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Improving Dispersion and Barrier Properties of Polyketone/Graphene Nanoplatelet Composites via Non-Covalent Functionalization using Aminopyrene Jaehyun Cho, Ikseong Jeon, Seong Yun Kim, Soonho Lim, and Jae Young Jho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10474 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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ACS Applied Materials & Interfaces

Improving Dispersion and Barrier Properties of Polyketone/Graphene Nanoplatelet Composites via Non-Covalent Functionalization using Aminopyrene Jaehyun Cho,†, ‡,# Ikseong Jeon,†,# Seong Yun Kim, ‡ Soonho Lim, ‡ and Jae Young Jho,*,† †

School of Chemical and Biological Engineering, Seoul National University, Seoul 08826,

Korea. ‡

Multifunctional Structural Composite Research Center, Korea Institute of Science and

Technology, Jeonbuk, 565-905, Korea. #

J. Cho and I. Jeon contributed equally to this work.

*E-mail: (J.Y.J) [email protected]

KEYWORDS: graphene nanocomposite, non-covalent functionalization, dispersion, interfacial interaction, mechanical property, barrier property, 3D micro-CT

ACS Paragon Plus Environment

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ABSTRACT A series of polyketone (PK) nanocomposite films with varying content of non-covalently functionalized graphene nanoplatelet with 1-aminopyrene (GNP/APy) are prepared by solution blending with a solvent of hexafluoro-2-propanol. GNP/APy, prepared by a facile method, can effectively induce specific interaction such as hydrogen bonding between the amine functional group of GNP/APy and the carbonyl functional group of the PK matrix. With comparison of GNP and GNP/Py as reference materials, intensive investigation on filler-matrix interaction is achieved. In addition, the dispersion state of the functionalized GNP (f-GNPs; GNP/Py and GNP/APy) in the PK matrix is analyzed by three dimensional non-destructive X-ray microcomputed tomography (3D micro-CT) and the increased dispersion state of those fillers results in significant improvement in the water vapor transmission rate (WVTR). The enhancement in WVTR of the PK/GNP/APy nanocomposite film at 1 wt % loading of filler leads to an approximately 2 times larger barrier performance compared to that of PK/GNP nanocomposite film and an approximately 92% reduction in WVTR compared to the case of pristine PK film. We expect that this facile method of graphene functionalization to enhance graphene dispersibility as well as interfacial interaction with the polymer matrix will be widely utilized to expand the potential of graphene materials to barrier film applications.


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1. INTRODUCTION Carbon nanomaterials such as carbon nanotube, graphene, and fullerene have been widely applied to enhance the performance of polymer composites. One of the carbon allotropes, graphene nanoplatelet (GNP), a two-dimensional single or multilayer of sp2-bonded carbon atoms packed in a honeycomb lattice, has attracted enormous amount of attention owing to its remarkable mechanical,1 electrical,2 thermal,3 and barrier properties.4 In particular, polymer composites based on GNP have been extensively investigated over several decades for enhanced properties of polymers, including lower mechanical properties as well as their potential to limit the need for thermal, electrical, and barrier property requiring fields.5-6 However, those properties of composites are strongly dependent on the size, dispersion state of fillers, and interaction between the polymer matrix and the fillers. Due to their extremely high surface to volume ratio, nano-sized fillers dispersed homogeneously in a polymer matrix provide high surface area for possible bonding with the matrix. Good interfacial adhesion between fillers and matrix leads to effective load transfer across the interfaces. Thus, to achieve exceptional properties of GNP incorporated polymer composites, homogeneous dispersion of GNP without aggregation and with good interfacial interaction with the polymer matrix are required. The homogeneous dispersion of GNP in the polymer matrix remains a big challenge because ππ* interactions of GNP cause the restacking of layers. In this regard, some approaches have been attempted to achieve uniform graphene dispersion in the polymer matrix by modifying the surface of GNP through either covalent or noncovalent functionalization.7 In general, graphite is covalently modified by chemical oxidation using strong oxidizing agents to produce graphene oxide (GO) with carboxylic acid groups located at its edges, and hydroxyl and epoxide groups at


