Polysaccharide Nanocomposites Reinforced with Graphene...

Polysaccharide Nanocomposites Reinforced with Graphene Oxideand Keratin-Grafted Graphene OxideClaramaría Rodríguez-Gonzaĺez,†,‡ Ana L. Martínez-Hernańdez,‡,§ Víctor M. Castaño,‡

Oxana V. Kharissova,*,† Rodney S. Ruoff,∇ and Carlos Velasco-Santos‡,§

†Universidad Autońoma de Nuevo Leoń, A.P. 101-F, Ciudad Universitaria, San Nicolaś de los Garza, Nuevo Leoń 66450, Mex́ico‡Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autońoma de Mex́ico, Boulevard Juriquilla 3001, Juriquilla,Queret́aro 76230, Mex́ico

§Divisioń de Estudios de Posgrado e Investigacioń, Instituto Tecnoloǵico de Queret́aro, Av. Tecnoloǵico S/N, Colonia CentroHistoŕico, Santiago de Queret́aro, Queret́aro 76000, Mex́ico

∇Department of Mechanical Engineering and the Texas Materials Institute, The University of Texas at Austin, 204 E. Dean KeetonSt., Austin, Texas 78712, United States

ABSTRACT: Nanocomposites of polysaccharide matrices, chitosan−starch, and carboxymethyl cellulose-starch reinforced withgraphene oxide and graphene grafted with keratin were developed. Composites films had been prepared for the casting/solventevaporation method. The interaction and distribution of graphene materials in the biopolymer matrices were analyzed by Fouriertransform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM), and thethermomechanical properties were examined using dynamic mechanical analysis. The nanocomposites of the chitosan−starchmatrix improved their mechanical properties substantially, with respect to the film without reinforcing, obtaining an increaseof 929% in the storage modulus (E′, 35 °C) with only 0.5 wt % of graphene oxide and outstanding increments in E′ at 150 and200 °C when keratin-grafted graphene oxide is incorporated (0.1 wt %). In contrast, the graphene oxide incorporated into thecarboxymethyl cellulose−starch matrix tends to decrease the stiffness of the film, behaving in a manner opposite to that ofnanocomposites of the chitosan−starch matrix. Similarly, the incorporation of graphene grafted with keratin shows a decrease inthe rigidity of the resulting material. In this way, the importance of compatibility between the graphene and the host matrix toachieve a fine control of interface and manipulate the final properties of the material is demonstrated.

1. INTRODUCTIONDuring the last two decades, significant efforts have been madetoward diminishing the impact of man on the environment.Following this tendency, the attempt has been made to replacepolymers derived from petroleum with biodegradable ones. Inthis sense, the biopolymers seem like a possible alternative tothis effort; nevertheless, some of their properties must beimproved to position them as materials that can be competitivewith the fossil derivatives. The technology of nanocomposites isan elegant answer to this problem. The nanocomposites consistof a polymeric matrix or continuous phase and a discontinuousphase or filler, where at least one dimension is

can be widely exploited to improve the interface between thegraphene and the polymer matrices. Chitosan (Ch), starch (S),and carboxymethyl cellulose (CMC) are natural polymers usedin this research that have hydroxyl and amine groups (onlychitosan) that could be useful to link the chemical moieties ofmodified graphene with natural polymer matrices. The chitosanis a polysaccharide obtained via partial deacetylation of thechitin.17 Its chemical structure is a random linear chain ofN-acetyl-D-glucosamine units (acetylated unit) and D-glucosamine(deacetylated unit) linked by β (1→4) linkages.3 It is nontoxic,biodegradable, biocompatible, and resistant to the microbialgrowth, and it displays good film-forming properties.18 The starchis a biopolymer that is produced by the plants in grain form,serving as reservoirs of energy. Extracted mainly from cereals andtubers, starch is composed of two α-D-glucopyranose homo-polymers: amylose and amylopectin.3 It is a polysaccharide that isinexpensive and biodegradable, and it has excellent gas-barrierproperties. The carboxymethyl cellulose is an anionic polymerderived from the cellulose.19 This biopolymer has diverseinteresting properties, including biocompatibility, nontoxicity,and biodegradability. The properties of the above-mentionedbiopolymers permit them to have important applications in thefields of medicine, water treatment, packaging, and pharmaceutics,as well as in the food industry. Despite these properties andadvantages, the starch films exhibit high sensitivity to moistureand poor mechanical properties, such fragility and rigidity.20,21 Toimprove the drawbacks, the blending of starch with variousbiopolymers has been investigated. Special attention has beengiven to chitosan, because its unique properties favor theformation of films with a high modulus, flexibility, and lowerwater permeability, as well as antimicrobial and antifiungalattributes.22,23 Such films can be used to develop biodegradablefilms for packaging and biomedical materials.23,24 The good inter-action between these biopolymers has been amply studied and isalso known.25−27 On the other hand, carboxymethyl celluloseexhibits excellent film-forming properties, and their chemicalsimilarity to the starch favor good compatibility between theirimproving mechanical properties and moisture resistance; despitethis fact, reports on CMC that has been modified with starch toform biocomposite films are scarcely found.28 Thus, thesepolymers could be find application in order to form novelecofriendly polymers; however, the mechanical properties of thesefilms still must be improved to make them suitable for possibleapplications, and nanofillers in this type of polymer play animportant role.In this paper, we report the synthesis of chitosan/starch

