The effect of the hydrolysis temperature on yield of MCC
Studies have shown that the hydrolysis of cellulose requires higher energy, so the hydrolysis temperature has a significant impact on the preparation of MCC [6, 12]. It can be seen from Fig. 1 that as the temperature increases, the yield of microcrystalline cellulose first increased and then decreased. When the temperature rose to 70–80℃, the yield of microcrystalline cellulose tends to stabilize and remains at about 87% When the temperature rose to 90℃, the system energy was too high. At this time, the glycosidic bond of cellulose was broken and was hydrolyzed to dissolved glucose, causing MCC loss. Therefore, the hydrolysis temperature should be maintained at 70–80°C. Considering the economics of the process, 70℃ was more appropriate.
Effect of the concentration of hydrochloric acid on yield of MCC
Due to the large activation energy required for the cellulose hydrolysis reaction, the reaction is difficult, and inorganic acid is usually added as a catalyst to reduce the activation energy of the reaction(Han et al. 2015). Considering the improvement of substrate reaction activity, product selectivity and cost, hydrochloric acid is the most suitable, so hydrochloric acid is used as the catalyst in this study. So, The reaction was caried in different concentrations of hydrochloric acid system. It can be seen from Fig. 2 that as the concentration of hydrochloric acid increases, the yield of MCC first increased and then decreased. When the concentration of hydrochloric acid was 5%, the acidity of the system was weak and the number of active sites was insufficient. In this condition, the rate of hydrolysis was much lower, so the product yield was low. With the increasing of the acidity of the solution, the rate of hydrolysis increased significantly. when the mass fraction of hydrochloric acid reached 10%, the yield was the highest. After that, as the concentration of hydrochloric acid increased, the hydrolysis of glycosidic bonds intensifies, exposing more reducing end groups and further accelerating the rate of hydrolysis, which leaded to excessive degradation and dissolution of cellulose, which reduced the yield of MCC.
It can be seen from Fig. 3 that in the early stage of acid hydrolysis (before 70 min), the main destruction of the hydrolysis reaction was the amorphous region of cellulose, which increased the crystallinity of cellulose, so the yield of MCC increased. However, if the reaction time was excessively extended, the crystalline area of cellulose will also be destroyed, that was to say, MCC will also undergo hydrolysis. When the degree of polymerization was reduced to a certain extent, it will be dissolved in the reaction system and lost. Therefore, the acid hydrolysis time was preferably selected as 70 minutes.
Reaction conditions: m(cellulose) = 2g; V(HCl) = 40ml; C(HCl) = 10%;T = 70℃;
As shown in Fig. 4, A is the Rubescens cellulose, B is the MCC of Rubescens, and C is the MCC standard sample (purchased from Xi'an Baichuan Biotechnology, purity 99%). It can be seen from the FTIR spectrum that the they all have the absorption peaks at 3400 cm− 1, 2920 cm− 1, 1634 cm− 1, 1370 cm− 1, 1040 cm− 1, and 897 cm− 1 in common represent the –OH stretching vibration peak, the –CH stretching vibration peak, and –C = O stretching vibration peak,–CH bending vibration peak, –C–O–C stretching vibration characteristic peak and alienation β-bond . It can be seen that the position of the basic characteristic peak of cellulose has not changed, indicating that the molecular structure of the microcrystalline cellulose was basically unchanged during the process of preparing microcrystalline cellulose, and it’s quality was basically consistent with the commercial microcrystalline cellulose.
It can be seen from Fig. 5 that the positions of the diffraction peaks of the rubescens microcrystalline cellulose and the MCC standard sample are the same, and diffraction peaks appear at 2θ = 15.4°, 22.5°, and 34.6°, and their crystal structure is type I(Wang et al. 2016), indicating that the preparation of microcrystalline cellulose was basically the same as the MCC standard. Analysised with jade software found that the crystallinity was slightly lower than MCC standard sample. The microcrystalline fiber microfilm had a diffraction peak at 2θ = 20.8°, and it was crystal structure was type II(Han et al. 2013), indicating that the crystalline structure of the cellulose has changed after the NMMO/H2O was dissolved into a membrane. After the membrane was formed, the diffraction intensity decreases, and the crystallinity drops to 58.03%. This was due to the strong polar oxygen atoms on the N-O in the NMMO solution attacking the hydrogen bond and breaking it during the dissolution process.
