GEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 2 2005
D.E. SUGDEN
ABSTRACT. This paper examines new geomorphological, chronological and modelling data on glacier fluctuations in southernmost South America in latitudes 46,55°S during the last glacial,interglacial transition. Establishing leads and lags between the northern and southern hemispheres and between southern mid-latitudes and Antarctica is key to an appreciation of the mechanisms and resilience of global climate. This is particularly important in the southern hemisphere where there is a paucity of empirical data. The overall structure of the last glacial cycle in Patagonia has a northern hemisphere signal. Glaciers reached or approached their Last Glacial Maxima on two or more occasions at 25,23 ka (calendar) and there was a third less extensive advance at 17.5 ka. Deglaciation occurred in two steps at 17.5 ka and at 11.4 ka. This structure is the same as that recognized in the northern hemisphere and taking place in spite of glacier advances occurring at a time of high southern hemisphere summer insolation and deglaciation at a time of decreasing summer insolation. The implication is that at orbital time scales the,northern' signal dominates any southern hemisphere signal. During deglaciation, at a millennial scale, the glacier fluctuations mirror an antiphase 'southern' climatic signal as revealed in Antarctic ice cores. There is a glacier advance coincident with the Antarctic Cold Reversal at 15.3,12.2 ka. Furthermore, deglaciation begins in the middle of the Younger Dryas. The implication is that, during the last glacial,interglacial transition, southernmost South America was under the influence of sea surface temperatures, sea ice and southern westerlies responding to conditions in the 'southern' Antarctic domain. Such asynchrony may reflect a situation whereby, during deglaciation, the world is more sensitized to fluctuations in the oceanic thermohaline circulation, perhaps related to the bipolar seesaw, than at orbital timescales. [source]

EXTRUSION COOKING OF BLENDS OF SOY FLOUR AND SWEET POTATO FLOUR ON SPECIFIC MECHANICAL ENERGY (SME), EXTRUDATE TEMPERATURE AND TORQUE

JOURNAL OF FOOD PROCESSING AND PRESERVATION, Issue 4 2001
M. O. IWE
Defatted soy flour and sweet potato flour containing 18% moisture were mixed in a pilot mixer, and extruded in an Almex-Bettenfeld single-screw extruder operated at varying rotational speed and die diameter. A central composite, rotatable nearly orthogonal design, which required 23 experiments for three factors (feed composition (fc), screw speed (ss) and die diameter (dd)) was developed and used for the generation of response surfaces. Effects of the extrusion variables on specific mechanical energy (SME), extrudate temperature (ET), and torque (T) were evaluated using response surface analysis. Results showed that product temperature increased with increases in die diameter, screw speed and feed composition. However, the effect of die diameter was greater than those of screw speed and feed composition. Decrease in die diameter with increase in sweet potato content increased torque. Screw speed exhibited a linear effect on torque. [source]

RHEOLOGY OF WHEAT DOUGHS FOR FRESH PASTA PRODUCTION: INFLUENCE OF SEMOLINA-FLOUR BLENDS AND SALT CONTENT

JOURNAL OF TEXTURE STUDIES, Issue 2 2000
DONATELLA PERESSINI
ABSTRACT Dynamic measurements were made with a controlled stress rheometer to study the viscoelastic properties (G', G', ,) of wheat doughs (45% wb water content) for fresh pasta production prepared with different blends of durum wheat semolina and common wheat flour with different concentrations of sodium chloride. Increasing the semolina and sodium chloride content, increased the strength and the solid-like behaviour of semolina-flour blends. The physical properties of dough were strongly dependent on particle size distribution and salt addition. By manipulating semolina-flour ratio and ionic strength, it was possible to obtain semolina-flour doughs with a rheological behaviour close to that of pure semolina dough. [source]

Measurement of Charge-Density Dependence of Carrier Mobility in an Organic Semiconductor Blend

