Switching Of Poly Research Articles

Effect of side-chain π-π stacking on the thermal conductivity switching in azobenzene polymers: a molecular dynamics simulation study.

The light switchable thermal conductivity displayed by some polymers makes them promising for applications like data storage, temperature regulation and light switchable devices. In this study, the mechanism of thermal conductivity switching in poly[6-(4-phenyldiazenyl phenoxy)hexyl metharylate] is studied using molecular dynamics (MD) simulations. The π-π stacking and amorphous polymer structures are specifically prepared through different simulation procedures, and the thermal conductivity of these structures is calculated. It is found that due to the π-π stacking structure, the thermal conductivity along the side-chain direction can change by 30-50% (from 0.34 to 0.51 W m-1 K-1). Through heat flux decomposition, it is found that the thermal conductivity change is dominated by the contribution from bonding interactions. This is because π-π stacking, which enforces the trans conformation, extends the side-chains of azobenzene polymers, making thermal transport in the side-chain direction more efficient. Along the polymer main-chain direction, the thermal conductivity is not affected by the π-π stacking of the side chains. This mechanism may be generalized to other types of polymers with azobenzene side-chains, which will develop a class of photo-responsive polymers.

Cation exchange behavior during the redox switching of poly (3,4-ethylenedioxythiophene) films

Poly (3,4-ethylenedioxythiophene), PEDOT, films were synthesized at room temperature by potentiodynamic and potentiostatic step deposition in aqueous solutions containing EDOT monomer and LiClO4. In some solutions, the effect of small amounts of sodium dodecylsulfate, SDS, on the polymerization rate of EDOT and on the stiffness of the obtained PEDOT film was studied. The obtained PEDOT films were transferred in aqueous solutions containing cations with different molar mass, such as H+, Li+, Na+, K+, and Cs+. The apparent molar masses of the exchanged species during potentiodynamic experiments were determined by in situ microgravimetry. These measurements underlined the importance of the electrolyte chosen for electropolymerization process. It is known that SDS anions can be trapped inside the polymer layer during electropolymerization, providing them with a cation exchange behavior. However, even if the PEDOT films were deposited from an electrolyte without SDS, they still acted as cation exchangers.

Effect of thiocyanate anions on switching of poly (N-isopropylacrylamide) tethered to nanoparticle surface

<sec>A recent experiment carried by Humphreys et al. (Humphreys B A, Wanless E J, Webber Grant B 2018 <i>J. Colloid Interface Sci</i>. <b>516</b> 153) shows that when poly (N-isopropylacrylamide) (PNIPAM) tethered to nanoparticle surface is immersed in potassium thiocyanate solution, the thiocyanate anions (SCN<sup>–</sup>) can increase the low critical solution temperature (LCST) of the PNIPAM below 500 mmol, though the LCST is reduced when at 1000 mmol. It is unclear why the SCN<sup>–</sup> increases the LCST at low concentration and reduces the LCST at high concentration. In this paper, using a molecular theory, we investigate the effect of SCN<sup>–</sup> on the switching and the structure of PNIPAM tethered to nanoparticle surface. In our model the PNIPAM-SCN<sup>–</sup> bonding (P—S bonds), electrostatic effects and their explicit coupling to the PNIPAM conformations are taken into consideration. We find that under the low SCN<sup>–</sup> concentration, as the SCN<sup>–</sup> concentration increases, the SCN<sup>–</sup> is associated with the PNIPAM chains through the PNIPAM—S bonds, and the PNIPAM segments become negatively charged, which makes electrostatic repulsion stronger and results in an increase in the LCST.</sec><sec>According to our model, the reduction of LCST at high SCN<sup>–</sup> concentration can be explained as follows: with the increase of SCN<sup>–</sup> concentration, more and more PNIPAM-SCN<sup>–</sup> bindings occur between SCN<sup>–</sup> and PNIPAM segments, which will lead the hydrophobicity of PNIPAM chains to increase. On the other hand, the P—S bonds have been filled at the high SCN<sup>–</sup> concentration, and the PNIPAM chains become more negatively charged. The increase of the SCN<sup>–</sup> is accompanied with an increase in the concentration of counterions (K<sup>+</sup>). The increase of counterion concentration will give rise to the counterion-mediated attractive interactions along the chains and electrostatic screening within the negatively charged PNIPAM, thus the LCST can be reduced when further increasing the SCN<sup>–</sup> concentration. The reduction of LCST can be attributed to the increased hydrophobicity of PNIPAM chains, or to the counterion-mediated attractive interaction along the chains and the screening of the electrostatic interactions. </sec><sec>By analyzing the distribution of PNIPAM segments near the critical temperature, we find that the distribution of volume fractions of the PNIPAM tethered to nanoparticle surface shows a maximum when the hydration of PNIPAM and PNIPAM-SCN<sup>–</sup> binding are stronger, which implies that a vertical phase separation may occur. Based on our theoretical model, a vertical phase separation and a two-step phase transition behaviors in the PNIPAM tethered to nanoparticle surface are predicted. We also analyze the height of the PNIPAM, which is a function of temperature at different SCN<sup>–</sup> concentrations, and then obtain the critical temperature of the two-step phase transition. The results show that the vertical phase separation and the two-step phase transition are promoted by competition between hydrophobicity, hydrophilicity and electrostatic effects due to the P—S bonds. Our theoretical results are consistent with the experimental observations, and provide a fundamental understanding of the effects of SCN<sup>–</sup> on the LCST of PNIPAM tethered to nanoparticle surface.</sec>

