Directionally Antagonistic Graphene Oxide-Polyurethane Hybrid

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Functional Nanostructured Materials (including low-D carbon)

Directionally Antagonistic Graphene OxidePolyurethane Hybrid Aerogel as a Sound Absorber Jung-Hwan Oh, Jieun Kim, Hyeongrae Lee, Yeonjune Kang, and Il-Kwon Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06361 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Directionally Antagonistic Graphene OxidePolyurethane Hybrid Aerogel as a Sound Absorber Jung-Hwan Oh,† Jieun Kim,§ Hyeongrae Lee,‡ Yeonjune Kang,‡* and Il-Kwon Oh†*

†

Creative Research Initiative Center for Functionally Antagonistic Nano-Engineering and

Graphene Research Center @ KAIST Institute for the NanoCentury, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡

Institute of Advanced Machines and Design, Department of Mechanical and Aerospace

Engineering, Seoul National University, Seoul 08826, Republic of Korea §

LG Chem, Ltd., 30 Magokjungang 10-ro, Gangseo-gu, Seoul, Republic of Korea

KEYWORDS: graphene; anisotropic; antagonistic; sound absorbing materials; porous materials

ACS Paragon Plus Environment

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ABSTRACT

Innovative sound absorbers, the design of which is based on carbon nanotubes and graphene derivatives, could be used to make more efficient sound absorbing materials, because of their excellent intrinsic mechanical and chemical properties. However, controlling the directional alignments of low-dimensional carbon nanomaterials, such as restacking, alignment, and dispersion, has been a challenging problem when developing sound absorbing forms. Herein, we present the directionally antagonistic graphene oxide-polyurethane hybrid aerogel we developed as a sound absorber, the physical properties of which differ according to the alignment of the microscopic graphene oxide sheets. This porous graphene sound absorber has a micro-porous hierarchical cellular structure with adjustable stiffness and improved sound absorption performance, thereby overcoming the restrictions of both geometric and function-orientated functions. Furthermore, by controlling the inner cell size and aligned structure of graphene oxide layers in this study, we achieved remarkable improvement of the sound absorption performance at low frequency. This improvement is attributed to multiple scattering of incident and reflection waves on the aligned porous surfaces, and air-viscous resistance damping inside interconnected structures between the urethane foam and the graphene oxide network. Two anisotropic sound absorbers based on the directionally antagonistic graphene oxide-polyurethane hybrid aerogels were fabricated. They show remarkable differences owing to the opposite alignment of graphene oxide layers inside the polyurethane foam and are expected to be appropriate for the engineering design of sound absorbers in consideration of the wave direction.


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INTRODUCTION

The morphological control of cellular structures with the accurate manipulation of the hierarchical cell pores is attracting increasing attention in the field of materials and mechanical engineering, offering several advantages such as an excellent strength-to-weight ratio, reversible compressibility and wave absorbing functionality.1-2 However, it is very difficult to synthesize micro-porous hierarchical cellular structures with two more ingredients because of the various challenging issues associated with restricted geometries and material properties.3-4 Most previous studies on the intracellular network in aerogel and form structures have been restricted to using a single material with polymer and nano-carbons or simply mixing various ingredients during the manufacturing process.5-6 Although the structural properties, such as the degree of cell opening and the diameter of the cell, were considered as the main controlling parameters, developing cost-effective and high sound absorbing cellular structures by structural manipulation still remained as challenging issues. To solve these challenging issues, many scientists have attempted to maximize the effects of sound absorption by using nanostructured open/closed cells, by adding graphene and carbon nanotubes (CNTs) into polymeric forms, and by producing multi-layered sound absorbing materials.7-9 However, the main problem presented by using graphene-inspired hybrids as sound absorbers is that low-dimensional graphene sheets restack each other occurring the small interconnecting pores.6 Moreover, their foam is disorderly arranged irrespective of the direction of incidence of sound waves resulting in a short pathway for an incident sound wave, indicating that incident sound wave is reflected rather than transmitted to the media foam.10 This short pathway limits the attenuation of sound wave and prevents it from traveling in the direction of


