A High-performance Structural Material Based on Maize Straw and Its Completely Biodegradable Composites of Poly (Propylene Carbonate)

A technique of chemical treatment combined with hot pressing is used to fabricate a high-performance structural material based on maize straw. The chemical structures and compositions of the densied maize straw are determined by ATR-FTIR, XRD, and TGA. The tensile strength and elongation at break of the densied straw can reach as high as 598.6MPa and 6.2%, which are approximately 9.3 times and 2.2 times higher than that of the natural straw, respectively. Furthermore, chemical treatment and hot pressing are essential steps for preparing the densied straw. The chemical treatment using a mixed alkali solution can remove the fractional lignin and hemicellulose but preserve most of the cellulose, thus enhancing the degree of crystallinity and the heating resistance of the densied straw. The SEM results prove that densication by hot pressing is committed to fabricating a structure material with high-performance from maize straw, which has eliminated the defects and reinforced the mechanical properties of the densied straw. At the molecular level, the hydrogen bonds between the aligned cellulose bers have bridged the neighboring cellulose bers, reinforcing the mechanical properties of the densied straw. After compositing the densied straw with a biodegradable poly (propylene carbonate), the mechanical properties of the composite are considerably improved, predicting a huge application prospect in automobile, construction, furniture, and even airplane elds.


Introduction
The development of sustainable materials is extremely important to meet the growing global demand for energy-saving materials and reduce negative environmental consequences. (Benítez et al. 2017) Simultaneously, the industrial community is also looking for lightweight integrated with high strength and toughness materials. (Ritchie 2011) Maize is one of the most important crops in the world.(X. Wang et al. 2021) A huge number of maize straws as crop residue is yielded every year, accompanying the great needs of maize. Although maize straws have been partly utilized (Croce et al. 2016) in biochar application, many maize straws are still wasted in burned (Xu et al. 2021) and mulched. The burned maize straws lead to environmental pollution and eventually cause a signi cant waste of natural resources. Thus, how to entirely utilize maize straw as a sustainable material is not only a scienti c problem but also a societal issue. Up to now, maize straw has been used in the areas of chemical (S. Chen et al. 2018;Lopez-Hidalgo et al. 2021;Ma et al. 2018) and biological engineering (Xiao Han et al. 2018;Pan et al. 2016), energy (Du et al. 2020;Zabed et al. 2016), composite materials (Tarres et al. 2020), and so on. For example, forage, fertilizer, bioethanol, furaldehyde et al. can be produced from maize straws. In this study, we focus on solving the issue of maize straw for a lightweight and sustainable material through a process of wasteto-energy.
Maize straw composited with polymers is a general method for biochar application. Some researchers had reported that the composite materials were fabricated from polyethylene (Tarres et al. 2020) and polypropylene (Delgado-Aguilar et al. 2018) reinforced with maize straw. In general, maize straws as a ller, its strength, and modulus determine the mechanical properties of the resulting composite (Ku et al. 2011;Pickering et al. 2016). Therefore, enhancing the strength and modulus of maize straw is quite important for fabricating high-performance polymer/maize straw composite. The mechanical performance of biomass materials such as natural wood and bamboo can be enhanced by pre-treatment with chemicals (G. Chen et al. 2020;Frey et al. 2018;Kabir et al. 2012;Mi et al. 2020; Y. Y. Wang et al. 2021), steam (Fang et al. 2011), heat (Fang et al. 2011;Poletto et al. 2014), bleaching and caramelization (Sharma et al. 2015), or by lling with resin followed by densi cation (Kalali et al. 2019). Recently, Hu and his coworkers (Song et al. 2018) reported a simple and effective strategy via chemical treatment followed by hot-pressing to directly transform bulk natural wood into a high-performance material with a more than tenfold increase in strength, toughness, and ballistic resistance. A similar method was also used to enhance the performance of bamboo . The tensile strength and modulus for the densi ed bamboo could reach as high as 1GPa and 75GPa, respectively. As the main biomass material, the mechanical performance of the maize straw is expected to be enhanced using the technique of chemical treatment combined with hot-pressing.
Poly (propylene carbonate) (PPC) is a kind of aliphatic polycarbonate synthesized from propylene oxide and carbon dioxide(Y. Chen et al. 2013;Wang E 2020). PPC is one of the most promising environmentfriendly and degradable synthetic polymeric material that has been widely applied in the elds of lm, barrier, and biomedical materials (Muthuraj et al. 2018;Qin et al. 2015). In order to enhance its thermal and mechanical properties, PPC has been composited with carbon ber(W. , wood (Nörnberg et al. 2014), SiC nanowires (Qu et al. 2020), and micro-brillated cellulose (Qi et al. 2014).
Previously, chlorinated PPC (CPPC) was used to improve the mechanical properties of PPC/straw our composites (Bin 2017). The tensile strength was increased by 38% when the mass ratio of straw our and CPPC was 30% and 1.8%, respectively. Hence, further improving the mechanical properties of PPC/maize straw composites and increasing the ratio of maize straw in composites for cost savings are the two signi cant issues in the application of the PPC industry.
In this study, a lightweight integrated with superstrength, high modulus, and toughness densi ed maize straw was fabricated via chemical treatment combined with hot pressing. Different treatments, including chemical treatment, cold and hot pressing, and chemical treatment combined with hot pressing, were individually evaluated. The chemical structures and compositions of the densi ed maize straw were determined by ATR-FTIR, XRD, and TGA. Combined with the analysis of surface microstructures by SEM, the reinforced mechanism of the densi ed maize straw was further discussed and proposed a mechanism of the compaction of voids in vertical and the alignment of cellulose micro brils in horizontal. Moreover, the densi ed maize straw was applied to improve the mechanical properties of PPC/densi ed maize straw composites.

