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Characterization of the in situ modified SiO2 in NR latex
Figure 3 shows the TEM images of the NR latex and in situ modified SiO2 generated in the NR latex. The NR particles in the NR latex had a diameter of about 1000 nm, while the in situ modified SiO2 by OTES and HDTMS generated in NR latex had an average particle size in the range of 720–770 nm with the network of SiO2 as a shell (yellow arrow) covered on the NR particles as a core. The hydrophobicity and steric effect of modified SiO2 networks cause the NR particle to shrink in size compared to the neat NR particle. The morphology of the in situ modified SiO2 generated in NR latex obtained in this study was similar to that in a previous work36.
Figure 3Representative TEM images of the (a) NR latex and (b, c) in situ modified SiO2 generated in NR latex with (b) OTES or (c) HDTMS.
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To confirm the successfully prepared in situ modified SiO2 generated in NR latex, 29Si solid-state NMR analysis was performed, with the results summarized in Fig. 4. The unmodified SiO2 spectrum showed peaks at − 109.81 ppm and − 100.70 ppm, which were attributed to Q4 and Q3, respectively. After modification, the intensities of the Q4 and Q3 peaks were significantly decreased and new signals appeared in the terms of the Tn group at − 64.54 and − 56.49 (Fig. 4b) ppm for the OTES-modified SiO2 and at − 64.38 and − 54.46 (Fig. 4c) ppm for the HDTMS-modified SiO2, which corresponded to T3 and T2, respectively. These clearly confirm that the condensation reaction between the alkyl silanes and the silanol groups on the SiO2 particles had occurred37,38 (Fig. 4f). It is interesting to note that after generating in situ modified SiO2 in NR latex, the T3 and T2 peaks disappeared and the intensity of the Q4 and Q3 peaks were significantly decreased. This may be due to the interference from the NR covering the SiO2 networks.
Figure 4Representative 29Si solid-state NMR of the (a) unmodified SiO2, (b) MSi-O, (c) MSi-H, (d) NR/MSi-O, and (e) NR/MSi-H; and (f) the proposed mechanism of modified SiO2 via sol–gel reaction.
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In addition, the FTIR spectra of the neat NR, in situ SiO2 generated in the NR latex, and the in situ modified SiO2 by OTES and HDTMS generated in the NR latex are shown in Fig. 5. The neat NR exhibited the asymmetric and symmetric stretching of C–H groups at 2960–2849 cm−1, C=C at 1639 cm−1, –CH2– symmetric stretching and –CH3 asymmetric stretching at 1445 cm−1, asymmetric –CH3 bending at 1375 cm−1, and C–H out-of-plane bending at 832 cm−1 of the cis-1,4-polyisoprene39,40,41. In the presence of the in situ formed SiO2 in the NR latex, new absorption peaks were observed at 1054, 798, and 447 cm−1 corresponding to the asymmetric stretching, symmetric stretching, and bending vibration of the Si–O–Si, respectively42,43. The band at 965 cm−1 was due to the stretching vibration of silanol (Si–OH) groups42,43. After SiO2 modification by OTES or HDTMS, the peak intensity for the absorption peaks of Si–O–Si and Si–OH was decreased compared to the unmodified ones. These results support that the in situ modified SiO2 particles had the –OH groups replaced by the long chain alkyl groups of OTES or HDTMS.
Figure 5Representative FTIR spectra of (a) neat NR, (b) in situ unmodified SiO2 generated in the NR latex, and (c) OTES- and (d) HDTMS-modified SiO2 generated in the NR latex.
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The static WCA of the SiO2 NPs before and after modification was also examined in order to confirm the sol–gel reaction of OTES and HDTMS. The results revealed that the static WCA of the SiO2 NPs before and after modification was significantly changed as follows: 118.3° (SiO2: Si), 134.8° (MSi-O) and 147.2° (MSi-H). Additionally, the NR/Si (in situ unmodified SiO2 in NR latex) CSSM showed the hydrophilic mesh with a static WCA of 54.3° compared to the NR/MSi-O (129.1°) and NR/MSi-H (134.0°) CSSMs [see in supplementary information (SI)]. These results confirmed that the in situ SiO2 was successfully modified on the surface of the NR latex by the long chain alkyl groups of OTES or HDTMS via a sol–gel reaction.
