One-Step Treatment for Upgrading Bleached Bamboo Pulp to Dissolving Pulp High Solvency in Green Alkali/Urea Aqueous Solution

Bleached bamboo pulp, as a kind of natural cellulose, has received significant attention in the field of biomass materials due to its advantages of environmental protection and the abundance of raw materials. Low-temperature alkali/urea aqueous system is a green dissolution technology for cellulose, which has promising application prospects in the field of regenerated cellulose materials. However, bleached bamboo pulp, with high viscosity average molecular weight (Mη) and high crystallinity, is difficult to dissolve in an alkaline urea solvent system, restraining its practical application in the textile field. Herein, based on commercial bleached bamboo pulp with high Mη, a series of dissolvable bamboo pulps with suitable Mη was prepared using a method of adjusting the ratio of sodium hydroxide and hydrogen peroxide in the pulping process. Due to the hydroxyl radicals being able to react with hydroxyls of cellulose, molecular chains are cut down. Moreover, several regenerated cellulose hydrogels and films were fabricated in an ethanol coagulation bath or a citric acid coagulation bath, and the relationship between the properties of the regenerated materials and the Mη of the bamboo cellulose was systematically studied. The results showed that hydrogel/film had good mechanical properties, as the Mη is 8.3 × 104 and the tensile strength of a regenerated film and the film have values up to 101 MPa and 3.19 MPa, respectively. In this contribution, a simple method of a one-step oxidation of hydroxyl radicals to prepare bamboo cellulose with diversified Mη is presented, providing an avenue for a preparation of dissolving pulp with different Mη in an alkali/urea dissolution system and expanding the practical applications of bamboo pulp in biomass-based materials, textiles, and biomedical materials.


Introduction
Natural cellulose from cotton, wood, and bamboo is the most abundant renewable resource on Earth [1][2][3]. Natural cellulose is considered to be the ideal substitute for current petroleum-based polymer materials [4,5]. However, cellulose is difficult to dissolve in common solvents, resulting from its unique microcrystal structures and strong hydrogen-bonding networks of an intra-and intermolecular nature [6][7][8]. Currently, numerous dissolution systems of cellulose have been reported, such as NaOH/CS 2 [9], N-methylmorpholine-N-oxide (NMMO) [10], ionic liquids (ILs) [11,12], alkali/urea [13,14], and lithium chloride/dimethylacetamide (LiCl/DMA C ) [15,16]. Even though NaOH/CS 2 is widely used to produce viscose, the process is time-consuming, has high energy consumption, and produces toxic by-products [17]. For solvents with a strong dissolving ability, including NMMO, LiCl/DMA C, and ionic liquids, industrial applications are still hindered by the high cost and the difficulties in solvent recycling [18]. Compared with

Preparation of Different Mη Bamboo Cellulose Solution
The 95 g LiOH·H 2 O/urea/deionized water (8:15:77 by weight) solution was precooled to −12.6 • C. An amount of 5 g dried cellulose sample was fleetly added into solvent system and vigorously stirred for 2-5 min, the dissolved bamboo cellulose solution was obtained. The clear solutions with different Mη were obtained after high-speed centrifugation (8000 rpm) of dissolved bamboo cellulose solution at 5 • C for 10 min.

Preparation of Bamboo Cellulose Hydrogels and Films with Different Mη
The 5 wt% bamboo cellulose solution was spread on a flat glass surface to form a solution film of cellulose with a thickness of 0.5 mm. Then, the liquid film of bamboo cellulose solution was immersed into a coagulation bath (ethanol or citric acid) for 30 min. The cellulose hydrogels with different Mη were obtained after gelation of bamboo cellulose aqueous solution. Moreover, the dry films of bamboo cellulose were prepared by washing with deionized water and drying the cellulose hydrogel at room temperature.

Characterization
The viscosity average molecular weight (Mη) was measured using an automatic viscometer (IV8100X, Hangzhou, China). An amount of 0.14 g of the prepared bamboo cellulose was added into 25 mL of deionized water and stirred for 10 min, forming the cellulose dispersion. Then, a completely dissolved cellulose solution was prepared after slowly adding 25 mL of copper ethylenediamine into the cellulose dispersion and stirring for 10 min. The intrinsic viscosity ([η]) of the bamboo cellulose was determined via a viscometer at 25 • C according to the following equation [46].