Materials and chemicals.
The raw (natural maize straw peel) was peeled from maize straws by hands taken from the southern suburb of Changchun City, Jilin Province, China. Poly (propylene carbonate) (PPC) was obtained from Taizhou Bangfeng Plastic Co., Ltd. (Wenling, Zhejiang, China). Sodium hydroxide (NaOH, AR) and sodium sul te (Na 2 SO 3 , AR) were obtained from Beijing Chemical Co., China.

Sample preparation
Densi ed maize straw: In the beginning, the natural maize straw was treated by a mixed solution (2.5mol/L NaOH and 0.4mol/L Na 2 SO 3 ) at 80°C for 0.5h to partially remove hemicellulose and partly lignin. Next, it was cleaned by ultrasonic for 15mins and then immersed into the deionized water for 2h to remove the residual lye as much as possible. After air-drying for a week, the samples were compressed under the pressure of 8MPa at 100°C for 0.5h to make it densi cation. The densi ed maize straw was obtained.

Compressed maize straw
The compressed maize straw was obtained directly by hot pressing under the pressure of 8MPa at 100°C for 0.5h.

Modi ed maize straw
The modi ed maize straw was obtained with the chemical pre-treatment but without hot pressing.

PPC/maize straw composite:
At rst, the densi ed maize straw was cut up into approximately 5mm-long fragments along the growth direction of the aligned vascular bers. Then, the straw fragments were smashed into powder by a grinder to separate the vascular bers. The mixture of PPC/densi ed maize straw was prepared as a certain mass ratio of 60/40. Next, the prepared mixture of PPC/densi ed maize straw was blended in a torque rheometer (XSS-300, Shanghai Kechuang Rubber Plastic Mechanical Equipment Co., Ltd, China) at 50rpm for 7min. At last, after compression molding under pressure of 8MPa at 100°C for 0.5h, the PPC/densi ed maize straw composite board (120mm⋅100mm⋅2mm) was obtained. By this method, PPC/natural maize straw composites, PPC/modi ed maize straw composites, and PPC/compressed maize straw composites were fabricated, respectively. It is noted that the composite boards were all cut into strips (50mm⋅10mm⋅2mm) for mechanical tests.

