Polymers exhibit a wealth of useful properties that make these materials uniquely applicable for a broad range of applications in all areas of technology and society. If commodity polymers were capable of self-repair upon fracture, many new applications could be sought out, enabling long-lasting properties, new functions, and substantial savings. This project seeks to understand the observed self-healing behavior in a type of commodity polymers (acrylate-based copolymers) which, in a certain intermediate compositional range undergo remarkable autonomous repair upon mechanical damage. If this unexplained observation is understood at a molecular level, it may offer high scientific and technological payoffs. This project will also explore the design of new copolymers with self-healing properties, which would reduce their consumption and have positive ecological impacts. This new fundamental knowledge may be used to explore other commodity polymers and at the same time train students in an interdisciplinary manner and prepare them to tackle global technological challenges.
This project aims to elucidate molecular origins of a surprising self-healing in commodity poly(methyl methacrylate/n-butyl acrylate), p(MMA/nBA), copolymers. This will be pursued by precisely synthesizing alternating, random, and block copolymers in the 45/55 - 55/45 MMA/nBA molar compositional range, correlating with the extent of self-healing, and identifying molecular events that govern this unique behavior. For that purpose, narrow dispersity copolymers with variable block sizes will be synthesized using atom transfer radical polymerization (ATRP) and heterogeneous radical polymerization (HRP). These experiments will allow to determine the role of the neighboring MMA and nBA copolymer repeating units on conformational changes during the damage-repair cycle and subsequent self-repair. The role of free radicals which can be potentially generated during mechanical damage as a function of copolymer molecular weight and copolymer topologies, and particularly their role in rebonding, will also be investigated. Since chain cleavage and/or slippage are expected to be copolymer-composition dependent, to identify molecular events responsible for self-healing, in-situ surface/interfacial chemical imaging (IR/Raman), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopy will be utilized. If intrinsic free radicals are generated during mechanical damage, they will act as dynamic nuclear polarization (DNP) agents to selectively enhance the magic-angle spinning (MAS) NMR signals of nuclei at the damage site. These measurements will be conducted as a function of copolymer topology, molecular weight, and dispersity, and will provide information about specific atomic-level conformation of the polymer chains, concentration levels of free radicals, and the nature of chemical groups at the site of damage. The planned experimental studies will parallel molecular dynamics simulations which will examine the role of van der Waals and other interactions on copolymer topologies and self-healing behavior.
