Polymerizable Nonconventional Gel Emulsions and Their Utilization in

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Polymerizable Nonconventional Gel Emulsions and Their Utilization in the Template Preparation of Low-Density, High-Strength Polymeric Monoliths and 3D Printing Jianfei Liu, Pei Wang, Yinan He, Kaiqiang Liu, Rong Miao,* and Yu Fang* Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, P. R. China

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ABSTRACT: New kinds of mechanically robust, porous polymeric materials with varied internal structures were produced using a newly designed water-in-oil (W/O) gel emulsion as a template. The density, internal structure, and mechanical properties of the porous materials can be easily and largely adjusted by varying the water content and the stabilizer (SiO2) amount in the relevant gel emulsions. Specifically, the density and compressive strength could be changed from 0.19 to 0.96 g/ cm3 and from a few MPa to more than 100 MPa, respectively. Importantly, the materials obtained demonstrated unusual low-frequency sound absorption properties. In addition, the gel emulsions created can be used as a new kind of 3D printable material, which allows polymerization after printing. In this way, complicated architectures can be produced.

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INTRODUCTION Bypassing the sophisticated traditional manufacturing techniques, 3D printing technology provides easier operation and promises new opportunities for personal design and the bottom-up fabrication of functional objects with complex, intricate shapes.1,2 During the past few years, much attention has been paid to this attractive area, and significant progress has been achieved. The material used in a 3D printing process plays a key role, as it determines both the printing technology to be chosen and the further applications of the printed products.3−5 Thus, the development of 3D printable materials has been one of the key issues in studies of 3D printing technology.6 To our knowledge, thermoplastics, photocurable materials, and metals have been widely used in 3D printing via the utilization of fused deposition modeling (FDM), stereolithography, and laser sintering techniques.7−9 As a new kind of 3D printable material, gel emulsions have recently been employed for 3D printing using the newly developed direct inkjet writing (DIW) technique. The reasons for paying attention to gel emulsions as new 3D printable materials mainly include two aspects: (1) the smart shear thinning property of gel emulsions at room temperature, including shear-thinning behavior to permit extrusion, a high shear elastic modulus, and a high shear yield strength, and (2) the unique architectures generated after polymerization of the printed objects, which broadens the applications of the produced products due to their larger surface area, lower density, and improved permeability.10−16 However, to be 3D printable materials, gel emulsions should possess a number of superior rheological properties. First, these materials should © XXXX American Chemical Society

exhibit a relatively low elastic shear modulus under a high shear stress, enabling stable flow through the deposition nozzle. Second, their storage moduli should be large enough to allow the extruded filament to deposit immediately, which avoids the collapse of the designed architecture to be produced. Third, the gel emulsions must be homogeneous to prevent clogging in the nozzle.17−19 These requirements may explain why the development of 3D printable gel emulsions is challenging. As reported in the literature, the rheological property of a gel emulsion is largely dependent upon the employed stabilizer, where the most widely used are surfactants,20 micro/ nanoparticles,21 amphiphilic polymers,22 and low-molecularmass gelators (LMMGs),23−25 and the composition of the system. Compared with others, micro/nanoparticles, in particular silica (SiO2), show great advantages because of their low toxicity, low cost, outstanding stability, reversible gel−sol phase transition, and nonpost-treatment after polymerization.26,27 In our previous work, we found that both acidified aramid fiber and acidified cotton could be used as efficient stabilizers to generate water-in-styrene (W/O) gel emulsions.28−30 However, as others reported, these gel emulsions cannot meet the requirements for 3D printing, which hindered their further applications.31,32 To improve the rheological properties of the systems, SiO2 nanoparticles were introduced, and we found that the properties of the systems were significantly Received: December 7, 2018 Revised: February 26, 2019

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DOI: 10.1021/acs.macromol.8b02610 Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Compositions of the Gel Emulsions and the Porous Monoliths As Produced from Polymerization of the Emulsionsa no.

continuous phase (μL)

OTES (μL)

SiO2b (w/v)

dispersed phase (μL)

yields (%)

compressive stress (MPa)

apparent density (g/cm3)

GE1(M1) GE2(M2) GE3(M3) GE4(M4) GE5(M5) GE6(M6)