Mechanical test Tensile test
During testing, the samples were clamped at both ends and stretched along the longitudinal direction of samples till the sample fractured with a constant test speed of 5mm/min at room temperature. Due to the limitation of the raw material, the length, width, and thickness of the test samples of natural, modi ed, compressed, and densi ed maize straw were 100mm, ≈3mm, and ≈ 0.3mm, respectively. The size of the test samples of neat PPC and PPC/natural maize straw composites, PPC/modi ed maize straw composites, PPC/compressed maize straw composites, PPC/densi ed maize straw composites was 50mm⋅10mm⋅2mm. All tests were conducted on the Instron 1121 Material Testing Machine, according to GB/T 1040.2-2006 (China national standard).

Three points bending test
The size of the test samples of PPC/natural maize straw composites, PPC/modi ed maize straw composites, PPC/compressed maize straw composites, and PPC/densi ed maize straw composites was 50mm⋅10mm⋅2mm. The span between the two bottom rollers was 20mm, and the speed of the top roller pressing down was 2mm/min. All tests were performed on the Instron 1121 Material Testing Machine, according to GB/T9341-2000 (China national standard).

Scanning electron microscope (SEM)
Cross-section of the natural and densi ed maize straw The natural maize straw and densi ed maize straw were rst placed in liquid nitrogen for 30 mins. After fracturing the samples quickly, the test samples of natural and densi ed maize straw were obtained.
Then, the microstructure of the cross-section of natural and densi ed maize straw with spraying gold were observed through a Field Emission Scanning Electron Microscope (FEG ESEM) (XL30, FEI COMPANY) at an acceleration voltage of 5kV.
Side face of the natural and densi ed maize straw The test samples were directly peeled from the natural and densi ed maize straw by hands. Then, the side face of the natural and densi ed maize straw with spraying gold could be observed.
The surface of the natural and densi ed maize straw The waxy layer of the natural maize straw must be removed before observing the surface of the natural maize straw. However, due to the waxy layer had been removed, the surface of densi ed maize straw with spraying gold could be observed directly.

X-ray Diffraction (XRD)
Before the test, the natural and densi ed maize straw were grinded into powder. The XRD curve was obtained by a Wide-angle X-ray Diffractometer (D8 ADVANCE, BRUKER Co., Germany) (operating voltage at 40kV, current at 40mA, CuKα, λ = 0.154nm) and the scanning angle was from 5° to 40°.

Attenuated Total Re ectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR)
Before the test, the natural and densi ed maize straw were rst grinded into powder. The powder was compressed into a sheet at room temperature. Then, the test samples of natural and densi ed maize straw were obtained. All tests were performed using a Fourier transform infrared spectrometer (BRUKER, ALPHA, PLATINUM-ATR) from 500cm − 1 to 4000cm − 1 at a spectral resolution of 4cm − 1 with a total of 32 scans.

Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG)
The tests were performed on a thermal analysis instrument (TA Instruments Q50, USA) from room temperature to 600°C at a rate of 10 K·min − 1 under a nitrogen atmosphere. Then, the statistics of TGA and DTG could be obtained directly.

Results And Discussion
To fabricate a kind of high-performance biomass material based on natural maize straw, a chemical treatment technology combined with hot pressing is used to obtain densi ed maize straw. The chemical treatment by a mixed alkali solution is used to remove the hemicellulose and lignin, called deligni cation. The hot pressing at a certain condition, called densi cation, is applied to increase the number of loading ber per unit volume and eliminate internal defects of the sample. The scheme of processing approach to transforming the natural maize straw into the densi ed maize straw is presented in Scheme 1. The natural, chemical modi ed, compressed without chemical treatment, and compressed with chemical treatment maize straw can be simpli ed as natural straw (NS), modi ed straw (MS), compressed straw (CS), and densi ed straw (DS), respectively.

