Polymer Mobility Research Articles

Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs.

Poly(vinyl alcohol) (PVA) and LiClO4, a solid polymer electrolyte with a glass transition temperature (Tg) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiClO4 are drifted into place by field-effect at T > Tg, providing n- or p-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is "locked" into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the Tg, and therefore the "locking" temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly(ethylene oxide) (PEO) and LiClO4. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO4 at top gate programming voltages of ±2 V (-6.3 ± 0.03 × 1013 cm-2 for electrons and 1.6 ± 0.3 × 1014 cm-2 for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO4 is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both I-V measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor.

Simulations of Morphology Evolution in Polymer Blends during Light Self-Trapping

Simulations are presented for binary phase morphologies prepared via coupling the self-trapping properties of light with photopolymerization induced phase separation in blends of reactive monomer and inert linear chain polymer. The morphology forming process is simulated based on a spatially varying photopolymerization rate, dictated by self-trapped light, coupled with the Cahn–Hilliard equation that incorporates the free energy of polymer mixing, degree of polymerization, and polymer mobility. Binary phase morphologies form with a structure that spatially correlates to the profile of the self-trapped beam. Attaining this spatial correlation emerges through a balance between the competitive processes entailed in photopolymerization-induced decreases in diffusion mobility and the drive for the blend components to phase separate. The simulations demonstrate the ability for a self-trapped optical beam to direct binary phase morphology along its propagation path. Such studies are important for controlling the...

Formation/Dissolution of Conductive Silver Filaments through an Ionic Liquid/Polymer Electrolyte Thin Film

Materials with reconfigurable optical properties are potential candidates for applications such as optical cloaking and wearable sensors. One approach to develop these materials is to create and destroy atomic-scale conductive channels in well-defined locations within a polymer film via field-effect. By applying an external field to a solid polymer electrolyte, silver filaments can be created and destroyed on demand. However, this approach requires a balance between fast ion mobility for fast filament formation/dissolution, along with robust mechanical properties for a flexible film with high elastic modulus. Unfortunately, the properties that promote fast ion transport (e.g., polymer mobility in a solid polymer electrolyte) also degrade the mechanical properties. To address this challenge, a UV-crosslinkable polymer is combined with an ionic liquid and a silver salt. The crosslinkable polymer provides mechanical structure, while the ionic liquid serves as a non-volatile plasticizer to increase ion transport. Poly(ethylene glycol) diacrylate (PEGDA) is the crosslinkable polymer, 1-butyl-3-methylimidazolium tetrafluoroborate is the ionic liquid, and AgBF4 is the silver salt. Ionic liquids are chemically and thermally stable with negligible vapor pressure, high ionic conductivity, and a wide electrochemical window. 100 nm thick films of 80/20 and 60/40 wt% PEGDA/IL are spin-coated, annealed, and measured by AFM inside an argon-filled glovebox where O2 and H2O are controlled to < 0.1 ppm. Using a conducting AFM tip as one electrode and a silver substrate as the other, automated measurement of hundreds of filament formation/dissolution events are made at predefined locations in a single experimental session. Filament formation occurs at a 3 V forward bias over timescales that range five orders of magnitude (Fig. (a)), while dissolution occurs at a 3 V reverse bias on the timescale of hundreds of milliseconds. As shown in Fig. (a), the formation time distributions for both 60/40 and 80/20 PEGDA/IL samples are bimodal: the fast formation times occur within a few milliseconds while the slow can require several hundreds of seconds. This is likely a consequence of IL-rich and poor regions within the film, as supported by AFM modulus measurements showing regions of high and low elastic modulus (Fig. (b)). Filaments are expected to form faster in regions of low modulus, and slower in regions of high modulus. This explanation is further supported by comparing the samples containing 60 versus 80 wt% PEGDA: the distribution shifts towards increasing formation times with increasing polymer concentration. These results show that the formation/dissolution kinetics can be controlled by controlling the microstructure of the film, and a more homogeneous microstructure will be required to narrow the distribution of formation/dissolution times. Acknowledgement: This worked was supported by the Defense Advanced Research Projects Agency (DARPA) Atoms to Products (A2P) program, grant #FA8650-15-C-7546. Figure 1