(Ch/S) and carboxymethyl cellulose/starch (CMC/S) nano-composites reinforced with GO and graphene oxide graftedwith keratin (GKGO). Thus, interesting effects of graphenematerials are observed, depending on the type of polymermatrix and the interfacial interactions, which are directlyrelated to the compatibility between polymers and nanoma-terials.

2. EXPERIMENTAL SECTION2.1. Materials. Chitosan powder from crab shells (with the

degree of deacetylation being ≥75%), salt of Na-CMC (with anaverage molecular weight of 250 000 g/mol), and rice starchwere supplied by Sigma−Aldrich. Sorbitol in aqueous solution(70 wt %) was purchased from Golden Bell Reagents. Graphitewas obtained from Electron Microscopy Sciences in the form ofbars, pulverized, and sieved to 300 mesh. Other reagents wereof analytic grade and were used as received.

2.1.1. Preparation of Graphene Oxide. Graphite oxide(GO) was prepared from graphite via the modified Hummersmethod, according to the literature reported by Stankovichet al.29 Briefly, 46 mL of cool (0 °C) concentrated sulfuric acid(H2SO4) placed into a round flask was mixed with 2 g ofgraphite powder. The suspension was stirred and maintained inan ice bath, followed by the slow addition of 6 g of KMnO4 tokeep the reaction temperature at

The Ch/S/GO mixture then was poured into expandiblepolystyrene plates. The film was dried, up to room temperature,during 120 h. The dry films were washed first with 1 Maqueous NaOH for 30 min to eliminate the remaining aceticacid and then with distilled water for an additional 30 min.After drying the films at room temperature for 360 h, they werepeeled from the plates and stored for later testing. The filmsthat contain graphene oxide grafted with keratin (GKGO1 andGKGO2) as nanofiller were prepared using the same methodand the same filler loading levels (0.1 and 0.5 wt %). Details ofthe formulations of the films and abbreviations used are shownin Table 1.

2.1.4. Preparation of Carboxymethyl Cellulose−Starch−Graphene Oxide and Carboxymethyl Cellulose−Starch−Graphene Oxide Grafted with Keratin Films. A Na-CMCsalt and starch were poured into a flask with 40 mL of distilledwater, and the mixture was heated at 90 °C for 5 min withvigorous stirring. The films contain a 70:30 carboxymethylcellulose/starch ratio. Previous tests allowed us to determinethis ratio to be the better ratio to use to obtain a uniform film.During the heating process, 0.5 mL of sorbitol (as a plastifier)was added. GO sheets were dispersed in 10 mL of distilledwater with a ultrasonic bath for 3 h. The GO filler loading levelswere 0.1 and 0.5 wt %. The CMC/S mixture was allowed tocool to room temperature under agitation. The dispersion ofGO sheets then was added to the cold CMC/S mixture anddispersed with constant stirring. The CMC/S/GO mixture waspoured in expandable polystyrene plates and dried to roomtemperature for 360 h. After drying, the films were peeled fromthe plates and stored for later testing. The films that containgraphene oxide grafted with keratin (GKGO1 and GKGO2) asnanofiller were prepared with the same method and with thesame filler loading level (0.1 and 0.5 wt %). Details of theformulations of the films and abbreviations used are shown inTable 2.2.2. Methods. Fourier transform infrared (FTIR) spectra of

the nanocomposite films in the attenuated total reflection(ATR) mode were obtained using a spectrometer (BrukerVector 33, model alpha-p) in the wavelength range of 4000−400 cm−1, with a resolution of 1 cm−1. The Raman spectra ofthe samples were measured on a Bruker dispersive RamanSenterra model. The laser wavelength was 785 nm, with apower of 100 mW. For each spectrum, an average of six scanswere performed with an integration time of 10 s over a range of80−3200 cm−1.

