melamine composite foam with enhanced sound absorption performance

Journal Pre-proof Hierarchical Pore Structure Based on Cellulose Nanofiber/Melamine Composite Foam with Enhanced Sound Absorption Performance Lu Shen (Methodology) (Formal analysis) (Investigation) (Data curation) (Writing - original draft) (Visualization), Haoruo Zhang (Methodology) (Investigation) (Data curation), Yanzhou Lei (Resources) (Validation), Yang Chen (Resources) (Validation), Mei Liang (Conceptualization) (Supervision) (Writing - review and editing) (Resources) (Project administration) (Validation), Huawei Zou (Conceptualization) (Supervision) (Writing - review and editing) (Resources) (Funding acquisition) (Project administration)

PII:

S0144-8617(20)31578-2

DOI:

https://doi.org/10.1016/j.carbpol.2020.117405

Reference:

CARP 117405

To appear in:

Carbohydrate Polymers

Received Date:

28 July 2020

Revised Date:

20 October 2020

Accepted Date:

26 October 2020

Please cite this article as: Shen L, Zhang H, Lei Y, Chen Y, Liang M, Zou H, Hierarchical Pore Structure Based on Cellulose Nanofiber/Melamine Composite Foam with Enhanced Sound Absorption Performance, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.117405

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Hierarchical Pore Structure Based on Cellulose Nanofiber/Melamine

Composite

Foam

with

Enhanced Sound Absorption Performance Lu Shen, Haoruo Zhang, Yanzhou Lei, Yang Chen, Mei Liang*, Huawei Zou*

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(The State Key Lab of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065)

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Graphical abstract

*

Corresponding author. Tel: +86-28-85408288; Fax: +86-28-85402465.

E-mail address: [email protected](Mei Liang) *

Corresponding author. Tel: +86-28-85408288; Fax: +86-28-85402465.

E-mail address: [email protected] (Huawei Zou) 1

Highlights

 Cellulose nanofiber was combined with melamine foam by cyclic freezingthawing.  The constructed hierarchical pore structure includes macropores and mesopores.  The complex pore structure enhanced the sound absorption over a broadband range.  The prepared composite foam has enhanced mechanical property.  The noise reduction coefficient of composite foam has an improvement of 80%.

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Abstract

For the preparation of high-performance sound absorption materials, the fabrication

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of hierarchical pore structure has proven to be an effective way. Herein, cellulose

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nanofiber (CNF) and melamine foam (MF) were combined by an environmentally friendly method for the first time, which endowed the final composite foam with

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both macropores and mesopores. The hierarchical pore structure was constructed by cyclic freezing-thawing, which enhanced the multiple reflections and micro-

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vibration of the sound waves, resulting in an obvious improvement in sound absorption performance. Specifically, compared with the unmodified MF, the sound

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absorption performance of composite foam with a thickness of 20 mm at 0.4 wt% CNF concentration showed an enhancement of about 107% at 500 Hz and the NRC

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(noise reduction coefficient) had an improvement of 80%. This work is expected to provide more inspiration for the design and preparation of high-performance sound absorption materials.

Keyword: cellulose nanofiber; hierarchical pore structure; cyclic freezing2

thawing; sound absorption performance

1. Introduction With the rapid development of industrial machines, airplanes, trains and so on， noise pollution has become more and more serious. It has been reported that longterm exposure of noise to the human body can cause hearing damage, annoyance, sleep disorders, cardiovascular diseases, and so on (Basner et al., 2014; Muzet,

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2007). For eliminating the unfavorable influence of noise on people, it is an effective method to utilize sound absorption materials to dissipate sound energy in the propagation path.

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Sound absorption materials are generally classified into two types: resonant

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sound absorption materials and porous sound absorption materials (Gai, Xing, Li, Zhang, & Wang, 2016; Tang & Yan, 2017). The resonance absorbing materials such

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as Helmholtz resonators (Cai, Guo, Hu, & Yang, 2014), membrane absorbers (Min, Nagamura, Nakagawa, & Okamura, 2013), and perforated plates (Zhao, Yu, & Wu,

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2016) have a narrow absorption frequency band that severely limit their applications. The porous sound absorption materials include natural fiber (Berardi & Iannace,

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2017), melamine foam (MF) (D'Alessandro, Baldinelli, Bianchi, Sambuco, &

