1:30pm - 2:00pmInvited
Molecular Design and Mechanical Behavior of Hyper-Confined Molecular Hybrids
Stanford University, United States
The exceptional mechanical properties of polymer nanocomposite hybrids are achieved through intimate mixing of the polymer and inorganic phases, which leads to spatial confinement of the polymer phase. The nature and degree of this confinement varies considerably, from macroscopic constraint in multilayer laminate systems to true nanoscale confinement in polymer nanocomposites. We probe the mechanical and fracture properties of polymers in the extreme limits of molecular confinement, where a stiff inorganic matrix phase confines the polymer chains to dimensions far smaller than their bulk radius of gyration. We show that polymers confined at such molecular length scales dissipate energy through a novel, confinement-induced molecular bridging mechanism in which individual confined polymer chains pull out from a nanoporous matrix. This mechanism contrasts with toughening processes in bulk and weakly-confined polymers and describes behavior that cannot be explained by existing entanglement-based theories of polymer deformation and fracture. We support the molecular bridging mechanism with a model that captures the associated nanomechanical processes, including the sliding friction of chain pullout, the deformation and stretching of confined polymer chains, and the eventual backbone scission of polymer molecules under extreme loads. We next describe the effects of controlling the interaction of the confined molecules with the confining matrix by surface functionalization of the matrix pores. Molecular mechanics models that describe the pore wall interactions are provided leading to unique insights into the frictional interaction of the polymer molecules with the matrix confinement itself. Finally, we have developed a new class of high-temperature light-weight hybrids that incorporate a polyimide molecular phase. We demonstrate remarkable polyimide filling, imidization and cross-linking reactions in the highly confined matrix pores despite the simultaneous imidization reactions that decrease their solubility and conformational freedom, and raise their glass transition temperature.
2:00pm - 2:30pmInvited
Carbon-based Nanolattice Materials
University of California, Irvine (UCI), United States
From the current perspective, there is little room for further expansion of the accessible material property space by classical material fabrication methods. Single one- and two-dimensional nanoscale objects, such as nanowires and thin films, are known to hold exceptional physical properties. Yet, their properties are intrinsically coupled to their small size and their solitary nature, and therefore can hardly be accessed in actual materials of practical volume. If nanowires and thin films are simply scaled up properties, which relate to surface to volume effects, get lost, when clustered in a composite interfaces dominate the overall performance. To overcome this dilemma one could think of regular three-dimensional networks constructed from nanowires or thin films, this is what nanolattice materials are. One may define a nanolattice material as a metamaterial which is capable to exploit size effects by means of its nanoscale lattice architecture. Ultimately its performance is maximized once all its relevant structural feature dimensions, from the unit cell size to the material thickness of individual lattice elements, are nanoscale.
In our work, we have fabricated carbon-based nano-architected lattice materials, by applying direct laser writing (DLW) and pyrolysis as well as subsequent techniques such atomic layer deposition. The combination of DLW and pyrolysis facilitates complex three-dimensional carbon-based ceramic structures of uniquely high resolution, with feature dimensions below 100 nm. This enables to take advantage of pronounced size-dependent mechanical effects such as in the strength, as well as non-mechanical size effects. With respect to these, we systematically analyzed the impact of processing parameters including, sample and feature size, temperature-time dependent graphitization or DLW directionality, as well as subsequent treatments such as post-DLW curing. An overview of applied manufacturing routs and relevant processing details will be given. The interplay of size-dependent effects with different topological and material design approaches are shown.
