Microstructure of CNCs
The SEM image (Figure 1(a)) of the prepared MCC shows that the diameter is about 10μm, and the length ranges from tens to hundreds of microns. During the hydrolysis process, acid can penetrate the cellulose network quickly and hydrolyze the amorphous region (Johar et al. 2012). At the same time, the crystalline region of cellulose is more resistant to weak acid hydrolysis, which is due to the stronger hydrogen bond interaction between adjacent cellulose molecules in the crystal region compared with the non-compact amorphous region. As shown in Figure 1(b), uniform spherical CNCs with a diameter of about 37 nm are observed under SEM. However, due to their small size, strong interaction, and large specific surface area, the agglomeration of nanoparticles is also obvious. TEM was further used to confirm the morphology and diameter of CNCs. Figure 1(b’) clearly shows that the prepared CNCs are spherical with a smooth surface and uniform diameter distribution.
Furthermore, the effects of reaction temperature and time on the diameter of CNCs were carefully studied. As shown in the distribution diagram in Figure1(c), the influence of temperature and time on the size of CNCs has a similar trend, that is, with the increase of time and temperature, the diameter decreases. The diameter decreases from 98 nm to 37 nm when the reaction time increases from 6 h to 12 h while the temperature is kept at 80 ℃. When the reaction is processed at 60 ℃ for 12 h, the particle size of CNCs is larger than 200 nm. Besides, the particle diameter gradually decreases to 32 nm while the temperature further increases to 90 ℃. However, due to the existence of high concentration sulfuric acid in the mixed acid, it is easy to cause CNCs to yellow under high reaction temperature and long reaction time. Therefore, the optimal condition of using 80 ℃ for 12h was selected as the befitting parameter to prepare CNCs in the following section.
Surface Morphology and Wetting Behaviors
The commercial PLA nonwoven membrane was modified via coating CNCs onto the surface of the PLA fibers by using PDA as an adhesive. The coating ratio of CNCs is closely related to the attaching weight of PDA which is due chiefly to the concentration of dopamine in the reaction solution. As shown in Figure 2, when the concentration of dopamine increases from 0.1 g/300 ml to 1 g/300 ml, the weight fraction of CNCs increases from 1% to 2.5%. With the lower concentration, the consumption of dopamine monomer can be timely replenished, so the weight fraction of CNCs increases. However, when the concentration of dopamine reaches a certain value of 0.5 g/300 ml, the change of weight fraction of CNCs tends to balance. The maximum weight fraction of CNCs is obtained as about 2.5%.
The microstructures of pristine and modified PLA nonwoven membranes were characterized by SEM. As shown in Figure 3, the pristine PLA nonwoven membrane has a 3D porous network structure building up of microfibers with a smooth surface and a diameter of about 20 μm. After treated with dopamine, a PDA layer appears obviously on the fiber. This layer of PDA may be broken and cracked due to bending and folding in the sample preparation. However, it can be seen from figure 3(b’) that the PDA layer is smooth and even in thickness in the relatively intact area. Further being attached by CNCs (~37nm), these nanoparticles are distributed on the surface of PLA fibers in the form of polydisperse aggregates. Besides, CNCs are not limited to the surface but infiltrated and fixed in the fiber network due to the large pore size of nonwovens. This will also reduce the pore size from 223 μm to 128 μm as shown in Figure 4. What’s more, the enlarged micrograph of figure 3(c’) clearly shows the rough surface of PLA fibers, which is in obvious contrast to the smooth fiber of pure PLA nonwoven membrane (figure 3(a’)). The protruding morphology will endow the CNCs/PDA/PLA nonwoven membrane a hierarchical rough structure, which is the key to constructing a superhydrophilic membrane.
Figure 5 shows the WCA of the original PLA nonwoven which is about 121 ± 1.9 °. The chemical structure of PLA consisting of hydrophobic ester groups determines that the nonwoven membrane presents a hydrophobic surface. After coating with PDA, the WCA of the PDA/PLA membrane decreases significantly to 58.5 ± 1.6 °. This is because the PDA film wrapped on the surface of PLA fiber contains a large number of hydrophilic groups such as hydroxyl groups and amino groups. However, the WCA of 58.5 ± 1.6 ° results in the PDA/PLA nonwoven membrane having hydrophilic and lipophilic properties at the same time, so it still cannot be used for oil/water separation. Upon being further attached CNCs, the roughness of the surface can amplify the original wetting characteristics to a certain extent, the WCA of CNCs/PDA/PLA nonwoven membrane reduces to 0 °, and water can quickly spread and penetrate the membrane. The transformation of PLA nonwovens from hydrophobic to superhydrophilic is realized. To the specific, the multi-level rough structure formed by CNCs on the surface of the PLA fiber and the hydrophilic groups (hydroxyl groups) rich in CNCs plays a synergistic role in the acquirement of superhydrophilic CNCs/PDA/PLA membranes.
