Showing papers on "Polymer blend published in 2006"

Efficient inverted polymer solar cells

Abstract: We investigate the effect of interfacial buffer layers—vanadium oxide (V2O5) and cesium carbonate (Cs2CO3)—on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester blend. The polarity of solar cells can be controlled by the relative positions of these two interfacial layers. Efficient inverted polymer solar cells were fabricated with the structure of indium tin oxide (ITO)/Cs2CO3/polymer blend/vanadium oxide (V2O5)/aluminum (Al). Short-circuit current of 8.42mA∕cm2, open-circuit voltage of 0.56V, and power conversion efficiency of 2.25% under a AM1.5G 130mW∕cm2 condition were achieved. The interfacial layers were also used to fabricate polymer solar cells using ITO and a thin gold (Au) layer as the transparent electrodes. The thickness of V2O5 layer (10nm) makes it an effective protective layer for the active layer so that ITO can be used for both the electrodes, enabling highly efficient transparent polymer solar cells (i.e., p...

Achieving High-Efficiency Polymer White-Light-Emitting Devices

Abstract: The external electroluminescence (EL) quantum efficiency (QEEL) of a polymer light-emitting diode (PLED) can be affected by the following four factors: a) charge balance, b) the efficiency of producing singlet excitons, c) photoluminescence quantum efficiency (QEPL), and d) the output coupling effect. The QEPL can approach unity and the efficiency of producing singlet excitons can be high in long-chain polymers. Therefore, the dominating factor for achieving high efficiency for a given polymer is the balance and confinement of electrons and holes. Unfortunately, most conjugated polymers have unbalanced charge-transport properties as the hole mobility is much larger than the electron mobility. In this manuscript, we report a general method to significantly increase the efficiency of PLEDs by controlling the charge injection and distribution through material processing and interface engineering in the device. By blending high-bandgap and low-bandgap polymers in proper ratios, we were able to introduce charge traps in the light-emitting polymer (LEP) layer. Similarly, by introducing an electron-injection/hole-blocking layer, we were able to enhance the minority carrier (electron) injection and confine holes to the emissive layer. Efficient and balanced charge injection, as well as charge confinement, are attained simultaneously, and as a result high-efficiency devices can be achieved. This is a simple yet powerful concept in enhancing the overall efficiency of PLEDs. To illustrated our concept, we have blended 0.25–2 % of poly[2-methoxy-5-(2′ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with poly(9,9-dioctylfluorene) (PFO) as the active polymer layer for PLEDs. A Cs2CO3 electron-injection (and hole-blocking) layer is used at the cathode interface. The emission from the device covers colors from white to yellow, depending on the blend ratio, with the highest peak efficiency being 16 lm W. To the best of our knowledge, this is the highest reported efficiency for a white-light emitting PLED. There are several benefits to using a polymer blend: 1) the low-bandgap LEP behaves as a dopant for energy transfer from the higher-bandgap LEP, 2) the low-bandgap LEP behaves as a charge-trapping site to trap (and confine) the injected charges, which is particularly important in the low-voltage regime where only one type of charge is often present, and 3) the trapped electrons in the low-bandgap LEP will eventually help with the injection of holes and lead to self-balanced charge injection. When this LEP blend system is coupled with an electron-injection (and hole-blocking) layer of Ca(acac)2 [4] (acac: acetylacetonate) or Cs2CO3 [5] at the cathode interface, holes are blocked within the LEP layer as well. As a result, both electrons and holes are effectively confined in the LEP layer rather than being extracted directly at the electrodes. Hence, efficient recombination occurs due to the overlapping distribution of electrons and holes (through formation of excitons). All of these factors can help to increase the efficiency of PLED devices. The schematic profile of the energy structure is shown in Figure 1.

Stereocomplexed polylactides (Neo-PLA) as high-performance bio-based polymers: their formation, properties, and application

Abstract: Polymeric materials prepared from renewable natural resources are now being accepted as “bio-based polymers”, because they are superior to the conventional petroleum-based polymers in reducing the emission of carbon dioxide. Among them, poly(L-lactide) (PLLA) prepared by fermentation and polymerization is paid an immediate attention. Although PLLA exhibits a broad range of physico-chemical properties, its thermal and mechanical properties are somewhat poorer for use as ordinary structural materials. For improving these inferior properties, a stereocomplex form consisting of PLLA and its enantiomer poly(D-lactide) (PDLA) has high potential because of showing high melting nature (230 °C). It can be formed by simple polymer blend of PLLA and PDLA or more easily with stereoblock polylactides (sb-PLA) which are PLLA/PDLA block copolymers. These novel PLA polymers, named “Neo-PLA”, can provide a wide range of properties that have never be attained with single PLLA. Neo-PLA retains sustainability or bio-based nature, because both monomers L- and D-lactic acids are manufactured from starch by fermentation. Copyright © 2006 Society of Chemical Industry