The lightweight densi ed straw with superstrength and high modulus and toughness
The mechanical performance of the natural, modi ed, compressed, and densi ed straw is shown in Fig. 1. For better comparison, the tensile strength-strain curve was selected instead of the typical stressstrain curve (Fig. S1) due to the different cross-sectional areas of the tested samples. The tensile strength of the four samples in Fig. 1a is proportional to the strain, which means the stress and the strain are linearly related before fracture. Correspondingly, the tensile strength and elongation at break of the densi ed straw are enhanced simultaneously ( Fig. 1b and 1c). Notably, the tensile strength and elongation at break of the densi ed straw have reached 598.6MPa and 6.2%, which is approximately 9.3 times and 2.2 times higher than that of the natural straw, respectively. The superstrength and the great elongation at break of the densi ed straw is the highlight of this study and can be comparable with the work of the densi ed wood and bamboo. Other researchers have reported that the tensile strength of the densi ed wood (≈ 549MPa) and the densi ed bamboo (≈ 770MPa) are both dramatically enhanced relative to the tensile strength of the natural wood (≈ 47MPa) and the natural bamboo (≈ 298MPa), respectively Song et al. 2018). However, the elongation at break of the densi ed wood (≈ 1.3%) and densi ed bamboo (≈ 1.8%) in those studies have no obvious changes after treatment. These discussions suggest that the tensile strength and elongation at break of the nature maize straw can be simultaneously and tremendously improved by the chemical treatment combined with hot pressing.
In addition, Young's modulus of the densi ed straw has reached to 16.6GPa that is about 5.9 times higher than that of the natural straw (Fig. 1d). The speci c strength of the densi ed straw has reached to 434MPa⋅g − 1 ⋅cm 3 that is almost 3.0 times higher than that of the natural straw (Fig. 1e), and the density of the densi ed straw has increased about two times after hot pressing (Fig. 1f). Therefore, the mechanical properties of densi ed straw with high-performance can compete with the mechanical properties of aluminum alloy. Generally, aluminum alloy is widely used in various elds due to the lightweight and high speci c strength. In comparison, four different industrial aluminum alloys are used to evaluate the superiority of the densi ed straw. Astonishingly, the tensile strength of the densi ed straw (598.6MPa) is completely higher than that of 5052-H112 (175MPa), 6061-T6 (290MPa), 2024-T3 (440MPa), and even the aircraft-grade aluminum alloy 7075-T6 (540MPa). For lightweight, the speci c strength of the densi ed straw (434MPa⋅g − 1 ⋅cm 3 ) is also higher than that of 5052-H112 (64MPa⋅g − 1 ⋅cm 3 ), 6061-T6 (106MPa⋅g − 1 ⋅cm 3 ), 2024-T3 (156MPa⋅g − 1 ⋅cm 3 ), and even 7075-T6 (191MPa⋅g − 1 ⋅cm 3 ).
Moreover, the pictures of the bending of the natural and densi ed straws are shown in Fig. 2. Clearly, the natural straw can be easily broken in a relatively small bending angle (Fig. 2b), while the densi ed straw can be bend into a circle but not broken (Fig. 2d). This phenomenon suggests that the densi ed straw still possesses excellent bending toughness after chemical treatment followed by hot pressing.
Above all, a lightweight and sustainable densi ed straw with excellent mechanical properties is fabricated by chemical treatment combined with hot pressing. The tensile strength and speci c strength of the densi ed straw are even higher than those of the commercial aluminum alloys, predicting a promising application as a lightweight material in the elds of automobile, construction, furniture, and even airplane.
3.2 Effect of different processing methods on tensile strength and modulus.
The high-performance densi ed straw in this work was fabricated by chemical treatment combined with hot pressing. Figure 3 shows the tensile strength and modulus of chemical treatment, cold pressing, hot pressing, and chemical treatment combined with hot pressing on natural straw. As a comparison, the performance of natural straw as a control experiment. The properties of natural straw are largely improved after different treatments that signi cantly affect the properties of tensile strength and Young's modulus. Obviously, chemical treatment combined with hot pressing has the greatest tensile strength and modulus, while none of the separate methods, such as chemical treatment, cold pressing, and hot pressing, can achieve best. These results give us a clear conclusion that the technique of chemical treatment and hot pressing are essential steps for preparing the densi ed straw.