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the basal plane.3, 8 Covalently functionalized GO with functional molecules such as aliphatic amines,9 isocyanates,10 and large aromatic molecules11 is an effective method to disperse GO in organic solvents. However, the intrinsic mechanical, electrical, and barrier properties of GNP are destroyed during the covalent functionalization, while the non-covalent functionalization method leads to less damage to the structure of GNP, preserving its intrinsic properties.12 The noncovalent functionalization of GNP can be achieved by following methods: wrapping with a polymer,13 small molecules containing aromatic structure via π-π*interactions,14 and with surfactants.15-16 This type non-covalently functionalized GNP disperses easily in polar aprotic solvents. One of the small molecules containing an aromatic structure, such as a pyrene derivative, is a powerful candidate to overcome the agglomeration of GNP and achieve good compatibility between GNP and the polymer matrix.17 The pyrene derivatives not only achieves good affinity with GNP,15, 18 but also the functional groups located at the edges of the pyrene basal plane possibly facilitate specific interactions with polymer matrices.19-20 Aliphatic polyketone (PK) is a terpolymer consisting of carbon monoxide (CO) and ethylene, and a small amount of polypropylene. PK is high-performance thermoplastic polymer with good chemical resistance, mechanical property, and barrier property.21-22 Especially for polymer barrier materials, PK, compared to the other polymers such as polyamide and PET, has received significant attention because of its outstanding performance at blocking water vapor molecules.21-22 Nevertheless, further efforts are necessary to overcome the limitation in the barrier property of polymer materials if these materials are to meet the high-criteria requirements for packaging applications. GNP is also suitable for gas barrier materials because of its atomically two-dimensional sheets and high aspect ratio (Af). For nanocomposite containing GNP can play vital role by blocking gas vapor diffusion with lengthening pathway for diffusing


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gas molecules.23 To achieve the longer and more tortuous path for the penetrant, a good dispersion state of GNP with high surface to volume ratio is required.24-25 In this work, we prepare PK composites with non-covalently functionalized GNP using pyrene derivatives to achieve improved GNP dispersion and barrier property. To investigate the effect of non-covalent functionalization using 1-aminopyrene (APy) on realistic size and dispersion of GNP in the PK matrix, 3D micro-CT analysis is performed. Finally, the relationships between the GNP dispersion state and the barrier properties of the PK composites were discussed.


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2. EXPERIMENTAL SECTION 2.1 Materials. Graphene nanoplatelet (GNP, Grade M-25) was obtained from XG Science (USA). 1-aminopyrene (98%, APy) and Pyrene (98%, Py) were purchased from TCI (Japan) and Sigma Aldrich (USA), respectively. Both of these materials were utilized as stabilizers, without any further purification. Polyketone (PK) powders (Grade M330A) were provided from Hyosung (S.Korea), and were used for the polymer matrix. The PK powder (Grade M330A) is composed of ethylene-propylene-carbonmonoxide terpolymer, including 6 mol % of propylene. The melt index is known to be 60 g/10 min at 220 °C. Hexafluoro isopropanol (HFIP) was purchased from Nanjing Chemical Material Corp (China) as a solvent for PK and used without further purification. 2.2 Preparation of Functionalized Graphene Nanoplatelet with 1-Aminopyrene (GNP/APy). For non-covalent functionalization of GNP with stabilizer, 70 mg of APy (and Py) and 70 mg of GNP were added to 35 ml of deionized (DI) water. Then, the dispersion was sonicated at 40 °C for 24 h and, in order to maintain the efficiency of the exfoliation process, the temperature was controlled by providing water manually every an hour so as not to exceed the set temperature. The dispersions were then transferred to the centrifugation apparatus, operated at 12000 rpm at 5 °C for 20 min; remaining supernatant was removed. Additional DI water was added to reach a total volume of 35 ml in a falcon tube; materials were then sonicated for another 10 min. This purification step of centrifugation with newly added DI water was repeated two times and materials were dried under vacuum oven at 90 °C for 2 days to obtain GNP/APy powder. As a reference material, GNP was sonicated without any stabilizer under the same conditions and GNP/Py was prepared using the same method.


