Miscible Blends Research Articles

Insights into molecular engineering of membranes based on fluorinated polyimide-polyamide miscible blends which do not obey the trade-off rule

This study focuses on the blending technique as a successful approach to improve gas separation performance of a series of polyimide (PI) - polyamide (PA) miscible blends by breaking the trade-off rule. Also, it brings an insight into the physico-chemical characteristics of these blends which were discussed in correlation with the polymer blend composition and structural elements of each component. Thus, the blends were analyzed with respect to their miscibility, morphology, crystallinity, optical transparency, dielectric and mechanical properties. The dielectric spectroscopy and dynamo-mechanical analysis were used to evidence the overall and local chain mobility and their influence on primary and secondary relaxations. The dielectric constant and conductivity of polymer blends were found to be notably higher when compared to the reference PI, increasing with the PA quantity. The gas permeation behavior of the blends was evaluated with respect to the influence of PA content on performances. It was found that gas permeability increases with the rise of PA component, following the simple mixing rule with negative deviation for the highest PA content. The enhanced permeability was obtained with a minimum loss of selectivity. The blend with 50 wt% of PA offered the best option of increased permeability and preserved selectivity.

Influence of ultrasound irradiation on the intrinsic viscosity of guar gum–PEG/rosin glycerol ester nanoparticles

Novel Guar Gum (GG) and polyethylene glycol (PEG) polymer blends with rosin glycerol ester (RE) nanoparticle was synthesized under ultrasonic irradiation at different composition ratios (1:1, 1:2, 1:4, 2:1, and 4:1). The intrinsic viscosities of the nanoparticles were investigated using ultrasound irradiation to determine the miscibility of the blends in solution as affected by salt, sonication time, temperature, and pH. The intrinsic viscosities of the nanosystems were compared with five different models, including Huggins, Kraemer, Tanglertpaibul-Rao, Higiro, and Rao. The Tanglertpaibul-Rao was the best model and the intrinsic viscosities of 138,27, 142,94 and 163,29 dl/g were reported for GG–PEG/RE (1:1,1:2,1:4), respectively. The viscosity results reveal that the blend containing 1:2 (GG–PEG/RE) was an optimum miscible blend. The miscibility behaviour of the polymeric nanoparticles was investigated using the voluminosity (VE), shape factor (υ), creaming index (CI) parameter, and the Krigbaum and Wall parameter (Δb), which account for the intermolecular interactions. When compared to the intrinsic viscosity results of the nanoparticles, the miscibility-improving effect of sub-300 nm GG-PEG/RE nanoparticles is clearly proven due to the ultrasonic effect. Scanning transmission electron microscopy (STEM) and Fourier transformed infrared (FTIR) spectroscopy were used for characterization of the polymeric nanoparticles.

Bio-Based PBT-DLA Copolyester as an Alternative Compatibilizer of PP/PBT Blends.

The aim of this work was to assess whether synthesized random copolyester, poly(butylene terephthalate-r-butylene dilinoleate) (PBT–DLA), containing bio-based components, can effectively compatibilize polypropylene/poly(butylene terephthalate) (PP/PBT) blends. For comparison, a commercial petrochemical triblock copolymer, poly(styrene-b-ethylene/butylene-b-styrene) (SEBS) was used. The chemical structure and block distribution of PBT–DLA was determined using nuclear magnetic resonance spectroscopy and gel permeation chromatography. PP/PBT blends with different mass ratios were prepared via twin-screw extrusion with 5 wt% of each compatibilizer. Thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis were used to assess changes in phase structure of PP/PBT blends. Static tensile testing demonstrated marked improvement in elongation at break, to ~18% and ~21% for PBT–DLA and SEBS, respectively. Importantly, the morphology of PP/PBT blends compatibilized with PBT–DLA copolymer showed that it is able to act as interphase modifier, being preferentially located at the interface. Therefore, we conclude that by using polycondensation and monomers from renewable resources, it is possible to obtain copolymers that efficiently modify blend miscibility, offering an alternative to widely used, rubber-like petrochemical styrene compatibilizers.

Pore Structure and Properties of PEEK Hollow Fiber Membranes: Influence of the Phase Structure Evolution of PEEK/PEI Composite.

