Effect of POSS on crystalline transitions and physical properties of polyamide 12

Polyamide 12/Trisilanolphenyl‐POSS (PA 12/POSS) composites were prepared via melt‐compounding. The effect of polyhedral oligomeric silsesquioxane (POSS) on crystalline structure and crystalline transition of PA 12 was investigated by wide‐angle X‐ray diffraction (WAXD) and real time fourier transform infrared spectroscopy (FTIR). WAXD results indicated that PA 12 crystallized into γ‐form as slowly cooling from melt and the presence of POSS did not influence the crystalline structure of PA 12. Both PA 12 and PA 12/POSS composites underwent Brill transitions when they were heated from room temperature to melt point. Real time FTIR patterns showed that an absorption band at 697 cm^?1 ascribed to Amide V (α) mode was emerged along with the disappearance of Amide VI (γ) band at 628 cm^?1 with the increase of the temperature for PA 12 and PA 12/POSS composites, which suggested that the γ‐form crystalline has transformed into α form. The Brill bands were identified and the transformed mechanism was discussed based on the real FTIR results. The addition of POSS enhanced the tensile strength and thermal stability of PA 12. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 121–129, 2009     

Repeated Downsian electoral competition

We analyze an infinitely repeated version of the Downsian model of elections. The folk theorem suggests that a wide range of policy paths can be supported by subgame perfect equilibria when parties and voters are sufficiently patient. We go beyond this result by imposing several suitable refinements and by giving separate weak conditions on the patience of voters and the patience of parties under which every policy path can be supported. On the other hand, we show that only majority undominated policy paths can be supported in equilibrium for arbitrarily low voter discount factors: if the core is empty, the generic case in multiple dimensions, then voter impatience leads us back to the problem of non-existence of equilibrium. We extend this result to give conditions under which core equivalence holds for a non-trivial range of voter and party discount factors, providing a game-theoretic version of the Median Voter Theorem in a model of repeated Downsian elections. J. Duggan was supported by the National Science Foundation, grant number 0213738, is gratefully acknowledged.     

Effective interactions in the proton optical-model potential

A six-parameter optical model is developed in which the real central part is calculated by folding several effective nucleon-nucleon interactions into matter distributions which reproduce single-particle binding energies and electron scattering data. A simple local approximation is made to take into account the exchange term. It is concluded that the density-dependent effective interactions derived from nuclear-structure calculations are also appropriate for nucleon-nucleus scattering. Off-shell effects are apparent from the worsening of the quality of fit for light nuclei.     

A rapidly convergent method for solving the static Hartree-Fock problem

An improved version of the direct density method for solving the static Hartree-Fock equations is presented. A simple relation with the imaginary time method is exhibited.     

N‐Heterocyclic Carbene Catalyzed (5+1) Annulations Exploiting a Vinyl Dianion Synthon Strategy

Direct polarity inversion of conjugate acceptors provides a valuable entry to homoenolates. N‐heterocyclic carbene (NHC) catalyzed reactions, in which β‐unsubstituted conjugate acceptors undergo homoenolate formation and C?C bond formation twice, have been developed. Specifically, the all‐carbon (5+1) annulations give a range of mono‐ and bicyclic cyclohexanones (31 examples). In the first family of annulations, β‐unsubstituted acrylates tethered to a divinyl ketone undergo cycloisomerization, providing hexahydroindenes and tetralins. In the second, partially untethered substrates undergo an intermolecular (5+1) annulation involving dimerization followed by cycloisomerization. While enantioselectivity was not possible with the former, the latter proved viable, allowing cyclohexanones to be produced with high levels of enantiopurity (most >95:5 e.r.) and exclusive diastereoselectivity (>20:1 d.r.). Derivatizations and mechanistic studies are also reported.     

Ring expansion reactions of 4-amino-1,1-dioxo-[1,2,3,5]-thiatriazoles

An unusual ring-expansion reaction of 4-amino-1,1-dioxo-[1,2,3,5]-thiatriazoles has been identified that produces the relatively rare 5-amino-1,1-dioxo-[1,2,4,6]-thiatriazines and. Initial alkylation of the thiatriazole with alpha-halo-esters at N-3 produces alpha-substituted esters which, under basic reaction conditions, undergo opening of the thiatriazole ring and re-closure to a thiatriazine ring. Similar alkylations of with diethyl chloromalonate and ethyl dichloroacetate lead to the loss of SO2 and the production of triazine and triazole, apparently by an initial alkylation at N-5. The reaction of with phenacyl bromides or a phenacyl dibromide forms fully unsaturated 5-amino-1,1-dioxo-[1,2,4,6]-thiatriazines.     

Synthesis and biological evaluation of anthranilamide-based non-peptide mimetics of ω-conotoxin GVIA

Non-peptide mimetics based on an anthranilamide ‘scaffold’ possessing fragments that mimic Lys2, Tyr13 and Arg17 in ω-conotoxin GVIA have been prepared. Compounds were assayed for binding to the voltage-gated calcium channels Ca_v2.2 (‘N-type’) and Ca_v2.1 (‘P/Q-type’) in rat brain. The primary synthetic target, 2-(6-amino-hexanoylamino)-5-(3-guanidino-propoxy)-N-[4-(4-hydroxyphenoxy)-phenyl]-benzamide (2a), exhibited low μM binding to Ca_v2.2 and was more than 30-fold selective for Ca_v2.2 over Ca_v2.1.     

Photoluminescence decay times in (AlGa)As  GaAs multiple quantum well heterostructures

Measurements are reported of the photoluminescence decay rate from 55 Å GaAs(Al_0.35Ga_0.65)As multiple quantum well samples prepared by molecular beam epitaxy. Results are reported for the temperature range 4–295 K. Additional measurements of the external photoluminescence efficiency of single thin layers of (AlGa)As lead us to conclude that at the higher temperatures the lifetime in the quantum wells is limited by non-radiative recombination at or close to the (AlGa)As barriers. Following studies of the decay at 4 K we offer a possible alternative explanation of the free exciton decay mechanism to that postulated previously.