940 940 940 940 940 940

40 40 40 40 40 40

8 8 8 8 8 8

50 500 1000 2000 3000 4000

99.6 99.4 99.5 98.2 98.6 98.3

102.2 48.1 24.6 19.4 7.3 6.2

0.96 0.66 0.49 0.31 0.24 0.19

a The continuous phase composition: St (640 μL, 68%, v/v), DVB (140 μL, 14.9%, v/v), EGDMA (140 μL, 14.9%, v/v), TMPTMA (20 μL, 2.1%, v/v), and the initiator AIBN (20 mg, 2%, w/v). bSiO2 is hydrophobic, and its specific surface area is ∼180 m2/g (8%, w/v, SiO2 accounts for the volume of the continuous phase).

enhanced.33 Further studies demonstrated that the internal structures and the viscoelastic properties of the gel emulsions could be widely tuned by a variation in either the SiO2 content in the systems or the ratio of water to styrene. In this way, gel emulsions with optimized stability, storage moduli, and shear thinning properties were obtained, which allow 3D printing to be described.34−37 The polymerization of the as-printed architectures resulted in various scaffolds with well-controlled mechanical strength and hierarchical porous internal structures. Importantly, the porous polymeric materials show anisotropic pores, including channels and cracks (cavities), which allow sound waves to enter the materials.38,39 Sound energy could be dissipated through thermal loss caused by the friction of air molecules with the pore walls. As such, these materials have demonstrated high sound absorption ability, especially at low frequencies, which is highly harmful to humans. This paper reports the details.

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parallel plate was used to avoid evaporation. All measurements were conducted at ambient temperature (25 °C). First, a stress sweep measurement at a fixed frequency (1 Hz) was conducted in a stress range of 0.1−1,000 Pa, which illustrates the mechanical strength of the gel sample. Second, an angular frequency sweep was conducted from 0.1 to 100 rad/s at a constant stress of 1.0 Pa that results in small strain, well within the linear regime. Viscometry measurements were performed at a shear rate of 0.01−100 s−1. Preparation of the Porous Polymeric Monoliths. The obtained gel emulsions were thoroughly degassed, then heated to 50 °C, and maintained at this temperature for 4 h to start prepolymerization. Next, the temperature was raised to 75 °C, and the emulsions were maintained at this temperature for another 4 h to complete the polymerization. The resulting monolith was removed from the test tube and initially washed with ethanol (5 mL) four times and then dichloromethane (5 mL) three times. Finally, the monolith was dried in an oven for 8 h (∼50 °C) and then at ambient temperature until its weight became constant. SEM Observation. The sizes of the pores and pore throats of the as-prepared porous polymeric monoliths were semiquantitatively calculated using images taken by a Quanta 200 scanning electron microscope (Philips-FEI, 15 kV and 10 mA). The correction for the average value taken from a random section through a sphere has been applied.40 Prior to observation, ∼1 cm3 of the material was mounted on a sample holder and sputtered with gold for 80 s to ensure sufficient conductivity. FTIR Measurement. These measurements were performed on a Bruker VERTEX 70 V infrared spectrometer. The testing scale ranged from 400 to 4000 cm−1 with 128 scans for each sample. The KBr pellet was obtained by mixing a small amount of the sample and anhydrous KBr powder. The FTIR measurements were performed at room temperature. Contact Angle Test. The contact angle of the monolith to water was measured in a routine way with a Dataphysics OCA 20 contactangle system at ambient temperature. Porosity Measurements. Mercury injection methods have been effectively applied to characterize the size and size distribution of the pores and evaluate their structures. M4 was chosen as a sample to conduct the measurements in an AutoPore 9500, which is a product of Micromeritics Instrument (American) Ltd. Mechanical Property Tests. The mechanical properties of the gels were measured using a Xie Qiang mechanical testing machine (CTM 2500 universal testing machine) at room temperature. All gels were prepared and aged for 24 h in the test. All gel samples for compression testing have a cylindrical shape of 12 mm in diameter and 24 mm in height. All samples were set on the lower plate and were compressed by the upper plate at a strain rate of 8.3%/min. 3D Printer Remolding. 3D printing was performed on a 3D Fused Deposition Modeling (FDM) printer (Aurora Technology Co., Ltd., China). Notably, the FDM printer was remolded before use and was changed into a direct ink writing (DIW) printer by replacing the original nozzle jetting with a conical needle. Moreover, during the 3D printing process, only the three-axis positioning stage and the control system were used. The heating system was not functional. Gel Emulsion Printing. The prepared gel emulsion was loaded into a 50 mL syringe barrel attached by drop-on-demand valves of the