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Rufini, 2018), polyurethane foam (Hyuk Park et al., 2017), ceramics (Du et al., 2020), fiberglass (Sun, Shen, Ma, & Zhang, 2015), etc. Such porous materials normally have advantages of wide sound absorption frequency range, convenient material selection and relatively simple processing, resulting in their increasing application in practical engineering (Berardi & Iannace, 2015; Yang, Kim, & Kim, 2003). Unfortunately, the porous sound absorption materials generally have poor 3

sound absorption performance within the low frequency range (100-800 Hz) and require to increase the thickness and weight for achieving a satisfactory sound absorption coefficient, which restricts their application with stringent requirements (C. Zhang, Li, Hu, Zhu, & Huang, 2012). Structural properties such as pore size are considered as the main controlling factors affecting the sound absorption performance of porous materials(Park et al.,

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2017). It has been demonstrated that both macropores and mesopores have varying degrees of influence on the sound absorption performance of materials by the existing research (Ghaffari Mosanenzadeh, Naguib, Park, & Atalla, 2015; Huang, Zhou, Xie, Yang, & Kong, 2014). However, the effect of only uniformly controlling

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the pore size of the materials on the sound absorption performance has certain

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limitations, especially in the low frequency range. Compared with porous materials with uniform structure, the hierarchical pore structure can provide more tortuous

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propagation paths and more reaction areas, which is more beneficial to improve the sound absorption performance and expand the sound absorption frequency range

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(Cao, Si, Wu, et al., 2019). Recently, several strategies to fabricate hierarchical pore structure have been reported, including the electrospinning technique (Cao, Si, Yin,

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Yu, & Ding, 2019), impregnation method (Lee & Jung, 2019), freeze-drying method

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(Simón-Herrero, Peco, Romero, Valverde, & Sánchez-Silva, 2019), templatedirected chemical vapor deposition (CVD) technique (Xue et al., 2017), salt-out method (Ghaffari Mosanenzadeh et al., 2015) and so on. For example, Cao et al. (Cao, Si, Yin, et al., 2019) used a direct electrospinning method to fabricate Polystyrene (PS) fiber sponges with a lamellar corrugated microstructure for sound absorption. Simón-Herrero et al (Simón-Herrero et al., 2019) synthesized aerogels 4

based on a ternary system of polyvinyl alcohol (PVA), nanoclay and thermally reduced graphene oxide (trGO) with enhanced sound absorption properties. To avoid materials collapse during the fabrication process and practical application, fabricating the hierarchical pore structure based on commercial foams through the impregnation method and freeze-drying method is an effective strategy (Liu et al., 2019; Nine et al., 2017; Oh, Kim, Lee, Kang, & Oh, 2018). For example,

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Lee et al. (Lee & Jung, 2019) reported that the polyurethane foam (PU) foam was immersed in the GO solution by a step-by-step vacuum-assisted process. The obtained PU foam with the hierarchical pore structure had a great enhancement in sound absorption. Nine et al. (Nine et al., 2017) immersed MF into the aqueous GO

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to evenly distribute GO in the open-cell network to form a GO‐ based lamella

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network for enhancing sound absorption. The resulting composite foam with a density of 24.12 kg/m3 and a thickness of 26 mm exhibited 60% enhancement over

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a broad absorption band range from 128 to 4000 Hz. However, the hierarchical pore structure based on commercial foams is mainly constructed by adding fillers such

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as GO, and the pore structure of the foam is only optimized on the macropores scale. The improvement of sound absorption performance brought by these methods is

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still difficult to meet the strict requirements for sound absorption materials.

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Cellulose is the most abundant and renewable biomass polysaccharide on earth. It is reported that the C6 primary hydroxyl groups of cellulose can be selectively converted into C6 sodium carboxylate groups in 2,2,6,6-tetramethylpiperidine (TEMPO) oxidation system (Isogai, Saito, & Fukuzumi, 2011). The obtained TEMPO-oxidized cellulose nanofiber (CNF) has the characteristics of high aspect ratio, low ζ-potential in water, high crystallinity and high elastic modulus (Lu et al., 5

2017; Rahmatika, Goi, Kitamura, Widiyastuti, & Ogi, 2019). It has been widely used in the fields of paper coating, barrier material and sensor applications (Alves, Ferraz, & Gamelas, 2019). But it is nearly no report about tempo-oxidized CNF used in sound absorption materials. In this work, CNF was combined with MF through the cyclic freezing-thawing method for the first time. The internal CNF films formed in this way can change the

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pore size on the macropores scale and affect the pore openness of MF. And some mesopores on the CNF films can be formed through the cyclic freezing-thawing method due to the close packing of CNF. The tortuosity and airflow resistance inside the materials can be adjusted by the formation of CNF films, which can provide

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more tortuous paths for sound waves, leading to more sound energy dissipation by

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air vibration. At the same time, the micro-vibration of the CNF films also can improve the consumption of energy, resulting in the improvement of sound

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absorption performance. More importantly, the hierarchical pore structure with both macropores and mesopores can increase the complexity of sound wave propagation

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and enhance the sound absorption over a broad absorption band range. This work is expected to provide more green approaches for the design and preparation of high-

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performance sound absorption materials.