Poly(ether ether ketone) (PEEK) hollow fiber membranes were successfully prepared from miscible blends of PEEK and polyetherimide (PEI) via thermally-induced phase separation (TIPS) with subsequent extraction of the PEI diluent. The phase structure evolution, extraction kinetics, membrane morphology, pore size distribution and permeability for the hollow fiber membrane were studied in detail. Extraction experiments, differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMA) studies showed that the heat treatment had a significant influence on the two-phase structure of PEEK/PEI, and that it was controlled by the crystallization kinetic of PEEK and the diffusion kinetic of PEI. As the annealing temperature increased, the controlling factor of the phase separation changed from PEEK crystallization to PEI diffusion, and the main distribution of the amorphous PEI chains were changed from the interlamellar region to the interfibrillar or interspherulitic regions of PEEK crystallization. When the annealing temperature increased from 240 °C to 280 °C, the extracted amount of PEI increased from 85.19 to 96.24 wt %, and the pore diameter of PEEK membrane increased from 10.59 to 37.85 nm, while the surface area of the PEEK membrane decreased from 111.9 to 83.69 m2/g. Moreover, the water flux of the PEEK hollow fiber membranes increased from 1.91 × 10−2 to 1.65 × 10−1 L h−1 m−2 bar−1 as the annealing temperature increased from 240 °C to 270 °C. The structure and properties of the PEEK hollow fiber membrane can be effectively controlled by regulating heat treatment conditions.

Exploring low-k dielectrics as structuring polymers for solid-state electrolyte-gated transistors

Abstract In this paper, we report the effect of using low-k polymer dielectrics as structuring polymers to prepare solid-state electrolyte insulators (SEGIs) for organic field-effect transistors (OFETs). Here, we use poly(3-hexylthiophene-2,5-diyl) (P3HT) as a semiconductor and explore poly(methyl methacrylate) (PMMA) and ethyl cellulose (ETC) polymer dielectrics as bulk structuring polymers for forming the SEGIs. Among the structuring polymers, PMMA-based SEGIs show favorable blend miscibility, exhibiting capacitance values of 20.61 ± 1.45 nFcm−2 at 1 Hz, whereas neat PMMA capacitors recorded 13.15 ± 0.55 nFcm−2. We achieve field-effect mobilities of 0.49 ± 0.06 and 0.05 ± 0.01 cm2 V−1s−1 in PMMA SEGI-2 and neat PMMA OFETs, respectively, operating at 10 V. The OFETs based on the PMMA SEGIs exhibit a relatively smaller hysteresis as compared to that of the ETC SEGI-gated OFETs. A comprehensive analysis of the PMMA SEGI devices using the transfer line method show a reduced contact resistance close to two orders of magnitude compared to that of the neat PMMA OFETs. This study provides a significant insight into the development of high-performance solid-state electrolyte-gated transistors by the selection and consideration of appropriate structuring polymers.

Confinement and localization effects revealed for thin films of the miscible blend poly(vinyl methyl ether)/polystyrene with a composition of 25/75 wt%⋆.

Thin films (200-7nm) of the asymmetric polymer blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) (25/75wt%) were investigated by broadband dielectric spectroscopy (BDS). Thicker samples ([Formula: see text]37 nm) were measured by crossed electrode capacitors (CEC), where the film is capped between Al-electrodes. For thinner films ([Formula: see text]37 nm) nanostructured capacitors (NSC) were employed, allowing one free surface in the film. The dielectric spectra of the thick films showed three relaxation processes ( [Formula: see text] -, [Formula: see text] - and [Formula: see text] -relaxation), like the bulk, related to PVME fluctuations in local spatial regions with different PS concentrations. The thickness dependence of the [Formula: see text] -process for films measured by CECs proved a spatially heterogeneous structure across the film with a PS-adsorption at the Al-electrodes. On the contrary, for the films measured by NSCs a PVME segregation at the free surface was found, resulting in faster dynamics, compared to the CECs. Moreover, for the thinnest films ([Formula: see text]26 nm) an additional relaxation process was detected. It was assigned to restricted fluctuations of PVME segments within the loosely bounded part of the adsorbed layer, proving that for NSCs a PVME enrichment takes place also at the polymer/substrate interface.