An analysis of physical models and numerical schemes for polymer flooding simulations

Chemical-enhanced oil recovery (CEOR) processes are nowadays commonly used by engineers to improve the recovery factor of an oil field. In this paper, we propose to investigate the physical and numerical singularities arising in the numerical simulation of polymer-enhanced oil recovery technique for oil fields. We assume that the polymer is only transported in the water phase or adsorbed on the rock. The polymer reduces the water-phase mobility and can change drastically the behavior of water oil interfaces. Due to its size, the polymer flows faster than water so that an inaccessible pore volume must be added in the model. After a brief description of different models for polymer adsorption, mobility reduction, and inaccessible pore volume, we discuss the mathematical singularities of the obtained system. In the framework of industrial reservoir simulators, we focus on a constant inaccessible pore volume model which may lead to severe mathematical and numerical singularities. We consider an IMPES scheme; we propose approximate Courant-Friedrichs-Levy (CFL) criteria which are required to obtain some numerical stability of the simulation. 1D numerical polymer flooding experiments are computed with a complete reservoir simulator to illustrate the validity of our approximate CFL criteria.

Optimization of polymer flooding design in conglomerate reservoirs

The polymer flooding can be a very promising, cost-effective enhanced oil recovery (EOR) strategy in the development of conglomerate oil reservoirs, especially when the reservoirs reach very high water-cut in production. The key to success for a polymer flooding is to use the full potential of mobility control under the premise of good compatibility between polymer and reservoirs. Polymer is easy to be adsorbed and also potentially clogs the formations especially when the pressure gradient is small in the reservoir. Meanwhile, polymer efficient viscosity has a huge impact on polymer mobility control. The method of polymer flooding optimization design is described in a formation of interest called Karamay conglomerate reservoir (Xinjiang Province, China). We’ve compared the pore structure difference between conglomerates and sandstones, measured hydrodynamic characteristic sizes of polymer molecular and ensured the maximum polymer molecular weight and concentration for corresponding permeability. Then different pore structure cores were used to investigate the rheology of polymer transport in conglomerates and sandstones. A model were built by using the water-oil relative permeability curves and polymer rheology experimental data, which was utilized to calculate the minimum viscosity of the polymer solution required for a high-performance mobility control in polymer flooding. In the study we have found that 1) conglomerate with the shallower depth and tortuous pore structure has about three times shear rates than single pore structure sandstone in the same flow velocity; 2) the viscosity range of a high-performance polymer for conglomerate reservoirs is determined to be 25.1cP−29.4cP at shear rate of 7.34s−1 for Upper Karamay formation, and 8.6cP−12.8cPs at shear rate of 7.34s−1 for Lower Karamay formation. It implies that polymer flooding should be using by injection of polymer solutions in each layer.

Controlling Ordered Structures of PS‐b‐P2VP Block Copolymer Thin Film by Tuning Solvent Evaporation Rate

SummarySelective solvents for each block were chosen to induce specific blocks of the Polystyrene‐block‐poly(2vinylpyridine) (PS‐b‐P2VP) block copolymer (BCP) thin film with the purpose of comparing the degree of block swelling affected by solvent vapors onto the thin film. Solvents with high boiling point have been preferred for this study, because these solvents evaporate slowly, and allowing the block copolymer to self‐assemble properly. Toluene and acetic acid were considered as suitable solvents for polystyrene (PS) and poly(2vinylpyridine) (P2VP), respectively. We prepared 1 wt% PS‐b‐P2VP BCP (79.0k‐b‐36.5k kg/mol) solution in a mixed solvent of toluene and tetrahydrofuran (THF) with a ratio of 7:3. Then, applied solvent annealing to the thin films with toluene and acetic acid at 21 °C for 3 hours, 6 hours and 9 hours. Thermal annealing to the thin films at 80 °C, 100 °C, 150°C, 200 °C and 250 °C for 24 hours was also applied. The morphology of the resulting thin film has been characterized using the atomic force microscopy (AFM). The formation of the block copolymer structure on thin film revealed highly affected by the evaporation rate owing to the decreased polymer mobility occurred by lower temperature of the solvent with high boiling point.

