EAGER: Exploring the Molecular Foundation for the Mechanics of Polymer Glasses

University Of Akron, Akron OH

Investigators

Abstract

NON-TECHNICAL SUMMARY: The project aims to address fundamental questions concerning the mechanical properties of common plastics (polymers) that have become a most important class of materials in modern society. Many billions of pounds of commercial products are manufactured annually from glassy polymers such as polystyrene, poly(methyl methacrylate), and polycarbonates. However, the mechanical strength of these glassy plastic materials has yet to be understood in terms of molecular parameters (i.e., through the behavior of their individual polymer molecules). For example, there is a distinct lack of knowledge about why materials such as polystyrene and poly(methyl methacrylate) are so brittle while some polycarbonates are not and can be highly deformed without breaking. This research aims to develop a molecular level understanding of such important mechanical behaviors as brittle fracture versus ductile deformation. Through an integrated approach combining experiments, theory, and molecular dynamics simulations, a molecular model will be subjected to tests to determine how it may or may not provide the framework on which to develop a coherent and comprehensive knowledge base for the mechanics of glassy polymers. If it is successful, it will provide a foundation that will not only enable improved mechanical performance of existing polymeric materials but also guide molecular design of next-generation polymeric materials, thus potentially bringing about significant economic benefits to society. This project will also contribute to the interdisciplinary education and training of graduate students and potentially be incorporated in courses and textbooks for undergraduate and graduate students as well. TECHNICAL SUMMARY: This research will explore answers to important questions on the mechanical behavior of polymeric materials such as why polymer glasses can be ductile and what determines the ultimate mechanical strength of amorphous polymer solids. The research program builds on and extends a newly proposed molecular model to describe large deformation behavior of polymeric glasses including yielding and fracture. The proposed work integrates both experiment and computer simulation to test the molecular model by identifying the essential ingredients in the theoretical description. Specifically, the following sub-projects will be pursued to improve current knowledge and understanding of glassy polymer mechanics: (A) Polymer glasses of low molecular weight (MW) and glassy polymer mixtures containing a low-MW component will be subjected to uniaxial compression to determine the possibility for ductile behavior and large scale plastic deformation. (B) Compression tests will be carried out below room temperature to find out whether there is also a brittle-ductile transition (BDT) in compression and to address the questions of why glassy polystyrene is ductile in compression at room temperature but brittle in tensile extension. (C) The BDT will be studied as a function of the extensional rate to explore how yielding may be a time-dependent activation process. (D) Molecular dynamics simulations will be conducted to explore the existence of load-bearing strands (LBS) in a chain network during external deformation and characterize the predicted emergence of activated phases surrounding the LBS for comparison with the physical picture contained in the molecular model.