Reactive blending of bisphenol-A polycarbonate with isosorbide-based polycarbonates: Effect of chain flexibility and compatibility

In this study the effect of flexible 1,4-cyclohexanedimethanol (CHDM) units on the reactive blending of poly(isosorbide-co-CHDM carbonate) (IcC-PC) with bisphenol-A polycarbonate (BPA-PC) was investigated and the contributions of chain flexibility and compatibility to transesterification were clarified. FTIR analysis revealed the presence of specific interactions between the CHDM segments of IcC-PC and the carbonyl group and phenyl rings of BPA-PC, leading to thermodynamic miscibility of the polymer blends when the CHDM content was higher than 50 mol% in IcC-PC. Addition of organotin catalysts during melt blending triggered transesterification between IcC-PC and BPA-PC, transforming the opaque blends into transparent alloys with a single Tg when the transesterification ratio reached a critical value. It was found that the reactivity of isosorbide (ISB) segments was higher than that of CHDM segments. The increase in the CHDM/ISB ratio first accelerated and then slowed down the transesterification, demonstrating that, despite of the low reactivity of the CHDM segments, the enhanced chain flexibility and compatibility were effective for promoting transesterification. Anyway, copolymerization of CHDM with ISB improved the compatibility of IcC-PC with BPA-PC, reduced the critical transesterification ratio for transparency, and remarkably shortened the blending time essential for the large-scale fabrication of transparent ISB-based polycarbonate alloys by twin-screw reactive extrusion.

Glass Transition Behaviors of Poly (Vinyl Pyridine)/Poly (Vinyl Phenol) Revisited.

We examined the composition and molecular weight dependence of the glass transition temperature in detail for two types of hydrogen bonding miscible blends: poly (2-vinyl pyridine)/poly (vinyl phenol) (2VPy/VPh) and poly (4-vinyl pyridine)/poly (vinyl phenol) (4VPy/VPh). Regarding the functional form of the glass transition temperature, Tg, as a function of the weight fraction, we found a weak deviation from the Kwei equation for 2VPy/VPh blends. In contrast, such a deviation was not observed for the 4VPy/VPh blend. By relating the difference in the functional forms of Tg between the two blend systems to the difference in hydrogen bonding ability, we proposed a modified version of the Kwei equation. As for the interaction parameter, q in the Kwei equation, clear molecular weight dependence was observed for 2VPy/VPh blends: the lower the VPh molecular weight in the oligomer level, the higher the q values, suggesting the higher hydrogen bonding formability near the polymer chain ends than the middle part of a polymer chain.

Nanofibrous vascular scaffold prepared from miscible polymer blend with heparin/stromal cell-derived factor-1 alpha for enhancing anticoagulation and endothelialization

There is an urgent clinical demand to develop a functional small diameter vascular graft with long-term patency, which mainly relies on the anticoagulation and endothelialization of the vascular graft. In addition, improved degradation of vascular graft provides more space for cell infiltration and facilitates remodeling of blood vessel. In this study, an elastic and biomimetic nanofibrous vascular scaffold with improved degradability was prepared by adding poly(lactic-co-glycolic acid) (PLGA) into the poly(L-lactic acid) (PLLA)/poly(L-lactide-co-ε-caprolactone) (PLCL) blend using thermally induced phase separation (TIPS) technique. The incorporation of PLGA also improved the hydrophilicity of the composite scaffold. Then the vascular scaffold was surface modified by combination of heparin and stromal cell-derived factor-1 alpha (SDF-1α). The results of whole blood clotting kinetics and plasma recalcification profiles indicated that heparinized modification significantly enhanced the anticoagulation of vascular scaffold. In vitro cell culture assays demonstrated the immobilized SDF-1α facilitated recruitment of endothelial progenitor cells (EPCs), migration and proliferation of mature endothelial cells, human umbilical vein endothelial cells (HUVECs), and expression of VE-cadherin/CD144 and endothelial nitric oxide synthase (eNOS) genes in HUVECs, thereby accelerating the endothelialization of the modified vascular scaffold. Besides, the nanofibrous vascular scaffold modified with heparin and SDF-1α inhibited the proliferation of human vascular smooth muscle cells (HVSMCs). Therefore, the phase separated nanofibrous vascular scaffold modified with heparin and SDF-1α shows the promising in vitro performance as a functional small diameter vascular graft.