The molecular weight distribution (D Mw/Mn ) of the bamboo cellulose was measured by liquid chromatography (e2695, Milford, MA, USA)/(2414RI, Waters, Milford, MA, USA). A mixed solution containing 0.025 g of bamboo cellulose and 3 mL of DMAc solvent was prepared and activated at 150 • C for 60 min. Then, LiCl (8 wt%) was added into the mixed solution at 100 • C for 60 min and maintained at 50 • C until the bamboo cellulose completely dissolved. Furthermore, the homogeneous solution was diluted to 0.5 wt% LiCl/DMAc with DMAc. The solution was filtered with a 0.22 µm Millipore filter membrane, and the test conditions were as follows: a flow rate of 0.6 mL/min, a column temperature of 80 • C, and a detector of 50 • C [47].
A quantitative study of hydroxyl radical (·OH) derived from alkaline solution by UV-Vis absorption spectroscopy was performed. As shown in Figure 1a, the mixed aqueous solution consisting of NaOH (4, 12 and 24 g), H 2 O 2 (0, 0.4, 1, and 2 g), and 23.4 g of DMSO was reacted at 100 • C for 60 min. Then, 1 mL of the reacted solution was diluted to 500 mL with pH = 4. Moreover, 1 mL of BB salt was added to 2 mL of the diluted solution and reacted for 10 min at room temperature. The diazo sulfone derivatives were extracted, separated, and detected by UVs spectrum (UV-1800, Shimadzu, Japan) at 415 nm in a range of 350-800 nm and a standard curve of CH 3 SOOH solution [48,49]. The chemical reactions in the process of the ·OH quantitative detection are shown in Figure 1b. The production of hydroxyl radicals in a high-temperature alkali/H 2 O 2 system is shown in chemical Equation (1). DMSO, as a radical scavenger, captured the ·OH to produce methane sulfinic acid (MSA), as shown in chemical Equation (2). Simultaneously, the diazo sulfone derivatives were formed through the reaction of MSA and Blue BB salt (chemical Equation (3)). The molecular weight distribution (DMw/Mn) of the bamboo cellulose was measured by liquid chromatography (e2695, Milford, MA, USA)/(2414RI, Waters, Milford, MA, USA). A mixed solution containing 0.025 g of bamboo cellulose and 3 mL of DMAc solvent was prepared and activated at 150 °C for 60 min. Then, LiCl (8 wt%) was added into the mixed solution at 100 °C for 60 min and maintained at 50 °C until the bamboo cellulose completely dissolved. Furthermore, the homogeneous solution was diluted to 0.5 wt% LiCl/DMAc with DMAc. The solution was filtered with a 0.22 µm Millipore filter membrane, and the test conditions were as follows: a flow rate of 0.6 mL/min, a column temperature of 80 °C, and a detector of 50 °C [47].
A quantitative study of hydroxyl radical (·OH) derived from alkaline solution by UV-Vis absorption spectroscopy was performed. As shown in Figure 1a, the mixed aqueous solution consisting of NaOH (4, 12 and 24 g), H2O2 (0, 0.4, 1, and 2 g), and 23.4 g of DMSO was reacted at 100 °C for 60 min. Then, 1 mL of the reacted solution was diluted to 500 mL with pH = 4. Moreover, 1 mL of BB salt was added to 2 mL of the diluted solution and reacted for 10 min at room temperature. The diazo sulfone derivatives were extracted, separated, and detected by UVs spectrum (UV-1800, Shimadzu, Japan) at 415 nm in a range of 350-800 nm and a standard curve of CH3SOOH solution [48,49]. The chemical reactions in the process of the ·OH quantitative detection are shown in Figure 1b. The production of hydroxyl radicals in a high-temperature alkali/H2O2 system is shown in chemical Equation (1). DMSO, as a radical scavenger, captured the ·OH to produce methane sulfinic acid (MSA), as shown in chemical Equation (2). Simultaneously, the diazo sulfone derivatives were formed through the reaction of MSA and Blue BB salt (chemical Equation (3). Thermal gravimetric analyses (TGA) and derivative thermogravimetry (DTG) were carried out with a Q50 thermal analyzer (NETZSCH Corp, Selb, Germany) in air with a heating range from 25 to 600 °C by 10 °C/min. Solid-state 13 C (Avance Neo, 400WB, Bruker, Billerica, MA, USA) cross-polarization magic-angle spinning ( 13 C CP/MAS NMR) spectra were recorded with a 4 mm double-resonance MAS probe. A sample spinning rate of 10.0 kHz, a contact time of 2 ms, and a pulse delay of 3 s were applied. The chemical compositions of the before and after prepared bamboo pulp were characterized with an Avatar Fourier Transform Infrared Spectrometer (FT-IR, Nicolet Company, Madison, WI, USA).