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2. Experimental Section 2.1. Materials

Softwood bleached kraft pulps were supplied by the Institute of Paper Science and Technology at Georgia Tech, USA. Melamine foam (MF) was obtained from Zhengzhou Fengtai Nano Material Co., Ltd. 2,2,6,6-tetramethy-1-piperidinooxy 6

(TEMPO, AR) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Hydrochloric acid (HCl) and sodium hypochlorite (NaClO) were provided by Sichuan Xilong Chemical Co., Ltd (China). Sodium bromide (NaBr) was provided by Tianjin Kemiou Chemical Reagent Co., Ltd. Sodium hydroxide (NaOH) was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China).

2.2. Fabrication of Cellulose Nanofiber (CNF)

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CNF was prepared from softwood bleached kraft pulps through TEMPOoxidized method in previous reports (H. Zhang et al., 2018). Generally, the chopped dry softwood bleached kraft pulps (15 g) and distilled water (1800 mL) were put

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into a big beaker with a mechanical stirrer for 24 h. TEMPO (0.24 g, 1.5 mmol) and

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NaBr (1.5 g, 15 mmol) were added as catalytic agents and were gradually dissolved in the slurry. The initiation of the oxidation reaction was driven by the participation

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of NaClO solution (8 wt%, 150 mmol) as a primary oxidant. 0.3 M HCl and 0.3 M NaOH were used to adjust the pH value, which was added to maintain the pH value

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at 10 during the reaction. Then, the reaction mixture was washed several times to make the pH value reach neutral. Finally, the TEMPO-oxidized CNF suspended in

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water was acquired by sonicating for 120 min and centrifuging for 5 min.

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2.3. Fabrication of Hierarchical Pore Structure Based on Cellulose Nanofiber (CNF) Commercially available MF was cut into a cylindrical shape with a diameter of 29 mm and a thickness of 20 mm by an engraving machine for sound absorption testing. Specifically, different concentrations by weight of CNF aqueous dispersion 7

were equipped for use in the next step. The pieces of MF were placed in CNF aqueous dispersion (20 mL) and squeezed three times with a glass stopper to make MF fully immersed. The mixtures with MF were bath sonicated for 1h, making CNF as evenly distributed as possible within the melamine skeleton. Finally, the prepared samples were placed at -20℃ for 9 h and room temperature for 15 h. After four cycles, they were directly frozen by liquid nitrogen for 10 min to a fully frozen state,

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followed by freeze-drying (-40℃, 68 h). The obtained composite foam by cyclic freezing-thawing was named as MF/CNF-FT0.1%, MF/CNF-FT0.2%, MF/CNFFT0.3%, MF/CNF-FT0.4% and MF/CNF-FT0.5% named for 0.1, 0.2, 0.3, 0.4 and 0.5 wt% of CNF, respectively. For comparison, the composite foam named MF/CNF

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was prepared by direct liquid nitrogen freezing instead of cyclic freezing-thawing,

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and other conditions were the same as the aforementioned method.

Figure 1 Fabrication procedure of MF/CNF-FT composite foam.

2.4. Characterization The microstructure of CNF was observed using Tecnai G2 F20 transmission electron microscope (TEM, FEI, USA) at an acceleration voltage of 120 kV. The 8

chemical structure of prepared CNF was detected by Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific, USA) from 4000 to 800 cm–1. The zetapotential of CNF suspension was measured using a Zetasizer Nano ZS (Malvern, U.K.) at 25°C. The UV–vis spectra of CNF with different concentrations were measured using the UV-3600 spectrophotometer (Shimadzu, Japan). The content of cellulose, hemicellulose and lignin in the softwood bleached kraft pulps and CNF

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was analyzed by the classical chemical titration methods (Hu et al., 2014; Z. Jiang, He, Li, & Hu, 2014). The prepared CNF aqueous solution was dried at 90°C before measurement, and each sample was tested three times. The structure and morphology of all the samples were characterized by Apteo S field emission

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scanning electron microscopy (SEM, Thermo Scientific, USA) at an acceleration

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voltage of 15 kV. The specific surface area was measured with N2 adsorption at 77 K using a surface area analyzer (TriStar3020, Micromeritics, USA). Thermal

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gravimetric analysis (TGA) was carried out on a TA Instruments TG209F1 (Netzsch, USA) under nitrogen purging at the heating rate of 10 °C/min from 30 to 800 °C.

