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Zero-Dimensional and Highly Oxygenated Graphene Oxide for Multifunctional Poly(lactic acid) Bionanocomposites Huan Xu, Karin H Adolfsson, Lan Xie, Salman Hassanzadeh, Torbjörn Pettersson, and Minna Hakkarainen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01524 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 8, 2016

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ACS Sustainable Chemistry & Engineering

Zero-Dimensional and Highly Oxygenated Graphene Oxide for Multifunctional Poly(lactic acid) Bionanocomposites Huan Xu,†,‡ Karin H. Adolfsson,† Lan Xie,§ Salman Hassanzadeh,† Torbjörn Pettersson,† and Minna Hakkarainen*,†

†

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Stockholm 100

44, Sweden

‡

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

Engineering, Sichuan University, Chengdu 610065, China

§

Department of Polymer Materials and Engineering, College of Materials and Metallurgy, Guizhou

University, Guiyang 550025, China

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

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ABSTRACT: The unique strengths of 2D graphene oxide nanosheets (GONSs) in polymer composites are thwarted by nanosheet agglomeration due to strong intersheet attractions. Here we reveal that shrinking the planar size to 0D graphene oxide quantum dots (GOQDs), together with the intercalation of rich oxygen functional groups, reduces filler aggregation and enhances interfacial interactions with the host polymer. With poly(lactic acid) (PLA) as a model matrix, atomic force microscopy colloidal probe measurements illustrated that a triple increase in adhesion force to PLA was achieved for GOQDs (234.8 nN) compared to GONSs (80.4 nN), accounting for the excellent exfoliation and dispersion of GOQDs in PLA, in contrast to the notable agglomeration of GONSs. Although present at trace amount (0.05 wt %), GOQDs made significant contribution to nucleation activity, mechanical strength and ductility, and gas barrier properties of PLA, which contrasted the inferior efficacy of GONSs, accompanied by clear distinction in film transparency (91% and 50%, respectively). Moreover, the GOQDs with higher hydrophilicity accelerated the degradation of PLA by enhancing water erosion, while the GONSs with large sheet surfaces gave a higher hydrolytic resistance. Our findings provide conceptual insights into the importance of the dimensionality and surface chemistry of GO nanostructures in the promising field of bionanocomposites integrating high strength and multifunction (e.g., enhanced transparency, degradation and gas barrier).

KEYWORDS:

Graphene

oxide,

Surface

functionalization,

Biobased,

Nanocomposite,

Multifunctional, Degradation


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INTRODUCTION Two-dimensional (2D) graphene oxide nanosheets (GONSs), offering a rich set of unique electrical, optical, thermal and mechanical properties,1 benefit the development of advanced structural and functional materials for emerging applications (e.g., electrodes for energy-storage devices, nano- and large-area electronics and opto-electronics, bacterial removal, and CO2 capture).2– 5

Incorporation of GONSs into polymer matrices for composite applications represents a

straightforward route to harness the strengths of nanostructured sheets opening up the possibilities for fabrication of multifunctional composites.6–10 The high specific surface area of GONSs (with a BET-measured value of over 600 m2 g–1)11 allows the creation of “nano-barrier walls” to resist gas diffusion;12 facilitates the nucleation of polymer chains at both the surfaces and edges of nanosheets;13,14 enables significant promotion in mechanical response,15,16 and even provides active platforms for efficient polymerization.17 The large surface area, on the other hand, causes challenges for exfoliation and dispersion of GONSs in the polymeric matrices.18 As an example, high concentrations of reduced GONSs (up to 7 wt %) led to a decline as high as 60% in tensile strength of polyetherimide composites, presumably due to unfavorable filler agglomeration that generated high stress concentrations.19 From the geometrical point of view, it is the 2D character of GONSs that extends the contact area between adjacent nanosheets, creates strong in-plane interaction through intersheet van der Waals forces (π−π stacking) and finally, drives the stack of numerous nanosheets into layered monoliths.20,21 This mechanism explains the re-aggregation of GONSs dispersed in liquid phase unless polymer stabilizers or surfactants are present.22 Within this context, shrinking the basal planes 3 ACS Paragon Plus Environment


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of the nanosheets along two dimensions until transformation to zero-dimensional (0D) graphene oxide quantum dots (GOQDs), which display an extremely low size ranging from a few nanometer to tens of nanometer, could be a promising strategy to decrease the intersheet attraction so as to reduce the driving force for nanosheet stacking.23 This strategy is elaborated by the approach to increase the distance between graphene nanosheets in all three dimensions after structural transformation to large quantum dots.24 Particularly, the quantum-confined excitons of graphene quantum dots point to the potential of strong coupling of electronic and photonic modes, with appealing implications for quantum information processing, smart optoelectronic devices, biological imaging, and ultrasensitive sensors.25–30 Integrating the strength and flexibility of polymer matrices with the functional properties of quantum dots could allow fabrication of composites with enhanced performance and multifunctionality. This is exemplified e.g. by the GOQD-enabled enhancement of mineralization in stimulated body fluids and crystallization control for poly(ε-caprolactone).31 Recent research shows large promise for this novel class of polymer nanocomposites including applications in luminescent films,32 fluorescent detection of paranitrophenol,33 tunable luminescence and electrical hysteresis behavior,34 supercapacitor electrodes,35 mechanical-to-electrical conversion,36 and solar cells.37 The prospect of GOQDs in composite application is further enhanced by the emergence of facile and efficient methods to prepare biomass-derived GOQDs.38–41 Assuming that the lower dimensionality pushes up the edge-to-surface ratio to suppress intersheet aggregation, GOQDs hold distinct advantages over GONSs in terms of the viability for homogeneous composite fabrication.18 This mechanism could be further enhanced by intercalation of oxygen functional groups to expand intersheet distance, readying easy exfoliation of GOQDs in mild 4 ACS Paragon Plus Environment

