Polystyrene Molecules Research Articles

Diffusiophoretically Mediated Nanopatterning by Solvent Nanodroplets on Polystyrene Surfaces.

Dissolution is a ubiquitous process in nature and industry. However, due to technical difficulties, the detailed dissolution process at the nanoscale has seldom been captured experimentally. In this study, we investigated the dissolution dynamics in the confinement of toluene surface nanodroplets on polystyrene (PS) thin films in oversaturated toluene/water mixture solutions. This was achieved by adjusting the immersion durations from several minutes to 9 h. Dissolution takes place upon the deposition of nanodroplets on the PS surfaces, leading to the formation of surface nanostructures. Interestingly, we found that the induced nanostructures underwent complex morphological changes, from complex nanocraters with central bulges and/or multiple rims to simple nanocraters. We speculate that diffusiophoresis plays a key role in the formation of the complex nanocraters, as it facilitates the transportation of dissolved PS molecules inside the nanodroplets. We believe this finding not only enhances our understanding of dissolution dynamics at the nanoscale but also holds promise for applications in dissolution-based nanopatterning.

Purification and Kinetics of Chlorogenic Acid from Eucommia ulmoides Oliver Leaves by Macroporous Resins Combined with First-Principles Calculation

Background: A simple and effective method to separate chlorogenic acid from Eucommia ulmoides leaves with macroporous resin was studied in this paper. Methods: In order to optimize the separation process of chlorogenic acid from Eucommia ulmoides leaves, dynamic adsorption and desorption experiments were carried out on a glass column filled with XDA-8 resin. Based on the First-principles calculation, the possible adsorption models were simulated. Results: Among the six macroporous resins, XDA-8 showed good adsorption/desorption capacity and a high adsorption/desorption ratio for chlorogenic acid. After being treated with XDA-8 resin once, the content of chlorogenic acid from the extraction increased by 525%, and the recovery of chlorogenic acid reached 85.36%. Conclusion: At 25°C, the adsorption behavior of chlorogenic acid on XDA-8 resin was consistent with the Langmuir isotherm model and pseudo-second-order kinetic model. Furthermore, by calculating the charge changes of the O atom at each position in the chlorogenic acid molecule and the H atom at the adsorption site in polystyrene molecule with resin skeleton, and combining with the electron cloud density distribution diagram of chlorogenic acid and resin skeleton, the adsorption of chlorogenic acid by XDA-8 resin is mainly due to the charge transfer, which causes the electron cloud to overlap.

From waste to energy storage: post-consumer waste expanded polystyrene/rGO composite as a high performance self-standing electrode for coin cell supercapacitors.

This research work aims to explore the potential usage of post-consumer waste expanded polystyrene (EPS) for the fabrication of self-standing electrodes by incorporating reduced graphene oxide (rGO) into it via a facile cost-effective mechanical mixing process. The π-π interaction between the expanded polystyrene and rGO is evidenced from FT-IR and Raman analysis. The elevated thermal stability of the EPS/rGO composite from thermogravimetric analysis (TGA) further confirms the interconnection between the rGO and EPS. This π-π stacking interaction between the rGO and the polystyrene molecules present in the polymer matrix enable the composite material to be interconnected throughout which is beneficial for the charge transport process. The symmetric coin cell supercapacitor fabricated using the EPS/rGO composite electrode can be operated with a high operating voltage of 1.6 V in aqueous KOH and Na2SO4 electrolytes. The devices fabricated with KOH and Na2SO4 electrolytes deliver an areal capacitance of 11.9 mF cm-2 and 10 mF cm-2 at the discharge current density of 0.1 mA cm-2. Further, the devices fabricated with the KOH and Na2SO4 electrolytes demonstrated remarkable rate capability of 87.1% and 99.5% after 10 000 continuous charge discharge cycles. This facile method of preparation without consuming energy or polluting the environment is a novel approach which can be scaled-up to large-scale fabrication of self-standing plastic electrodes for low-cost energy storage applications.

Pairing-specific microstructure in depletion gels of bidisperse colloids.

