA-t0)/G-% aerogel. Fig b reveals that the surface layers become coarser after carbonization. The high magnification SEM images (Fig. c, d, and e) show surface topology becomes irregularly wrinkled upon carbonization. Accepted Manuscript

25 Page of 0 Accepted Manuscript Fig. SEM micrographs of poly(sltb(hba-t0)/g-%) carbon aerogels at different microscope magnifications.

26 Page of 0 0 The possible contribution to these wrinkles may be the crumpling of graphene nanosheets that result from the reduced G and after the decomposition of soft segments of polybenzoxazine. Therefore, random distribution of G sheets can be observed (image f). The observations made with SEM are in good agreement with XRD results which show that G is highly exfoliated in the polymeric matrix.... Transmission Electron Microscopy (TEM) The structure and morphology of as-prepared polybenzoxazine/g aerogels were also investigated by means of transmission electron microscopy (TEM). TEM images of the neat poly(sltb(hba-t0)) are shown in Fig., while TEM images of polymerized and carbonized SLTB(HBA-t0)/G-% aerogels are shown in Fig. and respectively. a b 00 nm Fig. TEM (a) and HRTEM (b) images of poly(sltb(hba-t0)) aerogel; inset: SAED pattern (collected at area marked with red rectangle) reveals no crystalline structure. TEM analysis revealed the wide distribution of size and shape observed for the pores of the polybenzoxazine aerogel. Furthermore, the amorphous nature of the specimen was confirmed Accepted Manuscript

27 Page of 0 with Selected Area Electron Diffraction (SAED) and High Resolution TEM (HRTEM). Similar finds have been previously reported for neat polybenzoxazine-based porous carbons., EDS analysis showed minor presence of Cl impurities (>0.%). 0 a b Fig. TEM images of SLTB(HBA-t0)/G-% polymerized aerogel; (a) low magnification, (b) high magnification. Fig. shows TEM micrographs of SLTB(HBA-t0)/G-% polymerized aerogel. Concerning the polymerized SLTB(HBA-t0)/G-% aerogel, its morphology is similar to the polybenzoxazine aerogel, exhibiting a wide distribution of pore size and shape. Thin carbon layered structures with no aggregation of G were observed, as expected from previous work by Alhassan et al. 0 verall, the G sheets were found to be homogeneously dispersed into the polybenzoxazine amorphous structure. Similar finds are observed for the carbon aerogel containing wt % G, although in this case the ordering of the G sheets appeared to be higher; this was observed with HRTEM analysis at the edge of particles, where turbostratic and graphitic layers were observed (see Fig. b). Accepted Manuscript

28 Page of 0 a b Fig. TEM images of SLTB(HBA-t0)/G-% carbon aerogel: (a) low magnification, (b) high magnification; inset: SAED pattern (collected at area marked with red rectangle) reveals an exist crystalline structure due to presence of G. (c) (b) (a) ev Fig. 0 Background subtracted and deconvoluted EELS spectra at carbon K-edge for poly(sltb(hba-t0)) aerogel (a), SLTB(HBA-t0)/G-% polymerized aerogel (b) and SLTB(HBA-t0)/G-% carbon aerogel (c). Accepted Manuscript

29 Page of A strong indication of the increased G presence was provided with EELS analysis, shown in Fig. 0. Specifically, carbon hybridization was evaluated by acquiring the C K-edge for the three specimens mentioned above: poly(sltb(hba-t0)) aerogel, SLTB(HBAt0)/G-% polymerized aerogel and SLTB(HBA-t0)/G-% carbon aerogel. The characteristic peaks corresponding to sp hybridization (s π * at ev and s σ * at ev) are more prevalent in the carbonized and polymerized aerogels, due to G presence in the matrix. n the other hand, the polybenzoxazine aerogel exhibits significantly lower sp hybridization, which is mostly attributed to its amorphous nature. The overall of microstructure analysis can be summarized as the G sheets were found to be homogeneously distributed into the polybenzoxazine aerogels. In addition, G enhances the microstructure and reduces the pore size of polybenzoxazine aerogels, while the surface topology of G/polybenzoxazine aerogel becomes irregularly wrinkled upon carbonization due to the unique structure of G... Effect of G on polymerization behavior of benzoxazine monomer The influence of G content on the polymerization behavior of benzoxazine monomer was monitored and analyzed by DSC. As seen in Fig., the heat treatment of neat SLTB(HBAt0) and pristine G showed only one exothermic peak at 0 and 0 o C, respectively. The similarity of these two maxima is purely accidental. n the other hand, SLTB(HBA-t0)/G aerogels showed multiple overlapped or separated exothermic peaks with lower onset temperatures. This observation suggests that polymerization pathway of SLTB(HBA-t0)/G nanocomposites is different comparing with the neat SLTB(HBA-t0). This is hypothesized to be due to the catalytic effect caused by the surface species of G. Accepted Manuscript

