Abstract
Janus is the Roman god with two faces. Scientists use the name ‘Janus’, referring to a system of materials with opposite properties. Janus membranes are designed to have an asymmetric architecture such that the two components with incompatible chemistry function as one system. They have gained immense attention for their unique functionalities such as a uni-directional liquid or air penetration due to their anisotropic architecture. They have the potential for many applications such as waterproof breathable membranes, interface stabilizers, biological sensors, and drug delivery systems. There is yet another class of materials, which change molecular conformation in response to change in environmental conditions such as temperature. The behavior has made responsive material more prevalent in many applications such as drug delivery, cell culture, tissue engineering, actuators, sensors, etc. Independent research on the responsive materials and Janus constructs shows a high potential for a smart and self-sustaining system that responds to the environmental cues without an external source of energy. However, there has been no attempts in combining the responsive materials in the Janus system, which utilizes and enhances the advantages of each system. This is attempted for the first time in the thesis. This work fabricates a Janus membrane via electrospinning to bring about a multimodal system with thermoresponsive and directional wettability properties. The first part of the study builds a thermoresponsive membrane by optimizing the proportion of blending poly (vinylidene fluoride) (PVDF) in poly (N-isopropylacrylamide) (PNIPAM). The blend membrane is superhydrophilic below 32 °C and highly hydrophobic above 32 °C. Therefore, this membrane is not only as thermoresponsive as PNIPAM but also highly stable in water, making it a suitable candidate for water-based practical applications. The Janus membranes are investigated for directional liquid and vapor transmission. Liquid transmission through the Janus membrane is studied for ‘water diode’ and oil-water separation. The thickness of the individual layers within the membranes that influenced the wettability gradient are variable parameters for ‘water diode’. Our results show unidirectional water transmission at room temperature for a water droplet (5 µl) from the hydrophobic side. However, at elevated temperature, the thermal energy triggers a reversible change in the membrane, preventing the water transmission from both sides of the membrane. Directional water droplet flow is due to the hydrophilic thermoresponsive membrane that switches its wettability with the change in temperature of the system. At room temperature, the anisotropic property of the Janus membrane facilitates a pressure gradient for unidirectional liquid transport. At elevated temperatures, the anisotropic property reduces due to the hydrophobicity of the thermoresponsive membrane, which ceases the liquid transport. In the case of oil-water separation experiments, the hydrophilic blend layer separated the water from the oil in water emulsion. However, the Janus construct enables the membrane to have a dual performance of separating water from oil in water emulsion and oil from water in oil emulsion. Additionally, the Janus construct improves the separation efficiency and flux in comparison to non-Janus membranes. Finally, the Janus membranes are investigated for directional vapor transport. Based on our results from both direct and indirect (ISO 11092) measurement of vapor transmission, Janus constructs with PVDF facing the skin favors the vapor transmission at all temperatures. The vapor transmission from the flipped side is statistically lower due to the multimodal response of the membrane. We envision this membrane will find application in the field of protective textiles, filtration, and biomedical such as microfluidics, cell sorting, etc.