Crystallization behavior of plasticized poly(lactide) film by poly(l-lactic acid)-poly(ethylene glycol)-poly(l-lactic acid) triblock copolymer

Poly(l-lactic acid)-poly(ethylene glycol)-poly(l-lactic acid) triblock copolymer (PLLA-PEG-PLLA) prepared by simple esterification of l-lactic acid (l-LA) and diol-polyethylene glycol (diol-PEG) is proposed as an effective plasticizer for poly(lactide) film (PLA). Differential scanning calorimetry and X-ray diffraction analyses reveal improvement in PLA/PLLA-PEG-PLLA blend miscibility without PEG segment recrystallization in the PLA matrix. A comparative study on the performance of PLLA-PEG-PLLA with two different PLLA chain lengths clarifies that long PLLA-PEG-PLLA with balanced block lengths of PLLA and PEG segments performs a semicrystalline structure of PLA crystal without PEG-crystal formation, resulting in blend miscibility. This microstructure clearly induces the perfect α-form of the PLA crystal with increased degree of crystallinity to 50%. Additionally, the sufficiently long PLLA segment in triblock copolymer can get entangled with PLA chains, allowing to enhance melt strength of the PLA/long PLLA-PEG-PLLA blend. In contrast, short PLLA-PEG-PLLA is amorphous, obstructing the formation of the highly ordered PLA packing structure in the blend, and contributing to poor melt strength and mechanical weakness. Furthermore, the PLA/long PLLA-PEG-PLLA blend improves toughness with increased elongation at break to 80% while maintaining the relatively high tensile strength, as compared to that of PLA/PEG or PLA/short PLLA-PEG-PLLA films at the same blending weight ratio.

Nanoscale Restructuring of Polymer Materials to Produce Single Polymer Composites and Miscible Blends.

I summarize work conducted in our laboratories over the past 30 years using small host molecules to restructure polymer materials at the nanometer level. Certain small molecules, such as the cyclic starches cyclodextrins (CDs) and urea (U) can form non-covalent crystalline inclusion compounds (ICs) with a range of guest molecules, including many polymers. In polymer-CD- and -U-ICs, guest polymer chains reside in narrow channels created by the host molecule crystals, where they are separated and highly extended. When the host crystalline lattice is carefully removed, the guest polymer chains coalesce into a bulk sample with an organization that is distinct from that normally produced from its melt or from solution. Amorphous regions of such coalesced polymer samples have a greater density, likely with less chain entanglement and more chain alignment. As a consequence, after cooling from their melts, coalesced amorphous polymers show glass-transition temperatures (Tgs) that are elevated above those of samples prepared from their solutions or melts. Upon cooling from their melts, coalesced samples of crystallizable polymers show dramatically-increased abilities to crystallize more rapidly and much closer to their melting temperatures (Tms). These unique behaviors of polymers coalesced from their CD- and U-ICs are unexpectedly resistant to extended annealing above their Tgs and Tms. Taking advantage of this behavior permits us to create polymer materials with unique and improved properties. Among these are amorphous polymers with elevated Tgs and semi-crystalline polymers with finer more uniform morphologies. Improved mechanical properties can be achieved through self-nucleation with small amounts of the same polymer made rapidly crystallizable through coalescence from its CD- or U-IC. This can lead to single polymer composites with as-received polymer matrices and self-nucleated reinforcements. Through simultaneous formation and subsequent coalescence from their common CD–ICs, stable well-mixed blends can be achieved between any two or more polymers, despite their inherent immiscibilities. Such coalesced and well-mixed blends are also resistant to phase segregation when heated for extensive periods well above their Tgs and Tms.

Fabrication of topologically anisotropic microparticles and their surface modification with pH responsive polymer brush.

This paper describes the fabrication of topologically anisotropic cup shaped polylactide (PLA)/poly[methyl methacrylate‑co‑2‑(2‑bromopropionyloxy) ethyl methacrylate] (poly(MMA-co-BEMA)) (75/25) composite particles of ~6 μm size using electrojetting technique. An attempt was made to understand the mechanism of cup shape formation from the miscible blend by electrojetting. Both the solution parameters and the processing conditions affected the particles' shape which can be varied from cup shaped to discoid type. Surface initiated atom transfer radical polymerization (ATRP) of stimuli responsive DMAEMA (2‑dimethylamino ethyl methacrylate) was subsequently carried out for 1 h onto the surface of cup shaped particles to observe pH responsiveness of the modified anisotropic particles. Interestingly, morphology of the cup shaped particles was changed to elongated cup which did show significant swelling under acidic pH (swelling ratio:~1.6) and enhanced dye adsorption at specific pH as observed by optical microscope and confocal laser scanning microscope implying that DMAEMA polymerization happened onto the surface of the composite microparticles. The Raman microscopy and FTIR spectra obtained from the particles after polymerization further confirmed the immobilization of pH responsive poly(DMAEMA) brushes onto the cup shaped particles which may potentially function as triggered/targeted drug delivery vehicles. Moreover, the brush modified cup shaped particles were found to be two times more efficient in adsorbing dye compared to disc shaped one indicating a clear advantage of using cup shaped particles over other shapes for immobilizing/adsorbing charged species e.g. sensitive biomolecules.