The SEM images (Fig. 6) revealed the surface morphology of the SSM and the CSSMs after coating by PDMS and the in situ modified SiO2 generated in the NR latex. The surface of the pristine SSM was smooth and the pristine SSM was woven by a single layer of metal wires with an average diameter of 60 μm to form a square pore of ~ 106 μm. After coating, the average pore size of the NR/MSi-O and NR/MSi-H CSSMs tended to be decreased (Fig. 6b,c). The PDMS-CSSM (Fig. 6d) showed aggregates or agglomerates of aerosol SiO2 particles from the vapor deposition of PDMS35. The CSSM surface became rougher and the pore size of PDMS-CSSM was slightly decreased compared to the uncoated SSM. After coating with the in situ modified SiO2 generated in the NR latex, the NR latex covered the PDMS (Fig. 6e,f). In addition, both the aerosol SiO2 particles from PDMS and the in situ modified SiO2 were fused into large aggregates or agglomerates.
Figure 6Representative SEM images of the (a) pristine SSM, and the (b) NR/MSi-O, (c) NR/MSi-H, (d) PDMS, (e) PDMS/NR/MSi-O, and (f) PDMS/NR/MSi-H CSSMs.
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Comparison of the static WCA in the absence and presence of PDMS is shown in Fig. 7. In the absence of PDMS, the pristine (uncoated) SSM had a static WCA of 123.8°, demonstrating its hydrophobic nature. After coating with the in situ modified SiO2 generated in the NR latex, the static WCA of the CSSMs were increased to 129.1° for NR/MSi-O and 134.0° for NR/MSi-H. In the presence of PDMS, the static WCA of hydrophilic PDMS-CSSM became 0°due to the aerosol SiO2 particles that were formed after vapor deposition at 500 oC35. It is interesting to note that the hydrophobicity of PDMS/NR/MSi-O and PDMS/NR/MSi-H CSSMs significantly increased with a static WCA of 138.0° and 139. 7°, respectively. This means that the enhanced surface roughness induced by PDMS could increase the adhesion between the aerosol SiO2 particles from PDMS and the in situ modified SiO2, resulting in the increased hydrophobicity of the CSSMs. Accordingly, hydrophobic CSSMs (PDMS/NR/MSi-O and PDMS/NR/MSi-H) were successfully prepared by coating with PDMS and in situ modified SiO2 generated in the NR latex. Thus, not only the surface roughness of SSM but also the hydrophobic modified SiO2 NPs were the important factors to increase/decrease the hydrophobicity of CSSM.
Figure 7The static WCA of the (a) pristine SSM, and the (b) NR/MSi-O, (c) NR/MSi-H, (d) PDMS, (e) PDMS/NR/MSi-O, (f) and PDMS/NR/MSi-H CSSMs.
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Mechanical durability is an important required trait for the practical application of any hydrophobic CSSM. Therefore, a sandpaper-abrasion test was used in this study to test the robustness of the prepared SSM and CSSMs. The morphology of the SSM and CSSM samples after four cycles of the sandpaper-abrasion test was observed from the SEM images (Fig. 8a-d), where the surface was clearly damaged along the scratched direction in all cases (yellow dashed frames in Fig. 8). However, the weight loss was too low to reliably estimate. Accordingly, the relationship between the static WCA and the sandpaper-abrasion cycles was examined. The static WCA values of the CSSMs without PDMS were clearly decreased from the first cycle sandpaper-abrasion testing (129.1°–104.5° for NR/MSi-O and 134.0°–118.0° for NR/MSi-H), revealing a reduced hydrophobicity. The static WCA values for the CSSMs with PDMS were higher than those without, where the static WCA values of PDMS/NR/MSi-O and PDMS/NR/MSi-H seemed to remain almost constant after four cycles of the sandpaper-abrasion test (Fig. 8e). This result confirmed that PDMS acted as a binder to enhance the hydrophobicity of the CSSM and improve its mechanical durability. Therefore, the PDMS/NR/MSi-O and PDMS/NR/MSi-H CSSMs were selected for oil/seawater and chloroform/seawater separation to evaluate their potential for practical application.