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solvents.41 Following this conception, synthetic strategy for effectively transforming biopolymers or waste products to 0D GOQDs featuring ultrasmall dimensions (below 50 nm) and high oxygenation (e.g., hydroxyl and carboxyl functionalization) was established based on the microwave-assisted hydrothermal method.42,43 We examined the possibility to achieve multifunctional integration by introducing GOQDs into poly(lactic acid) (PLA)—the most representative model among the bioplastic candidates,44 with counterparts composited with traditional 2D GONSs that were produced from graphite using the modified Hummer’s method.14 To examine our hypothesis, we scaled the interaction levels between the two graphene oxide (GO) nanostructures and the host matrix, by virtue of direct adhesion force determination with colloidal probe technique and indirect spectroscopy characterization.45,46 This allowed us to reveal the underlying mechanisms for the significant morphological and performance distinctions in the two composite systems, ranging from the filler uniformity to the crystallization behavior, and mechanical, barrier and hydrolytic properties. These insights may assist fabrication of homogeneous, high-performance nanocomposites through dimensionality manipulation and surface functionalization for GO nanostructures, both of which are in favor of nanosheet−polymer interaction improvements.

EXPERIMENTAL SECTION Materials. A paper hand towel (Katrin Basic, Sweden) was used as the cellulose source. Commercial graphite powder under the trade name of SP-1 was supplied by Bay Carbon, USA. PLA under the trade name of 4032D, being characterized by a weight-average molecular weight of 11.9 × 104 g/mol and a number-average molecular weight of 6.6 × 104 g/mol, respectively, was procured



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from NatureWorks (USA). All chemical reagents in analytical grade were obtained from VWR, Germany. Preparation of GONSs and GOQDs. Starting from the graphite powder, GONSs were prepared via a modified Hummer’s method.14 As described in Figure 1A, graphite powder (10 g) and sodium nitrate (5 g) were initially mixed with 98 wt % sulfuric acid (230 ml) in a round-botttom flask in an ice bath using magnetic stirring. While maintaing the temperature below 20 °C, potassium permanganate (30 g) was slowly added. After reaction at 35 ± 2 °C for 0.5 h, deionized water (460 ml) was gradually added. The bath was heated to 98 °C for another 40 min to increase the oxidation degree of GONS product. The resulting bright-yellow suspension was diluted and further treated with a H2O2 solution (30 ml, 30%), followed by alternative centrifugation and careful washing to remove the excess of salt. The wet GONS powder was dried in a vacuum oven (50 °C). A fast, efficient and low-cost route was used to turn the cellulose-rich paper to the GOQDs by the microwave-assisted strategy developed in our group (Figure 1B).42,43 The paper (10 g) was soaked in dilute H2SO4 solution (0.01 g/mL), followed by direct submission to microwave heating for 2 hours (SynthWAVE, Milestone Inc., USA). The microwave reaction was conducted with a preset temperature of 180 °C and a pressure of 40 bars. The solid black carbon spheres were filtrated from the solution and washed with H2O (20 ml) to remove any remaining H2SO4. This was followed by oxidation of the carbon spheres in 70% HNO3 (~90 ml), first during 0.5 h of sonication at 45 °C and then 0.5 h at 90 °C with magnetic stirring to produce the small graphene oxide quantum dots according to previously reported protocol.43 The oxidized suspension was diluted and cooled down by adding 50 ml of cold H2O, followed by a fast rotary evaporation of acidic H2O in an oil bath at 6 ACS Paragon Plus Environment

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80 °C until reaching a pH of 6−7. Orange-red GOQD powder was obtained after drying the distilled suspension in vacuum at room temperature. Figure 1C compares the time consumption to prepare the two GO nanostructures. Microwave-assisted transformation of biomass signifies a green approach for the preparation of nanocarbon products with a largely shortened production cycle.

Figure 1. Preparation of GO nanostructures. (a) Schematic description of the modified Hummer’s method to prepare GONSs. (b) Microwave-assisted hydrothermal technique to prepare highly oxidized GOQDs. (c) Comparison of time consumption for the proposed two methods to prepare GO nanostructures.

Preparation of PLA Composite Films. The solution coagulation method was used to prepare the PLA composites containing 0.05 wt % GONSs and GOQDs, respectively (named GONS0.05 and GOQD0.05). Specifically, ultrasonically dispersed GONSs and GOQDs in ethanol were dropped in to the PLA/dichloromethane (VWR, Germany) solution. The coagulated composite then gradually precipitated from the solution. After complete drying, the coagulations were compression molded 7 ACS Paragon Plus Environment


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into films (~200 µm) at 200 ºC under fixed pressure of 5 MPa. Pure PLA was subjected to the same processing to make a control sample. Hydrolytic Degradation. The hydrolysis temperature was set at 60 °C to catalyze the degradation rate to reach distinguishable degradation patterns. Deionized H2O (VWR, Germany) was used as the hydrolysis medium for easy identification of degradation products by ESI-MS. Specifically, circular composite films with a diameter of 1 cm and an approximate weight of 25 mg were hydrolyzed in a temperature regulated oven in sealed vials containing 20 mL of H2O for up to 120 days. At predetermined time intervals, triplicate samples were taken from the test environment for mass loss, morphology and structure characterization.47 Transmission Electron Microscopy (TEM) Observation. The morphologies of nanofillers dispersed in ethanol and PLA matrix were examined using TEM. Droplets of the ethanol suspensions containing GONSs and GOQDs with the same concentration of 0.05 mg/mL were deposited onto a lacey carbon film 400 mesh copper TEM grid (Ted Pella, Inc.) and allowed to dry in ambient conditions prior to TEM imaging (Hitachi, 80 KeV). For the composite samples, ultrathin films with a thickness of 80 nm were obtained using a Leica cryo-ultramicrotome EM UC6 equipped with a diamond knife (Germany). Scanning Electronic Microscopy (SEM) Observation. An SE-4800 SEM (Hitachi, Japan), operating at a low accelerated voltage of 0.5 KeV to avoid high-energy electronic damage, was used to image morphologies for the GOQDs prepared from ethanol solutions (0.05 mg/mL), the atomic force microscopy colloidal probe coated by GOQDs, the crystals in isothermally crystallized composite films by etching the amorphous phase,48 the fracture surfaces after tensile failure, and the 8 ACS Paragon Plus Environment