Ferroelectric control of magnetism in P(VDF–TrFE)/Co heterostructure

Multiferroic magnetoelectric composite systems such as ferroelectric and ferromagnetic heterostructures have attracted an interest recently and have the potential applications such as four state memories, magnetic sensor and spintronics. In this study, we focus on the ferroelectric copolymer/ferromagnetic Co multiferroic film. The ferroelectric copolymer poly(vinylidene fluoride–trifluoroethylene) was fabricated by Langmuir–Blodgett deposition technique. Large magnetization changes of the Co films emerge in response to ferroelectric switching of poly(vinylidene fluoride–trifluoroethylene) controlled by applied electric field. The magnetization of the Co films was detected by the magneto-optical Kerr effect system. Although ferroelectric polymer poly(vinylidene fluoride–trifluoroethylene) have the lower piezoelectric coefficient about 30 pC/N than inorganic ferroelectrics, interface strain coupling is also the primary mechanism altering the induced magnetic anisotropy in the poly(vinylidene fluoride–trifluoroethylene)/Co heterostructure.

Exploring resistive switching in poly(4-vinylphenol)–graphene nano-composite films

In this study the organic resistive switching devices having sandwich structure of indium tin oxide (ITO)-coated glass/poly(4-vinylphenol) (PVP)–graphene composite/silver (Ag) were fabricated and characterized. The active layers were fabricated using blended, semiblend and layer-by-layer approaches, sandwiched between two electrodes. The film thicknesses of the active layers were measured to be about 200 nm. The surface morphology was characterized by field-emission scanning electron microscopy. Electrical current–voltage (I–V) analyses confirmed the memristive behavior of the sandwich devices. The effect of active layer fabrication approach was analyzed by comparing the resistive switching characteristics. The devices showed characteristic OFF to ON (high resistance to low resistance) transition at low voltages, when operated between ±2 V, characterized at 100 µA and 5 mA compliance currents. The memristive behavior of PVP–graphene active layer fabricated by blended approach showed more stability and robustness compared to non-blended approaches. The devices fabricated by blended approach exhibited a room temperature V–I hysteresis and ROFF/RON ≈ 5.

A photonic glucose biosensor for chronic wound prognostics.

The ability to monitor glucose levels in chronic wound fluid of diabetic patients is a promising theranostic approach in chronic wound healing. Phenylboronic acid polymers are glucose- and pH-responsive materials. In the presence of glucose, these polymers reversibly form cyclic boronate esters, changing the properties of the polymer and forming the basis of glucose sensing. In this report, poly(4-vinylphenylboronic acid) (PVPBA) was covalently grafted to the pores of porous silicon (pSi) films (pSi-PVPBA). Polymer switching in response to changing pH and glucose concentration was monitored by means of interferometric reflectance spectroscopy (IRS). We observed that a shift of the boronic acid equilibrium between the neutral and anionic form in the polymer translated into refractive index changes that could be detected as a variation of the effective optical thickness (EOT) of the pSi-PVPBA film. The pSi/polymer composite was further investigated as a platform for the detection of glucose. Using this sensing platform, we were able to detect glucose in a buffer solution as low as 0.15 mM and also in a wound fluid sample without encountering interferences.

Negative differential resistive switching in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) thin film through electrohydrodynamic atomization

Polymeric negative differential resistive (NDR) switching was explored based on the sandwiched structure of indium titanium oxide (ITO) coated polyethyleneterepthalate(PET)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/silver(Ag) through electrohydrodynamic atomization (EHDA) printing technique. The NDR switching in the fabricated device with the structure of ITO/PEDOT:PSS/Ag was analyzed through semiconductor device analyzer under polarity dependent bipolar sweeping voltage of less than \({\pm} 5~\mathrm{ V}\). Effect of the current compliance (CC) in the NDR switching of the fabricated switch has been demonstrated. Multiple resistive switching sweeps were taken to scrutinize the robustness of the fabricated device over 100 cycles. The non-volatility of the as-fabricated device was checked against different time stresses over 2500 s. The switching mechanism is proposed to be due to the transition between PEDOT+ and PEDOT0 chains. The current conduction mechanism involved in the PEDOT:PSS based NDR switches is attributed to the ohmic conduction at lower voltages, while space charge limited conduction and NDR effects were prominent due to the injection of carriers at higher voltages.