The thermal curve of MCC standard sample, Rubescens chinensis MCC and Rubescens chinensis MCC membrane measured at a heating rate of 10°C/min were shown in Fig. 6. The thermal decomposition behavior of the samples can be roughly divided into three intervals. The first stage was the micro-weight loss stage, which was mainly manifested by the volatilization of intermolecular bound water and additives.The second stage was the thermal decomposition stage which caused a significant weight loss. The third stage was the stability of carbon formation, the sample was basically carbonized at this stage, and the increase in temperature has a relatively small effect on the weight loss of the residue(Fahma et al. 2010)
From Fig. 6(a), we can see the thermal weight loss curve of Rubescens vulgaris MCC. Compared with the standard sample, in the first stage, the weight loss rate of Rubescens vulgaris MCC was slightly lower than the standard sample, indicating that it was more hydrophilic than the standard sample. The initial pyrolysis temperature was about 275℃, which was basically the same as the standard sample. It can be seen from the Fig. 6(b) that its maximum weight loss rate temperature was 327℃ ,which was 22℃ lower than the standard sample and the remaining residue rate was 8.9 %. Overall, the thermal stability of Rubescens vulgaris MCC was not much different from that of MCC standard. In sharp contrast, the initial pyrolysis temperature of Rubescens vulgaris MCC membrane was 151℃, and the maximum pyrolysis rate temperature was 218℃. The thermal stability after membrane formation was lower than before membrane formation, which may be due to the dissolution regeneration process. The hydrogen bond between the cellulose was not completely rebuilt after it was opened, and the decrease in crystallinity after dissolution also affects the thermal stability of the MCC membrane(Weng et al. 2017)
Influence of the content of MCC on the mechanical properties
Figure 7 shown the influence of the content of microcrystalline cellulose in the casting solution on the mechanical properties of the microcrystalline cellulose membrane. It can be seen that with the increase of the content of MCC in the casting solution, the tensile strength and elongation at break of the MCC membrane both show an increasing trend. When the content of MCC increased from 5–9%, the tensile strength of the membrane increased from 3.20 MPa to 8.38MPa, and the elongation at break increased from 13.79–26.72%. This was due to the increase in cellulose content in the casting solution, the increase in the number of molecules per unit volume, forming extrusion with each other, increasing the intermolecular microcrystalline entanglement, and the tighter intermolecular connection, thus increasing the mechanical properties of the membrane. However, as a separation membrane, the concentration of casting solution should not be too high, otherwise the membrane will be too dense, which will reduce the permeability and even lose the separation function.
Influence of MCC Content on the membrane’s hydrophilicity
The contact angle is one of the indexes to measure the hydrophilic or hydrophobic properties of the membrane. The hydrophilicity or hydrophobicity of the membrane has a certain influence on the application field of the membrane.
Contact angle of membranes with different MCC content
Therefore, it is of great significance to control the hydrophilic and hydrophobic properties of the membrane. From the perspective of cellulose structure, cellulose was an amphiphilic molecule, which has both a hydrophobic carbon ring and a hydrophilic hydroxyl group. Therefore, the hydrophilic and hydrophobic properties of the membrane are controllable by some means. Here, we examined the membranes formed by different cellulose contents of Rubescens in the casting solution, and the contact angles of water against membrane are also different. The contact angles were shown in Fig. 8 and Table 1. The data in Table 1 shown that as the concentration of cast cellulose increases, the contact angle increases, indicating that the hydrophobicity of the cellulose membrane increases simultaneously. This was because the higher the content of microcrystalline fibers in the casting liquid, the denser the membrane formed, the shrinkage of the membrane pores, the smaller the specific surface area, and the less the number of exposed hydroxyl groups, the weaker the hydrophilicity and the increased hydrophobicity.
The water flux and rejection rate of the membrane are important indicators to characterize the performance of the membrane. In this study, the flux and rejection rate of the membrane formed by the casting liquid with different MCC content to 1.0 g/L bovine serum albumin aqueous solution were determined under the pressure of 0.1MPa. The result was shown in Fig. 9.
The effect of the content of MCC on the separation performance of membrane
It can be seen from Fig. 9 that under the same pressure, with the increased of the cellulose content in the membrane casting solution, the water flux of the formed membrane shown a downward trend, and the rejection rate rate continues to increased. When it reached 7%, the water flux was still decline but the rejection rate hardly changes. This is due to the increase in cellulose content and the increase in the force between cellulose molecules, which will be more tightly bonded to each other when forming a membrane, and the membrane pore size will become smaller, resulting in smaller water molecular channels and reduced water flux. This was consistent with the above conclusion that the higher the cellulose content in the contact angle, the stronger the hydrophobicity of the cellulose membrane.
Therefore, selecting the appropriate cellulose content was of great significance to the separation effect of the regenerated cellulose membrane. Here, under a certain pressure, the solute rejection Q per unit time and unit area of the membrane represents the separation efficiency of the cellulose membrane.
It can be seen from Fig. 10 that the membrane formed when the MCC content was 5% has the highest rejection of bovine serum albumin, and its value can reach 37.23 g/(m2·h).
SEM analysis of membrane
The cellulose membrane with 5%MCC content with the best performance was taken as the test object, and its microstructure was analyzed using SEM. The plane and section structure were shown in Fig. 11. It can be seen from Fig. (a) that the surface of the membrane was smooth, flat and uniform without obvious structural defects. As can be seen from fig. (b) section, the section of the membrane shows a high density spongy shape, and the bonding between cellulose molecules was close, thus showing a high mechanical strength and separation performanc.