Title: A Single Multilayer Coating for Controlled Tailoring of Different Liquid Wettabilities *
4.3. Results and Discussions
4.3.1. Fabrication and characterization of the salt-assisted multilayer of NC
In the past, the 1,4-conjugate addition reaction was strategically exploited in the chemical modifications (dendritic amplification of functional groups or selective modification of polymeric microstructures)49,50 as well as the development of complex nanostructures (multilayer coatings, porous and moldable gels).51-53 In the Chapter 2, a chemically reactive multilayer (20 bilayers) coating of nanocomplex (NC) has been introduced following a covalent LbL deposition process, where the multilayer coating of NC display underwater superoleophobicity after appropriate chemical modification with selected hydrophilic small molecule—that is glucamine. However, such multilayer coating remained inappropriate to display superhydrophobicity, even after hydrophobic modification.
In this chapter, a single chemical avenue was designed by exploiting the 1,4-conjugate addition reaction (Fig. 4.1A) between acrylates of 5Acl and amine groups of BPEI (Fig. 4.1B) through LBL deposition (Fig. 4.1C) to develop a multilayer coating (Fig. 4.1D) that is capable of tailoring various special wettabilities including superhydrophobicity (Fig. 4.1E; in air) and superoleophobicity (Fig.
4.1F; under water). In that context, stable dispersions of NC were individually prepared by mixing BPEI/5Acl in methanol (Fig. 4.1B), both in the presence and absence of salt (0.5 mg ml-1). These dispersions of NC were later strategically used in the multilayer constructions (Fig. 4.1D) in association with the amine-containing branched polymer (BPEI). During the LbL deposition process, with increasing number of LbL deposition cycles, an expedited growth in the size of NC was observed in the presence of NaCl (0.5 mg mL-1), as confirmed by a DLS study as shown in Fig. 4.2A. Later, the
growths of multilayer coatings of NC that were prepared both in the presence and absence of salt, were monitored by measuring the thickness of the respective multilayer at regular intervals as shown in Fig.
4.2B. The growth of the multilayer of NC in the presence of salt was noticed to be exponential and yielded a thick (3.15 mm) multilayer coating after 9 bilayers (a pair of NC and BPEI layers are together denoted as a single bilayer) depositions. However, the growth of the multilayer of NC that were prepared in the absence of salt was noticed to be sluggish and the thickness of the coating (9 bilayers of NC/BPEI) was found to be only 830 nm. In comparison, the multilayer of the polymer that was directly constructed by sequential deposition of BPEI and 5Acl, provided a coating with 700 nm
Figure 4.1: (A) Schematic representation of the 1,4-conjugate addition reaction between primary amine and acrylate group. (B) Chemical structural of 5Acl (top panel) and BPEI (bottom panel). (C) Illustrating the formation of ‘reactive’
polymeric NC (in methanol) in presence of NaCl salt which was further extended for constructing a ‘reactive’
multilayer coating in combination with BPEI through covalent LbL deposition process. (D) Schematic representation of the salt-assisted multilayer of NC having residual acrylate groups. (E-F) Strategic post-chemical modifications of the multilayer of the NC with appropriate small molecules (ODA and glucamine) provided extremes of liquid wettabilities both in air (superhydrophobicity, (E)) and under water (superoleophobicity, (F)).
thickness after 80 bilayers (each bilayer has been referred as a pair of BPEI and 5Acl deposition) deposition.50 Thus, the strategic and rapid incorporation of the same components (BPEI and 5Acl) in the form of NC in the covalent multilayer through the 1,4-conjugate addition47–50 reaction allowed a high throughput synthesis of a thick and reactive polymeric coating. Next, the topography of the coatings was examined by field emission scanning electron microscope (FESEM) imaging. At low magnification, the multilayer coatings of NC that were prepared in the presence of salt, were found to have a prominent and random micro structure throughout the coating as shown in Fig. 4.3A-B.
Whereas, in the absence of salt, such dominated micro structures were not observed in the multilayer of NC, rather, comparatively smaller micro-domains appeared as evident from the images in Fig. 4.3D- E. These micro-domains in both of these multilayer coatings (that are prepared in presence and absence of salt) were developed due to random arrangements of granular NC structures as evident from the FESEM images at higher magnification in Fig. 4.3C,F. In contrast, the morphology of the multilayer of BPEI polymer that constructed in the presence of salt, was observed to be mostly featureless under FESEM as shown in Fig. 4.3G-I. As expected, the size of the granules is comparatively smaller in the
multilayer of NC that are prepared in the absence of salt as shown in Fig. 4.3F, which is consistent with the DLS study in Fig. 4.2A where the growth of NC was observed to be rapid in the presence of salt. However, the exact role of salt behind this expedited growth of NC is not understood yet. This accelerated growth of NC and the difference in the size of the granular structures would have eventually influenced the growth of the micro domains in the respective multilayer. Further, Fourier transform infra-red (FTIR) spectroscopy was used to investigate the residual chemical functionalities
Figure 4.2: (A) Illustrating the growth (size) of the NC in the presence (black line, 0.5 mg ml-1) and absence (red line) of salt (NaCl) in the LbL deposition process. (B) A comparison of the growth (thickness) of the multilayer of NC with LbL deposition cycles both in the presence (black line) and absence (red line) of salt in the NC solution (one of the dipping solutions in the LbL deposition process).
in the synthesized multilayer coatings (prepared in the presence of salt). The appearance of characteristic IR peaks at 1409 cm-1 and 1739 cm-1 for C-H deformation of vinyl group and carbonyl stretching, strongly suggested the existence of residual acrylate groups—which eventually made the multilayer coating chemically reactive as shown in Fig. 4.4 (black). A significant depletion in characteristic IR peak (ODA: blue curve and glucamine: red curve in Fig. 4.4) at 1409 cm-1 for the symmetric deformation of the C–H bond for the β carbon of the vinyl group with respect to another IR peak at 1739 cm-1 which corresponds to ester carbonyl stretching,49,50 was noticed in the multilayer of NC (in presence of salt) after successful post-chemical modifications of the multilayer coatings of NC with primary amine-containing small molecules (e.g., ODA and glucamine). This simple IR study
validating that the residual acrylate groups in the multilayer of NC allowed a successful and covalent post-modification with primary amine-containing small molecules (glucamine and ODA) through 1,4- conjugate addition reaction. Based on the past reports on the Michael addition reaction,47–50 this carbonyl stretching most likely appeared due to amino ester-type cross-linkages in the multilayer
Figure 4.3: (A-F) Field emission scanning electron microscope (FESEM) images of the multilayer (9 bilayers) of the NC/BPEI in low (A,D; scale bar = 20 μm), medium (B,E; scale bar: 15 μm) and high (C,F; scale bar: 500 nm) magnifications, which were constructed in presence (A-C) and absence (D-F) of salt. (G-I) FESEM images of multilayer coating of polymer (multilayer of 5Acl/BPEI prepared in presence of salt) in low (G, scale bar: 20 μm), medium (H; scale bar: 15 μm) and high (I, scale bar: 500 nm) magnifications.
through the repetitive 1,4-conjugate addition reaction between the amine and acrylate from BPEI and 5Acl, respectively. A very similar result was noticed with the multilayer of NC that are prepared in the absence of salt.