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film surfaces after hydrolysis. Specifically, the GOQD suspension was dropped onto a glass sheet and dried in vacuum. All the samples were sputter-coated with a 3.5 nm-thick gold layer prior to the SEM observations. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra in the range of 4000−600 cm−1 for GONSs and GOQDs powders and compression-molded composite films were recorded on a PerkinElmer Spectrum 2000 spectrometer (PerkinElmer Instrument) with 16 scans at a resolution of 4 cm−1. Energy Dispersive X-ray Spectrometry (EDS) Microanalysis. The elemental composition for GONSs and GOQDs was mapped on an EDS detector (X-MaxN, Oxford Instruments, UK) linked to the S-4800 SEM. The working voltage was 20 keV, and the samples were directly scanned without coating treatment. Ultraviolet-Visible (UV-Vis) Spectra. The transmittance and absorbance spectra of compress-molded composite films with the same thickness of ~200 µm were determined on a SHIMADZU UV-2550 spectrophotometer (Japan). Polarized Optical Microscopy (POM) Observations. To directly observe the filler dispersion in GONS0.05 and GOQD0.05, thin films with a thickness of ~20 µm were obtained using a Leica EM UC6 microtome and directly observed on an Optiphot 2 microscope equipped with a Leica digital camera. The films after isothermal crystallization at 130, 135 and 140 °C for 60 min on a Mettler FP82HT heating stage were directly taken for POM observation (see temperature protocol in Figure S1, Supporting Information).



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Atomic Force Microscopy (AFM) Topography Characterization. The compression-molded GOQD0.05 film and composite films hydrolyzed for 20, 40, 60, 80 and 100 days were directly taken for AFM characterization. The films adhered onto mica substrates were imaged using a Nanoscope Multimode 8 (Bruker AXS, Santa Barbara, USA) with a type E piezoelectric scanner. Images were acquired in tapping mode using RTESP Si cantilevers (Bruker Probes, Camarillo, USA) with a typical spring constant of 40 N/m. The image processing and roughness evaluation were carried out using the NanoScope Analysis software (Version 1.5). Differential Scanning Calorimeter (DSC) Characterization. The thermal behaviors of compression-molded composite films and hydrolyzed films (5~6 mg) were monitored by a Mettler Toledo DSC 820 under the nitrogen atmosphere (50 ml/min). The samples were steadily heated up to 200 °C and held this point for 5 min to remove thermal history, and finally cooled down to 40 °C. The heating and cooling rate were set at 10 °C/min. Two-Dimensional Wide-Angle X-ray Diffraction (2D-WAXD) Measurements. The crystalline morphologies of compression-molded composite films and melt crystallized films were determined by 2D-WAXD measurements using a home-made laboratory instrument (Bruker NanoStar, CuKα-radiation) in the Crystallography Lab, Department of Molecular Biology and Biotechnology, University of Sheffield. The X-ray beam with a wavelength of 0.154 nm was focused to a tiny area of 4 × 4 µm2, and the distance from sample to detector was fixed at 350 mm. The 2D diffraction patterns were collected by an X-ray CCD detector (Model Mar345, a resolution of 2300 × 2300 pixels, Rayonix Co. Ltd., USA). Following the method described in our earlier work, the diffraction intensity profiles were further processed to acquire quantitative analysis.49,50 10 ACS Paragon Plus Environment

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Nanofiller–Matrix Interaction Measurements. The interfacial adhesion between the nanofillers and pure PLA matrix was measured using the AFM colloidal probe technique,45,46 in which a Nanoscope Ⅲa Picoforce system (Veeco Instrument, Santa Barbara, USA) was used in the contact mode by ramping the modified probe up and down onto the matrix surface. A polystyrene particle was glued onto a tip-less silica cantilever CLFC-NOBO (Bruker, Camarillo, CA) with a typical width of 29 µm, a length of 97 µm, and a typical spring constant of 10.4 N/m, suitable spring constant for the cantilever was selected51 and the real spring constant was calibrated by thermal resonance method.52,53 For surface modification of the AFM probes, the probes were separately immersed in the ethanol suspensions containing GONSs and GOQDs at the same concentration (0.1 g/mL), and they were coated with the nanoparticles after 10 s of deposition. The probes after nanoparticle coating were then dried in vacuum. Note that the neat probe was directly used as a blank control. Performance Evaluation. Under the guidance of the Standard ASTM D3985, oxygen permeability coefficient (PO2) of composite films under various relative humidity was measured on an Oxtran 2/21 ML instrument at 23 °C. Following the ASTM standard D638, tensile testing were performed on an Instron universal test instrument (Model 5944, Instron Instruments, USA) with a load cell of 500 N at 23 °C and relative humidity of 50%. The crosshead speed was set at 5 mm/min and the gauge length was 20 mm. For the tensile and barrier property measurements, a minimum of 6 replicates for each sample were tested to obtain the average values with standard deviation.

RESULTS AND DISCUSSION



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Morphology and Surface Chemistry of GO Nanostructures. Figure 2 examines the morphological differences between GONSs and GOQDs, illustrating the distinction in dimensionality for these two classes of GO nanostructures. GONSs showed a typical 2D character with ultrathin sheets expanding up to a few micrometers (Figure 2a). A single-layer GONS was characterized by a thickness of approximately 1 nm, whereas strong intersheet attraction normally drove the in-plane stacking between several adjacent nanosheets (Figure S2). Figure 2b,c shows that the cellulose fibers from the paper were effectively transformed to dot-like nanostructures during the microwave and subsequent oxidation process. In ethanol, these GOQDs were uniformly dispersed and well extended, yielding the favorable exfoliation into individual dots (Figure 2d,e). The structure of GOQDs can be considered as a nanosized GO with varying stacking layers,31 as revealed by the size distinction between the diameter (around 50 nm) and height (up to 8 nm) of individual GOQDs (Figure 2f,g). It suggests that the GOQDs were, in essence, assembled by few-layer GO with extremely low planar dimension.27 Figure 2h indicates that the GOQDs were characterized by a narrow size distribution and an average diameter of 46 nm.