Fabrication and characterization of exfoliated chitosan–gelatin–montmorillonite nanocomposite nanofibers

Polymeric nanofibers as one of the most known nanotechnology products have huge potential applications in many fields due to their high aspect ratio and porosity, being capable of formation of three-dimensional structures and having great mechanical and biological properties. Chitosan is a natural abundant polymer which has attracted huge interests in biomedical and biological industries due to its biocompatible, biodegradable, and non-toxicity properties. However, electrospinning of chitosan is found to be a great challenge, blending it with other polymers such as gelatin was explored as means to improve the morphological deficiencies of chitosan nanofibers and facilitate its electrospinnability. On the other hand, montmorillonite (MMT) has been attracted great attention due to its remarkable improvement in properties of polymeric composites nanofibers. The main objective of this work was on effect of concretion of gelatin–chitosan blends and MMT on morphology of resulted nanocomposite nanofibers. The x-ray diffraction data demonstrated the exfoliation of MMT layers. The morphology of electrospun chiosan–gelatin–MMT composite nanofibers was characterized using scanning electron microscope (SEM). The miscibility of blend was determined using SEM and Fourier transform infrared spectrometer/attenuated total reflectance.

Effects of silanization and modification treatments on the stiffness and toughness of BF/SEBS/PA6,6 hybrid composites

To develop basalt fiber (BF)-reinforced PA6,6 composites with an optimal balance between stiffness and toughness, the effects of fiber surface treatments and the addition of rubber elastomers on the microstructural characteristics of these composites was investigated and their effects on their physico-mechanical properties was thoroughly characterized. It was confirmed that amine-silanized BF/SEBS-g-MAH/PA6,6 ternary composites have a high chemical affinity and that the constituent components are highly compatible. In contrast, MAH-silanized BF/SBBS-c-NH2/PA6,6 ternary composites shows that the fiber and rubber phases disperse separately, thereby enhancing both the stiffness and toughness of the composites individually. These two composite systems had both similarities and differences in terms of their mechanical properties, such as the strength, stiffness, toughness, and viscoelastic behavior, based on their microstructural characteristics. Compared with BF/PA6,6, the impact strength of the amine-silanized BF/SEBS-g-MAH/PA6,6 ternary composites was significantly improved, by 57.7%, but the tensile and flexural strengths decreased. Despite forming a miscible blend, the addition of SEBS-g-MAH only contributed to improving the toughness of the composites at the expense of stiffness. The tensile and impact strengths of the MAH-silanized BF/SBBS-c-NH2/PA6,6 ternary composites increased by 6.4% and 23.8%, respectively, thus demonstrating an improved balance between their stiffness and toughness. These results suggest that MAH-silanized BF/SBBS-c-NH2/PA6,6 ternary composites may be attractive materials for industrial applications because the silanized BFs and SEBS elastomers provide complementary reinforcement, thus efficiently improving the composites’ stiffness and toughness.

Multi-scale investigation on the phase miscibility of polylactic acid/o-carboxymethyl chitosan blends

Absract Polylactic acid (PLA), due to its high mechanical strength, biocompatibility, excellent biodegradability and good processing characteristics, can be used to print the bone scaffold with complex structures, but the poor cell affinity and inflammation resulted by the acidity of intermediate produce still pose a challenge. To address these issues, o-carboxymethyl chitosan (CMC) was used to modify PLA, and the phase miscibility of PLA/CMC blends with different component ratios was investigated by a multi-scale method. Here, molecular simulation and experimental methods were applied to predict the miscibility of polylactic acid and o-carboxymethyl chitosan blends. Results indicated that PLA and CMC are miscible when the concentration of CMC in blends below 20 wt%, and partially miscible at 30 wt% concentration of CMC, but completely immiscible at higher CMC content (>30 wt%). Conclusions from this work can be used to forecast the miscibility of PLA/CMC composites at different compositions and can provide a scientific basis for formula designs of PLA/CMC polymer composites for 3D printing artificial bone scaffolds.