The crystal peaks of the bamboo cellulose and the films with different Mη were determined using an X-ray polycrystalline diffractometer (D8, ADVANCE, Karlsruhe, Baden-Württemberg, Germany) with the conditions of the voltage, current, scanning range, Thermal gravimetric analyses (TGA) and derivative thermogravimetry (DTG) were carried out with a Q50 thermal analyzer (NETZSCH Corp, Selb, Germany) in air with a heating range from 25 to 600 • C by 10 • C/min. Solid-state 13 C (Avance Neo, 400WB, Bruker, Billerica, MA, USA) cross-polarization magic-angle spinning ( 13 C CP/MAS NMR) spectra were recorded with a 4 mm double-resonance MAS probe. A sample spinning rate of 10.0 kHz, a contact time of 2 ms, and a pulse delay of 3 s were applied. The chemical compositions of the before and after prepared bamboo pulp were characterized with an Avatar Fourier Transform Infrared Spectrometer (FT-IR, Nicolet Company, Madison, WI, USA).
The crystal peaks of the bamboo cellulose and the films with different Mη were determined using an X-ray polycrystalline diffractometer (D8, ADVANCE, Karlsruhe, Baden-Württemberg, Germany) with the conditions of the voltage, current, scanning range, and scanning rate being 40 kV, 40 mA, 2θ = 5-40 • , and 5 • /min, respectively. The crystallinity (χ C ) was calculated according to the ratio of the crystallization peak area to the integral area of the X-diffraction intensity curve of the cellulose samples and analyzed by the following Equation (2): where S a and S c are the crystalline areas and amorphous phases, respectively. The curve areas of the peaks were determined by integration, and they were recorded as the percentage of the crystalline peaks over the total area. The topography dimensions of the bamboo celluloses with different Mη were observed by environmental scanning electron microscopy (SEM, QUANTA 450, Gravenhage, South Holland, The Netherlands) under a voltage of 20 kV. The dissolution of the prepared bamboo cellulose samples with various Mη was systematically investigated using a polarized light microscope (Axiolab5, Zeiss, Oberkohen, Battenrunsberg, Germany) at room temperature. The viscosity of the cellulose solution was measured through a rotary viscometer (HAAKE, Viscotester3, Karlsruhe, Baden-Württemberg, Germany). The solubility of the bamboo cellulose was investigated via a weighing method (Equation (3)), and the specific operations were as follows: (i) forming a cellulose solution layer on flat glass surface substance, (ii) regeneration of cellulose solution in the 5 wt% H 2 SO 4 coagulation bath, and (iii) washing with deionized water to neutral and drying in the infrared oven for 10 min (WS70-1, Shanghai, China).
where M 1 is the mass of the added BP cellulose solution, M 2 is the dried sample, and A is the solubility. The particle size distributions of the diluted bamboo cellulose solutions were investigated by dynamic light scattering (DLS, 90 PALS, Brooke, Brooklyn, New York, USA). The bamboo cellulose solutions (c = 0.3 g/L) were filtrated through 0.45 µm Millipore filters. The hydrodynamic radius (R h ) was calculated according to the following Stokes-Einstein, Equation (4) [50].
where T is the temperature, K is the Boltzmann constant, η is the viscosity of solvent, and D T is the translational diffusion coefficient. The rheological behaviors of the different Mη solutions were characterized using a rheometer (HAAKE, RheoStress 600, Boston, MA, USA) with a gap of 500 µm from 20 • C to 80 • C. The cross-section morphologies of the cellulose hydrogels with different Mη were observed by SEM (Zeiss, Sigma 500, Oberkohen, Battenrunsberg, Germany) at an accelerating voltage of 10 kV. The strain-stress curves of the hydrogels and films were measured by means of a universal material testing machine (INSTRON5965, Boston, MA, USA).