Figure 8(a–d) Representative SEM images showing the surface morphologies after the fourth sandpaper-abrasion cycle of (a) NR/MSi-O, (b) NR/MSi-H, (c) PDMS/NR/MSi-O, and (d) PDMS/NR/MSi-H, (e) The static WCA of the hydrophobic CSSMs after the sandpaper-abrasion test.
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The separation of chloroform or crude oil from seawater was examined using the two hydrophobic CSSMs (PDMS/NR/MSi-O and PDMS/NR/MSi-H). The criteria for choosing the non-aqueous (oil) phase for separation was based upon density and immiscibility compared to seawater, where the density of chloroform (1.489 g/cm3) and crude oil (0.817 g/cm3) is higher and lower, respectively, than that of seawater (1.017 g/cm3). Therefore, the chloroform and crude oil easily phase separated below and above, respectively, the seawater phase. The experimental setup for the separation process is shown schematically in Fig. 9a,b. The test hydrophobic CSSM was held between two glass tubes and the separation device was placed vertically for chloroform (red solution)/seawater, while it was tilted (45° from the vertical) for separating the crude oil/seawater (blue solution). The respective mixture was poured onto the test CSSM and the chloroform or crude oil permeated through the CSSM under gravity, while the seawater was blocked on the mesh. The permeate flux of the hydrophobic CSSM in each condition is shown in Fig. 9c, which revealed that the permeate fluxes in the case of chloroform/seawater were higher than that of the crude oil/seawater mixture, due to their different viscosities. The viscosity of crude oil (99.6 cP)44 is higher than that of chloroform (0.514 cP)45, leading to the lower permeate flux and higher separation time. Moreover, the permeate flux with the PDMS/NR/MSi-H CSSM was significantly higher than that with the PDMS/NR/MSi-O due to their different hydrophobicity.
Figure 9(a, b) Schematic diagram showing the separation process of (a) chloroform/seawater and (b) crude oil/seawater. (c) The permeate fluxes through the CSSMs of chloroform/seawater and crude oil/seawater.
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The separation efficiency of the two hydrophobic CSSMs (PDMS/NR/MSi-O and PDMS/NR/MSi-H) is shown in Fig. 10a,b for 20 separation cycles. The separation efficiencies were up to 81.9% (PDMS/NR/MSi-O) and 99.3% (PDMS/NR/MSi-H) for the chloroform/seawater mixture. More importantly, after 13 cycles of separation, the separation efficiency was gradually reduced by about 10 and 20% after the 14th and 15th cycle, respectively, and significantly reduced to ~ 40% after the 16th cycle for PDMS/NR/MSi-O. Compared with the crude oil/seawater mixture, the separation efficiencies of both PDMS/NR/MSi-O and PDMS/NR/MSi-H were comparable at approximately 95–96% after 20 separation cycles.
Figure 10The (a, b) separation efficiencies of (a) chloroform/seawater and (b) crude oil/seawater, and (c–f) representative SEM images showing the surface morphologies after 20 separation cycles of (c, d) chloroform/seawater and (e, f) crude oil/seawater through (c, e) PDMS/NR/MSi-O and (d, f) PDMS/NR/MSi-H.
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The surface morphology of the two hydrophobic CSSMs (PDMS/NR/MSi-O and PDMS/NR/MSi-H) after the 20th separation cycle was examined from the SEM images (Fig. 10c–f). In the chloroform/seawater mixture, the NR was dissolved in the chloroform (yellow dashed frames in Fig. 10) making the CSSM surface easily damaged during the separation process. In contrast, the surface was only swelled and not damaged with the crude oil/seawater mixture. In conclusion, this material can be applied as a filter material to separate a crude oil/seawater mixture.
Finally, comparison of the WCA, separation efficiency, and reusability of several materials is summarized in Table 1. Although, the WCA of the optimal hydrophobic CSSM (PDMS/NR/MSi-H) in the present work was lower than that of others, the PDMS/NR/MSi-H CSSM exhibited a high separation efficiency and reusability. It is important to note that the separation efficiency of each material depends on several factors, such as the hydrophobicity of material or the oil properties.
Table 1 Comparison of oil/water separation efficiencies of various materials.Full size table
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