MechE PhD Dissertation Defense: Le Li


ABSTRACT: The properties of materials change in interesting ways when they are structured at the mesoscopic limit between macroscopic and molecular length scales. The mesoscopic properties of polymers in particular present a rich set of behavior because they potentially involve contributions that depend on numerous length scales such as the monomer size, persistence length of the polymer chain, the radius of gyration of polymer chains, and the average distance separating crosslinks between different polymer chains. It is critical to understand and leverage these size-dependent phenomena in order to realize the potential of polymers in nanotechnology applications spanning electronics, biomedicine, and soft robotics.

Here, we explore a combined experimental and computational method to investigate the mechanical properties of nanoconfined polymers. At a high level this process entails seeking regions where experiments disagree with continuum models such as finite element analysis (FEA) as these identify regions where mesoscopic structuring is potentially affecting material properties. For instance, we perform nanoindentation experiments of thin films that are understood to differ from bulk experiments due to the presence of a rigid support. Thus, we compute a correction factor to account for the influence of the substrate effect using finite element analysis (FEA), and provide a path for using nanoindentation to extract the modulus of both relatively stiff glassy polymer films and soft elastomer films. Comprehensive studies of glassy polymer films found that they consistently became more compliant when confined to films thinner than 100 nm, in agreement with molecular dynamic results. However, elastomer films were observed to stiffen when thinner than 1 μm. We propose a surface crosslinking model in which the films have a crosslink density that varies with depth because of surface oxidation and find this model to be in agreement with the observed increase in modulus of elastomers.

In addition to fundamental studies of polymer film mechanics, we explore the use of polymer thin films as photoactuators. In particular, composites of polydimethylsiloxane (PDMS) and carbon nanotubes (CNT) are known to physically deform when illuminated, however the mechanism of this effect is the subject of debate. We prepared thin films of PDMS-CNT composites and systematically studied their outof- plane motion when illuminated. By constructing a FEA-based model that accurately predicts the observed actuation dynamics and magnitude based upon a photothermal mechanism, we confirm that photothermal actuation is the cause of composite motion. These results have led to both an understanding of photoactuation of PDMS-CNT thin films and an ability to optimize thin film actuators for nanolithography applications. These results have also led us to propose and test novel structured actuators for enhanced actuation efficiency. Having defined a combined experimental and computational approach for studying the mechanics of nanoconfined polymers and their photoactuation behavior, this work has provided a more general understanding of polymers and how nanoscale confinement can be leveraged to improve our utilization of soft materials.

COMMITTEE: Professor Keith Brown, Advisor, ME/MSE/Physics; Professor Paul Barbone, Chair, ME/MSE; Professor Kamil Ekinci, ME/MSE; Professor Chuanhua Duan, ME/MSE; Professor Ophelia K. C. Tsui, Physics/MSE

Wednesday, Jul 24, 2019 at 10:00am until 12:00pm on Wednesday, Jul 24, 2019
Where 110 Cummington Mall, Room 245
Boston University