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Figure 2. Direct observation of GONSs and GOQDs. (a, b) TEM images of GONSs and GOQDs, and (c) shows the local observation for (b) at higher magnification. (d) SEM micrographs of GOQDs implying the uniform size distribution of individual GOQDs. The nanodots were possibly re-aggregated during the drying on the glass sheet. (e, f) 2D and 3D view of AFM height image of GOQDs, which produced (g) height profile measured along the green line in (e). (h) Diameter distribution of GOQDs based on TEM observation, yielding an average diameter of 46 nm. All samples were prepared from the GO/ethanol dispersions at the same concentration (0.05 mg/mL) after ultrasonication of 0.5 h.

The dimensionality distinction was accompanied by evident modification of surface chemistry in GOQDs, as demonstrated by FTIR measurements and EDS microanalysis (Figure 3a−c). In the FTIR spectra (Figure 3a), GONSs showed a sharp absorption at 3424 cm−1 due to the existence of free −OH,54 and the characteristic band at 1714 cm−1 with a moderate intensity assigned to the C＝O stretching vibration of −COOH units situated at the edges.14 Clearly, GOQDs exhibited much higher density of oxygen functional groups, as illustrated by the prominent, broad peak at the −OH stretching region (indicative of H-bonding), together with the strong absorptions occurring around 13 ACS Paragon Plus Environment


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1705 cm−1 and 1187 cm−1 ascribed to the existence of C＝O and C−O, respectively. This is mainly explained by the excessive oxidation during the acid treatment, enabling the incorporation of plentiful hydroxyl and carbonyl groups connected to the carbon atoms at the edges and surfaces of GOQDs.43 The chemical composition was likely further modified by the large edge-to-surface ratio of the nanodots, which increased the amount of oxygen-rich functional groups located at the edges, leading to an evident rise of O proportion at the surfaces of GOQDs compared to GONSs (Figure 3b,c and S3). Benefiting from the small size and high oxygenation, GOQDs presented rapid dispersion in ethanol, in sharp contrast to the poor dispersibility of GONSs (Figure 3d). Solution processability of nanofillers is an important factor when targeting homogeneous composites, conferring exfoliation and functionalization of nanofillers in appropriate solvents for composite fabrication.55–57 The excellent solution processability of GOQDs can, therefore, be perceived as an advantage over GONSs in the pursuit of homogeneous nanostructure-filled composites without using additional stabilizers such as surfactants.57


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Figure 3. Chemical composition of GO nanostructures. (a) FTIR spectra of GONSs and GOQDs showing a higher density of oxygen functional groups in GOQDs. EDS microanalysis for (b) GONSs and (c) GOQDs. An electron image showing the surface morphology, together with element maps for red-coded C and green-coded O, is presented for each sample. The atomic weight proportions of C and O are marked. Scale bar denotes 50 µm for all micrographs. (d) Digital photos of GONSs and GOQDs dispersed in ethanol (0.2 mg/mL) before and after ultrasonication, illustrating the improved solvent exfoliation for GOQDs.

Morphologies of GO Nanostructures in Composite Films. Figure 4a shows that pure PLA and GOQD0.05 presented similar optical properties, while GONS0.05 displayed lower transparence and darker color (see Figure S7 for quantitative spectroscopic measurements). The large differences in the optical properties between GOQD0.05 and GONS0.05 probably lie in the dispersion of the GO nanostructures. A number of small aggregates were observed in GONS0.05, in clear contrast to the inexistence of notable filler traces in GOQD0.05 (Figure 4b−e). Figure 4c reveals that the nanosheets were prone to aggregate into large entities by surface attraction and reached the size over tens of micrometers, making it difficult to recognize the layer structure. From the geometric point of view, the point-point contact between adjacent GOQDs contributed to a reduced attraction area, giving a more complete exfoliation and dispersion in PLA (Figure 4e,f). This could be assisted by generation of remarkable bond ligaments between the oxygen functional groups of GOQDs and PLA chains, creating a percolated domain of interphase that effectively enhanced the affinity of GOQDs to the host matrix.18 Both the TEM and AFM observations arrived at the assertion that the dimension of GOQDs in the composites exhibited a wide distribution ranging from tens of nanometers to ~200 nanometer—likely nanospheres assembled from few GOQDs (Figure 4e,f). Figure 4g manifests that the size of GOQDs presented a Gaussian distribution, yielding an average diameter of 90.2 nm. This 15 ACS Paragon Plus Environment


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value was approaching 2 times the size of individual GOQDs (Figure 2h), which offered a direct evidence for the proper dispersion of GOQDs in the PLA matrix. In the interest of designing and fabricating PLA nanocomposites, the essential considerations are not only the individual properties of nanofillers but also the assembled microstructures in the matrix as well as the interactions in between, in particular at high nanofiller contents. The easy manipulation on the dispersion of GOQDs allows the assertion that high concentrations of nanostructured building blocks could be homogeneously incorporated in the polymer matrices, in contrast to the unfavorable large agglomeration of concentrated GONSs.

Figure 4. Filler morphology in PLA composite films. (a) Digital photos of composite films with a plant flower in the background. POM images illustrating (b) the local agglomeration of GONSs in the host matrix, in contrast to (d) the proper dispersion of GOQDs. TEM micrographs showing (c) the aggregated nanosheets and (e) the relatively well dispersed nanodots in the composite films. The inset cartoons describe the “face contact” aggregation mode for GONSs and the “point contact” assembly mode for GOQDs. (f) AFM height image showing the morphology of GOQDs in the film, which was used to acquire (g) statistical analysis for the distribution of particle diameter, yielding an average value of 90.2 nm.