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2.3 Characterization of GNP/APy. The thickness of GNP and GNP/APy was identified using atomic force microscopy (AFM) (Surface Imaging Systems, NANO Station II) with tapping mode. In addition, morphological information was obtained by transmission electron microscope (TEM) (JEOL, JEM-2100F) under 200 kV applied voltage with selected area electron diffraction (SAED) technique. Raman spectra were obtained with a spectrometer (Horiba Jobin Yvon, LabRam Aramis) equipped with a CCD camera. The spectra were recorded under ambient conditions with an Ar-ion laser beam at an exciting radiation wavelength of 514.5 nm. The Raman excitation beam spot diameter is about 1.25 µm. The chemical composition was measured with electron spectroscopy for chemical analysis (ESCA) (SIGMA PROBE, ThermoVG) with a monochromatic Al-Kα X-ray source (15 kV, 100 W, 400 micrometer). The pass energy and the step size for the wide scan and the narrow scan were 50 eV and 1.0 eV, and 20 eV and 0.1 eV, respectively. Fluorescence emission spectra were acquired using a spectrofluorometer (JASCO, FP-6500) with a laser beam at an exciting wavelength of 350 nm; spectra were measured in the wavelength range of 370~680 nm. (see Supporting Information) 2.4 Fabrication of PK Nanocomposite Films by Solution Blending Method. The PK nanocomposite films were fabricated with PK powders and GNP/APy by solution blending method. First, GNP/APy was re-dispersed in 2 ml of HFIP with sonication at 40 °C for 2 h. PK powder was readily dissolved in 5 ml of HFIP with sonication at 40 °C for 2 h and prepared as a clear solution. The GNP/APy dispersions were mixed with the clear PK solution and then additionally sonicated under the same condition for 2 h to achieve a fully dispersed mixture. Composite sample series were prepared for 0, 0.5, 1, and 2 wt % of GNP/APy out of a total weight of 1 g. Each solution was cast onto a flat and clean glass plates, with a size of 10 × 10 cm2, and then maintained in a closed system to prevent the cast film from undergoing rapid


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evaporation of the HFIP solvent. Within the closed box, the cast films were slowly dried for 4 days. The dried films were transferred and sandwiched between two stainless steel plates, heated to 232 °C, in order to remove residual solvent from the slow drying process. The molten films between the two plates were vacuumed for 5 min and then pressed with a heat-compression machine under 14 psi for 2 min with a mold to achieve a 0.1 mm thickness; this was done to make the films have a regular thickness between 100 ~ 200 µm. There was no quenching period after the hot plates were removed from the compression machine; films were cooled for 5 min. In addition, for the reference films, PK nanocomposite films with fillers of GNP and GNP/Py were fabricated following the same method described above. 2.5 Characterization of PK Nanocomposite Films. A three dimensional non-destructive Xray micro-computed tomography (3D micro-CT) image for dispersion state of GNP and GNP/APy in PK matrix was scanned with a high-resolution X-ray micro-CT system (Bruker, Skyscan 1172). PK/GNP and PK/GNP/APy films were cut into sizes of 5 × 7 mm2. Scan data were acquired with an X-ray tube setting of 23 kV and 116 µA, and an exposure time of 4300 ms; scan parameters for the 1800 scan layers and 360 degrees were defined with a step size of 0.2 degrees, a pixel size of 1.36 µm, and no filter condition. The chain mobility of the PK nanocomposite films affected by the interaction between the PK matrix and its fillers were measured using a dynamic mechanical analyzer (DMA) (TA Instrument, Q800) for extension mode with at a frequency of 1 Hz at 2 °C /min for the temperature ramp rate. Morphological information about the PK nanocomposite films were obtained using X-ray diffraction (XRD) (Bruker, New D8 Advance) with a CuKa radiation (=1.5406 Å) source at a scan rate of 2 °/min for the range of 2θ = 10~40 °. The fracture surface microstructure of the PK nanocomposites was verified using field emission scanning electron microscopy (FE-SEM) (JEOL, JSM-6701F). For