We report the ensemble-averaged and pairing-specific network microstructure formed by short-range depletion attractions in hard sphere-like colloidal systems. Gelation is induced by adding polystyrene molecules at a fixed concentration to colloids with different colloid bidispersity ratios (α = 1, 0.72, and 0.60) across a range of volume fractions (0.10 ≤ ϕ ≤ 0.40). 3D confocal microscopy imaging combined with a scale-invariant feature transform algorithm show that monodisperse colloids pack more efficiently, whereas increasing the size disparity leads to looser, more disordered, and sub-isostatic packings. Categorizing the structures formed by small and large particles reveal that certain cluster configurations may be favored due to the complex interplay between the differences in particle surface areas and attractive potentials. These pairwise bonds assemble to affect the density of tetrahedral and poly-tetrahedral clusters in bidisperse systems. With the exception of non-percolating samples at ϕ = 0.10, increasing the gel volume fraction leads to an increase in the number of nearest neighbors. However, the internal density within each cluster decreases, possibly due to kinetic arrest from the deeper potential wells of tetrahedral clusters at low volume fractions in which vertices are primarily made out of larger particles.

Supramolecular additive-induced conformational transition of polymer chains via π-π interaction

Controlling the conformation of polymer chains throughout melt processing is a critical factor in the development and optimization of polymer materials for a variety of applications. In this study, we present a supramolecular strategy for modulating the conformational transition of polymer and investigate the structural evolution with supramolecular additive. It is found that minute amounts (0.07–0.9 wt%) of supramolecular nucleating agent, N, N′-dicyclohexylterephthalamide (DCHT), enable polystyrene (PS) chains to adopt more gauche conformation than the trans mode when processed from the melt, resulting in higher PS chain mobility and improved PS film transmittance. The conformational transition of PS chains shows dependence on the π-π interactions between PS and DCHT molecules. By controlling the critical assembly concentration and the self-assembly behavior of DCHT in PS, it is possible to tailor the ratio of gauche to trans states and the corresponding properties of PS. We anticipate that this study will advance the design and development of supramolecular additives for tailoring the structure and properties of other polymers.

Pd/C embedded crosslinked polystyrene fibers as an efficient catalyst for Heck reactions

AbstractPd/C embedded polystyrene fibers were successfully prepared by electrospinning. The polystyrene molecules were then cross‐linked by paraformaldehyde in sulfuric acid to improve the solvent resistance of composite fibers. SEM images conformed the preparation of uniform and smooth composite fibers. FT‐IR spectra demonstrated that the polystyrene molecules inside fibers have been sulphonated and crosslinked. Heck reactions were used to evaluate the catalytic performance of these novel composite fibers. The catalysis results show that this composite fiber mat catalyzed Heck reactions could be evidently promoted by using preferred reducing alcohol agent and solvent. Under the optimized reaction conditions, this composite fiber mat could effectively catalyze the Heck reactions of aromatic iodides with n‐butyl acrylate to afford the products with satisfied yields. Especially, compared with the particulate Pd/C catalyst, the separation and recycling of this fibrous catalyst from the reaction mixture were significantly improved due to the larger fibrous structure. At last, this fiber catalyst was successfully reused for eight times with little loss of initial catalytic activity, which was even better than the pristine Pd/C catalyst. Hence, embedment of particulate supported metal catalysts inside the crosslinked polystyrene fibers can effectively improve their catalytic performance and handiness.

Electrosynthesized nanostructured molecularly imprinted polymer for detecting diclofenac molecule