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Crystalline Morphology of PLA Induced by GO Nanostructures. Nanofiller-enabled improvements in polymer crystallization kinetics are of importance, not only because they yield insights into the fundamental changes in chain dynamics,58,59 but also because the associated gains in thermal and mechanical properties are critical for diverse applications.60,61 Figure 5a−c compares the crystalline morphologies for compression-molded composite films containing GONSs and GOQDs. During the steady heating traces, prominent exothermic peaks occurring at around 105 °C were exclusively observed for pure PLA and GONS0.05, likely due to the inferior crystallization ability (Figure 5a).62,63 In contrast to the low crystallinity for pure PLA and GONS0.05 (~5%), GOQD0.05 featured a substantially increased value up to 47.9%. During the melting of GOQD0.05, the generation of a prominent peak at the lower temperature offered evidence for the preferential heterogeneous nucleation of PLA with the aid of GOQDs, resulting in PLA crystals of higher density but relatively less order.20 The GOQD-assisted crystallization was well described by the melt crystallization behavior during the gradual cooling from the melts, accounting for the dramatically enlarged exothermic peaks (Figure 5b). The DSC results were supported by the 2D-WAXD measurements showing GOQD-enhanced diffraction intensities (Figure 5c and S8).64,65

Isothermal crystallization protocol was used to examine the underlying crystallization mechanisms induced by GONSs and GOQDs (Figure 5d,e). Figure 5d indicates that both GONSs and GOQDs were ready to provide active nucleating sites for pure PLA, conferring PLA with enhanced nucleation density. The density of nuclei developed in pure PLA was 1.8, 1.0 and 0.8 × 10–5 µm–2 at 130, 135 and 140 °C, respectively, which was increased to 3.0, 2.0 and 1.6 × 10–5 µm–2 for GONS0.05, and further promoted to 6.1, 3.2 and 2.8 × 10–5 µm–2 for GOQD0.05. The facilitated 17 ACS Paragon Plus Environment


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nucleation was responsible for the larger size of spherulites formed at high temperatures that suppressed self-nucleation of PLA, as exemplified by the evident rise of average radius climbing from 62 µm for pure PLA crystallized at 140 °C to 135 µm for GONS0.05 and 148 µm for GOQD0.05. Figure 5e and S9 offers quantitative insights into the crystal structure of PLA tailored by the addition of GO nanostructures, displaying increased diffraction intensities of α-form PLA crystals in comparison with those of pure PLA.65

The low resistance to heat deformation and gas/water permeation of PLA materials is, to a large extent, associated with the intrinsically poor crystallization ability.66 This lays down special challenges for the development of high-performance PLA, particularly by industrially feasible manufacturing. During conventional compression molding, the addition of GOQDs conferred sufficient control on the crystalline morphology, outperforming the GONSs that have shown potential promise to reinforce PLA.67,68 This is promising as the control of crystalline morphology represents a critical challenge for the development of high-performance PLA, with regard to the enhancement in heat resistance and barrier properties. Currently available nanofillers, such as nanoclay, nanotubes and graphene, have been tested to improve the crystallization kinetics of PLA.68–70 Normally this generates a need of surface modification for the nanofillers to enhance the interfacial interactions and to facilitate the nanofiller dispersion.71–73 Our results may shed lights into the importance of dimensionality and surface chemistry of nanostructured filler candidates for intimate interactions with polymer matrix.


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Figure 5. Crystalline morphologies induced by GO nanostructures. (a) DSC heating traces, (b) DSC cooling curves, and (c) 2D-WAXD patterns of compression-molded composite films, indicating the highly enhanced crystallization dynamics of PLA with the aid of GOQDs. (d) POM images demonstrating that GOQDs exhibited higher nucleating ability for PLA compared to GONSs. (e) 2D-WAXD patterns and (f) diffraction intensity profiles indicating increased diffraction intensities of PLA after addition of GO nanostructures, in which GOQD0.05 showed the highest crystallization ability. (D) Crystallinity as a function of crystallization temperature. Compared to GONSs, GOQDs showed a higher efficacy in promoting the crystallinity of PLA, especially at high temperatures (135 and 140 °C).

Figure 6 reveals the lamellar textures after isothermal crystallization. Figure 6a,a1 denotes that normal spherulites induced by the central nuclei were developed in pure PLA. Of interest is the irregular development of lamellae induced by GONSs and GOQDs (Figure 6b,c). The GONS-induced crystalline entities were ellipsoids assembled from random lamellae, rather than 19 ACS Paragon Plus Environment


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symmetrically developed spherulites (Figure 6b). In the geometrical center of the ellipsoids, Figure 6b1 reveals that GONSs were profoundly involved in triggering the generation of dense lamellae, as evidenced by the tight linkages between the nanosheets and lamellae. An inspection of Figure 6b2 manifests that the edges of GONSs provided active platforms to anchor and nucleate neighboring PLA chains, creating numerous ligaments between the nanosheets and PLA matrix. These findings supported the assertion proposed in the GONS-filled stereocomplex PLA system,14 assuming that both the basal planes and edges of nanosheets were involved in facilitating the nucleation activity of the matrix. Unexpectedly, a mixture of normal spherulites and dense random lamellae were observed in the crystallized GOQD0.05, displaying the prevailing development of crystalline regions as a result of the pronounced nucleating efficacy of GOQDs (Figure 6c). As for the normal spherulites, Figure 6c1 indicates that the nanodots, which were repelled into the amorphous regions during the development of two neighboring lamellae, seemingly facilitated the generation of these neighboring lamellae. The confined growth of regular spherulites by the impingement of adjacent random lamellae, causing the generation of linear edges in the growing fronts of spherulites (Figure 6c2). Instead of directional growth, these randomly distributed lamellae were characterized by very high density, primarily adopting the random nucleation induced by the homogeneously distributed nanodots (Figure 6c3). The random formation of dense lamellae was therefore probably associated with the high nucleation activity of GOQDs. The unique morphological transition from normal spherulites to compact random lamellae may contribute to the isotropic response to the external stress, as well as the suppression of undesirable propagation of cracks between spherulites due to the absence of interconnecting entities.74,75 20 ACS Paragon Plus Environment