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The sound absorption property of all the samples was performed by a Brüel & Kjær impedance tube 4206 type based on the two microphone transfer function method.

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The tested sample was a cylinder with a diameter of 29 mm and a thickness of 20

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mm, and each sample was tested three times to eliminate test errors. The compressive stress-strain measurements were performed under cyclic compression strain of 50% at a strain rate of 2 mm/min using a universal material testing machine

(Instron5667, USA).

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3. Results and discussion 3.1 Commercial MF and Prepared CNF The pore structure and morphology of unmodified MF is shown in Figure 2a. It is showed that the unmodified MF is a complete open-cell structure with threedimensional network, which has an average pore size of approximately 120 μm. The morphology of CNF is presented in the TEM image of Figure 2b and the CNF

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suspensions with the concentration of 0.5 wt% have the zeta-potential of -60.8 mV. This indicates that CNF has high dispersion ability in water due to the negative carboxylate groups on the surface. The component analysis results show that

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softwood bleached kraft pulps mainly contain 86.01% cellulose, 8.58%

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hemicellulose and very little lignin. After TEMPO-mediated oxidation, most of the hemicellulose originally present in the softwood bleached kraft pulps are removed

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from CNF as a water-soluble fraction using (Kuramae, Saito, & Isogai, 2014). The prepared CNF contains 94.37% cellulose and 3.24% hemicellulose. The FT-IR

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spectrum of CNF is shown in Figure 2c. The band observed at 3340 cm−1 is attributed to the O-H stretching. The absorption band at 1032 cm−1 is assigned to C-

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O-C vibrations of the glycosidic bridges. It can be proved that CNF was successfully

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synthesized (Niu et al., 2018). The absorbance spectra of CNF with different concentrations of UV-vis (200-500 nm) are presented in Figure 2d. CNF has an absorption peak near 250nm, which is consistent with the report (Fukuzumi, Saito, Iwata, Kumamoto, & Isogai, 2009). And the absorption peak intensity almost linearly increases with the concentration enhancing. According to Lambert-Beer law, the absorbance is proportional to the concentration (Zhou et al., 2017), which 10

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verified the different concentrations of CNF aqueous dispersion.

Figure 2 (a) SEM image of MF, (b) TEM image of CNF, (c) FT-IR spectrum of CNF,

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(d) UV-vis spectrum of CNF with different concentrations.

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3.2 Structure and Morphology

Morphological changes of modified MF that direct frozen by liquid nitrogen

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with different concentrations by weight (MF/CNF) can be observed in the SEM image of Figure 3 (a, c and e). The variation of microscopic structure from

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flocculent structure to flakes with the increasing content is observed in the SEM image of the cross section. CNF exists in the form of a flocculent structure around the skeleton of MF when the content is 0.1 wt%. However, CNF films begin to form when the concentration of CNF is 0.2 wt% and the membrane area gradually expands under the support of the MF skeleton with the content increasing, indicating that the self-assembly of CNF is enhanced, which resulted in the formation of large 11

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CNF films during freeze-drying.

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Figure 3 SEM images of MF/CNF and MF/CNF-FT with different concentrations of CNF: (a) MF/CNF-0.1%, (b) MF/CNF-0.1%FT, (c) MF/CNF-0.2%, (d)

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MF/CNF-FT0.2%, (e) MF/CNF-0.4%, (f) MF/CNF-FT0.4%. The cross-sectional SEM images of MF with different CNF concentrations by

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weight and cyclic freezing-thawing (MF/CNF-FT) are presented in Figure 3 (b, d,

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and f). The fabrication of CNF films due to the multiple self-assembly of CNF also is observed in the samples of MF/CNF-FT. However, there are some differences in the structure and morphology of CNF films between MF/CNF and MF/CNF-FT. Compared with MF/CNF, MF/CNF-FT begins to form the CNF films only when the concentration is 0.1 wt%. Meanwhile, when the concentration is greater than 0.2 wt%, the CNF films on MF/CNF-FT coexists with the flocculent structure, which 12

is observed in the enlarged SEM image of Figure 4a and b, indicating that CNF is superfluous. In particular, the CNF films prepared by different methods exhibit different morphology. As the schematic diagram of Figure 4e and f shows, the selfassembled CNF films of MF/CNF exhibit a large size, which are directly paved on the MF skeleton. But the CNF films of MF/CNF-FT are small and depend only on adjacent MF frameworks. The images of Figure 4c and d show an enlarged view of

