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.

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.

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.

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.

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.

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).

Solid-state self-template synthesis of Ta-doped Li2ZnTi3O8 spheres for efficient and durable lithium storage.

Ta-doped Li2ZnTi3O8 (LZTO) spheres (Li2ZnTi3-x Ta x O8; where x is the synthetic chemical input, x = 0, 0.03, 0.05, 0.07) are synthesized via solid-state reaction using mesoporous TiO2 spheres as the self-template. The majority of Ta5+ ions are uniformly doped into crystal lattices of LZTO through the Ti↔Ta substitution, and the rest forms the piezoelectric LiTaO3 secondary phase on the surface, as confirmed by X-ray diffraction refinement, Raman spectroscopy, density functional theory, and electron microscopy. Electrochemical impedance spectroscopy demonstrates that the Ta5+ doping creates rapid electronic transportation channels for high Li+ ion diffusion kinetics; however, the LiTaO3 surface coating is beneficial to improve the electronic conductivity. At the optimal x = 0.05, Li2ZnTi3-x Ta x O8 spheres exhibit a reversible capacity of 90.2 mAh/g after 2000 cycles with a high coulombic efficiency of ≈100% at 5.0 A/g, thus enabling a promising anode material for lithium-ion batteries with high power and energy densities.

Catalytically-active porous assembly with dynamic pulsating motion for efficient exchange of products and reagents.

Despite recent advances in the use of porous materials as efficient heterogeneous catalysts which operate through effectively trapping reagents in a well-defined space, continuously uptaking reagents to substitute products in the cavity for efficient product turnover still remains challenging. Here, a porous catalyst is endowed with 'breathing' characteristics by thermal stimulus, which can enable the efficient exchange of reagents and products through reversible stacking from inflated aromatic hexamers to contracted trimeric macrocycles. The contracted super-hydrophobic tubular interior with pyridine environment exhibits catalytic activity towards a nucleophilic aromatic substitution reaction by promoting interactions between concentrated reagents and active sites. Subsequent expansion facilitates the exchange of products and reagents, which ensures the next reaction. The strategy of mesoporous modification with inflatable transition may provide a new insight for construction of dynamic catalysts.

Supramolecular Nanopumps with Chiral Recognition for Moving Organic Pollutants from Water.

Since organic pollutants in water resources have raised concerns on aquatic ecosystems and human health, mechanical machines such as a nanopump for rapid and efficient removal of pollutants from water with regeneration properties remains a challenge. Here, a pH-responsive artificial pump from left-handed porous tubules into right-handed solid fibers was presented by the self-assembly of bent-shaped aromatic amphiphiles. The bent-shaped amphiphile with a pH-sensitive segment was demonstrated in aromatic hexameric macrocycles, which could contract into dimeric disks. Such a switchable aromatic pore with superhydrophobicity was well-suited for an efficient removal and controlled release of organic pollutants from water through pulsating motion. The removal efficiency is found to be 78% for ethinyloestradiol and 82% for bisphenol. Additionally, the pumping accompanied by chiral inversion was endowed with a rapid removal and convenient regenerable ability. The inflation from right-handed solid fibers into left-handed tubules for efficient removal pollutants was remarkably promoted by (-)-acidic enantiomer of malic acid, whereas the contraction with full desorption of pollutants was incisively responsive to alkaline with (+)-conformation. The kinetically regulable porous device with a chiral recognition will provide a promising platform for the construction of rapid responsible machine for sewage treatment.

Designing function-oriented artificial nanomaterials and membranes via electrospinning and electrospraying techniques.

The sister technologies, electrospinning and electrospraying provide a facile and universal synthesis method for the continuous preparation of nanostructured materials. Through adjusting the synthesis parameters, rich electrospun and electrosprayed nanomaterials, scaffolds, membranes with tunable composition (inorganic, polymeric, hybrid, etc.), shape (sphere, films, scaffold, etc.), morphology and inner structure (solid, hollow, core-shell, co-axial, etc.) can be selectively elaborated. This review provides an overview of the design of functional nanostructured materials, porous scaffolds and membranes by electrospinning and electrospraying techniques. Key experimental parameters and synthesis strategy are emphasized to reveal the synthesis-component-structure-property relationship and eventually realize the targeted functions through predictable synthesis. Potential applications in tissue engineering, medicine, membrane filtration and lithium battery are highlighted.