ADVANCED FUNCTIONAL MATERIALS, Issue 5 2010
Christopher G. Shuttle
Abstract Here, a new methodology for analyzing the charge-density dependence of carrier mobility in organic semiconductors, applicable to the low-charge-density regime (1014,1017,cm,3) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility µ is found to depend on charge density n according to a power law µ(n) , n,, where ,,=,0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra-band states. [source]

Rheological Investigation of Shear Induced-Mixing and Shear Induced-Demixing for Polystyrene/Poly(vinyl methyl ether) Blend

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 9 2004
Samy A. Madbouly
Abstract Full Paper: The phase behavior of polystyrene (PS) and poly(vinyl methyl ether) (PVME) blend has been investigated rheologically as a function of temperature, composition and oscillating shear rate as well as different heating rates. An LCST (lower critical solution temperature)-type phase diagram was detected rheologically from the sudden changes in the slopes of the dynamic temperature ramps of G, at given heating and shear rate values. The rheological cloud points were dependent on the heating rate, , and oscillating shear rate, . The cloud points shifted a few degrees to higher temperatures with increasing and reached an equilibrium value (heating rate independent) at ,°C/min. The phase diagrams of the blends detected at ,=,0.1 and 1 rad/s were located in lower temperature ranges than the quiescent phase diagram, i.e., oscillating shear rate induced-demixing at these two values for the shear rate. On the other hand, at ,=,10 rad/s, the phase diagram shifted to higher temperatures, higher than the corresponding values found under quiescent conditions, i.e., shear induced-mixing took place. Based on these two observations, shear induced-demixing and shear induced-mixing can be detected rheologically within a single composition at low and high shear rate values, respectively, and this is in good agreement with the previous investigation using simple shear flow techniques. In addition, the William, Landel and Ferry (WLF)-superposition principle was found to be applicable only in the single-phase regime; however, the principle broke-down at a temperature higher than or equal to the cloud point. Furthermore, different spinodal phase diagrams were estimated at different oscillating shear rates based on the theoretical approach of Ajji and Choplin. Spinodal phase diagrams at different oscillating shear rates. [source]

Effect of Organic Modification on the Compatibilization Efficiency of Clay in an Immiscible Polymer Blend

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 20 2005
Suprakas Sinha Ray
Abstract Summary: This communication describes the effect of organic modifier miscibility with the matrices, and the effect of the initial interlayer spacing of the organoclay, on the overall morphology and properties of an immiscible polycarbonate/poly(methyl methacrylate) blend. By varying the organic-modifier-specific interactions with the blend matrices at the same time as changing the initial interlayer spacing of the organoclay, different levels of compatibilization were revealed. The evidence for the interfacial compatibilization of the organoclay was assessed by scanning electron microscopy observations and was supported by differential scanning calorimetry analyses. The effect on the level of clay exfoliation was also examined. Differential scanning calorimetry scans of virgin, montmorillonite, and various organically modified montmorillonite-compatibilized 40PC/60PMMA blends [source]

The Effects of the Modified Starches on the Melting Flow and Biodegradation of the Starch/Glycerol Blend

MACROMOLECULAR SYMPOSIA, Issue 1 2008
Chia-I Liu
Abstract Summary: The purpose of this study was to improve the melting flow of starch/glycerol(GA) blends by modified starches. A variety of modified starches, which was treated by hydrolysis and acid hydrolysis, with and without ultrasonic treatment were used. The MFI (melt flow index) of blends increased from 0.5g/10min to 300g/10min when the addition of acid hydrolysis starch (0.3M CA-starch) was 70wt%. Their crystalline behaviors were analyzed by XRD results. The ultrasonic treatment has been proved to have the effect of hydrolysis without acids and synergistic effect on recrystalline. The SEM micrographs of the blend with the ultrasonic treatment starch gave the cleaving surface with comparison to the other blends. The weight loss of the blends with acid hydrolysis starches reached to 60,80% after one week biodegradation as the ultrasonic treatment was used. [source]

Antithrombogenic Polynitrosated Polyester/Poly(methyl methacrylate) Blend for the Coating of Blood-Contacting Surfaces