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the microscopic morphology of the interface between the membrane and the skeleton in MF/CNF and MF/CNF-FT. It can be observed that the CNF in the sample of MF/CNF-FT is entangled on the MF skeleton, and the binding with the MF skeleton is closer than that of MF/CNF. It may be because the CNF films in the

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MF/CNF sample are self-assembled by direct frozen by liquid nitrogen. As the

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concentration increases, the area of the CNF films gradually becomes larger and is only paved on the skeleton of MF. While MF/CNF-FT has undergone a process of

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cyclic freezing-thawing, the fluidity of the system is restored by thawing, providing a recombination opportunity for CNFs with poor connectivity. Therefore, multiple

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self-assembly makes the CNF films attach to the adjacent MF skeleton with entanglement interaction. At the same time, it also improves the effective utilization

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of CNF to result in the appearance of the flocculent structure. In addition, the density

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of MF/CNF and MF/CNF-FT is about 10~18 kg/m3. It shows that even at the high addition of CNF, the composites produced still have light features.

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Figure 4 SEM images of partial enlargement of (a) MF/CNF-FT0.3% and (b)

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MF/CNF-FT0.4%, (c) MF/CNF-0.2% and in (d) MF/CNF-FT0.2%, and the schematic diagrams of (e) MF/CNF and (f) MF/CNF-FT (The red circle in figure is

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the flocculent CNF, and the blue circle in figure is the interface between the CNF film and MF).

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Besides, the results of the specific surface area are shown in Figure 5. It can be presented that the specific surface area of MF/CNF-0.3% and MF/CNF-FT0.3% are 34.54 cm3/g and 27.30 cm3/g. This indicates that the specific surface area of the composite foams decreases undergone a process of cyclic freezing-thawing. And the specific surface area decreases as the concentration of CNF increases. This is because cyclic freezing-thawing provides multiple opportunities for self-assembly 14

to CNF and stronger and more ordered association among CNF can be established. At the same time, the increase of CNF concentration also makes the arrangement of CNF closer (F. Jiang & Hsieh, 2014). Therefore, the specific surface area and the size of the mesopores on the CNF films can be reduced (Kwon et al., 2020;

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Rodionova, Eriksen, & Gregersen, 2012).

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3.3 Thermal performance

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Figure 5 The specific surface area of MF/CNF and MF/CNF-FT composite foams.

Thermogravimetric analysis (TGA) is used to characterize the thermal stability

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of MF and MF/CNF-FT composite foams. It can be seen from Figure 6 that the weight loss of MF under N2 atmosphere is divided into four stages. The mass loss

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of the first stage that occurred below 100 °C can be attributed to the evaporation of water. The mass loss of the second stage at 145-350 °C is due to the elimination of formaldehyde from the ether bridge to form a methylene bridge. The mass loss of the third stage in the temperature range of 350-410 ℃ can be attributed to the breakdown of methylene bridges. The mass losses of the fourth stage that occurred more than 410 ℃ can be attributed to thermal decomposition of the triazine ring of 15

MF (Dashairya, Sahu, & Saha, 2019; Oribayo, Feng, Rempel, & Pan, 2017). Although there are some differences, MF/CNF has a similar thermogram with MF. MF/CNF has two thermal decomposition peaks and more mass losses in the second stage of mass losses. This is because the main mass losses of CNF occur between 190-350 °C, which can be observed in Figure 6. At the same time, more hydrogen bonds between CNF molecules can make CNF more compact, resulting in higher

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crystallinity and thermal stability (Poletto, Zattera, Forte, & Santana, 2012). Therefore, the mass losses of MF/CNF-FT increase and the thermal decomposition peaks move to high temperature with the improvement of CNF concentration in the second stage. Besides, because the residual weight of CNF is larger than that of MF,

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the residual weight of MF/CNF-FT increases at 800°C.

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Figure 6 (a) TGA and (b) DTG curves of MF, CNF, and MF/CNF-FT composite

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foams in N2 atmosphere.

