Influence of Cyclic Loading/Unloading on Segmental Dynamics of Polymer Glasses

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Non-Technical Summary: Polymer glasses (materials used in applications like automobile headlamp covers and modern composite aircraft) are important in technology because they are stiff, lightweight, and require little energy for manufacture. The response of polymer glasses to external forces (stretching vs. breaking) determines what materials can be used for particular applications but this response currently cannot be predicted. Experiments will be used to measure the rate at which polymer molecular motions occur when a force is applied to the polymer glass; in particular, experiments in which the glass is periodically stressed and unstressed will be utilized. Through comparison with models and computer calculations, these experiments will lead to the development of more reliable predictions of the mechanical properties of polymer glasses. Potentially this will have a significant economic impact by allowing broader utilization of these lightweight materials. Personnel supported by this project will work with the University of Wisconsin-Madison's PEOPLE program to prepare high-school students from under-represented groups for college. Two high-school science teachers will participate in the research and use their experience to guide high-school students in independent research projects. These teachers will also develop modules for use in Wisconsin public high schools that target concepts which are challenging for high-school students. Technical Summary: Polymer glasses are important in technology due to their low density, the low energy input required for molding, and their useful mechanical properties. This research aims to enhance the fundamental understanding of polymer glass deformation and to test current theoretical approaches and computer simulations; this should lead to the development of more reliable models for the prediction of the mechanical properties of polymer glasses. Segmental mobility in poly(methyl methacrylate) and poly(lactic acid) glasses will be measured before, during, and after mechanical deformation. Segmental mobility will be measured with an optical photobleaching method that provides access to the rotational correlation function of an ensemble of dilute probe molecules. Enhanced mobility is a key feature of the nonlinear response of polymer glasses to deformation. These experiments will utilize cyclic tensile loading/unloading deformations and more complex deformation protocols. These experiments will discriminate among theoretical models that all qualitatively explain deformation at constant strain rate. In addition, this work will advance the training of graduate students and high-school students, through the integration of materials research and education activities. Personnel supported by this project will work with the University of Wisconsin-Madison's PEOPLE program to prepare high-school students from under-represented groups for college. Two high-school science teachers will participate in the research and use their experience to guide high-school students in independent research projects. These teachers will also develop modules for use in Wisconsin public high schools that target concepts which are challenging for high-school students.