Title: Synthesis of Stretchable and Durable Underwater Superoleophobic Membrane for Filtration-based Oil/Water Separation *
3.3. Results and Discussions
3.3.1. Synthesis of stretchable underwater oil-repellent interface
The synthesis of functional polymeric nanomaterials has been recognized as an important avenue for tailoring various physical/chemical properties.38–44 In the Chapter 2, a chemically reactive
Figure 3.1: (A) Representing the catalyst-free facile 1,4-conjugate addition reaction between acrylate group and primary amine group. (B) The DLS study accounting the growth of the nanocomplex. (C) FTIR spectral analysis of chemically reactive multilayer coating before (black curve) and after (red curve) glucamine modification. (D-E) FESEM images of both the uncoated (C) and coated (D) fibrous substrate (scale bar: 4 µm).
nanocomplex (NC) that prepared using 1,4-conjugate addition reaction (Fig. 3.1A), was deposited following layer-by-layer (LbL) coating process for achieving bio-inspired durable liquid wettability.
In this Chapter 3, the ‘reactive’ NC was strategically deposited on the stretchable fibrous substrate through LbL deposition of BPEI polymer and NC (for 20 bilayers) for fabricating highly stretchable and robust ‘fish-scale’-mimicked membrane. As expected, the size of the polymeric NC gradually increased during the consecutive LbL deposition process (each bilayer consisted of two layers, one of NC and the other one of BPEI), which was confirmed by dynamic light scattering (DLS) study, as shown in Fig. 3.1B. After deposition of 20 bilayers, the available chemical functionalities present in the covalently cross-linked multilayer were investigated by a standard Fourier transform infrared (FTIR) spectroscopy study.44–46 The appearance of IR peaks at 1412 cm-1 and 1735 cm-1, which were attributed to the C–H deformation of the β-carbon of the vinyl groups and carbonyl stretching, respectively (Fig. 3.1C; black curve), revealed the presence of residual acrylate groups in
the polymeric coating. Further, these residual acrylate moieties were found to be highly reactive with primary amine-containing small molecules. It was observed that the IR peak intensity at 1412 cm-1, decreased significantly after post-modification of the same multilayer coating with hydrophilic small molecules (i.e., glucamine) via 1,4-conjugate addition reaction (Fig. 3.1C; red curve). Further, the existence of micro/nano featured topography that consisted of randomly aggregated granular polymeric domains on the smooth and native fibrous substrate was confirmed using FESEM imaging, as shown in Fig. 3.1D-E. The LbL deposition of granular NC on the fibrous substrate rendered the essential topography that conferred the ‘fish-scale’- mimicked wettability after the chemical modulation of the reactive multilayer-coated substrate (Fig. 3.2B) with glucamine. As a result, the coated fibrous substrate that instantly soaked the beaded oil droplet (red colour aided visual inspection)
Figure 3.2: (A-B) The scheme illustrating the post-covalent modification of ‘reactive’ polymeric multilayer coating on fibrous substrate with glucamine. (C-D,E-F) Digital images (C,E) and CA images (D,F) of beaded oil droplets (under water) on the ‘reactive’ multilayer coated fibrous substrate before (C-D) and after (E-F) covalent modification of with glucamine.
under water (Fig. 3.2C-D) became an extremely oil-repellent interface (Fig. 3.2E-F) with advancing OCA of 163.7º (Fig. 3.2F) after the post-modification of the same ‘reactive’ interface with glucamine (Fig. 3.2E). In the past, the elastomeric property of the selected fibrous substrate was exploited earlier for developing various functional materials.47 In recent past, polyurethane (PU) based fibrous/porous substrates, decorated with ‘lotus leaf’-inspired wettability, displayed a highly selective affinity towards oil/oily phases, and such materials were explored as oil-absorbent materials.48–50 However, in this chapter, the PU-based substrate was extended in the synthesis of fish-scale-mimicked underwater extreme oil-repellent membrane with impeccable durability. The multilayer-coated stretchable fibrous
substrate, which was post-modified with glucamine, was gradually deformed under applied tensile stress. The coated substrate was stretched up to 150% tensile strain without compromising the embedded anti-oil-fouling property under water, as shown in Fig. 3.3A,D-E. Even after such high
Figure 3.3: (A) Graphical representation of the change in underwater oil wettability on the biomimicked stretchable interface, where the tensile deformation was gradually increased from 0 to 200% under water. (B-G) Advancing (B,D,F) and receding (C,E,G) CA images of beaded (under water) oil droplet on the underwater superoleophobic interface after incurring tensile deformations with applied strains of 50% (B-C), 150% (D-E) and 200% (F-G), respectively.
tensile deformation, the coated substrate was capable of repelling the beaded oil droplet with advancing OCA above 160º and OCA hysteresis well below 10º. With further increase in the tensile strain from 150% to 175% of the as-synthesized material, a slight change in underwater OCA hysteresis (just above 10º; but the beaded oil droplet still repelled extremely with advancing OCA above 160º) was noticed, as shown in Fig. 3.3A. Upon further deforming the substrate with 200% tensile strain, the coated fibrous substrate became more adhesive with OCA hysteresis of 16 as shown in Fig. 3.3F-G.
Nevertheless, the stretched interfaces again restored their non-adhesive super-oil-repellence under water with an OCA hysteresis below 10º after releasing the tensile strain. As the bio-mimicked interface was capable of withstanding up to 150% of tensile deformation, so the same interface was manually and repetitively deformed with 150% tensile strain, and the underwater advancing OCA was examined on the stretchable interface after every 100 times of tensile deformations as shown in Fig.
3.4. The synthesized underwater superoleophobic coating continued to display non-adhesive extreme oil-repellence under water with advancing OCA of well above 160º and an OCA hysteresis of below 10º, even after deforming the interfaces with 150% tensile strain for 1000 times, as observed in Fig.
3.4.