Compatibilizing Bulk Polymer Blends by Using Organoclays

Abstract: We have studied the morphology of blends of PS/PMMA, PC/SAN24, and PMMA/EVA and compared the morphologies with and without modified organoclay Cloisite 20A or Cloisite 6A clays. In each case we found a large reduction in domains size and the localization of the clay platelets along the interfaces of the components. The increased miscibility was accompanied in some cases, with the reduction of the system from multiple values of the glass transition temperatures to one. In addition, the modulus of all the systems increased significantly. A model was proposed where it was proposed that in-situ grafts were forming on the clay surfaces during blending and the grafts then had to be localized at the interfaces. This blending mechanism reflects the composition of the blend and is fairly nonspecific. As a result, this may be a promising technology for use in processing recycled blends where the composition is often uncertain and price is of general concern.

The effect of crosslinking on the mechanical properties of polylactic acid/polycaprolactone blends

Abstract: The improvement of the brittle behavior of Polylactic acid (PLA) resin was studied by blending it with Polycaprolactone (PCL) resin. These materials were fabricated into the compressed films and injection moldings. The values of tensile modulus and strength were appropriate, judging from the rule of mixtures. However, the ultimate tensile strain was very small. Dicumyl peroxide (DCP) was added to this blend system to improve its ultimate tensile strain. It was found that the value of ultimate tensile strain peaked at low DCP concentration. The samples at low DCP contents show yield point and ductile behavior under tensile test. The impact strength of the optimum composition was 2.5 times superior to neat PLA, and ductile behavior such as plastic deformation was observed at its fracture surface. It was found that the carbonyl groups of the blend material with DCP were altered by using FTIR spectroscopy. Dynamic mechanical analysis data revealed the dual phase nature of PLA/PCL blend albeit with good interfacial adhesion, and the DCP enhanced the viscous property in PCL phase, which agreed with tensile ductility and impact strength. The mechanical properties of this blend are comparable to those of general purpose HIPS and ABS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1816–1825, 2006

Crystallization, morphology, and mechanical behavior of polylactide/poly(ε‐caprolactone) blends

Abstract: Optically pure polylactides, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), were blended across the range of compositions with poly(e-caprolactone) (PCL) to study their crystallization, morphology, and mechanical behavior. Differential scanning calorimetry and dynamic mechanical analysis (DMA) of the PLA/PCL blends showed two Tgs at positions close to the pure components revealing phase separation. However, a shift in the tan δ peak position by DMA from 64 to 57°C suggests a partial solubility of PCL in the PLA-rich phase. Scanning electron microscopy reveals phase separation and a transition in the phase morphology from spherical to interconnected domains as the equimolar blend approaches from the outermost compositions. The spherulitic growth of both PLA and PCL in the blends was followed by polarized optical microscopy at 140 and 37°C. From tensile tests at speed of 50 mm/min Young's modulus values between 5.2 and 0.4 GPa, strength values between 56 and 12 MPa, and strain at break values between 1 and 400% were obtained varying the blend composition. The viscoelastic properties (E′ and tan δ) obtained at frequency of 1 Hz by DMA are discussed and are found consistent with composition, phase separation, and crystallization behavior of the blends. POLYM. ENG. SCI., 46:1299–1308, 2006. © 2006 Society of Plastics Engineers

Origin of the enhanced performance in poly"3-hexylthiophene…: †6,6‡-phenyl C 61 -butyric acid methyl ester solar cells upon slow drying of the active layer

Abstract: The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester is investigated, combining charge transport measurements with numerical device simulations. Slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10−7m2V−1s−1 in the P3HT phase of the blend, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (∼300nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device.

Formation of a miscible supramolecular polymer blend through self-assembly mediated by a quadruply hydrogen-bonded heterocomplex

Abstract: A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-1,8-naphthyridine (2, DAN), which form an exceptionally strong quadruply hydrogen-bonding complex, are displayed at 1−10 mol % along the main backbone of PBMA and PS, respectively. 1H NMR studies show heterocomplexation between UG and DAN exclusively. This high-fidelity, high-affinity supramolecular connection of two different polymer coils at the molecular level produces a polymer blend. Blends containing different weight ratios of the polymers and mole percent of the recognition units were characterized by AFM and DSC experiments with no isolated domains observed and a single glass-transition temperature (Tg). The Tg is tunable by varying the weight ratio of the polymers in the blend. In addition, viscosity measurements, size-exclusion chromatography (SEC), and dynamic light-scattering (DLS) studies demonstrate the fo...