The morphology of the nanocomposites was examined usingscanning electron microscopy (SEM) (JEOL, Model JSM-6060LV) system under an accelerating voltage of 20 kV. In order toobtain samples for SEM observations, all the films werefractured on a universal tensile testing machine. The uniquepurpose of this procedure is to obtain a surface subject to strain.The fractured surfaces were coated with gold, using a sputtercoater (Electron Microscopy Sciences, Model EMS550), andthen were analyzed.Dynamic mechanical analysis (DMA) of the samples (cut in

rectangular shapes with dimensions of 20 mm in length and8 mm in width) were carried out with a TA Instruments ModelDMA Q800 system, using tension clamps. The thickness of thefilms was measured with a micrometer (Starrett No. 436-1 in.).Three thickness values were obtained along the length of thefilmstrip, and the mean values were 0.13 mm for the Ch/Snanocomposites and 0.15 mm for the CMC/S nanocomposites.The samples were tested over temperature ranges of 30−270 °C for Ch/S films and 30−220 °C for CMC/S films.A heating rate of 3 °C/min, with a constant frequency of 1 Hz,was applied. All treatments were made in duplicate.

3. RESULTS AND DISCUSSIONThe FTIR spectra of graphene oxide, neat Ch/S film, and thenanocomposite films with GO, GKGO1, and GKGO2, at 0.1and 0.5 wt %, are shown in Figures 1a and 1b, respectively.Characteristic infrared bands found in the neat Ch/S spectrumand the band shifts found in the nanocomposites spectrum, whichis referenced as the Ch/S spectrum, are shown in Table 3.Table 3 also shows the signals that confirm important shifts

in the nanocomposite spectra, taking, as reference, the Ch/Sfilm and GO spectra. It is possible to observe that the bandsthat are involved in these changes are related with ν(O−H),ν(CO), ν(N−H), ν(C−N), and ν(C−O). The shifts of themajority of these bands are associated with hydrogen bonds,because of the interaction between GO and the Ch/S polymerand might be considered as an evidence of the participation ofthese functionalities in the formation of interfacial interactionsbetween graphene and a polymeric matrix.34 Similar shifts havebeen found in starch−graphene oxide composites, indicatinginterfacial interactions.36 However, the peaks associated withCO and C−N vibration are more susceptible to shift in thecomposites than N−H and C−O. In addition, hydrogen bondsare also corroborated by the shifts in O−H vibrations, which

Table 1. Nomenclature and Description of Chitosan/Starch-Graphene Oxide Films

sampleidentification components

Ch/S chitosan and starchCh/S/GO1 chitosan and starch reinforced with graphene oxide at

0.1 wt %Ch/S/GO5 chitosan and starch reinforced with graphene oxide at

0.5 wt %Ch/S/GKGO11 chitosan and starch reinforced with graphene oxide

grafted with keratin condition 1 at 0.1 wt %Ch/S/GKGO15 chitosan and starch reinforced with graphene oxide



grafted with keratin condition 2 at 0.5 wt %

Table 2. Nomenclature and Description of CarboxymethylCellulose−Starch−Graphene Oxide Films

sample identification components

CMC/S carboxymethyl cellulose and starchCMC/S/GO1 carboxymethyl cellulose and starch reinforced with

graphene oxide at 0.1 wt %CMC/S/GO5 carboxymethyl cellulose and starch reinforced with

graphene oxide at 0.5 wt %CMC/S/GKGO11 carboxymethyl cellulose and starch reinforced with

graphene oxide grafted with keratin condition 1 at0.1 wt %

CMC/S/GKGO15 carboxymethyl cellulose and starch reinforced withgraphene oxide grafted with keratin condition 1 at0.5 wt %



Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie200742x | Ind. Eng. Chem. Res. 2012, 51, 3619−36293621

are present in all nanocomposite spectra. These facts indicatethat interactions between nanofillers and the polymer matrixare produced in both GO and grafted GO, showing that linksthat involve CO with O−H groups are more influenced thanN−H hydrogen bonds links. Taking these results into account,and because of the nature of keratin, it is supposed thannanocomposites with GO could produce more links with thematrix than keratin-grafted GO. The FTIR spectra of filmsreinforced at 0.5 wt % (Figure 1b) exhibit a behavior similar tothat of spectrum films filled at 0.1 wt %.The FTIR spectra of graphene oxide, neat CMC/S film, and

nanocomposite films at 0.1 and 0.5 wt % are shown in Figures 2aand 2b, respectively. Characteristic infrared bands found in theneat CMC/S spectrum and the band shifts found in the nano-composite spectrum, taking the CMC/S spectrum as a reference,are shown in Table 4. The remarks of infrared bands of GO werepreviously given in the table of the C/S films.Based on the band positions displayed in Table 4, it is

possible to observe only notable changes for the nano-composites in the peaks associated with ν(CO), situated at1366 cm−1 for the CMC/S spectrum, and the bands relatedwith ν(O−H). This indicates that possible interactions related

with hydrogen bonds also could be found in this type ofnanocomposite. However, notably, these spectra show muchless evidence of interfacial links than the Ch/S nanocomposites.The FTIR spectrum of nanocomposites films reinforced at0.5 wt % (Figure 2b) shows similar positions, compared withthe bands in films reinforced at 0.1 wt %.The Raman spectra of neat Ch/S film and nanocomposite

films at 0.1 and 0.5 wt % are presented in Figures 3a and 3b,respectively. Raman bands in neat Ch/S film and nano-composites spectra are shown in Table 5.In Table 5, it is possible to observe the bands in the nano-

composite spectra related with notable shifts, using the Ch/Sspectrum as a reference. The groups involved in these changescorrespond to ν(C−O−C) and δ(C−O−H) for the nano-composites reinforced with GO, and δ(C−OH) and chitosanskeleton for the nanocomposites containing grafted GO. Thiscorroborated the IR results, and the stronger influence ofhydrogen bonds in the interface of the nanocomposites; also, itis again evident that groups related with C−O vibration showdifferent links when keratin is present or not in GO. This factcould be influenced by the fact that the CO and O−H

Figure 1. FTIR spectra of neat chitosan/starch film (Ch/S), graphene oxide (GO) powder, and nanocomposite films (Ch/S/GO, Ch/S/GKGO1,and Ch/S/GKGO2) at (a) 0.1 wt % and (b) 0.5 wt %.

Table 3. Overview of Infrared Bands of Graphene Oxide, Neat Chitosan/Starch Film, and Ch/S/GO, Ch/S/GKGO11 andCh/S/GKGO21 Nanocomposites

Band Position [cm−1]

Ch/S Ch/S/GO Ch/S/GKGO11 Ch/S/GKGO21band

assignment remarks ref(s)

3354 3514 3514 3528 ν (O−H) assigned to the vibrations of this group linked to the saccharide chain of starch 322922 ν (C−H) associated with the stretching vibration of the ring methine hydrogen atoms 181647 1670 1670 1667 ν (CO) attributed to the amide group of the portion acetylated of chitosan 33, 341588 1592 1592 1592 ν (NH3

+) assigned to interaction of primary amide groups of chitosan and hydroxylgroups of starch

18

1374 1316 1311 1341 ν (C−N) attributed to the group amide III of chitosan 181014 1010 1010 1010 ν (C−O) associated with the stretching vibration of C−O in C−O−C groups in starch 32band position for graphene oxide [cm−1] band assignment remarks ref

3382−3169 ν(O−H) ascribed to the vibration of this group linked to basal planes of graphene 351700 ν(CO) corresponding to vibration of carboxyl groups 35