Modulation of the effective viscosity of polymer films by ultraviolet ozone treatment

Nanoconfinement of a polymer film affords a reduction of the glass transition temperature (Tg) and effective viscosity (ηeff). Early on, Prof. Tisato Kajiyama pioneered the idea of enhanced polymer mobility at the free surface. This concept is now well established, and accounts for the Tg and ηeff reductions of thin polymer films. To pay tribute to Prof. Kajiyama's seminal contribution, it is fitting to report in this special issue the use of ultraviolet ozone (UVO) to chemically modify the surface of polymer films and thereby alter their dynamical properties for the first time. Specifically, we show that with a brief exposure time of only 1.0 s under typical UVO treatment conditions, the ηeff of polystyrene (PS) films supported by silica changes from decreasing to rapidly increasing as the film thickness is decreased upon the UVO treatment. As such, the tendency of the films to spontaneously rupture on heating is significantly lessened. Considering the simplicity and inexpensiveness of the method, our results demonstrate that UVO treatment is an attractive method to refine the dynamical properties of polymer films and improve their thermal stability.

Role of Dynamical Heterogeneities on the Mechanical Response of Confined Polymer.

Confinement induces various modifications in the dynamics of polymers as compared to bulk. We focus here on the role of dynamical heterogeneities on the mechanics of confined polymers. Using a simple model that allows computation of the mechanical response over 10decades in frequency, we show that the local mechanical coupling controlling the macroscopic response in the bulk disappears in a confined geometry. The slowest domains significantly contribute to the mechanical response for increasing confinement. As a consequence, the apparent glass transition is broadened and shifted towards lower frequencies as confinement increases. We compare our numerical predictions with experiments performed on poly(ethylacrylate) chains in model filled elastomers. We suggest that the change of elastic coupling between domains induced by confinement should contribute significantly to the polymer mobility shift observed on filled systems.

Phase-Field Simulation of Polymerization-Induced Phase Separation: II. Effect of Volume Fraction and Mobility of Network Polymer

Phase-Field Simulation of Polymerization-Induced Phase Separation: II. Effect of Volume Fraction and Mobility of Network Polymer

Effects of Molecular Weight below the Entanglement Threshold on Interfacial Nanoparticles/Polymer Dynamics

This work deals with effects of polymer molecular weight, Wm, below the entanglement threshold, Wm,e, on molecular dynamics of polydimethylsiloxane (PDMS) adsorbed onto silica particles, employing differential scanning calorimetry (DSC) and two dielectric techniques: broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). The rigid amorphous polymer fraction at interfaces, RAFint, was found suppressed for larger Wm by all techniques in qualitative agreement with each other. Results on RAFint were supported by evaluating, for the first time, the coverage of hydroxyls at the surfaces of nanoparticles by polymer chains (S relaxation). The mobility of interfacial polymer (αint relaxation) was followed by BDS and TSDC, showing suppression of dynamics and cooperativity with decreasing Wm. We suggest that interfacial polymer fraction and dynamics are dominated by the concentration of polymer–particle contact points, the latter increasing for smaller Wm due to more free ch...