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the sample preparation, the dispersion of PK/GNP in the HFIP solution was cast into a mold with a thickness of 1 mm and dried in a vacuum oven. The coin-shape sample was then frozen with liquid nitrogen for 60 s and fractured into pieces. The surfaces of the fractured samples were coated with Pt for 120 s by sputtering equipment (TED PELLA, INC., high resolution sputter coater). The mechanical properties of the composites were measured with universal testing machine (UTM) (Lloyd, LR10K), following ASTM D638 type V for dog-bone specimen preparation. Before the test, all samples were dried in a vacuum oven at 60 °C for 2 days and test was conducted with 10 mm/min of cross-head speed at 24 °C, 50 RH% humidity condition. The water vapor transmission rates of the composites were measured with a permeation analyzer (MOCON, PERMATRAN-W 3/33) under 37 °C, 100 RH% humidity condition for 24 h, at which the permeation is saturated. For the sample preparation, the PK/GNP and PK/f-GNP films were cut into certain sizes and sandwiched between donut-shape aluminum disks, of which the diameter of the inner circle is 2.5 cm and the thickness is 150 µm. Specific interaction such as hydrogen bonding between the functional groups from GNP and GNP/APy or the GNP/Py and PK matrix was analyzed by Fourier transform infrared spectroscopy (FTIR) (Thermo Scientific, Nicolet 6700) in ATR mode with values of 32 for number of scans, 8 for resolution, and 650 to 4000 cm-1 for wavelength range. The microstructures of the PK/GNP and PK/f-GNP composites were analyzed by transmission electron microscopy (TEM) (JEOL, JEM-2100F) under 200 kV applied voltage. Thin sections of samples were cut using a diamond knife with a cryoultramicrotome, resulting in a thickness of 70~90 µm; sections were put onto carbon-coated copper grids (CF400-CU).


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3. RESULTS AND DISCUSSION 3.1 Preparation of GNP/APy via Non-Covalent Functionalization. Small molecules consisting of aromatic structures have been shown to allow intercalation and physisorption to the graphite through a π-π* interaction, which leads to an exfoliation of GNP flakes into fewer layer graphene. In this experiment, 1-aminopyrene (APy) and pyrene (Py) were utilized not only to effectively exfoliate the graphene nanoplatelet (GNP) by cleaving the GNP flakes into graphene, but also to prevent the exfoliated graphene from restacking by non-covalent π-π* interaction.14-15 A facile method of non-covalent functionalization of GNP has been demonstrated. Figure 1 depicts the overall scheme of GNP/APy dispersed in the PK matrix via specific interaction, such as hydrogen bonding, between the amine functional group of GNP/APy and the carbonyl functional group of PK. As shown in Figure 2, to investigate the effect of the non-covalent functionalization of GNP by pyrene molecules in the exfoliation of the GNP, the thicknesses of GNP/APy and GNP as reference were measured by AFM tapping mode. Compared to the thickness of GNP, a thinner thickness of GNP/APy was produced under the same sonication process. With the same preparation steps, average sheet thicknesses of the GNP and GNP/APy were found to be 25.01 nm and 7.21 nm, respectively. Also, it was confirmed that GNP has a lateral size of about 24.64 µm (Figure S1, see Supporting Information), consistent with the literature value for GNP (XG science, Grade M-25).26-27 With the thickness values obtained from the AFM images, the aspect ratio (Af) of GNP to GNP/APy could be said to have approximately increased from about 1000 to 3400 through non-covalent functionalization. For more detailed morphological information on GNP/APy, microscope images with SAED


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patterns were obtained by TEM, with results shown in Figure 3. Because there was exfoliation during the sonication process from GNP to GNP/APy, clear hexagonal SAED patterns were observed in the TEM images of GNP/APy, showing the material’s highly crystalline structure (Figure 3c). On the other hand, multi-layers of GNP as reference were observed in both the TEM image and in the corresponding SAED pattern (Figure 3a). Furthermore, in order to prove the direct exfoliation of GNP by pyrene moieties, fluorescence spectra (excited at 350 nm) of GNP/APy and GNP/Py in DI water in dilute concentration were obtained with increasing concentration of GNP (Figure S2, see Supporting Information). The graph showed excimer peaks at around 442 nm and 462 nm for GNP/APy and GNP/Py, respectively. The excimer emission peaks are remarkably quenched with increasing concentration of GNP. This effect is ascribed to the effective electron or energy transfer between the pyrene moieties and GNP. 28-29 In other words, when the pyrene molecules are exposed to the laser source, the excited electrons from the pyrene moieties are transferred to GNP by π-π* interaction; this could be evidence of the direct energy transfer of electrons, resulting in fluorescence quenching.28-29 Figure 4a shows the Raman spectra for the GNP and GNP/APy. It is notable that three characteristic peaks were observed for all, representing graphitic materials: a D band at 1354 cm1

implying defects in the sp2 domains of GNP basal plane, a G band at 1578 cm-1, which

corresponds to the graphitic region of GNP, and a 2D band at 2720 cm-1.30 The ratio of the intensity of the D band to the G band peak can be used as an estimation of the extent of the structural defects. That is, the ID/IG value for GNP was 0.03, implying that the GNP flakes have as low level of structural defects.30 On the other hand, GNP/APy showed an increased value of