Effects of Alkaline Peroxide Treatment for Bamboo Cellulose Mη
A schematic diagram of the bleached bamboo pulp upgrades to a high-solvency bamboo pulp in the alkali system by regulating Mη with OH originating from NaOH/H 2 O 2 is shown in Figure 2a. As shown in Figure 2b, hydroxyl radicals are able to react with hydroxyl groups at C6, C3, C2, and C1 of the cellulose chains in the alkaline solution system [51][52][53] and form various cellulose oxidation products ( Figure 2c). [37,54]. Consequently, the bamboo cellulose with lower Mη after de-polymerization has higher solubility in the alkali/urine system. The Fourier-transform infrared spectroscopy (FT-IR) of the bamboo pulps with different Mη (C14, C8.3, and C4.0) is shown in Figure 3a. The characteristic peaks at 3400 cm −1 , 2900 cm −1 , and 1640 cm −1 correspond to the O-H stretching vibration, C-H stretching vibration, and C=O stretching vibration, respectively. Evidently, the peak of C=O was significantly enhanced after the NaOH/H2O2 treatment, resulting from the formation o C=O groups that originated from the hydroxyls at C2, C3, and C6. Subsequently, the bamboo pulps of C4.0 and C14 were investigated by using the solid-state 13 C CP/MAS NMR spectrum. As shown in Figure 3b, the signals that appeared at δ105.3, δ89.1, δ84.6 δ75.1, δ72.6, and δ65.0 in the spectrum correspond to the C1, C4, C2, C3, C5, and C6 of the glucose, respectively. Comparing the spectra of the untreated and treated samples, three weak C=O signal peaks appeared at δ161.9, δ140.8, and δ134.1 in the treated samples which are responding to the C=O groups at C6′, C3′, and C2′. This result indicated tha the hydroxyl groups at corresponding positions on cellulose chains were partially oxi dized. The thermostability of the bamboo pulps with different Mη (C14, C8.3, and C4.0 was evaluated by TGA and DTG. In Figure 3c,d, the maximum decomposition tempera tures of the untreated and treated samples were 351.86 °C, 346.67 °C, and 341.24 °C, re spectively. Due to the formation, decomposition, and evaporation of the C=O groups originating from the hydroxyls at C2, C3, and C6, an advanced loss in weight was caused The Fourier-transform infrared spectroscopy (FT-IR) of the bamboo pulps with different Mη (C14, C8.3, and C4.0) is shown in Figure 3a. The characteristic peaks at 3400 cm −1 , 2900 cm −1 , and 1640 cm −1 correspond to the O-H stretching vibration, C-H stretching vibration, and C=O stretching vibration, respectively. Evidently, the peak of C=O was significantly enhanced after the NaOH/H 2 O 2 treatment, resulting from the formation of C=O groups that originated from the hydroxyls at C2, C3, and C6. Subsequently, the bamboo pulps of C4.0 and C14 were investigated by using the solid-state 13 C CP/MAS NMR spectrum. As shown in Figure 3b, the signals that appeared at δ105.3, δ89.1, δ84.6, δ75.1, δ72.6, and δ65.0 in the spectrum correspond to the C1, C4, C2, C3, C5, and C6 of the glucose, respectively. Comparing the spectra of the untreated and treated samples, three weak C=O signal peaks appeared at δ161.9, δ140.8, and δ134.1 in the treated samples, which are responding to the C=O groups at C6 , C3 , and C2 . This result indicated that the hydroxyl groups at corresponding positions on cellulose chains were partially oxidized. The thermostability of the bamboo pulps with different Mη (C14, C8.3, and C4.0) was evaluated by TGA and DTG. In Figure 3c,d, the maximum decomposition temperatures of the untreated and treated samples were 351.86 • C, 346.67 • C, and 341.24 • C, respectively. Due to the formation, decomposition, and evaporation of the C=O groups originating from the hydroxyls at C2, C3, and C6, an advanced loss in weight was caused.