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Figure 6. SEM observation of nucleic and lamellar structures in (a) pure PLA, (b) GONS0.05, and (c) GOQD0.05 after crystallization at 135 °C. Unlike the classic nucleation mechanism in pure PLA, random nucleation of PLA was induced by GONSs, involving (b1) the basal planes and (b2) the edges of GONSs. GOQDs were ready to induce the formation of (c1) normal spherulites and (c2, c3) random lamellae nucleated by GOQDs.

Interaction Properties between GO Nanostructures and PLA. From the interaction point of view, Figure 7 provides fundamental interpretation for the improved dispersion and crystallization kinetics in GOQD0.05. The adhesive behavior of PLA matrix in contact with the specific PS-based AFM tips onto which GONSs or GOQDs were coated showed high relevancy to the function of nanofillers (Figure 7a−c). The schematic illustration of the modified colloidal probe used in the AFM force interaction measurements is shown in Figure 7a, in which the PS sphere was coated by nanosheets or nanodots. Figure 7b,c indicates that the nanoparticles were homogeneously absorbed onto the PS probe, allowing the interfacial interactions between the GO nanostructures and the polymer chains to be measured. Figure 7d plots the typical force–distance curves on separation between the PLA matrix and the PS, or GONS- or GOQD-coated PS probes, and the adhesive 21 ACS Paragon Plus Environment


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interactions were determined by evaluating the maximum force required during separation of the probes from contact with the film. The lowest adhesion force of 25.9 ± 5.1 nN was observed for the pure PS probe, which was increased up to a tripled value of 80.4 ± 6.9 nN for the GONS-modified probe. Of interest is the additional significant enhancement of adhesion observed for the GOQD-modified probe, which exhibited the highest adhesive force of 234.8 ± 43.5 nN—nearly three times the adhesive force measured for GONSs. This distinction is assumed to arise from the two main features in GOQDs governing the interfacial adhesion that are closely associated with the surface chemistry and geometrics of nanofillers: (1) compared to GONSs, GOQDs carry a much higher concentration of functional groups that are ready to excite interactions with PLA chains; (2) from the geometric point of view, the GOQDs are probably featured by larger total interaction area with PLA chains, whereas most of the GONSs in the stacks are not available to interact with the film.76 This explains the generation of evident hydrogen bonding in the GOQD0.05 films, as described by a set of red shifts of ~3 cm‒1 for the characteristic bands of the chain backbone and functional groups (Figure 7e). Given the intimate filler−matrix interactions, the GOQDs were even stabilized in dichloromethane by the surrounding PLA chains through steric effects,77 yielding a homogeneous solution that presented good long-term stability (Figure 7f,g). However, the precipitation of GONSs was clearly observed due to the inferior interactions with PLA chains (Figure 7g).


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Figure 7. Evaluation of filler−matrix interacBons. (a) Schematic illustration of the friction force microscopy method devised to evaluate the interfacial interactions between nanofillers and PLA matrix. (b, c) SEM images showing the uniform coating of GOQDs on the probe. (d) Separation curves for pure PS, GONS-coated and GOQD-coated probes from the same PLA film substrate. (e) FTIR spectra of compression-molded composite films in the range of 4000−2500 cm‒1 (left) and 1500−900 cm‒1 (right). (f, g) Digital photos showing a comparison of dichloromethane solutions of the composites (0.5 g/mL).

Barrier and Mechanical Properties of Composite Films. The enhanced interfacial interactions and sufficient control of filler and crystalline morphologies in GOQD0.05 allow for the assumption of considerable performance enhancements, as examined in Figure 8. Substantial increase in the resistance to gas permeation was observed in GONS0.05 and GOQD0.05, as evidenced by the direct fall of oxygen permeability coefficient (PO2) in the dry state from 25.8 cm3 mm cm–2 day–1 atm–1 for pure PLA to around 14 cm3 mm cm–2 day–1 atm–1 for GONS0.05 and GOQD0.05 (Figure 8A). Upon 23 ACS Paragon Plus Environment


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increasing humidity, the composite films exhibited weakened barrier properties to different degrees. For pure PLA, the introduction of water molecules deteriorated the barrier ability significantly, with the PO2 gradually climbing up to 35.6 cm3 mm cm–2 day–1 atm–1 at the humidity of 50%—an increment of 38% compared to the dried atmosphere. The relative increase of PO2 was even larger for GONS0.05 (69%), reaching a PO2 of 21.4 cm3 mm cm–2 day–1 atm–1 at the humidity of 50%. In contrast, only slight variations of PO2 with the increase of humidity were found for GOQD0.05 (16.8 cm3 mm cm–2 day–1 atm–1 at the humidity of 50%), showing a notable decrease of 112% and 27% compared to the PO2 of pure PLA and GONS0.05, respectively. This distinction is suggested to originate from different underlying mechanisms to resist the gas/water permeation, as tentatively illustrated in Figure 8B,C. In the scenario of dry oxygen, Figure 8B manifests that GONSs serve as “nano-barrier walls” to resist the diffusion of oxygen molecules resulting in 99% decline in the PO2 of PLA films deposited with layer-by-layer assembled GONSs.78 For GOQD-filled PLA, the solubility of oxygen is pronouncedly lowered with the generation of dense PLA lamellae. The water molecules existing in pure PLA probably enlarge the inter- and intra-molecular free volume of PLA, creating more pathways to solubilize and diffuse oxygen/water (Figure 8C).79 As in the case of GONS0.05, the inferior interphase between nanosheets and PLA matrix is preferentially attacked by the penetration of water, which may open “gates” to allow the increased diffusion of oxygen, in analogous to the observation for nanocellulose-based barriers.80 On the contrary, the existence of rich ordered crystalline entities conferred the GOQD0.05 films high resistance to gas/water permeation due to the limited amorphous regions to diffuse, showing weak relation to ambient humidity.79