Impact of Side Chains of Conjugated Polymers on Electronic Structure: A Case Study.

Processing from solution is a crucial aspect of organic semiconductors, as it is at the heart of the promise of easy and inexpensive manufacturing of devices. Introducing alkyl side chains is an approach often used to increase solubility and enhance miscibility in blends. The influence of these side chains on the electronic structure, although highly important for a detailed understanding of the structure-function relationship of these materials, is still barely understood. Here, we use time-resolved electron paramagnetic resonance spectroscopy with its molecular resolution to investigate the role of alkyl side chains on the polymer PCDTBT and a series of its building blocks with increasing length. Comparing our results to the non-hexylated compounds allows us to distinguish four different factors determining exciton delocalization. Detailed quantum-chemical calculations (DFT) allows us to further interpret our spectroscopic data and to relate our findings to the molecular geometry. Alkylation generally leads to more localized excitons, most prominent only for the polymer. Furthermore, singlet excitons are more delocalized than the corresponding triplet excitons, despite the larger dihedral angles within the backbone found for the singlet-state geometries. Our results show TREPR spectroscopy of triplet excitons to be well suited for investigating crucial aspects of the structure-function relationship of conjugated polymers used as organic semiconductors on a molecular basis.

Influence of long chain branching on fiber diameter distribution for polypropylene nonwovens produced by melt blown process

In this work, linear isotactic polypropylene (L-PP) and long-chain branched polypropylene (LCB-PP) miscible blend, both having comparable weight average molecular weight, zero-shear viscosity, and polydispersity index, were used to produce nonwovens via melt blown technology in order to understand the role of long chain branching in the fiber diameter distribution. Basic morphological characteristics of produced nonwoven samples have been determined using digital image analysis of scanning electron microscope images considering different magnifications to capture nanofibers as well as microfibers. At the same air flow rate, polymer flow rate, and temperature, the average fiber diameters were the same, 1.6 μm, but the coefficient of variation, CV, was greater for the linear PP than for the blend. Material elasticity was assessed by reptation-mode relaxation time, λ, determined by fitting of deformation rate dependent shear viscosity by Cross and Carreau-Yasuda models as well as via fitting of frequency dependent loss and storage moduli master curve by a two-mode Maxwell model. It was found that λ is higher for LCB-PP in comparison with L-PP and the Cross model gives a meaningful relaxation time while the Carreau-Yasuda model does not despite giving a better numerical fit. Extensional rheology was assessed by the strain rate dependent uniaxial extensional viscosity (estimated from the entrance pressure drop using the Gibson method). The infinite shear to zero-shear shear viscosity ratio η∞/η0 (obtained directly from the shear viscosity data measured in a very wide shear rate range) was shown to be proportional to the maximum normalized extensional viscosity at very high extensional strain rates, ηE,∞/(3η0). η∞/η0 was related to temperature and basic molecular characteristics of given polymers via simple equation. It was observed that extensional viscosity for both samples first decreases with increased extensional strain rate to its minimum value at 200 000–400 000 1/s and then increases to plateau value, ηE,∞ (corresponding to the maximum chain stretch) at about 2 ⋅ 106 1/s. At low deformation rates, extensional viscosity is higher for LCB-PP in comparison with L-PP, but the trend is switched at very high deformation rates; ηE,∞ (and also ηE,∞/3η0) becomes lower for LCB-PP in comparison with L-PP. These results suggest that high stability of LCB-PP blend can be explained by its higher stretchability at very high deformation rates (occurring at the die exit where an intensive fiber attenuation takes the place) and its lower stretchability at medium and low deformation rates, at which melt/air inertia driven bending instability called whipping occurs.

