Immobilized Graphene Oxide Nanosheets as Thin ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

Immobilized Graphene Oxide Nanosheets as Thin but Strong Nano-Interfaces in Biocomposites Huan Xu, Lan Xie, Duo Wu, and Minna Hakkarainen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01703 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Immobilized Graphene Oxide Nanosheets as Thin but Strong Nano-Interfaces in Biocomposites

Huan Xu,†,§ Lan Xie,‡,§ Duo Wu,† and Minna Hakkarainen*,†

†

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology,

Stockholm 100 44, Sweden

‡

College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China

§

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials

Engineering, Sichuan University, Chengdu 610065, China

*Corresponding Author: [email protected] (M.H.)

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 52

ABSTRACT:

Graphene oxide (GO) nanosheets featuring high surface activity and large planar dimension may function as robust nano-interfaces in biocomposites, contributing to simultaneous promotion of mechanical and gas barrier properties. Here, a solution-processed, additive-free approach to immobilize few-layer GO nanosheets on starch granule surfaces (GO@starch) by hydrogen bonding is demonstrated. This approach enabled a straightforward pathway to remove the intersheet van der Waals forces (π−π stacking) that generally cause reaggregation and poor dispersion of GO in polymer matrices. Incorporation of GO@starch into poly(lactic acid) (PLA) allowed an interesting structure with few-layer nanosheets firmly immobilized at the PLA-starch interfaces. Inheriting the high aspect ratio and surface energy of GO, GO@starch distinctly strengthened the interfacial interactions with PLA, albeit present at ultralow GO concentrations (up to 0.03 wt %), facilitating the dispersion of GO@starch and nucleation of PLA. The morphological regulation rendered composite films with an impressive combination of high thermal stability, mechanical strength and oxygen resistance. A substantial increase of 280% in tensile strength (58.2 MPa) and a prominent decline of 82% in oxygen permeation coefficient (4.0 cm3 mm cm–2 day–1 atm–1) were achieved in the composites loaded with 30 wt % GO@starch in comparison with the counterpart. The cost-performance ratio for the nanostructured biocomposites was excellent even compared to the established packaging materials. The multiscale morphological regulation of sheet-like nanofillers by controlled exfoliation and immobilization of GO on micro-sized starch particle


Page 3 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surfaces, the simplicity of manufacturing, together with the versatility of the engineered composites should make our strategy broadly applicable to other material combinations.

KEYWORDS:

Poly(lactic

acid),

Biocomposite,

Graphene

oxide,

Nano-interfaces,

Mechanical properties, Barrier films

INTRODUCTION Two-dimensional graphene oxide (GO) and its derivatives have garnered intense scientific interest due to their exceptional mechanical, electronic and optical properties and exciting application potential in diverse fields from lithium-ion batteries to supercapacitors, catalysis, optics, hydrogen storage, sensors and polymer composites.1−4 The GO-enabled multifunction and versatility are directly related to the large specific surface area (SSA) of GO nanosheets (with a BET-determined value of over 600 m2 g–1),5 which allows storing and releasing of “particles” like ionic species, electric charges, hydrogen atoms and biological macromolecules.6,7 The large SSA of GO—when combined with its high mechanical strength,8,9 high resistance to gas/water permeation,10 rich oxygen functional groups on the planes and at the edges,11,12 and feasibility of functionalization—makes it an ideal platform for advancing mechanical performances and achieving multifunction for polymer composites,13−17 and for fabrication of robust barrier films,18 hybrid aerogels,19 and biomedical hydrogels.20,21 For example, the crystallization activity, mechanical ductility and barrier properties of bioplastics were notably promoted with low contents of GO (up to 0.5 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 52

wt %).1 The development of green GO from bio-based carbon resources, like cellulose and waste paper, further fertilizes the potential applications.22,23 Moreover, the integration of rational nanostructure design of GO nanosheets in polymeric composites offers great opportunities to tailor the composite properties.24 As a good example, a mechanically strong and electrically conductive thermoplastic polyurethane was created by introducing nacre-like layered GO and molybdenum disulfide nanosheets,25 while the cell morphology and mechanical flexibility and strength of polyimide foams were profoundly tailored by using GO precursors.26 The surface chemistry of GO is of immense importance for the application in polymer composites, which profoundly determines the morphology of nanosheets in the matrix (e.g., the dispersion, distribution and orientation) and thus the physicochemical properties of the composites (such as the homogeneity, strength, and thermal and electrical conductivity), with both set of properties linked to application-specific performance.27−29 The large SSA and high interaction activity of GO nanosheets, on the other side, frequently lead to undesirable aggregation of GO nanosheets in the polymeric matrix and thereby deterioration of performances, especially at high GO loadings.30 This has generated a demand for uniform dispersion and ordered organization of GO in the pursuit of flexible composites that inherit the advantageous strengths of nanosheets.31 Emerging as two main routes to enhance the property-harvesting capacity, solvent-based grafting and thermal reduction of GO have been applied to enlarge the interlayer spacing of GO and to improve the interfacial interactions between GO and polymeric matrix. Using “grafting-to” or “grafting-from” (e.g., esterification 4 ACS Paragon Plus Environment

Page 5 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and free radical polymerization), the former pathway proposes the covalent decoration of GO surfaces with functional small molecules or polymers such as styrene, amines and poly(ethylene glycol).32−34 A critical challenge lies in precise control of molecular weight and polydispersity, which is further hampered by the limited availability of functional architectures.35,36 As in the case of GO reduction, the decrease of aspect ratio and the creation of structural defects caused by high-temperature shock, to a large extent, negate any benefits associated with the nanoscopic merits of large-sized nanosheets.37,38 Seeking after a facile approach to facilitate the exfoliation and dispersion GO nanosheets, we propose the utilization of starch granules to anchor and support the nanosheets dispersed and extended in ethanol, creating GO-starch hydrogen bonding interactions instead of intersheet π−π stacking between GO nanosheets. It is hypothesized that the starch granules featuring rich oxygen functional groups are ready to serve as immobilization sites for GO to land on (GO@starch). Figure 1 describes the facile route to preparation of GO@starch and nanostructured poly(lactic acid) (PLA)/GO@starch biocomposites. Further experimental details are provided in the Supporting Information. To manipulate the morphology and regioselectivity of nanosheets, our key elements are (1) the use of trace amount of GO (0.1 wt % based on starch) to allow the full exfoliation and extension of nanosheets, and (2) the selection of micro-sized starch granules with an average diameter of 10.5 µm to appropriately support and settle the extended nanosheets. The GO@starch particles are then introduced into PLA with the GO contents ranging from 0.01 to 0.03 wt %—an exceptionally low level as compared to the existing GO reinforced polymeric composites. This contrasts recent use of 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 52

tenfold GO loadings (1 wt %) towards the encapsulation of starch, requiring additional etherification to create cationic starch before the encapsulation by GO.39

Figure 1. A facile solution processable route to prepare biocomposites with strong nano-interfaces. GO powders were initially sonicated in ethanol to exfoliate and extend the flakes. The dispersions were then introduced into starch/ethanol solution to allow the localization and immobilization of GO nanosheets onto starch. The GO@starch particles were compounded with PLA by solution mixing, followed by rapid evaporation and compression molding to PLA/GO@starch films (thickness of 200 μm) with GO@starch contents varying from 10 wt % to 20 wt % and 30 wt %. The digital photo shows high flexibility and light color for the composite film even with the highest filler loading. Traditional PLA/starch composites without addition of GO were prepared using the same method.