The creep process of the domain switching in poly(vinylidene fluoride-trifluoroethylene) ferroelectric thin films

The polarization switching behavior in poly(vinylidene fluoride-trifluoroethylene) thin films is studied by using a pulse transient current method. The dependence of the domain switching current on the coercive electric field was investigated. The charging current around the coercive field was found to be limited by domain switching instead of the series resistor in the measurement circuit because of the slow polarization switching in the films. The domain-switching process was explained by a creep model wherein the two-dimension domain walls motion in the transverse direction dominates the polarization switching process.

Thickening of Non-Ferroelectric Capacitive Layers with Enhanced Domain Switching Speed in Polyvinylidence Fluoride Copolymer Thin Films

The large coercive field of domain switching in poly(vinylidene fluoride-trifluoroethylene) thin films is in much part due to the contribution of nonpolar impurity phases rather than the manifestation of an intrinsic domain switching mechanism of the ferroelectric layer. We coincidentally derive the equivalent electrical capacitance for the total non-ferroelectric capacitive layers from either domain switching current transient or voltage dependence of the switched polarization. Unexpectedly, the non-ferroelectric capacitance reduces by more than 71% with the enhancement of domain switching speed spanning over 5 orders of magnitude in company with the continuous reduction of the remanent polarization, which suggests the thickening of the above capacitive layers with enhanced domain switching speed.

Temperature and acid/base-driven dual switching of poly(N-isopropylacrylamide) hydrogel with Azo dye

A thermoresponsive poly(NIPAM-co-Azo) hydrogel labeled with azo dye was prepared by typical radical copolymerization. The lower critical solution temperature (LCST) behavior was investigated by means of UV-vis spectroscopy which allows the measurement of the phase transition from 25 to 45 °C in aqueous solution. The poly(NIPAM-co-Azo) copolymer also exhibited sizeable color change when used a acid/base-induced molecular switch. This material can act as water soluble dual sensor for both temperature and acid/base.

Multilevel conductance switching in polymer films

Multilevel conductance switching in poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) films is demonstrated. A thin-film structure, ITO-coated glass/MEH-PPV/Al, has shown the ability to store a continuum of conductance states. These states are nonvolatile and can be switched reproducibly by applying appropriate programing biases above a certain threshold voltage. The electrical conductivity of the highest and lowest states can differ by five orders of magnitude. Furthermore, these devices exhibit good cyclic switching characteristics and retention times of several weeks.

Optical detection of ion diffusion in electrochromic poly(3,4-ethylenedioxy)thiophene film using microcantilever electrodes

We present measurements of microscale electrochromic switching of poly(3,4-ethylenedioxy)thiophene doped with poly(4-styrene sulfonate), thin film using microfabricated multi-point probe electrodes. After treatment with a dilute hydrochloric acid, a voltage bias above 3 V with respect to the ground potential of the film induced dark (light-absorbing) rings, which spread out from the anode on a time scale of seconds. The rate of expansion of the rings as well as the final diameter depended on the bias voltage. Using two micro four-point probes simultaneously, we measured with one probe the conductance of the film outside, near and inside a dark ring induced by a voltage applied to another probe and found the resistivity to be directly related to the observed absorbance of the film. The standard electrochromic mechanism of ion insertion was used to explain the observations. We anticipate this experimental approach to provide new insight into ion diffusion in electrochromic films, allowing real-time recording of diffusion dynamics on a microscale to be studied directly.

Metal‐induced optical sensing and optical switching in poly(pyridyl phenylene)

AbstractThe synthesis of a conjugated polymer containing pyridyl units in the polymer backbone was successfully carried out with 2,6‐dibromopyridine and a diboronic acid derivative via the Suzuki coupling reaction. This polymer had a number‐average molecular weight of 4000 and excellent solubility in conventional organic solvents; this suggested the effect of the incorporation of dialkoxy side chains. A metallochromic phenomenon with ferric ions was clearly observed in spin‐coated films and solutions of the polymer. The response of the absorption and the fluorescence changes of the polymer films were rapid and were reproduced more than 15 times. A lightly bound intermolecular interaction between the pyridine ligand and cation was presumed to be responsible for the facile optical switching property of the polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2444–2450, 2004

Thermally stimulated switching in electroactive polymer films

Electronic switching in poly(diphenylenephthalide) due to the thermal ionization of electron traps is studied. The method of thermally stimulated current and the method of thermally stimulated depolarization current are used. A correlation between spectra obtained by the two methods is established. The results are interpreted by using the temperature dependence of IR absorption spectra taken under conditions similar to those under which thermally stimulated phenomena are observed. Comparing data obtained, we assign electron traps to specific molecular groups of the polymer and infer the importance of the surplus space charge critical concentration for the onset of the high-conductivity state in the polymer.

