Dynamics of Entangled Networks in Ultrafast Perforation of Polystyrene Nanomembranes

The mechanical behavior of polymers at ultrahigh strain rates can be very different from their typical behavior because of the inertia effect, wave propagation, adiabatic processes, and rate-dependent polymer chain dynamics. Polymer chain entanglements play a significant role in the mechanical properties of polymers. By employing microscopic ballistic testing in vacuum, the high-strain-rate intrinsic behavior of polystyrene (PS) is demonstrated depending on its molecular weight and microprojectile’s perforation speed. Based on the experimentally quantified correlation between the specific penetration energy and the entanglement density, we predict the upper limits of the specific penetration energy of PS

Extreme Tribological Characteristics of Copolymers Induced by Dynamic Rheological Instability

Nonlinear tribological behavior of polymeric microparticles under extreme collision conditions is required for an in-depth understanding of advanced applications in the fields of defense, biomedicine, and manufacturing. Laser-induced projectile impact tests with an incidence angle of 45° are conducted to investigate the tribological response at the contact interface of block copolymers with glassy–rubbery phases and a stationary substrate to induce deformations with an ultrahigh strain rate. Morphological-phase-dependent tribological and rheological responses are quantified from the mechanical interactions involving adiabatic heating, plastic flow, and interfacial shear instability. An effective coefficient of friction mechanism that depends on the rheological transition activated by impact velocity is proposed to specify the rheological response of the copolymers

Extreme Plasticity, Adhesion, and Nanostructural Changes of Diblock Copolymer Microparticles in Cold Spray Additive Manufacturing

Using the laser-induced projectile impact testing (LIPIT), the extreme plastic and adhesive responses of polystyrene-polydimethylsiloxane block copolymer (BCP) microparticles are investigated to provide the ultra-high-strain-rate behavior of individual BCP feedstock powders during their collisions with a stationary substrate in the cold spray additive manufacturing process. The onset of BCP microparticle adhesion to the substrate is precisely predicted by the maximum coefficient of dynamic friction, quantified from the angled collisions, and by the spectra of the coefficients of restitution. This finding confirms the direct correlation between friction and adhesion mechanisms in the ultra-high-strain rate regime and its significance in the consolidation process of BCP feedstock powders. Furthermore, the impact-induced adiabatic shear flows create structural ordering of initially disordered nanostructures of the block copolymers consisting of glassy and rubbery domains while generating a temperature rise beyond their glass transition temperatures. In addition to the conventional strain-hardening effect in homopolymers, nanoscale morphological ordering can provide another strain-hardening mechanism of BCP feedstock microparticles in the cold spray of additive manufacturing

Dynamic Swelling of Tunable Full-Color Block Copolymer Photonic Gels <i>via</i> Counterion Exchange

One-dimensionally periodic block copolymer photonic lamellar gels with full-color tunability as a result of a direct exchange of counteranions were fabricated via a two-step procedure comprising the self-assembly of a hydrophobic block–hydrophilic polyelectrolyte block copolymer, polystyrene-b-poly(2-vinyl pyridine) (PS-b-P2VP), followed by sequential quaternization of the P2VP layers in 1-bromoethane solution. Depending on the hydration characteristics of each counteranion, the selective swelling of the block copolymer lamellar structures leads to large tunability of the photonic stop band from blue to red wavelengths. More extensive quaternization of the P2VP block allows the photonic lamellar gels to swell more and red shift to longer wavelength. Here, we investigate the dynamic swelling behavior in the photonic gel films through time-resolved in situ measurement of UV–vis transmission. We model the swelling behavior using the transfer matrix method based on the experimentally observed reflectivity data with substitution of appropriate counterions. These tunable structural color materials may be attractive for numerous applications such as high-contrast displays without using a backlight, color filters, and optical mirrors for flexible lasing

Conducting-Interlayer SiO_<i>x</i> Memory Devices on Rigid and Flexible Substrates

SiOx memory devices that offer significant improvement in switching performance were fabricated at room temperature with conducting interlayers such as Pd, Ti, carbon, or multilayer graphene. In particular, the Pd-interlayer SiOx memory devices exhibited improvements in lowering the electroforming voltages and threshold voltages as the number of inserted Pd layers was increased, as compared to a pure SiOx memory structure. In addition, we demonstrated that the Pd-interlayer SiOx junction fabricated on a flexible substrate maintained low electroforming voltage and mechanically stable switching properties. From these observations, a possible switching mechanism is discussed based on the formation of individual conducting paths at the weakest edge regions of each SiOx film, where the normalized bond-breaking probability of SiOx is influenced by the voltage and the thickness of SiOx. This fabrication approach offers a useful structural platform for next-generation memory applications for enhancement of the switching properties while maintaining a low-temperature fabrication method that is even amenable with flexible substrates