ARTIFICIAL ORGANS, Issue 4 2008
Amedea B. Seabra
Abstract:, A nitric oxide (NO) donor polyester containing multiple S-nitrosothiol (S-NO) groups covalently attached to the polymer backbone was synthesized through the esterification of poly(ethylene glycol) with mercaptosuccinic acid, followed by the nitrosation of the ,SH moieties. The polynitrosated polyester (PNPE) obtained was blended with poly(methyl methacrylate) (PMMA), yielding solid films capable of releasing NO. Scanning electron microscopy analysis showed that acrylic plates and stainless steel intracoronary stents can be coated with continuous and adherent PNPE/PMMA films. After an initial NO burst, these films release NO spontaneously in dry condition or immersed in aqueous solution at constant rates of 1.8 and 180 nmol/g/h, respectively, for more than 24 h at physiological temperature. PNPE/PMMA coated surfaces were shown to inhibit platelet adhesion when in contact with whole blood. These results show that PNPE/PMMA blend can be used for the coating of blood-contacting surfaces, with potential to inhibit thrombosis and restenosis after stenting. [source]

Perpendicularly Aligned, Size-and Spacing-Controlled Nanocylinders by Molecular-Weight Adjustment of a Homopolymer Blended in an Asymmetric Triblock Copolymer

ADVANCED FUNCTIONAL MATERIALS, Issue 15 2006
U. Ahn
Abstract Perpendicularly arrayed and size-controlled nanocylinders have been prepared by simply blending an asymmetric polystyrene- block -polyisoprene- block -polystyrene triblock copolymer with polystyrene (the minority component) homopolymers of different molecular weights. The preference for perpendicular orientation or hexagonal ordering of the nanocylinders over a large area in the asymmetric block copolymer can be controlled by adjusting the molecular weight of the blended homopolymer, and the perfection of hexagonal ordering of the perpendicular cylinders can be tuned by using a substrate whose surface tension is much different from that of the majority component of the block copolymer. Such highly controlled nanostructured block-copolymer materials, which have been obtained by a simple method independent of film thickness and interfacial tension between the blocks and the substrates, have wide-ranging commercial potential, e.g., for use in membranes and nanotemplates with size-tunable pores, bandgap-controlled photonic crystals, and other nanotechnological fields demanding a specific nanosize and nanomorphology. [source]

Aggregation of Fillers Blended into Random Elastomeric Networks: Theory and Comparison with Experiments

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 16 2006
Mustafa M. Demir
Abstract Summary: A theoretical model describing aggregation of filler particles in amorphous elastomers is proposed. The model is based on a counting technique originally used in genome analysis to characterize the size and distribution of overlapping segments randomly placed on a DNA molecule. In the present model, the particles are first assumed to aggregate randomly upon mixing into the elastomer and their sizes are calculated. The sizes and distributions of aggregates are also studied in the presence of attractive interparticle forces. Results of the proposed model are compared with experimental data on silica-filled end-linked poly(dimethylsiloxane) networks. Comparison of the theory and experiment shows that the random aggregation assumption where no attractive forces exist between the particles is not valid and a significant attraction between the silica particles is needed in the theory to justify the experimental data obtained using atomic force microscopy. For filler content below 1.45 vol.-%, the model agrees, qualitatively, with experiment and shows the increase in cluster size with increasing amount of filler. It also explains the increase in the dispersion of aggregate sizes with increasing amount of filler. Clustering of the primary silica particles in an imaginary volume of poly(dimethylsiloxane) network. [source]

