Deformation-induced changes in segmental dynamics of polymer glasses prepared by melt stretching and stress aging

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NON-TECHNICAL SUMMARY:Polymer glasses are important materials due to their high transparency, the low energy input required for molding them into particular shapes, their stiffness, and their resistance to breakage. Polymer glasses are used in applications as diverse as advanced aircraft composites and eyeglass lenses. The planned research will investigate how polymer glasses respond when they are subjected to large forces. In particular, the experiments will use lasers and microscopes to measure how the polymer molecules rearrange when a polymer glass is deformed. Current theories and computer simulations have identified this molecular motion as a key quantity that must be understood to determine if a polymer glass will stretch, deform, or break. The polymer glasses to be used will be processed in special ways (stretched at high temperature or deformed at low temperature) to critically test these simulations and theories. The proposed experiments should lead to more reliable models for the prediction of the mechanical properties of polymer glasses. This in turn may have a significant economic impact because of the broad potential of these lightweight materials. The students involved in this project will learn how to characterize polymer glasses and improve the properties of materials. All personnel supported by this grant will participate in a program to increase the number of highly qualified students from underrepresented groups that enter Ph.D. programs in the U.S.TECHNICAL SUMMARY:The deformation properties of polymer glasses are extremely challenging to predict as a result of the nonlinear nature of the deformation and the nonequilibrium nature of the glass. Here it is proposed to measure segmental mobility in polymer glasses during deformation using an optical photobleaching method that provides access to the rotational correlation function of an ensemble of dilute probe molecules. Deformation can increase segmental mobility in polymer glasses by more than a factor of 1000, and an understanding of this enhanced mobility is a key requirement for accurate modeling of the mechanical properties of polymer glasses. For the proposed work, polymer glasses will be prepared by melt quenching so that subsequent mobility measurements during deformation can disentangle the influences of yield and strain hardening on segmental mobility. The proposed work will also test whether deformed polymer glasses can show indications of overaging, i.e., physical aging that occurs at an accelerated rate as a result of applied stress. Finally, the generality of the results obtained in tensile deformation will be tested by performing optical measurements of segmental mobility during shear deformation. The proposed experiments can provide a molecular-level test of theoretical descriptions of polymer glass deformation. The experimental results obtained, in comparison with theory and simulation, should lead to an enhanced fundamental understanding of the deformation of polymer glasses. All personnel supported by this grant will participate in a program to increase the number of highly qualified students from underrepresented groups that enter Ph.D. programs in the U.S..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.