3.4 Compression Performance To make the materials are not easy to collapse in practical applications, mechanical properties are an important indicator of sound-absorbing materials. In 16

this work, to investigate the mechanical properties of prepared composite foams, the samples were subjected to cyclic compression testing. The cyclic compression curves of MF/CNF, MF/CNF-FT with different CNF contents and unmodified MF under the condition of 50% strain are shown in Figure 7a and b. At the beginning, due to the elastic bending of the MF framework and CNF films, the stress-strain curve showed a nearly linear elastic regime. Then the stress-strain curve appears a

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relatively flat platform caused by elastic buckling of the films of CNF. Under high strain, the stress-strain curve begins to increase sharply due to the densification of the composite foams (Cao, Si, Wu, et al., 2019). The compressive strength of MF/CNF-FT can reach 25 kPa. Some reports on the mechanical properties of

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cellulose composite foam also have been made by researchers. For example, Xu et

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al. (Xu et al., 2019) prepared composite foam based on the cellulose nanofiber/carbon nanotube, which had a compressive strength of 3kPa under the

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strain of 58%. Wu et al. successfully prepared the CNF/GNP hybrid-coated MF sponge (CG@MF), the compressive strength of which was 7 kPa under the strain of

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60%. Besides, it can be seen from the results that whether it is MF/CNF or MF/CNFFT, the compressive strength of the composite increases with the improved

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concentration of CNF. Fortunately, almost all modified samples except MF/CNF-

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0.1% have better compressive strength than unmodified MF. This is because the evenly distributed films of CNF inside the MF can act as a buffer material, which can effectively disperse and dissipate the concentrated stress (He et al., 2018). At the same time, the introduction of CNF resulted in a more condensed structure, effectively improving the compressive strength (Si, Yu, Tang, Ge, & Ding, 2014). Beyond that, the mechanical strength of MF/CNF-FT is higher than that of MF/CNF 17

when the CNF concentrations are the same. As shown by the schematic diagram of the compression process of MF/CNF and MF/CNF-FT in Figure 7e, MF/CNF-FT prepared by cyclic freezing-thawing has a distinct entanglement phenomenon between the CNF films and the MF skeleton, enhancing the mechanical interlocking effect of CNF and MF (Eyckens et al., 2020). Thence, the mechanical strength of MF/CNF-FT is improved more significantly than that of MF/CNF. The cyclic

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compression test was carried out to obtain more accurate mechanical properties of the composite under dynamic load, the results of which are presented in Figure 7c and d. When the compression is constant as 50% of the sample height, the stiffness of the sample tends to improve slightly as the number of cycles increases. Even after

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20 cycles of dynamics, the hysteresis of MF/CNF or MF/CNF-FT does not change

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significantly, indicating that the internal microstructure of the composite foams prepared by two methods doesn’t have the significantly collapse and break under

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the cyclic loading (Li et al., 2018).

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Figure 7 Mechanical properties of MF with CNF films under dynamic compression. Strain-stress curve of (a) MF/CNF and (b) MF/CNF-FT with different concentrations of CNF at 50% strain. The cycle compression test results of 20 cycles of (c) MF/CNF-0.2% and (d) MF/CNF-FT0.2% at 50% strain. (e) Schematic diagram of the compression process of MF/CNF and MF/CNF-FT.

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3.5 Sound Absorption Performance The sound absorption performance of MF/CNF and MF/CNF-FT with different concentrations of CNF is shown in Figure 8a and b. Compared with the unmodified MF, the sound absorption performance of MF/CNF with the CNF concentration of 0.1 wt% and 0.2 wt% is improved within 100-6400 Hz, and the sound absorption performance of MF/CNF-0.1% is the best outstanding of all. However, as the CNF

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increases further, the sound absorption performance of MF/CNF gradually deteriorates whether the test frequency is lower than 1600 Hz or higher than 4500 Hz. It also can be observed that each sample of MF/CNF has a characteristic

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absorption peak in the sound absorption curve, and the position of the peak

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gradually shifts to a higher frequency in the range of 1800 Hz to 4300 Hz. In addition, it can be seen from the shape of the sound absorption curve that the

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characteristic peaks are becoming more and more complete. The sound absorption performance of MF/CNF-FT has obvious advantages over MF/CNF. The sound

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absorption performance test results of MF/CNF-FT with different concentrations of CNF are presented in Figure 8b, and the sound absorption performance of

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unmodified MF is put together for comparison. The samples of MF/CNF-FT exhibit a different sound absorption property than the samples of MF/CNF. It is worth