EXPERIMENTAL SECTION

Materials. SiO2 (hydrophobic and the specific surface area is 180 m2/g) was purchased from Sigma-Aldrich, and styrene (St) and divinylbenzene (DVB) were chemically pure from Tokyo Chemical Industry Co., Ltd. These materials were purified to remove the preadded inhibitor by passing through a basic alumina column before use. After purification, the monomer (St) and the cross-linker (DVB) were stored in a freezer, if they were not used directly. 2,2′Azobis(isobutyronitrile) (AIBN, 97%) was purchased from SigmaAldrich and used without further purification. Ethylene glycol dimethacrylate (EGDMA), trihydroxymethylpropyltrimethyl acrylate (TMPTMA), and n-octyltriethoxysilane (OTES) were purchased from J&K Technology Co., Ltd. All solvents used in the studies were purified, as described in solvent handbooks. The water used throughout the study was doubly distilled. Other reagents, except those specially indicated, were used directly. Preparation of the Gel Emulsion. All the gel emulsions under study were prepared in a similar way. Typically, the oil or continuous phase was prepared in a test tube, which consists of the initiator AIBN, monomer St, and cross-linker DVB, EGDMA, and TMPTMA. A certain amount of SiO2 was added into the tube, and then a suitable amount of water was added. Finally, the mixture was shaken vigorously. The formation of a gel emulsion was confirmed by inverting the test tube to observe whether the mixture inside could still flow. The compositions of the gel emulsions are listed in Table 1. Characterization of the Gel Emulsion. Confocal laser scanning fluorescence microscopy images of the gel emulsion were taken on a TCS SP5 laser scanning confocal microscope. The structure of the probe used in the observation is shown in Scheme S1. The observation was conducted by using 365 and 550 nm as the excitation and emission wavelengths, respectively. Rheological Measurements. These measurements were performed with a stress-controlled rheometer (TA Instruments AR-G2) equipped with steel-coated parallel-plate geometry (20 mm diameter). The gap distance was fixed at 1000 μm. A steel cover matching with a B

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Figure 1. Rheological behaviors of the printable neat gel emulsion. (a) Log−log plots of viscosity as a function of the shear rate. (b) Log−log plots of the storage (G′) and loss (G″) moduli versus the shear stress applied to the gel emulsion under testing. (c) Evolution of G′ and G″ of the gel emulsion as functions of angular frequency. (d) Storage modulus and yield stress of the gel emulsion versus the water content. Note: the gel emulsion used in (a−c) is GE4 (cf. Table 1). peristaltic pump. The syringe barrel was connected to the conical needle of the remolded 3D printer with a thin pipe. The as-mentioned printing system was mounted beside the three-axis positioning stage. A computer program that uses G code files of the desired geometry was created in CAD. The software of the machine converted the input 3D model into a machine command, which controls the printing and motion. The inner diameter of the conical needle used was 0.4 mm (Huihuang Dispensing Company, China). The printing speed was set to 10 mm/s. Although the applied pressures were individually adjusted for each paste, they are generally in the range of a few kPa. Gel emulsions with both a dispersed phase (water) and a continuous phase (polymerizable oil) were used as the 3D printing materials. After each printing, a desired architecture was generated, and the gel emulsion with the shape was solidified at room environment. Then, the precast product was immersed into an oil bath to implement polymerization (∼6 h). The crude product obtained was washed with ethanol three times and finally dried at room temperature. Sound Absorption. The normal incident sound absorption coefficients of all samples were measured on a Brüel & Kjær impedance tube system (type 4206) via the two-microphone method. A loudspeaker was connected to the inlet side of the tube, and broadband sound was supplied to the tube through the loudspeaker. The downstream side of the pipe was equipped with a rigid portable piston. The materials to be tested were arranged on the front surface of the piston, and then the surface impedance was measured. The tested frequency band range was from 0 to 6.4 kHz. The samples tested exhibited a cylindrical shape with a diameter of 29 mm, but different samples had different thicknesses, which vary from 10 mm to 20, 30, 40, 50, and 60 mm.

