Shear Strain-Induced Two-Dimensional Slip Avalanches in Rhombohedral MoS2.

Slip avalanches are ubiquitous phenomena occurring in three-dimensional materials under shear strain, and their study contributes immensely to our understanding of plastic deformation, fragmentation, and earthquakes. So far, little is known about the role of shear strain in two-dimensional (2D) materials. Here we show some evidence of 2D slip avalanches in exfoliated rhombohedral MoS2, triggered by shear strain near the threshold level. Utilizing interfacial polarization in 3R-MoS2, we directly probe the stacking order in multilayer flakes and discover a wide variety of polarization domains with sizes following a power-law distribution. These findings suggest that slip avalanches can occur during the exfoliation of 2D materials, and the stacking orders can be changed via shear strain. Our observation has far-reaching implications for the development of new materials and technologies, where precise control over the atomic structure of these materials is essential for optimizing their properties as well as for our understanding of fundamental physical phenomena.

Role of the Intercrystalline Tie Chains Network in the Mechanical Response of Semicrystalline Polymers.

We examine the microscopic origin of the tensile response in semicrystalline polymers by performing large-scale molecular dynamics simulations of various chain lengths. We investigate the microscopic rearrangements of the polymers during tensile deformation and show that the intercrystalline chain connections known as tie chains contribute significantly to the elastic and plastic response. These results suggest that the mechanical behavior of semicrystalline polymers is controlled by two interpenetrated networks of entanglements and tie chains.

Correlation of structure and mechanical response in solid-like polymers.

Employing large scale molecular dynamics simulations, we measure the uniaxial tensile response of amorphous and semicrystalline states of a coarse-grained PVA bead-spring model. The response beyond the elastic limit encompasses strain-softening and strain-hardening regimes. To understand the underlying mechanisms of plastic deformation, we analyse conformational and structural changes of polymers. In particular, we characterise the volume distribution of crystalline domains along the stress-strain curve. The strain-softening regime in semicrystalline samples is dominated by deformation of crystalline parts, while strain-hardening involves unfolding and alignment of chains in both amorphous and crystalline parts. Comparing the tensile response of semicrystalline and amorphous polymers, we find similar conformations of polymers for both systems in the strain-hardening regime.

Plastic Deformation Mechanisms of Semicrystalline and Amorphous Polymers.

We use large-scale molecular dynamics simulations to investigate plastic deformation of semicrystalline polymers with randomly nucleated crystallites. The strain-softening regime is dominated by deformation of crystallites via reorientation of chain-folded lamellae toward the tensile axis, fragmentation of largest crystalline domains, and a partial loss of crystallinity. The strain-hardening regime coincides with unfolding of chains and recrystallization as a result of strain-induced chain alignment. These observed deformation mechanisms are consistent with experimental findings. We compare the tensile behavior of semicrystalline polymers with their amorphous counterparts at temperatures above and below the glass transition temperature.

A systematically coarse-grained model for DNA and its predictions for persistence length, stacking, twist, and chirality.

We introduce a coarse-grained model of DNA with bases modeled as rigid-body ellipsoids to capture their anisotropic stereochemistry. Interaction potentials are all physicochemical and generated from all-atom simulation/parameterization with minimal phenomenology. Persistence length, degree of stacking, and twist are studied by molecular dynamics simulation as functions of temperature, salt concentration, sequence, interaction potential strength, and local position along the chain for both single- and double-stranded DNA where appropriate. The model of DNA shows several phase transitions and crossover regimes in addition to dehybridization, including unstacking, untwisting, and collapse, which affect mechanical properties such as rigidity and persistence length. The model also exhibits chirality with a stable right-handed and metastable left-handed helix.