1624−1585 ν(CC) corresponding to the remaining sp2 character 351400 ν(C−O) assigned to the Carboxy groups 351228 ν(C−O) associated to the Epoxy groups 351059 ν(C−O) assigned to the Alkoxy groups 35



groups in GO are occupied by the grafted keratin. Changesfound in nanocomposites at 0.5 wt % show the same tendency.Raman spectroscopy of neat CMC/S film and nano-

composite films at 0.1 and 0.5 wt % are presented in Figures 4aand 4b, respectively. Characteristic Raman bands found in neatCMC/S film and nanocomposite spectra are shown in Table 6 inorder to observe possible changes.Contrary to the Ch/S nanocomposite spectra, where O−H

band shifts are observed, in the CMC/S nanocomposite

spectra, only light shifts are found, using the CMC/S spectrumas a reference; the bands involved in these changes are relatedto the backbone of cellulose and the glycosidic linkage. Thisindicates that GO groups could be produce some interactionsin polymer films; however, it is evident that the shifts found inCh/S film are stronger than those in CMC/S materials.The fracture surface morphology of the nanocomposite Ch/S

films was investigated via SEM, and the micrographs aredisplayed in Figure 5 at two magnifications, in order to show

Table 4. Overview of Infrared Bands of Graphene Oxide, Pristine Carboxymethyl Cellulose/Starch Film, and CMC/S/GO,CMC/S/GKGO11 and CMC/S/GKGO21 Nanocomposites


CMC/S CMC/S/GO CMC/S/GKGO11 CMC/S/GKGO21band


3283 3293 3296 3296 ν (O−H) assigned to the vibrations of this group linked to the saccharide chainof starch and carboxymethyl cellulose

19, 37

2920 ν (C−H) associated with the stretching vibration of the ring methine hydrogenatoms

18

1592 ν (CO) attributed to asymmetric stretching vibration of this group inCOO−Na+

33

1366 1410 1410 1413 ν (CO) attributed to symmetric stretching vibration of this group inCOO−Na+

33

1316 1321 ν (CO) associated with the stretching vibration in COO−1 381014 ν (C−O) associated with the stretching vibration of C−O in C−O−C groups in

starch32

Figure 3. Raman spectra of neat Ch/S film and nanocomposite films (Ch/S/GO, Ch/S/GKGO1, and Ch/S/GKGO2) at (a) 0.1 wt % and(b) 0.5 wt %.

Figure 2. FTIR spectra of neat carboxymethyl cellulose/starch film (CMC/S), graphene oxide (GO) powder and nanocomposites films (CMC/S/GO, CMC/S/GKGO1 and CMC/S/GKGO2) at (a) 0.1 wt % and (b) 0.5 wt %.



the differences between the nanocomposites and the polymer.The cross section of the pristine Ch/S film (Figure 5a(a′))

shows a homogeneous surface, without a separation of phasesbetween polysaccharides, which is an indication of the good

Figure 4. Raman spectra of neat CMC/S film and nanocomposites films (CMC/S/GO, CMC/S/GKGO1, and CMC/S/GKGO2) at (a) 0.1 wt %and (b) 0.5 wt %.

Table 5. Positions and Assignments of Characteristics Raman Bands of Pristine Ch/S Film and Nanocomposite FilmsReinforced with GO, GKGO1, and GKGO2 at 0.1 wt %


Ch/S Ch/S/GO Ch/S/GKGO11 Ch/S/GKGO21 band assignment(s) remarks ref(s)

362 365 365 ν (C−C−C) assigned at vibration of skeleton of chitosan 39−41480 ν (C−O−C) attributed to the symmetric vibration of the α-glucose ring; this

band is the dish most characteristic of starch in Raman spectra42, 43

940 936 ν (C−O−C) associated with the symmetric stretching vibration in amilosa α-1,4-glycosidic linkage

42, 43

1114 1110 ν (C−O), ν (C−O−C) corresponds to the symmetric vibration of these groups inglycosidic bond

40, 43

1320 1332 1332 D band assigned to this band in graphene; corresponds to the breathingmode of κ-point photons of A1g symmetry

44

1377 1456 1459 1459 δ (C−OH) attributed to the bending vibration of this group in starch 431600 1597 1592 G band attributed to this band in graphene; assigned to the first-order

scattering of the E2g phonon of sp2 C atoms

45

2627 2D band attributed to this band in graphene; attributed to the overtoneof the D line

44

2902 ν (C−H) assigned to the vibration of this group in starch and chitosan 40, 42

