High and low molecular weight crossovers in the longest relaxation time dependence of linear cis-1,4 polyisoprene by dielectric relaxations

The dielectric relaxation of cis-1,4 Polyisoprene [PI] is sensitive not only to the local and segmental dynamics but also to the larger scale chain (end-to-end) fluctuations. We have performed a careful dielectric investigation on linear PI with various molecular weights in the range of 1 to 320 kg/mol. The broadband dielectric spectra of all samples were measured isothermally at the same temperature to avoid utilizing shift factors. For the low and medium molecular weight range, the comparisons were performed at 250 K to access both the segmental relaxation and normal mode peaks inside the available frequency window (1 mHz–10 MHz). In this way, we were able to observe simultaneously the effect of molecular mass on the segmental dynamics—related with the glass transition process—and on the end-to-end relaxation time of PI and thus decouple the direct effect of molecular weight on the normal mode from that due to the effect on the monomeric friction coefficient. The latter effect is significant for low molecular weight (M w < 33 kg/mol), i.e., in the range where the crossover from Rouse dynamics to entanglement limited flow occurs. Despite the conductivity contribution at low frequency, careful experiments allowed us to access to the normal mode signal for molecular weights as high as M w = 320 kg/mol, i.e., into the range of high molecular weights where the pure reptation behavior could be valid, at least for the description of the slowest chain modes. The comparison between the dielectric relaxations of PI samples with medium and high molecular weight was performed at 320 K. We found two crossovers in the molecular weight dependence of the longest relaxation time, the first around a molecular weight of 6.5 ± 0.5 kg/mol corresponding to the end of the Rouse regime and the second around 75 ± 10 kg/mol. Above this latter value, we find a power law compatible with exponent 3 as predicted by the De Gennes theory

The effect of ultra-thin graphite on the morphology and physical properties of thermoplastic polyurethane elastomer composites

[EN] Composites of thermoplastic polyurethane (TPU) and ultra-thin graphite (UTG) with concentrations ranging from 0.5 wt.% to 3 wt.% were prepared using a solution compounding strategy. Substantial reinforcing effects with increased loadings are achieved. Compared to neat TPU, values for storage modulus and shear viscosity are enhanced by 300% and 150%, respectively, for UTG concentrations of 3 wt.%. Additionally, an enhancement of thermal properties is accomplished. The crystallization temperature and thermal stability increased by 30 C and 10 C, respectively, compared to neat TPU. Furthermore, the use of oxidized UTG (UTGO) with its added functional oxygen groups suggests the presence of chemical interactions between UTG and TPU, which additionally impact on the thermal properties of the corresponding composites. Controlling the oxidation degree, thus offers further possibilities to obtain composites with tailored properties. The presented approach is straightforward, leads to homogeneous TPU-UTG composites with improved materials properties and is especially suitable for commercial UTG materials and further up-scaled production.This research was supported by IMPIVA under Project (IMIDIP/2010/58), Spanish Ministry of Science and Innovation (MICINN) under Project MAT2010-15026, CSIC under Project 201080E124, and the Government of Aragon (DGA) and the European Social Fund (ESF) under Project DGA-ESF-T66 CNN. M.C. thanks MICINN and ESF for her Grant No. BES-2008-003503. Authors thank Merquinsa S.L. (Barcelona, Spain) and Avanzare S.L. (La Rioja, Spain) for kindly providing polyurethane and ultra-thin graphite samples, respectively.Menes, O.; Cano, M.; Benedito, A.; Giménez Torres, E.; Castell, P.; Maser, WK.; Benito, AM. (2012). The effect of ultra-thin graphite on the morphology and physical properties of thermoplastic polyurethane elastomer composites. Composites Science and Technology. 72(13):1595-1601. https://doi.org/10.1016/j.compscitech.2012.06.016S15951601721

Electronic Spectra & Acidity Constants of Schiff Bases Derived from 2-Amino-5-phynel-1,3,4-thiadiazole & Various Aromatic Aldehydes

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Effect of encapsulated SWNT on the mechanical properties of melt mixed PA12/SWNT composites

