This chapter includes the overview of the current thesis work thoroughly explained in the earlier chapters followed by the future work plan related to this study in various possible directions. The current thesis work comprises a detailed explanation of facile fabrication of two different amine-reactive polymeric multilayer coatings on different flexible and rigid substrates by strategically depositing a branched polymeric amine and reactive-nanocomplex (NC) (of BPEI/5Acl) following LbL deposition. A catalyst- free 1,4-conjugate addition reaction at ambient conditions was adopted for covalent cross-linking of the multilayer coating and its post covalent modification with desired and selected small molecules. The reactive multilayer coating consisting of 20 bilayers displayed a durable and bulk extreme anti-oil wettability, i.e. underwater superoleophobicity, after post-modification of the multilayer coating with selected hydrophilic small molecule—glucamine. This synthetic procedure provides an opportunity to tailor such underwater extreme oil repellency on various substrates irrespective of their physical state or compositions with impeccable durability. The LbL deposition process allowed to develop a substrate- independent coating of chemically reactive multilayer coating that remained capable of displaying extreme oil-repellency property under water. Next, I further extend this work by developing a durable and stretchable underwater superoleophobic membrane by constructing the multilayer coating on a stretchable fibrous substrate. The underwater anti-oil wetting property of the membrane remained unaltered after incurring various physical abrasions like, bending, twisting, winding, adhesive tape test, etc., exposure to various chemically harsh environments (like, pH (1, 12), artificial sea water, river water, etc.) and even after incurring 150% tensile strain repetitively for 1000 times. This membrane was highly capable of separating various oil-water mixtures, including bulk oil, sedimented oil and emulsified oil following gravity driven filtration-based separation process—even under diverse chemically complex conditions, and the efficiency of the oil-water separations was observed to be remained greater than 99 wt%. However, this multilayer coating displayed hydrophobicity after post-modification with hydrophobic octadecylamine (ODA) which has a long hydrocarbon tail that constitutes of 18-carbon atoms. Hence, it was assumed that due to the lack of essential topography, this multilayer coating was incapable of displaying superhydrophobicity even after low surface energy modification. Therefore, another reactive multilayer coating was developed in presence of NaCl salt, where the doping of appropriate amount of salt significantly accelerated the rate of the reaction which led to rapid growth of NC, resulting in formation of a highly porous and durable multilayer coating even after depositing less than half (9 bilayers) of the bilayers, in comparison to the other multilayer (20 bilayers) coating. This multilayer was displaying
superhydrophobicity after successful post-modification with same ODA. Besides, this single multilayer was designed with various wettability properties including, superhydrophobicity, underwater superoleophobicity and also other adhesive wettabilities both in air and underwater by changing the post- chemical modification of the residual acrylate moeity of the coatings with various primary amine- containing small molecules (having different hydrocarbon chain length) through 1,4-conjugate addition reaction. These as-synthesized multilayer coatings can withstand various physical insults, including adhesive tape test, sand drop test, extremes of temperatures (10°C, 100°C), etc. and exposure to various chemically harsh environments (like, pH (1, 12), artificial sea water, river water, etc.) as well as UV radiation. Moreover, the salt-doped ODA-treated multilayer coating was showing an extreme oil- absorption property under water due to the presence of continuous trapped air phase confined in the micro/nano grooves of the multilayer. Interestingly, the hydrophobic multilayer having discontinuous trapped air layer (consisting of 20 bilayers) was also exhibiting the underwater superoleophilicity property similar to the superhydrophobic multilayer. In that context, both these moderately hydrophobic and superhydrophobic multilayer coatings were thoroughly investigated to compare the performance of the embedded underwater superoleophilicity at harsh settings. The detailed studies validated that the hydrophobic multilayer coating having discontinuous trapped air layer was with superior stability than that of the superhydrophobic multilayer coating having continuous trapped airin stabilizing underwater superoleophilicity. The remarkable stability of underwater superoleophilicity of the hydrophobic multilayer led me to extend this work further to develop super-oil-absorbent by coating fibrous cotton substrate. This super-oil-absorbent was demonstrated to be highly efficient (separation efficiency of > 95 wt%) in separating various kind of oil/water mixtures, including floating oil, heavy oil and oil-in-water emulsion following both the selective absorption and the filtration processes, whereas the superhydrophobic multilayer-coated cotton failed to separate oil-in-water emulsion. This study conspicuously suggests that this hydrophobic super-oil-absorbent has more potential than superhyrophobic one as far as oil/water separation in practical scenarios are concerned.
Realizing the importance of this current research work, it can be clearly stated that this work can be further extended in various practically relevant biological and other environmental aspects. For instance, underwater superoleophobicity and superhydrophobicity properties had been explored for applications like anti-platelet adhesion, prevention of biofilm formation, etc. The chemically reactive multilayer coatings developed and explored for this thesis work is believed to have great potential for biologically important applications like blood-compatible materials because wettability plays a crucial role in protein
absorption, platelet activation/adhesion, and blood coagulation. In the future, these multilayer coating can be exploited for controlled prevention and promotion of platelet adhesion on medically relavent substrates.