Title: Synthesis of Chemically-Reactive Polymeric Multilayer Coating *
2.3. Results and Discussions
2.3.1. Synthesis and characterizations of reactive and covalent multilayer
the desired underwater superoleophobicity, and this material was further exploited in the demonstration of guided ‘No-loss’ transport of tiny oil-droplet, which was not possible to achieve with the bare glass tube.
superhydrophobic monolith. By taking advantage of the same facile 1,4-conjugate addition chemistry (Fig.
2.1A) , I have developed a thick and porous polymeric multilayer through layer-by-layer (LBL) deposition
of BPEI and ‘reactive’ NC as shown in Fig. 2.1E. Further, this polymeric multilayer of NC having residual acrylate group was post-modified with hydrophilic amine-containing small molecules (Fig. 2.2A) for achieving underwater superoleophobicity (Fig. 2.2B). At first, a composition of BPEI/5Acl (Fig. 2.1B) in
ethanol was optimized to obtain a stable dispersion of NC solution (Fig. 2.1D). However, the size of the NC was further grown during the course of LBL deposition process. Both the formation of NC and the
Figure 2.3: (A) Graphical depiction comparing the growth of NC with (Black) and without (red) LBL deposition process.
Inset images depicting the stable dispersion of NC solutions with (right-side vial) and without (left-side vial) LbL deposition process. (B) FTIR spectra for reactive NC before (black) and after (red) treatment with glucamine (denoted as
‘Glu-treated’).
Figure 2.2: (A) Schematic representing the post-chemical modification of the multilayer of NC with glucamine, a hydrophilic amine. (B) Schematic illustration of underwater superoleophobic multilayer after modification of multilayer of NC with glucamine.
growth of the NC were investigated by visual inspection and a DLS study as shown in Fig. 2.3A. A faint turbid solution of the NC with the size of ~151 nm rapidly transformed to a highly opaque and milky
solution (size ~459 nm) in a span of ~3 min during the LBL deposition process, and the size of the NC was observed to grow further with time without any sedimentation even after 21 min (size ~690 nm) (Fig.
2.3A: black curve). The same composition of BPEI/5Acl, which was not used in successive LbL deposition process, was also capable of forming NC, however, the growth of the NC was slow as in Fig.
2.3A: red curve. The accelerated growth of the NC in the BPEI/5Acl mixture during LbL deposition was likely due to the gradual and the additional transfer of the BPEI solution to the amine-‘reactive’ NC solution (mixture of BPEI/5Acl) through consecutive dipping of the substrates in the respective dipping solutions (Fig. 2.1E). Then, this dispersion of the NCs was thoroughly washed with ethanol prior to examination by FTIR spectroscopy. Two characteristic IR signatures at 1739 cm-1 and 1409 cm-1 (Fig.
2.3B; black curve) were noticed due to the carbonyl stretching and the symmetric deformation of the C–
H bond for the β-carbon of the vinyl groups, respectively. This IR study revealed the presence of residual acrylate groups in the polymeric NCs. Furthermore, the depletion of the characteristic peak intensity at 1409 cm-1 (Fig. 2.3B; red curve) for the residual acrylate moieties upon treatment with primary amine- containing small molecules (i.e. glucamine) unambiguously suggested the existence of residual chemical
Figure 2.4: The plot accounting the thickness of both the multilayer of NC (black line) and multilayer of BPEI (red line) with increasing the number of respective bilayers depositions.
reactivity (Fig. 2.3B) in the synthesized NC. The chemically reactive residual acrylate group in the polymeric NC provided an opportunity to develop a covalently cross-linked multilayer of NC in combination with the multiple amine-containing BPEI polymer through consecutive 1,4-Michael addition reactions (Fig. 2.1A). During LBL deposition process, no sedimentation of the NC solution was observed, rather a rapid growth of multilayer of NC was observed at ambient conditions. The thickness of the
multilayer increased exponentially with increasing the deposition cycles (Fig. 2.4; black curve). The thickness of the multilayer (20 bilayers) of NC was measured to be 2.2 µm (Fig. 2.4; black curve). In comparison to that, the LbL growth for the multilayer of BEPI, which was constructed by replacing the NC solution mixture with 5Acl solution, was observed to be significantly sluggish. The thickness of the multilayer was only 174.09 nm, even after 20 cycles of LbL depositions (Fig. 2.4; red curve), where at each cycle, each bilayer was referred to the sequential deposition of BPEI and 5Acl. This observation supported that the deposition of branched polymeric amine (BPEI) through covalent cross-linking with small molecules (5Acl) was much slower than the multilayer of NC. Furthermore, the topography of both the multilayer of NC and multilayer of BPEI was examined by FESEM imaging (Fig. 2.5A-H). After
Figure 2.5: (A-C, E-G) FESEM images of the reactive multilayer of NC in low magnification (A-C; scale bar = 10 μm) and high magnification (E-G; scale bar = 3 μm), after depositions of 10 bilayers (A, E), 15 bilayers (B,F) and 20 bilayers (C,G), where each bilayer consisted of sequential deposition of BPEI and NC. D,H) The FESEM images of multilayer of BPEI in both low (D; scale bar = 10 μm) and high (H; scale bar = 3 μm) magnifications, after 20 bilayers deposition of BPEI/5Acl.
deposition of 20 bilayers of NC, an arbitrary hierarchical and porous topography was observed under FESEM (Fig. 2.5C,G). Whereas, the topography of the multilayer of BPEI was noticed to be featureless and smooth (Fig. 2.5D,H), even after repeating deposition cycles for 20 times. The gradual change in the morphology in the multilayer of NC was exploited later in revealing the role of surface topography behind the underwater oil-wettability property.