Doctor of Philosophy, University of Akron, 2020, Polymer Science

Understanding the mechanism of adhesion and friction in soft materials is critical to the fields of transportation (tires, wiper blades, seals etc.), prosthetics and soft robotics. Most surfaces are inherently rough and the interfacial area between two contacting bodies depends largely on the material properties and surface topography of the contacting bodies. Johnson, Kendall and Roberts (JKR) derived an equilibrium energy balance for the behavior of smooth elastic spherical bodies in adhesive contact that predicts a thermodynamic work of adhesion for two surfaces in contact. The JKR equation gives a reversible work of adhesion value during approach and retraction. However, viscoelastic dissipation, surface roughness and chemical bonding result in different work of adhesion values for approach and retraction. This discrepancy is termed adhesion hysteresis. Roughness is undermined as a cause of hysteresis in adhesion studies. Recently, a continuum mechanics model has been developed that predicts the work of adhesion on rough surfaces with known roughness in the form of power spectral density (PSD) function. To test the above mentioned theoretical model, we have conducted JKR experiments between highly cross-linked smooth polydimethylsiloxane (PDMS) of four different elastic moduli and diamond surfaces of four different crystal sizes and roughness.The rough diamond surfaces are characterized for topography using stylus profilometry, atomic force microscopy and in-situ transmission electron microscopy combined to give a comprehensive PSD. Results suggest that the observed work of adhesion during approach is equivalent to energy required to stretch the PDMS network at the surface and in the bulk to form the real rough contact area. However, in retraction work of adhesion is found to be proportional to the ratio of excess energy spent in the loading-unloading cycle and the true contact area obtained from topography indicating conformal contact matching fracture mechani (open full item for complete abstract)

Committee: Ali Dhinojwala Ph. D. (Advisor); Mesfin Tsige Ph. D. (Committee Chair); Tevis Jacobs Ph. D. (Committee Member); Jutta Luettmer-Strathmann Ph. D. (Committee Member); Hunter King Ph. D. (Committee Member) Subjects: Engineering; Mechanics; Physics; Polymers; Science Education

Doctor of Philosophy, Case Western Reserve University, 2020, Macromolecular Science and Engineering

Additive manufacturing has been thriving rapidly in recent years to fabricate highly complex and well controlled architectures, yet challenges lie in the limited types of 3D printable materials, as well as the un-satisfying performance in the printed products. The dissertation focus on the diverse applications of inorganic fillers to formulate new materials that can be 3D printed into functional materials with enhanced properties. The uses of inorganic fillers are highly flexible, that they can act as rheology modifiers in both liquid resin for DIW printing and in solid resin for post processing, as property enhancement additives for thermo-mechanical performance, bio-compatibility and electrical conductivity, and as porogens for micro-sized pores production. Various functional materials with excellent performance have been fabricated by 3D printing, including super-elastic hierarchical PDMS and TPU foams, FDM printable isotropic epoxy/benzoxazine, rubber toughened epoxy, and bio-compatible TPU/PLA/GO nanocomposite. The freedom of structure design of 3D printing gives rise to well-controlled mechanical performance, improved elasticity, increased absorbance, and introduction of property gradient by combining multiple materials in one object. The newly formulated materials with enhanced performance can be used as functional materials instead of prototypes, and the well-designed architectures generated by 3D printing help to satisfy various demands to realize highly customized applications.

Committee: Rigoberto Advincula (Advisor); Hatsuo Ishida (Committee Member); Michael Hore (Committee Member); Alp Sehirlioglu (Committee Member) Subjects: Engineering; Materials Science; Nanotechnology; Polymers

PhD, University of Cincinnati, 2010, Arts and Sciences : Physics

Resilin is an elastomeric protein characterized by rubber-like elasticity, very high resilience and high fatigue lifetime. These outstanding material properties are conferred by multiple elastic repeats, similar to those found in other elastomeric proteins. In this thesis I use molecular dynamics to elucidate the effect of amino-acid sequence variation on the mechanical properties of resilin-like peptides. In particular, I address the role of disorder in the relaxed (unstretched) state and the amount of conformational entropy lost upon extension. I simulate model systems comprising multiple identical repeats from single elastic units observed in fruit fly and mosquito resilin gene products. The length of the simulated peptides ranges from 11 to 176 residues. In order to study the nature of the restoring force in resilin I use steered molecular dynamics (SMD) and fixed end simulations. I find a high level of disorder and lack of stable secondary structure for the well solvated relaxed state in all simulated peptides; these results are consistent with conclusions from circular dichroism spectra of resilin-like peptides. Structural parameters, computed from molecular dynamics trajectories, are compared with experimental NMR and SAXS results. While upon stretching the conformational entropy is significantly decreased, the enthalpy is estimated to remain essentially unchanged. I conclude that the restoring force is primarily of entropic origin and largely insensitive to the amino-acid composition of resilin-like elastic repeats. Finally, I build a coarse-grained model from all-atomic simulation of two repeats in mosquito resilin and apply it larger peptides in order to assess flexibility and the effect of cross-linking in multiple resilin-like polypeptides.

Committee: Thomas Beck PhD (Committee Chair); Jaroslaw Meller PhD (Committee Member); Rostislav Serota PhD (Committee Member) Subjects: Biophysics