Extreme Ductility in Freestanding Glassy Polymer Thin Films

In this dissertation the mechanical and thermodynamic properties of microtomed thin films of glassy polymers is investigated using unique techniques and methods developed for this purpose. The novel methodology developed led to the discovery of unexpected mechanical properties in glassy polymer thin films prepared via microtomy, primarily the extreme ductility in nominally brittle materials: polystyrene (PS) and poly(methyl methacrylate) (PMMA). To investigate the origins of this phenomenon, the technique was developed further to provide a means to probe temperature dependent properties of the films, with the specific interest in measuring the glass transition temperature (Tg). Temperature-controlled dynamic mechanical analysis (DMA) experiments were carried out, revealing Tg depression never before measured in glassy polymer thin films with thickness above 100 nm./nThe experimental methods are detailed in Chapter 1, beginning with the introduction of the micro-electromechanical system (MEMS) device, known as a push-to-pull (PTP) device. The PTP device was pivotal in the research herein but first required adaptations before it could be used to characterize polymeric materials, as its original design was intended for hard materials. The development of novel sample preparation techniques was also required to successfully utilize the PTP device for characterization of polymer thin films. These developments, as well as the challenges and limitations of the technique, are also outlined. This includes the topic of ion and electron beam irradiation effects, which are considerably more problematic and require additional precautions when compared to most hard materials, such as metals./nResults from PTP device testing of microtomed thin films of two polymer glasses, PS and PMMA, are presented and discussed in the proceeding chapters with a primary focus on the extreme ductility observed in both materials. These results include quantitative tensile testing, in situ and postmortem optical microscopy, postmortem transmission electron microscopy (TEM), and temperature-controlled DMA testing. Aside from the extreme ductility, analysis of the stress-strain relationship revealed a significant reduction of the elastic modulus and a thickness-dependent trend with respect to the strain softening amplitude. The quality of these results was verified by in situ optical microscopy, which also allowed for larger strains to be reached. Postmortem TEM investigations provided detailed imaging of the deformation microstructure, which showed a dependence on the thermal history of the film./nAnalysis of these results, in concert with current theory in polymer physics, are discussed and a hypothesis to describe the underlying mechanisms at work is proposed. In short, microtomy produces films with fractured surfaces containing a high concentration of chain ends and a distribution of reduced molecular weight (Mw). These attributes are believed to enhance segmental mobility of chains not only at the surface, but also those deeper within the film. Direct evidence and theoretical support for these claims are presented and discussed in this dissertation/nFinally, the potential applications of these findings as well as future research opportunities enabled by the development of the novel PTP device technique described herein are examined.

Advisor: Minor, Andrew M