Dr. Padmanabhan's research group works on a bottom-up approach to synthesizing materials with novel structure and novel properties. The workhorse model for self-assembly is the block copolymer amphiphile that contains at least two segments that repel each other, leading to a variety of microstructures -- each with unique properties. In particular, we are interested in obtaining ordered network structures from amphiphilic molecules containing at least two segments that repel each other, leading to a complex, network arrangement of molecules. While all porous networks are mechanically stable and have three- dimensional transport due to their high porosity, we target a special class of networks that have superior optical properties and can bend light in non-traditional ways for use in solar cells. Potential applications for the future are synthesizing negative refractive index materials and invisibility cloaks. But such networks are elusive due to their narrow range of stability in the phase diagram. We work on developing theoretical models to study these regions in the phase diagram and devise design strategies to expand the region of stability with a view toward informing experiments.
By adding a third block to the copolymer, networks can further be tuned by breaking the symmetry. Two such networks are embedded within a continuous matrix and under specific conditions, the networks can be made of completely different chemistries. The alternating gyroid is one such example of a morphology completely stable and formed by a triblock copolymer. We propose and show that addition of a selective homopolymer (shown in orange) can selectively expand one network and shrink the other network. Indeed, the alternating diamond and alternating plumber's nightmare can also form.
In addition to thermodynamic stability of ordered network structures, Dr. Padmanabhan has expertise in studying the dynamic assembly of ordered networks, and stability under flow, of bicontinuous colloidal gels under gravity. Colloidal gels have been heralded as novel self-healing materials due to the temporal nature of bonds, but their mechanical response under gravity has been poorly understood, yet has important implications in the shelf-life of these materials. By utilizing large- scale simulation to model their assembly and flow, we studied the micromechanical origins of gel collapse and connected the macroscopic response of the gel to its bond evolution and phase behavior.
Self-assembly of complex phases
Self-assembly offers a scalable manufacturing process to create materials with complex structure. Apart from polymers and colloids, we explore other materials such as liquid crystals and lipids, and apply targeted design strategies for their use in technological applications.
Our design approach is based on thermodynamics at the nano-scale: the interplay of enthalpic interactions and entropic considerations that determine the final structure. We use computational approaches such as coarse-graining to study a variety of length- scales.