2:30pm - 3:00pmInvited
Macroscopic and Microscopic Instabilities in Soft Fiber Composites
Technion - Israel Institute of Technology, Israel
We examine the response of fiber composites with hyperelastic phases subjected to compression along the fiber direction. The applied compressive loading may lead to development of elastic instabilities upon achieving the critical strain [1,2]. To estimate the critical strains corresponding to the onset of macroscopic instabilities associate with the loss of ellipticity, we obtain the homogenized response of transversely isotropic fiber composites, and evaluate the effective tensor of elastic moduli . We show that the onset of instabilities can be predicted by a closed form explicit expression in terms of the material properties and volume fractions of the constituents . The analytical estimations are compared with 3D finite element simulations, and an excellent agreement is reported . However, instabilities may develop at smaller length-scales first [1,3]; to capture the onset of these microscopic instabilities, we idealize the microstructure as 2D periodic layered composites, and perform the microscopic instability analysis [3,4,5]. We derive an explicit estimate for critical strain and critical wave length of the buckled wavy shapes. Finally, we experimentally observed the existence of these macroscopic and microscopic modes of instabilities in periodic layered structures [2,5], and in 3D fiber composites . The results for the critical strain and wavelength are found to be in good agreement with the theoretically and numerically predicted values. Moreover, we experimentally show that the microscopic instabilities in the composites with rate-dependent phases can be significantly tuned by the applied strain rate .
 Triantafyllidis and Maker. J. Appl. Mech., 52:794-800 (1985)
 Rudykh and deBotton. J. Elasticity, 106:123–147 (2012)
 Li, Kaynia, Rudykh and Boyce. Adv. Eng. Mater., 15 (10), 921-926 (2013)
 Rudykh and Boyce. Phys. Rev. Letters 112, 034301 (2014)
 Slesarenko and Rudykh. Soft Matter, 12:3677-3682 (2016)
 Slesarenko and Rudykh. J. Mech. Phys. Solids, 10.1016/j.jmps.2016.11.002 (2017)
3:00pm - 3:15pmOral
Biomimetic, Strong yet Tough Composites through 3D Printing
Singapore University of Technology and Design, Singapore
Nature has taught us fascinating strategies to fabricate novel materials that exhibit superior structural properties. For instance, natural composites such as the shells of mantis dactyl club have been found to exhibit remarkable strength and toughness, giving them an ability to absorb high impact energy. Here, we utilized polymer melt-electrospinning to mimic the 3D helicoidal complex architecture of the molecular-level structures found in these shells. From our tensile test results, the biomimetic 3D helicoidal fiber-structured architecture exhibited much higher toughness and ductility, but lower ultimate stress compared with unidirectional melt-electrospinning fibers and bulk sample in same material. After surface treatment with adding functional group (carboxyl and amino group) on each side of all the monolayer melt-electrospinning fiber samples, rebuilt biomimetic helicoidal structure presented much higher toughness than unidirectional melt-electrospinning fibers and bulk sample, accompanied with substantially improvement in its ultimate strength. These lightweight synthetic analogue materials enabled by 3D printing methodologies would display superior structural properties and functionalities such as high strength, extreme toughness and deformability.
3:15pm - 3:30pmOral
The Strength of Exfoliated Monolayer Graphene
National Graphene Institute and School of Materials, The University of Manchester, United Kingdom
A detailed investigation has been undertaken into the deformation and fracture behavior of one-atom-thick individual flakes of exfoliated monolayer graphene with the aid of in situ Raman spectroscopy. Individual single crystal graphene monolayer flakes with lateral dimensions of up to 50 μm were prepared by tape exfoliation and pressed on to the surface of a flat poly(methyl methacrylate beam. They were then deformed by bending the beam and the local stress in the flakes was determined from the shift of the Raman 2D band of the graphene monolayer. Good stress transfer was found between the beams and the graphene flakes through the van der Waals bonding. The stress was mapped over the whole of each flake in 0.5 μm steps in grids of up to 1000 data points, depending upon the size of the flake. Fracture of the flakes was observed from the development of cracks manifesting as a line of zero stress within the flakes, approximately perpendicular to the tensile axis. This fracture behavior was also correlated with the crystallographic orientation of the graphene determined from the splitting of the Raman G band during deformation and there was a general tendency for the cracks to follow the zig-zag direction within the flakes. It was found that the strength of the flakes tended to decrease with increasing flake width, indicating defect-controlled fracture. It fell to less than 5 GPa for the largest flakes in comparison with generally-accepted strength value for perfect graphene of the order of 130 GPa. The reasons for this behavior will be explained in terms of defects in the graphene flakes and the implications of this behavior for the use of graphene to reinforce nanocomposites will be discussed.