Table 6. Positions and Assignments of Characteristics Raman Bands of Neat CMC/S Film and Nanocomposite FilmsReinforced with GO, GKGO1, and GKGO2 at 0.1 wt %


CMC/S CMC/S/GO CMC/S/GKGO11 CMC/S/GKGO21band


359 ν (C−C−C) assigned to the vibration of the skeleton of both polymers 39480 ν (C−O−C) attributed to symmetric vibration of α-glucose ring; this band is the

dish most characteristic of starch in Raman spectra42, 43

916 921 ν (C−O−C) associated with vibration skeleton in CMC and glycosidic linkagesin starch

38, 43

1107 ν (C−C) assigned to symmetric vibration of glycosidic bonds in bothpolymers

40, 41

1320 1320 1323 D band assigned to this band in graphene; corresponds to the breathingmode of κ-point photons of A1g symmetry

44

1264 ν (C−O−C) attributed to the stretching vibration in carboxymethylatedpolymers

38

1328 δ (C−OH) assigned the bending vibration in starch 431412 1460 1460 1460 ν (C−C−C) associated with the vibration of the backbone of cellulose 40

1597 1600 1600 G band attributed to this band in graphene; assigned to the first-orderscattering of the E2g phonon of sp

2 C atoms45

2620 2620 2620 2D band attributed to this band in graphene; attributed at overtone of the Dline

44

2910 ν (C−H) assigned to the stretching vibration of this group in bothpolysaccharides

40, 43



interaction between two components.27 The cross section ofnanocomposite Ch/S/GO film at 0.1 wt % (Figure 5b(b′)) iscompletely different than that of the neat Ch/S film. The

fracture surface shows a rough structure, revealing a stronginterfacial adhesion and compatibility between the GO and thematrix. Similarly rough surfaces have been found on other

Figure 5. SEM images of fracture surfaces. Cross sections of (a-a′) neat Ch/S film, (b-b′) Ch/S/GO at 0.1 wt %, (c-c′) Ch/S/GKGO1 at 0.1 wt %,(d-d′) Ch/S/GKGO2 at 0.1 wt %, (e-e′) Ch/S/GO at 0.5 wt %, (f-f′) Ch/S/GKGO1 at 0.5 wt %, and (g-g′) Ch/S/GKGO2 at 0.5 wt %.



composites that contain graphene, indicating good adhesion.46

Also, in other recent studies, where chitosan is modified withgraphene oxide, a similar appearance of this type of com-posite has been observed, indicating that irregular surfaces areformed by the GO sheets covered by polymer.34,47 Note thatgraphene produce important changes in the morphology ofthe nanocomposite with very low loading levels of filler.Figures 5c(c′) and 5d(d′) show the fracture surface of Ch/S/GKGO1 and Ch/S/GKGO2 films at 0.1 wt %. The cross sectionof the films shows a rough structure and uniformity, indicating agood incorporation of keratin-grafted graphene oxide in thematrix. The film shows some microcracks, which are due to thefracture procedure.From the fracture surface of nanocomposite Ch/S/GO at

0.5 wt % (Figure 5e(e′)), it is possible to observe that this film

has a striated structure and many granulations, indicating adifferent behavior, with respect to the neat film. The Ch/S/GKGO1 and Ch/S/GKGO2 films at 0.5 wt % (Figures 5f(f′)and 5g(g′)) displayed a surface with essentially the samecharacteristics as the nanocomposite films at 0.1 wt %.The SEM micrographs of the fracture surface of nano-

composite CMC/S films are shown in Figure 6. The crosssection of pristine CMC/S film (Figure 6a(a′)) shows a roughand bulky surface, without a separation of phases betweenpolysaccharides, suggesting good integration between thecarboxymethyl cellulose and the starch. However, the majorityof the surfaces related with CMC/S/GO1, CMC/S/GO5, CMC/S/GKGO11, and CMC/S/GKGO15 films (Figures 6b(b′)−e(e′)) do not show structural differences, compared to the neat

Figure 6. SEM images of fracture surface. Cross sections of (a-a′) neat CMC/S film, (b-b′) CMC/S/GO at 0.1 wt %, (c-c′) C/S/GKGO1 at0.1 wt %, (d-d′) CMC/S/GO at 0.5 wt %, and (e-e′) C/S/GKGO1 at 0.5 wt %.



