Microscopic in situ observation of electromechanical instability in a dielectric elastomer actuator utilizing transparent carbon nanotube electrodes.

Electromechanical instability (EMI) restricts the performance of dielectric elastomer actuators (DEAs), leading to premature electrical breakdown at a certain voltage. However, macro-level observations using traditional carbon grease electrodes have failed to capture the detailed features of EMI. In this study, we investigated EMI at the microscopic scale by fabricating transparent and conductive single-walled carbon nanotube (SWCNT) electrodes. Our findings reveal that EMI predominantly occurs in highly localized regions with dimensions on the order of tens of micrometers. This snap-through instability is likely induced by pre-existing defects within the elastomer, such as air voids or conductive particles, which reduce the critical voltage required for EMI in the flawed areas. From the perspective of phase transition principles, these defects act as heterogeneous nucleation sites for new phase embryos, thereby lowering the energy barrier for the electromechanical phase transition (i.e., EMI) compared to homogeneous nucleation in an ideally impurity-free elastomer. This study clarifies the longstanding discrepancy between theoretically predicted deformation bursts and the experimentally observed macroscopic continuous expansion of DEAs under low pre-stretch conditions. Additionally, it underscores the critical importance of material purity in mitigating electromechanical instability.

Polyurethanes Modified by Ionic Liquids and Their Applications.

Polyurethane (PU) refers to the polymer containing carbamate groups in its molecular structure, generally obtained by the reaction of isocyanate and alcohol. Because of its flexible formulation, diverse product forms, and excellent performance, it has been widely used in mechanical engineering, electronic equipment, biomedical applications, etc. Through physical or chemical methods, ionic groups are introduced into PU, which gives PU electrical conductivity, flame-retardant, and antistatic properties, thus expanding the application fields of PU, especially in flexible devices such as sensors, actuators, and functional membranes for batteries and gas absorption. In this review, we firstly introduced the characteristics of PU in chemical and microphase structures and their related physical and chemical performance. To improve the performance of PU, ionic liquids (ILs) were applied in the processing or synthesis of PU, resulting in a new type of PU called ionic PU. In the following part of this review, we mainly summarized the fabrication methods of IL-modified PUs via physical blending and the chemical copolymerization method. Then, we summarized the research progress of the applications for IL-modified PUs in different fields, including sensors, actuators, transistors, antistatic films, etc. Finally, we discussed the future development trends and challenges faced by IL-modified PUs.

Crystallization-driven surface segregation and surface structures in poly(L-lactide)-block-poly(ethylene glycol) copolymer thick films.

In this work, we used poly(L-lactide)-block-poly(ethylene glycol) (PLLA-b-PEG) copolymer thick films to investigate the effect of crystallization on surface segregation, surface crystal orientation, and morphology by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), reflection optical microscopy (ROM), and two-dimensional grazing incident wide-angle X-ray scattering (2D GIWAXS) methods. ATR-FTIR results indicated that the surface fraction of PLLA block increased from 0.48 to 0.79 when T(c,PLLA) increased from 70 to 110 °C. Polarized ATR-FTIR and 2D GIWAXS results indicated that PLLA crystal lamellae preferentially oriented parallel to the film surface with increasing T(c,PLLA). The surface crystallinity of PLLA was almost independent of T(c,PLLA), while the surface crystallinity of PEG decreased with increasing T(c,PLLA). On the basis of surface crystal orientation and crystallization kinetics, we suggested that the excess of PLLA component at the surface was mainly dominated by a coupling effect of crystallization behavior and surface segregation.

Liquid crystalline assembly of coil-rod-coil molecules with lateral methyl groups into 3-D hexagonal and tetragonal assemblies.

In this paper, we report the synthesis and self-assembly behavior of coil-rod-coil molecules, consisting of three biphenyls linked through a vinylene unit as a conjugated rod segment and poly(ethylene oxide) (PEO) with a degree of polymerization (DP) of 7, 12 and 17, incorporating lateral methyl groups between the rod and coil segments as the coil segment. Self-organized investigation of these molecules by means of differential scanning calorimetry (DSC), thermal polarized optical microscopy (POM) and X-ray diffraction (XRD) reveals that the lateral methyl groups attached to the surface of rod and coil segments, dramatically influence the self-assembling behavior in the liquid-crystalline mesophase. Molecule 1 with a relatively short PEO coil length (DP=7) self-assembles into rectangular and oblique 2-dimensional columnar assemblies, whereas molecules 2 and 3 with DP of 12 and 17 respectively, spontaneously self-organize into unusual 3-dimensional hexagonal close-packed or body-centered tetragonal assemblies.

Eu3+-Doped Electrospun Polyvinylidene Fluoride-Hexafluoropropylene/Graphene Oxide Multilayer Composite Nanofiber for the Fabrication of Flexible Pressure Sensors.

The development of flexible materials with higher piezoelectric properties and electrostrictive response is of great significance in many applications such as wearable functional devices, flexible sensors, and actuators. In this study, we report an efficient fabrication strategy to construct a highly sensitive (0.72 kPa-1), red light-emitting flexible pressure sensor using electrospun Eu3+-doped polyvinylidene fluoride-hexafluoropropylene/graphene oxide composite nanofibers using a layer-by-layer technology. The high ß-phase concentration (96.3%) was achieved from the Eu3+-doped P(VDF-HFP)/GO nanofibers, leading to a high piezoelectricity of the composite nanofibers. We observed that a pressure sensor is enabled to generate an output voltage of 4.5 V. Furthermore, Eu3+-doped P(VDF-HFP)/GO composite nanofiber-based pressure sensors can also be used as an actuator as it has a good electrostrictive effect. At the same time, the nanofiber membrane has excellent ferroelectric properties and good fluorescence properties. These results indicate that this material has great application potential in the fields of photoluminescent fabrics, flexible sensors, soft actuators, and energy storage devices.

