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911.
Mikhail G. Zolotukhin Javier de Abajo Julio C. Alvarez Jose G. de la Campa Daniel R. Rueda 《Journal of polymer science. Part A, Polymer chemistry》1998,36(8):1251-1256
Linear polyaryl(ether ketones) containing tert-butyl pendent groups were prepared from aromatic hydrocarbons and aromatic diacid chlorides, both classes of monomers containing tert-butyl pendent groups. The polymers were prepared in high yield and high molecular weight by low-temperature precipitation polycondensation in 1,2-dichloroethane. The presence of meta-oriented moieties and bulky pendent groups played a beneficial role with regard to solubility, while the thermal transitions and thermal resistance were not greatly impaired relative to conventional all para-oriented polyaryl(ether–ketones). The current polyaryl(ether–ketones) showed glass transition temperatures in the range 170–240°C and decomposition temperatures, as measured by TGA, of about 500°C. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1251–1256, 1998 相似文献
912.
Julio A. Camarero Joanna Pavel Tom W. Muir 《Angewandte Chemie (International ed. in English)》1998,37(3):347-349
Extremely fast cyclization of the linear polypeptide precursor 1 takes place to form 2 . The reaction appears to be assisted by the native fold of 1 , which positions the reactive ends in close proximity. The circular topology has no influence on the folding or function of 2 . 相似文献
913.
J. M. Garcia de la Vega B. Miguel E. San Fabian 《International journal of quantum chemistry》1997,61(3):533-540
Density functional (DF) calculations of the tetrachloromethane cation and its most important competitive process, the formation of CCl+3, were carried out to explain the possible stability of CCl+4. From results obtained with B-LYP and B-P86 methods, it is possible to produce a slight Jahn-Teller (JT) effect for a Cs planar structure of the cation type CCl2(SINGLE BOND)Cl(SINGLE BOND)Cl+ compatible with the experimental data obtained by electron-spin resonance spectroscopy. A complex of C3v structure CCl+3(SINGLE BOND)Cl which is similar to the previous one found in CF+4 appears when symmetry-broken wave functions are used in HF-LYP and HF-P86 methods. Depending of the DF method employed, either one of the minima [Cs (planar) and C3v] is the most stable and competes with the dissociation of the molecular ion to give CCl+3. The JT stabilization energy is smaller when the JT active coordinates are considered. © 1997 John Wiley & Sons, Inc. 相似文献
914.
In a previous paper a general procedure for deriving stabilized finite element schemes for advective type problems based on invoking higher order balance laws over finite size domains was presented. This provides an expression for the element stabilization parameter in terms of the solution residual and its first derivatives in a kind of iterative or adaptative manner. Details of the application of this procedure to 1D and 2D advective–diffusive problems are given. Some examples of applications showing the potential of the new approach are presented. © 1997 John Wiley & Sons, Ltd. 相似文献
915.
Jan Janczak Genivaldo Julio Perptuo 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(6):o349-o352
The crystal structure of the title melaminium salt, bis(2,4,6‐triamino‐1,3,5‐triazin‐1‐ium) dl ‐malate tetrahydrate, 2C3H7N6+·C4H4O52−·4H2O, consists of singly protonated melaminium residues, dl ‐malate dianions and water molecules. The melaminium residues are connected into chains by four N—H⃛N hydrogen bonds, and these chains form a stacking structure along the c axis. The dl ‐malate dianions form hydrogen‐bonded chains and, together with hydrogen‐bonded water molecules, form a layer parallel to the (100) plane. The conformation of the malate ion is compared with an ab initio molecular‐orbital calculation. The oppositely charged moieties, i.e. the stacks of melaminium chains and hydrogen‐bonded dl ‐malate anions and water molecules, form a three‐dimensional polymeric structure, in which N—H⃛O hydrogen bonds stabilize the stacking. 相似文献
916.
917.
918.
Lucia H. Innocentini-Mei Julio R. Bartoli Rodrigo C. Baltieri 《Macromolecular Symposia》2003,197(1):77-88
Poly(3-hydroxybutyrate) (PHB) is a highly crystalline, biodegradable and biocompatible thermoplastic. However, its limited utilization as a commodity plastic is associated to both high cost and very poor mechanical properties. Blending PHB with a natural polymer, such as starch, is one way to improve its properties and to get low price raw materials, though they are not miscible since there are no strong interactions between the hydrophilic starch and the hydrophobic PHB. In this study binary blends of PHB were prepared with natural starch, starch-adipate and grafted starch-urethane derivatives. The PHB blends were characterized in terms of their mechanical and thermal properties. For all blends a decrease of the Young modulus was observed as compared to the pure PHB. However, blends containing natural starches and starch adipate resulted in brittle materials. A significant decrease of both glass transition temperature (Tg) and melting point (Tm) was observed for all formulations. The best results, lower modulus and Tg were obtained with grafted starch-urethane blends using poly(propylene glycol). 相似文献
919.
Roman Klinkner 《Nachrichten aus der Chemie》2002,50(11):1302-1304
920.
A t-(v, k, λ) covering is an incidence structure with v points, each block incident on exactly k points, such that every set of t distinct points is incident on at least λ blocks. By considering certain geometries over finite principal ideal rings, we construct infinite families of t-(v, k, λ) coverings having many interesting combinatorial properties. © 1999 John & Sons, Inc. J Combin Designs 7: 247–268, 1999 相似文献