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On examining the mechanical properties, it was found that the addition of GOQDs favored the promotion of strength and in particular ductility, whereas the GONS0.05 showed comparable strength and decreased elongation, compared to the pure PLA films (Figure 8D). Specifically, both pure PLA and GONS0.05 exhibited poor ductility, displaying low elongation at break of 7.6% and 5.9%, respectively. This was in contrast with the increased elongation recorded for GOQD0.05 (12.1%), accompanied by simultaneously improved tensile strength (63.2 MPa) compared to that of pure PLA and GONS0.05 (around 56 MPa). Figure 8E reveals that typical brittle fracture behavior was observed in both PLA and GONS0.05, showing smooth surfaces with little plastic deformation. Moreover, the aggregated GONS entities showed poor interfacial adhesion with the surrounding matrix, in which enormous crack propagation may be preferentially caused by the stress loading. The creation of poor interphase was the probable cause of inferior tensile properties for GONS0.05. Interestingly, brittle−to−ductile transition was observed in GOQD0.05, showing a large amount of plastic deformation involving the formation of fibrillar assemblies of extended chains. This unexpected transition, in essence, can be ascribed to the two main morphological features carried by GOQD0.05: (1) the appropriate crystallinity (over 40%) rendered the construction of stiff blocks in the crystalline regions, while the amorphous chains can be perceived as tenacious ligaments to provide sufficient ductility, exerting the structural function in analogous to the naturally strong and tough silkworm silk;81 (2) the formation of strong interfacial interactions between GOQDs and the matrix enabled the uniform dispersion of GOQDs and creation of firm polymer–filler ligaments, resulting in desirable reinforcing efficiency rather than preferred local fracture like in aggregated GONSs (Figure S11). 25 ACS Paragon Plus Environment


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Figure 8. Performance evaluation to demonstrate the exceptional multifunction of GOQDs. (A) PO2 as a function of relative humidity indicating the increased oxygen permeation with the existence of water molecules for pure PLA and GONS0.05. However, this relation was largely weakened in GOQD0.05. (B) Schematic explanations for the enhanced barrier properties in nanofiller-reinforced composites. The nanosheets were perceived as “barrier walls” to resist the diffusion of oxygen, while the formation of rich lamellae induced by GOQDs was responsible for the reduced gas solubility. (C) Schematic illustration explaining the different increase degrees of PO2 with the existence of water for composite films. (D) Typical stress–strain curves of composite films. (E) SEM images showing the fracture surfaces of composite films after tensile failure.

Long-Term Degradation Behavior of Composite Films. The acceleration effect of GOQDs, unlike the distinct suppression by GONSs (Figure S12), on the hydrolytic degradation of PLA is appraised from the surface morphology. It is apparent from Figure 9 that surface morphology of hydrolyzed films was altered by the addition of nanofillers, accounting for the variations in degradation rates. Nanofibrillation, through budding and subsequent one-dimensional growth of 26 ACS Paragon Plus Environment

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hydrolysis-induced PLA nanoparticles, was observed on the surface of pure PLA.82 This hypothesis is supported by the rise in the concentration of nanofibers with hydrolysis time. Compact cracks with a length up to tens of micrometers and a width of around 500 nm were observed in pure PLA after 120 days, serving as the direct pathway to induce water penetration. These cracks, however, were not generated on the surfaces of GONS0.05 that were covered by large sheets, resulting in the enhanced resistance to hydrolytic attack and thereby the decreased density of nanofibers and cracks. The presence of GOQDs was found to enhance the water penetration through the surface erosion, triggering the formation of dense wrinkles after 10 days and the shedding of micro-sized fragments after 20 days. The GOQD0.05 hydrolyzed for 120 days exhibited the highest density of cracks compared to pure PLA and GONS0.05, accompanied by the exposure of compact nanodots after the prominent erosion of matrix (Figure S17).



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Figure 9. Typical SEM images showing the surface morphology of composite films hydrolyzed for 10, 20, 80, and 120 days. Nanofibers were generated during the hydrolysis of pure PLA, and the density showed direct relation to degradation time. Nanosheets with high surface area were exposed onto GONS0.05 surfaces as proceeding of matrix erosion. Numerous wrinkles patterned the surfaces of hydrolyzed GOQD0.05, followed by the formation of fragments and cracks with higher densities compared to pure PLA or GONS0.05.

The surface morphology of degraded films was imaged on nanoscale to offer in-depth insights into the nanofiller-induced degradation mechanisms (Figure 10). For pure PLA, Figure 10a shows the formation of nanoparticles after 20 days, which were enlarged and elongated to stimulate the 28 ACS Paragon Plus Environment