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the cell walls. Therefore, in order to efficiently attenuate the incident sound waves, it is necessary to construct aligned spaces between the graphene walls intentionally arranged in the foam through structural manipulation, and to provide a passage through which sound waves can be converted into thermal energy. In attempts to address the abovementioned issues, three-dimensional (3D) graphene aerogel could lead to potential applications based on diverse unique cellular structures by controlling the cell size and aligning graphene oxide layers in ultra-light acoustic absorbing materials.11 Several reports about the preparation of 3D graphene aerogels have been published, regarding unique cellular structures by controlled pore direction and pore size; thereby these porous architectures are expected to have a significant influence on the specific engineering applications.12-20 Although unique 3D graphene aerogels could be achieved, it is still a great challenge to prepare graphene aerogels with high compressibility and excellent mechanical properties, due to the low controllability of macroscopic networks and the brittleness under low compression.16, 21 Therefore, the structural limitations of graphene aerogels as sound absorbing materials can be overcome by providing a high compressive polyurethane network as a supporting frame and precisely controlling the graphene arrangement to provide aligned spaces for sufficiently dispersing incident waves. However, the investigation of synthetic routes to anisotropic graphene aerogels is still in the beginning stage; the anisotropic behavior of pore-aligned cellular graphene oxide foreshadow its potential use in the dissipation of sound energy. Further, polyurethanebased graphene hybrids and aerogels for sound absorption applications have not yet been reported. To the best of our knowledge, the anisotropic behavior of a pore-aligned cellular graphene oxide-polyurethane foam has also not yet been studied.


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Herein, we report a directionally antagonistic graphene oxide-polyurethane hybrid aerogel with aligned pores and graphene oxide facesheets as a novel sound absorber that demonstrates high sound absorption ability at low frequency as well as excellent mechanical performance. Additionally, we developed an anisotropic graphene oxide porous sound absorber consisting of hierarchical cellular microstructures with a polyurethane frame supporting unidirectionally semi-open and well-arranged graphene oxide layers via simple directional freezing and freeze-drying methods. Furthermore, the new unique structure of anisotropic graphene oxide aerogels exhibits a greatly increased sound absorption ratio in the audible frequency region owing to the complex microcellular geometry, which induces high flow resistivity resulting in slow sound speed in the aerogels. Because of aligned graphene oxide walls, the uni-directionally semi-open cells of graphene oxide-polyurethane aerogel provides large surface area that can access more propagated sound wave and significantly attenuates sound energy, resulting in an efficient pathway for rapidly radiation of thermal energy on the surface of graphene oxide. Moreover, the improved sound absorption ratio is the result of micro-membrane vibrators of the well-arranged graphene oxide layers and multiple scattering processes of acoustic waves inside the gaps in the aerogel, compared to the existing bare polyurethane foam. This study culminated in the development of a new directionally anisotropic graphene oxide-polyurethane with an arrangement of graphene oxide layers as a macroscopic bulk sound absorbing material.22 Also, the directionally anisotropic graphene oxide-polyurethane composite aerogel exhibits different sound absorption performance according to changes of the arrangement of the micro-sized graphene oxide layers, indicating the effect of aligned micro walls and channels as a sound absorbing material. As a result, the obtained directionally antagonistic graphene oxidepolyurethane aerogels having hierarchical cellular microstructures with a polyurethane


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framework and uni-directionally semi-open pores are newly developed as a macroscopic bulk sound absorbing material with remarkably high sound absorption performance, opening a new approach to using graphene oxide aerogel in sound and vibration applications.

EXPERIMENTAL SECTION

Preparation of Directionally Antagonistic Graphene Oxide-Polyurethane Foam. A cleaned commercially available polyurethane sponge (PUS) was cut into pieces with a diameter of 29 mm to fit the size of the impedance tube for the purpose of conducting sound absorption rate measurements. Good adherence surfaces were achieved by twice subjecting the PUS to dip coating with a low-concentration graphene oxide solution. 100µm scale graphene oxide (100µGO) having an average size of particles around 66.5 µm was prepared from large (mm) scale graphite (LSG) flake following a modified Hummer-Tour method as described in the supporting information. Subsequently, the PUS was immersed in an aqueous solution of 100µGO with concentrations of 2 g/L and 4 g/L. The 100µGO sample frozen in PUS was prepared by orienting the graphene oxide in an ice-template by using liquid nitrogen to freeze the graphene oxide solution with which the PUS was completely filled. The conditions of process parameters were set to keep the size and growth rate of the ice crystals through repeated experiments under the same cooling rate. Specifically, the samples were frozen in less than an hour by conduction of heat without direct contact to liquid nitrogen. Only the condition of the ice crystals could be made different by changing the concentration of aqueous GO solutions as an input parameter. After lyophilization of the samples, a final directionally antagonistic graphene oxidepolyurethane sponge was obtained.