Highly Stretchable, Transparent and Adhesive Ionogel Based on Chitosan-Poly(acrylic acid) Double Networks for Flexible Strain Sensors.

A stretchable double-network (DN) ionogel composed of a physically crosslinked network of chitosan (CS) and a chemically crosslinked network of polyacrylic acid (PAA) was prepared in an ionic liquid ([EMIM][OAc]) using a one-step polymerization method. In this ionogel (CS/PAA), the CS and the PAA polymer chains served as backbones, which constructed an interpenetrating DN structure via numerous hydrogen bonds formed through the hydroxyl, amino and carboxyl groups on the polymer chains. The DN structure improves the mechanical properties of the ionogel. Therefore, the CS/PAA DN ionogel exhibited outstanding mechanical performance in many ways: tensile strength up to 2.04 MPa, strain range up to 1046% and the value of toughness up to 8.52 MJ/m3. The ionogel also showed good self-recovery performance, fatigue resistance, ability to work in a broad temperature range (-20~80 °C) and adhesion properties. As a flexible sensor, the CS/PAA DN ionogel showed high strain sensitivity (gauge factor = 6.235). It can sensitively detect human motion (such as joint-bending, vocal fold vibration, walking gait and other human body motions), revealing the practical application potential of flexible electronic devices.

Chitosan-based double cross-linked ionic hydrogels as a strain and pressure sensor with broad strain-range and high sensitivity.

A conductive hydrogel P(AAm-co-AA)/CS-Fe3+ with double cross-linked networks was fabricated using a one-step polymerization by UV irradiation and a soaking process in Fe(NO3)3 solution. In this hydrogel, the rigid chain of chitosan (CS) and the soft chain of copolymer P(AAm-co-AA) with acrylic acid (AA) and acrylamide (AAm) act as the backbone, among which large amounts of hydrogen bonds are formed by the amino, hydroxyl, and carboxyl groups on the two polymers. Ferric irons (Fe3+) are introduced and form coordination interactions with carboxyl and amino groups of the polymers. The double cross-linked interactions in the system can enhance the tensile strength and toughness of the hydrogel. Thus, the prepared P(AAm-co-AA)/CS-Fe3+ hybrid network hydrogel shows excellent mechanical properties in many aspects: a strength of up to 550 kPa, a broad strain-range up to 800%, fast self-recovery ability (30 min), and low hysteresis strain (<100%). The conductive hydrogel demonstrates high strain sensitivity (gauge factor (GF) = 6.6 at a strain of 700%) as a flexible sensor. Human movements (for example, finger bending, vocal cord vibration, and other human activities) can be sensitively detected using the P(AAm-co-AA)/CS-Fe3+ hydrogel sensor.

A Flexible Multifunctional PAN Piezoelectric Fiber with Hydrophobicity, Energy Storage, and Fluorescence.

Lightweight, flexible, and hydrophobic multifunctional piezoelectric sensors have increasingly important research value in contemporary society. They can generate electrical signals under the action of pressure and can be applied in various complex scenarios. In this study, we prepared a polyacrylonitrile (PAN) composite fiber doped with imidazolium type ionic liquids (ILs) and europium nitrate hexahydrate (Eu (NO3)3·6H2O) by a facile method. The results show that the PAN composite fibers had excellent mechanical properties (the elongation at break was 114% and the elastic modulus was 2.98 MPa), hydrophobic self-cleaning ability (water contact angle reached 127.99°), and can also emit light under UV light irradiation red fluorescence. In addition, thanks to the induction of the piezoelectric phase of PAN by the dual fillers, the composite fibers exhibited efficient energy storage capacity and excellent sensitivity. The energy density of PAN@Eu-6ILs reached a maximum of 44.02 mJ/cm3 and had an energy storage efficiency of 80%. More importantly, under low pressure detection, the sensitivity of the composite fiber was 0.69 kPa-1. The research results show that this PAN composite fiber has the potential to act as wearable piezoelectric devices, energy storage devices, and other electronic devices.

Visualizing crystal structure evolution of electrode materials upon doping and during charge/discharge cycles in lithium-ion batteries.

Here we propose a systematic approach to reliably visualize the crystal structure evolution of electrode materials of lithium-ion batteries (LIBs) during cyclic charge/discharge process. Using anodic Ta5+-doped Li2ZnTi3O8 (LZTO) spheres as an example, this protocol describes the doping state modeling by density functional theory (DFT) calculation, their crystal structure parameter determination by X-ray diffraction (XRD) refinement, and formation energy by electron density calculation. This protocol also details the in-situ XRD technique and date processing to visualize the cycling reversibility of Ta5+-doped LZTO. For complete details on the use and execution of this profile, please refer to Ma et al. (2021).

Concentric ring pattern formation in a competing crystallization and phase separation process.

Spherulitic patterns usually form in the single process of crystallization in polymer blends. But when phase separation intervenes under deep quench, the radial growth of the initial spherulitic patterns tends to invert into concentric alternating crystalline-/amorphous-rich ring structures. Within crystalline-rich regions, lateral lamellae orient in the tangential direction rather than in the usual radial direction. We demonstrate the determining factor for this first observed phenomenon is the concentration deviation enhanced phase separation dynamics at the growth interface of crystals.