Exploring ultimate mechanical characteristics of polymers, from molecular to fracture mechanics

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NON-TECHNICAL SUMMARYMechanical failure of polymers in all forms (including the familiar plastics) due to fracture limits the range of applications and increases the costs of polymeric products. This project will attempt to clarify the nature of polymer fracture in order to identify means to increase the threshold for failure. This could lead to reduction of the annual consumption (several hundred billion pounds) of petroleum-based plastics toward the goal of building a more sustainable economy by lowering the pressure for landfills. The project will explore and show the benefits of elucidating a connection between the resistance to fracture and the material strength that can be improved through chemical and physical means. Specifically, fracture behavior of polymeric materials will be examined in a different than conventional way to better illustrate the cause for fracture. The contrast between existing knowledge and new perspectives would be incorporated in a textbook on all aspects of polymer mechanics that the PI plans to work on. The project will also help to fill the knowledge gap in this area by training a new generation of scientists and engineers who can generate new solutions to existing and future challenges facing the properties and applications of polymeric materials. TECHNICAL SUMMARYThe planned project will investigate the fracture behavior of polymeric materials through establishment of a stress criterion based on birefringence measurements. The overall objective is to identify more effective means to increase the toughness of polymers. This cannot be achieved without proper explanation of why toughness is a material parameter and what dictates its magnitude. The PI plans to explore an intimate connection between material mechanical strength and toughness. For polymers in all forms, i.e., plastic, elastomeric and thermosetting, their brittle fracture behavior will be investigated from an angle that focuses on the stress state at the crack tip. The investigations will take into account the following effects. Enhancement of chain network through pre-melt stretching increases ductility of glassy amorphous polymers. Preservation of chain network during crystallization avoids brittle fracture in glassy semicrystalline polymers (e.g., crystalline polyethylene terephthalate). Structural changes to increase load-bearing strands per unit area result in stronger elastomers. The proposed research will gather more evidence relevant to a scenario where stress state at the crack tip explicitly dictates polymer fracture instead of Griffith-Irwin type energy balance argument. The entire project will attempt to answer why toughness is a material constant and what factors determine its magnitude for polymers in their various forms. For improvement of ductility and increased toughness of polymers an effective strategy will be explored through enhancement of structure of load-bearing chain network to increase the intrinsic mechanical strength.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.