Sorption and diffusion of water vapor in polyimide films

Sorption and diffusion of water vapor are investigated gravimetrically for polyimide films. The activity dependence of the solubility and diffusion coefficients, S and D, respectively, is classified under four types: (1) constant S and D type, (2) dual-mode sorption and transport type, (3) dual-mode type followed by a deviation due to a plasticization effect at high vapor activity, and (4) constant S and D type followed by a deviation due to water cluster formation at high activity. For the dual-mode type, the Henry's law component is much larger than the Langmuir component except at low activity, and therefore deviation in behavior from the first type is small. S is larger for polyimides with higher content of polar groups such as carbonyl, carboxyl, and sulfonyl. D is larger for polyimides with a higher fraction of free space, with some exceptions. The polyimide from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and dimethyl-3,7-diaminodibenzothiophene-5, 5-dioxide belongs to the third type and displays both large S and large D. The polyimide from 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 4,4′-oxydianiline belongs to the fourth type, and has the largest D but rather small S because of the hydrophobic C(CF₃)₂ groups. © 1992 John Wiley & Sons, Inc.     

Lewis acid mediated claisen-type rearrangement of aryl dienyl ethers

In the presence of BF₃·.OEt₂, aryl dienyl ethers were rearranged under mild conditions to afford dienyl phenols in good yields.     

Variational principle for gravitational equations of the Bianchi identity type

It is shown that a square invariant of the Weyl conformal curvature tensor can lead to a Lagrangian in a variational principle for a gravitational equation in vacuum of the Bianchi identity type which is compatible with the Einstein equation. Moreover we show that such a Lagrangian implicitly includes a conformally invariant theory characterized by two gauge fields and the metric tensor.     

Water content distribution in a polymer electrolyte membrane for advanced fuel cell system with liquid water supply

To better understand the operation of a new fuel cell design, we used magnetic resonance imaging (MRI) to measure the water content distribution in a polymer electrolyte membrane under fuel cell operation with and without a supply of liquid water. The supply of liquid water to the membrane improved the cell performance by increasing the water content in the membrane and thus reducing the electrical resistance of the membrane. The study also showed that MRI is a promising method to investigate the distribution of water in the membrane of a fuel cell under operating conditions.     

Evidence for strain glass in the ferroelastic-martensitic system Ti(50-x)Ni(50+x)

We report here "strain glass," a new glassy phenomenon in ferroelastic-martensitic system of Ni-rich intermetallic Ti(50-x)Ni(50+x) (x > 1), where local strain is frozen in disordered configuration below a critical temperature Tg. The ac elastic modulus shows a minimum at Tg, which exhibits logarithmic frequency dependence following Vogel-Fulcher relationship, and the corresponding internal friction shows a frequency-dependent peak located at a lower temperature. In situ high-resolution transmission electron microscopy observations reveal uncorrelated nanoclusters of martensiticlike phase, randomly frozen in the otherwise untransformed parentlike matrix. Being parallel to spin glass and relaxor, strain glass may shed new light on the fundamental physics of glass and lead to the discovery of novel properties.     

Novel stereoselective entry to 2'-beta-carbon-substituted 2'-deoxy-4'-thionucleosides from 4-thiofuranoid glycals

2'-Beta-methyl- and 2'-beta-hydroxymethyl-2'-deoxy-4'-thionucleosides have been synthesized through PhSeCl-mediated electrophilic glycosidation using 4-thiofuranoid glycals having carbon substituents at the C2-position as a glycosyl donor. Preparation of these glycals were carried out by means of the C2 lithiation of 1-chloro-4-thiofuranoid glycal with LTMP followed by the Birch reduction of the chlorine atom. [reaction: see text]