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the for ID/IG ratio of 0.07. In addition, the ID/IG ratio of GNP/Py was 0.10 which was the maximum value (Table S1). The higher the ID/IG value, the more structural defects occur in the GNP, resulting from the asymmetric breathing modes of the six-atom rings that occur due to noncovalent interaction between the GNP and the pyrene moieties.18, 31 The chemical compositions of GNP/APy in comparison with that of GNP were also analyzed by XPS to confirm that non-covalent bonding via π-π* interaction is remained after the washing step. Figure 4b and 4d shows the C 1s spectra of the GNP and the GNP/APy, respectively. Each peak was deconvoluted with assignments of binding energy as follows: graphitic carbon (284.5 eV), C-O (285.3 eV), C=O (286.6 eV), O-C=O (288.0 eV), and π-π* transition (290~291 eV).18, 32

From the peak area calculation, it was observed that the areas corresponding to graphitic

carbon (284.5 eV) were 57.6% and 60.0% of the total area of the GNP and GNP/APy, respectively (Table S1). The increase in the graphitic carbonaceous composition means that the residual pyrene moieties after the washing process firmly formed non-covalent bonding with GNP.32 These results agree well with our Raman results (Figure 4a). In addition, Figure 4c shows the N 1s spectra of GNP/APy; the deconvoluted peak assignments of the binding energy are as follows: -NH2 (399.3 eV) and –NH (401.4 eV).33 The clear N 1s spectra of GNP/APy provide confirmation of the firmly formed non-covalent bond between the GNP and APy. Additionally, atomic percentages of N atoms for each sample were 0.11 At. % and 2.04 At. % for the GNP and GNP/APy, respectively (Table S1). 3.2 Effect of GNP/APy on Dispersion and Interfacial Interaction in PK Nanocomposite Films. The homogeneous dispersion and morphology of fillers are significant factors affecting the final properties of the composite material. In order to observe the comprehensive dispersion


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state of fillers in the polymer matrix, 3D micro-CT scan analysis was conducted. PK nanocomposite films at 1 wt % loading of GNP, GNP/Py, and GNP/APy, with thickness of about 150 µm, were cut into samples with dimensions of 5 × 7 mm2 for X-ray scanning. Figure 5 shows the 3D micro-CT images for overall state of filler dispersion for the GNP, GNP/Py, and the GNP/APy in the PK matrix. (scale bars, 250 µm). It is noteworthy that the state of agglomeration of particle is more prominent in PK/GNP (Figure 5a, 5b) than in PK/GNP/Py (Figure 5c, 5d) and in PK/GNP/APy (Figure 5e, 5f) as can be seen by the larger matrix area where fillers are sparse. In other words, PK/GNP/APy showed the most homogeneous dispersion state of the fillers over other fillers, which can be attributed to the combinational effect from both increased interfacial interaction and effective exfoliation process applied for GNP/APy. Because the detecting resolution of the sample scan was 1.36 µm, a size below the resolution limit may not appear in the 3D micro-CT images. Using XRD analysis, it was possible to qualitatively observe how the GNP, GNP/Py, and GNP/APy are dispersed in the PK matrix. Figure 6a describes the XRD patterns of the PK/GNP, PK/GNP/Py, and PK/GNP/APy nanocomposites with its fillers, individually. There was a sharp and intense peak observed for GNP at 2θ = 26.7 °, corresponding to an interlayer spacing of 0.34 nm. In contrast, GNP/Py and GNP/APy both showed relatively less strong and broad peak at 2θ = 21.6 ° (corresponding to an interlayer spacing of 0.41 nm). PK as matrix showed diffraction peaks at 2θ = 21.4 ° (110), 25.5 ° (200), 31.3 ° (210), and 31.3 ° (β form), corresponding to the crystalline phases of PK.34 As can be noted in Figure 6a, the PK/GNP film showed a still existing diffraction peak from GNP at 2θ = 26.7 °, indicating that the interlayer spacing did not change and remained in aggregated form during the fabrication of the composite film. In contrast,