Spontaneous bilayer phase separations of spin-coated polymer blend thin films: A neutron reflectivity study

This study investigates the spontaneous formation of phase-separated bilayer structures of polymer blend thin films upon spin coating. Neutron reflectivity is used to measure the bilayer structure of deuterated polystyrene (dPS) and polymethylmethacrylate (PMMA) blend thin films with various molecular weights (M w ). The results indicate that the binary polymer blend forms a well-defined phase-separated bilayer structure of a dPS-rich layer on top of a PMMA rich layer with clear interfaces between the layers. The interfacial width is in reasonable agreement with the results of self-consistent field theory modified to account for capillary waves. The blend with higher M w exhibits broader interfaces than the theoretical predictions, probably due to the reduced mobility of mixed polymers in the solvent during spin coating. Formation of thin blend films with parallel phase separation can provide a viable route to potential applications of the active layer in bilayer heterojunction devices for polymer photovoltaic applications.

Molecular mobility with respect to accessible volume in Monte Carlo lattice model for polymers

Abstract A three-dimensional cubic Monte Carlo lattice model is considered to test the impact of volume on the molecular mobility of amorphous polymers. Assuming classic polymer chain dynamics, the concept of locked volume limiting the accessible volume around the polymer chains is introduced. The polymer mobility is assessed by its ability to explore the entire lattice thanks to reptation motions. When recording the polymer mobility with respect to the lattice accessible volume, a sharp mobility transition is observed as witnessed during glass transition. The model ability to reproduce known actual trends in terms of glass transition with respect to material parameters, is also tested.

Role of Near Substrate and Bulk Polymer Morphology on Out-of-Plane Space-Charge Limited Hole Mobility.

Charge transport is a central issue in all types of organic electronic devices. In organic films, charge transport is crucially limited by film microstructure and the nature of the substrate/organic interface interactions. In this report, we discuss the influence of active layer thickness on space-charge limited hole transport in pristine polymer and polymer/fullerene bulk heterojunction thin films (∼15-300 nm) in a diode structure. According to the results, the out-of-plane hole mobility in pristine polymers is sensitive to the degree of polymer chain aggregation. Blending the polymers with a fullerene molecule does not change the trend of hole mobility if the polymer tends to make an amorphous structure. However, employing an aggregating polymer in a bulk heterojunction blend gives rise to a marked difference in charge carrier transport behavior compared to the pristine polymer and this difference is sensitive to active layer thickness. In aggregating polymer films, the thickness-dependent interchain interaction was found to have direct impact on hole mobility. The thickness-dependent mobility trend was found to correspond well with the trend of fill factors of corresponding bulk heterojunction solar cells. This investigation has a vital implication for material design and the development of efficient organic electronic devices, including solar cells and light-emitting diodes.

Main‐chain alternating fullerene and dye oligomers for organic photovoltaics

AbstractThis work demonstrates for the first time that it is possible to prepare alternating oligomers, containing both dyes and fullerenes in repeating structures, that act as electron acceptors in bulk heterojunction devices. A sterically controlled azomethine ylide cycloaddition polymerization is employed with either C60 or phenyl‐C61‐butyric acid methyl ester (PCBM) and the dye diketopyrrolopyrrole (DPP). The former results in low molecular weights of around 5600 g mol−1, whereas the latter, PCBM, enables the formation of more soluble chains with higher molecular weights of ca 11 200 g mol−1. Remarkably, cyclic voltammetry shows that the incorporation of PCBM into the main‐chain raises the lowest unoccupied molecular orbital by ca 380 meV due to the in‐chain bis‐additions. The observation of the complete quenching of DPP fluorescence by the fullerene moiety, combined with computer modelling studies, indicates both electron and energy transfers between intra‐chain moieties. Proof‐of‐concept devices show low efficiencies most likely due to as‐yet‐unoptimized preparation and structures, but hint at the possibilities of these novel bi‐functionalized, in‐chain fullerenes due to their high Voc of 0.89 V with an example low‐bandgap polymer, KP115, and reasonable charge mobilities of ca 1 × 10−4 cm2 V−1 s−1, making this new class of materials of strong interest for applications. Furthermore, their good thermal stability to above 300 °C and their stabilization of photovoltaic devices against thermal degradation confirm that this new pathway to a wide range of dye/fullerene structures is extremely promising. © 2016 Society of Chemical Industry