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Characterization and Instruments. The normal incident sound absorption coefficient of all samples was characterized using a Brüel & Kjær impedance tube, type 4206. A cylindrical sample of 29 mm in diameter was used to measure the frequency bandpri from 50 to 6.4 kHz via the two-microphone method. Airflow resistivity measurements based on ASTM C4522-03 were conducted in a laminar flow state (0.5 to 50 mm/s). The microstructure of all samples was observed by Field emission scanning electron microscopy (Model: SU8230, HITACHI). Sound absorption coefficient of all samples was characterized by using the B & K impedance tube 4206 type. A cylindrical sample of 29 mm in diameter was used to measure the frequency band of 50 to 16000 Hz by the two-microphone method. The compressive stress-strain measurement were conducted under cyclic compression strain 10% at a strain rate of 1 mm min-1 using an AGS-X (Shimadzu, Japan). The dimensions of the tested samples were 15 mm x 15 mm x 15 mm. Spectral analysis of graphene oxide was characterized by FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific, USA), Raman spectra were collected with a High resolution Raman (LabRAM HR Evolution Visible, HORIBA, France).

RESULTS AND DISCUSSION

The entire synthetic route to the directionally antagonistic graphene oxide-polyurethane hybrid aerogel is shown in Figure 1. Large scale graphite (LSG) having a diameter of a few millimeters was used as a precursor.23 The graphene oxide, at a scale of approximately 100 µm (called 100µGO), was prepared by the modified Hummers-Tour method from the LGS starting material.24 Using the directional freezing method, the fabricated 100µGO forms a directionally


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well-aligned microstructure of graphene oxide aerogel inside a polyurethane foam (PUF). In this process, solid-state 100µGO is separated during the formation of ice crystals from water in a fluid state. The 100µGO sheets are interconnected with neighboring ice structures and are forced to form a continuous porous frame according to the growth direction of the ice crystals, as shown in Figure 1.25 After the 100µGO sheets are shaped into microscopic graphene oxide foam, the 100µGO frozen in the PUF is dried. The two different graphene oxide porous sound absorbers that are produced as described here, are directionally antagonistic and anisotropic, and are classified as Parallel G-PUF and Perpendicular G-PUF according to the direction of the graphene oxide layers in the PUF. Parallel G-PUF means that the graphene oxide layers inside the polyurethane foam are aligned parallel to the direction of acoustic waves, whereas Perpendicular G-PUF means that the graphene oxide walls are arranged perpendicular to the direction of sound waves. To determine the necessity of the polyurethane framework, it would be better to provide the sound absorption coefficient of the pure graphene aerogel. However, if a graphene oxide micropore-structure is created, cutting with a laser cutter or a heat cutter will cause the samples to be sparked or broken, making it difficult to be cut into a uniform cylindrical sample. It also depends on the size of the beaker in the process of pure graphene aerosol synthesis, and it is difficult to make a sample to fit the impedance tube size because the volume of sample is reduced during hydrogel formation. Therefore, as shown in Figure 1, the directional freezing method proposed in this study is based on preliminarily cut polyurethane framework, which has been finally filled with the graphene oxide aerogel, so that it has the advantages of keeping the shape and size constant. The structural morphologies of the bare polyurethane foam and graphene oxide-polyurethane composite aerogels are shown in Figure 2. Figures 2a, 2b, and 2c show SEM images of the bare