The compositions and structural characterizations of natural and densi ed straws.
Since the densi ed straw had superior mechanical performance compared to the natural straw, the compositions and structures of the densi ed straw were characterized. Figure 4a shows the ATR-FTIR spectra of the natural and densi ed straws. The absorption peaks at around 1507cm − 1 and 1604cm − 1 are attributed to the aromatic ring vibrations of the lignin, and the absorption peak at around 1238cm − 1 is attributed to the C-O-C stretching of aromatic ether linkages of the lignin (Poletto et al. 2014;Rehman et al. 2013). These three peaks at around 1238cm − 1 , 1507cm − 1 , and 1604cm − 1 of the densi ed straw become weak, which is attributed to the partial removal of lignin after chemical treatment. Besides, the peak at 1732cm − 1 of the densi ed straw is almost disappeared, which is due to the disappearance of C = O linkage in hemicellulose, suggesting that the hemicellulose in the natural straw is nearly removed after chemical treatment. The peak of cellulose at 1420cm − 1 is still reserved in the densi ed straw, and the hydroxy groups and hydrogen bonds at 3320 cm − 1 are also preserved in the densi ed straw. The natural straw is mainly composed of abundant cellulose, hemicellulose, and lignin. (He et al. 2020;Kambli et al. 2017;Sirviö et al. 2017) Theoretically, lye can dissolve lignin and hemicellulose but not cellulose. Thus, the partial lignin and hemicellulose of the natural straw can be easily removed by a mixed alkali solution (Song et al. 2018). Finally, the cellulose can be mostly reserved in the densi ed straw.
Moreover, the XRD pattern is used to check the crystalline cellulose of the natural and densi ed straws in Fig. 4b. Cellulose contains crystalline and amorphous phases connected with intra-and inter-molecular hydrogen bonds, whereas hemicellulose and lignin are amorphous. (Nishiyama et al. 2002;Poletto et al. 2014;Reddy et al. 2005;Rehman et al. 2013;Szczęśniak et al. 2020;J. Wang et al. 2021;Ye et al. 2020) Thus, the most preserved cellulose in the densi ed straw plays a vital role in mechanical properties, especially for the crystalline cellulose. (Benítez et al. 2017) The Segal method is selected to calculate the degree of crystallinity of the natural and densi ed straws, which assumes that the peak intensity I 200 is contributed by the crystalline cellulose and amorphous regions, while the peak intensity I AM is entirely contributed by the amorphous regions. (Poletto et al. 2014;Rehman et al. 2013;Thygesen et al. 2005;Q. Wang et al. 2013) Thus, the degree of crystallinity of the cellulose can be calculated as followed.
The calculated degree of crystallinity of the cellulose in the natural and densi ed straw is 48.6% and 62.3%, respectively. Clearly, the degree of crystallinity of the densi ed straw is greater than that of the natural straw.
For studying the heating resistance, the TGA and DTG of the natural, modi ed, compressed, and densi ed straws are shown in Fig. 4c and 4d. The reported degradation temperature of cellulose (315-400°C), hemicellulose (220-315°C), and lignin (150-900°C) are different. (Yang et al. 2007) In Fig. 4c, the initial degradation temperatures of the modi ed (314.7°C) and the densi ed (308.0°C) straws are higher than those of the natural (287.9°C) and the compressed (288.0°C) straws. Besides, the temperatures of the maximum degradation rate of the modi ed and densi ed straws are also obviously higher than those of the natural and compressed straws (Fig. 4d). Thus, chemical treatment using lye but not hot pressing to the natural straw can improve the heating resistance of the modi ed and the densi ed straws because of the removal of the partial lignin and hemicellulose and the reservation of the cellulose. In summary, the structures and compositions of the densi ed straw suggest that the chemical treatment using a mixed alkali solution can remove the fractional lignin and hemicellulose but preserve most of the cellulose, thus enhancing the degree of crystallinity and the heating resistance of the densi ed straw.