SiO2 was employed as the stabilizer. In a typical study, a mixture of styrene (St), divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA), and trihydroxymethylpropyltrimethyl acrylate (TMPTMA) was used as the continuous phase, and water was used as the dispersed phase. St is a monomer, and DVB is a cross-linker. EGDMA and TMPTMA were introduced to improve the stability of the system and the mechanical strength of the final materials to be produced. The basic constitutions of the gel emulsions are shown in Table S1. The effects of the stabilizer, SiO2, content on gel emulsion formation and the rheological properties of the systems are shown in Figures S1 and S2, respectively. The results from the water effect study are depicted in Figures S3 and S2. As seen from Table 1, the water content in the gel emulsions can vary from 4.8% to 80.3%. In other words, the volume fraction of the dispersed phase in the systems could be much lower than 74%, which is the critical value of conventional gel emulsions.41−43 This observation suggests that the systems are not general Pickering emulsions, but something like lowmolecular-mass gelator (LMMGs)-based gel emulsions, which have never been reported previously.44 Interestingly, the storage modulus and the yield stress of the systems are not simply affected by the water content. The two values first increased with increasing water content, but the values then decreased with a further increase in the volume fraction of the dispersed phase, indicating that the rheological behavior of the gel emulsions could be largely tuned by alternating the stabilizer content and the dispersed phase volume fraction (Figure S4). Further rheological studies showed that the viscosity of the gel emulsion decreased with increasing shear rate (Figure 1a), and similarly, both the storage and the loss moduli decreased

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RESULTS AND DISCUSSION Preparation and Rheological Behaviors of Gel Emulsions. To prepare 3D printable gel emulsions with polymerizable organics as the continuous phase (oil phase), C

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Macromolecules with increasing shear stress. Moreover, further increasing the shear stress could result in a gel-to-sol phase transition (Figure 1b). All these properties are favorable for 3D printing.45,46 Frequency scanning revealed that the gel emulsions are stable within, at least, an angular frequency range of 0.1−100 rad s−1, and moreover, the value of G′ is approximately an order of magnitude higher than that of G″ within the whole frequency range studied (Figure 1c). The large difference between G′ and G″ enables the stable flow of the gel emulsion in the extrusion process when it is used as a 3D ink. As seen from Figure 1b, G″ will exceed G′ at a higher shear stress as the latter decreased faster, which allows the gel to leave the nozzle in liquid form. Fortunately, the gel recovers immediately when the shear stress is removed, a necessity for retaining printed complicated 3D architectures. To understand the effect of the water content on the processability of the gel emulsions, the G′ and τ (yield stress) of the systems were quantitatively measured at different water contents. Of the compositions studied, the one with 2,000 μL of water shows much superior rheological properties than the others, where the gel emulsion has the highest G′ and lowest loss factor G″/G′ (cf. Figure S4 and Figure 1d). Thus, this gel emulsion (GE4) was chosen for the 3D printing study. Notably, GE4 is so stable that ten-month storage under ambient conditions has little effect on the exterior of the gel, as evidenced by the pictures depicted in Figure S5. 3D Printing Process. With the prepared gel emulsion (GE4) as ink, different architectures were produced under ambient conditions using a 3D printing technique. The printing process is shown in Scheme 1, and a real printing scene is visually demonstrated in Video S1. For real printing, GE4 was first loaded into a syringe attached to the drop-ondemand valves of a peristaltic pump, which was connected to (via a thin pipe) the nozzle jet mounted beside the three-axis positioning stage of the 3D printer. As expected, the filaments