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polyurethane foam, which has high resilient properties under cyclic loads with a weight of 45 kg. The polyurethane structure used in this study is a macroscopic porous structure in which a plurality of open cells and a closed cell are partially formed. Figures 2d and 2e provide a schematic view of the directionally antagonistic graphene oxide-polyurethane sound absorbers, defined according to the cutting surfaces. As can be seen on the left in Figure 2d, the acoustic wave enters uniformly along the x-axis in Parallel G-PUF in which the graphene oxide layers are stacked in the z-direction. Perpendicular G-PUF has graphene oxide layers arranged in lines perpendicular to the direction of acoustic waves toward the origin along the z-axis, as shown on the right in Figure 2d. The cutting plane is the xz-plane perpendicular to the y-axis. Figures 2f and 2i show cross-sectional views of Parallel G-PUF along the xz-plane. The graphene oxide layers can be seen to be well aligned inside the polyurethane frame. Cross-sectional views of Perpendicular G-PUF, with 90° rotation after cutting in the xz-plane, are shown in Figures 2g and 2j. The graphene oxide sheets are interconnected and well arranged in a row along the xdirection perpendicular to the direction of acoustic waves. When an acoustic wave passes through Perpendicular G-PUF, a large portion of reflection occurs due to the graphene oxide wall. A schematic diagram of the directionally antagonistic graphene oxide sound absorber cut along the yz-plane perpendicular to the x-axis is shown in Figure 2e.26 Inside the large frame of the polyurethane foam, a long passage was formed with an array of graphene oxide sheets; the sound waves enter this tunnel and sound absorption occurs due to micro-vibrations of the aligned graphene oxide layers and multiple scattering phenomena, as shown in Figures 2h and 2k. The formation of the directional graphene oxide-polyurethane foam was determined by analyzing the concentration of an aqueous solution of the graphene oxide. The concentrations of graphene oxide are 1 g/L, 2 g/L, and 4 g/L; the polyurethane foam is soaked in a solution of the


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appropriate concentration, and the unique well-aligned microstructure of graphene oxide layers is formed through the directional freezing method. The 100µm graphene oxide particles are to be entrapped in an aqueous graphene oxide dispersion, where the ice crystals immediately grow in a lamellar shape. Due to the viscosity effect on the ice nucleation and growth in a perpendicularly packed arrangement, these particles are aligned.16,

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Figures 3a and 3d show the

microstructure of the graphene oxide-polyurethane foam at a concentration of 1 g/L. Lowdensity graphene oxide is not conducive for the formation of well-aligned graphene oxide layers inside the polyurethane cell. The SEM images reveal that directionally well-aligned graphene oxide aerogels are formed when the concentration of graphene oxide solution exceeds 2 g/L, as shown in Figures 3b and 3e. Graphene oxide sheets were interconnected and stacked at intervals of more than 100 micrometers between parallel pores inside the polyurethane foam. Figures 3c and 3f show graphene oxide-polyurethane aerogels obtained with graphene oxide with a concentration of 4 g/L. At concentrations below 4 g/L, the spacing of the graphene oxide layers did not widen, whereas at the concentration of 4 g/L, the graphene oxide layers were arranged at spacing intervals of less than 100 micrometers and formed tightly inside the polyurethane form. The normal incidence sound absorption coefficients of all samples were obtained using the Brüel & Kjær impedance tube. Figure 4a provides a schematic diagram of the experimental setup used to measure the surface impedance of the samples. The sound absorption coefficients up to 6.4 kHz were measured by using an impedance tube with a diameter of 29 mm. A loud speaker was connected to the inlet side of the tube, and broadband sound was supplied to the tube through the loud speaker. The downstream side of the pipe was equipped with a rigid portable piston. The front surface of the rigid piston body was arranged with homogeneous sound absorbing material of different thicknesses to obtain the surface impedance.


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The effect of the direction of alignment of samples on the sound absorption performance was investigated as shown in Figure 4b. The parallel sample of the graphene oxide-polyurethane sound absorber, with a thickness of 45 mm, exhibits excellent absorption performance in the broadband frequency range. Compared with the bare polyurethane foam, the graphene oxidepolyurethane sound absorber shows superior performance only because of the alignment of graphene oxide layers inside the polyurethane foam. As the density of the graphene oxide increases, the sound absorption also tends to increase. This tendency indicates that the formation of a structural arrangement of the graphene oxide layers inside the polyurethane cell is strongly dependent on the concentration of graphene oxide. In particular, in the frequency band below 1,000 Hz, all values of the normal incidence sound absorption coefficient of bare polyurethane foam (PUF) remained below 0.2. On the other hand, the Perpendicular G-PUF sample, which has vertical graphene oxide walls, dissipated much more acoustic wave energy than did the bare PUF sample, as shown in Figure 4b. The normal incidence sound absorption coefficient of the Perpendicular G-PUF is increased compared to that of the bare PUF, except for the area in the vicinity of the peak value of PUF. Furthermore, compared with the other samples, the experimental results of the Parallel G-PUF were shifted to the low-frequency region. The measured peak frequencies of each sample were 1,910 Hz, 2,360 Hz, and 4,230 Hz for the Parallel G-PUF, Perpendicular G- PUF, and PUF, respectively. The normal incidence sound absorption coefficient at the peak increased from 0.83 for PUF to 0.91 for the Perpendicular GPUF, and to 0.98 for the Parallel G-PUF. When the graphene oxide layer is densely formed inside the polyurethane foam, it is more effective for low frequency noise damping because of its complicated microcellular geometry. Generally, in a high density foam, there is more spaces for sound waves to contact into the narrow interconnecting pores. As a result, the Parallel G-PUF