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noting that all samples of MF/CNF-FT have better sound absorption performance than unmodified MF within the whole sound absorption frequency range. The sound absorption curve shows a trend of increase and then decrease in the middle and high frequency ranges (800-6400 Hz). The samples of MF/CNF-FT0.1%, MF/CNFFT0.2% and MF/CNF-FT0.3% have optical sound absorption performance that sound absorption coefficient can reach above 0.9 in the range of 3500-6400 Hz, 20

which almost achieve complete sound absorption. Furthermore, the sound absorption performance of MF/CNF-FT is gradually increasing in the low frequency range (100-800 Hz) with the CNF concentration increasing. In particular, when the sound absorption frequency is at 500 Hz, the MF/CNF-FT sound absorption frequency is optimally increased by almost 150% compared to the unmodified MF, which is a very obvious improvement within the low frequency range (100-800 Hz).

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The NRC (noise reduction coefficient) of different samples with the same thickness of 20 mm are shown in Figure 8c, which is the average of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz. The NRC values of MF/CNF-0.1% and MF/CNF-FT0.4% are 0.42, 0.45, which are about 68% and 80% improvement than

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MF, respectively.

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In addition, to identify the superiority of the hierarchical pore structure with both macropores and mesopores for the preparation of sound absorption materials,

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some previously reported acoustic absorbers and their sound absorption performances are listed in Table 1. Compared with other works, the obtained

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MF/CNF-FT0.4% and MF/CNF-FT0.5% in our work presented relatively higher sound absorption performance with the same thickness or thicker thickness.

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Traditional sound-absorbing materials, such as natural rubber foam, sheep wool,

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and natural coir fiber, etc. have poor overall sound-absorbing properties and higher density (Navacerrada, Fernández, Díaz, & Pedrero, 2013; Nine et al., 2017). They consume sound energy gradually by the friction loss of air and the viscous dissipation of the skeleton. And the general method to fabricate the hierarchical pore structure based on commercial foam only adjusts the sound absorption performance by optimizing the macropores scale. However, the hierarchical pore structure in the 21

MF/CNF-FT composite foam contains macropores and mesopores due to the close packing of nanoscale CNF and cyclic freezing-thawing process. The interconnected CNF films inside the MF can optimize the pore opening of the structure, which can greatly enhance the propagation path of sound waves. More importantly, the small mesopores on the CNF films of prepared composite foam can efficiently improve the sound absorption performance by enhancing the multiple scattering and

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reflection of sound waves. At the same time, this unique structure is beneficial to broaden the sound absorption range. It is indicated that the prepared MF/CNF-FT

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composite foam is a good candidate for sound absorption.

22

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Figure 8 Sound absorption performance of (a) MF/CNF and (b) MF/CNF-FT with different concentrations of CNF, (c) the noise reduction coefficient (NRC) of different samples with the thickness of 20 mm.

23

Table 1 Comparison of our composite foams with other common acoustic absorbers. Density

Thickness

Acoustic absorption coefficient

Materials

Ref. (mm)

500 Hz

1000 Hz

2000 Hz

4000 Hz

MF/CNF-0.1%

11.17

20

0.29

0.40

0.83

0.85

This work

MF/CNF-FT0.4%

16.00

20

0.29

0.58

0.80

0.99

This work

MF/CNF-FT0.5%

18.17

20

0.37

0.55

0.67

0.93

This work

MF

9.42

20

0.14

0.29

0.49

0.75

This work

PSFS-10

6.63

20

0.10

0.57

—

GO-PU50

50.52

30

—

0.40

0.99

MFGO-1

12.39

20

0.18

0.32

0.56

Aluminum foam

—

20

0.06

0.10

fibrous

31.2

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Kenaf fibers

Sheep wool

20

20

ur

10.0 assemblies

20.04

0.26

0.12

30

0.13

0.18

0.29

—

(Cao, Si, Yin, et al., 2019)

0.85

(Lee & Jung, 2019)

0.82

(Nine et al., 2017)

-p 0.31

re

25

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≈60

Polyurethane foam Kapok

—

na

Natural rubber foam

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(kg/m3)

0.12

0.46

(Navacerrada et al., 2013) (Tomyangkul, Pongmuksuwan,

0.08 Harnnarongchai,

&

Chaochanchaikul, 2016) 0.77

—

(Sung, Kim, & Kim, 2016) (Xiang, Wang, Liua, Zhao, &

0.30

0.62

0.92 Xu, 2013) (Lim, Putra, Nor, & Yaakob,

0.32

0.26

0.36

0.55 2018) (Zach,

20

0.18

0.33

0.54

Korjenic,

Petránek,

— Hroudová, & Bednar, 2012)