Evaluation of the stability of blends of sunflower and rice bran oil

EUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 5 2007
Samia Mezouari
Abstract Blends of sunflower oil (SFO) and rice bran oil (RBO) were evaluated for their stability. Additionally, known amounts of natural antioxidants extracted from RBO were added to SFO, and their protective effect was compared to that of the blends. The results found indicate that by raising the amount of RBO, from 10 to 50%, an increase of OLO, OLP, PPL, OOO, PPO, OPO, 18:1 and 16:0 occurred, followed by a decrease of LLL, LLO, and 18:2. These changes in fatty acid and triacylglycerol (TAG) composition led to an increase of the oil stability index at 120,°C and a reduction of polymer TAG formation in the heated blends at 180,°C during 8,h. A comparable protective effect of natural antioxidants to that of blending was observed in a 50,:,50 blend, by remarkably increasing the induction period. [source]

Interaction of Osteoblasts with Macroporous Scaffolds Made of PLLA/PCL Blends Modified with Collagen and Hydroxyapatite,

ADVANCED ENGINEERING MATERIALS, Issue 8 2009
Halil Murat Aydin
To mimic natural bone, a tissue engineering scaffold was developed that combines inorganic and organic components of natural bone, its pore diameter, and its interconnected structure. Collagen was coated onto a PLLA/PCL scaffold and hydroxyapatite particles were delivered throughout the polymer matrix much more easily than with other techniques thanks to the porosity-forming method of combining two porogens, namely, salt leaching and supercritical CO2 extraction. Compared with other coating techniques, this procedure can be performed readily and homogeneous 3D hydroxyapatite coating was achieved. [source]

Porous Structures: In situ Porous Structures: A Unique Polymer Erosion Mechanism in Biodegradable Dipeptide-Based Polyphosphazene and Polyester Blends Producing Matrices for Regenerative Engineering (Adv. Funct.

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2010
Mater.
Abstract Synthetic biodegradable polymers serve as temporary substrates that accommodate cell infiltration and tissue in-growth in regenerative medicine. To allow tissue in-growth and nutrient transport, traditional three-dimensional (3D) scaffolds must be prefabricated with an interconnected porous structure. Here we demonstrated for the first time a unique polymer erosion process through which polymer matrices evolve from a solid coherent film to an assemblage of microspheres with an interconnected 3D porous structure. This polymer system was developed on the highly versatile platform of polyphosphazene-polyester blends. Co-substituting a polyphosphazene backbone with both hydrophilic glycylglycine dipeptide and hydrophobic 4-phenylphenoxy group generated a polymer with strong hydrogen bonding capacity. Rapid hydrolysis of the polyester component permitted the formation of 3D void space filled with self-assembled polyphosphazene spheres. Characterization of such self-assembled porous structures revealed macropores (10,100 ,m) between spheres as well as micro- and nanopores on the sphere surface. A similar degradation pattern was confirmed in vivo using a rat subcutaneous implantation model. 12 weeks of implantation resulted in an interconnected porous structure with 82,87% porosity. Cell infiltration and collagen tissue in-growth between microspheres observed by histology confirmed the formation of an in situ 3D interconnected porous structure. It was determined that the in situ porous structure resulted from unique hydrogen bonding in the blend promoting a three-stage degradation mechanism. The robust tissue in-growth of this dynamic pore forming scaffold attests to the utility of this system as a new strategy in regenerative medicine for developing solid matrices that balance degradation with tissue formation. [source]

In situ Porous Structures: A Unique Polymer Erosion Mechanism in Biodegradable Dipeptide-Based Polyphosphazene and Polyester Blends Producing Matrices for Regenerative Engineering