• Effective technique to generate nanostructured polymer on glassy carbon electrodes. • Micropatterned molecular imprinted polymer film for detecting diclofenac. • A cavity-confined electrochemical growth of PEDOT in the presence of DCF. • Simultaneous extraction of PS beads and DCF by dipping in THF solution at once. • A larger active surface decorated GC provides more accessible binding sites. This work describes the preparation and application of micropatterned molecular imprinting in conductive polymer film for the detection of diclofenac molecules. A fast and cost-effective technique for generating nanostructured polymer on glassy carbon electrode (GCE) by combining nanosphere lithography (NSL) and electrochemical polymerization is reported. Firstly, GCE is modified by a close-packed monolayer of polystyrene (PS) spheres sized 500 nm in diameter, which subsequently serves as a template for the electrochemical deposition of poly(3,4-ethylene dioxythiophene) (PEDOT) accompanied by diclofenac (DCF). The functional monomers of 3,4-ethylene dioxythiophene (EDOT) were electropolymerized in the presence of sodium diclofenac (DCF-Na) as a target molecule to prepare a molecularly imprinted polymer (MIP) electrode. After dissolving PS and DCF-templated molecules simultaneously in tetrahydrofuran (THF) solvent, the micropatterned structure of PEDOT is generated on the GCE surface. This as-prepared MIP surface is applied to test the recognition of DCF molecules in phosphate-buffered saline (PBS) solution. Good linearity of peak current recorded by differential pulse voltammograms (DVP) after exposure to different DCF concentrations was obtained with a limit of detection (LOD) of 77.5 nM. The sensitivity of the as-prepared MIP electrode is 5 times larger than that of the control electrode made by non-molecular imprinted polymer (NIP). It opens a pathway to significantly improve the sensitivity and the quality of conventional electrochemical sensors for the detection of Diclofenac.

CARS spectra of polystyrene sheet at excitation of fs pulses and chirped pulses

Two-beam coherent anti-Stokes Raman scattering (CARS) spectroscopy setup is built. Measurements of CARS spectra generated from polystyrene (PS) sheet under excitation of femtosecond (fs) pulses and chirped pulses are demonstrated. The pump-Stokes time delay is adjusted at fixed step of 6 fs and transient CARS spectra is measured at every pump-Stokes time delay. Two-dimensional plots depicting intensity of fs CARS, chirped Stokes CARS, chirped pump/probe CARS and spectral focusing CARS as a function of the wavenumber and the pump-Stokes time delay are realized. Excitation situation of Raman resonances in fingerprint region of polystyrene molecule is discussed in conditions of un-chirped and chirped CARS spectra. Capacity of chirped Stokes CARS, chirped pump/probe CARS and spectral focusing CARS in improving spectral resolution over fs CARS is systematically analyzed.

Cluster-Induced Desorption/Ionization of Polystyrene: Desorption Mechanism and Effect of Polymer Chain Length on Desorption Probability.

Soft cluster-induced desorption/ionization of polystyrene oligomers was investigated with respect to application in mass spectrometry. Clear peak progressions corresponding to intact polystyrene molecules were observed in the mass spectra, and no fragmentation was detected; efficient desorption was deduced from quartz crystal microbalance measurements. Molecular dynamics (MD) simulations of the process revealed that even in the case of the nonpolar polystyrene molecules cluster-induced desorption proceeds via dissolvation in the polar clusters. Experimentally, a significantly lower desorption efficiency was observed for polystyrene molecules with larger chain length. Taking into account MD simulations and further experiments with mixed samples consisting of long- and short-chain polystyrene oligomers, the reduced desorption efficiency for longer chain polystyrene molecules was attributed to a stronger entanglement of the larger polystyrene molecules.

A Microscopic View of Macromolecule Transfer in the Vacuum Using Gas and Bismuth Clusters

