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Séminaires à venir

Deformation/Perforation Mechanisms and Energy Absorption of Glassy Polymer and Multiwall Carbon Nanotube Thin Films at Supersonic Strain Rates

Edwin L. Thomas, Department of Materials Science and NanoEngineering Rice University, Houston, Texas USA

le vendredi 27 septembre à 11h à l’amphi Boreau

 

We investigate the energy absorption characteristics and associated deformation behavior of free standing thin films of multiwall carbon nanotubes (MWCNTs) and glassy polymers using a micro-projectile impact test. Target films with thicknesses between 30-300 nm are impacted by micron size silica spheres at projectile velocities ranging from 300 m/s to 900 m/s, corresponding to kinetic energies up to 20 nJ. The deformation features are characterized by electron microscopy to deduce energy dissipation mechanisms operative at the extremely high strain rates ( 107 s-1). The quasi-static properties of the 2D isotropic network of meandering MWCNT nanofibers are quite modest but at the extreme strain rates and large strains of ballistic impact, the deformation behavior of the mat results in unprecedented energy absorption per unit mass of the target mat. As the projectile moves forward, the MWCNT tubes and tube bundles are straightened and pulled into the impact region. The increased friction associated with the amplified surface interactions as well as secondary and covalent bonding occurring between the translating principal tubes distributes the load around the impact region and raises the load on those portions of the tubes adhering to the sphere surface. The subsequent large back-deflection of the impact region slows the advancing projectile as KE is converted into elastic stretching energy of the network and ultimately fracture of many principal tubes. The deformation results in very high specific energy absorption of 7-9 MJ/kg, greater than any other material under micro-ballistic impact.

Freestanding glassy polystyrene (PS) films also show unexpectedly large energy absorption at extreme rates of loading. The more mobile and less entangled near-surface regions of the PS facilitate crazing and dramatically increase craze multiplication and subsequent growth with accompanying large adiabatic temperature rise of the highly deforming film. By grafting the PS chains to nanoparticle (NP) surfaces creating several hundred covalently anchored polymer chains to individual silica NPs, both the well-entangled coronal regions between NPs and the giant “NP crosslinks” improve the stress transfer through the composite. The single component nanocomposite PS grafted NP films ( 1% v/v, 16nm diameter SiO2 NPs) show 25% enhanced high kinetic energy absorption per unit mass of the target film over the previous high specific energy absorption of the thin, freestanding homopolymer PS films ( 3 MJ/kg).

A unified framework to understand ductility in glassy polymers : from crazing, brittle-to-ductile transition to rubber-toughened polymers

Shi-Qing Wang, Faculty of Department of Polymer Science, University of Akron, Akron, OH

le vendredi 11 octobre à 14h à l’amphi Boreau

 

Abstract
In my lab, we focus on building a molecular-level understanding of polymer mechanics in both liquid and solid states. This is a journey that involves three episodes or steps : a. Phenomenology and conceptual foundation of polymer melt rheology, b. Molecular mechanics of polymer glasses, c. Brittleness and ductility of semicrystalline polymers. The latter two subjects can only be understood after the molecular foundation [1] for polymer melt rheology has been established. In this talk, I will concentrate on subject b, exploring how we can understand the remarkable ductility of glassy polymers. In a pedagogical way, I will explain why a valid theory to explain yielding of glassy polymers must address when the polymers fail to remain ductile, i.e., unable to yield and undergo brittle fracture. The universally applied Eyring idea of activation alone is powerless to provide the foundation for the molecular mechanics of glassy polymers. Rich experiences with melt rheology have provided us the crucial ingredients to formulate the basis [2] for all aspects of mechanical behavior of polymeric glasses including brittle-ductile transition, crazing and rubber-toughening.


References
[1] Nonlinear polymer rheology : macroscopic phenomenology and molecular foundation, S. Q. Wang, Wiley (2018)
[2] A phenomenological molecular model for brittle-ductile transition and yielding of polymer glasses, S. Q. Wang et al., J. Chem. Phys. 141, 094905 (2014).


Biography
Shi-Qing Wang was educated in China and arrived in the US in fall 1982 through the Chinese-US Physics Examination and Application (CUSPEA) program after earning a BS in Physics in Wuhan University. He spent the first nine months at University of Rochester before transferring to University of Chicago where received his Ph.D. degree in Physics in 1987, working with Karl Freed on renormalization group calculations of linear viscoelastic properties of polymer solutions. After two years of postdoctoral research at University of California at Los Angeles with William Gelbart, Shi-Qing Wang joined in fall 1989 the faculty of Macromolecular Science and Engineering at Case Western Reserve University, and rose to Full Professor in 1998. Since fall 2000, he has been on the faculty of Department of Polymer Science at University of Akron. His research interest is polymer physics and engineering. First half of his career efforts has resulted in a comprehensive monograph on polymer rheology : Nonlinear Polymer Rheology – Macroscopic phenomenology and Molecular foundation (Wiley, 2017). He is a fellow of both APS and AAAS.