On the microexplosion mechanisms of burning droplets blended with biodiesel and alcohol

Microexplosions are reported to occur frequently in droplet combustion of biodiesel mixed with alcohol, which are nominally miscible. We found, however, that they could be prevented if the standing time of fuel samples before burning was sufficiently long. To realize the microexplosion mechanisms, experiments based on suspending droplets and free-falling droplets were conducted. It was found that, before ignition, heterogeneous sites, which appeared similar to small drops, emerged from the droplet surface and subsequently merged into a bigger one, which then deposited at the bottom of the droplet. The diameter of this heterogeneous drop enlarged with increasing ambient humidity. At the same humidity, by using methanol, ethanol and isopropanol, respectively, the experiments showed that the merged heterogeneous drop of biodiesel/methanol was the largest and the smallest one was generated by that mixed with isopropanol. These heterogeneous sites were likely caused by the hygroscopic characteristics of alcohol fuels, which were supposed to mix well with biodiesel fuel and treated as a miscible blend in the first place. The miscibility, however, was destroyed by the absorbed water and phase separation occurred, leading to microexplosions when the droplet was burned. On the other hand, the interior heterogeneous drop vanished after sufficiently long time of standing prior to burning, and consequently no microexplosion was observed during the whole combustion process. For the mixture fuel of alcohol, methanol had higher propensity to absorb water and needed longer standing time for dissolution of the heterogeneous drop, as compared to ethanol and isopropanol, which had lower hygroscopicity. From the results, the heterogeneous sites inside the blended droplet were concluded unambiguously to be the primary cause of microexplosions during the combustion process, leading to the highest burning rate at equi-volume proportion of mixed fuels. In contrast, elimination of the heterogeneous sites in the burning droplets could prohibit microexplosions and yielded the highest burning rate at 25% fraction of alcohol.

Green polymeric materials: On the dynamic homogeneity and miscibility of furan-based polyester blends

Polyesters from 2,5-furandicarboxylic acid are considered as biobased alternatives to their terephthalate counterparts. This work is focused on polymer blends comprised of furan-based polyesters differing by one or two methylene groups in their repeat units. Blends of Poly(ethylene 2,5-furandicarboxylate) (PEF) and poly(propylene 2,5-furandicarboxylate) (PPF) displayed a single composition-dependent glass temperature (Tg). Dielectric spectroscopy results revealed dynamically homogeneous mixtures when the backbones differ by a single methylene unit and dynamically heterogeneous mixtures when they differ by two methylene units. The most interesting finding was the single segmental dynamics in the PEF-PPF and poly(butylene furanoate)-poly(propylene furanoate) (PBF-PPF) blends. For comparison, homologous terephthalate blends were also investigated. Finally, reactive blending was applied in PEF-PBF blends and enhanced compatibility was achieved even after short times due to transesterification reactions at elevated temperatures. With increasing duration of melt-mixing the thermodynamic miscibility between PEF and PBF increased and ultimately a random copolymer was obtained.

Polymer Electrolyte Blend Gate Dielectrics for High-Performance Ultrathin Organic Transistors: Toward Favorable Polymer Blend Miscibility and Reliability.

We report on systematic mobility enhancements in electrolyte-gated organic field-effect transistors (OFETs) by thinning down the active layer and exploiting polymer solid-state electrolyte gate insulators (SEGIs). The SEGI is composed of homogeneous poly(vinylidene fluoride- co-hexafluoropropylene) [P(VDF-HFP)] polymer solution-ion gel blends of high areal capacitance of >10 μF cm-2 at 1 Hz. By scaling up the poly(3-hexylthiophene) (P3HT) semiconducting layer by 1 order of magnitude (5-50 nm), an ultraviolet photoelectron spectroscopy examination reveals a downward vacuum-level shift generating a substantial hole injection barrier that originates from different interfacial dipole layer formations. The ultrathin (5.1 nm) P3HT FETs outperformed the other devices, exhibiting stable device characteristics with a highest field-effect mobility of >2 cm2 V-1 s-1 (effective mobility of 0.83 ± 0.05 cm2 V-1 s-1), on/off ratio of ∼106, low threshold voltage of <-0.6 V, and low gate-leakage current levels of ∼105 below the on-current levels in 10 μm channel length devices. We observed a positive threshold voltage shift in the P3HT/SEGI FETs with decreasing semiconductor thickness. The aforementioned mobility is at least 10 times greater than that of neat P(VDF-HFP) devices. The significant FET performance is attributed to a better insulator/semiconductor interface, efficient hole injection from the Au electrode resulting in a low contact resistance of <500 Ω cm, and boosted charge-carrier densities in the transistor channel. This work demonstrates an excellent approach for carrier mobility enhancement and reliability assessment in low-voltage-operated electrolyte-gated OFETs.