RESULTS AND DISCUSSION The micro-sized starch granules were assumed to function as supports and immobilizers of the exfoliated nanosheets, conferring adequate coating of starch surfaces with large nanosheets. To prove this hypothesis, we conducted direct transmission electron microscopy (TEM) and scanning electronic microscopy (SEM) observations of GO dispersed in ethanol and starch granules wrapped by GO (Figure 2). Figure 2A manifests that the combined 6 ACS Paragon Plus Environment

Page 7 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


approach of low GO concentration and ultrasonication enabled the complete exfoliation of GO flakes to nanosheets with large planar dimension exceeding 1 µm. The direct observation was supported by dynamic light scattering (DLS) measurements,40 showing that the lateral size distribution of GO displayed a Gaussian distribution with an average dimension of 1199 nm. Figure 2B compares the surface morphology of starch and GO@starch. In contrast to the smooth surfaces of plain starch granules, it is apparent that the GO@starch was closely wrapped by large, exfoliated GO nanosheets, displaying enormous wrinkles and ripples. The large surface area and high surface energy of individual nanosheets conferred full coverage of starch surfaces, and even formation of linkages between the adjacent starch granules. The specific structural features of the peanut-like assembled GO@starch are revealed in Figure 2C. Driven by the strong interfacial interactions, the nanosheets were anchored and immobilized onto the starch granule surfaces (Figure 2C1), serving also as the joint connection between two adjacent starch particles (Figure 2C2). Figure 2C3 illustrates the coating of starch by few-layer GO by exposing the ultrathin edges of GO nanosheets under high electron energy penetration. Our process, thus, results in well-defined exfoliation and extension of GO nanosheets that fully cover the starch surfaces and modify the surface chemistry and surface activity.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 52

Figure 2. Direct observation of GO and GO@starch. (A) TEM micrograph of an individual GO nanosheet in ethanol (0.05 mg/ml), suggesting that the basal planes of GO were sufficiently extended in the dilute solution. The inset bar graph depicts the size distribution determined by DLS, showing a Gaussian distribution with an average dimension of 1199 nm, in good correlation with TEM observation. (B) SEM images comparing the surface morphologies of pristine starch and GO@starch. Extended, large GO nanosheets were tightly attached onto the starch, and even bonded together neighboring starch granules. (C) TEM micrograph showing two starch particles wrapped and connected by GO nanosheets, displaying the peanut-like appearance. Specifically, (C1) shows that the starch was closely wrapped by extended nanosheets as pointed out by the arrow, (C2) reveals that 8 ACS Paragon Plus Environment

Page 9 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the large nanosheets acted as a binder to connect the neighboring starch granules, and (C3) indicates the presence of thin layer of nanosheets after planar breakdown using high electron energy.

The structural modification of starch after coating by GO was determined by two-dimensional wide-angle X-ray diffraction (2D-WAXD) measurements, as shown in Figure 3A,B. Although displaying the similar diffraction patterns (Figure 3A), the diffraction peaks of starch were slightly shifted to right in GO@starch (Figure 3B), presumably as a result of decreased spacing due to the stacked, integrated starch molecules by tight attachment of GO nanosheets (Figure 2B). Although GO in the condensed state showed the (001) reflection at 2θ = 9.8° (Figure S5),41 apparently there existed no diffraction reflection from GO in GO@starch, which was attributed to the sufficient exfoliation and extension of nanosheets on the starch granule surfaces.42 The underlying interactions between starch and GO were envisaged by the evident shifts of the characteristic peaks in Fourier transform infrared spectroscopy (FTIR) spectra (Figure 3C). Both GO and starch are rich in oxygen functional groups, exciting the prominent, broad peaks in the range of 3600–3000 cm–1. In the C–H stretching region, starch was characterized by the distinguished absorption peak located at 2927 cm–1, which witnessed a blue shift of 3 cm–1 in GO@starch. This was accompanied by a more distinct blue shift of –COOH units situated at the edges of GO whose stretching frequency shifted from 1716 cm–1 for GO to 1737 cm–1 for GO@starch. Sharing the same fundamental formation mechanism with classic red-shifting hydrogen bond,43 improper blue-shifting hydrogen bonds can be caused by strong intermolecular/intramolecular 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 52

interactions.44 While recognizing the characteristic peaks of starch and GO, we attempted to image the distribution of GO nanosheets in GO@starch (Figure 3D). The absorbance patterns of band 1737 cm–1, indicative of the presence of GO, were exclusively observed in GO@starch. Of interest was the associated spatial distribution of starch and GO nanosheets detected in GO@starch, indicating that the nanosheets were closely bonded to starch.

Figure 3. Structural characterization of GO@starch. (A) 2D-WAXD patterns and (B)

1D-WAXD intensity profiles of pure starch and GO@starch. Full exfoliation and uniform dispersion of nanosheets led to the absence of characteristic diffraction peaks of GO. The right shift of diffraction peaks indicated that the attachment of 10 ACS Paragon Plus Environment

Page 11 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GO sheets slightly lowered the lamellar spacing of starch. (C) Integrated FTIR spectra in the two specific wavenumber ranges of (C1) 3700−2700 cm−1 and (C2) 1900−1500 cm−1. (D) 2D-FTIR imaging patterns comparing the absorbance intensity at wavenumbers of 2927 cm−1 and 1737 cm−1, which were characteristic bands for starch and GO, respectively. Scale bar denotes 100 μm.

Quantitative element mapping by energy dispersive X-ray spectrometry (EDS) microanalysis scanned over the surfaces of GO, starch and GO@starch provided evidence of the immobilization of GO nanosheets on starch surfaces (Figure 4). Figure 4A shows the spatial distribution of carbon (C) and oxygen (O) for GO powder, starch and GO@starch particles. A C/O ratio of 1.56 was determined for GO (Figure 4B), which is in good agreement with typical values as reported by X-ray photoelectron spectroscopy measurements.45 A clear increase in C/O ratio was observed for GO@starch (1.26) compared to pure starch (1.11), giving evidence of the attachment of GO nanosheets onto the starch surfaces that pushed up the C proportion. The detection depth of EDS method and the gap between the C/O ratios of GO and GO@starch indicates that the surface is covered by GO with a limited number of layers.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 52

Figure 4. Immobilization of GO on starch evidenced by EDS microanalysis. (A)

Electron images overlaid with color-coded element maps (red for C and green for O). Scale bar denotes 10 μm. (B) EDS spectra showing the atomic weight proportions. The C/O ratio of GO@starch lay between those of GO and starch— and closer to that of starch, indicating the existence of only a few nanosheet layers of GO wrapping the starch. The individual electron images and element maps are presented in Figure S7.

Next we examined the filler morphologies in the composite films, including the starch dispersion, filler-matrix adhesion and GO distribution (Figure 5). As shown in Figure 5A, an irregular distribution of starch particles was observed in PLA/starch30, showing a higher concentration of starch in the central region but sparse filler contents close to the film surface. This indicates poor adhesion between PLA matrix and starch that failed to prevent filler agglomeration, as also implied by the large gaps generated between the smooth starch particles and the matrix. This represents a fundamental challenge for the enhancement of 12 ACS Paragon Plus Environment

Page 13 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mechanical properties in PLA/starch composites, principally arising from the intrinsic hydrophilicity of starch and the smooth surface that features low specific surface energy.46 By contrast, GO@starch particles were uniformly dispersed in the matrix even at the highest filler content, without obvious concentration variations along the thickness direction. The enhanced interfacial interaction, mainly attributed to GO-enabled contribution to ligament generation bridging starch and PLA, should be responsible for the homogeneous dispersion of GO@starch in the matrix. While addition of higher starch content to PLA lowers the cost of composite fabrication, two critical challenges lie in strengthening the filler-matrix interphase and controlling the filler dispersion in the matrix, both of which profoundly affect the macroscopic properties of composites.47 As a general approach to improve the inferior interphase between PLA and starch, surface modification of starch by grafting with polymer chains, such as epoxy resins48 and poly(ethylene glycol),49 has been developed. In a recent progress, recycled multifunctional PLA was utilized as effective compatibilizer in PLA/starch composites.50 The grafting method, however, likely pushes up the production costs and prolong the production cycle, imposing undesirable restrictions on the industrial development of low-cost PLA/starch biocomposites. On examining the distribution of GO in the composites, it was found that starch firmly immobilized the nanosheets through strong surface anchoring, leading to the preferential localization of nanosheets at the interfaces rather than immigration to the matrix (Figure 5B). Moreover, this distinct regioselectivity showed weak relation to the filler concentrations, allowing the assertion that the function of nanosheets could be well exerted to advance the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 52

composite performances even at high filler loadings. An inspection of Figure 5B indicates that starch was wrapped by few-layer GO in accordance with the EDS results.