The microstructures of the natural and densi ed straws
The appearance of the natural straw and densi ed straw is shown in Fig. 5a and 5b, respectively. For studying the microstructures of the natural and densi ed straws, the SEM images of the natural and densi ed straws in three different directions are shown in Fig. 5c-5n. In the direction of cross-section (TW plane), plenty of holes exist on the cross-section of the natural straw ( Fig. 5c and 5d). However, the holes in the cross-section of the densi ed straw (Fig. 5e) are collapsed, and the cell walls are tightly intertwined with each other, leading to the formation of the complete densi ed structures (Fig. 5f) in the enlarged image of the densi ed straw. In the side face, the obvious layer structure and the interstices between layers are presented in images of the natural straw ( Fig. 5g and 5h). However, the images of the densi ed straw show a different view. The interstices among the layers in Fig. 5i become unobservable, and lots of micro brils are pulled out from the bulk during peeling (Fig. 5j). These morphological changes form a pretty densi ed and hard-to-peel structure in the side face of the densi ed straw. Thus, the densi cation of structures in the cross-section and the side face is the main reason for enhancing density and improving the tensile strength. As mentioned above in Fig. 1, the density of the densi ed straw increases from 0.43 to 1.38 g cm − 3 , and the tensile strength is tremendously improved more than ninefold. On the surface perpendicular to the pressure direction, sharply aligned fabric structures are found in the images of the densi ed straw (Fig. 5m and 5n) compared to the loose and disordered bers in the images of the natural straw ( Fig. 5k and 5i). The aligned cellulose bers are also contributed to the densi ed packing, hence improving the mechanical properties of the densi ed straw in a different direction (Fig. S2). Therefore, microstructural densi cation is the critical factor to fabricate a structure material with highperformance from maize straw. On the one hand, a densi ed microstructure helps increase the number of the loading bers per unit volume and thus obtain a highly compressed material. On the other hand, the processing of densi cation is an elimination of defects that makes the densi ed straw more perfect.
3.5 The in uence of the pressure on the mechanical properties.
Two steps with chemical treatment and hot pressing can achieve a high-performance structural material based on maize straw. The step of chemical treatment provides an e cient strategy to remove the partial lignin and hemicellulose and preserve the most cellulose, enhancing the crystalline degree and the alignment of the cellulose bers of the densi ed straw. The other step, hot pressing, is a technique to perform densi cation. The in uence of the pressure on the mechanical properties of the densi ed straw is investigated in Fig. 6. The tensile strength (Fig. 6a) and Young's modulus (Fig. 6c) of the densi ed straw have the greatest values with the pressure of 8MPa. When the pressure is lower than 8MPa, it is insu cient to densify and achieve high-performance; while the pressure is higher than 8MPa, the densi ed and aligned structures probably are slightly destroyed, leading to a bit of decline in strength and modulus. Besides, the elongation at break of the densi ed straw in Fig. 6b rst increases with pressure and then essentially levels off from 6MPa. Hence, for balancing the energy-saving and the mechanical property, 8MPa is the optimum pressure when the temperature is 100°C for 0.5h during hot pressing.
3.6 The reinforced mechanism of the densi ed straw.
Generally, amorphous hemicellulose and lignin are directly linked to the cellulose micro brils. (Berglund et al. 2020;Szczęśniak et al. 2020) The scheme of the surface microstructures of the natural straw is shown in Fig. 7a, and the SEM image is below. The microstructures of the natural straw are loose, and lots of small hemicelluloses and lignin patches are linked to the cellulose bers. After chemical treatment, the patches on the surface of the natural straw are drastically diminished via the removal of the hemicellulose and lignin (Fig. 7b). Lastly, after hot pressing, the densi ed straw shows a rather compact and ordered microstructure ( Fig. 7c and SEM image is below). The surface bers of the densi ed straw are recognizable and crosswise arranged, and nearly no mixture of hemicellulose and lignin is found in the SEM image. These results also verify that the two steps combined with chemical treatment and hot pressing have eliminated the defects and reinforced the mechanical properties of the densi ed straw.
Furthermore, the hydrogen bonds between the aligned cellulose bers (Fig. 7d and 7e) have bridged the neighboring cellulose bers (Xiaoshuai Han et al. 2019), which is the reinforced mechanism of the densi ed straw at the molecular level for constructing a structure densi ed straw with superstrength, high modulus, and extraordinary elongation at break. 3.7 The Composites of PPC and the densi ed straw.
As aforementioned, the densi ed straw has been constructed as a structural material with highperformance in tensile and speci c strength, modulus, and elongation at a break. Here, the densi ed straw is used as a ller to reinforce the mechanical properties of the biodegradable PPC and cut costs. The technological process of the PPC and the densi ed straw composite is shown in Fig. 8a, and the mechanical properties of different samples are presented in Fig. 8b-8e. Taking PPC/densi ed straw (PPC/DS, 60/40wt%) composite as an example, the densi ed straw is rst blended with PPC in a mixer.
After compressing molding, the PPC/DS composite is obtained. The mechanical properties of the PPC/DS composite are improved compared with the sample of the neat PPC and other composites samples. Speci cally, the tensile strength and Young's modulus of the PPC/DS composite is 31.9MPa (Fig. 8b) and 1812MPa (Fig. 8d), which is about 157.3% and 124.8% higher than that of the neat PPC, respectively. Moreover, the speci c strength and the density of the PPC/DS composite is considerably improved. The densi ed straw can be used as a reinforced ller with PPC to fabricate a high-performance whole biodegradable composite.