Transferring large nonvolatile molecules in the vacuum is key to both their characterization by mass spectrometry (ion mobility, etc.) and their use as nanofabrication building blocks in soft-landing-type experiments. Recently, our group performed the successful transfer and redeposition of intact and bioactive lysozyme (14 kDa) with large gas cluster ion beams, in the absence of a solvent or matrix, demonstrating that such beams could serve as tools for macromolecule manipulation. A number of fundamental questions arose in relation with this experimental proof-of-concept, concerning for instance the maximum molecular size for successful transfer, the effect of the cluster projectile incidence angle, the dependence of molecular dissociation on the projectile energy and size, the internal energy uptake ultimately leading to metastable decay reactions, or the influence of the surface interactions on desorption. To address these questions, a series of molecular dynamics simulations were conducted involving cluster impacts on an adsorbed globular polystyrene (PS) macromolecule of 60 kDa, with a mass and shape comparable to those of a protein such as bovine serum albumin. Conditions of intact desorption of this PS molecule by Ar clusters are identified, in terms of energy per atom and incidence angle, and the results are compared to 30 keV Bi5 impacts, commonly used for molecular analysis and 2D imaging in our time-of-flight secondary ion mass spectrometry experiments. When Ar cluster impact conditions are appropriate for intact desorption, the take-off angle of the macromolecule is larger than the projectile incidence. Bombardment by a massive methane projectile (6.6 × 104 methane molecules) does not induce qualitatively different results, and in general, the energy per atom or energy per unit mass remains the deciding factor concerning the survival or dissociation of the bombarded molecule. In contrast with large gas clusters, Bi5 projectiles largely fail at desorbing the intact macromolecules, because of the detrimental effect of the collision cascade and the insufficient momentum transferred by the crater expansion for molecular lift off. Additionally, 10 keV Ar5000 projectiles with 45–75° incidence with respect to the surface normal directly transfer momentum to the macromolecule via their backscattered Ar atoms and small clusters, inducing molecular desorption with center-of-mass velocities of the order of 1 km/s. The implications for future experiments are discussed.

Self-Assembly of Nanocrystals into Ring-like Superstructures: When Shape, Size, and Material Do Not Matter.

This manuscript describes a universal method for the spontaneous self-assembly of nanostructures ranging from 2-4 nm spherical particles to ∼440 nm long anisotropic nanorods into ring-like superstructures. The nanostructures composed of Au, Pt, and Pd as surface materials were synthesized in an aqueous cetyltrimethyl ammonium bromide (CTAB) solution. The ligand exchange technique with 4-mercaptophenol was applied to replace CTAB from the surface of nanostructures with a functional thiol. The esterification reaction was carried out to covalently attach carboxy-terminated long-chain polystyrene (PS) molecules to the surface of nanostructures. The high grafting density of PS chains around nanocrystals made them highly soluble in a wide range of organic solvents. When a drop of nanostructure solution in a volatile nonpolar solvent was dried on a solid surface, the nanostructures spontaneously arranged themselves in the form of ring-like assemblies. The condensation of microscopic water droplets from the atmosphere on the surface of an evaporating solvent creates templates for the self-assembly of nanostructures into rings. We demonstrate that this self-assembly method is highly universal and can be extended to various nanostructures regardless of their shapes, sizes, and surface materials.

Hydrophobic Molecule Monolayer Brush-Tethered Zinc Anodes for Aqueous Zinc Batteries.

Aqueous zinc batteries are of great interest as a rechargeable energy storage system, particularly owing to the low cost and high safety of aqueous electrolytes, as well as the high capacity of zinc anodes. Unfortunately, the wide commercialization of aqueous zinc batteries is impeded by the irreversible water reduction and irregular zinc evolution issues on the anode side. Hereby, a hydrophobic and ultrathin polystyrene molecule brush layer is tethered onto the surface of zinc metal anodes to tackle the above limitations. Experimental investigations reveal that the waterproof artificial layer can sustain fast interfacial ionic transportation, minimize hydrogen evolution, and smoothen Zn deposition, thus conferring enhanced electrochemical performance to the as-protected Zn anode in both symmetric Zn//Zn cells and Zn//LiV3O8 full cells.

Diagnostics of atomic- molecular interactions in nano-compositions organics-inorganics by means of infrared spectroscopy

This paper prolongs our studies of the formation and properties of the organic-inorganic scintillating compositions from activated polystyrene and inorganic nanoparticles from cesium iodid and sulfate. The compositions from the solution of polystyrene in benzene and nano- or micro-particles of cesium iodide were solidified in the form of thin films. Then some of the films were subjected to rolling. The variations of the infrared transmission spectra of the compositions were measured either during their solidification or after the solidification and rolling. The new absorption band was revealed in the process of the solidification and disappeared after it. The fractal-like structures of parallel rows were formed in the films. Quasi-periodical oscillations of the transmission spectra were found in several infrared regions ascribed to the interaction of the infrared radiation with the fractal super-structure of the films. The modifications of the absorption spectra found in our experiments are attributed to the interactions of polystyrene molecules with benzene molecules and nanoparticles of cesium iodide.