Styrene maleic anhydride copolymer (SMA) encapsulated single wall carbon nanotubes (SWNT) are melt mixed with PA12 in order to disperse SANT more homogeneously. The mechanism is found to be a reactive coupling between amine end groups of PA12 and maleic anhydride functionality of SMA during melt mixing of PA12 with SMA modified SWNT. This leads next to a better dispersion to an enhanced interfacial adhesion between PA12 and SWNT as indicated by enhanced rheological, dynamic mechanical, and tensile properties of the PA12/SWNT composites. (C) 200

Fire behaviour of polyamide 6/multiwall carbon nanotube nanocomposites

Nanocomposites of polyamide 6 with 5 wt.% multiwall carbon nanotubes are investigated to clarify their potential as regards the fire retardancy of polymers. The nanocomposites are investigated using SEM, electrical resistivity, and oscillatory shear rheology. The pyrolysis is characterized using thermal analysis. The fire behaviour is investigated with a cone calorimeter using different external heat fluxes, by means of the limiting oxygen index and the UL 94 classification. The fire residue is characterized using SEM. The comprehensive fire behaviour characterization not only allows the materials' potential for implementation in different fire scenarios and fire tests to be assessed, but also provides detailed insight into the active mechanisms. The increased melt viscosity of the nanocomposites and the fibre-network character of the nanofiller are the dominant mechanisms influencing fire performance. The changes are found to be adjuvant with respect to forced flaming conditions in the cone calorimeter, but also deleterious in terms of flammability

Optical urethrotomy under local anaesthesia is a feasible option in urethral stricture disease

The aim of our study was to assess the feasibility of performing optical urethrotomy for urethral stricture disease under local anaesthesia. A total of 33 patients with radiologically proven urethral stricture underwent optical urethrotomy by a single operator under local anaesthesia. Of these patients, 23 (70%) had stricture involving the corpora spongiosum and 18 (55%) of the patients were dependent on supra-pubic catheters. The procedure was successful in 30 cases (91%). The procedure was very well tolerated (average visual analogue pain score of 2/10) with an extremely low complication rate. The large number of patients with urethral stricture disease and the premium on operating time on formal theatre slates encouraged us to perform these procedures under local anaesthetic. Although most patients had severe stricture disease, the majority of cases were successful and very well tolerated. Optical urethrotomy under local anesthesia could be a viable option in the absence of formal theatre time and the facilities to perform general anaesthesia. </jats:p

Shear Stability of EOR Polymers

Summary An experimental study of shear stability of several high-molecular-weight polymers used as mobility-control agents in EOR projects has been performed in well-controlled conditions. The shearing device was made of a capillary tube with an internal diameter (ID) of 125 μm, through which polymer solution was injected at a controlled rate. The setup enables a precise measurement of the shear rate to which the polymer macromolecule is submitted. The degradation rate was measured by the viscosity loss induced by the passage into the capillary tube. The shear rate was gradually increased up to 106 sec–1 while checking degradation rate at each stage. Different commercial EOR polymer products were submitted to the test with polyacrylamide backbone and different substitution monomer groups. All macromolecules behave as flexible coils in solution. The parameters investigated were Molecular weight (between 6 and 20×106)Nature of substitution group (acrylate, ATBS/sulfonate, nVP/ vinyl-pyrrolidone)Salinity Polymer shear degradation increases with molecular weight and salinity, but decreases with the presence of acrylate, ATBS, and nVP. All results can be interpreted in terms of chain flexibility. The highly flexible polyacrylamide homopolymer is the most sensitive to shear degradation. Introduction of acrylate groups in the polymer chain induces some stability because of the rigidity provided by charge repulsion, which vanishes in the presence of high salinity because of the screening of acrylate negative charges. ATBS and VP groups, which are larger in size, provide significant chain rigidity, and thus better shear stability. It is also shown that some very-high-molecular-weight polymers, after passing the shearing device, attain a final viscosity lower than lower-molecular-weight products with the same chemical composition. This factor has to be taken into account in the final choice of a polymer for a given field application. As a comparison, although less popular today than 2 decades ago, xanthan gum (XG), which behaves like a semirigid rod, is shown to be much less sensitive to the shear-degradation test than the coiled polyacrylamides (Sorbie 1991).</jats:p