Conclusions
In this work, a lightweight densi ed maize straw with superstrength, high modulus, and toughness based on maize straw peel is successfully fabricated by chemical treatment combined with hot pressing. The tensile strength (598.6MPa) and elongation at break (6.3%) of the densi ed straw are simultaneously enhanced about 9.3 times and 2.2 times than those of natural maize straw, respectively. The Young's modulus (16.6GPa) of the densi ed straw is about 5.9 times than that of the natural straw. The tensile strength and speci c strength (434MPa g-1 cm3) of the densi ed straw are even higher than those of the commercial aluminum alloys. The chemical treatment and hot pressing are essential steps for preparing the densi ed straw and improving the mechanical properties. The chemical treatment using a mixed alkali solution can remove the fractional lignin and hemicellulose but preserve most of the cellulose, thus enhancing the degree of crystallinity and the heating resistance of the densi ed straw. The densi cation by hot pressing is committed to fabricating a structure material with high-performance from maize straw, which has eliminated the defects and reinforced the mechanical properties of the densi ed straw. The microstructural densi cation in the cross-section and the side face is the main reason for enhancing density and improving the tensile strength. At the molecular level, the hydrogen bonds between the aligned cellulose bers have bridged the neighboring cellulose bers, reinforcing the mechanical properties of the densi ed straw and constructing a structural material with superstrength, high modulus, and extraordinary elongation at break. Lastly, the densi ed straw is successfully used to composite with PPC for constructing a whole biodegrade material. The mechanical properties of the composite have been reinforced that predicts a huge prospect in automobile, construction, furniture, and even airplane elds. ASSOCIATED CONTENT The stress-strain curve (Fig. S1), the tensile strength, elongation at break, and Young's modulus perpendicular to the growth direction of the natural and densi ed straw (Fig. S2) are shown in SUPPORTING INFORMATION.

Figure 3
The tensile strength and Young's modulus of different processing methods on natural straw.

Figure 4
The ATR-FTIR spectra (a) and XRD pattern (b) of natural and densi ed straw; the TGA (c) and DTG (d) of natural, modi ed, compressed, and densi ed straws.

Figure 5
The pictures of the natural straw (a) and densi ed straw (b); the SEM pictures of the cross-section (in the TW plane) of the natural straw (c, d) and densi ed straw (e, f), the side face (in the TL plane) of the natural straw (g, h) and densi ed straw (i, j), the surface (in the LW plane) of the natural straw (k, l) and densi ed straw (m, n).

Figure 6
The tensile strength (a), elongation at break (b), and Young's modulus (c) of the samples under different pressure at 100°C for 0.5h.

Figure 7
The changes of micro ber arrangement of natural straw (a), modi ed straw (b), and densi ed straw (c) during deligni cation and densi cation; the crystalline region and amorphous region of the cellulose micro brils (d) and hydrogen bonds between cellulose molecules (e).