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with void spaces induces high flow resistivity and can warp a path of sound wave propagation as a high tortuosity, making the sound speed slow inside of it.6-7 The difference in sound absorption performances according to the orientation of the graphene oxide (Parallel G-PUF and Perpendicular G-PUF versus PUF) is clearly shown in Figure 4b and the sound absorption mechanism in the polyurethane backbone can be clearly explained in detail, as shown in Figure 7. As shown in the tendency of the sound absorption along the direction of graphene oxide alignment, the concentration of graphene oxide is a very important factor influencing sound absorption. The dependency of the graphene oxide concentration on the normal incidence sound absorption coefficient is clearly shown in Figures 4c and 4d. As the concentration of both samples increased from 2 g/L to 4 g/L, the sound absorption performance improved slightly. The sound absorption performance depends on the density and orientation of the porous material. If the graphene oxide layer is formed to be thinner and the air layer between the graphene oxide layers is densely formed, the thinner plate-like graphene oxide layers are expected to be more likely to vibrate with much larger amplitude. Furthermore, the sound absorption rate will be greatly increased because the wave energy is transformed into the structural vibration of the graphene oxide layers. Because the graphene oxide concentration is high and the spacing of the graphene oxide layers formed on the polyurethane backbone is narrow, the sound absorption performance is also greatly improved. This is related to the difference in the flow resistivity depending on the graphene oxide concentration, as shown in Figure 6b. Figure 5 shows the results of sound absorption tests that were carried out with thickness variations of 15, 30, and 45 mm for Parallel G-PUF and Perpendicular G-PUF (density: 4 g/L) samples. As the thickness increases, the sound absorption performance increases in the low-


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frequency band. In addition, the parallel samples, in which graphene oxide layers are aligned parallel to the direction of the sound waves, are more effective than the perpendicular samples, in which graphene oxide layers are arranged normal to the acoustic waves. As the thickness of the graphene oxide-polyurethane foam increases, the sound absorption coefficient greatly increases in the low- and mid-frequency range. An increase in the thickness of the porous graphene oxide sound absorber enlarges the effective frequency range of the material as a sound absorbing material, which is an important condition for improving the sound absorption rate. The extent to which the acoustic samples can absorb the sound energy was investigated by calculating the noise reduction coefficient (NRC) of each sample, as shown in Figure 6a. The NRC is the average of the normal incidence sound absorption coefficient at four 1/3 octave frequencies (250, 500, 1,000, and 2,000 Hz), indicating the standard index of sound damping materials. As the concentration of the Parallel G-PUF samples increases, the sound absorption performance increases from 0.451 at 2 g/L to 0.667 at 4 g/L. The Perpendicular G-PUF samples have NRCs of 0.45 to 0.474 at concentrations of 2 g/L and 4 g/L, respectively. Because the NRC of the bare PUF is 0.217, the NRC of the Parallel G-PUF-4 is greatly improved (by as much as 212.42 %). Regarding the propagation of acoustic waves, the flow resistivity of the graphene oxidepolyurethane foams was measured to observe the effects of the alignment and density of graphene oxide. The airflow resistivity was measured by preparing samples with diameters of 100 mm and a thickness of 15 mm. The sound absorbing material is attached to the fixing part and inserted in the middle of the channel. The flow resistivity represents the extent to which the flow in air, which is propagated through the fluid portion of the porous material, is prevented from progressing. Based on ASTM C4522-03, a volumetric flow rate is applied such that the air