Figure 5. Filler dispersion and distribution in PLA composite films. (A) SEM

micrographs of cryogenically fractured surfaces implying desirable filler distribution and filler-matrix interactions in PLA/GO@starch30, in clear contrast to the non-uniform dispersion of starch particularly in the vicinity of film surface and poor interfacial interactions observed for PLA/starch30. (B) SEM images examining


Page 15 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanosheet distribution in GO@starch-filled composite films, in which the matrix was etched by washing with dichloromethane. Few-layer nanosheets were firmly immobilized onto starch particles, without traces of free sheets.

The firm interactions between GO and starch was assumed to result in the preferential localization of GO on starch surfaces rather than in the PLA matrix. Figure 6 provides evidence for this assertion by mapping separately the distribution and relative proportion of C and O elements for the matrix and filler sections of the composites (Figure 6A). Figure 6B illustrates that a similar C/O ratio was determined for the matrix regions of PLA/starch30 and PLA/GO@starch30 (2.83 and 2.85, respectively). The C/O ratio in the filler regions increased from 1.48 for PLA/starch30 to 1.63 for PLA/GO@starch due to the coating of starch by GO. These values are in close relation to those observed for pure starch and GO@starch, while the presence of some PLA matrix likely contributed to higher C proportion as compared to pure starch and GO@starch for which C/O ratios of 1.11 and 1.26 were measured, respectively.

Figure 6. GO distribution in composite films determined by EDS. (A) Electron

images overlaid with color-coded element maps (red for C and green for O), 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 52

separately scanned over the PLA matrix and fillers. Scale bar denotes 5 μm. (B) EDS spectra showing the atomic weight proportions. In the matrix, PLA/starch30 and PLA/GO@starch30 shared a very similar C/O ratio. The C/O ratio of fillers increased from 1.48 for PLA/starch30 to 1.63 for PLA/GO@starch due to the coating of GO. The individual electron images and element maps are presented in Figure S9.

GO nanosheets have been demonstrated to serve as effective nucleating platforms to anchor and nucleate stereocomplex PLA chains, contributing to the distinct promotion of stereocomplex crystallization.2,51 The addition of GO@starch is, thus, expected to inherit the merits of GO and offer active nucleating platforms for PLA and eventually, boost the crystallinity. This effect is illustrated in Figure 7. The absence of diffraction excitation for PLA crystals in PLA/starch composites can be explained by the inherently poor crystallization ability of PLA (Figure 7A,B).52−56 In PLA/GO@starch composites, however, the formation of rich α-form crystals of PLA, accounting for the diffraction peaks at 15.0°, 16.7°, 19.1° and 22.5°, were clearly observed.57 Figure 7C reveals that for PLA/GO@starch20 compact PLA lamellae packed in the edge-on manner were induced by the GO planes, in clear contrast to the inexistence of crystalline entities for PLA/starch20. The nanosheet-assisted anchoring and immobilization of neighboring PLA chains were evidenced by the notable rise of glass transition temperature (Tg) of PLA/GO@starch films (Figure 7D). This also explains the significant increase in crystallinity, climbing to around 50% for PLA/GO@starch from below 15% for PLA/starch composites. During the steady cooling (Figure 7E), the PLA/GO@starch composites were characterized by the notably enhanced crystallization kinetics, as indicated 16 ACS Paragon Plus Environment

Page 17 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by the increased crystallization temperatures (Tc) and crystallization enthalpies (∆Hc).58,59 The GO-enabled nucleation activity for PLA has been highlighted recently in binary PLA/GO composites, in which over 0.1 wt % GO was normally added to distinguish the nucleation efficacy of GO.33,60 As such, undesirable aggregation of nanosheets was frequently observed. In our case, few-layer GO nanosheets were extended to function as robust nano-interfaces exerting robust control over interfacial crystalline morphology.

Figure 7. Evaluation of crystalline morphologies of compression-molded composite

films. (A) 2D-WAXD patterns and (B) diffraction intensity profiles indicating large crystallinity promotion assisted by GO@starch. (C) SEM images showing the directional growth of PLA lamellae induced by the nanosheet, while no crystalline entities were traced in the normal composite. The amorphous phase was etched in alkaline solution.61 (D, E) DSC traces recorded during steady heating and cooling,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 52

respectively. The values of Tg and crystallinity were marked in (D). The presence of GO@starch particles strengthened the interactions with neighboring PLA chains, leading to the rise of Tg, and pushing the crystallinity of PLA up to ~50%. The substantial increase in Tc and ΔHc was indicative of enhanced melt crystallization with the aid of extended GO. All the enthalpies were normalized based on the PLA contents. (a) PLA/starch10, (b) PLA/GO@starch10, (c) PLA/starch20, (d) PLA/GO@starch20, (e) PLA/starch30, and (f) PLA/GO@starch30.

Figure 8 clearly shows that the decoration of starch with GO nanosheets favors creation of tenacious interfacial adhesion, accounting for the greatly promoted tensile properties. Figure 8A demonstrate the notable decrease in tensile strength caused by the addition of starch, gradually falling from 29.4 MPa for PLA/starch10 to 15.3 MPa for PLA/starch30. This is in line with inferior mechanical performances generally reported for PLA/starch composites, where drastic decreases of ~50% in tensile strength after the addition of 30 wt % starch were commonly observed.62 Instead, the addition of GO@starch significantly pushed up the tensile strength of PLA composites, reaching the highest level of 58.2 MPa for PLA/GO@starch30—a rise of over 280 % compared to PLA/starch30. This distinction was accompanied by the clear promotion in stiffness and ductility (Figure 8B,C), as illustrated by the increase of 65% and 210% in the Young’s modulus and elongation for PLA/GO@starch20 (1562 MPa, 6.5%) in comparison to those of PLA/starch20, respectively. It is worth noting that the ductility of PLA films was not sacrificed with the addition of PLA/GO@starch. This has rarely been reported for plasticizer-free composites and can 18 ACS Paragon Plus Environment

Page 19 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


greatly benefit the extension of PLA applications. In a previous study by Che et al., an increase of ~6 MPa in yield strength of PLA was observed after addition of 10 wt % GO encapsulation modified starch, while substantial decline of ductility was recorded (~75% decrease in elongation at break) compared to pure PLA.39As illustrated in Figure 8D, the well-defined control of GO morphology on starch exerts more effective function in advancing the composite strength, compared to the traditional chemical modification of starch surface chemistry. It results in high strength for PLA/GO@starch composite comparable with pure PLA, while the grafting approaches generally failed to compensate the decline in strength as reported in the conventional PLA/starch composites. The combination of strength, stiffness and ductility observed in GO@starch composites, in essence, originated from the enhanced interfacial adhesion, with GO nanosheets bridging the neighboring two phases (Figure 8E).63 −65 In contrast to the smooth fracture surfaces and inferior interfacial interactions existing in pure starch-filled composites, GO nanosheets were tightly adhered onto the starch to create strong interfacial ligaments between the PLA matrix and starch particles in GO@starch composites. This is of importance to enhance the stress transfer between the particles and the matrix, which even triggered the formation of long, resilient nanofibers in the matrix during the external deformation. This mechanism was further enhanced by possible fracture of nanosheets on the starch surface (Figure S13), propelling the dissipation of plenty of energy due to the intrinsically high elastic modulus and breaking strength of GO nanosheets (~200 and ~10 GPa, respectively, measured for multilayer nanosheets with a thickness of tens of nanometer).66 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 52

Figure 8. Mechanical properties of composite films. (A) Tensile strength, (B) tensile

modulus and (C) elongation at break demonstrating the superior tensile properties of PLA/GO@starch in comparison with PLA/starch. (D) Comparison of tensile strength for modified PLA/starch composites using our method and covalent grafting, including epoxidized itaconic acid (EIA)-g-starch,47 epoxidized cardanol (Epicard)-g-starch,47 combined use of starch and starch-g-PLA (5 wt % and 10 wt %),67 starch-g-poly(ethylene glycol) (PEG),48 maleic anhydride (MA)-g-starch,61 and starch-g-PLA.68 (E) SEM images of fracture surfaces after tensile failure suggesting strong interfacial bonding and even formation of numerous ligaments in PLA/GO@starch films.