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PK/GNP/Py and PK/GNP/APy films both showed disappearing diffraction peak at around 2θ = 26.7 °, implying that an expanded interlayer spacing and dispersion of GNP/APy in PK matrix is remained. The disappearing peak could be also the result of the increased interlayer spacing caused by the non-covalent functionalization of GNP and certain degree of exfoliation.35 Figure 6b shows DMA tan δ curves where the peak maximum of the curve represents the glass transition temperature (Tg) affected by interfacial interaction between GNP/APy and the PK matrix. When there is a strong interaction between the fillers and the polymer matrix, the chain mobility of the polymer at the interface affects Tg.36 As shown in Figure 6b, different values of dissipation factor (tan δ) were observed in the case of neat PK and PK nanocomposites and corresponding values are list in Table S2. (See Supporting Information) The neat PK presented Tg of 26.1 °C. With the introduction of GNP, GNP/Py, and GNP/APy to the PK, the Tg of those PK nanocomposites changed due to a combinational effect of friction and interfacial interaction between the polymer chain and the fillers. Compared to Tg of neat PK, the PK/GNP and PK/GNP/Py nanocomposites showed increased Tg at 34.2 °C and 36.0 °C, respectively. The incorporation of GNP leads to interruption for polymer chain mobility and further slight increase in Tg from PK/GNP to PK/GNP/Py may be attributed to increased interfacial area due to improved dispersion.36 It is noteworthy that the PK/GNP/APy nanocomposite showed the highest Tg at 41.5 °C, meaning that together with the larger interfacial area and additional interfacial interaction, presumably hydrogen bonding, between GNP/APy and PK leads to extra chain mobility interruption.7 Another evidence of hydrogen bonding interaction in PK/GNP/APy was also explained to a certain degree by FT-IR study in Figure 7. The FT-IR spectra of PK, PK/GNP/Py, and


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PK/GNP/APy clearly show that there is a shift of the carbonyl functional group of PK. The peak representing the carbonyl functional group was originally at 1690 cm-1 and only shifted to a higher wavenumber of 1695 cm-1 in PK/GNP/APy, whereas there is no shift in the PK/GNP/Py sample. This is plausible evidence that there is specific interaction, particularly hydrogen bonding interaction, in PK/GNP/APy.37 This behavior was explained by another group regarding the specific interaction between PK and the functionalized graphene oxide.37-39 In order to obtain explicit information on filler interaction with the polymer matrix, the fracture surfaces of the PK/GNP, PK/GNP/Py, and PK/GNP/APy nanocomposites were examined. Figure 8 shows SEM images, revealing the difference in degree of interfacial interaction between the filler and the polymer matrix in these systems. It should be noted that while there is cleanly protruded GNP with a relatively weakly wrinkled structure of the fracture surface of PK/GNP and PK/GNP/Py (Figure 8b, 8c), there were extraordinarily wrinkled structures of fillers in PK/GNP/APy (Figure

8d), indicating stronger filler and polymer interactions. This may result

from the non-covalently functionalized amine functional group on the GNP/APy surface having a hydrogen bonding interaction with the carbonyl functional group of the PK chain, which has been reported by other groups as well.7, 36 3.3 Properties of PK Nanocomposite Films. The mechanical properties of polymer nanocomposites were found to be highly affected by interaction between the filler and the polymer, as well as by the dispersion state of the fillers and the filler content. The effects of GNP/APy on the tensile properties of the PK nanocomposite films were shown in Figure 9 and list in Table S2. (See Supporting Information) The tensile property test was measured according to ASTM D638 method with type V. Also, for reproducibility of the results, at least four


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specimens of each type were tested. Figure 9a shows the Young’s modulus, the stiffness of a material measured at a relatively low deformation stage of a tensile test, of the PK nanocomposite films. The Young’s modulus of PK/GNP/Py and PK/GNP/APy were more effectively improved as the fillers were incorporated, compared to PK/GNP. Since the Young’s modulus is mainly affected by the filler loading and its aspect ratio, the increased aspect ratio of the non-covalently exfoliated GNP/Py and GNP/APy led to higher stiffness in PK composites. In addition, although the Young’s modulus is not so much affected by the interfacial interaction between the filler and the matrix due to the fact that the Young’s modulus is measured at an earlier stage of deformation,40 PK/GNP/APy show the greatest improvement in Young’s modulus compared to PK/GNP and PK/GNP/Py. For theoretical approximation of filler aspect ratio (Af), the Halpin-Tsai model for randomly oriented discontinuous fiber reinforced composites was utilized.41-42 Hence, our system can be explored mathematically to determine theoretical Young’s modulus of the composites at specific aspect ratio by using the equation. (Equation S1, See Supporting Information)42 As can be seen in Figure 9a, an increasing trend of the Young’s modulus was seen in all the films, following the ‘rule of mixture’.43 The theoretically calculated Young’s modulus of PK/GNP/APy with 1 wt% loading of GNP/APy was 2.26 GPa (Af = 3400); this could vary with the filler aspect ratio. The experimentally measured Young’s modulus for PK/GNP/APy with 1 wt% loading was 2.15 GPa, which is 95.1% of the theoretical value. The experimentally measured values of Young’s modulus show relatively good agreement with the model line until 1 wt% of filler loaded in PK composites. At above 1 wt% of the filler loading, the increasing stiffness trend did not follow the model lines, which may be attributed to the agglomerated structure of the fillers, which leads to a


