Three-dimensional (3D) network polymers are an important family of materials. For many applications of 3D networks it is important to combine high modulus, high tensile strength, and high extensibility. 1 Rigid network materials tend to fail after only a short extension. While flexible elastomers are more extensible, they usually have low moduli and exhibit shallow stress-response. Although many engineering approaches and chemical modifications 2 have been developed to improve mechanical properties of network polymers, it remains a challenge to design ideal networks that have a combination of desired properties. Among chemical methods, interesting work has been reported on using interchain hydrogen bonding to improve polymer physical properties. 3 Given the importance of cross-linker structure on mechanical properties of network polymers, it is surprising that there is very limited investigation on designing molecularly engineered cross-linkers to enhance network properties. Herein we introduce a novel biomimetic design of a reversibly unfolding modular cross-linker that can increase elastomer stiffness without sacrificing extensibility leading to a dramatic tensile strength enhancement.Our biomimetic concept is based on the modular design observed in many biopolymers, such as the skeletal muscle protein titin and connective proteins in both soft and hard tissues, that have a remarkable combination of strength and elasticity. 4 Single-molecule nanomechanical studies have revealed that their combined mechanical properties originate from their unique modular structure, which sequentially unfolds upon deformation providing the molecular mechanism to sustain high force (strength) and to yield high elongation (elasticity). 5 Inspired by nature, our group has been mimicking this modular domain strategy in the pursuit of synthetic polymers with advanced mechanical properties. A number of biomimetic modules have been successfully designed and incorporated into linear polymers. 6,7 Single-molecule and bulk properties validated our biomimetic concept of using modular structures to enhance polymer properties. This communication describes the first example of introducing a reversibly unfolding modular cross-linker into 3D networks to enhance their mechanical properties.Our concept is illustrated in Figure 1. A stress applied from any direction to a 3D network will be ultimately transferred across the individual network junctions, where biomimetic modules can be reversibly unfolded. Since it requires significant forces to unfold the modules held by strong multiple hydrogen bonds, further extension can be gained without sacrificing the modulus. In addition, we propose that the enhanced energy dissipation capability by unfolding the biomimetic modules should lead to significant increases in both * zguan@uci.edu. Supporting Information Available:Synthesis and characterization of cross-linker and polymers, MALDI-TOF MS, MTS stressstrain experiments. This material is available free of charge at http//:pubs.acs.org. The biomimet...

Many of Earth's great earthquakes occur on thrust faults. These earthquakes predominantly occur within subduction zones, such as the 2011 moment magnitude 9.0 eathquake in Tohoku-Oki, Japan, or along large collision zones, such as the 1999 moment magnitude 7.7 earthquake in Chi-Chi, Taiwan. Notably, these two earthquakes had a maximum slip that was very close to the surface. This contributed to the destructive tsunami that occurred during the Tohoku-Oki event and to the large amount of structural damage caused by the Chi-Chi event. The mechanism that results in such large slip near the surface is poorly understood as shallow parts of thrust faults are considered to be frictionally stable. Here we use earthquake rupture experiments to reveal the existence of a torquing mechanism of thrust fault ruptures near the free surface that causes them to unclamp and slip large distances. Complementary numerical modelling of the experiments confirms that the hanging-wall wedge undergoes pronounced rotation in one direction as the earthquake rupture approaches the free surface, and this torque is released as soon as the rupture breaks the free surface, resulting in the unclamping and violent 'flapping' of the hanging-wall wedge. Our results imply that the shallow extent of the seismogenic zone of a subducting interface is not fixed and can extend up to the trench during great earthquakes through a torquing mechanism.

(2014), Experimental investigation of strong ground motion due to thrust fault earthquakes, J. Geophys. Res. Solid Earth, 119, 1316-1336, doi:10.1002 Abstract Thrust fault earthquakes are studied in a laboratory earthquake setup previously used to investigate analog strike-slip seismic events. Dynamic mode II ruptures are generated along preexisting faults in an analog material, Homalite H-100, and their interaction with the free surface is studied for both sub-Rayleigh and supershear rupture speeds. High-speed digital photography and laser velocimeter diagnostics are used synergistically to identify and study the ground velocity signatures caused by the various features of the generated ruptures. The obtained surface-normal motions of both sub-Rayleigh and supershear ruptures show substantial asymmetry between the hanging and footwall, with the hanging wall experiencing much larger velocity amplitudes. The main features of the surface velocity traces at various stations can be explained by the calculated arrivals of various waves and fronts-Mach cones, P and S waves, and sub-Rayleigh features. In both the sub-Rayleigh and supershear cases, the arrival of the rupture tip generates a prominent Rayleigh wave traveling along the simulated Earth's surface. Supershear events feature larger amplitudes of ground shaking profiles. All signatures in the surface motion records attenuate and broaden with increasing distance from the fault trace. The signatures corresponding to the arrival of the Mach fronts attenuate with distance at a slower rate than those from sub-Rayleigh ruptures. The arrival of the updip supershear rupture at the free surface creates a downdip propagating slip feature with its own Mach cone. These additional Mach fronts further amplify ground shaking on the hanging and footwalls.

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Motivated by the need to evaluate the seismic response of large capacity gravity energy storage systems (potential energy batteries) such as the proposed frictional Multiblock Tower Structures (MTS) recently discussed by Andrade et al. [1], we apply Buckingham's Pi Theorem [2] to identify the most general forms of dimensionless numbers and dynamic similitude laws appropriate for scaling discontinuous multiblock structural systems involving general restoring forces resisting inertial loading. We begin by introducing the dimensionless “mu-number” (μN) appropriate for both gravitational and frictional restoring forces and then generalize by introducing the “arbitrary restoring force number” (RFN). RFN is subsequently employed to study similitude in various types of discontinuous or discrete systems featuring frictional, gravitational, cohesive, elastic and mixed restoring forces acting at the block interfaces. In the process, we explore the additional consequences of inter and intra-block elasticity on scaling. We also formulate a model describing the mechanism of structural signal transmission for the case of rigid MTS featuring inter-block restoring forces composed of elastic springs and interfacial friction, introducing the concept of “structural speed”. Finally, we validate our results by demonstrating that dynamic time-histories of field quantities and structural speeds between MTS models at various scales are governed by our proposed similitude laws, thus demonstrating the consistency of our approach.