Page 21 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The deformation-induced formation of numerous nanofibrils in PLA/GO@starch composites makes a potentially important contribution for preventing failure around starch particles—where stress concentration is frequently created. The intrinsic brittleness of PLA led to limited deformation in the bulk matrix, whereas GO@starch particles were twined by ultrafine nanofibrils that featured high length of up to tens of micrometer and large flexibility that permitted vine-like crimp (Figure 9A). An inspection of the particle surfaces revealed some interesting morphological features: (1) the surface of GO@starch was highly wrinkled due to the firm coating by GO nanosheets (Figure 9Aa1); (2) different from the filler-free section, the matrix adhered to GO@starch appeared to present much higher degree of plastic deformation, indicating that the enhanced filler-matrix interaction was primarily triggered by nanosheets (Figure 9Aa1); (3) the birth and growth of nanofibrillar ligaments appeared to be originated at the surface of GO@starch, leading to gradually decreased diameter toward the fibril tip (Figure 9Aa2); (4) the band-like long roots of nanofibrils were likely a result of simultaneous deformation of GO nanosheets along the matrix nanofibrillation (Figure 9Aa2). Overlapping these morphological features of nanofibrillar ligaments, Figure 9B schematically depicts a possible mechanism to explain the deformation-induced formation of ligaments in the vicinity of GO@starch particles. Given the presence of dense PLA lamellae induced by active surfaces of GO@starch (crystallinity of over 50%, Figure 7), the composites can be perceived as a “rigid-soft” system composed of coiled amorphous chains capable of viscoelastic motion and ordered crystalline blocks that hold adjacent coils together. When a stress is applied to such a system, the first deformation involves orientation of the 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 52

amorphous chains, which subsequently drives alignment of lamellae along the tensile direction. With the increase of deformation, nanofibrillar ligaments are gradually developed, comprising highly oriented bundles with substructures of oriented lamellae and amorphous chains. Another fundamental assumption related to the nanofibrillation mechanism is the possible slip of GO nanosheets on starch surfaces, along with the planar deformation under high tension, which are finally involved in the simultaneous nanofibrillation of polymer chains. This essentially lies in the combination of strength and toughness of GO nanosheets, providing the readiness to bridge starch particles and as-stretched polymer phase. The formation nanofibrillar ligaments in the vicinity of GO@starch particles would, thus, be a consequence of appropriate crystalline morphology in the matrix and firm immobilization of large, few-layer GO nanosheets on starch surfaces. The GO-enabled ligament formation explains the notable promotion in strength and toughness with increase of GO@starch loadings, although the degree of plastic deformation is reduced due to the decrease of matrix content.


Page 23 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Potential formation mechanism for the nanofibrillar ligaments in

PLA/GO@starch composites. (A) SEM micrographs of fracture surfaces of PLA/GO@starch10 after tensile failure, and local observation of GO@starch particles revealing the existence of numerous wrinkles (a1) and the high elasticity and firm adhesion of nanofibrils rooted at starch surface (a2). (B) Schematic description of formation mechanism of nanofibrillar ligaments during tensile deformation. On the surfaces of GO@starch, compact PLA lamellae were generated, serving as numerous cross-linkers to bond neighboring chains and to render the formation of deformation-induced nanofibrils consisting of stretched chains and oriented lamellae.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 52

In addition to the mechanical property promotion, incorporation of GO@starch into PLA rendered multifunction for composite films (Figure 10). Figure 10A shows that both the decomposition onset temperature (Tonset) and the decomposition maximum rate temperature (Tmax) were significantly promoted by the incorporation of nanostructured GO, as exemplified by the 18.7 °C higher Tonset and 23.6 °C higher Tmax for PLA/GO@starch30 compared to PLA/starch30. This could be ascribed to the impressive thermal conductivity of GO nanosheets (~5000 W m–1 K–1),69 conferring thermal shielding by rapidly transferring heat through the large nanosheets. In addition, the generation of dense ordered PLA lamellae may contribute to the enhancement of thermal resistance.70 Next, we characterized the resistance to oxygen permeation for the composite films, as described in Figure 10B. For PLA/starch composites, the decline of gas solubility within the reduced PLA matrix was responsible for the gradual drop of PO2,71 falling to 21.7 cm3 mm cm–2 day–1 atm–1 for PLA/starch30 from the initial value of 35.6 cm3 mm cm–2 day–1 atm–1 for pure PLA film. The oxygen permeation was further reduced by the incorporation of nanosheets, displaying a direct drop of PO2 from 9.4 cm3 mm cm–2 day–1 atm–1 for PLA/GO@starch10 to 4.0 cm3 mm cm–2 day–1 atm–1 for PLA/GO@starch30. This distinction arose from the three main morphological features in PLA films governing the oxygen permeation that was closely associated with the solubility and diffusion of oxygen molecules (Figure 10C): (1) the well-extended nanosheets on surfaces of starch granules rendered the construction of compact “nano-barrier walls” to resist diffusing molecules (Figure 5B); (2) the solubility of oxygen in the matrix was decreased with regard to the GO-assisted formation of rich crystalline regions (Figure 7);72,73 (3) in 24 ACS Paragon Plus Environment

Page 25 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PLA/starch composites oxygen molecules were allowed to preferentially diffuse through the interfacial gaps between pure starch and PLA (Figure 5A), these “gates”, however, were closed in PLA/starch@GO by the tight attachment of GO, as well as the generation of rich PLA lamellae (Figure 7C). Addition of nanofillers, especially with sheet-like structures, has been perceived as a widely applicable approach to improve the barrier properties of PLA. As described in Figure 10D, a high concentration of nanofillers, ranging from 0.1 wt % to 20 wt %, has been used to trigger significant barrier property improvement. This may, to some extent, limit the composite applications with regard to the production costs, dark color, non-uniform nanofiller distribution and environmental issues that could be caused by the use of high filler loadings. By sufficiently controlling the morphology of GO sheets in PLA, we achieved drastic decline of PO2, up to 88.8%, with trace amount of GO (0.03 wt %)—a very low concentration for polymeric composites. Of commercial significance in terms of production costs and gas barrier properties is the cost-performance effectiveness of the nanostructured PLA/GO@starch films, even when compared to the commercially available films (Figure 10E). Polyethylene films generally fall into the range of low barrier properties, as exemplified by a high PO2 for high density polyethylene films used for packaging chocolates (21.5 cm3 mm cm–2 day–1 atm–1), which is in the same level with pure PLA (35.6 cm3 mm cm–2 day–1 atm–1). The incorporation of GO@starch could render PLA films into the category of high-barrier packaging materials comparable to poly(ethylene terephthalate) that is characterized by the lowest PO2 among all the examined commercial films (5.2 cm3 mm cm–2 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 52

day–1 atm–1). With respect to the cost of raw materials, the advantages of PLA composites based on low-cost GO@starch are overwhelming. Pure PLA has a commercial price of around 3000 $/t, which can be pushed down to 2190 $/t by adding 30 wt % GO@starch (~300 $/t). This largely narrows the cost gap between PLA and other traditional petroleum-based packaging materials (around 2000 $/t), rendering the composite films competitive to the available commercial packaging films.