The processability of bamboo pulp is typically determined by their solubility. Mη is one of the most important parameters for the efficient dissolution of bamboo pulps. Regulating the Mη of bamboo cellulose with different dosages (mass fraction) of NaOH and H 2 O 2 is displayed in Figure 4a-c. Compared with only a NaOH solution, Mη is easier to regulate by a mixed solution of NaOH and H 2 O 2 treatment. The ·OH was able to oxidize hydroxyl groups of cellulose in the position of the C1, C2, C3, and C6, resulting in the de-polymerization of the cellulose and forming lower Mη cellulose. Generally, as a fixed dosage of NaOH (1wt%, 3wt%, and 6 wt%), the amount of ·OH increased with the increased concentration of H 2 O 2 (from 0~1 wt%). Similarly, the number of hydroxyl radicals produced at different NaOH concentrations was in disaffinity. Significantly, the amount of reactive ·OH was less than that in real production, because ·OH is easily quenched at different alkali concentrations. The results showed that the D Mw/Mn of the bamboo cellulose was determined by the synergistic effect of the mixed solution of NaOH and H 2 O 2 , as seen in Figure (Table S1). The processability of bamboo pulp is typically determined by their solubility. Mη is one of the most important parameters for the efficient dissolution of bamboo pulps. Regulating the Mη of bamboo cellulose with different dosages (mass fraction) of NaOH and H2O2 is displayed in Figure 4a-c. Compared with only a NaOH solution, Mη is easier to regulate by a mixed solution of NaOH and H2O2 treatment. The ·OH was able to oxidize hydroxyl groups of cellulose in the position of the C1, C2, C3, and C6, resulting in the de-polymerization of the cellulose and forming lower Mη cellulose. Generally, as a fixed dosage of NaOH (1wt%, 3wt%, and 6 wt%), the amount of ·OH increased with the increased concentration of H2O2 (from 0~1 wt%). Similarly, the number of hydroxyl radicals produced at different NaOH concentrations was in disaffinity. Significantly, the amount of reactive ·OH was less than that in real production, because ·OH is easily quenched at different alkali concentrations. The results showed that the DMw/Mn of the bamboo cellulose was determined by the synergistic effect of the mixed solution of NaOH and H2O2, as seen in Figure 4d. For instance, the DMw/Mn could be changed in the range of 4.20 to 2.99, as in a mixed solution consisting of 0.4 wt% H2O2 and NaOH with different dosages. The alkali peeling reaction on the bamboo cellulose reacted by means of the outside-to-inside, causing the DMw/Mn to decline slowly after the reaction speed was accelerated by using increasing alkali concentrations. Moreover, the results obtained by A standard curve of the relationship between the concentration of MSA (C MSA ) and absorbance is presented in Figure 5a. The formula was Abs = 0.0021 × C MSA + 0.3651, with R 2 = 0.9937. Based on this, the total amount of ·OH was calculated, as shown in Figure 5b. The absorbance was up to 1.486 in the mixed solution of 6 wt% NaOH and 0.5 wt% H 2 O 2 , and the absorbance increased with the increasing H 2 O 2 mass fraction. Moreover, this linear rule was also compounded in the mixed solution containing 1 wt% NaOH and 3 wt% NaOH systems ( Figure S1). Due to the fact that the detected products had a short lifetime and that the reactive radical species were difficult to detect, the detected data are approximate values in the three cases. The relationship between the amount of ·OH and Mη is shown in Figure S2: the more·OH produced, the lower the Mη.  A standard curve of the relationship between the concentration of MSA (CMSA) and absorbance is presented in Figure 5a. The formula was Abs = 0.0021 × CMSA + 0.3651, with R 2 = 0.9937. Based on this, the total amount of ·OH was calculated, as shown in Figure 5b. The absorbance was up to 1.486 in the mixed solution of 6 wt% NaOH and 0.5 wt% H2O2, and the absorbance increased with the increasing H2O2 mass fraction. Moreover, this linear rule was also compounded in the mixed solution containing 1 wt% NaOH and 3 wt% NaOH systems ( Figure S1). Due to the fact that the detected products had a short lifetime and that the reactive radical species were difficult to detect, the detected data are approximate values in the three cases. The relationship between the amount of ·OH and Mη is shown in Figure S2: the more·OH produced, the lower the Mη.   A standard curve of the relationship between the concentration of MSA (CMSA) and absorbance is presented in Figure 5a. The formula was Abs = 0.0021 × CMSA + 0.3651, with R 2 = 0.9937. Based on this, the total amount of ·OH was calculated, as shown in Figure 5b. The absorbance was up to 1.486 in the mixed solution of 6 wt% NaOH and 0.5 wt% H2O2, and the absorbance increased with the increasing H2O2 mass fraction. Moreover, this linear rule was also compounded in the mixed solution containing 1 wt% NaOH and 3 wt% NaOH systems ( Figure S1). Due to the fact that the detected products had a short lifetime and that the reactive radical species were difficult to detect, the detected data are approximate values in the three cases. The relationship between the amount of ·OH and Mη is shown in Figure S2: the more·OH produced, the lower the Mη.