A Bioinspired Swimming and Walking Hydrogel Driven by Light-Controlled Local Density.

A hydrogel exhibits a real-time depth-controllable swimming motion via light-mediated modulation of local density to mimic the volume changes found in the bladders of fish. Moreover, other motions, e.g., rolling, somersaulting, and bipedal-like walking, can also be realized by designing or combining gel shapes, and the location of light.

"Brill Transition" Shown by Green Material Poly(octamethylene carbonate).

Poly(octamethylene carbonate) (POMC), as the eighth member of the newly developed biodegradable aliphatic polycarbonate family, demonstrates a reversible crystal-crystal transition, which is highly similar to Brill transition extensively studied in the nylon family. With the dipole-dipole interaction in POMC much weaker than the hydrogen bonding, POMC exhibits its "Brill transition" temperature at around 42 °C, much lower than nylons. The two crystalline structures of POMC at below and above the transition temperature can be identified. The transition of POMC is largely associated with the reversible conformation change of methylene sequences from trans-dominated at low temperatures to trans/gauche coexistence at high temperatures.

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.

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.

High performance graphene oxide based rubber composites.

In this paper, graphene oxide/styrene-butadiene rubber (GO/SBR) composites with complete exfoliation of GO sheets were prepared by aqueous-phase mixing of GO colloid with SBR latex and a small loading of butadiene-styrene-vinyl-pyridine rubber (VPR) latex, followed by their co-coagulation. During co-coagulation, VPR not only plays a key role in the prevention of aggregation of GO sheets but also acts as an interface-bridge between GO and SBR. The results demonstrated that the mechanical properties of the GO/SBR composite with 2.0 vol.% GO is comparable with those of the SBR composite reinforced with 13.1 vol.% of carbon black (CB), with a low mass density and a good gas barrier ability to boot. The present work also showed that GO-silica/SBR composite exhibited outstanding wear resistance and low-rolling resistance which make GO-silica/SBR very competitive for the green tire application, opening up enormous opportunities to prepare high performance rubber composites for future engineering applications.

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.

Multiple peak reference method for polarized Fourier transform infrared-attenuated total reflection spectroscopy to determine the three-dimensional orientation of a uniaxially drawn poly(trimethylene 2,6-naphthalate) film.

The polarized Fourier transform infrared-attenuated total reflection (FT-IR-ATR) technique is a very useful infrared spectroscopic method for characterizing the three-dimensional orientation of thick samples, to which the conventional transmission FT-IR method cannot easily be applied. To quantitatively analyze polarized FT-IR-ATR spectra, the dependence of the optical contact between the sample and the ATR crystal during the clamping and reclamping processes must be controlled. In this work, a new multiple peak reference (MPR) method is proposed and used to carry out a three-dimensional analysis of a poly(trimethylene 2,6-naphthalate) (PTN) polymer sample. The conventional single peak reference (SPR) analysis technique cannot be applied to such a sample due to the lack of an established reference peak. A new artificial reference band was generated by the MPR method using two different infrared bands, at 1602 cm(-1) and 917 cm(-1), in identical spectra with a combination constant of 0.95. The new artificial reference band was successfully used to calculate the three-dimensional orientation of various infrared bands of a uniaxially drawn PTN film, which has not previously been studied by three-dimensional orientation analyses using the polarized FT-IR-ATR method.

A new method for the time-resolved analysis of structure and orientation: polarization modulation infrared structural absorbance spectroscopy.

Polarization modulation infrared linear dichroism (PM-IRLD) is often used for measurements of molecular orientation with high sensitivity and good time resolution. However, PM-IRLD is unable to provide the structural absorbance spectrum because it does not measure separately the parallel and perpendicular spectra. Here we propose a new method, named polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS), to overcome this limitation of PM-IRLD. PM-IRSAS measures the dichroic difference and structural absorbance spectra simultaneously and, therefore, allows quantitative analysis of molecular orientation and conformation with 200 ms time resolution. The PM-IRSAS method was first validated through comparison with conventional polarized FT-IR spectroscopy using drawn polymer films. Second, it was demonstrated that the PM-IRSAS method can provide a quantitative analysis of dynamic orientation and conformation changes in PET films during deformation and crystallization processes.