Exploring Polymer-Metal Ion Inspired Aqueous Interfaces using Molecular Dynamic Simulation and Nonlinear Laser Vibrational Spectroscopy

Abstract

The incorporation of metal ions with polymer structures develops unique properties for material functionalities, such as polymer electrolytes and polyelectrolytes, and is ubiquitously involved in electrochemical, biomedical, and rheological applications. The polymer-metal ion interaction is crucial for the performance of material functionalities in the application across the various junctions of interfaces. However, there is often a lack of surface understanding regarding the interfacial behavior and molecular conformation that facilitate their application. This gap needs to be addressed distinctly to correlate the fundamental attributes of the involved processes. We have worked with water-soluble polymer and mono-, di-, tri-valent ions, aiming to acknowledge the adaptability, structural flexibility, amphiphilicity, and site-specificity exhibited by polymer molecules. The current thesis work delves into the significance of interfaces in deciphering molecular-scale phenomena. This includes exploring structural and ion conductive properties, perturbations in surface tensiometry, surface adsorption propensity, molecular kinetics, to understand binding interactions at the interface, surface density, molecular orientation, fluid interface and the role of the surrounding environment in modulating molecular structure. Several research works done in analyzing the involved fundamental intermolecular force of interactions have been performed using FTIR, electrochemical impedance spectroscopy, molecular dynamic simulation. However, they expose bulk-involved information and lacks in effectively providing interface-inspired interactions in the aqueous surfaces. This information is crucial for utilizing polymer metal ion coordination or complexation to develop fine-tuned functional surfaces/interfaces. This could potentially be identified through an extremely interface sensitive methodology known as sum frequency generation vibrational spectroscopy. It has the capability to offer insights into highly aligned molecular systems that are both IR- and Raman-active. Simultaneously, it allows exploration of information related to structural composition, molecular interaction, kinetics, orientation, and the impact on the neighboring environment in a single experimental configuration at the interface. We additionally utilized computational approach to predict the motion of polymer, ions and its structural evidence due to the efficient ability of MD simulation. Therefore, this study has the potential to offer a new perspective on the field of polymer-metal ion interactions, an area that remains relatively unexplored within the SFG community. We have also extended our studies on tensiometry, zeta-potential, ATR-FTIR spectroscopy, and rheometer to gather information related to the bulk-features. We have characterized the phosphazene-based polymer in LiCl salt, various monovalent and divalent salts, as well as the polyacrylamide-based polymer in trivalent salts at the interface. Based on our research findings, we conclude that the presence of polymer-metal ion interaction at the interface has a profound impact on the organization of water molecules at the aqueous interface. Through the molecular recognition of interface molecules, we realized that both polymer electrolytes and the Weissenberg effect are the result of surface intermolecular interactions and hold significance in various surface applications. Additionally, it includes, the interplay of different intermolecular forces such as electrostatic, van der Waals, and hydrogen bonding play an important contribution in the fine-tuning of interfaces. Our work could offer significant contribution towards the rapidly growing domains of soft functional materials, electrochemical devices, micro-fluidic device fabrications and rheology control domain. This creates a new frontier for research in interfacial science, inviting further exploration through a range of experimental approaches.