film, indicating pore interaction between the nanofillers and thematrix.Dynamic mechanical analysis (DMA) provides information

about the viscoelastic behavior in a range of temperatures.Figure 7 shows the storage modulus (E′) for neat Ch/S film

and nanocomposites at 0.1 wt %. It is possible observe that allthe films shows higher E′ values, compared to that of the neatCh/S film. When the matrix is reinforced with GO at 0.1 wt %,the storage modulus increases from 196 MPa to 767 MPa at35 °C. As the temperature increases, the nanocomposite filmdisplays a gradual decrease in storage modulus, showing behaviorsimilar to that of the Ch/S film. The Ch/S/GKGO11 and Ch/S/GKGO21 nanocomposites exhibit a completely differentbehavior. The storage modulus at 35 °C is 322 MPa for Ch/S/GKGO11 and 252 MPa for Ch/S/GKGO21, and the storagemodulus gradually increases and achieves maxima in E′ at 164and 135 °C, respectively. It is known that Ch/S film loses freewater at ∼100 °C,18 and there exist more interactions with freewater of hydroxyl groups than amine groups in this type ofpolymer.48 These later statements could explain the effect inDMA of E′ in the temperature range. Note that, at highertemperature, there is a loss of OH groups in Ch/S, belonging tofree water; thus, one can suppose that this fact allowsmore interaction between bound OH of the polymer and theamine groups in keratin. The suggestion is also supported bythe hydrophobic nature of keratin,12 since, although more freewater is eliminated, there exist the possibility to produce a linkbetween hydrophobic groups in keratin with polymer chains;therefore, an increase in the stiffness is reached. In addition,the assumption agrees with the IR and Raman results, wherenanocomposites with grafted GO show interactions in theamide groups and nanocomposites with GO show evidence ofinteractions in CO, O−H moieties. This fact also explainsthe high storage modulus of Ch/S/GO at 35 °C. Such behaviorindicated that GO and keratin-grafted graphene oxide inter-acted with the matrix in different ways. As previously mentioned,the different interactions between the nanofillers and the matrixproduced an important synergistic effect at different temperatures.Figure 8 display the storage modulus (E′) for neat Ch/S film

and nanocomposites at 0.5 wt %. It can be seen the behavior ofthe nanocomposite films is more uniform compared with thesamples reinforced at 0.1 wt %. In this case the storage modulusis substantially improvement with the increase in content ofGO nanofiller. When the matrix was reinforced with GO, the

storage modulus increased from 193 to 2033 MPa at 35 °C. Asthe temperature increases, the Ch/S/GO film displays a typicalgradual decrease in storage modulus. For the Ch/S/GKGO15and Ch/S/GKGO25 films the storage modulus at 35 °C is 796and 467 MPa, respectively. In contrast with their counterpartreinforced at 0.1 wt %, the storage modulus gradually decreasedat higher temperatures suggesting that the higher content ofgrafted nanofiller may modify the interfacial interactions due tothe hydrophobic nature of keratin. Dynamic storage modulushas been improved in a wide range of temperatures for all thenanocomposites, however, the Ch/S/GO film shows an importantimprovement respect to the neat film. This improvement in themechanical properties suggesting a good distribution of the GOand graphene oxide grafted with keratin in the matrix as well as agood interfacial contact which also agree with the interactionsfound by IR and Raman spectroscopy.The temperature dependencies of storage modulus for neat

CMC/S and nanocomposite films at 0.1 and 0.5 wt % aredisplayed in Figure 9. Clearly, the thermomechanical propertiesof these nanocomposite films are totally different, compared tothe Ch/S films. While, in Ch/S films, the adhesion of nano-fillers significantly improved the storage modulus in CMC/Sfilms, E′ decreased. When the CMC/S matrix was reinforcedwith GO at 0.1 wt % (Figure 9a), the stiffness decreased. Thestorage modulus decreased from 1255 to 784 MPa comparedwith neat CMC/S film and the same effect is observed with theCMC/S/GKGO11 and CMC/S/GKGO22 films. The storagemodulus at 35 °C for the CMC/S/GKGO11 film is 965 MPa,and for the CMC/S/GKGO21 film, it is 774 MPa. The storagemodulus is partially recovered with the increase in nanofillercontent. The nanocomposite CMC/S/GO film at 0.5 wt %(Figure 9b) displayed a storage modulus of 1126 MPa at atemperature of 35 °C. The CMC/S/GKGO15 and CMC/S/GKGO25 films have a slightly higher storage modulus,compared with CMC/S/GO film; however, at higher temper-atures, the films have less stiffness. These results indicated thatthe graphene oxide produces a different effect, leading to anincrease in E′ for the Ch/S film and a decrease in E′ for theCMC/S film, which also is in agreement with the light inter-actions found by IR and Raman and the insignificant morphologicalchanges found in SEM.This behavior also exhibits the importance of compatibility

