STUDY OF SELF-ASSEMBLY AND STIMULI RESPONSIVE BEHAVIOR IN POLYMERIC AND DENDRITIC AMPHIPHILES

Natural processes are intricately detailed and able to convert molecular-level events into macroscopic or visually observable properties. This is made possible through multiple interactions at the molecular level, signaling due to covalent and non-covalent interactions, and supramolecular networks that rely on dynamic, non-equilibrium structures. Detailing processes in this manner is the current quest for material science research and designing materials for this purpose is usually via a key process known as self-assembly. Self-assembly is a process in which a material with varied components organizes itself into a particular pattern due to various specific inter and intra molecular interactions. By understanding this process, scientists have developed methods to predict the patterns formed (morphologies) depending on the molecular construct of the material, and impart a broad range of responsive behavior to these self-assemblies, for several different applications. The goal of this thesis was to investigate the different parameters and stimuli that affect the morphology of these assemblies, either transforming them, perturbing them or, destroying them completely, and the outcome of this event. Polymeric and dendritic amphiphiles were chosen as scaffolds for this purpose of building stimuli responsive assemblies, as they provide handles for innovative modifications and engineering of various stimuli-sensitive groups. Both offer unique advantages, polymers offering low critical aggregation concentrations and ease of synthesis, and dendrimers having uniform dispersity and exact synthetic reproducibility. Amphiphilic homo-polymers were designed to respond to an environmental pH change, and degrade, expelling the non-covalently held contents. However, a temporal degradation of its backbone, resulted in a change of its hydrophilic-lipophilic balance (HLB) and hence, its self-assembled morphology. Light-sensitive amphiphilic block co-polymers, with an ability to form equilibrium assemblies when dispersed in aqueous media were also explored. The reversible conformational change brought about by the light-actuation, in one azobenzene molecule placed at the interface of the hydrophilic and hydrophobic blocks, was able to transduce motions throughout the polymer chains, resulting in a change in the material’s permeability. This system’s ability to function far-from-equilibrium, i.e. work only in the presence of an energy input and rest in a dormant state in its absence, was also explored. This thesis also investigated the response of this polymer to stimuli such as pH and proteins that brought about an irreversible covalent modification to the system, as opposed to the light stimulus. Signaling through non-covalent based interactions, akin to nature, causing release of non-covalent guest contents was also explored using dendritic and block co-polymer amphiphiles. In the case of the dendrimer, the looming challenge of bacterial targeting was addressed by harnessing a membrane protein-ligand interaction. Block co-polymers were used to understand the effect of non-covalent binding on the fidelity of an assembly with a high degree of chain-entanglements and a densely hydrophobic core

Temporal and Triggered Evolution of Host–Guest Characteristics in Amphiphilic Polymer Assemblies

An amphiphilic polymer with cleavable side chain and main chain functional groups has been designed and synthesized. Specific cleavage of either of its functional groups was found to have an effect on the morphology of the assembly. Degradation of the main chain is shown to cause morphology of the supramolecular assembly to evolve with time from a micelle-like assembly to a vesicular assembly. On the other hand, stimulus-induced cleavage of the side chains causes these nanoassemblies to disassemble. These temporal (main chain) and triggered (side chain) degradation processes have implications in the design of degradable polymers as supramolecular scaffolds for biological applications