Flexible pressure sensors with a high sensitivity over a broad linear range can simplify wearable sensing systems without additional signal processing for the linear output, enabling device miniaturization and low power consumption. Here, we demonstrate a flexible ferroelectric sensor with ultrahigh pressure sensitivity and linear response over an exceptionally broad pressure range based on the material and structural design of ferroelectric composites with a multilayer interlocked microdome geometry. Due to the stress concentration between interlocked microdome arrays and increased contact area in the multilayer design, the flexible ferroelectric sensors could perceive static/dynamic pressure with high sensitivity (47.7 kPa, 1.3 Pa minimum detection). In addition, efficient stress distribution between stacked multilayers enables linear sensing over exceptionally broad pressure range (0.0013-353 kPa) with fast response time (20 ms) and high reliability over 5000 repetitive cycles even at an extremely high pressure of 272 kPa. Our sensor can be used to monitor diverse stimuli from a low to a high pressure range including weak gas flow, acoustic sound, wrist pulse pressure, respiration, and foot pressure with a single device.

We present detailed results about the structural, conformational, rheo-optical, and topological properties of an entangled of C 400 H 802 linear polyethylene (PE) melt over a wide range of shear rates (covering both the linear and the highly nonlinear viscoelastic regimes) from direct nonequilibrium molecular dynamics (NEMD) simulations of a large system containing 192 chains (corresponding to 79200 interacting atomistic units). We discuss results for (i) the probability distribution of the mean-square chain end-to-end distance and its radius of gyration, (ii) the conformation tensor, (iii) the material functions in steady shear (viscosity, normal stress differences, nonequilibrium shear compliance, hydrostatic pressure), (iv) the flow birefringence, (v) the orientation angle and order parameter, (vi) the interaction energies and their relative importance, (vii) the intermolecular pair distribution function, and (viii) the intrinsic molecular shape of the chains (represented by the isosurface plots in terms of their monomer number density), all as a function of flow strength. A detailed primitive path (PP) analysis has allowed us to examine how the flow field alters the statistical properties of the underlying topological network of the melt (probability distribution functions and mean values of PP contour length, of the number and size of entanglement strands, etc.). Our results reveal significant distortions of all these distributions due to applied flow. One of the most important results of our work is that as the shear rate is increased, the average value of the contour length goes through a maximum and the number of entanglements per chain exhibits a shear-thinning behavior which bears many similarities with the corresponding behavior of the shear viscosity. Overall, most of the computed rheological properties of the C 400 H 802 melt change in a nonlinear way with the applied shear rate due to the simultaneous effect of (a) chain orientation and stretching, (b) chain rotation and tumbling under shear, and (c) chain disentanglement.

The gradient stiffness between stiff epidermis and soft dermis with interlocked microridge structures in human skin induces effective stress transmission to underlying mechanoreceptors for enhanced tactile sensing. Inspired by skin structure and function, we fabricate hierarchical nanoporous and interlocked microridge structured polymers with gradient stiffness for spacer-free, ultrathin, and highly sensitive triboelectric sensors (TESs). The skin-inspired hierarchical polymers with gradient elastic modulus enhance the compressibility and contact areal differences due to effective transmission of the external stress from stiff to soft layers, resulting in highly sensitive TESs capable of detecting human vital signs and voice. In addition, the microridges in the interlocked polymers provide an effective variation of gap distance between interlocked layers without using the bulk spacer and thus facilitate the ultrathin and flexible design of TESs that could be worn on the body and detect a variety of pressing, bending, and twisting motions even in humid and underwater environments. Our TESs exhibit the highest power density (46.7 μW/cm), pressure (0.55 V/kPa), and bending (∼0.1 V/°) sensitivities ever reported on flexible TESs. The proposed design of hierarchical polymer architectures for the flexible and wearable TESs can find numerous applications in next-generation wearable electronics.

Atomistic configurations of model unentangled ring polyethylene (PE) melts ranging in chain length from C 24 up to C 400 have been subjected to detailed molecular dynamics (MD) simulations in the isothermal-isobaric statistical ensemble at temperature T = 450 K and P = 1 atm. Strictly monodisperse samples were employed in all cases. We present and discuss in detail simulation results for a variety of structural, thermodynamic, conformational and dynamic properties of these systems, and their variation with chain length. Among others, these include the mean chain radius of gyration, the pair correlation function, the intrinsic molecular shape, the local dynamics, the segmental mean square displacement (msd), the chain center-of-mass self-diffusion coefficient D G , the chain terminal relaxation time τ d , the characteristic spectrum of the Rouse relaxation times τ p , and the dynamic structure factor S(q,t). In all cases, the results are compared against the corresponding data from simulations with linear PE melts of the same chain length (the linear analogues) and the predictions of the Rouse theory for polymer rings which we derive here in its entirety. The Rouse theory is found to provide a satisfactory description of the simulation findings, especially for rings with chain length between C 50 and C 170 . An important finding of our work (from the observed dependence of D G , τ p , ζ, and η 0 on chain length N) is that PE ring melts follow approximately Rouse-like dynamics even when their chain length is as long as C 400 ; this is more than twice the characteristic crossover chain length (∼C 156 ) marking the passage from Rouse to reptation dynamics for the corresponding linear PE melts. In a second step, and by mapping the simulation data onto the Rouse model, we have managed to extract the friction coefficient ζ and the zero-shear rate viscosity η 0 of the simulated ring melts. Overall, and in agreement with previous theoretical and experimental studies, our simulation results support that the structure of ring polymers in the melt is more compact than that of their linear analogues due to their nonconcatenated configurations. Additional results for the intermolecular mer-mer and center-of-mass pair correlation functions confirm that the effective correlation hole effect is more pronounced in melts of rings than in melts of linear chains.