Highly Efficient Pure-White-Light-Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties†

Abstract: Light-emitting diodes exhibiting efficient pure-white-light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8-naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8-naphthalimide components and optimizing the relative content of 1,8-naphthalimide derivatives in the resulting polymers, white-light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4-ethyleiledioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de I'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11900 cd m(-2), a current efficiency of 3.8 cd A(-1), a power efficiency of 2.0 lm W-1. an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m(-2).

Two calorimetric glass transitions do not necessarily indicate immiscibility: The case of PEO/PMMA

Abstract: Differential scanning calorimetry has been used to examine blends of a poly(ethylene oxide) (PEO), Mn = 300 g/mol, and a poly(methylmethacrylate) (PMMA), Mn = 10,000 g/mol, across the complete composition range. The relatively low molar mass of the PEO minimizes interference from crystallization. In the midrange of composition, ∼25–70% PEO, two broad, but distinct, glass transitions are resolved. These are interpreted as distinct glass transitions of the two components, as anticipated by the self-concentration model of Lodge and McLeish. The composition dependence of the observed transitions is well described by the self-concentration approach, using lengthscales of approximately two-thirds of the Kuhn length. The results are compared with previous measurements on PEO/PMMA blends and other miscible systems. The principal, general conclusion is that one should actually expect two glass transitions in a miscible polymer blend or polymer solution; the rule of thumb that two transitions indicate immiscibility is incorrect. Furthermore, attempts to rationalize two transitions on the basis of incomplete segmental mixing, or other unspecified “nanoheterogeneity,” may not be justified in many cases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 756–763, 2006

Morphology of multi-component polymer systems: single chain in mean field simulation studies

Abstract: Recent work exploring phase separation and self-assembly in multicomponent polymer fluids using a particle-based self-consistent field simulation method is reviewed. The computational method is placed in the context of classical molecular dynamics and Monte Carlo simulations as well as field-theoretic approaches. Its potential is illustrated by applications ranging from spinodal decomposition in symmetric polymer blends and the ordering of diblock copolymers in the bulk to more complex phenomena such as solvent evaporation from thin polymer films and the fabrication of three-dimensional bicontinuous diblock copolymer morphologies via reconstruction on patterned substrates.

Efficient polymer:polymer bulk heterojunction solar cells

Abstract: An organic bulk heterojunction photovoltaic device based on a blend of two conjugated polymers, a polyphenylenevinylene as the electron donor and a red emitting polyfluorene as the acceptor, is presented with a maximum external quantum efficiency of 52% at 530nm and a power conversion efficiency, measured under AM1.5G, 100mW∕cm2 conditions, of 1.5% on an active area of 0.36cm2.

An Overview of Starch-Based Plastic Blends from Reactive Extrusion

Abstract: The North American market for biodegradable plastics in 2005 was estimated to be around 60 million pounds (27 kt). Starch-based polymer blends were expected to account for 30 million pounds (14 kt) with significantly lower growth rates than other biodegradable polymers such as polyesters. The main hurdle in the growth of starch-based products is the thermodynamic immiscibility and non-wetting of starch with other polymers which leads to serious deterioration of mechanical properties at >25-30 wt% starch. Higher amounts of starch in the blends entail adding suitable functional groups on starch and other polymers in the blend to make them more compatible. The primary challenge is to develop fast reaction chemistries that can be transformed into viable processes and integrated into existing process lines with economically viable formulations. This article briefly reviews some of the most promising chemistries available for the reactive extrusion of starch-based polymer blends (biodegradable/non-biodegradable).

Dynamic mechanical and thermal properties of plasticized poly(lactic acid)

Abstract: The blends of low molecular weight triacetin (TAC) and oligomeric poly(1,3-butylene glycol adipate) (PBGA) were used as multiple plasticizers to lubricate poly(lactic acid) (PLA) in this study. The thermal and mechanical properties of plasticized polymers were investigated by means of dynamic mechanical analysis and differential scanning calorimetry. Atomic force microscopy (AFM) was used to analyze the morphologies of the blends. Multiple plasticizers were effective in lowering the glass transition temperature (T-g) and the melting temperature (T-m) of PLA. Moreover, crystallinity of PLA increased with increasing the con-tent of multiple plasticizers. Tensile strength of the blends decreased following the increasing of the plasticizers, but increased in elongation at break. AFM topographic images showed that the multiple plasticizers dispersed between interfibrillar regions. Moreover, the fibrillar crystallite formed the quasicrosslinkings, which is another cause for the increase in elongation at break.