Value Addition to Food Industry By-Products and Wastes (Deoiled Rice Bran and Banana Peel) by Optimizing Pellets' Formulation Using Response Surface Methodology: Characterisation and Classification by PCA Approach

This study aimed to optimize the pellets formulation (deoiled rice bran (DOB), banana peel powder (BPP) and plasticizers viz. cashew nut shell liquid (CNSL) and glycerol). The maximum hardness and bulk density as desirable responses were obtained for pellets having 100g DOB, 100g BPP, 12% of CNSL and 12% glycerol. Specific mechanical energy during extrusion was higher (85.565 kJ/kg) for pellets containing CNSL. On the basis of Hausner ratio (1.068-1.070), pellets were classified as free flowing. Pellets with CNSL were found durable and more shelf-stable. Scanning electron micrographs revealed that the intermolecular interactions of pellets containing CNSL were better with smoother surface and minimum voids as compared to pellets containing glycerol. Thermal properties revealed that pellets with glycerol had reduced glass transition temperature as compared to pellets with CNSL owing to higher polymer mobility in glycerol. Significant correlations were observed among physical, mechanical, aerodynamic, and functional properties of pellets. Practical Applications Banana peels are obtained in excess amount which gets accumulated in bulk. Therefore, value addition to banana peels by developing pellets can be a better option. Characterization of the durability (engineering, functional, aerodynamic, thermal and morphological) will provide an insight of the quality attributes of the pellets during various processing operations (handling, transportation, injection molding). These pellets can be further injection molded products into various products e.g., biodegradable planters.

High Performance All-Polymer Solar Cells by Synergistic Effects of Fine-Tuned Crystallinity and Solvent Annealing.

Growing interests have been devoted to the design of polymer acceptors as potential replacement for fullerene derivatives for high-performance all polymer solar cells (all-PSCs). One key factor that is limiting the efficiency of all-PSCs is the low fill factor (FF) (normally <0.65), which is strongly correlated with the mobility and film morphology of polymer:polymer blends. In this work, we find a facile method to modulate the crystallinity of the well-known naphthalene diimide (NDI) based polymer N2200, by replacing a certain amount of bithiophene (2T) units in the N2200 backbone by single thiophene (T) units and synthesizing a series of random polymers PNDI-Tx, where x is the percentage of the single T. The acceptor PNDI-T10 is properly miscible with the low band gap donor polymer PTB7-Th, and the nanostructured blend promotes efficient exciton dissociation and charge transport. Solvent annealing (SA) enables higher hole and electron mobilities, and further suppresses the bimolecular recombination. As expected, the PTB7-Th:PNDI-T10 solar cells attain a high PCE of 7.6%, which is a 2-fold increase compared to that of PTB7-Th:N2200 solar cells. The FF of 0.71 reaches the highest value among all-PSCs to date. Our work demonstrates a rational design for fine-tuned crystallinity of polymer acceptors, and reveals the high potential of all-PSCs through structure and morphology engineering of semicrystalline polymer:polymer blends.

Controlled release of liraglutide using thermogelling polymers in treatment of diabetes.

