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

Rheological properties and percolation in suspensions of multiwalled carbon nanotubes in polycarbonate

This paper is concerned with several issues related to the rheological behavior of polycarbonate/ multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.Peer reviewed: YesNRC publication: Ye