Quantifying the Polymer Physics of Mechanical Deformation in Ultra-thin Polymer Glasses

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NON-TECHNICAL SUMMARY Numerous technologies rely upon the properties and performance of thin polymer films. For example, membranes used in clean water applications are comprised of thin polymer layers through which water is pushed to separate and remove unwanted contaminants. Although currently useful, this technology remains energetically unfavorable due to engineering limits with regard to the mechanical strength of polymer thin films. Beyond membranes, the efficiency and lifetime of many other applications are also limited by the mechanical properties of polymer thin films; however, direct knowledge of these properties in thin films is extremely limited. Changes in polymer materials when they are processed into films comprised of only a few molecules in thickness are well known, but how these changes impact mechanical strength remains unclear. The proposed project will overcome current challenges with directly measuring mechanical properties in ultra-thin polymers, leading to new fundamental data and knowledge that can help guide the design and synthesis of new materials for better membranes, alternative power sources, as well as many other technologies. The proposed research will provide a strong foundation for the education and training of multiple graduate students and undergraduate researchers. To extend the impact of this project to the K-12 education level, new materials science and engineering curriculum materials will be developed to be integrated into a workshop, called BioInspire!. This program is designed to teach lessons of materials science and engineering and biology in the context of bioinspired innovation, where science, engineering, and art can be used to engage a broad, diverse group of students.TECHNICAL SUMMARYThis project will develop fundamental knowledge of the polymer physics associated with the mechanical properties in ultra-thin films of polymer glasses. Although changes in the mobility of glassy polymer chains in a surface-dominated regime has been studied extensively, the mechanical properties in ultrathin polymer films have only been studied to a limited extent. Here, a new measurement device, called the Ultrathin Film Tensile tester (UFT), will be used to measure the complete stress-strain relationship for polymer thin films under uniaxial extension conditions. The proposed measurements will provide a complete understanding of mechanical responses in ultra-thin polymer films by quantifying elastic and dissipative processes through monotonic and cyclic loading histories across a wide strain and temperature range, as well as controlled fracture experiments in the dimensionally-constrained regime. Three materials systems, including polystyrene (PS), polycarbonate (PC), and blends of polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide) (PS/PPO), will be studied, allowing molecular structure and mechanical responses to be tuned systematically. The findings are expected to provide new insight with regards to how polymer mobility changes near a surface can alter the constitutive response of a polymer, how changes in entanglement density due to dimensional geometric constraints alters fracture mechanisms, and how geometric constraints on polymer molecules may alter the onset of dissipative or plastic deformations. The findings from this project will have a significant impact on fundamental polymer physics, as well as on the development of new characterization methods that can aid the development of advanced materials for thin film applications.