Tailoring Size and Shape of beta-Sheet Nanocrystals for Crosslinking and Reinforcement of Elastomers

University Of Akron, Akron OH

Investigators

Abstract

NON-TECHNICAL SUMMARY. Elastomers are rubbery materials that are produced on the scale of 200 million tons annually worldwide. Their applications range from daily goods (for example tires) to defense (for example sonar domes of navy ships) and to biomedicine (for example coatings of artery stents). This NSF-supported research/educational team led by a chemist (Jia) and a physicist/chemical engineer (Foster) combines inspirations from biological systems and lessons from chemical and physical principles to develop the next generation of elastomers. A central focus is to reduce the size of hard particles that strengthen the elastomers to the nanometer scale. The research team will advance our fundamental understanding of how these reinforcing elements produce elastomers that are strong, stiff, and extensible and that have programmed capability to dissipate energy. This scientific knowledge can be used for a number of applications, e.g. tires that are safe, durable, and fuel-efficient. The elastomers developed can also potentially be directly applied to make medical devices safer. Parallel to the research effort, the program will train undergraduate and graduate students in this interdisciplinary area. The team will carry out outreach activities aimed at attracting domestic talent to careers in science and technology and particularly in polymer-related areas using aspects of elastomers as the primary content materials. TECHNICAL SUMMARY. The ability to manipulate atoms and molecules to form hierarchical structures with precisely controlled size and shape is central to nanoscience. Beta-sheet nanocrystals exist in both natural and synthetic elastomers and function as crosslinks and provide reinforcement. However, their morphologies are drastically different in these two circumstances. In natural elastomers (i.e., silks), they are particulates with all three dimensions smaller than 10 nm. In synthetic elastomers (e.g., polyurethanes), they have been found to be fibrous with the longest dimension, in the hydrogen-bonding direction, ranging from hundreds of nanometers to microns. In silks, the size control is attributed to specific amino acid sequences and an exquisite reeling process. Controlling the size and aspect ratio of beta-sheet nanocrystals is an unresolved challenge for synthetic systems. This research pursues this central quest of nanoscience in an important area of soft materials, elastomers, to realize material properties otherwise unattainable. The scientific approach of the research is multifaceted, involving synthesis, characterization, and mechanical studies across the molecular, supramolecular, and nanometer scales. Based on their recent success in reducing the longest dimension of n-beta-sheet nanocrystals in a series of oligo(beta-alanine)-grafted polyisobutylenes to well below 100 nm, the research team will expand their ability to regulate the size and aspect ratio of the beta-sheet nanocrystals without an elaborate aminoacid sequence and to elucidate the reinforcing characteristics of the two morphologically contrasting beta-sheet nanocrystals. The polymer brush at the interface of the nanocrystal will be another focus as it is critical for the morphological control and likely plays an important role in reinforcement as well. The specific objectives of the planned research are to: (1) synthesize monodisperse oligo(beta-alanine)-grafted polyisobutylenes that form particulate nanocrystals with still smaller aspect ratios as well as those that form fibrous nanocrystals. (2) characterize the structures and morphologies of the beta-sheet nanocrystals including the polymer brush attached to the nanocrystal surface. (3) elucidate the reinforcing characteristics and study the reinforcing mechanisms of the particulate and fibrous nanocrystals.