Figure 10. Performance evaluation to demonstrate the multiple property

enhancement for PLA/GO@starch. (A) TGA curves manifesting the substantial promotion of thermal stability achieved by the incorporation of GO. The inset cartoon depicts

preferential heat transfer through the interconnected


Page 27 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nano-interfaces, while the inset table lists the values of Tonset and Tmax. (B) PO2 values demonstrating the reduced gas permeation after addition of starch, which was further pushed down by GO nano-interfaces. (C) Schematic illustration of oxygen molecules following a relatively easy path through PLA/starch and a tortuous path through PLA/GO@starch. The construction of compact high-barrier nano-interfaces—combined with the increased crystallinity and enhanced interfacial adhesion—was assumed to decrease the solubility and diffusion of gas molecules, which explained the elevated resistance to gas permeation in PLA/GO@starch composites. (D) Comparison of percentage decline of PO2 as a function of the concentration of sheet-like nanofillers. Showing prominent enhancement in barrier properties at very low GO concentrations, the nanostructured PLA/GO systems stand out among the PLA composites based on GO,74 graphene,75 and montmorillonite (C16-MMT). 76 (E) Comparison of PO2 and raw material cost between PLA/GO@starch films and commercial films existing in the market. Taking into consideration the trace amount of GO (up to 0.03 wt %), the cost of GO was not included.

CONCLUSIONS A thin layer of GO nanosheets could be immobilized onto starch granule surfaces by hydrogen bonding to create active GO@starch surfaces. Once incorporated into PLA matrix, GO@starch allowed the creation of nano-interfaces between PLA matrix and starch particles,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 52

without immigration of GO into PLA matrix. Although the GO layer was ultrathin and GO was present at ultralow concentration (up to 0.03 wt %), these nano-interfaces made a significant contribution to strengthen the PLA-starch interfacial interactions and the facilitated uniform dispersion of GO@starch in PLA matrix. Driven by the high surface activity of high-aspect-ratio nanosheets, a large promotion in crystallinity for PLA/GO@starch films (around 50%) was demonstrated, clearly surpassing that of PLA/starch composites (below 15%). These morphological regulations culminated in multiple property enhancement including mechanical strength, thermal stability and gas barrier properties. Specifically, the combination of high tensile strength (58.2 MPa), improved elongation (6.1%) and low PO2 (4.0 cm3 mm cm–2 day–1 atm–1) were demonstrated for PLA/GO@starch30, outperforming PLA/starch30 with an increase of ~280% in both strength and elongation, and a drastic decline of 81.6% in PO2. Our approach also enables easy yet effective control on the morphology of 2D nanosheets, which makes fabrication of the multifunctional composites straightforward, and holds great potential to suite other 2D fillers beyond GO.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Experimental details, AFM characterization of GO, UV-vis measurement of GO@starch dispersions, SEM observation of GO@starch, WAXD characterization of GO, full-range FTIR spectra and individual EDS maps of GO, starch and 28 ACS Paragon Plus Environment

Page 29 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GO@starch, optical micrographs examining the filler dispersion in composite films, individual EDS maps of composite films, WAXD characterization of pure PLA film, stress-strain curves of composite films, SEM images of fracture surfaces for pure PLA and composite films after tensile failure (PDF)

AUTHOR INFORMATION Corresponding Author *M. Hakkarainen, Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are deeply indebted to Dr. Patrick Baker from the Department of Molecular Biology and Biotechnology, University of Sheffield for his kind help during the X-ray measurements. H.X., L.X. and D.W. are grateful to the financial support from the China Scholarship Council (CSC) for studying abroad.

REFERENCES 1. Xu, H.; Feng, Z.-X.; Xie, L.; Hakkarainen, M. Graphene Oxide-Driven Design of Strong and Flexible Biopolymer Barrier Films: From Smart Crystallization Control to Affordable Engineering. ACS Sustainable Chem. Eng. 2016, 4, 334-349.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 52

2. Hassanzadeh, S.; Adolfsson, K.H.; Wu, D.; Hakkarainen, M. Supramolecular Assembly of Biobased Graphene Oxide Quantum Dots Controls the Morphology and Induces Mineralization on Poly(ε-caprolactone) Films, Biomacromolecules 2016, 17, 256-261. 3. Shen, Y.; Wang, H.; Liu, J.; Zhang, Y. Enhanced Performance of a Novel Polyvinyl Amine/Chitosan/Graphene Oxide Mixed Matrix Membrane for CO2 Capture. ACS Sustainable Chem. Eng. 2015, 3, 1819-1829. 4. Yang, J.; Wu, J.-X.; Lü, Q.-F.; Lin, T.-T. Facile Preparation of Lignosulfonate–Graphene Oxide–Polyaniline Ternary Nanocomposite as an Effective Adsorbent for Pb(II) Ions. ACS Sustainable Chem. Eng. 2014, 2, 1203-1211. 5. McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater.

2007, 19, 4396-4404. 6. Chen, D.; Zhu, G.; Lin, J.; Liu, J.; Huang, S. Renewable Reduced Graphene Oxide-Based Oil-Absorbent Aerosols: Preparation and Essential Oils Absorption Ability. ACS Sustainable Chem. Eng. 2015, 3, 1428-1433. 7. Li, X.; Han, Y.; Ling, Y.; Wang, X.; Sun, R. Assembly of Layered Silicate Loaded Quaternized Chitosan/Reduced Graphene Oxide Composites as Efficient Absorbents for Double-Stranded DNA. ACS Sustainable Chem. Eng. 2015, 3, 1846-1852.


Page 31 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8. Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457-460. 9. Park, S.; Lee, K.-S.; Bozoklu, G.; Cai, W.; Nguyen, S. T.; Ruoff, R. S. Graphene Oxide Papers Modified by Divalent Ions—Enhancing Mechanical Properties via Chemical Cross-Linking. ACS Nano 2008, 2, 572-578. 10. Musico, Y. L. F.; Santos, C. M.; Dalida, M. L. P.; Rodrigues, D. F. Surface Modification of Membrane Filters Using Graphene and Graphene Oxide-Based Nanomaterials for Bacterial Inactivation and Removal. ACS Sustainable Chem. Eng. 2014, 2, 1559-1565. 11. Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228. 12. Cheng, Z.; Liao, J.; He, B.; Zhang, F.; Zhang, F.; Huang, X.; Zhou, L. One-Step Fabrication of Graphene Oxide Enhanced Magnetic Composite Gel for Highly Efficient Dye Adsorption and Catalysis. ACS Sustainable Chem. Eng. 2015, 3, 1677-1685. 13. Chen, Y.; Zhang, S.; Liu, X.; Pei, Q.; Qian, J.; Zhuang, Q.; Han, Z. Preparation of Solution-Processable Reduced Graphene Oxide/Polybenzoxazole Nanocomposites with Improved Dielectric Properties. Macromolecules 2015, 48, 365-372. 14. Tang, Z.; Kang, H.; Shen, Z.; Guo, B.; Zhang, L.; Jia, D. Grafting of Polyester onto Graphene for Electrically and Thermally Conductive Composites. Macromolecules 2012, 45, 3444-3451.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 52

15. Thakur, S.; Karak, N. Ultratough, Ductile, Castor Oil-Based, Hyperbranched, Polyurethane Nanocomposite Using Functionalized Reduced Graphene Oxide. ACS Sustainable Chem. Eng. 2014, 2, 1195-1202. 16. Ni, T.; Xu, L.; Sun, Y.; Yao, W.; Dai, T.; Lu, Y. Facile Fabrication of Reduced Graphene Oxide/Polypyrrole Composite Hydrogels with Excellent Electrochemical Performance and Compression Capacity. ACS Sustainable Chem. Eng. 2015, 3, 862-870. 17. Xiang, Z.; Zhou, X.; Wan, G.; Zhang, G.; Cao, D. Improving Energy Conversion Efficiency of Dye-Sensitized Solar Cells by Modifying TiO2 Photoanodes with Nitrogen-Reduced Graphene Oxide. ACS Sustainable Chem. Eng. 2014, 2, 1234-1240. 18. Wang, L.; Lian, W.; Yao, H.; Liu, H. Multiple-Stimuli Responsive Bioelectrocatalysis Based on Reduced Graphene Oxide/Poly(N-isopropylacrylamide) Composite Films and Its Application in the Fabrication of Logic Gates. ACS Appl. Mater. Interfaces 2015, 7, 5168-5176. 19. Liu, W.; Cai, J.; Li, Z. Self-Assembly of Semiconductor Nanoparticles/Reduced Graphene Oxide (RGO) Composite Aerogels for Enhanced Photocatalytic Performance and Facile Recycling in Aqueous Photocatalysis. ACS Sustainable Chem. Eng. 2015, 3, 277-282. 20. Li, J.; Xie, J.; Gao, L.; Li, C. M. Au Nanoparticles–3D Graphene Hydrogel Nanocomposite to Boost Synergistically In Situ Detection Sensitivity toward Cell-Released Nitric Oxide. ACS Appl. Mater. Interfaces 2015, 7, 2726-2734.