Thickness dependence of ferroelectric polarization switching in poly(vinylidene fluoride–trifluoroethylene) spin cast films

In poly(vinylidene fluoride–trifluoroethylene) copolymer spin cast films, it has been observed that the polarization switching time increases as the film thickness is reduced to below 1 μm. We will show that this change with film thickness can be divided into two thickness regimes, i.e., those above 120–150 nm and those below that thickness. For films thicker than 120–150 nm, the change in the switching behavior is due to interface effects that can be modeled by an effective interface layer with lower dielectric constant that is in series with the film. For films below 120–150 nm thickness, there is an additional and very large increase of the switching time with reduced film thickness. This additional effect is caused by the precipitous drop of the crystallinity in films at this thickness range.

Temperature-controlled ionic conductivity switching in poly[oligo(oxyethylene) methacrylate]/ poly (ethylene oxide) layered film

The present study is aimed to design the temperature-driven switching of the ionic conductivity in polymer electrolytes. First, thermotropic liquid crystalline molecule, 4'-ethoxybenzylidene-4-n-butylaniline (EBBA), was sandwiched with poly(oligo(oxyethylene) methacrylate)(PMEO) containing LiClO 4. A change in the ionic conductivity was observed by heating due to crystal/nematic phase change, but it was observed only in the first heating. On the other hand, the ionic conductivity of PEO sandwiched system showed drastic ionic conductivity jump which was attributed to the phase transition of the PEO. The temperature dependence of the ionic conductivity showed a good reproducibility in this system. The phase transition temperature was affected by the added salt concentration. This was comprehensible as freezing point depression of PEOs. The switching temperature was controllable in the range from −13°C to 52°C by the molecular weight of PEO and added salt concentration in the PEO. The formation of ion pair and aggregate ion was never observed by Raman spectroscopy in the range of salt concentration used in this experiment.

Direct evidence for ferroelectric switching in poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) crystals

Direct evidence for ferroelectric switching in poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) crystals

Switching in Poly(N-vinylcarbazole) Thin Films: Effect of γ-Ray Irradiation and Heat Treatment

The effects of preirradiation with γ-rays or preheat treatment on the switching characteristics of poly(N-vinylcarbazole) (PVCz) films were studied. The dark conductivity and threshold voltage increased when PVCz was irradiated. The magnitude of the effects of irradiation depend on the state of PVCz. PVCz irradiated with γ-rays in a film form showed a higher conductivity and threshold voltage than films cast after irradiation in powder form. Purification of irradiated PVCz by precipitation from a benzene solution reduced the effects of γ-irradiation. Preheat treatment increases crystallinity and conductivity but not the threshold voltage.

Effect of Lamellar Structure on Ferroelectric Switching in Poly (Vinylidene Fluoride)

The solitary-wave model of ferroelectric switching (poling) in $\ensuremath{\beta}$-phase poly(vinylidene fluoride) is developed to allow comparison with recent measurements of the detailed time dependence of this process. This model is found to be consistent with these measurements.

Temperature Effect on Ferroelectric Polarization Switching in Poly(vinylidene fluoride)

Ferroelectric polarization switching experiments were carried out on Form I crystal of poly(vinylidene fluoride) over a temperature range from −76°C to 125°C and at applied electric fields E up to 200 MV m1. At 25°C, notable polarization switching currents which decayed within a few ms were observed at E≥75 MV m−1. These currents became more intensive with increasing applied electric field. The maximum switching current density Jmax and the maximum switching time τmax could be expressed approximately by Jmax (Am−2)=0.72 (E−75) and τmax−1 (s−1)=37(E−75), respectively, as also in the case with common ferroelectric crystals such as BaTiO3 and Rochelle salts. The polarization switching behavior changed remarkably with temperature and could be separated into three parts at about −44°C and 60°C. These temperatures coincide well with the reported β (glass transition) and α (crystalline) relaxation temperatures, respectively. This fact indicates that the polarization switching phenomenon in poly(vinylidene fluoride) has close correlation with molecular chain motions. Finally, molecular motions responsible for the polarization switching in the three temperature regions are discussed.