Periodic Bicontinuous Composites for High Specific Energy Absorption

We report on the mechanical behavior of an interpenetrating carbon/epoxy periodic submicrometer-scale bicontinuous composite material fabricated following the design principles deduced from biological composites. Using microscopic uniaxial compressive tests, the specific energy absorption is quantitatively evaluated and compared with the epoxy/air and carbon/air precursors. The carbon/epoxy material demonstrates extremely high specific energy absorption up to 720 kJ/kg and shear-dominant interphase interactions from the interlocked hard (carbon) and soft (epoxy) phases. Such bicontinuous nanocomposites are a new type of structural metamaterial with designed cell topology and mechanical anisotropy. Their inherent small length scale can play a critical role in prohibiting segregated mechanical responses leading to flaw tolerance

Mechanics of Nanoindentation on a Monolayer of Colloidal Hollow Nanoparticles

We explore the collective mechanical behavior of monolayer assemblies composed of close-packed arrays of hollow silica nanoparticles using a spherical nanoindentor. Seven types of well-defined hollow nanoparticles are studied with their radii ranging from 100 to 300 nm and shell thickness ranging from 14 to 44 nm. Micromechanical models reveal the underlying deformation mechanisms during indentation, where the consecutive contacting of the indentor with an increasing number of nanoparticles results in a nonlinear increase in the indentation force with penetration depth. Each contacted hollow nanoparticle successively locally bends, flattens, and then locally buckles. The effective indentation modulus of the monolayer film, which is obtained by a Hertzian fit to the experimental data, is found to be proportional to the elastic modulus of the nanoparticle shell material and scales exponentially with the ratio of particle shell thickness t to radius R to the power of 2.3. Furthermore, we find that for a constant film density with the same t/R of the constituent nanoparticles, smaller particles with a thinner shell can provide a higher effective indentation modulus, compared to their larger diameter and thicker shell counterparts. This study provides useful insights and guidance for constructing high-performance lightweight nanoparticle films and coatings with potential applications in tailoring stiffness and mechanical energy absorption

Entanglement Density-Dependent Energy Absorption of Polycarbonate Films via Supersonic Fracture

The fracture behavior of glassy polymers is strongly coupled to molecular parameters such as entanglement density as well as extrinsic parameters such as strain rate and test temperature. Here we use laser-induced projectile impact testing (LIPIT) to study the extreme strain rate (≈107 s–1) puncture behavior of free-standing polycarbonate (PC) thin films. We demonstrate that changes to the PC molecular mass and the degree of plasticization can lead to substantial changes in the specific puncture energy. We relate these changes to the alteration of the entanglement density of the polymer that determines the underlying failure mechanism as well as the size of the deformation zone

Enhanced Energy Dissipation in Periodic Epoxy Nanoframes

Periodic nanostructures fabricated by interference lithography can be precisely designed to have a specific cell geometry, topology, and porosity in contrast to typical stochastic cellular materials. We use nanoindentation to elucidate the mechanical characteristics of the nanoframe as a function of its relative density and model the deformation behavior via numerical simulations. The nanoframe exhibits a scaling exponent of relative modulus versus relative density of 1.26, which is less sensitive than for conventional foams. Moreover, the nanoframe shows large mechanical energy dissipation/volume (up to 4.5 MJ/m3), comparable to the highest values achieved in the conventional polymer foams but at a far smaller strain. Counterintuitively, a nanoframe of smaller relative density can dissipate more energy per volume because the geometry of the nanoframe evolves during deformation to engage more of the material in plastic deformation. The results demonstrate how geometrical control at the nano- and microstructural scale can tailor modulus and energy dissipation and suggest means for engineering of mechanically superior materials in the future

Three-Dimensional Networked Nanoporous Ta₂O_5–<i>x</i> Memory System for Ultrahigh Density Storage

Oxide-based resistive memory systems have high near-term promise for use in nonvolatile memory. Here we introduce a memory system employing a three-dimensional (3D) networked nanoporous (NP) Ta₂O_5–<i>x</i> structure and graphene for ultrahigh density storage. The devices exhibit a self-embedded highly nonlinear <i>I–V</i> switching behavior with an extremely low leakage current (on the order of pA) and good endurance. Calculations indicated that this memory architecture could be scaled up to a ∼162 Gbit crossbar array without the need for selectors or diodes normally used in crossbar arrays. In addition, we demonstrate that the voltage point for a minimum current is systematically controlled by the applied set voltage, thereby offering a broad range of switching characteristics. The potential switching mechanism is suggested based upon the transformation from Schottky to Ohmic-like contacts, and <i>vice versa</i>, depending on the movement of oxygen vacancies at the interfaces induced by the voltage polarity, and the formation of oxygen ions in the pores by the electric field