Separation Performance and Water Adsorption
The superhydrophilic surface wettability and porous internal structure of CNCs/PDA/PLA nonwoven membrane make it a superior choice for oil/water separation and water adsorption. As shown in Figure 6(a) and (a’), the separation performance of CNCs/PDA/PLA nonwoven membrane was investigated by using n-hexane/water mixtures. In the separation process, water can quickly wet the CNCs/PDA/PLA nonwoven membrane and penetrate through the membrane, while oil is trapped on the upper surface of the membrane. The separation efficiency can be as high as 99% indicating outstanding oil/water separation performance. Reusability is an important aspect to evaluate the durability and environmental protection of oil/water separation materials especially for these coated with particles, which are easy to wear and tear in repeated use. 100 cycles of separation tests were carried out and it is found that after 100 cycles of repeated use, the separation efficiency is still above 98% stating exceptionally well in durability (Figure 6(b)). This makes modified PLA nonwoven membrane become a very practical environmental protection material for oil/water separation.
Water adsorption is also discussed since it is important for some absorbent and water retaining materials. As shown in Figure 6(c), the water absorption ratio of the original PLA nonwoven is only about 270%. After being modified by PDA, the water absorption ratio of PDA/PLA membranes reaches 778%. It further enhances up to 1000% by attaching CNCs nanoparticles for CNCs/PDA/PLA membranes. The water absorption ratio is closely related to the wetting behavior of the membrane. For the pristine PLA nonwoven membrane, water is difficult to withholding in the material. Whether it is modified by PDA or further coated by CNCs, the modified PLA nonwovens have a good water retention effect. Also, the flux maintains as high as 3710 Lm-2h-1 for the CNCs/PDA/PLA membrane.
The excellent mechanical properties are the basis of improving the practicability of PLA nonwoven membrane as oil/water separation materials. In particular, the flexibility of nonwovens tends to deteriorate after post-treatment, which has adverse effects on the storage, handling, and use of membranes. The mechanical strength of pure PLA nonwoven membrane is only 114 N due to the disorderly arrangement of fibers (Figure 7(a)). For nonwovens, the bond fastness of interlaced fibers is the key factor affecting mechanical strength. Since PDA can not only wrap on the surface of the fiber but also adhere to the intersection of the fibers, which plays the role of chemical crosslinking, the mechanical strength of the PDA/PLA nonwoven membrane is increased to 150N. It is further increased to 168N for the CNCs/PDA/PLA membrane, which is 47% higher than that of the original PLA sample. Also, the modified PLA nonwoven membrane maintains good flexibility, and the elongation at break is about 60%. This should be ascribed to the uniform loading of PDA and CNCs on the fiber surface, which is conducive to the stability and uniform deformation of the nonwoven membrane.
The mechanical strength of oil/water separation materials will be damaged after repeated use, which will affect its service life. The mechanical strength of CNCs/PDA/PLA nonwoven membrane after a certain number of uses was carefully studied. After observation of the trend of mechanical strength with the using times, there is a sudden decrease when used more than 40 times. The microstructure of fiber cross-section of CNCs/PDA/PLA nonwoven membrane after 1 and 40 applications is compared in Figure 8. The surface of PLA fiber in the membrane used for 1 time is relatively smooth without observed defects. However, after 40 times of use, it can be found that there are irregular cavities with diameters ranging from several nanometers to hundreds of nanometers in the enlarged SEM image (Figure 8(b’)). This corresponds well to the decrease in mechanical properties. Nevertheless, the decrease of mechanical strength is as low as 15% after 50 cycles. Even after 90 times of repeated use, the mechanical strength of CNCs/PDA/PLA nonwoven membrane is still higher than that of pure PLA membrane, showing excellent durability.