CAREER: Stimuli-Responsive Self-Assembly of Supramolecular Block Copolymers: Hierarchical Structures and Kinetic Pathways

Mississippi State University, Mississippi State MS

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports theoretical and computational research integrated with education to investigate how complex polymers assemble and contribute to developing design principles for complex polymer materials. Self-assembly is a process in which molecules or atoms organize as building blocks into ordered structures because of interactions among the components. This process occurs in the assembly of molecules or monomers into molecular building blocks that can themselves assemble in sequences to form long chain-like molecules known as polymers. Polymers are literally everywhere – they make up plants, plastics, the food we eat, and even our own bodies. Through increasingly precise control over the monomer sequences and molecular architectures, polymers have been transformed from large-scale commodity plastics into precision self-assembly building blocks that can undergo pre-programmed organization into materials with more complex structures. It is likely that further expanding the scope of polymers over a broad spectrum of possible bonding among monomers, new self-assembly processes can emerge, which will bring about functions that are unrealizable with conventional covalent polymers. The focus of this project is to study the process of self-assembly of the so-called supramolecular block copolymers -- a class of the “hybrid bonding” polymers made of structural units of covalent polymers connected into well-defined architectures through temperature-responsive reversible bonds. The PI aims to understand the fundamental connection between the reversibility of bonds at the molecular level and the temperature-responsive feature in the self-assembly process, both at equilibrium and in dynamical processes. By using computer simulations and quantifying the influence of the reversible bonds on the self-assembly process, this project is aimed to identify new pathways for making materials and for materials design. A goal is to formulate rules to incorporate predictable structural and kinetic properties into polymeric materials. As part of the educational component of the project, the PI aims to integrate classroom learning, research projects, and community service by developing a Community-Engaged Learning course “Introduction to the Mesoscale Simulations of Polymers for Engineering Applications" for senior undergraduates and graduate students. This effort will be in collaboration with the local water facility to test/develop wastewater treatments based on recyclable polymers. The goal of the Community-Engaged Learning course is three-fold: (1) to promote community awareness in training of the next-generation work force; (2) to cultivate and foster lasting partnership between the university and the city of Starkville; and (3) to develop technologies that are potentially beneficial to the local community. TECHNICAL SUMMARY This award supports theoretical and computational research integrated with education to investigate self-assembly of supramolecular block copolymers and contribute to developing strategies for the design of new polymeric materials. It is of central importance to expand the scope of polymeric materials over a broad spectrum of possible bonding energies among monomers, for not only creating novel structures but most importantly access to previously unknown functions. Supramolecular block copolymers represent a class of such hybrid bonding polymers in which structural units of covalent polymers are connected into well-defined architectures via supramolecular bonds. Assisted by the reversible supramolecular interactions, it is envisaged that supramolecular block copolymer self-assemblies may exhibit more diverse morphologies, stimuli-responsivity and dramatically reduced annealing times/temperatures comparing to their covalent analogues. In terms of the fundamental aspects, it is however unclear through what mechanisms these envisaged functions and properties will be brought about. It becomes particularly relevant for self-assemblies under nonequilibrium conditions, where the dynamic nature of supramolecular block copolymers may lead to dramatically different self-assembly pathways. A lack of understanding of these fundamental aspects had restricted efforts of experiment to develop hybrid bonding polymers with predictable structural and kinetic properties to case-by-case attempts within narrow parameter ranges. This project aims to address these unmet needs from a computational perspective, by revealing the link between reversibility of the supramolecular interactions and the structural and kinetic behaviors of supramolecular block copolymers self-assemblies. Specifically, this project will consider the supramolecular comb-coil diblock copolymers made of covalent diblock copolymer backbone and low-molar-mass oligomer additives end-attached to the backbone via supramolecular bonds. The self-assembly of supramolecular comb-coil deblock copolymers is known to produce thermo-responsive hierarchical structures with built-in functionality. The focus of this project is to understand the effects of the strong yet reversible supramolecular bonds on the thermodynamic stability of microphases, the kinetics of structure formation during processing, and the kinetic pathways for stimuli-responsive structure transitions. To provide the base for the kinetics studies, the equilibrium morphologies of the self-assembly of supramolecular comb-coil deblock copolymers will first be determined. The kinetics of the structure ordering process during the thermal annealing may then be investigated and kinetic pathways of the thermo-responsive order-order transitions will be determined. The minimum free energy path will be identified using particle-based coarse-grained computer simulations; it will also enable the influence of supramolecular bonds on the free-energy landscape of the self-assembly to be quantified. The predicted behaviors of the self-assembly based on the minimum free energy path will be verified in experiments as part of a collaboration with experiment groups. Results from this project will enable strategies for the design of new polymeric materials with adaptable structural and kinetic properties. This project is jointly funded by the Division of Materials Research through the Condensed Matter and Materials Theory program, and the Established Program to Stimulate Competitive Research (EPSCoR). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.