Page 33 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21. Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutiérrez, M. C.; del Monte, F. Three Dimensional Macroporous Architectures and Aerogels Built of Carbon Nanotubes and/or Graphene: Synthesis and Applications. Chem. Soc. Rev. 2013, 42, 794-830. 22. Adolfsson, K. H.; Hassanzadeh, S.; Hakkarainen, M. Valorization of Cellulose and Waste Paper to Graphene Oxide Quantum Dots. RSC Adv. 2015, 5, 26550-26558. 23. Hassanzadeh, S.; Adolfsson, K. H.; Hakkarainen, M. Controlling the Cooperative Self-Assembly of Graphene Oxide Quantum Dots in Aqueous Solutions. RSC Adv. 2015, 5, 57425-57432. 24. Ouyang, W.; Sun, J.; Memon, J.; Wang, C.; Geng, J.; Huang, Y. Scalable Preparation of Three-Dimensional Porous Structures of Reduced Graphene Oxide/Cellulose Composites and Their Application in Supercapacitors. Carbon 2013, 62, 501-509. 25. Wan, S.; Li, Y.; Peng, J.; Hu, H.; Cheng, Q.; Jiang, L. Synergistic Toughening of Graphene Oxide–Molybdenum Disulfide–Thermoplastic Polyurethane Ternary Artificial Nacre. ACS Nano 2015, 9, 708-714. 26. Xu, L.; Jiang, S.; Li, B.; Hou, W.; Li, G.; Memon, M. A.; Huang, Y.; Geng, J. Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility. Chem. Mater. 2015, 27, 4358-4367. 27. Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568-571.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 52

28. Ning, N.; Ma, Q.; Liu, S.; Tian, M.; Zhang, L.; Nishi, T. Tailoring Dielectric and Actuated Properties of Elastomer Composites by Bioinspired Poly(dopamine) Encapsulated Graphene Oxide. ACS Appl. Mater. Interfaces 2015, 7, 10755-10762. 29. Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. 30. Kim, H.; Abdala, A. A.; Macosko, C. W. Graphene/Polymer Nanocomposites. Macromolecules 2010, 43, 6515-6530. 31. Potts, J. R.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S. Graphene-Based Polymer Nanocomposites. Polymer 2011, 52, 5-25. 32. Lin, Y.; Jin, J.; Song, M. Preparation and Characterisation of Covalent Polymer Functionalized Graphene Oxide. J. Mater. Chem. 2011, 21, 3455-3461. 33. Sun, Y.; He, C. Synthesis and Stereocomplex Crystallization of Poly(lactide)–Graphene Oxide Nanocomposites. ACS Macro Lett. 2012, 1, 709-713. 34. Beckert, F.; Rostas, A. M.; Thomann, R.; Weber, S.; Schleicher, E.; Friedrich, C.; Mülhaupt, R. Self-Initiated Free Radical Grafting of Styrene Homo- and Copolymers onto Functionalized Graphene. Macromolecules 2013, 46, 5488-5496. 35. Layek, R. K.; Nandi, A. K. A Review on Synthesis and Properties of Polymer Functionalized Graphene. Polymer 2013, 54, 5087-5103. 36. Zhang, B.; Zhang, Y.; Peng, C.; Yu, M.; Li, L.; Deng, B.; Hu, P.; Fan, C.; Li, J.; Huang, Q. Preparation of Polymer Decorated Graphene Oxide by γ-Ray Induced Graft Polymerization. Nanoscale 2012, 4, 1742-1748. 34 ACS Paragon Plus Environment

Page 35 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37. Compton, O. C.; Nguyen, S. T. Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials. Small 2010, 6, 711-723. 38. Wang, Z.; Xu, D.; Huang, Y.; Wu, Z.; Wang, L.; Zhang, X. Facile, Mild and Fast Thermal-Decomposition Reduction of Graphene Oxide in Air and Its Application in High-Performance Lithium Batteries. Chem. Commun. 2012, 48, 976-978. 39. Nuona, A.; Li, X.; Zhu, X.; Xiao, Y.; Che, J. Starch/Polylactide Sustainable Composites: Interface Tailoring with Graphene Oxide. Composites, Part A 2015, 69, 247-254. 40. Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and Exfoliation of Isocyanate-Treated Graphene Oxide Nanoplatelets. Carbon 2006, 44, 3342-3347. 41. Chen, S.; Zhu, J.; Wu, X.; Han, Q.; Wang, X. Graphene Oxide−MnO2 Nanocomposites for Supercapacitors. ACS Nano 2010, 4, 2822-2830. 42. Fang, M.; Wang, K.; Lu, H.; Yang, Y.; Nutt, S. Single-Layer Graphene Nanosheets with Controlled Grafting of Polymer Chains. J. Mater. Chem. 2010, 20, 1982-1992. 43. Scheiner, S.; Kar, T. Red-versus Blue-Shifting Hydrogen Bonds: Are There Fundamental Distinctions? J. Phys. Chem. A 2002, 106, 1784-1789. 44. Hobza, P.; Havlas, Z. Blue-Shifting Hydrogen Bonds. Chem. Rev. 2000, 100, 4253-4264. 45. Chen, W.; Yan, L.; Bangal, P. R. Preparation of Graphene by the Rapid and Mild Thermal Reduction of Graphene Oxide Induced by Microwaves. Carbon 2010, 48, 1146-1152. 46. Zhang, J.-F.; Sun, X. Mechanical Properties of Poly(lactic acid)/Starch Composites Compatibilized by Maleic Anhydride. Biomacromolecules 2004, 5, 1446-1451. 35 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 52

47. Teixeira, E. d. M.; Curvelo, A. A.; Corrêa, A. C.; Marconcini, J. M.; Glenn, G. M.; Mattoso, L. H. Properties of Thermoplastic Starch from Cassava Bagasse and Cassava Starch and Their Blends with Poly(lactic acid). Ind. Crops Prod. 2012, 37, 61-68. 48. Xiong, Z.; Ma, S.; Fan, L.; Tang, Z.; Zhang, R.; Na, H.; Zhu, J. Surface Hydrophobic Modification of Starch with Bio-Based Epoxy Resins to Fabricate High-Performance Polylactide Composite Materials. Compos. Sci. Technol. 2014, 94, 16-22. 49. Wang, J.; Zhai, W.; Zheng, W. Poly(ethylene glycol) Grafted Starch Introducing a Novel Interphase in Poly(lactic acid)/Poly(ethylene glycol)/Starch Ternary Composites. J. Polym. Environ. 2012, 20, 528-539. 50. Wu, D.; Hakkarainen, M. Recycling PLA to Multifunctional Oligomeric Compatibilizers for PLA/Starch Composites. Eur. Polym. J. 2015, 64, 126-137. 51. Xu, H.; Wu, D.; Yang, X.; Xie, L.; Hakkarainen, M. Thermostable and Impermeable “Nano-Barrier Walls” Constructed by Poly(lactic acid) Stereocomplex Crystal Decorated Graphene Oxide Nanosheets. Macromolecules 2015, 48, 2127-2137. 52. Inkinen, S.; Hakkarainen, M.; Albertsson, A.-C.; Södergård, A. From Lactic Acid to Poly(lactic acid) (PLA): Characterization and Analysis of PLA and Its Precursors. Biomacromolecules 2011, 12, 523-532. 53. Ma, P.; Shen, T.; Xu, P.; Dong, W.; Lemstra, P. J.; Chen, M. Superior Performance of Fully Bio-Based Poly(lactide) via Stereocomplexation-Induced Phase Separation: Structure Versus Property. ACS Sustainable Chem. Eng. 2015, 3, 1470-1478.


