Studies of magnetic resonance phenomena in polymers. II. Molecular motions in poly(methyl methacrylate) as studied by spin-probe and spin-label methods

Thin films of spin-probed and spin-labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) in the temperature range of 77–520°K. The rotational correlation times of nitroxides used as spin probes and labels have been determined as a function of temperature from which activation energies are also determined. The nitroxide rotational times are found to strongly correlate with local segmental and side-chain motions of the host PMMA matrix. Five discrete molecular motions are detected in PMMA along with their activation energies as measured: side-chain CH₃ rotation (1 Kcal/mol), main-chain CH₃ rotation (2 Kcal/mol), ester side-chain COOCH₃ rotation around the C? C bond (4 Kcal/mol), main-chain C? C bond rotation (6 Kcal/mol), and side-chain OCH₃ rotation around the C? O bond (18 Kcal/mol). The activation energies determined by ESR are consistent with the potential-energy barriers calculated theoretically for various rotations in PMMA. It is concluded that the probe and label rotational motions do respond to localized, small-scale segmental and side-chain motions of host polymers but are relatively ineffective in response to the large-scale segmental motion with an activation energy larger than 20 Kcal/mol in the case of PMMA.     

Heat transfer enhancement through liquid film evaporation into countercurrent moist air flow in a vertical plate channel

A numerical analysis has been carried out to study the heat removal process from hot channel plate through liquid film evaporation into a countercurrent air flow. The influences of the wall heat flux, the inlet Reynolds number of liquid film and the inlet Reynolds number of moist air on the transfer characteristics are investigated detailedly. The Results show that the interface latent heat transfer associated with the film vaporization causes a temperature drop of the heated plate in the entry region of air flow, which is more significant for a system with higherq _w , lowerRe _l,in or largerRe _{c, in} . The overall temperature rise of the heated wall is rather small, as compared with the case without interface latent heat transfer. In addition, the difference in results obtained by the one-dimensional and two-dimensional methods is substantial.Die numerische Untersuchung bezieht sich auf den Wärmetransportprozeß von heißen, plattenförmigen Kanalwänden durch Flüssigfilmverdampfung in gegenströmende Luft. Die Einflüsse des Wärmeflusses, der Reynolds-Zahlen, des Flüssigkeitfilms und der Feuchtluft (jeweils am Eintritt) auf das Wärmeübertragungsverhalten werden eingehend untersucht. Die Ergebnisse zeigen, daß die bei der Filmverdampfung eingespeicherte latente Wärme eine Temperaturabnahme der Heizplatte am Eintritt der Luft bewirkt, die mit den Wärmefluß und steigender Reynolds-Zahl für Feuchtluft zunimmt. Die gesamte Temperaturerhöhung der beheizten Wand ist sehr gering im Vergleich mit dem Fall ohne Latentwärmeaustausch. Darüber hinaus resultieren erhebliche Unterschiede in den Ergebnissen, je nachdem, ob eindimensionale oder zweidimensionale Methoden angewandt werden.The financial support of this study by the engineering division of the National Science Council, Taiwan, R.O.C., through the contract NSC 82-0401-E-150-049 is greatly appreciated.     

An improved preparation of 3,5-bis(trifluoromethyl)acetophenone and safety considerations in the preparation of 3,5-bis(trifluoromethyl)phenyl Grignard reagent

An improved and efficient bromination of 3,5-bis(trifluoromethyl)benzene was developed. A safe and reliable preparation of the potentially explosive 3,5-bis(trifluoromethyl)phenyl Grignard and 3-trifluoromethylphenyl Grignard reagents, from the precursor bromides, is described. Reaction System Screening Tool (RSST) and Differential Thermal Analysis (DTA) studies suggest these trifluoromethylphenyl Grignard reagents can detonate on loss of solvent contact or upon moderate heating. When prepared and handled according to the methods described herein, these Grignard reagents can be safely prepared and carried on to advanced intermediates.     

New light on quantum dot cytotoxicity

As quantum dots are beginning to be used for in vivo imaging, the question of their long-term effect on cell viability is becoming critical. In this issue of Chemistry & Biology, Lovri? and colleagues examine the likely role of reactive oxygen species in quantum dot cytotoxicity .