Multi-arm Star Polyisobutylenes. V. Characterization of Multi-arm Polyisobutylene Stars by Viscometry,Pour Points,Electron Microscopy,and Ultrasonic Shear Degradation

Abstract

Structure/property relationship of multi-arm star polyisobutylenes [?-(PIB) _n s] were characterized by a variety of techniques, including vis-cometry, pour points, electron microscopy, and ultrasonic degradation. The intrinsic viscosity of ?-(PIB) _n s changes very little with temperature in the 30 to 100°C range, whereas that of linear PIBs of the same molecular weight increases strongly with temperature. Kinematic viscosity measurements of select ?-(PIB) _n s gave viscosity indices in excess of 130. The viscosity of ?-(PIB) _n s is mainly determined by the molecular weight of the arms and much less by the number of arms or overall molecular weights. Electron microscopy of ?-(PIB) _n s indicates a compact spherical morphology, a conclusion that was substantiated by radius of gyration measurements. Pour points of ?-(PIB) _n s are ?27°C. Ultrasonic studies gave insight into the mechanism of shear degradation of ?-(PIB) _n s. These characteristics render ?-(PIB) _n s of interest as rheology control additives for motor oils.     

The Effect of the Electron Density Distribution of Guest on the Entropy Change During Complex Formation of Calix[6]arene Hexasulfonate Host with ortho- and para-cresols as Guests

The π–π interaction-based inclusion complexation of calix[6 Danil de Namor, A. F., Cleverly, R. M. and Zapata-Ormachea, M. L. 1998. Chem. Rev., 98: 2495[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]]arene hexasulfonate as host with neutral aromatic guest molecules was studied in aqueous media. To vary the distribution of electron density on the guest's aromatic rings, the phenol parent compound was substituted in the para- or ortho- positions with CH₃ group. To study the interaction between calixarene and the guests, PL, DSC and quantum-chemical methods were used. The results indicate 1:1 stoichiometry of the formed host-guest complexes. Although the enthalpy change during complex formation of calixarene with p- or o-cresol are the same, the Gibbs free energy change is significantly higher in the case of calixarene—o-cresol complexes. This property is due to the unexpected entropy change during the complex formation. Using molecular dynamic calculations, a guest-induced redistribution of the electron density on the calixarene rings, followed by the restructuring of the solvent molecules was identified as a background of this unexpected entropy change at molecular level.     

Influence of the degree of ionization and molecular mass of weak polyelectrolytes on charging and stability behavior of oppositely charged colloidal particles

Positively charged amidine latex particles are studied in the presence of poly(acrylic acid) (PAA) with different molecular masses under neutral and acidic conditions by electrophoresis and time-resolved dynamic light scattering. Under neutral conditions, where PAA is highly charged, the system is governed by the charge reversal induced by the quantitatively adsorbing polyelectrolyte and attractive patch-charge interactions. Under acidic conditions, where PAA is more weakly charged, the following two effects come into play. First, the lateral structure of the adsorbed layers becomes more homogeneous, which weakens the attractive patch-charge interactions. Second, polyelectrolyte adsorption is no longer quantitative and partitioning into the solution phase is observed, especially for PAA of low molecular mass.     

Charging and stability of anionic latex particles in the presence of linear poly(ethylene imine)

Charging properties and colloidal stability of negatively charged polystyrene latex particles were investigated in the presence of linear poly(ethylene imine) (LPEI) of different molecular masses by electrophoresis and dynamic light scattering (DLS). Electrophoretic mobility measurements illustrate that LPEI strongly adsorbs on these particles leading to charge neutralization at isoelectric point (IEP) and charge reversal. Time-resolved DLS experiments indicate that the aggregation of the latex particles is rapid near the IEP and slows down away from this point. Surprisingly, the colloidal stability does not depend on the molecular mass, which indicates that the adsorbed LPEI layer is rather homogeneous.     

Fungal cyclooligomer depsipeptides: from classical biochemistry to combinatorial biosynthesis

This review surveys the biological activities and the iterative and recursive biosynthetic mechanisms of fungal cyclooligomer depsipeptides, and their structural diversification by various combinatorial biosynthetic methods.     

Palmarumycins BG1-BG7 and preussomerin BG1: establishment of their absolute configurations using theoretical calculations of electronic circular dichroism spectra

Palmarumycins BG1-BG7 (1-7), seven new compounds related to palmarumycins, were isolated from the aerial parts of Bruguiera gymnorrhiza as well as a new preussomerin derivative BG1 (8). The structures of these compounds were determined mainly by the analysis of their NMR and MS data, and their relative configurations were assigned on the basis of their (3)J(H,H) coupling constants. Compounds 4 and 7 have a sulfate group that is unprecedented among members of spirodioxynaphthalene-type natural products. The absolute configurations of 1-8 were determined by TDDFT CD calculations of the solution conformers. Compound 5 displayed inhibitory activity against HL 60 and MCF-7 cell lines.     

Geometry and models in chemistry

Madame Curie was awarded her second Nobel Prize in 1911 and on this occasion it was in chemistry. Honoring the centennial of this event, the world celebrates chemistry in 2011. Chemistry serves the world every minute every year and the world reciprocates with paying attention to this often cursed but uniquely useful science during this particular year.     

Sensitive and robust electrophoretic NMR: instrumentation and experiments

Although simple as a concept, electrophoretic NMR (eNMR) has so far failed to find wider application. Problems encountered are mainly due to disturbing and partly irreproducible convection-like bulk flow effects from both electro-osmosis and thermal convection. Additionally, bubble formation at the electrodes and rf noise pickup has constrained the typical sample geometry to U-tube-like arrangements with a small filling factor and a low resulting NMR sensitivity. Furthermore, the sign of the electrophoretic mobility cancels out in U-tube geometries. We present here a new electrophoretic sample cell based on a vertically placed conventional NMR sample tube with bubble-suppressing palladium metal as electrode material. A suitable radiofrequency filter design prevents noise pickup by the NMR sample coil from the high-voltage leads which extend into the sensitive sample volume. Hence, the obtained signal-to-noise ratio of this cell is one order of magnitude higher than that of our previous U-tube cells. Permitted by the retention of the sign of the displacement-related signal phase in the new cell design, an experimental approach is described where bulk flow effects by electro-osmosis and/or thermal convection are compensated through parallel monitoring of a reference signal from a non-charged species in the sample. This approach, together with a CPMG-like pulse train scheme provides a superior first-order cancellation of non-electrophoretic bulk flow effects.     

Estimation of effective doses to cavers based on radon measurements carried out in seven caves of the Bakony Mountains in Hungary

Nowadays, as the practice of extreme sports is spreading, potholing is becoming more and more popular. As a result, both the number of cavers and the time spent in the caves have been on the rise. There are some cavers known to have spent some 5000 h in caves over a span of 10 years. In poorly ventilated caves, radon exhalated from cave rocks and deposits may accumulate and cause significant doses to cavers.In this study, the radon concentration in seven caves in the Bakony Mountains, Hungary, was measured by continuous and integrated measurement devices. Measured values for the different caves were rather different, and varied between 50 and 24,000 Bq m^?3. The average radon concentration over the measurement period was approximately 10,000 Bq m^?3 in five of the seven caves inspected.By assuming an average of 470 h year^?1 spent in caves, effective doses to cavers were estimated. The expected annual effective dose, in case of an equilibrium factor of 0.6, was 19.7 mSv.