of the gel emulsion exhibited accurate and stable 3D features because of their excellent rheological properties. With this technique and the materials, free-standing woodpiles and other complicated objects with complicated structures were built (Figure 2a−f). Polymerization of the objects in an oil bath resulted in permanent structures. Scanning electron microscopy (SEM) of M4 studies revealed that the produced products possess, as expected, hierarchical porous internal structures (Figure 2g−i). The applications of porous materials are highly dependent on their internal structures, and we further examined the effect of the water and SiO2 contents on the internal structures of the polymerized gel emulsions (Figure 3 and Figure S6). As expected, all the monoliths prepared in this way possess porous internal structures, but for the monoliths produced from the systems with increasing water content, the pores changed from closed cells to interpenetrated ones (Figure 3). For the effect of SiO2, all the monoliths produced from the systems with different stabilizer contents show closed pore structures, and the pore size and size distribution do not change very much (Figure S6). These results may not be difficult to understand because increasing the dispersed phase (water) volume ratio must be accompanied by a decrease in the thickness of the continuous phase layer, which is no doubt becoming increasingly thinner and more vulnerable to breaking, resulting in more and larger throats between the neighboring pores within the polymerized monolith. As for why SiO2 showed little effect on the internal structures of the monoliths asprepared, the results can be understood by considering the nature of the gel emulsions, which as mentioned above are not conventional Pickering emulsions, but they are sol/gel two phase systems that are water droplets trapped within the gelled continuous phase. This unique structure compels the SiO2 particles to always stay within the continuous phase. The only difference between the gel emulsions is the stability of the gelled continuous phase. The thickness of the walls between the droplets of the dispersed phase does not change with a variation in the stabilizer content, which may explain why the internal structures of the monoliths are characterized by closed cells. As mentioned above, silica nanoparticles were introduced not only to make the oil−water two phase systems become gels but also to enhance the mechanical stability of the monoliths to be prepared. To evaluate the effect, compression stress− strain measurements were conducted. The results are provided in Figure S7. The maximum compression strength of the materials increased with increasing SiO2 content in the systems. The water content effect on the compression strength of the monoliths was also studied, and the results are shown in Figure S8. The water content shows a great effect upon the mechanical strength of the prepared monoliths. As expected, the strength of the materials decreases with increasing water content in the gel emulsions, which is one of the reasons for the decreased density of the materials. Structural Characterizations. To better understand the relationship between the internal structure of the gel emulsion and the properties of the polymerized monolith, monolith no. 4 (M4) and its corresponding gel emulsion (GE4) were chosen as examples for the study. As depicted in Figure 4a, the gel emulsion does possess water-in-oil (W/O) structure, as evidenced by the fluorescence microscopy image shown in the figure, where a hydrophobic perylene bisimide derivative (Scheme S1) was adopted as the probe, which is believed to

Scheme 1. Schematic Representation of the Procedures Adopted for the Preparation of Porous Polymeric Monoliths through 3D Printing of Stable Gel Emulsionsa

a

The initial oil phase with SiO2 (route I) was added with a dispersed phase (route II) to form a gel emulsion ink that is suitable for DIW. Porous structures were obtained after polymerization of the architecture from DIW of the gel emulsion. The inset shows the proposed stabilization mechanism of the gel emulsion; namely, particle-stabilized gel emulsions are created via interfacial adsorption of the hydrophobic SiO2 particles. D

DOI: 10.1021/acs.macromol.8b02610 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. Photographs of the 2D and 3D structures of the gel emulsion by DIW-based printing. (a) The picture shows that the gel emulsion is extruded through a robot-assisted nozzle to construct different 3D architectures. (b) Snapshots of the gel emulsion in a syringe. (c) Schematic illustration of nozzle-based printing techniques and the deposition of the material on a substrate to produce the first layer. (d, e) Images of a first layer of an architecture of different shapes. (f) Top view image of a printed grid structure after polymerization and drying. (g−i) SEM images of the 3D architecture with different magnifications.

Figure 3. SEM micrographs of the monoliths prepared via polymerization of the gel emulsions of the same continuous phase composition but different water contents: M1 (50 μL), M2 (500 μL), M3 (1000 μL), M4 (2000 μL), M5 (3000 μL), and M6 (4000 μL). Composition of the continuous phase: SiO2 (8%, w/v, for the volume of the continuous phase). The compositions of these monoliths are listed in Table 1.

stay only in the oil phase. The contact angle (for water) measurement demonstrated that M4 is superhydrophobic as the value exceeds 162°, which is understandable as the components, St, DVB, and EGDMA, are all hydrophobic in nature (Figure 4b). The porosity of the monolith was studied

using an automatic mercury injection apparatus. The results are shown in Figure 4c. The total pore volume is ∼1.7 mL/g, and the average pore diameter is ∼11.8 μm, which is roughly in accordance with the results from SEM observations (Figure 3 and Figure S6). A compressive test on M4 revealed that the E

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Figure 4. (a) Fluorescence confocal image of a typical gel emulsion (GE4). (b) Water droplet (2 μL) on the monolith (M4) with a water contact angle of 162.1° (left); the picture on the right shows drops of CuSO4 aqueous solution (blue) and dyed dichloromethane (red) added onto the monolith. (c) Pore diameter distribution of a typical porous polymeric monolith (M4). (d) Compressive strength−strain curve of monolith M4.