Diffusion Coefficients and Activation Energies of Diffusion of Organic Molecules in Polystyrene below and above Glass Transition Temperature.

General Purpose Polystyrene (GPPS) and High Impact Polystyrene (HIPS) is used in packaging food as well as for technical products. Knowledge of the diffusion behavior of organic molecules in polystyrene (PS) is important for the evaluation of the diffusion and migration process. Within this study, diffusion coefficients were determined in GPPS and HIPS below and above the glass transition temperature. Diffusion coefficients were determined from desorption kinetics into the gas phase using spiked GPPS and HIPS sheets as well as from permeation kinetics through a thin GPPS film. Overall, 187 diffusion coefficients were determined in GPPS and HIPS at temperatures between 0 °C and 115 °C. From the temperature dependency of the diffusion coefficients 45 activation energies of diffusion EA and the pre-exponential factor D0 were determined. As expected, the activation energies of diffusion EA show a strong dependency from the molecular volume of the investigated substances. At the glass transition temperature, only a slight change of the diffusion behavior were observed. Based on EA and D0, prediction parameters for diffusion coefficients were established.

Surface-Enhanced Raman Scattering and Surface-Enhanced Infrared Absorption by Plasmon Polaritons in Three-Dimensional Nanoparticle Supercrystals.

Surface-enhanced vibrational spectroscopy strongly increases the cross section of Raman scattering and infrared absorption, overcoming the limited sensitivity and resolution of these two powerful analytic tools. While surface-enhanced setups with maximum enhancement have been studied widely in recent years, substrates with reproducible, uniform enhancement have received less attention although they are required in many applications. Here, we show that plasmonic supercrystals are an excellent platform for enhanced spectroscopy because they possess a high density of hotspots in the electric field. We describe the near field inside the supercrystal within the framework of plasmon polaritons that form due to strong light-matter interaction. From the polariton resonances we predict resonances in the far-field enhancement for Raman scattering and infrared absorption. We verify our predictions by measuring the vibrations of polystyrene molecules embedded in supercrystals of gold nanoparticles. The intensity of surface-enhanced Raman scattering is uniform within 10% across the crystal with a peak integrated enhancement of up to 300 and a peak hotspot enhancement of 105. The supercrystal polaritons induce pairs of incoming and outgoing resonances in the enhanced cross section as we demonstrate experimentally by measuring surface-enhanced Raman scattering with multiple laser wavelengths across the polariton resonance. The infrared absorption of polystyrene is likewise enhanced inside the supercrystals with a maximum enhancement of 400%. We show with a coupled oscillator model that the increase originates from the combined effects of hotspot formation and the excitation of standing polariton waves. Our work clearly relates the structural and optical properties of plasmonic supercrystals and shows that such crystals are excellent hosts and substrates for the uniform and predictable enhancement of vibrational spectra.

Morphological and Elastic Transition of Polystyrene Adsorbed Layers on Silicon Oxide.

Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. More in detail, standard contact mode, force versus distance spectroscopy and ultrasonic force microscopy have been employed to obtain spatially-resolved maps and, thus, observe the physisorption of polystyrene on native silicon oxide substrate in function of time. Thick films, spin coated from a toluene solution, have been annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires 24 h. The pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion measurements show that the films present a reduced snap-off and the formation of a meniscus between tip and surface for annealing time up to 8 h. On the other hand, elastic measurements allow us to observe a progressive increase of the elastic modulus, with a complete transition for annealing time above 20 h. This is indication that a dense packing of the polystyrene molecules occurs, in line with the predictions of current models on the kinetics of irreversible adsorption. LAY DESCRIPTION: Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. Thick polystyrene films, spin coated from a toluene solution, have been thermally annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires twenty-four hours. The adsorption pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion and elastic measurements have allowed us to observe a progressive increase of the packing density of the polystyrene molecules, in agreement with the predictions of current models on the kinetics of irreversible adsorption.