Natural coir fiber

153

30

0.28

0.84

24

0.73

0.82

(Nine et al., 2017)

3.6 Sound Absorption Mechanism The sound waves can be reflected, absorbed or transmitted when striking on the surface of the materials (Bujoreanu, Nedeff, Benchea, & Agop, 2017). The propagation of sound waves through porous materials through three mechanisms: visco-inertial, thermal, and structural mechanisms (Mohamed et al., 2018; Wang et al., 2020). The sound energy can be dissipated by the viscous damping of the

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melamine skeleton and the friction loss of air vibration when sound waves pass through the composite foams (Rahimabady et al., 2017; Wang et al., 2020). Therefore, the CNF films formed inside the MF have a great impact on the

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propagation of sound waves, leading to the sound absorption performance of the composite foams with suitable concentrations of CNF can be significantly

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optimized.

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Taban et al. (Taban et al., 2020) have studied the sound absorption performance of kenaf fibers based on the Johnson-Champoux-Allard (JCA) model. The results

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of this research showed that structural parameters such as tortuosity, flow resistance, and porosity had an important influence on sound absorption performance. To

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clarify the sound absorption mechanism of MF/CNF-FT composite foam, the predicted mechanism diagram is shown in Figure 9. In the high frequency range,

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sound absorption of the elastic porous materials is an adiabatic process, which causes heat loss through the friction between the air driven by sound waves and the irregular pore walls (Sagartzazu, Hervella-Nieto, & Pagalday, 2008). A large number of interconnected CNF films inside the MF slightly reduce the porosity and small mesoporous are fabricated on the films due to the close packing of CNF, which increases the tortuosity and flow resistivity of materials. This can effectively 25

increase the multiple reflections and extend the propagation path of sound waves, resulting in the enhancement of contact areas for air friction and the consumption of sound energy. At the same time, CNF films reduce the pore opening of MF, which increases the scattering of sound waves (Hyuk Park et al., 2017; Nine et al., 2017). In the low frequency range, it is an isothermal process when the elastic porous materials absorb the sound, which dissipates energy by heat exchange. The low-

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frequency sound damping of porous materials is especially sensitive to the pore structure (Taban et al., 2020). This complex microstructure containing more smaller and more irregular pores increases the contribution of viscosity loss, which is caused by the friction between the air and the inner surface of pores (Wu et al., 2017).

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Besides, when the sound wave approaches the narrow gap between CNF films, the

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micro-vibration of CNF films caused by the air further dissipates the sound energy in the form of thermal energy (Oh et al., 2018; Oh, Lee, Umrao, Kang, & Oh, 2019).

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Therefore, MF/CNF-FT composite foam exhibits excellent sound absorption

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ur

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performance within the low frequency range.

Figure 9 Schematic illustration of sound absorption of MF/CNF-FT.

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4. Conclusions In this work, the hierarchical pore structure with both macropores and mesopores for enhanced sound absorption was fabricated by combining the CNF and MF for the first time through cyclic freezing-thawing. The composite foam prepared by cyclic freezing-thawing showed more excellent sound absorption performance in broadband acoustic absorption and mechanical properties than that

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prepared by direct freeze-drying due to the multiple self-assembly of CNF. The sound absorption performance of composite foam with a thickness of 20 mm (CNFFT0.4%) showed a maximum enhancement of about 107% at 500 Hz compared with

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unmodified MF and achieved an average 94% absorption of sound waves in the range of 4000-6000 Hz. Therefore, it is anticipated that fabricating the hierarchical

re

pore structure based on the commercial foam by cyclic freezing-thawing has

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guiding significance for future research and the composite foam can be used as a

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sound absorber in rail transportation, industrial, residential building, etc.

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Credit authorship contribution statement

Lu Shen: Methodology, Formal analysis, Investigation, Data curation, Writing-original draft,

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Visualization. Haoruo Zhang: Methodology, Investigation, Data curation. Yanzhou Lei: Resources, Validation. Yang Chen: Resources, Validation. Mei Liang: Conceptualization, Supervision, Writing-review & editing, Resources, Project administration, Validation. Huawei Zou: Conceptualization, Supervision, Writing-review & editing, Resources, Funding acquisition,

27

Project administration.

Acknowledgements

The authors would like to thank the financial support of the Key Technology Research and Development Program of Sichuan Province (No. 2019YFG0488), and

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thank the Analytical & Testing Center of Sichuan University for supporting SEM testing.

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