30 Page of 0 0 Fig. DSC thermograms of SLTB(HBA-t0)/G aerogels with different G content. It has been reported that both phenolic and carboxylic groups exist on the G surface. Also, both phenolic and carboxylic acids are effective benzoxazine polymerization initiators and/or catalysts., As a result, increase in the G contents decreases the onset polymerization temperature of the SLTB(HBA-t0)/G aerogels. The recent report by Gu et al. demonstrated that G has a catalytic effect on the ring-opening polymerization of benzoxazine; however, their DSC investigation did not show de-oxygenation peak of G. n the other hand, the exothermic peak of G at 0 o C observed in this study is in agreement with reported results. 0, Zhao et al. found that de-oxygenation of G has a large single exothermic peak at 00 o C. Furthermore, Alhassan et al. 0 reported two exothermic peaks for benzoxazine/g nanocomposites. It is noteworthy to mention that presence of G in all cases has catalytic effect on the ring-opening polymerization of the benzoxazine monomer. The results in Fig. reveal that the maximum temperature of the polymerization exotherm on DSC and the area under the Accepted Manuscript

31 Page 0 of peak are significantly shifted to lower values. As an example, wt% G the onset temperature and maximum temperature of neat SLTB(HBA-t0) decreased from and 0 o C to and o C, respectively, indicating that G has significant catalytic activity on benzoxazine polymerization. DSC results also show that addition of G to benzoxazine matrix has catalytic effect on reducing the polymerization enthalpies of aldehyde-terminal benzoxazine monomer. eat aldehyde-terminal benzoxazine and G show exotherm with enthalpies of and 0 J/g, respectively. Accordingly, for instant, theoretical values of enthalpies for benzoxazine polymerization of samples containing % and % of G should read and J/g, respectively. However, the DSC results indicated that the polymerization enthalpies of same nanocomposite samples are and J/g, respectively... Thermal stability of G reinforced aerogels Polybenzoxazine-based porous materials including aerogels exhibit anomalous thermal stability.,, Lorjai et al. studied the thermal decomposition of neat polybenzoxazine in comparison to the corresponding aerogel. Polybenzoxazine aerogel exhibited a much higher degradation temperature and char yield than the corresponding bulk polybenzoxazine. Thermal stability of the obtained aerogels after polymerization was evaluated by TGA and DTG. TGA profiles of pristine G, poly(sltb(hba-t0)) aerogel, and poly(sltb(hba-t0)/g) aerogels are shown in Fig. and the results are summarized in Table. Accepted Manuscript 0

32 Page of 0 Resedual weight, % Pristine G 00 SLTB(HBA-t0)/G-0% SLTB(HBA-t0)/G-% 0 SLTB(HBA-t0)/G-% SLTB(HBA-t0)/G-0% Temperature, o C Fig. TGA thermograms of aerogels obtained with different G content. Table Summary of TGA data of the aerogels with different G content. Temperature of Decomposition Sample code T max ( o C) Td ( o C) Td 0 ( o C) T 0 ( o C) T 0 ( o C) Char Yield (%) eat G 0 0 ± 0 wt% G 0 ± wt% G 0 ± wt% G ± 0 wt% G 0 ± The results indicate that the poly(sltb(hba-t0)/g) has weight loss at lower temperature than the neat poly(sltb(hba-t0)) due to the low temperature weight loss of G; however, the char yield is higher with G because of the higher char yield of G than Drivative weight loss, %/ o C Accepted Manuscript