Comparison of organic – inorganic nano-compositions created by different deformation – chemical techniques

Comparative studies of compositions from polystyrene and inorganic crystals of cesium sulfate and cesium iodide obtained by different methods gave new experimental facts and versions about particularities of structures and properties of these materials. The methods of creation of these compositions were based on two deformation – chemical techniques: solidification of micro-fibers from solutions of polystyrene with cesium sulfate and cesium iodide particles and introduction of polystyrene as the doping substance into single crystals of cesium iodide and potassium chloride by means of a ball rolling. In both cases micro- and nanosize singularities of the mass transfer were observed during the forming of these compositions. The microfibers formed by solidification of doped solutions of polystyrene turned out to be split to sets of nano – filaments. In the process of the dynamical centrifuging they formed the bundles parallel to the axis of the fiber, whereas the slow pulling of the fibers from the solution with a thin glass stick created saw-like chains of short pieces of the nano-filaments going along the axis of the fiber. These geometries were regulated by the ratios of compressive and tensile internal stresses, the chemical content of the components, lengths of the polystyrene molecules, viscosity of the solution. Superfast and superdeep mass transfer in the form of nano- and micro-filaments observed in our experiments is attributed to the soliton-like propagation provided by non-linear changes of the properties of the matrix in the vicinity of the filament. Hence wide sets of various nanostructures required by new trends of electronics, optics, etc. can be prepared by rather economic techniques.

Charge trapping in the films blended with polystyrene and different cyano-substituted spirofluorenes organic small molecules

We investigated the charges trapping and retention processes of the blend electret films of polystyrene and different cyano-substituted spirofluorenes organic small molecular material (PS/SFXs) by using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) technologies. The series of blend films present a promising charge storage property and a potential application in the charge trapping memory devices. We focused on the influences of the prepared parameters and the storage conditions for the samples on their charge storage properties. We found that water molecules in the films will influence the injection and retention processes of the trapped charges in the blend PS/SFXs films. The enhanced localization ability of the trapped charges is observed in the film prepared with high annealing temperature. The thicknesses of the films have an obvious effect on the charge injection process while having no discernible effect on the retention process. These results are very important for the studies of PS/SFXs in memory devices. The results show that the polarization and net charge transfer coexist for charge trapping in PS/SFXs films, while the symmetry of molecular structure of SFXs plays an important role in trapped charges stability.

Enhanced Electrical Properties of Polyethylene-Graft-Polystyrene/LDPE Composites.

Nanocomposite dielectrics show a great potential application in high voltage direct current cables for their obvious improvements in electrical properties. In the present manuscript, nanocomposite composed of low-density polyethylene and nanoscale polystyrene particles is studied by using low-density polyethylene grafted with polystyrene molecule. Fourier-transform infrared spectra reveal successful grafting of the polystyrene molecule onto the low-density polyethylene chain and the scanning electron microscope image shows the homogeneously dispersed nanoscale polystyrene particles. The presence of the polystyrene nanoparticles obviously improves the dielectric properties, such as the direct current breakdown strength and space charge inhibition. The conductivity and thermally stimulated current characteristics imply the deep traps in the composite increase obviously. Density functional theory calculation reveals that the grafted polystyrene can accommodate both shallow and deep electron carriers, and the depth of the hole traps are as deep as 2.07 eV.

Flexible organic field-effect transistors based on 6,13-bis(triisopropylsilylethynyl) pentacene/polystyrene blend film prepared by electrostatic spray deposition

In this study, we fabricated flexible organic field-effect transistors (OFETs) (bottom-gate and top-contact architecture) based on the blend film composed of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) as a π-conjugated small-molecule semiconductor and polystyrene (PS) as a polymer insulator. Blend solutions including both the TIPS pentacene and PS molecules were electrosprayed onto bottom-gate electrodes which were patterned on polyethylene naphthalate films. The devices exhibited superior electrical characteristics due to well-defined vertical phase separation between the TIPS pentacene crystalline layer (top) and PS insulator (bottom). This study will contribute to a facile one-step printing process to produce high-performance flexible OFETs.