Solubility of Different Mη Bamboo Cellulose in LiOH/Urea/Aqueous Solution
The physicochemical parameters of the resultant bamboo cellulose with different Mη are displayed in Table 1. The results showed that the parameters of the α-cellulose content and the whiteness increased, the hemicellulose content and the cellulose yield decreased, and the other indexes (ash, Fe 3+ , and dichloromethane extract) changed slightly. However, χ c exhibited a trend of first increasing and then decreasing, resulting from the fact that the degree of damage to the crystalline and amorphous regions of the cellulose was different during the alkaline oxidation treatment. Thus, cellulose dissolution required unwrapping and devitrification, causing the crystallinity to affect the cellulose dissolution in an alkali/urea aqueous solvent.  The relationship between the solubility and the Mη of the bamboo cellulose in the LiOH/urea aqueous system was investigated, as shown in Figure 7a and Table S3. The solubility of the bamboo cellulose decreased from 100% to 34.5% with increased Mη from C4.0 to C14. Moreover, the stability of the dissolved solution of the bamboo cellulose with different Mη was further studied by means of analyzing rheology, as shown in Figures 7b  and S4. The results showed that the gel point decreased from 67.75 °C to 33.05 °C with increasing Mη (Figure 7b) because the thermal motion and the effective collision of the cellulose molecule chains were increased with increasing temperatures. In addition, the loss modulus (G″) and storage modulus (G′) increased with the increasing Mη during the The relationship between the solubility and the Mη of the bamboo cellulose in the LiOH/urea aqueous system was investigated, as shown in Figure 7a and Table S3. The solubility of the bamboo cellulose decreased from 100% to 34.5% with increased Mη from C4.0 to C14. Moreover, the stability of the dissolved solution of the bamboo cellulose with different Mη was further studied by means of analyzing rheology, as shown in Figures 7b and S4. The results showed that the gel point decreased from 67.75 • C to 33.05 • C with increasing Mη (Figure 7b) because the thermal motion and the effective collision of the cellulose molecule chains were increased with increasing temperatures. In addition, the loss modulus (G ) and storage modulus (G ) increased with the increasing Mη during the heating process from 20 • C to 80 • C ( Figure S4). Subsequently, DLS was used to observe the size of agglomerates of the bamboo cellulose solution with different Mη, as shown in Figure 7c and Table S3. The R h increased from 106.8 nm to 215.72 nm, with Mη increasing from C4.0 to C14. The formation of large aggregates was due to increased cellulose molecular weight. Furthermore, the results of rotary viscometer of the prepared cellulose solution with different Mη were shown in Figure 7d. The viscosity of the bamboo cellulose increased from 323.4 mPa·s to 12,449.8 mPa·s with increasing Mη from C4.0 to C8.3. However, the viscosities at C10 and C14 were less than the highest value at C8.3. Due to bamboo celluloses (C10 and C14) having massive hydroxyl groups and chain entanglement, the solution viscosity is increased and difficulties in separation by centrifugation arise.

Structure and Mechanical Properties of Hydrogels and Dry Films
The structure-mechanical properties relationships of the regenerated cellulose materials with different Mη were investigated. The cross-section SEM images of the regenerated cellulose materials, both in ethanol coagulation bath ( Figure S5a-e) and in citric acid coagulation bath ( Figure S5f-j), were generated. The Mη ranged from C5.1, C6.7, C8.3, and C10 to C14, respectively. Due to the lowest viscosity and good flowability of the cellulose solution at C4.0, the regenerative cellulose material was uniform. It was difficult to characterize the actual performance of the materials, as shown in Figure S6a,b. The different Mη of the bamboo cellulose caused different amounts of hydroxyl groups in the cellulose chain. Furthermore, several structures of the regenerated cellulose materials were formed through the parallel aggregation of intra-or intermolecular hydrogen bonds. The bamboo cellulose solutions with high dissolution rates in the alkali/urea system, such as C5.1, C6.7, and C8.3, formed regenerated materials with a dense porous/layer structure. On the contrary, the cellulose solution with low solubility (C10 and C14) formed porous-structured materials. Moreover, the regeneration rate of the cellulose solution in ethanol was slower than citric acid and formed cellulose with a denser structure.