33 Page of polybenzoxazine. TGA results of G show two regions of weight loss. The first weight loss occurred below 00 o C, which is due to the evaporation of the adsorbed moisture. The second weight loss is ascribed to the decomposition of functional groups. The results in Table indicated that the maximum degradation temperature of neat G is o C, which is much lower than that for the neat polybenzoxazine. The lower degradation temperature is attributed to the low thermal stability of soft segment of the functional groups of G. This investigation is in agreement with literature. The previous studies reported that the thermal decomposition of instable oxygen-containing functional groups occurring at a temperature of 00 o C. The TGA results showed that stability of G is shifted to about 00 o C after incorporation with benzoxazine, which ascribed to the strong interaction of benzoxazine structure with the functional groups of G as early discussed. As seen in Table, the maximum rate degradation temperature (T max ) of G is o C. However, the, 0, 0 and 0 % weight loss temperatures (T d, T d0, T d0, and T d0 ) of poly(sltb(hba-t0)/g) aerogels increased with G-loading. For example, T d and T d0 for poly(sltb(hba-t0)/g-%) are 0 and o C, respectively, whereas T d and T d0 for poly(sltb(hba-t0)/g-0%) are and o C. The improved properties of polymerized nanocomposite aerogels may be due to the strong interactions with G as discussed earlier. Additionally, the G reinforced aerogels exhibited much lower degradation behavior at higher temperatures (>00 o C) than the neat polybenzoxazine aerogel, resulting in increased char yields, which is important property for preparing carbon aerogels. As seen in Table, the char yield at 00 o C of polybenzoxazine aerogel increased from to % by loading 0 wt% G. The char yield of G is high since it is mostly carbon. Thus, increased the char yield of the obtained samples is expected. Therefore, polybenzoxazine/g aerogels not only exhibit improved Accepted Manuscript

34 Page of 0 0 morphology than the neat polybenzoxazine aerogel, but also show excellent precursor for carbon aerogels. The char yield of benzoxazine aerogels increased significantly with G content, primarily due to their strong interaction. To show the synergistic effect of G-loading on the char yield of polybenzoxazine aerogels, the theoretical char yield for each nanocomposite aerogels was calculated. The theoretical char yield was obtained based on the composition of the sample and the actual char yield for neat both G and polybenzoxazine, poly(sltb(hba- t0)). The theoretical char yield value was calculated according to the Eq. (): %= %+ % () Where: M total is the theoretical value of char yield. M PBZ is the char yield during TGA run of neat polybenzoxazine, PBZ. X PBZ is the mass fraction of polybenzoxazine in the sample. M G is the char yield during TGA run of neat G. Char yield, % Synergistic effect Experimental Theoretical Accepted Manuscript 0 0 Graphene oxide content, % Fig. Theoretical and measured char yield for polybenzoxazine aerogels with different G content.

35 Page of In order to investigate whether interactions existed between aldehyde-functional benzoxazine and G, the theoretical and experimental values of char yield was compared. Fig. displays the expected and experimental char yields of the nanocomposite aerogels. The char yields of the nanocomposites are higher than the theoretical prediction, which is attributed to the synergism between polybenzoxazine and G due to their strong interactions. To summarize the TGA results, these nanocomposite aerogels derived from G-reinforced polybenzoxazine exhibit more efficient for carbon aerogels than aerogels obtained from neat polybenzoxazine due to the increase in the char yield with increasing the G content. Furthermore, the PBZ aerogels are mechanically stable due to the unique feature of PBZ for cross-linking ability. In addition, presence of aldehyde groups enhances PBZ/G interactions as investigated earlier. As a result of the network structure, the cross-linking contributes stiffness to the aerogels. These advantages make PBZ/G hybrid aerogels as one of the strong candidates of aerogels for many applications.. CCLUSIS ovel polybenzoxazine nanocomposites with three-dimensional (D) porous materials have been successfully prepared by blending of star-like telechelic aldehyde-terminal-benzoxazine (SLTB(HBA-t0)) with graphene oxide (G) nanosheets. The D porous solids have been developed via freeze-drying of various colloidal dispersion of SLTB(HBA-t0)/G. Physicochemical interactions between aldehyde-terminal benzoxazine and G interphases matrices led to hybridization. These interactions between functional groups of both SLTB(HBA-t0) and G are important in the formation of networks structure. FT-IR results indicated that during the heat treatment of SLTB(HBA-t0)/G aerogels, the aldehyde groups Accepted Manuscript

