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Connie Roth

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Stress during Thermal Cooling on Physical Aging of Polymer Glasses

How stress imparts mobility to glasses is an ongoing subject of debate in soft matter.  Many experiments and simulations have shown that stress or strain applied to polymers, colloids, granular materials, etc. in the glassy state leads to enhanced mobility.  In some cases, such deformation can even appear to erase past physical aging as if "rejuvenating" the glass.  How deformation imparts mobility to glasses is a process often described by an potential energy landscape 'tilting' mechanism, where the applied stress is thought to reduce energy barriers, allowing the system to transition to a higher energy state. 
 

Stability of Polymer Glasses Vitrified Under Stress

      Laura A.G. Gray and C.B. Roth, Soft Matter 2014, 10, 1572-1578. [Link]

We have investigated for the first time the impact of applying stress during vitrification, i.e., formation of a glass during thermal cooling, on the subsequent physical aging.  We find that the subsequent stability of the glassy system is affected by the stress applied on cooling, even after the stress has been removed.  The data show an initial plateau in aging rate at low stresses that quickly transitions to a much higher aging rate at high stresses.  Above a minimum threshold in stress, the aging rate appears to plateau at a higher value indicative of a less stable glass.  Following the potential energy landscape picture used to describe nonequilibrium glasses, we suggest that a glassy system formed under high stress is left trapped in a higher, shallower energy minimum with a corresponding faster physical aging rate. 
 

Physical Aging of Polymer Films Quenched and Measured Free-Standing via Ellipsometry: Controlling Stress Imparted by Thermal Expansion Mismatch between Film and Support

      Justin E. Pye and Connie B. Roth, Macromolecules 2013, 46, 9455-9463. [Link]

We have also demonstrated that similar stress effects on cooling can lead to changes in physical aging rates even when the stress imparted arises inherently from how the film is supported.  We have calculated the stress imparted to free-standing polystyrene films on cooling when supported on rigid frames of different materials.  This stress is independent of film thickness, but depends on the thermal expansion mismatch between the film and rigid frame.  We demonstrate that the physical aging rate is independent of film thickness, but correlates with the difference in thermal expansion between the polystyrene and frame material, and hence the stress imparted to the film on cooling.