Figure 5. Sound absorption performance of monolith M4. (a) Schematic representation of the impedance tube test with the definition of a directionally antagonistic porous material sound absorber. (b) Sound absorption curves of porous materials with different thicknesses of 10, 20, 30, 40, 50, and 60 mm. (c, d) Two earplugs and an ear produced using a 3D printing technique with GE4 as the printable materials.

and the relevant reactants were measured, and the results are depicted in Figure S9. Clearly, M4 is characterized by absorption peaks belonging to St, DVB, EGDMA, and nOTES. In particular, the characteristic absorptions of St are dominated in the spectrum of M4, confirming the fact that it is the main component of the monolith. In addition, the

compression strength could reach 19.4 MPa at which the strain is 10%, suggesting that the material is mechanically strong, even though its apparent density is only 0.31 g/cm3 (Figure 4d). However, notably, the material is relatively brittle. To characterize the chemical structures and compositions of the monoliths, the FTIR spectra of the example monolith, M4, F

DOI: 10.1021/acs.macromol.8b02610 Macromolecules XXXX, XXX, XXX−XXX

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after extrusion and can therefore be used as a new kind of 3D printable material. By use of the DIW-based 3D printing technique and polymerization, a variety of complex structures were produced. SEM studies revealed the hierarchical porous internal structures of the polymerized materials, which enables highly efficient energy dissipation, as demonstrated by the favorable sound absorption performance, especially at low frequencies, which can be taken as urgently needed nextgeneration sound-absorbing materials.

appearance of the peaks within the range from 1680 to 1620 cm−1 in the polymeric material can be taken as an indication of the presence of a carbon−carbon double bond, which could be a result of incomplete polymerization. Sound Absorption. From the above studies, the lightweight polymeric monolith has rich porous and coherent network structures, which should be favorable for sound absorption. As reported in the literature,38,39 the pore structure and pore distribution of a porous material are closely related to its sound absorption property, as they have a great influence on sound energy dissipation. This is why open pore foams are widely used for noise control in applications for use in buildings and transportation because of their capability to dissipate sound energy over a wide frequency range. Figure 5a provides a schematic diagram of the experimental setup used to measure the surface impedance of the samples. Figure S10a shows a conceptual morphology of porous monoliths made from gel emulsions. As shown, it contains various pores with closed, partially closed and open cell structures, and the sound energy will be reflected, absorbed, and transmitted (Figure S10b). Sound absorption coefficient α is used to quantify the dissipation ability of a sound absorbing material, which can be tested by the impedance tube method, as used in the present test, where α is defined as the ratio of the absorbed sound energy to the total incident sound energy (eq 1). α=1−

Er + E t E = a Ei Ei

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02610.

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Figures S1−S10, Table S1, and Scheme S1 (PDF) Video S1 (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]. *E-mail [email protected]. ORCID

Kaiqiang Liu: 0000-0001-7069-566X Yu Fang: 0000-0001-8490-8080 Notes

The authors declare no competing financial interest.

(1)

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Figure 5b shows the results of sound absorption tests performed with M4 as an example monolith, in which samples of different thicknesses (10, 20, 30, 40, 50, and 60 mm) were used. Referring to the figure, the results revealed that the sound absorption coefficient generally increases with increasing sample thickness. Notably, the sound absorption curve shows a peak at approximately 200 Hz, and for the sample with a thickness of 60 mm, the absorption coefficient could reach 0.78, indicating that M4 is a good low-frequency (180−500 Hz) sound absorber, which has been rarely reported in the literature. Motivated from these results and the printable property of the gel emulsion, two earplugs and an ear (Figure 5c,d) were 3D printed to illustrate the potential application of the as-prepared materials in noise absorption. The results are not surprising because the materials under testing possess hierarchical porous internal structures with the major pore sizes ranging from 10 to 30 μm and minor pore sizes ranging from 1 to 2 μm as well as pore throats of different diameters, which makes the materials possess larger contact areas with air molecules, resulting in a greatly enhanced ability to dissipate sound energy. In addition, the unique internal structures must facilitate the propagation of sound waves into the interiors of the materials, which are no doubt favorable for the absorption of sound of different frequencies. It is anticipated that the as-prepared materials will play roles in the areas with the particular requirement for the absorption of low-frequency sound, such as improving the quality of living environments for residents.

ACKNOWLEDGMENTS We acknowledge the funding from the Natural Science Foundation of China (21527802, 21673133, 21603139, 21872091, and 21820102005), 111 project (B14041), and Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33).

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CONCLUSION In summary, a new SiO2-based W/O gel emulsion displaying strong shear thinning properties and high viscosity at low shear rates was developed. The gel emulsion enables shape retention G

DOI: 10.1021/acs.macromol.8b02610 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02610 Macromolecules XXXX, XXX, XXX−XXX