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2010
Meng Deng
Abstract Synthetic biodegradable polymers serve as temporary substrates that accommodate cell infiltration and tissue in-growth in regenerative medicine. To allow tissue in-growth and nutrient transport, traditional three-dimensional (3D) scaffolds must be prefabricated with an interconnected porous structure. Here a unique polymer erosion process through which polymer matrices evolve from a solid coherent film to an assemblage of microspheres with an interconnected 3D porous structure is demonstrated for the first time. This polymer system is developed on the highly versatile platform of polyphosphazene-polyester blends. Co-substituting a polyphosphazene backbone with both hydrophilic glycylglycine dipeptide and hydrophobic 4-phenylphenoxy group generates a polymer with strong hydrogen bonding capacity. Rapid hydrolysis of the polyester component permits the formation of 3D void space filled with self-assembled polyphosphazene spheres. Characterization of such self-assembled porous structures reveals macropores (10,100 ,m) between spheres as well as micro- and nanopores on the sphere surface. A similar degradation pattern is confirmed In vivo using a rat subcutaneous implantation model. 12 weeks of implantation results in an interconnected porous structure with 82,87% porosity. Cell infiltration and collagen tissue in-growth between microspheres observed by histology confirms the formation of an in situ 3D interconnected porous structure. It is determined that the in situ porous structure results from unique hydrogen bonding in the blend promoting a three-stage degradation mechanism. The robust tissue in-growth of this dynamic pore forming scaffold attests to the utility of this system as a new strategy in regenerative medicine for developing solid matrices that balance degradation with tissue formation. [source]

Ultrathin, Organic, Semiconductor/Polymer Blends by Scanning Corona-Discharge Coating for High-Performance Organic Thin-Film Transistors

ADVANCED FUNCTIONAL MATERIALS, Issue 17 2010
Hee Joon Jung
Abstract A new thin-film coating process, scanning corona-discharge coating (SCDC), to fabricate ultrathin tri-isopropylsilylethynyl pentacene (TIPS-PEN)/amorphous-polymer blend layers suitable for high-performance, bottom-gate, organic thin-film transistors (OTFTs) is described. The method is based on utilizing the electrodynamic flow of gas molecules that are corona-discharged at a sharp metallic tip under a high voltage and subsequently directed towards a bottom electrode. With the static movement of the bottom electrode, on which a blend solution of TIPS-PEN and an amorphous polymer is deposited, SCDC provides an efficient route to produce uniform blend films with thicknesses of less than one hundred nanometers, in which the TIPS-PEN and the amorphous polymer are vertically phase-separated into a bilayered structure with a single-crystalline nature of the TIPS-PEN. A bottom-gate field-effect transistor with a blend layer of TIPS-PEN/polystyrene (PS) (90/10 wt%) operated at ambient conditions, for example, indeed exhibits a highly reliable device performance with a field-effect mobility of approximately 0.23 cm2 V,1 s,1: two orders of magnitude greater than that of a spin-coated blend film. SCDC also turns out to be applicable to other amorphous polymers, such as poly(, -methyl styrene) and poly(methyl methacrylate) and, readily combined with the conventional transfer-printing technique, gives rise to micropatterned arrays of TIPS-PEN/polymer films. [source]

Triplet Formation in Fullerene Multi-Adduct Blends for Organic Solar Cells and Its Influence on Device Performance

ADVANCED FUNCTIONAL MATERIALS, Issue 16 2010
Clare Dyer-Smith
Abstract In organic solar cells, high open circuit voltages may be obtained by choosing materials with a high offset between the donor highest occupied molecular orbital (HOMO) and acceptor lowest unoccupied molecular orbital (LUMO). However, increasing this energy offset can also lead to photophysical processes that compete with charge separation. In this paper the formation of triplet states is addressed in blends of polyfluorene polymers with a series of PCBM multi-adducts. Specifically, it is demonstrated that the formation of such triplets occurs when the offset energy between donor ionization potential and acceptor electron affinity is ,1.6 eV or greater. Spectroscopic measurements support a mechanism of resonance energy transfer for triplet formation, influenced by the energy levels of the materials, but also demonstrate that the competition between processes at the donor,acceptor interface is strongly influenced by morphology. [source]

Integration of Density Multiplication in the Formation of Device-Oriented Structures by Directed Assembly of Block Copolymer,Homopolymer Blends