36 Page of possibly oxidases and form carboxyl moieties. Therefore, ester formation results from chemical interaction SLTB(HBA-t0) and G via H and -CH moieties. Furthermore, a strong hydrogen bonding is occurred between aldehyde-terminal benzoxazine and G. The unique combination of intramolecular and intermolecular hydrogen bonding contributes to the properties of polybenzoxazine/g aerogels. The XRD investigations indicated that G is exfoliated in benzoxazine matrix up to wt%, while slight residual G aggregation was observed with 0 wt% G. Morphological studies show that the aerogels exhibit porous nanostructure features. The presence of G in polybenzoxazine aerogels plays a significant role in enhancing the morphological properties. The SEM images reveal that G increases the layered structure of the aerogels and surface roughness. SEM micrographs of carbon aerogels show wrinkled and disordered G-like sheets. The TEM results indicate good dispersion of G in the polybenzoxazine matrix and pores have been observed in the nanometer scale. The DSC results indicated that addition of G decreases the polymerization temperature of the SLTB(HBAt0). Compared to the neat polybenzoxazine aerogel, the nanocomposite aerogels show better thermal stability. The 0% weight loss temperature (T d0 ) for aerogel and composite aerogels containing,, and 0 wt% G are,, and 0 o C, respectively, while T d0 for neat polybenzoxazine aerogel is o C. In addition, the char yield increased non-linearly with increasing G content, showing synergism. Hybridization of benzoxazine aerogels with G as highly porous structure of nanofiller provides special thermal and morphological properties of a new class of nanocomposite aerogels, which are expected to find various applications, such as in adsorption, catalytic, and conductive applications. Accepted Manuscript

37 Page of Supporting Information See supporting information (SI) for experimental details of synthesis and characterizations of G and SLTB(HBA-t0), additional FT-IR spectra of SLTB(HBA-t0)/G-% hybrid aerogel. This material is available free of charge via the Internet at ACKWLEDGMETS The Ministry of Education of Libya is acknowledged for the financial support in the form of a national scholarship to Almahdi Alhwaige. The authors thank Prof. Dr. Schiraldi at Macromolecular Science and Engineering, Case Western Reserve University for his kind support. REFRECES [] S. S. Kistler, Phys. Chem., (), -. [] C. Tan, B. M. Fung, J. K. ewman, and C. Vu, Adv. Mater. 00,, -. [] J. R. Johnson III, J. Spikowski, and D. A. Schiraldi, ACS Appl. Mater. Interfaces 00,, 0-0. [] S. M. Alhassan, S. Qutubuddin, and D. Schiraldi, Langmuir 00,, -0. [] A. I. Cooper and A. B. Holmes, Adv. Mater.,, 0-. [] P. Lorjai, S. Wongkasemjit, T. Chaisuwan, and A. M. Jamieson, Polym. Degrad. Stab. 0,, 0-. [] H. Ishida, in Handbook of Benzoxazine Resins, H. Ishida and T. Agag, Eds., Elsevier, Amsterdam 0, -. [] T. Takeichi, R. Zeidam, and T. Agag, Polymer 00,, -. Accepted Manuscript

38 Page of [] B. Kiskan,.. Ghosh, and Y. Yagci, Polym. Int. 0, 0, -. [0].. Ghosh, B. Kiskan, and Y. Yagci, Prog. Polym. Sci. 00,, -. [] Y. H. Wang, C. M. Chang, and Y. L. Liu, Polymer 0,, 0-. [] F. W. Holly and A. C. Cope, J. Am. Chem. Soc.,, -. [] X. ing and H. Ishida, J. Polym. Sci. Part A: Polym. Chem.,, -. [] A. Chernykh, J. Liu, and H. Ishida, Polymer 00,, -. [] P. Lorjai, T. Chaisuwan, and S. Wongkasemjit, J. Sol-Gel Sci. Technol. 00,, -. [] P. Katanyoota, T. Chaisuwan, A. Wongchaisuwat, and S. Wongkasemjit, Mater. Sci. Eng. B, 00,, -. [] U. Thubsuang, H. Ishida, S. Wongkasemjit, and T. Chaisuwan, Microporous and Mesoporous Mater., 0,, -. [] T. Chaisuwan, T. Komalwanich, S. Luangsukrerk, and S. Wongkasemjit, Desalination, 00,, 0-. [] K. Pakkethati, A. Boonmalert, T. Chaisuwan, and S. Wongkasemjit, Desalination 0,, -. [0] D. A. Rubenstein, H. B. Lu, S. S. Mahadik, and W. J. Biomater. Sci., Polym. Ed. 0,, -. [] Q. C. Ran, Q. Tian, C. Li, and Y. Gu, Polym. Adv. Technol. 00,, 0-. []. Bitinis, M. Hernandez, R. Verdejo, J. M. Kenny, and M. A. M. Lopez, Adv. Mater. 0,, -. [] S. R. Hostler, A. R. Abramson, M. D. Gawryla, S. A. Bandi, and D. A. Schiraldi, Int. J. Heat Mass Transfer 00,, -. [] X. Fu and S. Qutubuddin, Polymer 00,, 0-. Accepted Manuscript