Modeling of a high frequency ultrasonic transducer using periodic structures

Solidly mounted integrated transducers with a Bragg cell inserted between the piezoelectric film and the substrate are investigated for high frequency ultrasonic applications. A numerically stable recursive one dimensional transmission/reflection model was used to analyze the behavior of the periodic structure. This theoretical analysis includes the study of the influence of the acoustic properties of the constitutive layer, the effect of the number of cells and their arrangement. A 35 MHz integrated transducer consisting in a PZT ceramic laid down on a Au/PZT Bragg cell deposited on a porous substrate was fabricated and characterized. Both theoretical and experimental results highlight the interest of using a periodic structure for high frequency ultrasonic applications.     

2‐Amino­thia­zole and 2‐amino­thiazolinone derivatives

The reaction of different substituted α‐cyano­oxiranes with thio­urea resulted in the formation of the 2‐amino­thia­zolinone derivative 2‐amino‐5‐(2,5‐di­methoxy­phenyl)‐1,3‐thia­zol‐4(5H)‐one, C₁₁H₁₂N₂O₃S, (I), and the 2‐amino­thia­zole derivative ethyl 2‐amino‐5‐(2,5‐di­methoxy­phenyl)‐1,3‐thia­zole‐4‐carboxyl­ate, C₁₄H₁₆N₂O₄S, (II). The geometries of the two crystallographically independent mol­ecules in (II) are nearly identical but mirror related. The crystal structures of both compounds contain two types of intermolecular hydrogen bonds.     

Theoretical modeling of the heme group with a hybrid QM/MM method

The quality of the results obtained in calculations with the hybrid QM/MM method IMOMM on systems where the heme group is partitioned in QM and MM regions is evaluated through the performance of calculations on the 4‐coordinate [Fe(P)] (P = porphyrin), the 5‐coordinate [Fe(P)(1−(Me)Im)] (Im = imidazole) and the 6‐coordinate [Fe(P)(1−(Me)Im)(O₂)] systems. The results are compared with those obtained from much more expensive pure quantum mechanics calculations on model systems. Three different properties are analyzed—namely, the optimized geometries, the binding energies of the axial ligands to the heme group, and the energy cost of the biochemically relevant out‐of‐plane displacement of the iron atom. Agreement is especially good in the case of optimized geometries and energy cost of out‐of‐plane displacements, with larger discrepancies in the case of binding energies. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 282–294, 2000     

Enzyme-catalyzed hydrolysis of unsaturated polyester networks. I. study of the hydrolysis of a precursor: Poly(1,2-propanediyl fumarate)

Hydrolytic degradation of poly(1,2-propanediyl fumarate) was carried out in mild experimental conditions using Chromobacterium viscosum lipase as catalyst. The influence of pH, temperature, enzyme concentration, and enzyme inactivation on the initial hydrolysis rate has been analyzed. Enzymatic and chemical processes were compared. This study is an introduction to the enzyme-catalyzed hydrolysis of networks based on unsaturated polyesters. © 1997 John Wiley & Sons, Inc.     

Dodecyl dodecanamide as a model for the study of polyamide n-chlorination. Unusual behaviors of the reaction system

The N-chlorination of dodecyl dodecanamide (DDA) by ter-butyl hypochlorite (tBuOCl) was carried out at 25°C in methylene chloride. The conversion is determined by ¹H-NMR and iodometric titrations of N-chlorinated DDA (N-Cl DDA). Curves of conversion-time plots present singular profiles whose shapes are strongly depending on the reagent and product concentrations.     

Synthesis and characterization of thermotropic poly(alkylene fumarates)-block-poly(arylene fumarates)

Thermotropic poly(2,2′-oxydiethylene fumarate)-block-poly(2-methyl-1,4-phenylene fumarate) C is synthesized by interfacial polycondensation of α,ω-dichlorocarbonyl-poly(2,2′-oxydiethylene fumarate) and fumaroyl chloride with 2-methyl-1,4-dihydroxybenzene. A study of each block is carried out on the corresponding homopolymers as models of flexible A and rigid B blocks. Copolymer C and models are characterized by ¹H and ¹³C NMR, I.R. spectroscopy, SEC, DSC, TGA, X-Ray diffractometry and electron microscopy. A phase segregation is observed in C when B content is above 50%. Copolymer C exhibits a nematic mesophase and can be crosslinked.     

Aluminate and Magnesiate Complexes as Propagating Species in the Anionic Polymerization of Styrene and Dienes

The influence of MgR₂ and AlR₃ additives on alkyllithium initiators in the anionic polymerization of butadiene has been investigated in non polar solvents. A strong decrease of the diene polymerization rate in the presence of the two Lewis acids was observed, similarly to that observed in the retarded anionic polymerisation of styrene. With n,s-Bu₂Mg, the percentage of 1,2 vinyl units increases with the [Mg]/[Li] ratio. This behavior is specific to magnesium derivatives bearing secondary alkyl groups and likely results from the additional complexation of lithium species by free dialkylmagnesium and/or a 1,4- to 1,2- chain end isomerization process during chain exchanges between polybutadienyl active chains and dormant ones attached to magnesium species. These reversible exchanges also lead to the formation of one supplementary chain by initial dialkyl magnesium which acts as reversible chain transfer agent. On the contrary with the R₃Al/RLi systems the number of chains is only determined by the concentration of initial alkyllithium and no modification of the polybutadiene microstructure compared to lithium initiators (1,4 units = 80%) is noticed. Dialkyl magnesiate complexes with alkali metal derivatives (i.e. alkoxide) are also able to influence the stereochemistry of the styrene insertion during the propagation reaction. Polystyrenes with different tacticities ranging from predominantly isotactic (85% triad iso) to syndiotactic (80% triad syndio) can be obtained with these initiators.