The mechanical properties of the regenerated cellulose materials (hydrogels and dry films) with different Mη were investigated. Figure 8a summarizes the tensile stress of the

Structure and Mechanical Properties of Hydrogels and Dry Films
The structure-mechanical properties relationships of the regenerated cellulose materials with different Mη were investigated. The cross-section SEM images of the regenerated cellulose materials, both in ethanol coagulation bath ( Figure S5a-e) and in citric acid coagulation bath ( Figure S5f-j), were generated. The Mη ranged from C5.1, C6.7, C8.3, and C10 to C14, respectively. Due to the lowest viscosity and good flowability of the cellulose solution at C4.0, the regenerative cellulose material was uniform. It was difficult to characterize the actual performance of the materials, as shown in Figure S6a,b. The different Mη of the bamboo cellulose caused different amounts of hydroxyl groups in the cellulose chain. Furthermore, several structures of the regenerated cellulose materials were formed through the parallel aggregation of intra-or intermolecular hydrogen bonds. The bamboo cellulose solutions with high dissolution rates in the alkali/urea system, such as C5.1, C6.7, and C8.3, formed regenerated materials with a dense porous/layer structure. On the contrary, the cellulose solution with low solubility (C10 and C14) formed porous-structured materials.
Moreover, the regeneration rate of the cellulose solution in ethanol was slower than citric acid and formed cellulose with a denser structure.
The mechanical properties of the regenerated cellulose materials (hydrogels and dry films) with different Mη were investigated. Figure 8a summarizes the tensile stress of the cellulose hydrogels in different coagulation baths, including ethanol and citric acid. The result indicated that the stress of the cellulose hydrogel in ethanol was higher than in citric acid. This phenomenon corresponded to the structure of cellulose materials (in Figure S5), in which the greater stress comes from the denser structure. Moreover, the fracture of the stress-strain curve increased gradually from 2.44 to 3.19 MPa and 30.64% to 41.78% with increasing Mη (C5.1 to C8.3) in the ethanol coagulation bath. The relationship between the Mη and the stress-strain is exhibited in Figure S7a, and the mechanical properties of the regenerated cellulose hydrogels are summarized in Table S4. Furthermore, as shown in Figure 8b, the tensile stresses of the cellulose films in two coagulation baths (ethanol and citric acid) with the same Mη (C8.3) are presented. The results showed that the stress of the regenerated cellulose film in citric acid was higher than in ethanol. This phenomenon is contrary to the mechanical properties of the cellulose hydrogels, resulting from the reorganization of the internal structure caused by the occurrence of moisture. In addition, the stress-strain of the regenerated cellulose films increased first and then decreased, similar to the trend seen with the cellulose hydrogels. The stress-strain of cellulose was maximal at values of 101.66 MPa and 5.69 %, with a Mη vaue of C8.3 ( Figure S7b and Table S5). Therefore, the corresponding cellulose solution has good fluidity and the regenerated cellulose materials have excellent mechanical properties.  Figure S7b and Table S5). Therefore, the corresponding cellulose solution has good fluidity and the regenerated cellulose materials have excellent mechanical properties. The XRD data of the regenerated cellulose film are presented in Figure S8a,b. The tested samples produced three characteristic peaks at 12°, 20°, and 22.3° (2θ), corresponding to the (11 ̅ 0), (110), and (200) planes of the cellulose II crystalline form, respectively. The crystallinity (χ C ) of the cellulose film regenerated in citric acid first increased (47.82% to 60.49%) and then decreased (60.49% to 42%), and the highest value of 60.49% corresponded to C8.3. Moreover, the χ C of the cellulose films regenerated in ethanol also first increased and then decreased with increased cellulose Mη. The highest value of χ C is 49.69% (C10), as shown in Figure S8c. The results demonstrate that the bamboo cellulose dry films have a higher χ C when regenerated in citric acid than in ethanol, corresponding to the mechanical properties of the cellulose film. Therefore, these results could confirm that crystallinity is also an important parameter for mechanical performance and that only cellulose with appropriate Mη has a good crystallinity.

