Crack Propagation in Self-Healing Polymer Gels with High Toughness

Northwestern University, Evanston IL

Investigators

Abstract

Completion of this project will result in the development of gels with exceptional mechanical toughness that also have a self-healing capability. Application areas for these types of materials are quite diverse and range from artificial cartilage to protection systems that rely on the ability of a material to dissipate large amounts of energy under repeated loading conditions. The general design strategy for the synthesis of tough polymer gels is based on recent work with 'double network' gels consisting of a relatively high-modulus primary network, and an independent secondary network with a much lower modulus. In the proposed work the secondary network is based on a self-assembling triblock copolymer gel, and the primary network originates either from additional ionic crosslinking of the secondary network, or from the development of a co-continuous silicate network or array of silica nanoparticles. An additional outcome of the work is the refinement of the nonlinear elastic fracture mechanics analysis needed to provide a general understanding of crack propagation in highly deformable solids. The self-healing capability of the gels originates primarily from the use of non-covalent bonds in the formation of the primary and secondary networks. Additional healing mechanisms are available in the silicate systems because of the formation of covalent bonds that are able to undergo reversible hydrolysis and condensation reactions. Development and understanding of these hybrid silicate/organic gels will impact a broad range of fields in materials science. In order to accomplish these goals the PI and co-PI propose a coherent plan that involves the synthesis of the primary and secondary networks and the use of finite element methods to understand the stress fields in the vicinity of a growing crack. Connections will also be made to analytic models that are more generally accessible to the broader scientific and technical communities interested in the fracture toughness of highly deformable materials. Non-Technical Summary Many naturally-occurring materials have mechanical properties that are highly optimized and have not yet been duplicated in synthetic materials. One of the most intriguing properties of some of these naturally-occurring materials is their 'self-healing' capability that enables them to recover their mechanical integrity after they have been damaged. Developing strategies for achieving the balance of properties possessed by these natural materials is one of the aims of this project. The focus is on relatively 'soft' materials, similar to those that make up the soft tissues of plants and animals. Cartilage is an excellent example of a desired combination of materials properties. Cartilage lubricates the joints by presenting a low friction interface between bone surfaces, even when the bones are pressed against one another with very high forces. The design strategy utilized in this project will enable this combination of properties to be obtained in a synthetic material. The work is a collaborative effort involving mechanical modeling, mechanical testing, and the synthesis and processing of new materials. Educational outreach activities are planned with local museums in the Chicago area, beginning with the Chicago Botanic Garden and Shedd Aquarium. These institutions bring expertise in reaching the public at the broadest and most general level. These collaborations will enable the principles being applied in the proposed work to be understood at a qualitative level by people with a natural curiosity about their natural environment.