between the graphene oxide and the polymer matrix tomanipulate the final properties of the material. Therefore,the storage modulus of the CMC/S and Ch/S polymer at35 °C with graphene oxide paper with 4 wt % of water is

Figure 7. Temperature versus dynamic storage modulus (E′) of neatCh/S film and the nanocomposite Ch/S/GO, Ch/S/GKGO11, Ch/S/GKGO21 films.

Figure 8. Temperature versus dynamic storage modulus (E′) of neatCh/S film and the nanocomposite Ch/S/GO, Ch/S/GKGO15, Ch/S/GKGO25 films.



considered (E′ ≈ 23 GPa (ref 49)). The calculated values ofnanocomposites, considering the rules of mixture, would be 210and 311 MPa for Ch/S composites reinforced with 0.1 and0.5 wt % of GO, respectively, and the experimental valuesobtained correspond to 767 and 2033 MPa, respectively. Incontrast, in the caswt %e of CMC/S composites, the calculatedvalues correspond with 1270 and 1350 MPa for nano-composites with 0.1 and 0.5 wt % GO, respectively, while theobtained values in DMA are 784 MPa for composite with 0.1wt % and 1119 MPa for composite with 0.5 wt %.

4. CONCLUSIONS

In summary, films of chitosan/starch (Ch/S) and carboxy-metyhl cellulose reinforced with GO (CMC/GO) andgraphene oxide grafted with keratin (GKGO) have beensuccessfully prepared via the casting/solvent evaporationmethod. According to the results from the FTIR analysis, allthe Ch/S nanocomposites undergo significant shifts in thebands related with some functional groups (such as OH, CO,and C−N). However, in other functional groups (such as N−Hand C−O), only light shifts are found. These facts indicate thatCO···OH links could predominant in comparison with N−H···OH bonds. Therefore, a higher interaction with the hanginggroups of GO by hydrogen bonds could be produced.Meanwhile, in the GKGO, the interaction with the matrixcould be produced by both CO and N−H; however, becauseof the nature of proteins, N−H bonds could be predominant.This fact suggests possible stronger interactions in GOcomposites than in the GKGO composites.In Raman spectroscopy, the groups involve in changes

correspond to ν(C−O−C) and δ(C−O−H) in the nano-composites reinforced with GO, and δ(C−OH) and chitosanskeleton in nanocomposites containing grafted GO. Thiscorroborated the IR results, as well as the stronger influenceof hydrogen bonds in the interface of the nanocomposites.The dynamic materials analysis DMA and SEM results

corroborated the links found in the IR and Raman spectros-copies. Composites with GO presents high storage moduli andthe composites with GKGO increase their storage moduluswhen free water is evaporated. SEM analysis of samplesconfirmed that the fillers in the Ch/S nanocomposites have agood interaction with the matrix for the capacity they have tochange the roughness of the polymer. Further, SEM analysis in

CMC/S nanocomposites shows not change in morphology ofpolymer.The results presented in this work demonstrate the importance

of the compatibility between the nanofillers and the polymericmatrix to manipulate the final properties of a material.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +52 81 83 29 40 30, ext. 6167. Fax: +52 81 83 52 29 44.E-mail address: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are grateful Dr. Genoveva Hernańdez Padroń for herassistance with the FT-IR and Raman spectroscopy. We alsothank Miss Alicia del Real for her assistance with the SEMmicroscopy.

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Figure 9. (a) Temperature versus storage modulus for neat CMC/S and nanocomposite CMC/S/GO1, CMC/S/GKGO11, and CMC/S/GKGO21films. (b) Temperature versus storage modulus of neat CMC/S film and nanocomposites films at 0.5 wt %.



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