ADVANCED FUNCTIONAL MATERIALS, Issue 8 2010
Guoliang Liu
Abstract Non-regular, device-oriented structures can be directed to assemble on chemically nanopatterned surfaces such that the density of features in the assembled pattern is multiplied by a factor of two or more compared to the chemical pattern. By blending the block copolymers with homopolymers and designing the chemical pattern rationally, complicated structures such as bends, jogs, junctions, terminations, and combined structures are fabricated. Previously, directed assembly of block copolymers has been shown to enhance the resolution of lithographic processes for hexagonal arrays of spots and parallel lines, corresponding to the bulk morphologies of block copolymer systems, but this is the first demonstration of enhanced resolution for more complicated, device-oriented features. This fundamental knowledge broadens the range of technologies that can be served by the directed assembly of block copolymers. [source]

Solar Cell Blends: High-Resolution Spectroscopic Mapping of the Chemical Contrast from Nanometer Domains in P3HT:PCBM Organic Blend Films for Solar-Cell Applications (Adv. Funct.

ADVANCED FUNCTIONAL MATERIALS, Issue 3 2010
Mater.
The inside cover image presents a near-field spectroscopic map of a P3HT:PCBM solar cell blend film obtaining using a parabolic-mirror-assisted optical microscope. This technique allows for simultaneous recording of both morphological and spectroscopic information, and on page 492 Xiao Wang et al. use it to investigate local molecular distribution, photoluminescence quenching efficiency, and other key properties of the P3HT:PCBM blend film. [source]

White Electroluminescence by Supramolecular Control of Energy Transfer in Blends of Organic-Soluble Encapsulated Polyfluorenes

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2010
Sergio Brovelli
Abstract Here, it is demonstrated that energy transfer in a blend of semiconducting polymers can be strongly reduced by non-covalent encapsulation of one constituent, ensured by threading of the conjugated strands into functionalized cyclodextrins. Such macrocycles control the minimum intermolecular distance of chromophores with similar alignment, at the nanoscale, and therefore the relevant energy transfer rates, thus enabling fabrication of white-light-emitting diodes (CIE coordinates: x,=,0.282, y,=,0.336). In particular, white electroluminescence in a binary blend of a blue-emitting, organic-soluble rotaxane based on a polyfluorene derivative and the green-emitting poly(9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) is achieved. Morphological and structural analyses by atomic force microscopy, fluorescence mapping, µ-Raman, and fluorescence lifetime microscopy are used to complement optical and electroluminescence characterization, and to enable a deeper insight into the properties of the novel blend. [source]

SPEEK/Polyimide Blends for Proton Conductive MembranesPresented at the 1st CARISMA Conference, Progress MEA 2008, La Grande Motte, 21st,24th September 2008.

FUEL CELLS, Issue 4 2009
H. Maab
Abstract A series of membranes, based on sulphonated poly(ether ether ketone) (SPEEK)/polyimide (PI) blends, was prepared at different casting conditions. They were characterised by SEM, FTIR, DMTA, DSC, TGA, water/methanol pervaporation and impedance spectroscopy. The membranes prepared at 130,°C from blends with 10, 20 and 30,wt.-% of PI are homogeneous, and the methanol permeabilities decreased from 28,×,10,10,kg,m,s,1,m,2 (plain SPEEK) to 7.21, 2.61 and 0.55,×,10,10,kg,m,s,1,m,2, respectively. This corresponds to a 4- to 57-fold methanol crossover reduction. With this improvement, by the introduction of PI, the power density of SPEEK-based membranes in DMFC tests could be greatly improved. [source]

Charge Transfer Excitons in Polymer/Fullerene Blends: The Role of Morphology and Polymer Chain Conformation

ADVANCED FUNCTIONAL MATERIALS, Issue 22 2009
Markus Hallermann
Abstract Here, it is shown how carrier recombination through charge transfer excitons between conjugated polymers and fullerene molecules is mainly controlled by the intrachain conformation of the polymer, and to a limited extent by the mesoscopic morphology of the blend. This experimental result is obtained by combining near-infrared photoluminescence spectroscopy and transmission electron microscopy, which are sensitive to charge transfer exciton emission and morphology, respectively. The photoluminescence intensity of the charge transfer exciton is correlated to the degree of intrachain order of the polymer, highlighting an important aspect for understanding and limiting carrier recombination in organic photovoltaics. [source]

Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends

ADVANCED FUNCTIONAL MATERIALS, Issue 19 2009
Christopher R. McNeill
Abstract Here, studies on the evolution of photophysics and device performance with annealing of blends of poly(3-hexylthiophene) with the two polyfluorene copolymers poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,,2,,-diyl) (F8TBT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100,°C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140,°C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing-incidence small-angle X-ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140,°C-annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as-spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200,°C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize. [source]

Tunable Injection Barrier in Organic Resistive Switches Based on Phase-Separated Ferroelectric,Semiconductor Blends,

ADVANCED FUNCTIONAL MATERIALS, Issue 19 2009
Kamal Asadi
Abstract Organic non-volatile resistive bistable diodes based on phase-separated blends of ferroelectric and semiconducting polymers are fabricated. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor,electrode contact and, hence, the resistance of the comprising diodes. Comparison between the on- and off-current of the switching diodes, with the current measured for semiconductor-only diodes reveals that the switching occurs between bulk-limited, i.e., space-charge-limited, and injection-limited current transport. By deliberately varying the HOMO energy of the semiconductor and the work-function of the metal electrode, it is demonstrated that injection barriers up to 1.6,eV can be surmounted by the ferroelectric polarization yielding on/off current modulations of more than five orders of magnitude. The exponential dependence of the current modulation with a slope of 0.25,eV/decade is rationalized by the magnitude of the injection barrier. [source]

The Energy of Charge-Transfer States in Electron Donor,Acceptor Blends: Insight into the Energy Losses in Organic Solar Cells

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2009
Dirk Veldman
Abstract Here, a general experimental method to determine the energy ECT of intermolecular charge-transfer (CT) states in electron donor,acceptor (D,A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D,A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open-circuit voltage (Voc) of photovoltaic devices in D,A blends via eVoc,=,ECT,,,0.5,eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1,,,CT) occurs when Eg,,,ECT,>,0.1,eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT,,,T1) can occur when ECT,,,ET,>,0.1,eV. From these relations, it is concluded that in D,A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc,=,Eg,,,0.6,eV) and ii) the singlet,triplet energy gap should be ,EST,<,0.2,eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area. [source]

Scanning Kelvin Probe Microscopy on Bulk Heterojunction Polymer Blends

ADVANCED FUNCTIONAL MATERIALS, Issue 9 2009
Klára Maturová
Abstract Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub-100,nm resolution on the phase-separated active layer of polymer-fullerene (MDMO-PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport. [source]

Vertical Phase Separation in Poly(3-hexylthiophene): Fullerene Derivative Blends and its Advantage for Inverted Structure Solar Cells

ADVANCED FUNCTIONAL MATERIALS, Issue 8 2009
Zheng Xu
Abstract A method which enables the investigation of the buried interfaces without altering the properties of the polymer films is used to study vertical phase separation of spin-coated poly(3-hexylthiophene) (P3HT):fullerene derivative blends. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis reveals the P3HT enrichment at the free (air) surfaces and abundance of fullerene derivatives at the organic/substrate interfaces. The vertical phase separation is attributed to the surface energy difference of the components and their interactions with the substrates. This inhomogeneous distribution of the donor and acceptor components significantly affects photovoltaic device performance and makes the inverted device structure a promising choice. [source]

Silicone,Poly(hexylthiophene) Blends as Elastomers with Enhanced Electromechanical Transduction Properties,

