Effects of Energetic and Entropic Disparities on the Properties of Elastomers

Cornell University, Ithaca NY

Investigators

Abstract

TECHNICAL SUMMARY The goal of the proposed work is to provide a detailed and basic understanding on how entropic and energetic effects can be harnessed to manipulate elastic behavior of end-linked polymer networks. From a design perspective, energetic effects are readily tunable by introducing chemical disparity in selected components, while entropic inter-chain interactions can be strongly influenced by changing the molecular weight distribution and the extent of entanglements in the system. Accordingly, model networks will be synthesized under different conditions and characterized to address three questions: 1) To what extent can one enhance the toughness of networks that exhibit entropy-driven order-disorder microphase segregation by coupling such segregation with energetic forces arising from a chemical disparity between chains and crosslinks? 2) What is the effect of the entropic disparity brought about by chain length multimodality on network structure and mechanical properties? 3) What is the interplay between chain interspersion and entanglements (both entropic chain interactions) on rubber elasticity and toughness? - A judicious choice of cross-linkers in poly(diethylsiloxane) elastomers can result in energetic attraction between the cross-links and lead to increased toughness in these materials. Synthetic and characterization efforts along with coarse-grained computer models will be carried out for this purpose. Secondly, deuterium NMR measurements on samples where different components of multimodal elastomers are labeled will be used along with computer simulations to provide an understanding of the increased toughness of multimodal structures. Finally, experiments and simulations will be carried out to investigate the different roles of trapped entanglements and chain interspersion on the mechanical properties of elastomers. NON-TECHNICAL SUMMARY Elastomers are elastic materials such as those encountered in soft contact lenses, tires, rubber bands, certain biomedical devices, O-rings, etc. This research focuses on how the properties of the polymer molecules that make up these elastomers affect their end-use and what can be done to improve these materials or develop new ones. Of particular interest would be the synthesis of an elastomer that mimics the mechanical behavior of natural substances, like the muscle protein titin, that exhibit the ability to absorb large amounts of deforming energy without breaking (e.g., for applications entailing soft materials under severe stresses). Students, both graduate and undergraduate, will be directly involved in this research that will provide them with educational and research training in the general area of polymer science and engineering. The Principal Investigators will participate in educational outreach activities such as Materials Workshops for upstate New York high school teachers and the Curie Academy for high school girls.