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less dispersed state of the filler and to the decreased load transfer under tensile stress.44-45 Figure 9b shows the tensile strength of the PK nanocomposite films with varying amount of fillers. Since the tensile strength is largely affected by the quality of the interfacial interaction between the filler and the matrix, the highest tensile strength in PK/GNP/APy (67.3 MPa) with 1 wt % loading over PK/GNP (58.0 MPa) and PK/GNP/Py (59.2 MPa) means GNP/APy has good adhesion to the PK matrix. The overall tensile strength trend of PK/GNP/APy showed increments until a 1 wt% level of the filler and showed a decreased tensile strength at 2 wt %. The filler may be aggregated and thus may act as a stress concentrator, which would make it unable to dissipate the stress.44-45 Figure 9c shows the elongation at break trend of PK nanocomposite films. When rigid fillers were added, elongation at break typically decreased.5 This behavior is similar to the brittle fracture behavior, which can be easily observed in composite materials with weak interfacial interaction between two different materials.46-47 It is noteworthy that while elongation of all PK nanocomposites decreased, PK/GNP/APy showed the least decrease in elongation at break value, which can be explained by the strong interfacial bonding formation between the GNP/APy and PK matrix. Again, the stronger interfacial interaction may be attributed to specific interaction such as hydrogen bonding between the amine functional group on the GNP/APy surface and the carbonyl functional group on the backbone of the PK chain.37 The WVTR of the PK nanocomposites drops when the in-plane pathway, called tortuosity, of the small permeant molecules is increased. The way to increase the tortuosity is to adopt a high aspect ratio and a well dispersed 2D material in the matrix to effectively increase the tortuosity.23 In this study, the water vapor barrier properties of the PK nanocomposite films (thickness is


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about 150 µm for all films) were measured when the WVTR was saturated and stabilized. Figure 10 describes the decrease of WVTR with respect to the filler content and the degree of filler functionalization. It can be seen that the WVTR values of PK/GNP, PK/GNP/Py, and PK/GNP/APy at 1 wt % of filler loading are 4.2, 2.6, and 2.3 g/m2-day, respectively. Also, the WVTR of the PK/GNP/APy composite with 2 wt% loading of filler decreased to 2.0 g/m2 day from 28 g/m2 day, which is an approximately 92.8% reduction compared to the value of PK matrix. For qualitative investigation of the effect of filler dispersion as well as the filler aspect ratio (Af) on the WVTR, we invoked a simple model that can predict the WVTR of a composite depending on the filler orientation, the filler dispersion state, and the aspect ratio of the filler. The increased tortuosity was calculated according to the model using the Nielsen equation. (Equation S2, See Supporting Information)25 Note that the WVTR values obtained with PK/GNP nanocomposite films closely followed the straight model line (Af = 1000), which agrees well with values theoretically approximated from tensile modulus. Moreover, the WVTR values obtained with PK/GNP/APy nanocomposite films were the closest to the dashed model line (Af = 3400) among other composite films. In addition, at 2 wt % loading of all fillers, the resulting values are slightly out of the predicted line; this can be ascribed to the poor dispersion and to the stacking of the fillers, which lead to a decrease of the overall tortuosity, leading to higher WVTR results.


