RESUMO
In this paper, the experimental characterization of the viscoelastic properties of thermoplastic polyurethane (TPU) samples through creep experiments is presented. Experiments were conducted at different constant temperature levels (15, 25, and 35 ∘C), for three different tensile stress levels (0.3, 0.5, and 0.7 MPa), and at different physisorbed water contents, providing access to: (i) the temperature dependency of creep parameters and (ii) the assessment, if behavior is indeed viscoelastic. The physisorbed water content was achieved by exposing virgin samples to environments with relative humidity ranging from 0 to 80 percent until mass stability was reached. Creep tests were conducted immediately afterwards with this particular humidity level. The main results of this study are as follows. The temperature dependency of the obtained creep parameters is well described in Arrhenius plots. With regard to water content, two prototype material responses were observed in the experimental program and accurately modeled using the following fractional-type models: (i) Scott Blair-type (i.e., power-law-type) only behavior, pronounced for the combination of low water content/low temperature; (ii) combined Scott Blair plus Lomnitz (i.e., log-type) behavior for high water content/high temperature. This change in behavior associated with certain thresholds for the specified environmental conditions (temperature and relative humidity) may indicate the initiation of hydrogen bond breakage and rearrangement (carbamate H-bonds and physisorbed water H-bonds). Regarding the short-term or quasi-instantaneous behavior, the Scott Blair element seems highly appropriate and may be better suited than the standard elastic model: the Hookean spring. We associated Scott Blair behavior with the load-induced, quasi-instantaneous re-arrangement of polymer network chains. The secondary viscoelastic mechanism associated with the Lomnitz element, hydrogen bond breakage and rearrangement, comes into play for higher temperatures and/or higher physisorbed water contents. In this case, the contribution of the two constitutive elements is well separated due to the large number of the characteristic time of the Lomnitz element, much larger than the respective value for the Scott Blair element.
RESUMO
The structure of [IrCl2(C58H51N3P4)]Cl·5.5CH3CN or [IrCl2(NHCHPh)(((dppm)C(N2dppm))-κ 3P,C,P)]Cl·5.5CH3CN [3, dppm = bis-(di-phenyl-phos-phino)methane; systematic name: di-chlorido(1,1,3,3,7,7,9,9-octa-phenyl-4,5-di-aza-1,3λ5,7λ4,9-tetra-phosphanona-3,5-dien-6-yl-κ2 P 1,P 9)-(phenyl-methanimine-κN)iridium(III) chloride aceto-nitrile hemihendeca-solvate], resulting from an oxygen-mediated cleavage of a triazeneyl-idene-phospho-rane ligand producing a diazo-methyl-ene-phospho-rane and a nitrene moiety, which in turn rearrange via a Staudinger reaction and a 1,2-hydride shift to the first title complex, involves a six-coordinate IrIII complex cation coordinated by a facial PCP pincer ligand, a benzaldimine and two chlorido ligands. The pincer system features a five- and a seven-membered ring, with the central divalent carbon of the PCP pincer ligand being connected to a phosphine and a diazo-phospho-rane. The chlorido ligands are positioned trans to the central carbon atom and to the phospho-rus donor of the seven-membered ring of the pincer system, respectively. A chloride ion serves as counter-ion for the monocationic complex. The structure of [IrI(C26H22N2P2)(C26H22P2)(C6H7N)]I(I3)·0.5I2·CH3OH·0.5CH2Cl2 or [IrI(NHCHPh)((dppm)C(N2)-κ 2P,C)(dppm-κ 2P,P')]I(I3)·0.5I2·CH3OH·0.5CH2Cl2 {4, systematic name: (4-diazo-1,1,3,3,-tetra-phenyl-1,3λ4-diphosphabutan-4-yl-κP 1)iodido[methyl-enebis(di-phenyl-phosphine)-κ2 P,P'](phenyl-methanimine-κN)iridium(III) iodide-triiodide-di-chloro-methane-iodine-methanol (2/2/1/1/2)}, accessed via treatment of the triazeneyl-idene-phospho-rane complex [Ir((BnN3)C(dppm)-κ 3P,C,N)(dppm-κ 2P,P')]Cl with hydro-iodic acid, consists of a dicationic six-coordinate IrIII complex, coordinated by a bidentate diazo-methyl-ene-phospho-rane, a benzaldimine, a chelating dppm moiety and an iodido ligand. The phospho-rus atoms of the chelating dppm are trans to the central carbon atom of the diazo-methyl-ene-phospho-rane and the iodide ligand, respectively. Both an iodide and a triiodide moiety function as counter-ions. The aceto-nitrile solvent mol-ecules in 3 are severely disordered in position and occupation. In 4, the I3 - anion is positionally disordered (ratio roughly 1:1), as is the I- anion with a ratio of 9:1. The di-chloro-methane solvent mol-ecule lies near a twofold rotation axis (disorder) and was refined with an occupancy of 0.5. Another disorder occurs for the solvent methanol with a 1:1 ratio.
RESUMO
The crystal structure of the methanol hemisolvate of 5,5-dibromobarbituric acid (1MH) displays an H-bonded layer structure which is based on N-Hâ¯O=C, N-Hâ¯O(MeOH) and (MeOH)O-Hâ¯O interactions. The barbiturate molecules form an H-bonded substructure which has the fes topology. 5,5'-Methanediylbis(5-bromobarbituric acid) 2, obtained from a solution of 5,5-dibromobarbituric acid in nitromethane, displays a N-Hâ¯O=C bonded framework of the sxd type. The conformation of the pyridmidine ring and the lengths of the ring substituent bonds C5-X and C5-X' in crystal forms of 5,5-dibromobarbituric acid and three closely related analogues (X = X' = Br, Cl, F, Me) have been investigated. In each case, a conformation close to a C5-endo envelope is correlated with a significant lengthening of the axial C5-X' in comparison to the equatorial C5-X bond. Isolated molecule geometry optimizations at different levels of theory confirm that the C5-endo envelope is the global conformational energy minimum of 5,5-dihalogenbarbituric acids. The relative lengthening of the axial bond is therefore interpreted as an inherent feature of the preferred envelope conformation of the pyrimidine ring, which minimizes repulsive interactions between the axial substituent and pyrimidine ring atoms.