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nanofibrillation (D60 and D100). This direct observation supported the conjecture proposed by Akashi et al., in which the PLA nanoparticles assembled from the low-molecular-weight chains and oligomers were assumed to join and grow into nanofibers.82 In essence, it shared the same nanofibrillation mechanism induced by hydrolytic degradation, successively involving the regularization of chain conformation and the self-assembling of adjacent nanospheres of PLA.83 The nanoparticles formed in GONS0.05 were characterized by a decreased size compared to pure PLA, principally resulting from the suppressed degradation rate. The imaged areas were extremely tiny (500 nm × 500 nm), which was responsible for the failure to trace GONSs. On the surfaces of degraded GOQD0.05, a higher concentration of nanoparticles was generated with a uniform distribution of size. These nanoparticles probably originated from GOQDs or degraded PLA. Particularly interesting is the lowest surface roughness (Rq) of around 5.8 nm observed for GOQD0.05, showing weak relation to the degradation time (Figure 10b). The roughness was moderately increased for GONS0.05, and reached the highest level for pure PLA. This observation seems to contradict with the general assumption that the addition of fillers leads to increase in the surface roughness.18 Figure 10c illustrates the hydrolytic mechanisms in an endeavor to understand the unexpected variations of surface roughness. On the surfaces of pure PLA, the water penetration was preferentially induced in the amorphous regions, because crystalline entities show higher resistance to hydrolytic attack.47,84,85 This explains the rise of roughness to 16.8 nm after 60 days (Figure 10b), which was followed by the gradual fall as a result of the eventual erosion of lamellae—verified by decrease in the crystallinity after hydrolysis for 40 days (Figure S12). This effect was weakened with 29 ACS Paragon Plus Environment


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the interference of GONSs, serving as a protective outer layer of the film surface to resist the water penetration. It was thus anticipated that lower hydrolytic erosion would be observed in GONS0.05, and thus decreased roughness. The improved hydrophilicity for GOQDs was assumed to facilitate the water penetration and therefore the hydrolytic degradation. This could have led to the suppression of roughness variations caused by the preferential erosion of amorphous phase. Furthermore, the homogeneous dispersion of GOQDs conferred the uniform surface erosion occurring gradually with proceeding hydrolysis, accounting for the steady roughness level regardless of degradation time.

Figure 10. Degradation mechanisms induced by GO nanostructures. (A) Representative AFM phase (left) and 3D height (right) images showing the surface morphology for degraded films with a scanning area of 500 nm × 500 nm. (B) Surface roughness (Rq) of degraded film from AFM height images, showing the lowest level of ~5.8 nm for GOQD0.05 films irrespective of degradation time. (C) Schematic illustration for the roughness distinction on the hydrolyzed film surfaces, assuming that the amorphous phase of PLA was preferentially attacked, while the GONSs attached onto the surfaces 30 ACS Paragon Plus Environment

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locally weakened water erosion and the hydrophilic GOQDs facilitated the hydrolysis proceeding in a uniform manner.

CONCLUSIONS The relationship between the dimensionality and surface chemistry of GO nanostructures and the function in PLA nanocomposites was established using 2D GONSs with microsized planes and 0D GOQDs with an average dimension of only 46 nm as model compounds. The small dimension, together with high density of oxygen functional groups, not only conferred GOQDs with increased dispersability in common solvents but also afforded intimate interactions with PLA matrix. By means of AFM force measurements, three times higher adhesion force was demonstrated for the PLA/GOQDs system (234.8 nN) in comparison with the PLA/GONSs system (80.4 nN). This explained the greatly facilitated exfoliation and dispersion of GOQDs in composite films, whereas the numerous intersheet attractions led to undesirable aggregation of GONSs reducing the interactions with PLA, accounting for the large distinction between the transparency of GOQD0.05 (91%) and GONS0.05 (50%). The complete exfoliation of ultrasmall GOQDs in PLA, together with the intimate interactions established in between, gave rise to the formation of dense lamellae with random distribution, showing a much higher nucleation efficacy compared to GONSs. In addition to the facilitated crystallization, GOQDs significantly enhanced the tensile and oxygen barrier properties of PLA, in contrast to the inferior contribution from GONSs. Furthermore, the tensile strength (63.2 MPa) and elongation at break (12.1%) of GOQD0.05 were 13% and 105% higher than those of GONS0.05, respectively, being accompanied by a lower dependency of moisture on resisting oxygen permeation. Of interest is the moderately increased degradation rate of PLA by the 31 ACS Paragon Plus Environment


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more hydrophilic GOQDs that facilitated water attack, whereas the relatively more hydrophobic GONSs with large sheet surfaces on film surfaces provided enhanced hydrolytic resistance. Nanoscale imaging of the hydrolyzed surfaces afforded the underlying degradation mechanisms from the perspective of surface roughness. This work, by strategically manipulating the dimensionality and intercalating functional groups for GO nanostructures, paves the way to homogeneous bionanocomposites with tunable optical, mechanical and barrier properties.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Temperature protocol for isothermal crystallization, AFM observation of GONSs, EDS spectra of GONSs and GOQDs, digital photo of composite coagulations, SEM image of neat PS AFM probe, TGA curves of QONSs and GOQDs, UV-vis spectra of compression-molded films, WAXD intensity profiles of compression-molded and isothermally crystallized composites, SEM images of lamellar textures in composites, SEM observation of fracture surfaces, mass residue, thermal parameters and pH values recorded during hydrolysis, DSC heating and cooling profiles of degraded composites, SEM images and 2D AFM height images of hydrolyzed surfaces (PDF)

AUTHOR INFORMATION Corresponding Author


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*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. is grateful to the financial support from the China Scholarship Council (CSC) for studying abroad. The Swedish Research Council (VR) is acknowledged for the financial support (contract grant number 2014-4091).

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85. Andersson, S. R.; Hakkarainen, M.; Inkinen, S.; Södergård, A.; Albertsson, A.-C. Customizing the Hydrolytic Degradation Rate of Stereocomplex PLA through Different PDLA Architectures. Biomacromolecules 2012, 13 (4), 1212-1222.



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For Table of Contents Only

Zero-Dimensional and Highly Oxygenated Graphene Oxide for Multifunctional Poly(lactic acid) Bionanocomposites Huan Xu,†,‡ Karin H. Adolfsson,† Lan Xie,§ Salman Hassanzadeh,† Torbjörn Pettersson,† and Minna Hakkarainen*,†

Synopsis: A combination of dimensionality shrinkage and oxygen functionalization enables new possibilities for graphene oxide in the field of biobased nanocomposites.


Zero-Dimensional and Highly Oxygenated ... - ACS Publications

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