Page 37 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


54. Yin, H.-Y.; Wei, X.-F.; Bao, R.-Y.; Dong, Q.-X.; Liu, Z.-Y.; Yang, W.; Xie, B.-H.; Yang, M.-B. Enhancing Thermomechanical Properties and Heat Distortion Resistance of Poly(L-lactide) with High Crystallinity under High Cooling Rate. ACS Sustainable Chem. Eng. 2015, 3, 654-661. 55. Liu, Z.; Luo, Y.; Bai, H.; Zhang, Q.; Fu, Q. Remarkably enhanced impact toughness and heat resistance of poly (L-lactide)/thermoplastic polyurethane blends by constructing stereocomplex crystallites in the matrix. ACS Sustainable Chem. Eng. 2016, 4, 111-120. 56. Bai, J.; Wang, J.; Wang, W.; Fang, H.; Xu, Z.; Chen, X.; Wang, Z. Stereocomplex Crystallite-Assisted Shear-Induced Crystallization Kinetics at a High Temperature for Asymmetric Biodegradable PLLA/PDLA Blends. ACS Sustainable Chem. Eng. 2016, 4, 273-283. 57. Bao, R.-Y.; Yang, W.; Wei, X.-F.; Xie, B.-H.; Yang, M.-B. Enhanced Formation of Stereocomplex Crystallites of High Molecular Weight Poly(L-lactide)/Poly(D-lactide) Blends from Melt by Using Poly(ethylene glycol). ACS Sustainable Chem. Eng. 2014, 2, 2301-2309. 58. Xu, H.; Yang, X.; Xie, L.; Hakkarainen, M. Conformational Footprint in Hydrolysis-Induced Nanofibrillation and Crystallization of Poly(lactic acid). Biomacromolecules 2016, DOI: 10.1021/acs.biomac.5b01636. 59. Huang, Y.; Chang, R.; Han, L.; Shan, G.; Bao, Y.; Pan, P. ABA-Type Thermoplastic Elastomers Composed of Poly(ε-caprolactone-co-δ-valerolactone) Soft Midblock and Polymorphic Poly(lactic acid) Hard End blocks. ACS Sustainable Chem. Eng. 2016, 4, 121-128. 37 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 52

60. Wang, H.; Qiu, Z. Crystallization Kinetics and Morphology of Biodegradable Poly(L-lactic acid)/Graphene Oxide Nanocomposites: Influences of Graphene Oxide Loading and Crystallization Temperature. Thermochim. Acta 2012, 527, 40-46. 61. Xu, H.; Xie, L.; Hakkarainen, M. Beyond a Model of Polymer Processing-Triggered Shear: Recounciling Shish-Kebab Formation and Control of Chain Degradation in Sheared Poly(L-lactic acid). ACS Sustainable Chem. Eng. 2015, 3, 1443-1452. 62. Xiong, Z.; Yang, Y.; Feng, J.; Zhang, X.; Zhang, C.; Tang, Z.; Zhu, J. Preparation and Characterization of Poly(lactic acid)/Starch Composites Toughened with Epoxidized Soybean Oil. Carbohydr. Polym. 2013, 92, 810-816. 63. Yuan, D.; Chen, Z.; Xu, C.; Chen, K.; Chen, Y. Fully Biobased Shape Memory Material Based on Novel Cocontinuous Structure in Poly(lactic acid)/Natural Rubber TPVs Fabricated via Peroxide-Induced Dynamic Vulcanization and in Situ Interfacial Compatibilization. ACS Sustainable Chem. Eng. 2015, 3, 2856-2865. 64. Ma, P.; Xu, P.; Zhai, Y.; Dong, W.; Zhang, Y.; Chen, M. Biobased Poly(lactide)/ethylene-co-vinyl Acetate Thermoplastic Vulcanizates: Morphology Evolution, Superior Properties, and Partial Degradability. ACS Sustainable Chem. Eng. 2015, 3, 2211-2219. 65. Dong, W.; He, M.; Wang, H.; Ren, F.; Zhang, J.; Zhao, X.; Li, Y. PLLA/ABS Blends Compatibilized by Reactive Comb Polymers: Double Tg Depression and Significantly Improved Toughness. ACS Sustainable Chem. Eng. 2015, 3, 2542-2550.


Page 39 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


66. Cao, C.; Daly, M.; Chen, B.; Howe, J. Y.; Singh, C. V.; Filleter, T.; Sun, Y. Strengthening in Graphene Oxide Nanosheets: Bridging the Gap between Interplanar and Intraplanar Fracture. Nano Lett. 2015, 15, 6528-6534. 67. Chen, L.; Qiu, X.; Deng, M.; Hong, Z.; Luo, R.; Chen, X.; Jing, X. The Starch Grafted Poly(L-lactide) and the Physical Properties of Its Blending Composites. Polymer 2005, 46, 5723-5729. 68. Yang, X.; Finne-Wistrand, A.; Hakkarainen, M. Improved Dispersion of Grafted Starch Granules Leads to Lower Water Resistance for Starch-g-PLA/PLA Composites. Compos. Sci. Technol. 2013, 86, 149-156. 69. Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902-907. 70. Xu, H.; Xie, L.; Chen, J.-B.; Jiang, X.; Hsiao, B. S.; Zhong, G.-J.; Fu, Q.; Li, Z.-M. Strong and Tough Micro/Nanostructured Poly(lactic acid) by Mimicking Multifunctional Hierarchy of Shell. Mater. Horiz. 2014, 1, 546-552. 71. Evans, C. M.; Singh, M. R.; Lynd, N. A.; Segalman, R. A. Improving the Gas Barrier Properties of Nafion via Thermal Annealing: Evidence for Diffusion through Hydrophilic Channels and Matrix. Macromolecules 2015, 48, 3303-3309. 72. Tsuji, H.; Tsuruno, T. Water Vapor Permeability of Poly(L-lactide)/Poly(D-lactide) Stereocomplexes. Macromol. Mater. Eng. 2010, 295, 709-715.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 52

73. Fujie, T.; Kawamoto, Y.; Haniuda, H.; Saito, A.; Kabata, K.; Honda, Y.; Ohmori, E.; Asahi, T.; Takeoka, S. Selective Molecular Permeability Induced by Glass Transition Dynamics of Semicrystalline Polymer Ultrathin Films. Macromolecules 2013, 46, 395-402. 74. Huang, H.-D.; Ren, P.-G.; Xu, J.-Z.; Xu, L.; Zhong, G.-J.; Hsiao, B. S.; Li, Z.-M. Improved Barrier Properties of Poly(lactic acid) with Randomly Dispersed Graphene Oxide Nanosheets. J. Membr. Sci. 2014, 464, 110-118. 75. Pinto, A. M.; Cabral, J.; Tanaka, D. A. P.; Mendes, A. M.; Magalhães, F. D. Effect of Incorporation of Graphene Oxide and Graphene Nanoplatelets on Mechanical and Gas Permeability Properties of Poly(lactic acid) Films. Polym. Int. 2013, 62, 33-40. 76. Chang, J. H.; An, Y. U.; Sur, G. S. Poly(lactic acid) Nanocomposites withVarious Organoclays.I. Thermomechanical Properties, Morphology, and Gas Permeability. J. Polym. Sci., Polym. Phys. 2003, 41, 94-103.