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propagating through the fluid phase of the porous material is in a laminar flow state (0.5 to 50 mm/s); the resulting pressure difference is measured to obtain the flow resistivity. As expected, the flow resistivity increased as the concentration of graphene oxide in the sample increased and as the graphene oxide layer became denser. The measured flow resistivity of the highest density Perpendicular G-PUF-4 was found to be 92,535 Rayls/m, which is slightly higher than the value of 90,381 Rayls/m of the Parallel G-PUF-4. However, considering the standard deviation, according to the orientation of graphene oxide, there is no difference in the flow resistivity between the Parallel G-PUS-4 and the Perpendicular G-PUS-4. As a result, the flow resistivity did not change greatly depending on the orientation of graphene oxide. However, as the concentration of graphene oxide increases, the graphene oxide is densely arranged and the flow resistivity increases. Likewise, the sound absorption performance tends to increase with an increase in the flow resistivity, especially in the low frequency band. Therefore, although the flow resistivity has a similar value depending on the directionality of the graphene oxide at 4 g/L, it can be seen that the sound absorption performance varies depending on the arrangement characteristics of the graphene oxide layers, as shown in Figure 4b. Thus far, we have investigated the sound absorption coefficient according to the graphene oxide concentration and directionality and the thickness of the samples. Now, we have to speculate on the sound absorbing mechanism in the directionally antagonistic graphene oxidepolyurethane hybrid aerogel. Absorbing a sound wave means energy dissipation of the acoustic waves, and eventual conversion of the kinetic energy of the medium particle into heat energy.29 The acoustic wave propagation was absorbed by the principle of thermal damping and viscoelastic frame damping when passing through graphene oxide and polyurethane microcellular structures, as well as tortuosity in wave propagation on the graphene oxide surface, and


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micro-vibration of narrow graphene oxide layers with heat extraction.10 As shown in Figure 7a, the Perpendicular G-PUF will produce wave reflections and transmission while passing through the graphene oxide layers. In addition, as the sound waves enter the gaps between the graphene oxide walls, the fluctuation of the acoustic pressure will induce micro-vibration of the aligned graphene oxide layers. On the other hand, Figure 7b shows the sound absorption principle when a sound wave enters the inner void of the Parallel G-PUF. Here, multiple scattering phenomena strongly reduce the intensity of an acoustic wave passing through the highly porous graphene oxide-polyurethane foam, resulting in greatly increased sound absorption. The acoustic pressure is attenuated because of viscous friction and thermal conduction from the polyurethane frame and the graphene oxide microstructure surrounding the frame.30 Furthermore, from the viewpoint of viscoelastic frame damping, the molecular friction of the solid domain transforms sound waves in the vibration mode to heat loss. This is because the heat generated by the thermal conductivity of graphene oxide is rapidly released to the outside.31 In addition, from a structural point of view, the path of wave propagation changed because of the long passage created by the arrangement of the graphene in the polyurethane frame; the tortuosity of the wave propagation increased the scattering phenomena, resulting in enhanced sound absorption ability. Finally, as the sound waves approach the narrow gaps between the graphene oxide layers, the air starts to oscillate considerably, and the kinetic energy of the sound waves is lost to friction between the air and the graphene oxide layers and to the micro-vibration of the graphene layer. The mechanical stiffness and the hysteresis of the directional graphene oxide-polyurethane foam was verified by performing cyclic compression tests and comparing the results to those for bare polyurethane foam.32 The Parallel G-PUF was arranged parallel to the direction in which the


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compression load was applied. That is, the Parallel G-PUF was aligned in the z-direction, as shown in Figure 8a. On the other hand, the graphene oxide layers in the Perpendicular G-PUF were arranged perpendicular to the direction of the compressive load. Figure 8c shows the first cycle responses of the Parallel G-PUF samples with different graphene concentrations under dynamic compression tests. The results clearly indicate that the increase in the graphene oxide loading increases the mechanical strength of all samples. The high-density Parallel G-PUF sample shows much higher stiffness under 10% compressive strain because of the stiffening effect of the aligned graphene oxide layers.33 Moreover, the Parallel G-PUF shows superior compression stiffness compared to the Perpendicular G-PU, as shown in Figure 8d. Figures 8e and 8f show the cyclic compression responses of the Parallel G-PUF and the Perpendicular G-PUF samples, respectively. Interestingly, even though the Perpendicular G-PUF has much lower mechanical stiffness than Parallel G-PUF, the hysteresis became much smaller as the number of cycles increased. We speculate that the graphene oxide layers in the Perpendicular G-PUF are repeatedly crushed without remarkable damage under cyclic compressive loads.34 The directionally antagonistic graphene oxide-polyurethane foam can absorb kinetic energy during mechanical deformations because of this hysteresis mechanism.21 The energy loss coefficient was calculated by obtaining the ratio between the dissipated energy and the work performed by compression. The PUF has an energy loss coefficient of 0.31. In Figure 8c, the energy loss coefficient of the Parallel G-PUF-4 can be seen to be 0.63; that value for the Parallel G-PUF-2 is 0.47. Similarly, the Perpendicular G-PUF-4 has an energy loss coefficient of 0.48 and that of the Perpendicular G-PUF-2 is 0.34. The dissipated energy originated from van der Waals adhesion between the graphene oxide layers and the friction between the polyurethane support structure and graphene, or the movement of air through the