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4. CONCLUSIONS GNP and non-covalently functionalized GNPs (f-GNPs) were solution blended with PK. The AFM images demonstrated that the aspect ratio of the GNP and f-GNPs were 1000 and 3400, respectively. By obtaining 3D micro-CT images, the extent of the dispersion of the fillers in the composites was directly monitored. Not only the dispersion of fillers, but also the interfacial interaction between the fillers and the polymer matrix play significant roles in enhancing the composite properties. Dynamic mechanical analysis and infrared spectroscopy data demonstrated that there is a substantially recognizable chemical interaction between GNP/APy and PK via hydrogen bonding, whereas GNP and GNP/Py have only a physical interaction. Moreover, mechanical and barrier properties of PK nanocomposites were measured to confirm the overall statement on the extent of filler dispersion and interfacial interaction. From the mechanical property analysis, it was observed that the PK/GNP/APy nanocomposite films had an increased tensile strength and the least decrease in elongation at a break, which means that the GNP/APy has an effective interfacial interaction with the PK matrix over GNP or GNP/Py. In addition, WVTR measurements confirmed that PK/GNP/APy nanocomposite films were superior to both PK/GNP and PK/GNP/Py nanocomposite films; this may have been due to a higher aspect ratio of f-GNPs and better dispersion state of GNP/APy in the PK matrix compared to that of GNP and GNP/Py. The WVTR would be more dependent on the length of tortuosity, which can be affected by the dispersion state with a higher aspect ratio and interfacial interaction between the filler and matrix.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Summarized data for analysis of GNP particles and PK/GNP composites (Table S1 and S2); SEM images (Figure S1); Fluorescence analysis (Figure S2); TEM images (Figure S3); microCT images (Figure S4); Equations for predicting properties compared to experimental measurements (Equation S1 and Equation S2)

AUTHOR INFORMATION Corresponding Author *(J.Y.J.) E-mail: [email protected].

Author Contributions #

J. Cho and I. Jeon contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the Institute of Chemical Processes (ICP) from Seoul National University and funded by the Fundamental R&D Program for Technology of World Premier Materials (WPM) from the Ministry of Knowledge Economy, Republic of Korea. We acknowledge the financial support from a Korea Institute of Science and Technology internal project (institutional program).


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Figure 1. Schematic diagram for non-covalently functionalized GNP/APy and its interaction mechanism with PK matrix.


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Figure 2. AFM images of (a) GNP and (b) GNP/APy on a mica substrate. The height profiles of (c) GNP and (d) GNP/APy.


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Figure 3. Representative TEM micrographs of (a-b) GNP and (c-d) GNP/APy in different magnifications. The inset images are the corresponding SAED patterns.


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Figure 4. Characterization of non-covalently functionalized GNP with APy: (a) Raman spectra with ID/IG ratio of GNP and GNP/APy, (b) XPS C 1s spectra of GNP, (c) XPS N 1s spectra of GNP/APy and (d) XPS C 1s spectra of GNP/APy.


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Figure 5. Representative 3D micro-CT images: (a-b) top and side view of PK/GNP nanocomposite film (1 wt %), respectively, (c-d) PK/GNP/Py nanocomposite film (1 wt %), respectively, and (e-f) PK/GNP/APy nanocomposite film (1 wt %), respectively. Scale bars, 250


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µm.

Figure 6. Characterization of filler dispersion in PK nanocomposites and interfacial interaction with PK matrix: (a) Left: XRD profiles of GNP, GNP/Py, GNP/APy, neat PK, PK/GNP nanocomposite (0.5 wt %), PK/GNP/Py nanocomposite (0.5 wt %), and PK/GNP/APy nanocomposite (0.5 wt %). Right: enlarged view of the XRD profile between 20 and 28 degrees. (b) DMA tan δ curves for PK, PK/GNP nanocomposite (1 wt %), PK/GNP/Py nanocomposite (1 wt %), and PK/GNP/APy nanocomposite (1 wt %).


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Figure 7. FT-IR spectra of PK/GNP, PK/GNP/Py, and PK/GNP/APy nanocomposites.


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Figure 8. Representative fracture surface SEM images of (a) neat PK, (b) PK/GNP nanocomposite (1 wt %), (c) PK/GNP/Py nanocomposite (1 wt %), and (d) PK/GNP/APy nanocomposite (1 wt %).


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Figure 9. Mechanical properties measurements: (a) Young’s modulus, (b) Tensile strength, and (c) Elongation at break of PK nanocomposites with GNP, GNP/Py, and GNP/APy in varying filler content.


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Figure 10. Water vapor transmission rate (WVTR) measurements for PK nanocomposites with GNP, GNP/Py, and GNP/APy in varying filler content. (test environment for WVTR measurement at 23 °C and 100 % relative humidity; film thickness: about 150 µm).


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