In treatment of diabetes, it is much desired in clinics and challenging in pharmaceutics and material science to set up a long-acting drug delivery system. This study was aimed at constructing a new delivery system using thermogelling PEG/polyester copolymers. Liraglutide, a fatty acid-modified antidiabetic polypeptide, was selected as the model drug. The thermogelling polymers were presented by poly(ε-caprolactone-co-glycolic acid)-poly(ethylene glycol)-poly(ε-caprolactone-co-glycolic acid) (PCGA-PEG-PCGA) and poly(lactic acid-co-glycolic acid)-poly(ethylene glycol)-poly(lactic acid-co-glycolic acid) (PLGA-PEG-PLGA). Both the copolymers were soluble in water, and their concentrated solutions underwent temperature-induced sol-gel transitions. The drug-loaded polymer solutions were injectable at room temperature and gelled in situ at body temperature. Particularly, the liraglutide-loaded PCGA-PEG-PCGA thermogel formulation exhibited a sustained drug release manner over one week in both in vitro and in vivo tests. This feature was attributed to the combined effects of an appropriate drug/polymer interaction and a high chain mobility of the carrier polymer, which facilitated the sustained diffusion of drug out of the thermogel. Finally, a single subcutaneous injection of this formulation showed a remarkably improved glucose tolerance of mice for one week. Hence, the present study not only developed a promising long-acting antidiabetic formulation, but also put forward a combined strategy for controlled delivery of polypeptide.

Nanosecond-time-scale reversibility of dilation induced by carbon dioxide sorption in glassy polymer membranes

Many models have been proposed to describe the sorption isotherms of small gases in glassy polymers at equilibrium. However, the sorption kinetics to reach this equilibrium are dependent on the nature of the penetrant and the swelling of the matrices. In the present work, large-scale molecular dynamics (MD) simulations are used to describe CO2 sorption-induced dilation kinetics at very short time scales in 6FDA-6FpDA polyimide membranes. The latter are subjected to two cycles of CO2 sorption and post-degassing relaxation, which mimick the sorption-degassing experiments with large integral pressure steps. The glassy membranes dilate during sorption, mainly because of an increase in the longitudinal strain. Upon degassing, they systematically relax to a metastable state, with some residual extra-volume and improved solubility for the penetrant, which depends on the swelling behavior history. These subtle changes agree with the qualitative model based on variations in the relative fractions of Fickian and relaxational dilation sites as a function of the swelling history. Both sorptions also exhibit differences at very short times, with a enhanced void-space and concentration of CO2 sorbed along with a lower polymer mobility for the conditioned one, even if competitive sorption counterbalances the expected increase in gas mobility.

Local orientational mobility in regular hyperbranched polymers.

We study the dynamics of local bond orientation in regular hyperbranched polymers modeled by Vicsek fractals. The local dynamics is investigated through the temporal autocorrelation functions of single bonds and the corresponding relaxation forms of the complex dielectric susceptibility. We show that the dynamic behavior of single segments depends on their remoteness from the periphery rather than on the size of the whole macromolecule. Remarkably, the dynamics of the core segments (which are most remote from the periphery) shows a scaling behavior that differs from the dynamics obtained after structural average. We analyze the most relevant processes of single segment motion and provide an analytic approximation for the corresponding relaxation times. Furthermore, we describe an iterative method to calculate the orientational dynamics in the case of very large macromolecular sizes.

Quantum propagation of electronic excitations in macromolecules: A computationally efficient multiscale approach

We introduce a theoretical approach to study the quantum-dissipative dynamics of electronic excitations in macromolecules, which enables to perform calculations in large systems and cover long time intervals. All the parameters of the underlying microscopic Hamiltonian are obtained from \emph{ab-initio} electronic structure calculations, ensuring chemical detail. In the short-time regime, the theory is solvable using a diagrammatic perturbation theory, enabling analytic insight. To compute the time evolution of the density matrix at intermediate times, typically $\lesssim$~ps, we develop a Monte Carlo algorithm free from any sign or phase problem, hence computationally efficient. Finally, the dynamics in the long-time and large-distance limit can be studied combining the microscopic calculations with renormalization group techniques to define a rigorous low-resolution effective theory. We benchmark our Monte Carlo algorithm against the results obtained in perturbation theory and using a semi-classical non-perturbative scheme. Then we apply it to compute the intra-chain charge mobility in a realistic conjugate polymer.