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porous structure.21 In Figure 8e, it can be observed that the energy loss coefficient, which ranges from 0.63 to 0.41 after the first 10 compression cycles, results in impinging the stress softening behavior due to bending of the graphene walls and cyclic loading. Moreover, this indicates energy dissipation caused by the buckling of microstructures of aligned graphene oxide layers and friction and adhesion between graphene oxide layers.35 On the other hand, the Perpendicular G-PUF shows good mechanical robustness and its energy loss coefficient gradually stabilizes. The energy loss coefficient of the Perpendicular G-PUF-4 is 0.48, 0.44, and 0.4 in the first, second, and tenth cycles, respectively. Interestingly, when the graphene concentrations of the Parallel G-PUF and the Perpendicular G-PUF are the same, the energy loss coefficients initially differ, but converge to almost the same value as the cycle compression test progresses, indicating the remarkable structural resilience of the directionally antagonistic graphene oxide-polyurethane hybrid aerogel.

CONCLUSION

In summary, we report a directionally anisotropic graphene oxide-polyurethane hybrid foam as a sound absorber with excellent performance in terms of energy dissipation. A macroscale sound absorber was realized by synthesizing a much larger scale graphene oxide (100µGO) sample with a diameter of ~100 µm from a large (mm) scale graphite (LSG) flake by using a modification of the Hummer-Tour method. The graphene oxide layers inside each polyurethane open cell were aligned using the simple directional freezing method, providing two directionally antagonistic structures: parallel-aligned versus perpendicular-aligned graphene oxide cellular structures in polyurethane foam according to the direction of graphene alignment. Two different


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Parallel and Perpendicular G-PUFs were defined considering the direction of incidence of the sound waves and these two samples were tested to evaluate their sound absorption performance and cyclic compression ability. This unique graphene oxide-polyurethane cellular microstructure, with uni-directionally aligned graphene and a semi-open arrangement, shows exceptional sound absorbing performance, and opens a new approach to the use of graphene aerogel in noise and vibration applications. We remain hopeful that the directionally anisotropic graphene oxidepolyurethane foam with excellent energy dissipation capacity would meet the tight requirements needed for next-generation sound absorbing materials.


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FIGURE CAPTIONS Figure 1. Schematic illustration of synthetic route to directionally antagonistic graphene oxidepolyurethane sound absorber, indicating fabrication steps to directionally aligned graphene oxide-polyurethane foam. Figure 2. Morphology of directionally antagonistic graphene oxide-polyurethane sound absorber. (a,b,c) Bare polyurethane foam. (d,e) Schematic of directionally antagonistic graphene oxide-polyurethane sound absorber with cutting planes. (f,i) SEM image of the Parallel G-PUF cut in xz plane. (g,j) SEM image of the Perpendicular G-PUF rotated by 90˚ after cutting in xz plane. (h,k) SEM image of the Parallel G-PUF cut in yz plane. Figure 3. SEM images of graphene oxide layers inside polyurethane foam with three different concentrations of graphene oxide. Concentrations of (a,d) 1 g/L, (b,e) 2 g/L, (c,f) 4 g/L. Figure 4. Normal incident sound absorption coefficient. (a) Schematic of impedance tube test with definition of directionally antagonistic graphene sound absorber according to the direction of the incident sound wave (Thickness of all samples is 45 mm). (b) Comparison of sound absorption coefficient between the Parallel G-PUF and the Perpendicular G-PUF at identical concentration of 4 g/L. (c) Comparison of sound absorption coefficient of the Parallel G-PUF when the concentration is varied between 2 g/L and 4 g/L. (d) Comparison of sound absorption coefficient of the Perpendicular G-PUF when the concentration is varied between 2 g/L and 4 g/L. Figure 5. Dependency of the normal incident sound absorption coefficient on the thickness of samples. (a) Comparison of sound absorption coefficient of the Parallel G-PUF with increasing