Page 41 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only Immobilized Graphene Oxide Nanosheets as Thin but Strong Nano-Interfaces in Biocomposites Huan Xu,†,§ Lan Xie,‡,§ Duo Wu,† and Minna Hakkarainen*,†

Synopsis: Exfoliated immobilized GO nanosheets function as strong nano-interfaces in PLA/starch composites, making a key contribution to mechanical and barrier properties.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. A facile solution processable route to prepare biocomposites with strong nano-interfaces. GO powders were initially sonicated in ethanol to exfoliate and extend the flakes. The dispersions were then introduced into starch/ethanol solution to allow the localization and immobilization of GO nanosheets onto starch. The GO@starch particles were compounded with PLA by solution mixing, followed by rapid evaporation and compression molding to PLA/GO@starch films (thickness of 200 µm) with GO@starch contents varying from 10 wt % to 20 wt % and 30 wt %. The digital photo shows high flexibility and light color for the composite film even with the highest filler loading. Traditional PLA/starch composites without addition of GO were prepared using the same method. 210x50mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 42 of 52

Page 43 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Direct observation of GO and GO@starch. (A) TEM micrograph of an individual GO nanosheet in ethanol (0.05 mg/ml), suggesting that the basal planes of GO were sufficiently extended in the dilute solution. The inset bar graph depicts the size distribution determined by DLS, showing a Gaussian distribution with an average dimension of 1199 nm, in good correlation with TEM observation. (B) SEM images comparing the surface morphologies of pristine starch and GO@starch. Extended, large GO nanosheets were tightly attached onto the starch, and even bonded together neighboring starch granules. (C) TEM micrograph showing two starch particles wrapped and connected by GO nanosheets, displaying the peanut-like appearance. Specifically, (C1) shows that the starch was closely wrapped by extended nanosheets as pointed out by the arrow, (C2) reveals that the large nanosheets acted as a binder to connect the neighboring starch granules, and (C3) indicates the presence of thin layer of nanosheets after planar breakdown using high electron energy. 240x172mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Structural characterization of GO@starch. (A) 2D-WAXD patterns and (B) 1D-WAXD intensity profiles of pure starch and GO@starch. Full exfoliation and uniform dispersion of nanosheets led to the absence of characteristic diffraction peaks of GO. The right shift of diffraction peaks indicated that the attachment of GO sheets slightly lowered the lamellar spacing of starch. (C) Integrated FTIR spectra in the two specific wavenumber ranges of (C1) 3700−2700 cm−1 and (C2) 1900−1500 cm−1. (D) 2D-FTIR imaging patterns comparing the absorbance intensity at wavenumbers of 2927 cm−1 and 1737 cm−1, which were characteristic bands for starch and GO, respectively. Scale bar denotes 100 µm. 253x246mm (156 x 156 DPI)


Page 44 of 52

Page 45 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Immobilization of GO on starch evidenced by EDS microanalysis. (A) Electron images overlaid with color-coded element maps (red for C and green for O). Scale bar denotes 10 µm. (B) EDS spectra showing the atomic weight proportions. The C/O ratio of GO@starch lay between those of GO and starch—and closer to that of starch, indicating the existence of only a few nanosheet layers of GO wrapping the starch. The individual electron images and element maps are presented in Figure S7. 243x121mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Filler dispersion and distribution in PLA composite films. (A) SEM micrographs of cryogenically fractured surfaces implying desirable filler distribution and filler-matrix interactions in PLA/GO@starch30, in clear contrast to the non-uniform dispersion of starch particularly in the vicinity of film surface and poor interfacial interactions observed for PLA/starch30. (B) SEM images examining nanosheet distribution in GO@starch-filled composite films, in which the matrix was etched by washing with dichloromethane. Fewlayer nanosheets were firmly immobilized onto starch particles, without traces of free sheets. 195x183mm (300 x 300 DPI)


Page 46 of 52

Page 47 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. GO distribution in composite films determined by EDS. (A) Electron images overlaid with colorcoded element maps (red for C and green for O), separately scanned over the PLA matrix and fillers. Scale bar denotes 5 µm. (B) EDS spectra showing the atomic weight proportions. In the matrix, PLA/starch30 and PLA/GO@starch30 shared a very similar C/O ratio. The C/O ratio of fillers increased from 1.48 for PLA/starch30 to 1.63 for PLA/GO@starch due to the coating of GO. The individual electron images and element maps are presented in Figure S9. 236x90mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Evaluation of crystalline morphologies of compression-molded composite films. (A) 2D-WAXD patterns and (B) diffraction intensity profiles indicating large crystallinity promotion assisted by GO@starch. (C) SEM images showing the directional growth of PLA lamellae induced by the nanosheet, while no crystalline entities were traced in the normal composite. The amorphous phase was etched in alkaline solution.61 (D, E) DSC traces recorded during steady heating and cooling, respectively. The values of Tg and crystallinity were marked in (D). The presence of GO@starch particles strengthened the interactions with neighboring PLA chains, leading to the rise of Tg, and pushing the crystallinity of PLA up to ~50%. The substantial increase in Tc and ∆Hc was indicative of enhanced melt crystallization with the aid of extended GO. All the enthalpies were normalized based on the PLA contents. (a) PLA/starch10, (b) PLA/GO@starch10, (c) PLA/starch20, (d) PLA/GO@starch20, (e) PLA/starch30, and (f) PLA/GO@starch30. 300x171mm (156 x 156 DPI)


Page 48 of 52

Page 49 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Mechanical properties of composite films. (A) Tensile strength, (B) tensile modulus and (C) elongation at break demonstrating the superior tensile properties of PLA/GO@starch in comparison with PLA/starch. (D) Comparison of tensile strength for modified PLA/starch composites using our method and covalent grafting, including epoxidized itaconic acid (EIA)-g-starch,47 epoxidized cardanol (Epicard)-gstarch,47 combined use of starch and starch-g-PLA (5 wt % and 10 wt %),67 starch-g-poly(ethylene glycol) (PEG),48 maleic anhydride (MA)-g-starch,61 and starch-g-PLA.68 (E) SEM images of fracture surfaces after tensile failure suggesting strong interfacial bonding and even formation of numerous ligaments in PLA/GO@starch films. 470x342mm (156 x 156 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Potential formation mechanism for the nanofibrillar ligaments in PLA/GO@starch composites. (A) SEM micrographs of fracture surfaces of PLA/GO@starch10 after tensile failure, and local observation of GO@starch particles revealing the existence of numerous wrinkles (a1) and the high elasticity and firm adhesion of nanofibrils rooted at starch surface (a2). (B) Schematic description of formation mechanism of nanofibrillar ligaments during tensile deformation. On the surfaces of GO@starch, compact PLA lamellae were generated, serving as numerous cross-linkers to bond neighboring chains and to render the formation of deformation-induced nanofibrils consisting of stretched chains and oriented lamellae. 228x163mm (300 x 300 DPI)


Page 50 of 52

Page 51 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Performance evaluation to demonstrate the multiple property enhancement for PLA/GO@starch. (A) TGA curves manifesting the substantial promotion of thermal stability achieved by the incorporation of GO. The inset cartoon depicts preferential heat transfer through the interconnected nano-interfaces, while the inset table lists the values of Tonset and Tmax. (B) PO2 values demonstrating the reduced gas permeation after addition of starch, which was further pushed down by GO nano-interfaces. (C) Schematic illustration of oxygen molecules following a relatively easy path through PLA/starch and a tortuous path through PLA/GO@starch. The construction of compact high-barrier nano-interfaces—combined with the increased crystallinity and enhanced interfacial adhesion—was assumed to decrease the solubility and diffusion of gas molecules, which explained the elevated resistance to gas permeation in PLA/GO@starch composites. (D) Comparison of percentage decline of PO2 as a function of the concentration of sheet-like nanofillers. Showing prominent enhancement in barrier properties at very low GO concentrations, the nanostructured PLA/GO systems stand out among the PLA composites based on GO,74 graphene,75 and montmorillonite (C16-MMT). 76 (E) Comparison of PO2 and raw material cost between PLA/GO@starch films and commercial films existing in the market. Taking into consideration the trace amount of GO (up to 0.03 wt %), the cost of GO was not included. 224x168mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Exfoliated immobilized GO nanosheets function as strong nano-interfaces in PLA/starch composites, making a key contribution to mechanical and barrier properties. 205x94mm (300 x 300 DPI)


Page 52 of 52

Immobilized Graphene Oxide Nanosheets as Thin ... - ACS Publications

Recommend Documents