The Nanomechanical and Viscoelastic Responses of Ultrathin Polymer Films

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NON-TECHNICAL SUMMARY:The technological infrastructure that provides science and engineering solutions to the rapidly growing nanotechnology area is of considerable national interest. The present work addresses fundamentals of the nanoconfinement behavior of materials that form the basis of the relevant enabling technologies. One important set of problems addressed relates to the engineering properties (such as stiffness and yield strength) of nanometer-thick films that are in freely standing form and, consequently, cannot be readily measured. The only method available for making such measurements in such materials is a bubble-inflation method developed in the PI's laboratory that allows testing of extremely small quantities of material, especially nanometer-thin polymer films. The work investigates the engineering properties of freely standing polymer films deep in the glassy state with particular emphasis on yield behavior. These studies will be the first to provide film thickness and temperature dependence of yield in freely standing films. Also, in the freely standing films, a large enhancement in the material stiffness is observed and, recently, conflicting theoretical models of the stiffening behavior have appeared to explain the phenomenon. Such predictions are, of course important to nanomaterial design and use, and the present work will establish the range of validity of these theories. Molecular architecture effects will also be investigated. Finally, the nanobubble inflation experiment permits investigations of novel materials that were previously unachievable due to their extremely small quantities. In this case, the investigators will study ultrastable polymer glasses made by physical vapor deposition (PVD) and that can be made more stable than even a 20 million year old amber glass. This high stability allows the interrogation of a long-standing question whose resolution is fundamental to theories of glasses and, in particular, how to make long-term predictions of their behavior in applications to important areas such as advanced composites and adhesives. TECHNICAL SUMMARY:The behavior of ultrathin polymer films remains an intense area of investigation, but most studies have been limited to the case of substrate-supported films even though studies suggest much larger effects occur in the freely standing state. The present work tests three aspects of freely standing ultrathin films using the TTU bubble inflation method and takes advantage of the method's capability of making viscoelastic measurements on extremely small quantities of material to study the response of an ultrastable polymer glass made by physical vapor deposition (PVD). One important set of problems addressed relates to the engineering properties, such as modulus and yield strength of nanometer thick films that are in freely standing form and, consequently, not readily measured. The only method available for making such measurements in such materials is a bubble inflation method that allows testing of extremely small quantities of material, especially ultrathin or nano-metric polymer films. The work investigates the engineering properties of freely standing polymer films deep in the glassy state with particular emphasis on yield behavior. Also, in freely standing films, a large modulus enhancement is observed and, recently, conflicting theoretical models of the stiffening behavior have appeared to explain the phenomenon. Such predictions are, of course important to nanomaterial design and use and the present work will establish the range of validity of these theories. Branched polymers have been shown to exhibit different nanoscale behavior from linear counterparts upon confinement on a supporting layer and the TTU bubble inflation method will be used to examine the effects of branching and unentangled polymer chain length on the viscoelastic properties of freely standing ultrathin films. Finally, it remains controversial whether or not the dynamics (relaxation time or viscosity) in glass-forming liquids, including polymers, diverge at a finite temperature. The PI's group has now demonstrated the first PVD ultrastable polymer glass that can be used to determine the upper bound relaxation times in a fashion similar to prior work with a 20 million year old amber but over a larger "window" of temperatures because the PVD polymer has a fictive temperature at least 50 K below the glass transition temperature, and optimization of the PVD conditions offers the possibility of an even larger testing window. Should the experiment be successful, it will provide further experimental data that can challenge theories of the behavior of glass-forming systems.