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thickness of 15, 30, and 45 mm. (b) Comparison of sound absorption coefficient of the Perpendicular G-PUF with increasing thickness of 15, 30, and 45 mm. Figure 6. Noise reduction coefficient (NRC) and flow resistivity. (a) Comparison of NRC values of the Parallel G-PUF and the Perpendicular G-PUF according to concentration of 100µGO. (b) Comparison of air flow resistivity of samples. Figure 7. Schematic illustration of mechanism of sound absorption in directionally antagonistic graphene sound absorber. (a) Perpendicular G-PUF where wave reflection occurs. (b) Parallel GPUF where wave absorption occurs. Figure 8. Mechanical properties of directionally antagonistic graphene microstructures under dynamic compression. Schematic illustration of (a) Parallel G-PUF and (b) Perpendicular G-PUF when compressive load is applied. Strain-stress curve of (c) Parallel G-PUF and (d) Perpendicular G-PUF at different concentrations. Results of cycle compression test of (e) Parallel G-PUF-4 and (f) Perpendicular G-PUF-4 at 10% strain.


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ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge via ACS Publications website at http://pubs.acs.org. Detailed synthesis of 100µGO, analysis of size distribution of 100µGO, chemical analysis of 100µGO, BET analysis of graphene oxide-polyurethane aerogel, two-microphone transferfunction method, and measurement of flow resistivity

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y. Kang) * E-mail: [email protected] (I.K.Oh) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This

work

was

partially

supported

by

Creative

Research

Initiative

Program

(2015R1A3A2028975) funded by National Research Foundation of Korea (NRF).


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Table of Contents Graphic

A directionally antagonistic graphene oxide-polyurethane hybrid aerogel, with aligned pores and graphene facesheets as a novel sound absorber, demonstrates high sound absorption ability at low frequency as well as excellent mechanical performance


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Figure 1. Schematic illustration of synthetic route to directionally antagonistic graphene oxidepolyurethane sound absorber, indicating fabrication steps to directionally aligned graphene oxide-polyurethane foam.


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Figure 2. Morphology of directionally antagonistic graphene oxide-polyurethane sound absorber. (a,b,c) Bare polyurethane foam. (d,e) Schematic of directionally antagonistic graphene oxide-polyurethane sound absorber with cutting planes. (f,i) SEM image of the Parallel G-PUF cut in xz plane. (g,j) SEM image of the Perpendicular G-PUF rotated by 90˚ after cutting in xz plane. (h,k) SEM image of the Parallel G-PUF cut in yz plane.


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Figure 3. SEM images of graphene oxide layers inside polyurethane foam with three different concentrations of graphene oxide. Concentrations of (a,d) 1 g/L, (b,e) 2 g/L, (c,f) 4 g/L.


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Figure 4. Normal incident sound absorption coefficient. (a) Schematic of impedance tube test with definition of directionally antagonistic graphene sound absorber according to the direction of the incident sound wave (Thickness of all samples is 45 mm). (b) Comparison of sound absorption coefficient between the Parallel G-PUF and the Perpendicular G-PUF at identical concentration of 4 g/L. (c) Comparison of sound absorption coefficient of the Parallel G-PUF when the concentration is varied between 2 g/L and 4 g/L. (d) Comparison of sound absorption coefficient of the Perpendicular G-PUF when the concentration is varied between 2 g/L and 4 g/L.


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Figure 5. Dependency of the normal incident sound absorption coefficient on the thickness of samples. (a) Comparison of sound absorption coefficient of the Parallel G-PUF with increasing thickness of 15, 30, and 45 mm. (b) Comparison of sound absorption coefficient of the Perpendicular G-PUF with increasing thickness of 15, 30, and 45 mm.


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Figure 6. Noise reduction coefficient (NRC) and flow resistivity. (a) Comparison of NRC values of the Parallel G-PUF and the Perpendicular G-PUF according to concentration of 100µGO. (b) Comparison of air flow resistivity of samples.


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Figure 7. Schematic illustration of mechanism of sound absorption in directionally antagonistic graphene sound absorber. (a) Perpendicular G-PUF where wave reflection occurs. (b) Parallel GPUF where wave absorption occurs.


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Figure 8. Mechanical properties of directionally antagonistic graphene microstructures under dynamic compression. Schematic illustration of (a) Parallel G-PUF and (b) Perpendicular G-PUF when compressive load is applied. Strain-stress curve of (c) Parallel G-PUF and (d) Perpendicular G-PUF at different concentrations. Results of cycle compression test of (e) Parallel G-PUF-4 and (f) Perpendicular G-PUF-4 at 10% strain.


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Directionally Antagonistic Graphene Oxide-Polyurethane Hybrid

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