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008
F. Carpi
Abstract Dielectric elastomers are progressively emerging as one of the best-performing classes of electroactive polymers for electromechanical transduction. They are used for actuation devices driven by the so-called Maxwell stress effect. At present, the need for high-driving electric fields limits the use of these transduction materials in some areas of potential application, especially in the case of biomedical disciplines. A reduction of the driving fields may be achieved with new elastomers offering intrinsically superior electromechanical properties. So far, most attempts in this direction have been focused on the development of composites between elastomer matrixes and high-permittivity ceramic fillers, yielding limited results. In this work, a different approach was adopted for increasing the electromechanical response of a common type of dielectric elastomer. The technique consisted in blending, rather than loading, the elastomer (poly(dimethylsiloxane)) with a highly polarizable conjugated polymer (undoped poly(3-hexylthiophene)). The resulting material was characterised by dielectric spectroscopy, scanning electron microscopy, tensile mechanical analysis, and electromechanical transduction tests. Very low percentages (1,6 wt %) of poly(3-hexylthiophene) yielded both an increase of the relative dielectric permittivity and an unexpected reduction of the tensile elastic modulus. Both these factors synergetically contributed to a remarkable increase of the electromechanical response, which reached a maximum at 1 wt % content of conjugated polymer. Estimations based on a simple linear model were compared with the experimental electromechanical data and a good agreement was found up to 1 wt %. This approach may lead to the development of new types of materials suitable for several types of applications requiring elastomers with improved electromechanical properties. [source]

Phase Segregation in Thin Films of Conjugated Polyrotaxane, Poly(ethylene oxide) Blends: A Scanning Force Microscopy Study,

ADVANCED FUNCTIONAL MATERIALS, Issue 6 2007
L. Sardone
Abstract Scanning force microscopy (SFM) is used to study the surface morphology of spin-coated thin films of the ion-transport polymer poly(ethylene oxide) (PEO) blended with either cyclodextrin (CD)-threaded conjugated polyrotaxanes based on poly(4,4,-diphenylene-vinylene) (PDV), ,-CD,PDV, or their uninsulated PDV analogues. Both the polyrotaxanes and their blends with PEO are of interest as active materials in light-emitting devices. The SFM analysis of the blended films supported on mica and on indium tin oxide (ITO) reveals in both cases a morphology that reflects the substrate topography on the (sub-)micrometer scale and is characterized by an absence of the surface structure that is usually associated with phase segregation. This observation confirms a good miscibility of the two hydrophilic components, when deposited by using spin-coating, as suggested by the luminescence data on devices and thin films. Clear evidence of phase segregation is instead found when blending PEO with a new organic-soluble conjugated polymer such as a silylated poly(fluorene)- alt -poly(para -phenylene) based polyrotaxane (THS,,-CD,PF,PPP). The results obtained are relevant to the understanding of the factors influencing the interfacial and the intermolecular interactions with a view to optimizing the performance of light-emitting diodes, and light-emitting electrochemical cells based on supramolecularly engineered organic polymers. [source]

Hybrid Solar Cells from Regioregular Polythiophene and ZnO Nanoparticles,

ADVANCED FUNCTIONAL MATERIALS, Issue 8 2006

Abstract Blends of nanocrystalline zinc oxide nanoparticles (nc-ZnO) and regioregular poly(3-hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer,metal oxide bulk-heterojunction solar cells. Thermal annealing of the spin-cast films significantly improves the solar-energy conversion efficiency of these hybrid solar cells to ,,0.9,%. Photoluminescence and photoinduced absorption spectroscopy demonstrate that charge-carrier generation is not quantitative, because a fraction of P3HT appears not to be in contact with or in close proximity to ZnO. The coarse morphology of the films, also identified by tapping-mode atomic force microscopy, likely limits the device performance. [source]

Generation of Compositional-Gradient Structures in Biodegradable, Immiscible, Polymer Blends by Intermolecular Hydrogen-Bonding Interactions,

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2005
B. Hexig
Abstract A biodegradable, immiscible poly(butylenes adipate- co -butylenes terephthalate) [P(BA- co -BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film-thickness direction has been successfully prepared in the presence of a low-molecular-weight compound 4,4,-thiodiphenal (TDP), which is used as a miscibility-enhancing agent. The miscibilities of the P(BA- co -BT)/PEO/TDP ternary blend films and the P(BA- co -BT)/PEO/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA- co -BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier-transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient-phase structures in immiscible polymer-blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers. [source]