Conclusions
In conclusion, a one-step method of the treatment of NaOH/H2O2 to regulate the Mη of commercial, bleached bamboo pulp with high Mη, realizing the preparation of various bamboo pulps with a low Mη (C4.0, C5.1, C6.7, C8.3, and C10) and efficient dissolving in a LiOH/urea aqueous solution, was developed. Moreover, the solution concentration of NaOH/H2O2 was systematically studied, indicating that the higher the concentration, the lower the molecular weight. In addition, the reason for the decreased Mη of the bamboo cellulose was revealed, resulting from the effect of the reaction of ·OH with the hydroxyl The XRD data of the regenerated cellulose film are presented in Figure S8a,b. The tested samples produced three characteristic peaks at 12 • , 20 • , and 22.3 • (2θ), corresponding to the (10), (110), and (200) planes of the cellulose II crystalline form, respectively. The crystallinity (χ C ) of the cellulose film regenerated in citric acid first increased (47.82% to 60.49%) and then decreased (60.49% to 42%), and the highest value of 60.49% corresponded to C8.3. Moreover, the χ C of the cellulose films regenerated in ethanol also first increased and then decreased with increased cellulose Mη. The highest value of χ C is 49.69% (C10), as shown in Figure S8c. The results demonstrate that the bamboo cellulose dry films have a higher χ C when regenerated in citric acid than in ethanol, corresponding to the mechanical properties of the cellulose film. Therefore, these results could confirm that crystallinity is also an important parameter for mechanical performance and that only cellulose with appropriate Mη has a good crystallinity.

Conclusions
In conclusion, a one-step method of the treatment of NaOH/H 2 O 2 to regulate the Mη of commercial, bleached bamboo pulp with high Mη, realizing the preparation of various bamboo pulps with a low Mη (C4.0, C5.1, C6.7, C8.3, and C10) and efficient dissolving in a LiOH/urea aqueous solution, was developed. Moreover, the solution concentration of NaOH/H 2 O 2 was systematically studied, indicating that the higher the concentration, the lower the molecular weight. In addition, the reason for the decreased Mη of the bamboo cellulose was revealed, resulting from the effect of the reaction of ·OH with the hydroxyl groups on the bamboo cellulose. Interestingly, the mechanical properties of the regenerated bamboo cellulose materials increased with increased Mη (Mη < C10), as the Mη of C8.3 had the best mechanical properties; the tensile stress of bamboo cellulose film was up to 101 MPa and the strength of the bamboo cellulose hydrogel was about 3.19 MPa. In this work, a simple method to prepare bamboo cellulose with diversified Mη is presented, and the regenerated cellulose materials with appropriate Mη possess excellent mechanical properties. It is believed that this study can provide significant guidance for creating a cellulose pulp with a suitable Mη for an alkali/urea aqueous system. This study also expands the practical applications of bamboo pulp in biomass-based materials, textiles, and biomedical materials.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/polym15061475/s1, Table S1: The Mη and D Mw/Mn of bamboo pulps treated with NaOH and H 2 O 2 solutions of different mass fractions; Figure S1: The absorbance of diazo salt changed with the increase of H 2 O 2 under the conditions of 1 wt% NaOH (a) and 3 wt% NaOH (b); Figure S2: The relationship between the hydroxyl radicals and the Mη of bamboo pulp in three alkali conditions: 1 wt% (a), 3 wt% (b) and 6 wt% (c); Figure S3: (a) The SEM images of bamboo cellulose with different Mη. Optical pictures (b) and optical microscope photographs (c) of bamboo cellulose solutions with different Mη; Table S2: The Mη of bamboo pulps are corresponding to cellulose bundle chain size; Table S3: Solubility and DLS of bamboo cellulose solutions with different Mη; Figure S4: Gel temperature of bamboo cellulose solutions with different Mη; Figure S5: The cross-section SEM images of the regenerated cellulose materials in ethanol coagulation bath (a-e) and citric acid coagulation bath (f-j); Figure S6: The regenerated cellulose hydrogels of C4.0 (Mη, 1.0 × 10 4 ) in ethanol coagulation bath (a) and citric acid coagulation bath (b); Figure S7: The stress-strain curves of regenerated hydrogels (a) and dry films (b) of bamboo cellulose with different Mη; Table S4: The stress-strain data of the regenerated cellulose hydrogels with different Mη; Table S5: The stress-strain data of the regenerated bamboo cellulose films with different Mη; Figure

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.